WO2016063280A2

WO2016063280A2 - Stimulated emission and laser effects in optically pumped plasma

Info

Publication number: WO2016063280A2
Application number: PCT/IL2015/051034
Authority: WO
Inventors: Lev Nagli; Michael Gaft
Original assignee: Laser Distance Spectrometry Ltd.
Priority date: 2014-10-20
Filing date: 2015-10-20
Publication date: 2016-04-28
Also published as: WO2016063280A3

Abstract

A system for collimated light generation including a substrate disposed in open space, an energy source providing energy to the substrate, thereby causing formation of a plasma on the substrate in the open space, and an optical pump providing optical pumping to the plasma, such that stimulated emission of collimated light by the plasma occurs.

Description

STIMULATED EMISSION AND LASER EFFECTS IN OPTICALLY PUMPED PLASMA

REFERENCE TO RELATED APPLICATIONS

Reference is hereby made to U.S. Provisional Patent Application 62/066,074 , entitled OPTICALLY PUMPED LASER INDUCED PLASMA, filed October 20, 2014, the disclosure of which is hereby incorporated by reference and priority of which is hereby claimed pursuant to 37 CFR 1.78(a)(4) and (5)(i).

FIELD OF THE INVENTION

The present invention relates generally to media exhibiting stimulated emission and more specifically to plasma media exhibiting stimulated emission and laser effects.

BACKGROUND OF THE INVENTION

Various types of media exhibiting stimulated emission and operating as lasers are known in the art.

SUMMARY OF THE INVENTION

The present invention seeks to provide novel systems and methods for the generation of collimated light by optically pumped plasma in open space, and for laser effects based thereon.

There is thus provided in accordance with a preferred embodiment of the present invention a system for collimated light generation including a substrate disposed in open space, an energy source providing energy to the substrate, thereby causing formation of a plasma on the substrate in the open space, and an optical pump providing optical pumping to the plasma, such that stimulated emission of collimated light by the plasma occurs.

Preferably, the system for collimated light generation also includes an optical resonator cavity, the plasma operating as a laser when disposed in the optical resonator cavity.

In accordance with a preferred embodiment of the present invention, the energy source includes a laser and the energy includes laser radiation.

Preferably, the system for collimated light generation also includes a cylindrical lens for focusing the laser radiation on the plasma.

In accordance with another preferred embodiment of the present invention, the energy source includes a high voltage pulse generator and the energy includes high voltage pulses.

Preferably, the substrate includes a positive electrode connected to the high voltage pulse generator, the positive electrode including a substrate material.

Preferably, the system for collimated light generation also includes a ground electrode formed by the substrate material, the plasma being formed between the positive electrode and the ground electrode.

Preferably, the plasma includes a multiplicity of plasmas.

In accordance with a further preferred embodiment of the present invention, the system for collimated light generation also includes a delay generator connected to the energy source and the optical pump, for coordinating provision of the energy and the optical pumping.

Preferably, the optical pumping is delayed with respect to the provision of the energy by 2 - 10 microseconds.

Preferably, the optical pumping is delayed with respect to the provision of the energy by

4 - 5 microseconds.

Preferably, the optical pump includes an optical parametric oscillator. Alternatively, the optical pump includes a flash-lamp or a laser diode.

In accordance with a preferred embodiment of the present invention, the collimated light has an emission direction and the optical pumping is incident on the plasma in a direction generally parallel to the emission direction.

Preferably, the system for collimated light generation also includes a dichroic mirror, the optical pumping being reflected by the dichroic mirror onto the plasma.

In accordance with another preferred embodiment of the present invention, the collimated light has an emission direction and the optical pumping is incident on the plasma in a direction generally perpendicular to the emission direction.

Preferably, the system for collimated light generation also includes a focusing lens for focusing the optical pumping on the plasma.

Additionally or alternatively, the system for collimated light generation also includes a cylindrical lens for focusing the optical pumping on the plasma.

Preferably, the optical resonator cavity includes a forward fully reflecting mirror and a backward partly reflecting mirror, the plasma being disposed between the forward fully reflecting mirror and the backward partly reflecting mirror.

Preferably, the forward and backward mirrors include broadband reflecting mirrors.

Preferably, at least one of the mirrors includes a metal-coated surface.

Preferably, the metal-coated surface includes a metal-coated glass surface. Alternatively, the metal-coated surface includes a metal-coated quartz surface.

Preferably, the surface is concave. Alternatively, the surface is convex or substantially flat.

Preferably, the forward and backward mirrors include selective reflecting and transmitting mirrors.

Preferably, the backward partly reflecting mirror is 90% reflective.

Preferably, the substrate is at ambient temperature.

Preferably, the substrate includes at least one chemical element.

Preferably, the at least one chemical element is conductive. Alternatively, the at least one chemical element is non-conductive.

Preferably, the substrate includes an alloy. Preferably, the least one chemical element belongs to the 13 group of elements of the periodic table.

Additionally or alternatively, the at least one chemical element belongs to the 14^th group of elements of the periodic table.

Preferably, the substrate has a spontaneous transition probability of at least 10 7 sec -^" 1.

There is further provided in accordance with a preferred embodiment of the present invention a method for generating collimated light including disposing a substrate in open space, delivering energy from an energy source to the substrate, thereby causing formation of a plasma on the substrate in the open space, an optically pumping the plasma by an optical pump, such that stimulated emission of collimated light by the plasma occurs.

Preferably, the method also includes disposing the plasma in an optical resonator cavity, the plasma operating as a laser when disposed in the optical resonator cavity.

Preferably, the method also includes providing a cylindrical lens for focusing the laser radiation on the plasma.

Preferably, the substrate includes a positive electrode receiving the high voltage pulses, the positive electrode including a substrate material.

Preferably, the method also includes providing a ground electrode cooperating with the positive electrode, the ground electrode being formed by the substrate material, the plasma being formed between the positive electrode and the ground electrode.

Preferably, the plasma includes a multiplicity of plasmas.

In accordance with yet another preferred embodiment of the present invention, the method also includes providing a delay generator connected to the energy source and the optical pump, for coordinating provision of the energy and the optical pumping.

Preferably, the optical pumping is delayed with respect to the provision of the energy by 4 - 5 microseconds. In accordance with yet a further preferred embodiment of the present invention, the optical pump includes an optical parametric oscillator.

Alternatively, the optical pump includes a flash-lamp or laser diode.

In accordance with a preferred embodiment of the present invention the colhmated light has an emission direction and the optical pumping is incident on the plasma in a direction generally parallel to the emission direction.

Preferably, the method also includes providing a dichroic mirror for reflecting the optical pumping onto the plasma.

Preferably, the method also includes providing a focusing lens for focusing the optical pumping onto the plasma.

Additionally or alternatively, the method also includes providing a cylindrical lens for focusing the optical pumping onto the plasma.

Preferably, at least one of the mirrors includes a metal-coated surface.

Preferably, the metal-coated surface includes a metal-coated glass surface.

Alternatively, the metal-coated surface includes a metal-coated quartz surface.

Preferably, the backward partly reflecting mirror is 90% reflective.

Preferably, the substrate is at ambient temperature.

Preferably, the substrate includes at least one chemical element.

Preferably, the at least one chemical element is conductive. Alternatively, the at least one chemical element is non-conductive. Preferably, the substrate includes an alloy.

Preferably, the at least one chemical element belongs to the 13^th group of elements of the periodic table.

There is additionally provided in accordance with still another preferred embodiment of the present invention a laser including an optical resonator cavity, a substrate disposed in open space, in the optical resonating cavity, an energy source providing energy to the substrate, thereby causing formation of a plasma on the substrate in the open space and an optical pump providing optical pumping to the plasma, such that stimulated emission of laser light by the plasma occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

Fig. 1 is a highly simplified schematic illustration of a system for collimated light generation constructed and operative in accordance with a preferred embodiment of the present invention;

Fig. 2 is a simplified partially schematic, partially block-diagram illustration of a system for collimated light generation, constructed and operative in accordance with another preferred embodiment of the present invention;

Fig. 3 is a simplified partially schematic, partially block-diagram illustration of a system for collimated light generation, constructed and operative in accordance with a further preferred embodiment of the present invention;

Fig. 4 is a simplified partially schematic, partially block-diagram illustration of a system for collimated light generation, constructed and operative in accordance with yet another preferred embodiment of the present invention;

Fig. 5 is a simplified partially schematic, partially block-diagram illustration of a system for collimated light generation, constructed and operative in accordance with yet a further preferred embodiment of the present invention;

Fig. 6 is a simplified graphical representation of pulse synchronization in a system of any one of the types shown in Figs. 1 - 5;

Figs. 7A, 7B, 8 and 9 are simplified respective graphical representations of emission spectra generated by an Aluminum substrate in a system of any one of the types shown in Figs. 1 - 5 and of the energy dependence thereof, and an energy generation scheme diagram corresponding thereto;

Figs. 10 and 11 are simplified respective representations of emission spectra generated by a Thallium substrate in a system of any one of the types shown in Figs. 1 - 5 and an energy generation scheme diagram corresponding thereto;

Figs. 12 and 13 are simplified respective representations of emission spectra generated by a Germanium substrate in a system of any one of the types shown in Figs. 1 - 5 and an energy generation scheme diagram corresponding thereto; Figs. 14 and 15 are simplified respective representations of emission spectra generated by a Titanium substrate in a system of any one of the types shown in Figs. 1 - 5 and an energy generation scheme diagram corresponding thereto;

Figs. 16A, 16B and 17 are simplified respective first and second graphical representations of emission spectra generated by an Iron substrate in a system of any one of the types shown in Figs. 1 - 5 and an energy generation scheme diagram corresponding thereto;

Fig. 18 is a simplified graphical representation of emission spectra generated by a Titanium Aluminium alloy in a system of any one of the types shown in Figs. 1 - 5; and

Fig. 19 is a simplified partially schematic, partially block-diagram illustration of a system for collimated light generation, constructed and operative in accordance with still another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to Fig. 1 , which is a highly simplified schematic illustration of a system for collimated light generation, constructed and operative in accordance with a preferred embodiment of the present invention.

As seen in Fig. 1, there is provided a collimated light generation system 100, preferably including a substrate 102 having a surface 104. An energy source 106 preferably delivers energy to substrate 102, thereby causing formation of a plasma 108 on substrate 102. As seen in Fig. 1, substrate 102 is preferably disposed in open space and plasma 108 is created on substrate 102 in open space. The system of the present invention, in contrast to conventional plasma formation techniques, thus does not require substrate 102 to be enclosed in a container, nor to undergo heating, leading to highly advantageous features of the present invention which will be described in greater detail henceforth.

Substrate 102 may be conductive or non-conductive and may comprise, by way of example only, a chemical element, alloy or compound. Various embodiments of substrate 102 will be set forth hereinbelow, including elements of the 13^th and 14^th groups of the periodic table as well as Calcium, Titanium, Zirconium, Iron and Nickel. It is understood, however, that these embodiments are exemplary only and that substrate 102 may comprise any material capable of having a plasma induced thereupon in open space and without heating, by way of an energy input to substrate 102. The energy input to substrate 102 from energy source 106 may be termed an induction beam since it leads to the induction of a plasma on substrate 102, and is generally indicated by a pair of arrows 110 in Fig. 1.

Energy source 106 may comprise an optical energy source, such as a laser, as will be exemplified henceforth with reference to Figs. 2 and 3. Energy source 106 may alternatively comprise an electrical energy source such as a high voltage pulse generator, as will be exemplified henceforth with reference to Figs. 4 and 5. It is appreciated, however, that other appropriate energy sources may alternatively be employed in the system of the present invention for inducing a plasma 108 on substrate 102.

System 100 further preferably includes an optical pump 120 generating a pulsed optical output, which optical output is generally indicated in Fig. 1 by an arrow 122. Optical output 122 is preferably delivered to plasma 108 following the formation of plasma 108. Optical output 122 may be termed a pumping beam 122, operative to optically pump plasma 108.

In operation of system 100, the optical pumping of plasma 108 leads to the optical excitation of plasma 108, such that population inversion and hence stimulated emission of a collimated light beam 130 from plasma 108 occurs. Optically pumped plasma 108 thus itself becomes a source of collimated light due to stimulated emission effects therein. As seen in Fig. 1, collimated light beam 130 represents a radiation beam emitted by plasma 108 in an emission direction, following the optical pumping thereof. The emission of collimated light by an optically pumped plasma in open space and at ambient temperature is a highly advantageous feature of a preferred embodiment of the present invention.

In a preferred embodiment of the present invention shown in Fig. 1, optical pump 120 preferably generates pumping beam 122 in a direction generally perpendicular to both the substrate 102 and plasma 108 thereon and spatially offset therefrom. Pumping beam 122 is preferably directed into plasma 108 by way of reflection at a dichroic mirror 132. The emitted plasma radiation beam 130 thus emerges generally parallel to the direction of incidence of pumping beam 122 on plasma 108. This parallel spatial relationship between the incoming optical pumping beam causing stimulated emission by the plasma and the stimulated emission beam itself may be termed 'longitudinal pumping' and will be described in greater detail henceforth with reference to Figs. 2 -5.

Optical pump 120 may alternatively be arranged with respect to substrate 102 and plasma 108 such that the pumping beam 122 of optical pump 120 orthogonally enters plasma 108, in which case the emitted plasma radiation beam emerges generally perpendicular to the direction of incidence of pumping beam 122 on plasma 108. This perpendicular spatial relationship between the incoming optical pulse causing stimulated emission by the plasma and the stimulated emission itself may be termed 'transverse pumping' and will be described in greater detail henceforth with reference to Fig. 19.

Optical pump 120 may, by way of example, be embodied as an Optical Parametric

Oscillator (OPO), a flash-lamp, a laser diode or any other suitable source of optical pumping. It is appreciated that the direction of pumping beam 122 towards plasma 108 by means of dichroic mirror 132 is illustrative only and that other methods may be employed in order to direct the output of optical pump 120 towards plasma 108, as will be well known to those skilled in the art. It has been found that high spontaneous emission transition probabilities in the plasma, at least equal to approximately 10 7 sec -^" 1 may be required in order for stimulated emission by the plasma to occur. It has further been found that plasma 108 may exhibit a relatively high population inversion following optical pumping and hence have a correspondingly large amplification coefficient, of the order of, for example, 100 per cm. As a result of the large optical gain possible in plasma 108, plasma 108 may in some cases operate as a stimulated emission medium having detectable collimated light output without the additional need for an optical resonator cavity in order to increase amplification. Hence, system 100 is not shown in Fig. 1 to include an optical resonator cavity, since system 100 may generate detectable collimated light without requiring the inclusion of an optical resonator cavity therein. The output of system 100 may, however, be augmented by the placement of system 100 within an optical resonator cavity, such that system 100 may operate as a laser, as will be detailed henceforth.

It is appreciated that although both induction beam 110 generated by energy source 106 and pumping beam 122 generated by optical pump 120 are seen in Fig. 1, induction beam 110 and pumping beam 122 are preferably not both simultaneously present in system 100. As explained above, pumping beam 122 is delivered to plasma 108 only following the formation of plasma 108 and is preferably delayed by several microseconds with respect to induction beam 110, in order to allow time for cooling of plasma 108 and the formation of neutral atoms therein. The preferably delay between induction beam 110 and pumping beam 122 is substrate-dependent and may be influenced by the thermal and electrical properties of substrate 102. Further details concerning the temporal distribution of the induction and pumping beams 110, 122 will be provided henceforth with reference to Fig. 6.

It is thus appreciated that the optical pumping of plasma 108 in open space allows plasma 108 to operate as a stimulated emission medium having detectable collimated light output, without requiring the presence of an optical cavity. Furthermore, optically pumped plasma 108 exhibits laser effects when inserted in an optical cavity. The stimulated emission and laser effects of the optically pumped plasma of the present invention may be highly advantageous for use in investigating characteristics of plasma plumes, in remote sensing systems and in the creation of coherent light sources spanning a broad spectral range. The formation of plasma 108 in open space and at ambient temperature highly simplifies the system and makes it significantly more compact, resulting in system 100 being particularly well suited for use in remote sensing of minerals and explosives.

Reference is now made to Fig. 2, which is a simplified partially schematic, partially block-diagram illustration of a system for collimated light generation, constructed and operative in accordance with another preferred embodiment of the present invention.

As seen in Fig. 2, there is provided a system 200 including a substrate 202 disposed in open space and having a surface 204. Surface 204 is illustrated in Fig. 2 as comprising a generally flat planar surface. However, it is appreciated that surface 204 may have a variety of other topologies including undulating and uneven topologies.

An energy source 206 preferably delivers energy to substrate 202. Here, by way of example, energy source 206 may be embodied as a laser 206 delivering laser pulses to substrate 202 and thereby causing induction of a plasma 208 on surface 204 of substrate 202. Induction laser 206 may be a Quantel-Ultra laser manufactured by Quantel- USA Inc., of Montana, USA, preferably operative to deliver pulses having a wavelength of approximately 1064 nm, an energy of approximately 50 mJ and a duration of approximately 7 ns. It is appreciated, however, that these laser characteristics are provided by way of example only and that lasers having a variety of operating wavelengths, pulse energies and pulse durations may be employed in preferred embodiments of the present invention in order to create plasma 208 on substrate 202.

As seen in Fig. 2, plasma 208 is created on surface 204 of substrate 202 in open space and, in contrast to conventional plasma formation techniques does not require substrate 202 to be enclosed in a container, nor to undergo heating. The formation of plasma 208 on substrate 202 in open space and at ambient temperature rather than in a closed, heated container, leads to highly advantageous features of the present invention, as will be detailed henceforth.

The laser input to substrate 202 from induction laser 206 may be termed a laser induction beam since it leads to the induction of a plasma on substrate 202 and is generally indicated by a set of arrows 210 in Fig. 2. Laser induction beam 210 may be focused on surface 204 of substrate 202 by a cylindrical lens 212 having a focus length F of approximately 15 cm and placed at a height of approximately 12 cm above surface 204. The presence of cylindrical lens 212 preferably serves to shape laser induction beam 210 and preferably produces a beam 214 lying in a plane normal to surface 204 and having an elliptical waist with long and short axes of approximately 5 mm and 0. 5 mm respectively. Plasma 208 is preferably created in open space on surface 204 of substrate 202 as a result of absorption of laser induction beam 210 by substrate 202. Plasma 208 is preferably in the form of a semi-ellipsoidal plasma plume and may have axes of approximately 5 x 0.5 xl mm.

System 200 further preferably includes an optical pump 220 generating an optical output, which optical output constitutes a pumping beam with respect to plasma 208 and is generally indicated in Fig. 2 by a set of arrows 222. Pumping beam 222 is delivered to plasma 208 following the formation of plasma 208. The relative timing of laser induction beam 210 and pumping beam 222 is preferably controlled by a delay generator 224, which delay generator 224 is preferably connected to both induction laser 206 and optical pump 220. The time delay between the formation of plasma 208 as a result of the delivery of laser induction beam 210 to substrate 202 and the delivery of pumping beam 222 to plasma 208 is substrate-dependent and will be further elaborated hereinbelow with reference to Fig. 6.

As seen in Fig. 2, optical pump 220 may be embodied as an OPO such as, by way of example only, an OPO of the type OPOTEK - VIBRANT 355 II- LD, manufactured by OPOTEK Inc., of California, USA. The OPO may, by way of example, deliver pulses having a pulse energy of approximately 5 mJ tunable in the spectral range of 210 - 280 nm and a pulse energy of approximately 10 mJ in the spectral range of 410 - 600 nm, the pulses having a 5 ns duration and spectral line width of approximately 5cm^"1.

In operation of system 200, pumping beam 222 optically excites plasma 208 such that population inversion and hence stimulated emission of a collimated light beam 230 from plasma 208 occurs. Collimated light beam 230 represents a radiation beam emitted by the optically pumped plasma 208. In order for stimulated emission to occur in optically pumped plasma 208, it has been found that high spontaneous emission transition probabilities in the plasma at least equal to approximately 10 7 sec -^" 1 are required. It has further been found that plasma 208 may exhibit a relatively high population inversion following optical pumping and hence have a correspondingly large amplification coefficient, of the order of, for example, 100 per cm. As a result of the large optical gain possible in plasma 208, plasma 208 may in some cases operate as a stimulated emission medium having detectable output without the additional need for an optical resonator cavity in order to increase amplification, as seen in the case of system 200 in Fig. 2, in which no optical cavity is present and detectable plasma radiation 230 exits directly from plasma 208. In the embodiment of the present invention shown in Fig. 2, OPO 220 generates pumping beam 222 in a direction generally perpendicular to both the surface 204 and plasma 208 thereon and spatially offset therefrom. Pumping beam 222 emerging from OPO 220 is reflected at a dichroic mirror 232 in a direction towards plasma plume 208. Dichroic mirror 232 preferably fully reflects pumping beam 222 but transmits the longer wavelength collimated radiation 230 emitted by plasma 208. Reflected pumping beam 222 thus preferably enters plasma plume 208 generally parallel both to surface 204 and to the long waist axis of induction beam 214. The emitted plasma radiation beam 230 preferably emerges generally parallel to the direction of the incoming pumping beam 222. Due to the parallel spatial relationship between the incoming optical pumping beam causing stimulated emission by the plasma and the stimulated emission beam itself, the optical pumping of system 200 may be termed a longitudinal pumping system.

Pumping beam 222 is preferably focused on plasma plume 208 at a distance of approximately 300 μπι above surface 204, by way of a lens 234. Lens 234 may be placed at a distance of approximately 28 cm from a center of the plasma plume 208 and may be interposed between OPO 220 and dichroic mirror 232. Lens 234 may create a waist diameter of pumping beam 222 of approximately 0.6 mm along the long axis of the plasma plume semi-ellipsoid 208.

Optical pump 220 may alternatively be arranged with respect to substrate 202 and plasma 208 such that the pumping beam 222 of optical pump 220 orthogonally enters plasma 208, whereby that emitted plasma radiation beam 230 emerges generally perpendicular to the direction of the incoming pumping beam 220. Such a transverse arrangement will be described in greater detail henceforth with reference to Fig. 19.

It is appreciated that although in system 200 optical pump 220 is embodied as an OPO, optical pump 220 may alternatively be embodied as other suitable sources of optical pumping, such as a flash-lamp or a laser diode. It is further appreciated that the optical elements used to direct pumping beam 222 towards plasma 208, namely dichroic mirror 232 and lens 234, are exemplary only and that pumping beam 222 may be longitudinally directed into plasma 208 by other optical elements, as will be apparent to one skilled in the art. Furthermore, it understood that the inclusion of lens 234 in system 200 is optional and that lens 234 may be obviated, should the output of optical pump 220 have required optical characteristics.

Notwithstanding the possible operation of system 200 as a collimated light generator without the need for the inclusion of an optical resonator cavity therein, the output of system 200 may be augmented by the placement of system 200 within an optical cavity. Such an arrangement allows system 200 to exhibit a laser effect, as seen in the case of a system 300 shown in Fig. 3.

As seen in Fig. 3, system 200 may be disposed within an optical resonator cavity 302, which optical resonator cavity 302 may comprise a forward fully reflecting mirror 304 and a backward partly reflecting mirror 306. Forward fully reflecting mirror 304 is shown herein, by way of example, as having a concave configuration and backward partly reflecting mirror as having a flat configuration. It is appreciated, however, that forward and backward mirrors 304, 306 may alternatively have a concave, convex or a flat configuration. It is appreciated that the formation of optical resonator cavity 302 by mirrors 304 and 306 is exemplary only and that other optical resonators may be used as are known in the art, such as those described in H. Kogelnik and T. Li "Laser beams and Resonators ", Applied Optics vol. 8, (1966) 1550-1567.

Forward and backward mirrors 304 and 306 are preferably formed as broadband metal- coated glass or quartz mirrors. Particularly preferably, the applied metal coating comprises aluminum and forward and backward mirrors 304, 306 comprise Al, UV enhanced mirrors.

Optical resonator cavity 302 may be a non-selective, broadband resonator cavity supporting generation of optical emission over a wide spectral range spanning UV to IR. In the case that it is desirable to generate selective emission of one selected wavelength, forward and backward mirrors 304 and 306 may be formed as dichroic mirrors having reflectance wavelengths coinciding with the desired selected line wavelength.

Backward partly reflecting mirror 306 preferably has approximately 90% reflectance and may be placed approximately 75 mm behind the center of the plasma plume 208. The preferable distance between backward partly reflecting mirror 306 and the plasma plume 208 preferably depends on the mechanical features of system 300, including the holders of the substrate 202 and dichroic mirror 232. It is appreciated that the distance between the backward partly reflecting mirror 306 and the center of the plasma plume 208 may be reduced to less than 75 mm, depending on the mechanical characteristics of system 300.

It is appreciated that although systems 200 and 300 are shown herein as two distinct preferred embodiments of the present invention, systems 200 and 300 may be conveniently combined in a single physical system by the addition of a removable non-reflecting optical shutter to system 300. The removable non-reflecting optical shutter may be interposed between plasma 208 and forward fully reflecting mirror 304, such that when the removable non-reflecting optical shutter is present, forward fully reflecting mirror 304 is blocked and no optical cavity is formed. In this case, backward partly reflecting mirror 306 acts as an attenuating filter. Upon removal of the removable optical shutter, forward fully reflecting mirror 304 becomes functional and an optical cavity, such as optical cavity 302, is formed.

It is appreciated that induction laser 206 may be replaced by other energy sources capable of inducing a plasma on substrate 202 by way of delivery of energy thereto, as will be exemplified with reference to Figs. 4 and 5 below.

Reference is now made to Fig. 4, which is a simplified partially schematic, partially block-diagram illustration of a system for collimated light generation, constructed and operative in accordance with a further preferred embodiment of the present invention.

As seen in Fig. 4, there is provided a system 400 including a substrate 402 disposed in open space. Here, by way of example, substrate 402 is embodied as a plate 402 having a first set of high voltage positive needle electrodes 403 extending therefrom. Plate 402 and positive needle electrodes 403 are preferably disposed opposite and spaced apart from a ground electrode plate 404 having a second set of high voltage needle electrodes 405 extending therefrom. Plates 402 and 404 and first and second sets of high voltage needle electrodes 403 and 405 are all preferably formed by the same substrate material.

An energy source 406 preferably delivers energy to first set of high voltage electrodes 403 and is preferably connected thereto. Here, by way of example, energy source 406 is embodied as a high voltage pulse generator 406 delivering high voltage electrical signals to first set of high voltage electrodes 403 and thereby causing induction of a set of plasmas strips 408 between first and second sets of electrodes 403 and 405. By way of example, energy source 406 may be a high voltage pulse Generator GP 30-01-2 manufactured by Montena Co. of Switzerland. Energy source 406 may be operative to deliver high voltage pulses of variable voltages of up to 30kV at frequencies of up to 1kHz, with a 1 ns pulse duration and peak current of 600 A at 50 Ohm.

It is appreciated that needle electrodes 403, 405 may optionally be replaced by a pyramid electrode arrangement, in which case plasma strips 408 would be formed as an elongate plasma volume rather than set of discrete plasma strips, as illustrated in Fig. 4. The distance between first and second sets of electrodes 403 and 405 and the strength of the high voltage electrical signals are preferably selected so as to prevent the occurrence of air spontaneous electric breakdown. By way of example, an inter-electrode separation of 5 mm and pulse voltage of 15kV may be used.

A discharge triggering electrode 410 is preferably provided in close proximity to the pointed ends of first set of electrodes 403, in order to facilitate creation of a synchronized electric discharge plasma. By way of example, discharge electrode 410 may be placed at a distance of 0.5 mm from the pointed ends of first set of electrodes 403 and fed by IkV electric pulses from high voltage generator 406. Electric impulses between discharge electrode 410 and first set of electrodes 403 preferably cause synchronized high voltage discharges between first and second set of electrodes 403 and 405, thereby producing electric plasma 408. Plasma 408 preferably contains atoms, ions and electrons both from air and from substrate 402 and, following a substrate-dependent delay, may be subjected to optical pumping and thus form a stimulated emission or laser medium.

Plasma 408 is preferably in the form of a multiplicity of cylindrical plasma plumes, each having a length of approximately 5mm and diameter of approximately 0.2mm. The formation of plasmas 408 on first and second sets of high voltage electrodes 403, 405 in open space and at ambient temperature rather than in a closed, heated container as is conventionally required in plasma formation techniques, is a highly advantageous feature of a preferred embodiment of the present invention.

System 400 further preferably includes an optical pump 420 generating an optical output, which optical output constitutes a pumping beam with respect to plasmas 408 and is generally indicated in Fig. 4 by a set of arrows 422. Pumping beam 422 is delivered to plasmas 408 following the formation of plasmas 408. The relative timing of the high voltage pulse produced by high voltage pulse generator 406 and pumping beam 422 is preferably controlled by a delay generator 424, which delay generator 424 is preferably connected to both high voltage pulse generator 406 and optical pump 420. The time delay between the formation of plasmas 408 as a result of the delivery of a high voltage pulse to substrate 402 and the delivery of pumping beam 422 to plasmas 408 is substrate-dependent and will be further elaborated hereinbelow with reference to Fig. 6. As seen in Fig. 4, optical pump 420 may be embodied as an OPO such as, by way of example only, an OPO of the type OPOTEK - VIBRANT 355 II- LD, manufactured by OPOTEK Inc. of California, USA. The OPO may, by way of example, deliver pulses having a pulse energy of approximately 5 mJ tunable in the spectral range of 210 - 280 nm and a pulse energy of approximately 10 mJ in the spectral range of 410 - 600 nm, the pulses having a 5 ns duration and spectral line width of approximately 5cm^"1.

In operation of system 400, pumping beam 422 optically excites plasmas 408 such that population inversion and hence stimulated emission of a collimated light beam 430 from plasmas 408 occurs. Collimated light beam 430 represents a radiation beam emitted by the optically pumped plasmas 408. In order for stimulated emission to occur in optically pumped plasmas 408, it has been found that high spontaneous emission transition probabilities in the plasma, at least equal to approximately 10 7 sec -^" 1 are required. It has further been found that plasmas 408 may exhibit a relatively high population inversion following optical pumping and hence have a correspondingly large amplification coefficient, of the order of, for example, 100 per cm. As a result of the large optical gain possible in plasmas 408, plasmas 408 may in some cases operate as a stimulated emission medium having detectable output without the additional need for an optical resonator cavity in order to increase amplification, as seen in the case of system 400 in Fig. 4, in which no optical cavity is present and detectable plasma radiation 430 exits directly from plasmas 408.

In the embodiment shown in Fig. 4, OPO 420 generates pumping beam 422 in a direction generally perpendicular to both the surface 404 and plasmas 408 thereon and spatially offset therefrom. Pumping beam 422 emerging from OPO 420 is preferably reflected at a dichroic mirror 432 in a direction towards plasma plumes 408. Dichroic mirror 432 preferably fully reflects pumping beam 422 but transmits the longer wavelength collimated radiation 430 emitted by plasma 408. Reflected pumping beam 422 thus preferably enters plasma plumes 408 generally parallel to a surface of plate 402. The emitted plasma radiation beam 430 preferably emerges generally parallel to the direction of the incoming pumping beam 422. Due to the parallel spatial relationship between the incoming optical pumping beam causing stimulated emission by the plasmas and the stimulated emission beam itself, the optical pumping of system 400 may be termed a longitudinal pumping system. Pumping beam 422 is preferably focused on plasma plumes 408 at the center of plasma strip 408 by way of a beam reducer 434 that is preferably functional to reduce the OPO beam diameter from about 5mm to about 2mm. Beam reducer 434 may be interposed between OPO 420 and dichroic mirror 432.

It is appreciated that although in system 400 optical pump 420 is embodied as an OPO, optical pump 420 may alternatively be embodied as other suitable sources of optical pumping, such as a flash lamp or a laser diode. It is further appreciated that the optical elements used to direct pumping beam 422 towards plasmas 408, namely dichroic mirror 432 and beam reducer 434, are exemplary only and that pumping beam 422 may be longitudinally directed into plasma 408 by other optical elements, as will be apparent to one skilled in the art. Furthermore, it understood that the inclusion of beam reducer 434 in system 400 is optional and that beam reducer 434 may be obviated, should the output of optical pump 420 have required optical characteristics.

Notwithstanding the possible operation of system 400 as a collimated light generator without the need for the inclusion of an optical cavity therein, the output of system 400 may be augmented by the placement of system 400 within an optical cavity. Such an arrangement allows system 400 to exhibit laser effects, as seen in the case of a system 500 shown in Fig. 5.

As seen in Fig. 5, system 400 may be disposed within an optical cavity 502, which optical cavity 502 preferably comprises a forward fully reflecting mirror 504 and a backward partly reflecting mirror 506. Forward fully reflecting mirror 504 is shown herein, by way of example, as having a concave configuration and backward partly reflecting mirror 506 is shown herein as having a flat configuration. It is appreciated, however, that forward and backward mirrors 504, 506 may alternatively have concave, convex or flat configurations. It is further appreciated that the formation of optical resonator cavity 502 by mirrors 504 and 506 is exemplary only and that other optical resonators may be used as are known in the art, such as those described in H. Kogelnik and T. Li "Laser beams and Resonators ", Applied Optics vol. 8, (1966) 1550-1567.

Forward and backward mirrors 504 and 506 are preferably formed as broadband metal- coated glass or quartz mirrors. Particularly preferably, the applied metal coating comprises aluminum and forward and backward mirrors 504, 506 comprise Al, UV enhanced mirrors.

Optical resonator cavity 502 may be a non-selective, broadband resonating cavity supporting generation of optical emission over a wide spectral range spanning UV to IR. In the case that it is desirable to generate selective emission of one selected wavelength, forward and backward mirrors 504 and 506 may be formed as dichroic mirrors having reflectance wavelengths coinciding with the desired selected line wavelength.

Backward partly reflecting mirror 506 preferably has approximately 90% reflectance and may be placed approximately 75 mm behind the center of the first closest plasma plume. The preferable distance between backward partly reflecting mirror 506 and the plasma plumes 408 preferably depends on the mechanical construction of system 500, including the holders of the substrate 402 and dichroic mirror 432. It is appreciated that the distance between the backward partly reflecting mirror 506 and the center of the plasma plume 408 may be reduced to less than 75 mm, depending on the mechanical features of system 500.

It is appreciated that although systems 400 and 500 are shown herein as two distinct preferred embodiments of the present invention, systems 400 and 500 may be conveniently combined in a single physical system by the addition of a removable non-reflecting optical shutter to system 500. The removable non-reflecting optical shutter may be interposed between plasmas 408 and forward fully reflecting mirror 504, such that when the removable non- reflecting optical shutter is present, forward fully reflecting mirror 504 is blocked and no optical cavity is formed. In this case, backward partly reflecting mirror 506 acts as an attenuating filter. Upon removal of the removable optical shutter, forward fully reflecting mirror 504 becomes functional and an optical cavity, such as optical cavity 502, is formed.

It is further appreciated that although both plasma induction beam 110 and pumping beam 122 are seen in Fig. 1, both plasma induction beam 210 and pumping beam 222 are seen in

Figs. 2 and 3, and both high voltage pulse generator output and pumping beam 422 are seen in

Figs. 4 and 5, the plasma induction pulse and pumping pulse are preferably not simultaneously present in the system of the present invention. This is because the pumping beams 122, 222, 422 may be delivered to the plasma only following the formation of the plasma as a result of absorption of the plasma induction signal by the substrate. Furthermore, the onset of optical pumping is preferably delayed with respect to formation of the plasma by several microseconds, in order to allow time for the plasma to cool and neutral atoms to be formed therein. The preferable delay between the induction pulse and the pumping pulse is substrate-dependent and may depend on the thermal and electrical properties of the substrate and hence the plasma. An exemplary synchronization scheme between induction pulses and corresponding pumping beams 122, 222, 422 is presented in Fig. 6. As seen in Fig. 6, a pumping pulse represented by a first plot 602 is delayed with respect to an induction pulse represented by a second plot 604. An inter-pulse separation D between the induction pulse and pumping pulse of approximately 2 - 10 microseconds has been found to be preferable, depending on the specific substrate and induction mechanism employed.

It is appreciated that although a third plot 606 representing an emission pulse appears to have a maximum coinciding with that of first plot 602 representing the pumping pulse at time to, this is simply due to the time-axis scale used in Fig. 6. An additional plot 608 is seen following induction pulse 604, which additional plot 608 represents spontaneous plasma emission decaying within the time delay D.

Experimental Results for Longitudinally Pumped Plasma

Data showing collimated light generation in the form of stimulated emission and resultant laser effects exhibited by longitudinally optically pumped plasma, generated in accordance with the above described systems and methods, are now presented with reference to Figs. 7 A - 18. The data set forth hereinbelow were obtained using a system of the type described in reference to Figs. 2 and 3. It is appreciated, however, that the results are generally representative of optically pumped plasma stimulated emission and laser performance in a system of any one of the types described in reference to Figs. 1 - 5, constructed and operative in accordance with preferred embodiments of the present invention.

Data of the types set forth hereinbelow may be collected from a system of any one of the types described in reference to Figs. 1 and 5 by way of collection of the plasma radiation by an optical fiber and delivery thereof to a spectrometer combined with a rapid ICCD camera for data acquisition. A spectrometer such as a Shamrock -303i-A spectrometer, manufactured by Andor

Technology Inc. of Belfast, UK and an ICCD camera such as a DH720-25F03 camera, manufactured by Andor Technology Inc. of Belfast, UK, are well suited for this purpose.

Respective spectral and temporal resolutions of O.lnm and 1 ns may be used. Laser spot images of the types described hereinbelow may also be seen on a luminous screen placed behind the partly reflecting mirrors 306, 506 of the systems of Figs. 3 and 5. The data presented hereinbelow is divided into three groups, in accordance with the category to which the substrate material and hence plasma belong, as follows: substrate elements belonging to the 13^th group of the periodic table, substrate elements belonging to the 14^th group of the periodic table and miscellaneous substrate elements.

Data group 1 : Emission spectra and laser effects in plasma formed on a substrate belonging to the 13^th group of the periodic table: Aluminium (Al), Gallium (Ga), Indium (In) and Thallium mi

An experimental set up of the type illustrated in Figs. 2 and 3 was employed with these substrates. In the case of the Al substrate, optical pumping was commenced 4 microseconds following the creation of the plasma plume, which time delay has been found to be optimum for Al in this system. Optical pumping with 256.8 nm focused OPO laser pulses was found to create strong, well-collimated emission beams at only 394.4 nm in both forward and backward directions. Optical pumping with 257.5 nm focused OPO laser pulses was found to create strong, well-collimated emission beams at only 396.2 nm in both forward and backward directions.

Al plasma emission spectra for the systems of Figs. 2 and 3, respectively with and without the presence of an optical resonator cavity, are respectively presented in Figs. 7A and 7B using arbitrary intensity units. Fig. 7A presents Al plasma plume emission under pumping at 256.8 nm and 257.5 nm measured in a backward direction, in the absence of an optical resonator cavity. Pumped emission measured in other directions, for example normal to the direction of the pumping beam, was not found to show any notable effects. In the case of pumped emission measured in other directions, both 394.4 nm and 396.2 nm emission exist simultaneously, independent on whether an excitation wavelength of 256.8 nm or 257. 5 nm is used.

Fig. 7B presents Al plasma plume emission under pumping at 256.8 nm and 257.5 nm measured in a direction behind the partly reflecting mirror 306 in the presence of an optical resonator cavity and under the same geometrical conditions as those relevant to Fig. 7A. As appreciated from a comparison of the relative intensities of the emission spectra of Figs. 7A and 7B, plasma emission intensity increased approximately 10 fold for 394.4 nm emission and 20 fold for 396.2 nm emission when an optical resonator cavity was included in the system. Emission lines of Fig. 7A were found to be up to five orders of magnitude stronger than corresponding Laser Induced Breakdown Spectroscopy (LIBS) emission lines, measured under similar geometric conditions but without optical pumping. Furthermore, emission lines of Fig. 7B were found to be up to six orders of magnitude stronger than corresponding LIBS emission lines measured under similar geometric conditions. These results show a highly significant enhancement in stimulated emission and laser effects due to the optical pumping of the induced Al plasma.

The Full-Width Half Maximum (FWHM) of the optically pumped emission lines was found to be approximately 0.13 nm in comparison to a FWHM of corresponding LIBS plasma emission lines of approximately 0.25 nm. The FWHM of the optically pumped emission lines may in fact be even narrower than 0.13 nm, since the measurement of the FWHM was limited by the 0.1 nm spectral resolution of the optical system.

The strong collimated emissions shown in Figs. 7A and 7B were found to last only for the duration of the OPO pumping pulse of approximately 4 - 5 ns. This is significantly shorter than the spontaneous emission decay time, estimated to be equal to approximately 10 ns. Measurements of the pumped emission after a delay of greater than about 5 ns with respect to the start of the pumping pulse were found to show disappearance of the strong, collimated emission.

The emission lines under pumping of Figs. 7A and 7B were found to exhibit strong linear polarization parallel to the pumping light polarization and to be completely un-polarized following pumping, in resemblance to unpolarized conventional LIBS emission.

These experimental results demonstrate that Al plasma pumped at 256.8 and 257.5 nm emits collimated stimulated emission in both forward and backward directions at 394.4 and 396.2 nm, respectively. Furthermore, placement of the Al plasma plume inside of an optical resonator leads to the creation of a laser with an additional increase in the backward beam intensity. Such a laser may be termed a Laser-Induced Plasma Laser (LIPL).

The energy dependence of the 396.2 nm plasma plume lasing line on a 257.5 nm pumping pulse energy is presented in Figure 8. As appreciated from consideration of Fig. 8, the plasma emission line exhibits characteristic laser behavior, with a 1 μΐ threshold and estimated generation efficiency of about 10^" . The same Al plasma lasing lines at 394.4 nm and 396.2 nm in an Al plasma plume having the above-described characteristics were also observed under pulsed pumping at 226.3 and 226.9 nm, 236.7 and 237.3 nm or 265.3 and 266.0 nm pulses, respectively.

The emission spectra of the Al plasma plume may be best understood with reference to an Al atom plasma laser transition scheme, showing the various optical transitions in Al atoms leading to photon emission and hence collimated beam generation. An Al atom plasma laser transition scheme is shown in Fig. 9. As seen in Fig. 9, the transition scheme is a classical, three energy level generation scheme. For example, excitation at 226.35 or 226.9 nm from the 3p ( P1/2 or 2 ls raises electrons to the upper 3s 26d 2

P3/2) leve D3/2 level, as indicated by arrows 902. From these excited states, the system rapidly decays non-radiatively to the intermediate emitting

(3s 2"4s) 2 "Si/2 level. These non-radiative decays are indicated in Fig. 9 by wavy arrows 904.

Spontaneous emissions (transitions 4s 2 2

S ₂—3p P ₂ 394.4 nm in the case of 226.35 nm excitation or 4s 2 Si_{/ 2}—3p 2

P3/2 _, 396.2 nm in the case of 226.9 nm excitation ) precede the stimulated emissions responsible for build-up of light in the plasma plume laser.

Further theoretical consideration is required in order to understand the appearance of separate lasing lines at 394.4 nm and 396.2 nm as a result of pumping from the slightly split Pi_/2 or P3/2 ground states of Al atoms, which slightly split ground states are only separated by 0.013 eV. Strong separation between these lasing lines is possible only if stimulated transition is more probable than non-radiative transition between closely spaced split ground levels and the mechanism responsible for the appearance of these lasing lines thus requires further clarification.

Al plasma laser medium involves high optical transition probabilities (A_y > 10 7 s -^" 1 ) and therefore has a high optical gain coefficient The optical gain coefficient OC21 for lasing at 396.2 nm under pumping at 266.0 nm was estimated using equations (1) to (3), as follows:

2₁ = ΑΝ_{2 1} σ_{2 1} ( l)

2 2

wherein O21 is the cross section of the transitions 4s Si_/2— 3p P3/2 and equal to approximately 10^"14 cm², and AN 21 is the population inversion of the lasing level 2 (4s ²Si_{/ 2}).

2 2

Assuming that non-radiative decays 4d D3/2, 5/2 → 4s S1/2 are significantly faster than any radiative decays, then before threshold is reached the following approximation applies:

AN_{2 l}≥ N₀ ^^ (2) wherein No is the Al atom concentration in Al plasma and is equal to about 10¹⁶ cm^"3 for Al atoms in the experimental conditions, W 3 is the absorption probability in transition 3p

2 2 7 -1 2 2

P3/2^→4d D5/2, and A₂₁ =9.8 sec ^" is the spontaneous transition 4s S1/2— *^■ 3p P3/2 probability.

Wi3 may be estimated using the following equation:

W₁₃ = ii0₁₃ /S (3)

wherein is the number of incident photons per second, equal to approximately 3x10 23

n

photons/sec in the experimental conditions, S is the waist area of the pumping beam and is equal to approximately 310^"3 cm² and ΟΊ3 is the cross section of the pumping transitions 3p ²P3/₂— »4d

2 ual to approximately 3x10 -^"15 cm 2

D5/2 and is eq

Using these data, a relatively high population inversion and a correspondingly large amplification coefficient of Q¾_/~100 cm^"1 were found. This large optical gain shows that an Al LIPL is capable of operating as a stimulated emission medium, with no optical resonator cavity being required in order to detect the output thereof.

Furthermore, it has been found that in the case that an optical cavity is present, only alignment of the forward fully reflecting mirror is significant in order for lasing operation to occur. This means that an Al LIPL may work as a one -pass laser, with the backward 90% reflectance mirror serving primarily to improve laser operation and in particular to improve laser beam divergence.

In addition to the three energy level generation scheme exhibited by Al LIPL, LIPL have also been found to exhibit the more efficient four and three-energy level scheme, in which generation transition occurs from the pumped excited level. The four and three-energy level scheme is exhibited, by way of example, by a Tl plasma acting as a Tl LIPL.

Figs. 10 and 11 respectively show the plasma emission spectra and energy level generation scheme for a Tl plasma, as generated by a system of the type shown in Fig. 3. As seen in Figs. 10 and 11, under pumping at 258.0 nm indicated by an arrow 1102 three generation lines of Tl LIPL are seen to exist simultaneously, namely 535 nm corresponding transitions 7s S1/2→

2 2 2

6p P3/2, 377.6 nm corresponding to transitions 7s S1/2→ 6p Ρχ/₂ and 323 nm corresponding to transitions 8s 2 Si_/2→ 6p 2

P_3/2. The emission line at 535 nm is due to generation by a classical four energy level generation scheme, the emission line at 377.6 nm is due to generation by a classical three energy level generation scheme and the emission line at 323 nm is due to a generation transition occurring from the pumped excited level. Generation transitions occurring from the pumped excited level appear to be the most common generation scheme in LIPL.

Additional pumping and lasing wavelengths and corresponding transition probabilities for Ga and In LIPL are presented in Table 1, provided at the end of this results section.

Data group 2: Emission spectra and laser effects in plasma formed on a substrate belonging to the 14^th group of the periodic table: Germanium (Ge), Tin (Sn), and Lead (Pb)

An experimental set up of the type illustrated in Figs. 2 and 3 was employed with these substrates, which were found to exhibit effective stimulated emission and lasing. Figs. 12 and 13 respectively show the plasma emission spectra and energy level generation scheme for Ge LIPL. As seen in Figs. 12 and 13, under pumping at 249.8 nm indicated by an arrow 1302 in Fig. 13, two generation lines at 303.9 nm and 422.7 nm are seen to exist. As seen in Fig. 13, pumping at 253.3 nm produces generation lines of the same wavelength as those produced by pumping at 249.8 nm. Additionally, pumping at 265.16 nm produces a generation line at 326. 9 nm. It will be appreciated from consideration of the energy transitions shown in Fig. 13 that all lasing transitions are due to three energy level generation scheme transitions occurring from the pumped excited level.

Additional pumping and lasing wavelengths and corresponding transition probabilities for Sn and Pb LIPL are presented in Table 1.

Data group 3: Emission spectra and laser effects in plasma formed on a substrate comprising one of a miscellaneous group of elements: Calcium (Ca), Iron (Fe), Titanium (Ti), Zirconium (Zr) and Nickel (Ni)

An experimental set up of the type illustrated in Figs. 2 and 3 was employed with these substrates, which were found to exhibit effective stimulated emission and lasing. Figs. 14 and 15 respectively show the plasma emission spectra and energy level generation scheme for Ti LIPL. As seen in Figs. 14 and 15, under pumping at 252.05 nm a generation line of 479.6 nm is seen to exist and under pumping at 254.2 nm a generation line of 480.5 nm is seen to exist.

As noted above with respect to the behavior of Al LIPL, separate Ti LIPL generation lines of 479.6 nm, corresponding to transitions 4p 3 Di→4s ³ Po and 480.5 nm corresponding to transitions 4p 3 ³

D3→4s P₂ are created by transitions from closely spaced excited levels, separated by only 0.006eV. As for the Al LIPL, stimulated transitions probabilities must be much higher than non-radiative transitions between closely spaced energy levels, which understanding requires additional theoretical explanation.

Figs. 16A and 16B respectively show a portion of the plasma emission spectra for Fe LIPL and Fig. 17 shows a corresponding portion of an energy level transition diagram. As appreciated from consideration of Figs. 16A, 16B and 17, Fe plasma exhibits multiple excitations under optical pumping, thus operating over a wide spectral range spanning ultraviolet up to green-yellow. As a result of the unique, multiple excitations exhibited by Fe plasma, many emission lines may be simultaneously generated under a given pumping wavelength, using a non-selective resonator such as mirrors. Furthermore, specific desired generation lines may be obtained by employing a selective resonator in combination with an Fe plasma.

Fig. 18 shows a plasma emission spectra for a TiAl alloy. As appreciated from consideration of Fig. 18, the TiAl alloy plasma exhibits two excitations, one of which corresponds to the Al component therein and the other one of which corresponds to the Ti component therein. These results demonstrate the applicability of systems of the present invention to alloys and compound materials, as well as to pure elements.

Additional pumping and lasing wavelengths and corresponding transition probabilities for Ca, Fe, Zr and Ni LIPL are presented in Table 1, below.

Pumping Pumping transitions Pumping Emission Emission transitions Emission wavelength configuration probability wavelength configuration probability (nm) xlO ¹ (nm) xlifsec ¹

Al (3s²3p ²Pi/₂) ground state

256.8 3p ²Pi/₂→²D_3/2 1.92 394.4 5s ²Si_{/ 2}→ 3p ²Pi_/2 4.99

257.5 3p ²P3/₂→²D₅/₂ 3.6 396.2 5s ²Si_/2→3p ²P_3/2 9.85

265.25 3p ²Pi_/2→²Si_/2 1.42 394.4 5s ²Si_{/ 2}→3p ²Pi_/2 4.99

Table 1

Reference is now made to Fig. 19, which is a simplified partially schematic, partially block-diagram illustrations of a system for collimated light generation, constructed and operative in accordance with another preferred embodiment of the present invention.

As seen in Fig. 19, there is provided a system 1900 including a substrate 1902 disposed in open space and having a surface 1904. Surface 1904 is illustrated in Fig. 19 as comprising a generally flat planar surface. However, it is appreciated that surface 1904 may have a variety of other topologies including undulating and uneven topologies.

An energy source 1906 preferably delivers energy to substrate 1902. Here, by way of example, energy source 1906 may be embodied as a laser 1906 delivering laser pulses to substrate 1902 and thereby causing induction of a plasma 1908 on surface 1904 of substrate 1902. Induction laser 1906 may be a Quantel-Ultra laser manufactured by Quantel USA Inc., of Montana, USA, preferably operative to deliver pulses having a wavelength of approximately 1064 nm, an energy of approximately 50 mJ and a duration of approximately 7 ns. It is appreciated, however, that these laser characteristics are provided by way of example only and that lasers having a variety of operating wavelengths, pulse energies and pulse durations may be employed in preferred embodiments of the present invention in order to create plasma 1908 on substrate 1902.

As seen in Fig. 19, plasma 1908 is created on surface 1904 of substrate 1902 in open space and, in contrast to conventional plasma formation techniques does not require substrate 1902 to be enclosed in a container, nor to undergo heating. The formation of plasma 1908 on substrate 1902 in open space and at ambient temperature rather than in a closed, heated container, is a highly advantageous feature of a preferred embodiment of the present invention.

The laser input to substrate 1902 from induction laser 1906 may be termed a laser induction beam since it leads to the induction of a plasma on substrate 1902 and is generally indicated by a set of arrows 1910 in Fig. 19. Laser induction beam 1910 may be focused on surface 1904 of substrate 1902 by a cylindrical lens 1912 having an F- number of approximately 15 cm and placed at a height of approximately 12 cm above surface 1904. The presence of cylindrical lens 1912 preferably serves to shape laser induction beam 1910 and preferably produces a beam 1914 lying in a plane normal to surface 1904 and having an elliptical waist with long and short axes of approximately 5 cm and 0. 5 mm.

Plasma 1908 is preferably created in open space on surface 1904 of substrate 1902 as a result of absorption of laser induction beam 1910 by substrate 1902. Plasma 1908 is preferably in the form of a semi-ellipsoidal plasma plume and may have axes of approximately 5 x 0.5 x 1 mm.

System 1900 further preferably includes an optical pump 1920 generating an optical output, which optical output constitutes a pumping beam with respect to plasma 1908 and is generally indicated in Fig. 19 by a set of arrows 1922. Pumping beam 1922 is delivered to plasma 1908 following the formation of plasma 1908. The relative timing of laser induction beam 1910 and pumping beam 1922 is preferably controlled by a delay generator 1924, which delay generator 1924 is preferably connected to both induction laser 1906 and optical pump 1920. The time delay between the formation of plasma 1908 as a result of the delivery of laser induction beam 1910 to substrate 1902 and the delivery of pumping beam 1922 to plasma 1908 is substrate-dependent, as is detailed above with respect to Fig. 6.

Optical pump 1920 may be embodied as an OPO such as, by way of example only, an OPO of the type OPOTEK - VIBRANT 355 II- LD, manufactured by OPOTEK Inc. of California, USA. The OPO may, by way of example, deliver pulses having a pulse energy of approximately 5 mJ tunable in the spectral range of 210 - 280 nm and a pulse energy of approximately 10 mJ in the spectral range of 410 - 600 nm, the pulses having a 5 ns duration and spectral line width of approximately 5cm^"1. In operation of system 1900, pumping beam 1922 optically excites plasma 1908 such that population inversion and hence stimulated emission of a collimated light beam 1930 from plasma 1908 occurs. Collimated light beam 1930 represents a radiation beam emitted by the optically pumped plasma 1908.

It is a particular feature of the preferred embodiment of the present invention of Fig. 19 that OPO 1920 generates pumping beam 1922 such that pumping beam 1922 enters plasma 1908 in a direction generally perpendicular to the laser output beam 1930 of plasma 1908,. This is in contrast to the pumping arrangement of the systems shown in Figs. 1 - 5, in which the pumping beam enters the plasma in a direction generally parallel to the direction of the emitted collimated light.

Pumping beam 1922 is preferably focused on plasma 1908 by way of a cylindrical lens 1932 disposed between OPO 1920 and substrate 1902. Due to the perpendicular spatial relationship between the incoming optical pumping beam causing stimulated emission by the plasma and the stimulated emission beam itself, the optical pumping arrangement of system 1900 may be termed a transverse pumping system, in contrast to the longitudinal pumping systems of Figs. 1 - 5.

Transverse pumping systems such as system 1900 may be advantageous in comparison to longitudinal pumping systems such as systems 100 - 500, due to the pumping and laser beams being spatially separated in transverse pumping systems, thereby obviating the need for the dichroic mirror preferably found in longitudinal pumping systems. Furthermore, the pumping beam used in transverse systems may be more powerful than those in longitudinal systems, since in transverse systems there is no risk of the pumping beam damaging the front mirror of the optical cavity.

System 1900 further preferably includes an optical resonator cavity 1940, within which optical resonator cavity 1940 substrate 1902 is preferably disposed. Optical resonator cavity 1940 preferably comprises a forward fully reflecting mirror 1942 and a backward partly reflecting mirror 1944.

It is appreciated that mirrors 1942 and 1944 may have a flat, concave or convex configuration. It is further appreciated that the formation of optical resonator cavity 1940 by mirrors 1942 and 1944 is exemplary only and that other optical resonators may be used as are known in the art, such as those described in H. Kogelnik and T. Li "Laser beams and Resonators ", Applied Optics vol. 8, (1966) 1550-1567.

Forward and backward mirrors 1942 and 1944 are preferably formed as broadband metal- coated glass or quartz mirrors. Particularly preferably, the applied metal coating comprises aluminum and forward and backward mirrors 1942, 1944 comprise Al, UV enhanced mirrors.

Optical resonating cavity 1940 may be a non-selective, broadband resonating cavity supporting generation of optical emission over a wide spectral range spanning UV to IR. In the case that it is desirable to generate selective emission of one selected wavelength, forward and backward mirrors 1942 and 1944 may be formed as dichroic mirrors having reflectance wavelengths coinciding with the desired selected line wavelength.

Backward partly reflecting mirror 1944 preferably has approximately 90% reflectance and may be placed approximately 20mm away from the center of plasma plume 1908. In transverse pumping system 1900 distances between fully reflecting mirror 1942 and partly reflecting mirror 1944 depend on the mechanical parameters of the system and may be highly minimized up to the plasma plume 1908 ellipsoid long axis.

Precise alignment of optical resonator cavity 1940 has been found to be required for plasma laser operation in system 1900. Transverse pumping systems, such as system 1900, have been found to have a greater pumping threshold and lower efficiency than comparable longitudinal pumping systems, such as systems 100 - 500. The pumping threshold of system 1900 has been found to be approximately 3 mJ, in contrast to a ΙμΙ threshold in the case of longitudinal pumping and the estimated efficiency of system 1900 has been found to be approximately 10 -^"3 , in contrast to a 10 -^"2 threshold in the case of longitudinal pumping.

It is appreciated that as a result of the lower efficiency of transverse pumping system 1900 in comparison to that of longitudinal pumping systems 100 - 500, emission spectra data obtained for transverse pumping system 1900 generally resemble those obtained for longitudinal pumping systems 100 - 500, as displayed in Figs. 7A - 18, but with lower emission efficiencies.

It is appreciated that although in system 1900 optical pump 1920 is embodied as an

OPO, optical pump 1920 may alternatively be embodied as other suitable sources of optical pumping, such as a flash-lamp or a laser diode. It is further appreciated that the use of cylindrical lens 1932 to direct pumping beam 1922 towards plasma 1908 is exemplary only and that pumping beam 1922 may be directed into plasma 1908 by other optical elements or may require no additional optical elements, as will be apparent to one skilled in the art and depending on system requirements.

It is additionally appreciated that induction laser 1906 may be replaced by other energy sources capable of inducing a plasma on a substrate by way of delivery of energy thereto. Thus, by way of example, induction laser 1906 may be replaced by a high voltage energy source used in conjunction with an electrode substrate arrangement of the type illustrated in Figs. 4 and 5.

It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly claimed hereinbelow. Rather, the scope of the invention includes various combinations and subcombinations of the features described hereinabove as well as modifications and variations thereof as would occur to persons skilled in the art upon reading the forgoing description with reference to the drawings and which are not in the prior art.

Claims

1. A system for collimated light generation comprising:

a substrate disposed in open space,

an energy source providing energy to said substrate, thereby causing formation of a plasma on said substrate in said open space; and

an optical pump providing optical pumping to said plasma, such that stimulated emission of collimated light by said plasma occurs.

2. A system for collimated light generation according to claim 1, and also comprising an optical resonator cavity, said plasma operating as a laser when disposed in said optical resonator cavity.

3. A system for collimated light generation according to claim 1 or claim 2, wherein said energy source comprises a laser and said energy comprises laser radiation.

4. A system for collimated light generation according to claim 3, and also comprising a cylindrical lens for focusing said laser radiation on said plasma.

5. A system for collimated light generation according to claim 1 or claim 2, wherein said energy source comprises a high voltage pulse generator and said energy comprises high voltage pulses.

6. A system for collimated light generation according to claim 5, wherein said substrate comprises a positive electrode connected to said high voltage pulse generator, said positive electrode comprising a substrate material.

7. A system for collimated light generation according to claim 6, and also comprising a ground electrode formed by said substrate material, said plasma being formed between said positive electrode and said ground electrode.

8. A system for collimated light generation according to claim 7, wherein said plasma comprises a multiplicity of plasmas.

9. A system for collimated light generation according to any one of the preceding claims, and also comprising a delay generator connected to said energy source and said optical pump, for coordinating provision of said energy and said optical pumping.

10. A system for collimated light generation according to claim 9, wherein said optical pumping is delayed with respect to said provision of said energy by 2 - 10 microseconds.

11. A system for collimated light generation according to claim 10, wherein said optical pumping is delayed with respect to said provision of said energy by 4 - 5 microseconds.

12. A system for collimated light generation according to any one of the preceding claims, wherein said optical pump comprises an optical parametric oscillator.

13. A system for collimated light generation according to any one of claims 1 - 11, wherein said optical pump comprises a flash-lamp.

14. A system for collimated light generation according to any one of claims 1 - 11, wherein said optical pump comprises a laser diode.

15. A system for collimated light generation according to any one of the preceding claims, wherein said collimated light has an emission direction and said optical pumping is incident on said plasma in a direction generally parallel to said emission direction.

16. A system for collimated light generation according to claim 15, and also comprising a dichroic mirror, said optical pumping being reflected by said dichroic mirror onto said plasma.

17. A system for collimated light generation according to claim 2, wherein said collimated light has an emission direction and said optical pumping is incident on said plasma in a direction generally perpendicular to said emission direction.

18. A system for collimated light generation according to claim 15, and also comprising a focusing lens for focusing said optical pumping on said plasma.

19. A system for collimated light generation according to claim 17, and also comprising a cylindrical lens for focusing said optical pumping on said plasma.

20. A system for collimated light generation according to claim 2, wherein said optical resonator cavity comprises a forward fully reflecting mirror and a backward partly reflecting mirror, said plasma being disposed between said forward fully reflecting mirror and said backward partly reflecting mirror.

21. A system for collimated light generation according to claim 20, wherein said forward and backward mirrors comprise broadband reflecting mirrors.

22. A system for collimated light generation according to claim 20 or claim 21, wherein at least one of said mirrors comprises a metal-coated surface.

23. A system for collimated light generation according to claim 22, wherein said metal-coated surface comprises a metal-coated glass surface.

24. A system for collimated light generation according to claim 22, wherein said metal-coated surface comprises a metal-coated quartz surface.

25. A system for collimated light generation according to any of claims 22 - 24, wherein said surface is concave.

26. A system for collimated light generation according to any of claims 22 - 24, wherein said surface is convex.

27. A system for collimated light generation according to any of claims 22 - 24, wherein said surface is substantially flat.

28. A system for collimated light generation according to claim 20, wherein said forward and backward mirrors comprise selective reflecting and transmitting mirrors.

29. A system for collimated light generation according to any one of claims 20 - 28, wherein said backward partly reflecting mirror is 90% reflective.

30. A system for collimated light generation according to any one of the preceding claims, wherein said substrate is at ambient temperature.

31. A system for collimated light generation according to any one of the preceding claims, wherein said substrate comprises at least one chemical element.

32. A system for collimated light generation according to claim 31, wherein said at least one chemical element is conductive.

33. A system for collimated light generation according to claim 31, wherein said at least one chemical element is non-conductive.

34. A system for collimated light generation according to claim 31, wherein said substrate comprises an alloy.

35. A system for collimated light generation according to claim 31, wherein said at least one chemical element belongs to the 13^th group of elements of the periodic table.

36. A system for collimated light generation according to claim 31, wherein said at least one chemical element belongs to the 14^th group of elements of the periodic table.

37. A system for collimated light generation according to any one of the preceding claims, wherein said substrate has a spontaneous transition probability of at least 10⁷sec^_1.

38. A method for generating collimated light comprising:

disposing a substrate in open space;

delivering energy from an energy source to said substrate, thereby causing formation of a plasma on said substrate in said open space; and

optically pumping said plasma by an optical pump, such that stimulated emission of collimated light by said plasma occurs.

39. A method for generating collimated light according to claim 38, and also comprising disposing said plasma in an optical resonator cavity, said plasma operating as a laser when disposed in said optical resonator cavity.

40. A method for generating collimated light according to claim 38 or claim 39, wherein said energy source comprises a laser and said energy comprises laser radiation.

41. A method for generating collimated light according to claim 40, and also comprising providing a cylindrical lens for focusing said laser radiation on said plasma.

42. A method for generating collimated light according to claim 38 or claim 39, wherein said energy source comprises a high voltage pulse generator and said energy comprises high voltage pulses.

43. A method for generating collimated light according to claim 42, wherein said substrate comprises a positive electrode receiving said high voltage pulses, said positive electrode comprising a substrate material.

44. A method for generating collimated light according to claim 43, and also comprising providing a ground electrode co-operating with said positive electrode, said ground electrode being formed by said substrate material, said plasma being formed between said positive electrode and said ground electrode.

45. A method for generating collimated light according to claim 44, wherein said plasma comprises a multiplicity of plasmas.

46. A method for generating collimated light according to any one of claims 38- 45, and also comprising providing a delay generator connected to said energy source and said optical pump, for coordinating provision of said energy and said optical pumping.

47. A method for generating collimated light according to claim 46, wherein said optical pumping is delayed with respect to said provision of said energy by 2 - 10 microseconds.

48. A method for generating collimated light according to claim 47, wherein said optical pumping is delayed with respect to said provision of said energy by 4 - 5 microseconds.

49. A method for generating collimated light according to any one of claims 38 - 48, wherein said optical pump comprises an optical parametric oscillator.

50. A method for generating collimated light according to any one of claims 38 - 48, wherein said optical pump comprises a flash-lamp.

51. A method for generating collimated light according to any one of claims 38 - 48, wherein said optical pump comprises a laser diode.

52. A method for generating collimated light according to any one of claims 38 - 51, wherein said collimated light has an emission direction and said optical pumping is incident on said plasma in a direction generally parallel to said emission direction.

53. A method for generating collimated light according to claim 52, and also comprising providing a dichroic mirror for reflecting said optical pumping onto said plasma.

54. A method for generating collimated light according to claim 39, wherein said collimated light has an emission direction and said optical pumping is incident on said plasma in a direction generally perpendicular to said emission direction.

55. A method for generating collimated light according to claim 52, and also comprising providing a focusing lens for focusing said optical pumping onto said plasma.

56. A method for generating collimated light according to claim 54, and also comprising providing a cylindrical lens for focusing said optical pumping onto said plasma.

57. A method for generating collimated light according to claim 39, wherein said optical resonator cavity comprises a forward fully reflecting mirror and a backward partly reflecting mirror, said plasma being disposed between said forward fully reflecting mirror and said backward partly reflecting mirror.

58. A method for generating colhmated light according to claim 57, wherein said forward and backward mirrors comprise broadband reflecting mirrors.

59. A method for generating colhmated light according to claim 57 or claim 58, wherein at least one of said mirrors comprises a metal-coated surface.

60. A method for generating colhmated light according to claim 59, wherein said metal-coated surface comprises a metal-coated glass surface.

61. A method for generating colhmated light according to claim 59, wherein said metal-coated surface comprises a metal-coated quartz surface.

62. A method for generating collimated light according to any one of claims 59 - 61, wherein said surface is concave.

63. A method for generating collimated light according to any one of claims 59 - 61, wherein said surface is convex.

64. A method for generating collimated light according to any one of claims 59 - 61, wherein said surface is substantially flat.

65. A method for generating collimated light according to claim 57, wherein said forward and backward mirrors comprise selective reflecting and transmitting mirrors.

66. A method for generating collimated light according to any one of claims 57 - 65, wherein said backward partly reflecting mirror is 90% reflective.

67. A method for generating collimated light according to any one of claims 38 - 66, wherein said substrate is at ambient temperature.

68. A method for generating collimated light according to any one of claims 38 - 67, wherein said substrate comprises at least one chemical element.

69. A method for generating collimated light according to claim 68, wherein said at least one chemical element is conductive.

70. A method for generating collimated light according to claim 68, wherein said at least one chemical element is non-conductive.

71. A method for generating collimated light according to claim 68, wherein said substrate comprises an alloy.

72. A method for generating collimated light according to claim 68, wherein said at least one chemical element belongs to the 13^th group of elements of the periodic table.

73. A method for generating collimated light according to claim 68, wherein said at least one chemical element belongs to the 14^th group of elements of the periodic table.

74. A method for generating collimated light according to any one of claims 38 - 73, wherein said substrate has a spontaneous transition probability of at least 10 7 sec -^" 1.

75. A laser comprising:

an optical resonator cavity;

a substrate disposed in open space, in said optical resonator cavity;

an optical pump providing optical pumping to said plasma, such that stimulated emission of laser light by said plasma occurs.