WO1992004747A1 - Appareil laser a electrons libres avec film semi-conducteur - Google Patents

Appareil laser a electrons libres avec film semi-conducteur Download PDF

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Publication number
WO1992004747A1
WO1992004747A1 PCT/US1991/006189 US9106189W WO9204747A1 WO 1992004747 A1 WO1992004747 A1 WO 1992004747A1 US 9106189 W US9106189 W US 9106189W WO 9204747 A1 WO9204747 A1 WO 9204747A1
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Prior art keywords
laser apparatus
laser
radiation
resonator
resonant cavity
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PCT/US1991/006189
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English (en)
Inventor
John E. Walsh
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Trustees Of Dartmouth College
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Publication of WO1992004747A1 publication Critical patent/WO1992004747A1/fr

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/0903—Free-electron laser

Definitions

  • This invention relates generally to apparatus and methods for generating coherent
  • electromagnetic radiation and, more particularly, relates to tunable free electron laser (FEL)
  • Spectroscopy applications include surface science, condensed-matter phenomena, excitations in high T C superconductor, laser chemistry, radio astronomy, diagnostics in thermonuclear plasmas, conformational excitations in biomolecules, gas phase spectroscopy (rotational excitations).
  • Technology development activities include submm radar, Radar modeling, countermeasures for FIR systems, space communications, compact high-gradient accelerator research, plasma heating, isotope separation.
  • Harmonics from these negative resistance devices can be used to produce low levels of power in the submm regime. It is possible to use them for some spectroscopic applications. They can be used directly or mixed with FIR lasers for use at still shorter wavelengths. These systems are usually delicate and difficult to use. Furthermore, the power is extremely low ( ⁇ 1 ⁇ w). Other "negative resistance" sources such as superlattice structures are interesting, but the power available in the submm region is also very low and it may not be possible to extend their operation into the FIR regime.
  • the gyrotron is a relatively new entry to the coherent source population. It is an extremely attractive option when high power in the mm range is required. Because of their potential for application to ECRH in fusion plasma heating, they have been extensively developed. However, once again, as the operating wavelength approaches 1 mm, they encounter fundamental limitations. It is in part due to this that plans for ECRH experiments at 1 mm were changed in order to employ the more highly-developed 2 mm gyrotrons. At shorter wavelengths,
  • harmonic gyrotrons have not been operated at both high harmonic with a shorter ( ⁇ 1 mm) fundamental wavelength. Hence, it is not yet clear whether or not operation at submm - FIR wavelength is possible.
  • CARM cyclotron autoresonance maser
  • free-electron laser was first introduced to describe a device which employed a spatially-periodic magnetic field (undulator) as a means of coupling a highly-relativistic electron beam to the electromagnetic field. Since the operation relied upon two doppler upshifts and thus wavelength scaled inversely with the square of the relative e-beam energy, these devices were generally termed beam energy tunable. At high beam energy however, varying the beam energy is not straightforward.
  • the highly relativistic version of the device was first operated at a wavelength between 3 and 4 ⁇ m and in subsequent experiments, operation of RF-linac-driven FEL's the operating wavelength has been extended to approximately 40 microns. Extension to longer wavelengths with the highly energetic beam drive is not attractive. It requires a long period and thus also
  • Electrostatic accelerator-driven FEL's also operate in the submm - FIR. However, these devices are large and thus are available only as a user-facility-based source.
  • a further object of the invention is to provide laser devices that are compact in size, produce substantial output power, and have enhanced frequency stability.
  • Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.
  • the foregoing objects are attained by the invention, which provides laser apparatus including resonator elements for defining a resonant cavity in which stimulated radiation can propagate to generate coherent electromagnetic laser radiation.
  • the resonator elements include at least one semiconductor element having at least a first surface. A beam of electrons is directed over the surface of the resonator elements
  • the passage of the electrons produces stimulated radiation from the semiconductor element, which propagates in the resonant cavity to generate coherent electromagnetic laser radiation.
  • the coherent radiation can be tuned through a spectral range of approximately 1
  • the resonator elements also include a resonant cavity element having first and second ends, the first and second ends being positioned so that the beam enters the resonant cavity element at the first end and exits the resonant cavity element at the second end.
  • First and second mirror elements are positioned in proximity to an interior portion of the first and second end, respectively, for reflecting the stimulated radiation.
  • the electron beam can be generated by constant or pulsed sources, including high-brightness cathode elements or RF accelerators.
  • electromagnetic generate a magnetic field in proximity to the electron beam, to constrain the electron beam to pass over the surface of the semiconductor in a direction substantially parallel to the surface, in selected proximity to the surface.
  • the output of the laser can be tuned by control elements that vary electron beam energy, or by bias elements that apply a bias voltage to the semiconductor element, to vary the wavelength of the coherent electromagnetic laser radiation.
  • the invention can utilize various resonant cavity configurations.
  • the surface of the invention can utilize various resonant cavity configurations.
  • semiconductor can be flat or curved, and multiple semiconductor elements can be employed in a
  • the surface of the coupling medium can include a grating pattern characterized by selected grating height and separation.
  • the semiconductor element can include silicon, germanium, or gallium arsenide.
  • structure can include silicon with dopant, or silicon deposited on a plastic, quartz, sapphire, or metal substrate.
  • a further aspect of the invention includes vacuum elements for evacuating a region about the resonant cavity element, and electron absorbing elements situated proximate the exit end of the resonant cavity.
  • FIG. 1 is a schematic diagram depicting the Cerenkov laser mechanism, and particularly the relationship between an electron moving over a surface and the z-component of the electric field;
  • FIG. 2 is a schematic diagram depicting the relationship between the beam and the film
  • FIG. 3 is a graph showing dispersion effects in a free electron laser according to the invention, for silicon, gallium arsenide, and germanium;
  • FIG. 4 is a graph showing tuning curves in a free electron laser according to the invention, for silicon, gallium arsenide, and germanium;
  • FIG. 5 is a graph showing the single particle lineshape function vs. phase mismatch, for a free electron laser according to the invention.
  • FIG. 6 is a graph showing gain vs. beam energy for germanium, gallium arsenide, and silicon in a free electron laser according to the invention
  • FIG. 7. is a schematic diagram depicting a semiconductor resonator structure in accordance with the invention.
  • FIG. 8 is a schematic diagram depicting a mirror configuration useful in a practice of the invention, in which slight curvature is introduced to match the Gaussian mode wavefront and improve transverse confinement;
  • FIG. 9 is a schematic diagram depicting a mirror configuration useful in a practice of the invention, in which adiabatic confinement of the evanescent mode ensures that the end mirrors reflect the entire mode ;
  • FIG. 10 is a schematic diagram depicting a square profile distributed Bragg reflector useful in a practice of the invention, in which alternate cross-sections and varying index materials exhibit similar pass and stop bands that are employed as a reflector;
  • FIG. 11 is a schematic diagram showing a free electron laser beam/resonator configuration in accordance with the invention.
  • FIGS. 12A and 12B are two views of a resonator constructed in accordance with the
  • FIG. 13 depicts a tapered surface guide useful in one practice of the invention
  • FIG. 14 is a graph depicting the dispersion effects of the tapered guide of FIG. 13;
  • FIG. 15 is another embodiment of a resonator in accord with the invention, utilizing a
  • FIG. 16 is another embodiment of a resonator in accord with the invention, utilizing a
  • FIGS. 17A and 17B show a klystron-type resonator/beam configuration in accord with the invention
  • FIG. 18 is a graph depicting a high-voltage output pulse from an electron beam source useful in one practice of the invention.
  • FIG. 19 sets forth the effects of various high-brightness cathodes useful as electron beam sources in one practice of the invention.
  • FIG. 20 is a schematic diagram depicting a grating coupled oscillator (GCO) laser mechanism, and particularly the relationship between an electron moving over a surface and the z-component of the electric field;
  • GCO grating coupled oscillator
  • FIG. 21 is a schematic diagram depicting the relationship between the beam and the grating
  • FIG. 22 depicts a GCO oscillator in accord with the invention.
  • FIG. 23 is a schematic diagram depicting a GCO configuration in accord with the invention.
  • FIGS. 1 and 2 show a laser in accord with the invention.
  • the device uses an electron beam moving over the surface of a semiconducting film to produce coherent electromagnetic radiation.
  • the anticipated operating range is the submm-FlR region of the spectrum and the electron beam energies required are modest (20-200kV).
  • threshold energy increases as the relative dielectric constant of the film material decreases, and thus high-energy beams (hundreds of kV-MV) have been used.
  • high-energy beams hundreds of kV-MV
  • the basic theory of operation is the same. An electron moving near the film surface with a velocity which exceeds that of light in the film material, emits both spontaneous and stimulated
  • Cerenkov radiation A portion of the stimulated Cerenkov radiation is trapped in a resonator formed by mirrors which terminate a length of surface waveguide. If the gain is sufficient, a coherent Cerenkov wave grows to saturation.
  • the use of higher-index semiconducting materials for the surface guide reduces the beam energy required and leads to a physically smaller device, which is attractive for many applications.
  • a further advantage of the semiconducting films lies in the fact that their dc conductivity is large enough to eliminate problems associated with charge buildup on the film surface.
  • FIGS. 1 and 2 A basic embodiment of the invention is illustrated on FIGS. 1 and 2.
  • An electron moving above and parallel to the surface of a dielectric film of index n, with a velocity (c ⁇ o ) which is greater than the speed of light in the film material (c/n), will produce a spontaneous Cerenkov radiation wake.
  • the stimulated Cerenkov radiation will increase the stored energy in the fields. If the resonator Q is sufficient, a coherent oscillation will grow until nonlinear effects cause saturation.
  • the central issues are dispersion and tuning, coupling and gain, the constraints imposed by beam quality, and
  • the electron beam couples predominantly to the longitudinal electric field, or to the TM cavity modes.
  • the dispersion function for these modes takes the form:
  • ⁇ d/c and kd are the angular frequency and axial wavenumber scaled in units of film thickness d.
  • the dimensionless coupling strength for the beam in the resonator may be defined in terms of an active Q:
  • J is the modulated current produced by the electric field E and E is the energy stored in the resonator.
  • J bo is the peak beam current density
  • (mc 3 /e 17 KA)
  • f c is a coupling factor
  • the gain line shape, F R (kL) (FIG. 5), is given by
  • the width of the gain line determines the minimum acceptable energy spread.
  • a schematic illustration of a beam distribution is shown on FIG. 5. It is clear from this sketch that all dephasing effects, when converted to equivalent phase spread, must satisfy
  • the function f c which appears in the expression for g, is a dimensionless measure of coupling. It depends upon the beam cross-section and the mode profile. As a first approximation it is convenient to assume that the distribution is rectangular with thickness ⁇ bx and a lower edge located a distance ⁇ above the surface. For this case:
  • Values of ⁇ , which must be maintained in order to meet the condition ⁇ 1, are listed in Table 1. The presence of a gap does lead to a decrease in the gain, but two
  • v g is the group velocity, d ⁇ /dk.
  • the group velocity and operating frequency ⁇ are dictated by the excited mode, and are taken from the dispersion relation.
  • the active beam quality of eq. (6) must exceed that of the cavity for lasing.
  • Minimizing 1/Q cav delineates the resonator-dependent areas of concern.
  • L should be large relative to ⁇ . It is also clear that r 1 r 2 should be as high as possible while still permitting output coupling. Finally the material losses should be small.
  • resonator design issues are: macroscopic dimensions, mode confinement, mirror reflectivity, and material losses.
  • the propagating wave is more accurately described by a Gaussian mode than by a plane wave. It is realistic to assume Gaussian-type solutions in the y-z plane. These are characterized by a minimum spot size with diameter W O .
  • the waist at a distance z is defined by
  • the degree of mode confinement in the x-z plane can be obtained by invoking the ray optic picture of guided wave propagation.
  • a guided ray propagates along the resonator via successive reflections at the two interfaces.
  • a schematic of this ray is included in FIG. 7.
  • the mode is well confined.
  • the wave penetrates into the region above the guide a distance on the order of 1/q.
  • the effective mode width is
  • a well-confined mode has d eff very close to d;
  • the CML relies on the evanescent tail above the guide for electron/mode coupling.
  • a high resonator quality factor relaxes the material loss and laser gain requirements of the FEL.
  • One way of improving the Q is to increase the mirror reflectivities. Only integral mirrors are treated. They eliminate the losses associated with coupling out of the guide while offering the
  • FIGS. 8, 9 and 10 Several different mirror designs are shown in FIGS. 8, 9 and 10 and discussed below. The flexibility of mirror design enables us to choose the reflectivities we require for optimum CML performance.
  • the simplest Fabry-Perot cavity is formed by cleaving and polishing the ends of the dielectric resonator.
  • the dielectric/air interface can provide sufficient reflectivity, r, to achieve threshold.
  • Typical r values are about 30-40% for Si, GaAs and Ge. If an improved r is desired, metal can easily be evaporated onto the end face.
  • evanescent tail is decreased.
  • the mode is pulled into the dielectric where it is more effectively reflected.
  • the reverse is accomplished if the guiding layer is thinned adiabatically: the wave is accelerated and the evanescent tail grows until the entire wave is launched into the vacuum.
  • adiabatic by assuming that the change occurs over a minimum of 10 wavelengths. This is a practical distance for the entire FIR region.
  • Another possible reflector is a periodic grating which could be fabricated on the resonator ends (as shown in FIG. 10). It is known as a
  • DBR distributed Bragg reflector
  • V T threshold energy required for lasing
  • the gain usually peaks at about twice that voltage. A higher index reduces the voltage required to reach the
  • Cerenkov threshold and also reduces the voltage necessary to operate at a given wavelength. This reduction in energy facilitates the search for an appropriate electron beam source. It also reduces the charging problems associated with high voltage beams.
  • the choice of a semiconductor guiding layer grew from the search for a higher index material, but has become an engineering advantage. Techniques common to integrated circuit and micro-mechanics structure fabrication provide the basis for
  • This gap wavelength separates the absorption into two regions. The first corresponds to ⁇ g where
  • ⁇ g is characterized by free carrier absorption, lattice absorption, intra-band
  • Free-carrier absorption is a consideration at any temperature in the FIR. Energy is lost as free carriers are first excited, and then transfer energy to the surroundings via scattering.
  • N is the free carrier concentration
  • ⁇ is the lasing wavelength
  • m* is the free carrier effective mass in the semiconductor
  • n is its index of
  • ⁇ is the scattering lifetime. This absorption is less significant at low temperatures and short wavelengths. Lattice absorption must also be considered for infrared wavelengths at most temperatures. It will be increasingly important for very lightly doped semiconductor resonators at wavelengths in the vicinity of 10 ⁇ m,, where free-carrier absorption has diminished. Excitation of semiconductor resonator lattice phonon modes via coupling to the beam or the emitted laser radiation is clearly possible in crystalline resonators, and is proportional to a power of the lasing wavelength. The exponent of ⁇ can vary between 1.5 and 2.5, depending upon whether acoustic or optical phonon modes are excited.
  • Polycrystalline or amorphous resonators may be necessary to overcome crystalline lattice absorption limitations.
  • Intra-band absorption can be important in p-type semiconductors. As with free-carrier
  • Impurity absorption will be unimportant at temperatures near 300K. However, near 77K,
  • freeze-out of free carriers onto dopant impurity sites will allow the reverse process to occur via absorption of photons with energy near the impurity ionization energy.
  • temperature lowering is used as a means to control dielectric constant, this mechanism will need consideration.
  • V T ( ⁇ T -1)mc 2 (27) are 23.1, 21.0 and 16.8 kV respectively.
  • Electrons in this energy range can be produced either with a conventional cathode material such as LaB 6 , or a high-brightness field-emitting cathode structure.
  • the drift section separating the films can take a number of forms including a cut-off guide, lossy guide, or the introduction of dispersive magnetic fields.
  • a wedged film can be used in any configuration to slow the phase velocity of the wave to match the electron loss and thus enhance the electronic efficiency.
  • the dielectric constant for semiconductors can be tuned via a number of mechanisms including applying a voltage or changing the temperature. Photolithography allows for essentially limitless (on the CML scale)
  • optical properties of typical semiconductor materials Si, Ge and GaAs in the submillimeter to infrared portion of the spectrum are ideally suited for the SFCL application.
  • Their indices of refraction are relatively high (3.5-4) and published loss rates are acceptably low ( ⁇ bulk ⁇ 0.15 cm -1 ).
  • the invention can be practiced with a number of resonator designs, as depicted in FIGS. 11 through 17B.
  • the resonator in FIGS. 11, 12A and 12B is formed by a parallel plate waveguide which is terminated with cylindrical section mirrors. This structure supports planar-cylindrical Gaussian modes.
  • a narrow semiconductor strip loads the central portion of the resonator and the beam enters and exits through apertures in the mirrors. If needed, additional feedback in the form of Bragg reflector sections may be added in the mirror apertures.
  • the addition of the strip grating to the open parallel-plate resonator adds a space harmonic modulation to the cavity modes and the slow-wave components of these fields couple the bema to the resonator. If needed, Bragg sections and an
  • top surface film can also be included in these Phase One SFCL resonators.
  • the film thickness is typically a factor of 5-15 smaller than the desired operating wavelength. In the spectral region selected for the preliminary experiments, these "films" can actually be
  • Thinner pieces will require some surface polishing after mounting in the resonator. At the shorter end of the spectral range of interest (10's of ⁇ rn), films will be grown directly on substrates.
  • Advanced "integrated" resonators can take a wide variety of forms.
  • One example is shown in FIG.
  • a film is tapered in such a way that it is thicker on the end, and after a suitable transition length, it is terminated with a reflecting surface (M).
  • M reflecting surface
  • the guide wavelength decreases, which in turn draws a greater fraction of the mode into the film where it is then reflected.
  • FIGS. 15 and 16 Two additional options for second-generation resonators are shown in FIGS. 15 and 16.
  • the mode is launched from the film surface by a short section of grating and in the other (FIG. 16), transverse containment is provided by a trough configuration.
  • a grating section is employed as a launching structure, additional
  • the trough section guide can be terminated with
  • resonator designs can be segmented (or klystron-like) structures. Examples are shown on FIGS. 17A and 17B.
  • the segmented structure provides separate sections for velocity modulation and energy extraction. Thus the length of high-precision film can be reduced. Gain will be enhanced and tapered output couplers may be used to increase the
  • the resonator can incorporate a grating strip in place of the semiconductor strip to couple the beam to the resonator.
  • a grating-coupled oscillator GCO
  • FIGS. 20 and 21 The basic GCO geometry is depicted in FIGS. 20 and 21.
  • FIG. 20 illustrates the relationship between a beam electron and the axial component of the electric field.
  • the electron moves over the surface at a distance x o above the top of the grating and couples "synchronously" to space harmonics with nearly the same velocity.
  • FIG. 21 shows an electron beam having a thickness ⁇ bx located a distance ⁇ above the surface of the grating.
  • the factors which govern coupling strength may be
  • the operating wavelength of the GCO is determined by Equation 32 and the dispersion relation of the bound modes on the grating.
  • the dispersion relation itself will be a universal self-similar function of the grating period and the other grating parameters:
  • the threshold condition is a minimum when
  • FIG. 22 A perspective view of the GCO resonator configuration is shown in FIG. 22.
  • a narrow strip of grating loads the center region of a resonator formed by parallel-plate waveguide and cylindrical section mirrors. The beam enters and exits through apertures in the mirrors and the radiation exists along the beam. After exiting the resonator, a transverse magnetic field deflects the electron beam and the radiation proceeds along a light pipe into a
  • a short pulse with higher peak power is a more desirable configuration
  • a second consequence of increasing the beam energy is a decrease of gain (approximately as the ratio of the initial and final energy to the fifth power). This will result in a substantial change in final energy is in the 5 Mev range. It can be offset in part by an increase in the interaction length and in part by an increase in beam current density. The latter can also be substantial. Unlike the
  • the modern microwave-produced beams are capable of attaining J b values in the 1-2 KA/cm 2 range.
  • subsections rely on beam current densities in the 20-200 A/cm 2 range. These values can be achieved with conventional cathode and beam focussing
  • J b 10 3 A/cm 2
  • J b 10 3 A/cm 2
  • This choice is actually conservative (1.5 - 3 ka/cm 2 have been reported).
  • Current densities in this 10 3 A/cm 2 range can also be obtained from what has become known as the "microwave gun”.
  • These devices are now used as injectors for linear accelerators. They produce very high current density small diameter beams in the 0.5 - 5 Mev range.
  • Both conventional (e.g., LaB 6 ) thermionic and high-brightness photo-cathodes have been used as emitters. In either case, the emerging current will have the high-peak short-pulse structure that is characteristic of linear
  • FIG. 19 A preliminary assessment of the potential impact of high-brightness cathodes on SFCL operation is presented in FIG. 19.
  • beam radii from 1-100 urn are listed and in the second column, the total beam current in the diameter 2r b is displayed. The current has been computed assuming that an average of 10 3 A/cm 2 over the cross-section has been achieved.
  • the beam power at beam voltages of 100 kV and 1 mV are listed in columns 3 and 4. These values provide guidance as to the output power that could be obtained at a given efficiency ( ⁇ may be expected to fall in the 0.1 - 10% range).
  • microwave-gun-driven SFCL may still smaller.
  • the basic SFCL discussed above consists of a resonator which is formed by a semiconductor film surface waveguide and either internal or external mirrors. The latter provide feedback and output coupling.
  • the structures may be driven by a variety of electron beam generators and they are expected to produce tunable, coherent radiation over a range extending from 10's of ⁇ m through to the mm region of the spectrum.
  • the output power and pulse structure will depend upon the beam generator option chosen. Estimated values of the saturation level yield typical efficiency values in the 0.1-10% range and output levels in the W-KW (pulsed) region are
  • the semiconductor film resonators have a number of unique features which lend themselves to SFCL operation. Among them are: low loss in the spectral range of interest, the potential for forming integrated resonators, high relative indices of refraction, and the ability to bleed charge via their finite dc resistivity. These will be investigated and exploited during the second and third years of the project. Finally, during the third year, integration of modern high-brightness cathode

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Optics & Photonics (AREA)
  • Lasers (AREA)

Abstract

L'invention se rapporte à un appareil laser à électrons libres avec film semi-conducteur, qui utilise un faisceau d'électrons et un résonateur quasi-optique ouvert et chargé d'un film semi-conducteur, en vue de produire un rayonnement électro-magnétique cohérent dans des longueurs d'onde de l'ordre du sous-millimètre et dans l'infrarouge lointain.
PCT/US1991/006189 1990-08-31 1991-08-29 Appareil laser a electrons libres avec film semi-conducteur WO1992004747A1 (fr)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20230029210A1 (en) * 2021-07-22 2023-01-26 National Tsing Hua University Dielectric-grating-waveguide free-electron laser

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4727550A (en) * 1985-09-19 1988-02-23 Chang David B Radiation source
US4874953A (en) * 1988-10-06 1989-10-17 California Institute Of Technology Method for generation of tunable far infrared radiation from two-dimensional plasmons

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4727550A (en) * 1985-09-19 1988-02-23 Chang David B Radiation source
US4874953A (en) * 1988-10-06 1989-10-17 California Institute Of Technology Method for generation of tunable far infrared radiation from two-dimensional plasmons

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20230029210A1 (en) * 2021-07-22 2023-01-26 National Tsing Hua University Dielectric-grating-waveguide free-electron laser
US12015236B2 (en) * 2021-07-22 2024-06-18 National Tsing Hua University Dielectric-grating-waveguide free-electron laser

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