JP2003066501A - Laser modulation device and laser modulation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain single pulse laser light of subfemto-second (10<-16> sec) order with a simple device constitution. SOLUTION: Nanosecond pulse laser light 2 of 1st wavelength is made incident on solid hydrogen 1 and nanosecond pulse laser light 3 of 2nd wavelength is made incident on the solid hydrogen 1 at a prescribed angle θ from the laser light 2 to generate quantum coherence and nanosecond pulse laser light 4 of 3rd wavelength is made incident on an area in which the quantum coherence is generated at a prescribed angle α from the laser light 3, so that femto-second pulse laser light 4 of the 3rd wavelength is modulated to single subfemto-second pulse laser light 5.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a laser modulation device and a laser modulation method for obtaining a single sub-femtosecond pulsed laser beam.

[0002]

2. Description of the Related Art Conventionally, for example, in a titanium-sapphire laser, a pulse width (time) is 10 femtoseconds (10) by using a so-called mode-locking technique.
A pulse laser of about 10-15 seconds) has been realized, and a pulse laser of about 4 femtoseconds has been reported as an experimental example.

At present, it is desired to realize a pulse laser having a shorter pulse width, for example, on the order of sub-femtosecond (10-16 seconds), and various proposals have been made.

[0004]

By the way, in order to realize a sub-femtosecond laser having a pulse width shorter than that of the femtosecond pulse laser as described above, such an ultrashort pulse should be Fourier-transformed. It is necessary to obtain a corresponding wideband sideband series.

The inventors of the present invention have already generated high-order sidebands by injecting nanosecond pulse lasers of different predetermined wavelengths coaxially in solid hydrogen.
It is proposed that a wideband sideband series can be obtained.

However, when sub-femtosecond laser light is obtained using the wide band sideband series obtained in this way, this sub-femtosecond laser light is emitted as a pulse train at intervals of 8 femtoseconds. It is difficult to extract a single pulse, and there is a problem in putting it into practical use as a pulse laser.

Therefore, the present invention has been proposed in view of the above situation, and a laser modulator and a laser modulation method capable of obtaining a single sub-femtosecond pulsed laser beam with a simple device configuration. Is to provide.

[0008]

In order to solve the above-mentioned problems, a laser modulator according to the present invention comprises solid hydrogen and means for injecting a nanosecond pulse laser of a first wavelength into the solid hydrogen. , A means for generating a quantum coherence by injecting a nanosecond pulsed laser of a second wavelength with respect to solid hydrogen at a predetermined angle with respect to a nanosecond pulsed laser of a first wavelength, and quantum coherence is generated. Third in the area
Means for injecting the femtosecond pulse laser of the second wavelength into the nanosecond pulse laser of the second wavelength at a predetermined angle, and the femtosecond pulse laser of the third wavelength into a single sub-femtosecond pulse. It is characterized in that it is modulated into laser light.

In the laser modulation method according to the present invention, a nanosecond pulse laser having a first wavelength is incident on solid hydrogen, and a nanosecond pulse laser having a second wavelength is irradiated on the solid hydrogen. A nanosecond pulse laser of a first wavelength is made incident at a predetermined angle to generate quantum coherence, and a femtosecond pulse laser of a third wavelength is used to generate a quantum coherence in the region where the quantum coherence is generated. It is characterized in that the femtosecond pulse laser of the third wavelength is modulated into a single sub-femtosecond pulse laser beam by making the second pulse laser incident at a predetermined angle.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

[Structure of Laser Modulator] As shown in FIG. 1, a laser modulator according to the present invention comprises a solid hydrogen 1 and a nanosecond (ns) pulse laser having a first wavelength with respect to the solid hydrogen 1. 2 means for injecting 2 and a nanosecond pulse laser 3 having a second wavelength longer than the first wavelength with respect to the solid hydrogen 1 and a predetermined angle θ with respect to the nanosecond pulse laser 2 having the first wavelength. Means for generating quantum coherence by injecting the laser light and a femtosecond (fs) pulse laser 4 having a third wavelength in the region where the quantum coherence is generated, with respect to a nanosecond pulse laser 3 having a second wavelength. Means for making the light incident at a predetermined angle α.

In this laser modulator,
The femtosecond pulse laser 4 of the third wavelength is modulated into a single sub-femtosecond pulse laser beam 5.

The outputs of the nanosecond pulse lasers 2 and 3 of the first and second wavelengths are on the order of millijoules (mJ). In addition, the femtosecond pulse laser 4 of the third wavelength
Output is on the order of nanojoules (nJ).

Further, the laser modulation method according to the present invention is a method which can be implemented in the laser modulation apparatus according to the present invention, in which the nanosecond pulse laser 2 of the first wavelength is made incident on the solid hydrogen 1, and The nanosecond pulse laser 3 having a second wavelength, which is longer than the first wavelength, is incident on the solid hydrogen 1 at a predetermined angle with respect to the nanosecond pulse laser 2 having the first wavelength to generate a quantum. The coherence is generated, and the femtosecond pulse laser 4 having the third wavelength is incident on the region where the quantum coherence is generated, at a predetermined angle with respect to the nanosecond pulse laser 3 having the second wavelength. The femtosecond pulse laser beam 4 having the wavelength is modulated into a single sub-femtosecond pulse laser beam 5.

The modulated sub-femtosecond pulse laser light 5 is emitted from the solid hydrogen 1 on the extension line of the incident direction of the femtosecond pulse laser 4 of the third wavelength, and the nanoseconds of the first and second wavelengths are emitted. Since the pulse lasers 2 and 3 are emitted in a direction different from the emission direction from the solid hydrogen 1, the nanosecond pulse lasers 2 and 3 having the first and second wavelengths can be easily separated.

In these laser modulation devices and methods,
The first and second wavelengths are, for example, 355 each.
nm (3rd harmonic of YAG laser) and 416 nm (2nd harmonic of titanium-sapphire laser) can be used,
In this case, the angle θ is 0.29 degrees and the angle α is
Is preferably 1.65 degrees, but there is a wide allowable range for the angle θ from the characteristics of solid hydrogen, as will be described later.

The first and second wavelengths are, for example, 738 nm (titanium-sapphire laser), respectively.
And 1064 nm (YAG laser) can be used, and in this case, it is desirable that the angle θ is 1.14 degrees and the angle α is 2.57 degrees. Has a wide tolerance.

The third wavelength is, for example, 800 nm
(Titanium-sapphire laser) can be used. As the femtosecond laser light having the third wavelength, a femtosecond laser beam having a pulse width of about 10 femtoseconds, which has been practically used, can be used.

[Solid Hydrogen] Solid hydrogen is a low temperature molecular crystal composed of hydrogen molecules. The density of solid hydrogen is 1000
Equivalent to hydrogen gas at atmospheric pressure. The outstanding feature of this solid hydrogen is that the interaction between hydrogen molecules is weak and the binding energy between molecules is much smaller than the energy of rotation and vibration of H2 molecule. Therefore, the hydrogen molecule has the degree of freedom of rotation and vibration even in solid hydrogen as in free space.

The hydrogen molecule has almost the same vibration and rotation frequency as in free space even in the condensed phase, but in the solid state,
Both vibration and rotation show unique behavior. First, it is the ground state of solid hydrogen, which is ν = 0 for all molecules,
It is in a state of vibration and rotation of J = 0. ν = 0, J
Since the state of = 0 does not degenerate including the nuclear spin, the ground state is a single state, and its wave function is represented by the direct product of the wave functions of each ground state molecule. On the other hand, in the first excited state of vibration, since there is hopping interaction of vibrational excitation between adjacent molecules, its eigenstate is represented by the Bloch state and forms an exciton band called vibron.

The ground state which is a single state is, so to speak, k = 0.
Since it can be regarded as the Bloch state of, the vibrons of k = 0 need only be considered in the optical transition (selection rule Δk = 0) accompanied by vibrational excitation in solid hydrogen.

In other words, considering the interaction with light, solid hydrogen can be modeled as a system in which the center of gravity of free hydrogen molecules is fixed at the lattice point of the crystal, and the solid density is well defined as well as the isolated system. It can be regarded as a material system that also has quantum qualities. The phase relaxation of vibron is 3 MHz (FWHM) converted into a spectrum line width by setting the concentration of ortho hydrogen, which is an impurity, to 0.1% or less and the temperature to 6 K (-267 ° C) or less. Below,
It is as small as the phase relaxation of atoms and molecules in the gas phase, or rather smaller when the Doppler width is taken into consideration. Therefore, the phase matching condition, which is indispensable in the optical modulation process, is automatically satisfied, and the vibron transition is extremely excellent as a work transition for controlling the optical response using quantum coherence.

Therefore, as described above, with respect to the angle θ formed by the nanosecond pulse lasers of the first and second wavelengths,
Wide tolerance range is obtained.

[Quantum Coherence] Three Raman-type (Λ-type) three quasi-states consisting of metastable levels 1 and 2 and level 3 which is an electric dipole permissible with respect to these levels 1 and 2. The quantum coherence generated between level 1 and level 2 due to the formation of so-called "dark resonance" in the level system can be considered as an extension of the far resonance system. In this far resonance system, as shown in FIG.
The ground level and the metastable level are referred to as a and b for convenience of formulation.

In the resonance system, a negative coherence (dark state) can be prepared through the quantum interference effect via a relaxation phenomenon such as spontaneous emission, but in the far resonance system having a very large detuning, it disappears by optical absorption. Since there is no bright state, a different method is needed to control the sign of coherence. In other words, in this far-resonance system, if the detuning of the two-photon transition is selected to be a net negative value including the AC Stark shift, and the laser field is adiabatically operated, a negative coherence (dark state) is obtained. Can be generated.

[Highly Efficient and Arbitrary Sideband Generation] In the state where quantum coherence is generated in the far resonance system, the beating process between this coherence and the incident laser light results in the generation of multiple sidebands on plus and minus sides. .

Further, the conversion efficiency of plus-minus side bands reaches 25% to 30%, and this efficiency does not depend on the intensity of the pulse laser to be incident.

That is, by using the large coherence of the bipron generated in solid hydrogen, any arbitrary light can be highly efficiently modulated beyond the traditional phase matching framework.

[Generation of Wideband Sideband and Sub-Femtosecond Pulse] As described above, in the far resonance system, the large coherence and the beat process with the laser beam are inevitably the generation process of the higher-order sideband. If there is a proper phase relationship between the sidebands, the wideband sideband series behaves as an ultrashort optical pulse train in the time domain. Considering the spread of the sideband series in the far resonance system, the ultrashort pulse can also be in the sub-femtosecond region.

That is, as described above, by preparing the coherence with a negative sign, compression occurs in the pulse train, and a high intensity subfemtosecond optical pulse can be generated. In order to generate a large coherence, a large number of photons are essentially required, so that an incident pulsed laser is in the nanosecond range. This means pulse modulation in a huge dynamic range from nanosecond to sub-femtosecond on the time scale.

The coherence of the far resonance system interacts with any optical pulse and can efficiently generate sidebands. That is, if the coherence prepared by using the nanosecond pulse laser and the femtosecond pulse laser are beaten, the modulation from the femtosecond pulse laser to the sub femtosecond pulse laser is performed.

[Structure of Experimental Device and Method of Creating Solid Hydrogen]
As the experimental apparatus that constitutes the above laser modulator, as shown in FIG. 2, a first vacuum container 7 that contains a sapphire substrate 6 that serves as a substrate that supports the solid hydrogen 1 is used. The first vacuum container 7 has an outer wall portion 8 made of a heat insulating material.
Liquid nitrogen (liqN2) shield wall 9 in the outer wall portion 8 and liquid helium (liq) in the liquid nitrogen shield wall 9
qHe) It has a triple structure with the shield wall 10, and the inside is kept vacuum by a vacuum pump (not shown).

The liquid nitrogen shield wall 9 is kept at 77K (about -196 ° C) by being strongly in thermal contact with a liquid nitrogen tank (not shown). The liquid helium shield wall 10 is kept at 4K (about -269 ° C) by being strongly in thermal contact with the liquid helium tank 11.

The sapphire substrate 6 has a thickness of 10
Liquid helium tank 1 formed in a flat plate shape of about 0 μm
The cryogenic tank 12 connected to No. 1 is supported via the copper plate 12a. The copper plate 12a has a thickness of, for example, 5
It is a flat plate of about mm, and is fixed to the cryogenic bath 12 by screwing or the like. The copper plate 12a and the sapphire substrate 6 are joined by the indium layer 12c in a state of being stacked with the indium layer 12c interposed therebetween. The sapphire substrate 6 is attached to the copper plate 12a.
A through hole 12b is provided for passing the nanosecond pulse lasers 2 and 3 having the first and second wavelengths and the femtosecond pulse laser 4 having the third wavelength, which are transmitted through and are incident on the solid hydrogen 1. .

The cryogenic tank 12 has a hollow structure, and liquid helium from the liquid helium tank 11 is supplied to the inside little by little from one end side. Then, the cryogenic tank 12 sucks the internal liquid helium from the other end side by a pump (not shown), so that 1.5
It is cooled to about K. And the sapphire substrate 6 also
It is cooled to about 1.5 K, which is the same level as the cryogenic tank 12.

As a method for producing solid hydrogen, first, liquid normal hydrogen having a purity of 99.9% or more is used at a temperature slightly higher than the melting point (at atmospheric pressure, 13.8 K) using an iron oxide catalyst. It is converted to para hydrogen (P-H2). Then, this parahydrogen is supplied to the nozzle 13 in the first vacuum container 7.
Through the surface of the sapphire substrate 6 cooled to about 1.5K. Then, this sapphire substrate 6
On the surface of the, solid hydrogen 1
Is formed. Since the solid hydrogen 1 becomes thicker as it grows, the supply of parahydrogen may be stopped when the solid hydrogen 1 reaches a predetermined thickness of about 100 μm. Thus, the thin plate-shaped solid hydrogen 1 supported on the surface of the sapphire substrate 6 is formed.

Then, the nanosecond pulse lasers 2 and 3 having the first and second wavelengths and the femtosecond pulse laser 4 having the third wavelength are incident on the first vacuum vessel 7 into the first vacuum vessel 7. An entrance hole 14 is provided for this purpose. Further, the first vacuum container 7 has an emission hole for emitting the sub-femtosecond laser beam 5 modulated through the solid hydrogen 1 at a position opposite to the incident hole 14 with the sapphire substrate 6 interposed therebetween. 15 are provided.

As shown in FIG. 3, the entrance hole 14 is closed by a first quartz plate 16 having a thickness of about 5 mm. The first quartz plate 16 has a small hole 17 through which the femtosecond pulse laser 4 of the third wavelength passes. The thickness of the first quartz plate 16 is 1
A second quartz plate 18 having a size of about 00 μm is overlapped and bonded, and the sealed state in the first vacuum container 7 is maintained.

In this incident hole 14, the first and second
The nanosecond pulse lasers 2 and 3 of the wavelength are sequentially transmitted through the second and first quartz plates 18 and 16 and are incident into the first vacuum container 7, and the femtosecond pulse laser 4 of the third wavelength is ,
The light is transmitted through only the second quartz plate 18 and is incident on the inside of the first vacuum container 7.

A second vacuum container 19 forming an experimental space is connected to the emission hole 15. The inside of the second vacuum container 19 is connected to the first vacuum container 7 via the emission hole 15.
It is connected to the inside, and is maintained in a vacuum like the inside of the first vacuum container 7.

The nanosecond pulse lasers 2 and 3 having the first and second wavelengths and the femtosecond pulse laser 4 having the third wavelength, which have entered the first vacuum container 7, are provided with through holes 12 in the copper plate 12a.
After passing through the inside of b, the light is incident on the solid hydrogen 1 through the sapphire substrate 6 from the light incident surface side (the surface on which the solid hydrogen 1 is not formed) of the sapphire substrate 6. Then, the single sub-femtosecond pulsed laser light 5 modulated in the solid hydrogen 1 is emitted to the experimental space 20 in the second vacuum container 19 via the emission hole 15. The solid hydrogen 1 to the experimental space 20 is a vacuum space, and the single sub-femtosecond pulsed laser light 5 reaches the experimental space 20 without changing its state. Various experiments using the single sub-femtosecond pulsed laser light 5 can be performed in this experimental space 20.

[Modulation Process] In the laser modulator according to the present invention, as described above, the femtosecond pulse laser light is modulated to the sub femtosecond pulse laser light. This modulation is shown in FIG. As described above, the quantum coherence is performed with the progress of the femtosecond pulsed laser light in the solid hydrogen in which the quantum coherence is generated. That is, in the example shown here (when the first and second wavelengths are 355 nm and 416 nm, respectively, and the angle θ is 0.29 degrees),
Since the pulse width becomes 0.3 femtoseconds after proceeding 100 μm, if the thickness of the solid hydrogen is 100 μm, a good single sub-femtosecond pulsed laser beam can be obtained.

[Modulation Efficiency] The modulation efficiency of the laser modulator according to the present invention is shown in FIG. 5 when the first and second wavelengths are 355 nm and 416 nm, respectively, and the angle θ is 0.29 degrees. Thus, at a travel distance of 100 μm in solid hydrogen of a femtosecond pulse laser, 1
It exceeds 110% and reaches 110%.

[0044]

As described above, in the laser modulation apparatus and the laser modulation method according to the present invention, the nanosecond pulsed laser of the first wavelength is made incident on the solid hydrogen, and the solid hydrogen is irradiated at the first time. Second wavelength longer than 1 wavelength
A nanosecond pulsed laser having a wavelength of 1 μm is incident on the nanosecond pulsed laser having a first wavelength at a predetermined angle to generate quantum coherence. The femtosecond pulse laser of the third wavelength is modulated into a single sub-femtosecond pulse laser beam by making the second pulse laser enter the nanosecond pulse laser of the second wavelength at a predetermined angle. be able to.

That is, the present invention can provide a laser modulation device and a laser modulation method that can obtain a single sub-femtosecond pulsed laser beam with a simple device configuration.

[Brief description of drawings]

FIG. 1 is a plan view showing a configuration of a laser modulator according to the present invention.

FIG. 2 is a vertical cross-sectional view showing the configuration of an experimental device that constitutes the laser modulator.

FIG. 3 is a vertical cross-sectional view showing a configuration of a main part of the experimental apparatus.

FIG. 4 is a graph showing a process in which laser light is modulated in the laser modulator.

FIG. 5 is a graph showing modulation efficiency in the laser modulation device.

[Explanation of symbols]

1 solid hydrogen, 2 first wavelength nanosecond pulsed laser,
3 Second-wavelength nanosecond pulse laser, 4th-wavelength femtosecond pulse laser, 5 Sub-femtosecond pulse laser light

Continued front page    (72) Inventor Pham Lekien             3-73-1, Chofugaoka, Chofu-shi, Tokyo NOBU             Chofugaoka 303 F-term (reference) 2K002 AB33 BA02 CA02 DA01 GA04                       HA23                 4E068 CA03 CD05                 5F072 HH07 KK11 QQ07 SS08

Claims (2)

[Claims] 1. Solid hydrogen, means for injecting a nanosecond pulsed laser of a first wavelength to the solid hydrogen, and a second wavelength of a wavelength longer than the first wavelength of the solid hydrogen. The nanosecond pulsed laser of the present invention is incident on the nanosecond pulsed laser of the first wavelength at a predetermined angle,
Means for generating quantum coherence, and means for making a femtosecond pulse laser of a third wavelength incident on the region where the quantum coherence is generated at a predetermined angle with respect to the nanosecond pulse laser of the second wavelength. A laser modulation device comprising: a femtosecond pulse laser beam of the third wavelength, which is modulated into a single sub-femtosecond pulse laser beam. 2. A nanosecond pulsed laser of a first wavelength is made incident on solid hydrogen, and a nanosecond pulsed laser of a second wavelength longer than the first wavelength is made incident on the solid hydrogen. A nanosecond pulsed laser having a wavelength of 1 is made incident at a predetermined angle to generate quantum coherence, and a femtosecond pulsed laser having a third wavelength is applied to the region where the quantum coherence is generated. A laser modulation method characterized in that the femtosecond pulse laser of the third wavelength is modulated into a single sub-femtosecond pulse laser beam by making the nanosecond pulse laser incident at a predetermined angle. .