The development of high pulse energy and high repetition rate lasers based on Yb:YAG ceramics is expected to achieve high average power in areas not previously achieved by high energy diode pumped solid state lasers (DPSSL). Such lasers are of interest for advanced materials processing, surface treatments such as laser peening, and pumping ultra-intense lasers for compact radiation and particle sources. The choices of gain media, amplifier geometry, thermal management, and extraction architecture are important aspects for development of a scalable high repetition rate and high energy laser system. We are aiming to develop a pulse energy of 100 J, repetition rate of 100 Hz using conductive-cooled Yb:YAG active-mirror amplifier with a liquid-nitrogen cooling. We report on the status of the development our laser.
We had developed a unique porous thin films by a special coating method1. In this technique, two dielectric materials A and B having different refractive indices nA and nB ,where nA>nB are simultaneously deposited in vacuum on a substrate such as fused silica or optical glasses. Then the coated surface is processed in ultra-pure water which preferentially dissolves the material B. These processes result in a porous thin film
which has gradient refractive index and has the antireflection (AR) property over broad
bandwidth. The porous coating obtained by this method cannot apply depositing a multilayered dielectric thin film. We have developed a novel method. The present technique, a dielectric material D and a plastic P are simultaneously deposited in vacuum on a heated-substrate such as fused silica, ceramic or optical glasses. Then
the coated surface forms an adaptively mixed thin film ( AMTF ) with dielectric material and plastic. In this coating process, plastics partially evaporate due to the heated-substrate. The refractive index of the coated AMTF mainly decided by the mixing ratio of the dielectric material and plastic. In our samples the damage threshold was confirmed to be 115 J/cm2 at 10 ns and λ=1064 nm. The band width of AMTF with MgF2 and Teflon (AMTF: MgF2 ) was confirmed to cover from 200 to 8000 nm. This AMTF: MgF2 can be applicable not only to AR thin film, but to a high reflectance mirror and polarizer in various high intensity laser syetems.
1K.Yoshida, H.Yoshida, Y.Kato, and C.Yamanaka, Appl.Phy.Lett.47,911(1985)
We demonstrated beam shaping to top-flat and square by phase-only Spatial Light Modulator (SLM) and spatial frequency filtering. Spatial phase distribution of a femtosecond laser beam was modulated by a phase grating pattern reflecting a transfer function for beam shaping. By filtering the higher spatial frequency component at Fourier plane in 4f system, the spatial amplitude distribution of the zero-order beam was shaped to top-flat and square. This result enables us to fabricate large area and uniform devices by using multi-shot processing.
Contamination of optics observed in LFEX compression chamber was a critical problem for maintaining high damage
threshold and high optical performance for mirrors and gratings in the vacuum environments. We conducted a study for
understanding this problem, and we found important knowledge on the nature of contamination, namely materials of
contaminants, source of contamination, the invasion mechanism, and removal method of contamination. We also found
the samples for the damage test is easily contaminated in the storage environment. This means the optical coating
accumulate contaminations even in the air. So we tested in-situ damage test in a gas flowing chamber with controlled
contaminants. The degradation was time-dependent phenomena, and proportional to the vapor
pressure of
contaminants. Several materials were tested, and even in water vapor, the damage threshold was decreased about 10%.
We also found out two methods for removing contaminations from the coatings. According to these studies, our
conclusion is special treatments should be used for knowing the intrinsic damage threshold of the coatings.
We have fabricated a 410 x 468 mm size deformable mirror with 100 Bimorph piezoceramic actuators for the LFEX
laser system at Osaka University. In the case of Bimorph-type deformable mirrors, the mirror surface had to be polished
and coated after bonding the piezoceramic actuators to the rear side of the thin mirror substrate. This provides a good
surface figure, but the coating temperature for the high-reflection mirror was strictly limited because of the thermal
fragility of piezoceramic actuators. The mirror substrate with the actuators was polished, and an ion-assisted multilayer
dielectric coating was produced at 60 degrees Celsius with our 80-inch coating chamber. The flatness of the mirror just
after coating was 7 μm, and reduced by aging to 3.2 μm when the mirror was assembled. The surface figure of the
assembled mirror with 20 piezostack bonded actuators is demonstrated and a laser-induced damage threshold tested with
a witness sample is also reported.
LFEX is the world’s largest high-energy petawatt laser. So far it delivers 3 kJ/1 ps and is planed to finally deliver 10 kJ/10-20 ps. It has been constructed and became partially operational since 2008, and with full beams since 2014. LFEX is synchronized to nsec Gekko-XII laser for variety of experiments with nsec and psec simultaneous laser beams irradiating the targets for fast ignition and other high-energy density physics.
In high power laser systems, the laser-induced damage threshold (LIDT) in optical coating is very important parameters for obtaining high performances. Recently, LIDT was found to have strong temperature dependences in the bulk, surface of substrates, and in coatings. These temperature dependences of LIDT were carefully measured, and the damage formation model was constructed regarding to this temperature dependence. To explain this temperature dependence of LIDT, the temperature dependences of the initial electron generation and electron multiplication in the avalanche process were taken into account. On the other hand, LIDT in optical coating is very sensitive to organic contaminations accumulated in coating layers during storage and using condition. This paper also introduces the oil-contamination problem in LFEX laser system for First Ignition scheme in the laser fusion. We have analyzed contaminants and evaluated the effects of the contamination. We also developed new cleaning methods to remove contamination from the coating, and we have succeeded to prevent the degradation in LIDT for the duration of evacuation with Silica-gel in the chamber. The quantitative analysis of contamination on LIDT was made. We have investigated the characteristics of LIDT in dielectric coatings under the controlled contamination with several materials.
We have developed high average power MOPA laser system with SHG unit on the table top size (3 × 1.5m). At the wavelength 1064nm has been obtained the max average output power of 715W. We have achieved the average power 180W at the wavelength 532nm, the pulse width of about 100ns, the frequency of 1kHz. And the power efficiency of the SHG from the wavelength of 1064nm to 532nm was obtained about 25.6%.
Double-clade crystalline fiber waveguide (CFW) has been produced by using adhesive-free bond (AFB®) technology. The waveguide consists of a 1 at.% Yb:YAG core, un-doped YAG inner cladding and ceramic spinel outer cladding. It is a direct analog of the conventional double-clad glass fiber laser in the crystal domain. Signal gain of 45 or 16.5 dB has been measured in a preliminary master oscillator power amplifier (MOPA) experiment. Due to the high laser gain and the weak Fresnel reflection at the uncoated waveguide ends, the CFW even starts self-lasing above a certain pump power. Laser output power of 4 W in the backward propagation direction has been measured for input pump power of 44 W. After considering the same amount of forward propagated laser power, the laser efficiency to the absorbed pump power is estimated to be about 44%. In principle, CFW can have extremely large single mode area for high efficiency and high power laser applications. So far, Single mode area < 6700 μm2 has been demonstrated in Er:YAG CFWs.
In high power laser system, laser-induced damage threshold (LIDT) in optical coating is very important for obtaining
high performances. The dependence of LIDT on the pulse duration and the repetition rate are well known phenomena.
But recently, LIDT was found to have strong temperature dependences in the bulk, surface of substrates, and in coatings.
This temperature dependence of LIDT was carefully measured, and the damage formation model was constructed
regarding to this temperature dependence. This paper introduces LFEX laser system for First Ignition scheme in the
laser fusion. A large-scale pulse compression chamber was designed and constructed, and segmented grating system has
been employed for large-scale pulse compressor. This compressor provided good pulse compression performances, but
we observed a heavy oil-contamination of optics in this chamber. We have analyzed contaminants and evaluated the
effects of the contamination. We also developed new cleaning methods to remove contamination from the coating, and
the quantitative analysis of contamination on LIDT was made. We have investigated the characteristics of LIDT in
dielectric coatings under the controlled contamination. LIDT of coating drops to 1/2 in the saturated toluene vapor at
room temperature.
We are developing a high-repetition and high-average-power Nd:YAG laser amplifier pumped by fiber coupled LDs in order to apply to laser machining of carbon composite materials such as CFRP (Carbon Fiber Reinforced Plastic). Final target is 1.5 kW output at wavelength of 1064 nm by 8 kW LD pumping. The amplifier consists of several Nd:YAG ceramic thin discs on a non-doped ceramic YAG block. At first, we measured wave-front distortion and small-signal gain of a prototype amplifier whose target is more than 500 W output.
Laser-induced damage threshold (LIDT) in optical coating is very sensitive to organic contaminations accumulated in coating layers during storage and using condition. The sources of contamination are commonly exists, and optical coatings are easily contaminated regardless to the environment pressure, LIDT at ns region decreased largely by contamination, but LIDT at ps seems insensitive. In this study, we have investigated the influence of contamination of optical coating on LIDT and other optical properties. We examined several kinds of coating to clarify the sensitivity to the contamination. Degradations of LIDT were commonly observed in e-beam deposition, IAD and IBS. Some coatings changed spectral characteristics by contamination, and other coatings did not change. Some samples were contaminated as received condition, and some were very clean. Furthermore, we have investigated the characteristics of LIDT in dielectric coatings under the controlled contamination. LIDT of coating drops to 1/2 in the saturated toluene vapor at room temperature.
Huge numbers of 2D- or 3D- nanostructures can be generated by interfering ultra-short pulse laser processing in a single
shot. Unit structures are nanowhiskers, nanoneedle, nanobump, nanomesh of metal. The distribution of these
nanostructures are according to interference pattern, which can be controlled and designed by the number of beams,
correlation angle, amplitude ratio and phase shifts between the beams. In this paper, we simulated the interference
pattern with different combinations of these parameters. Our technique is useful for fabrication of metamaterials, in
which designed unit structures are in designed periodic patterns.
Interfering ultra-short pulse laser processing can make nano-structures on metallic thin films. The unit nano-structures
are nano-waterdrop, nanocrown, nanobump etc.. They change according to the character of target and the interference
pattern. An interference pattern of four beams, diffracted by a transmission grating, is like a simple matrix. We generated
arranged periodic structures different from the past experiments, by changing the configuration of four interfering beams.
Parameters of an interference pattern are wavelength, correlation angle, difference of intensities and phase shift between
the beams. As a result, complicated or duplicated structures can be generated.
Interfering femtosecond laser can induce periodic induction of energy on a thin film deposited on a substrate, and
periodic thermal process is induced. This results in liquid motion of target, such as melting, inflation, flow and shrink,
and then it freezes due to temperature fall by thermal radiation and conduction. The resultant structures are nanobump,
nano-whisker, nano-waterdrop, nano-crown, and the shapes can be controlled by laser fluence, thin film thickness,
substrate material, etc.. The size of some structures is smaller than 10 nm in curvature radius, and the aspect ratio is over
20. In addition, duplicated structure of two shapes, or double density structures can be generated in a single shot of laser
irradiation by controlling the phase shift and power ratio between interfering beams. In the case of duplicated structure,
the density of nano-structures is doubled, and two different nano-structures appear alternately. These structures will be
useful in nanotechnology, especially in meta-material technology.
A heavy oil-contamination was observed on the optical components in LFEX pulse compressor. This contamination
came from the wall of compression chamber, and the damage threshold of the mirror dropped to 1/2 or1/3 of the original
value. The same contamination was observed in different compression chambers in our institute. The contamination
materials were identified as Paraffin-oil and DBP (Di-n-butyl phthalate). Several cleaning schemes were tried, but no
significant improvement was obtained. Finally, we found well-baked silica gel placed in the vacuum chamber improved
the contamination very much. In a small vacuum chamber, the damage threshold increase by 3 times, and this result
indicated the contamination of damage test sample. We also tried to remove contamination with dipping optics in the
water-alcohol mixture, and we obtained almost the same improvements with the silica gel.
Top down technology of ultra-short pulse laser processing was applied to induce liquidly process and generate new
nanostructures such as nanobump, bead-on-bump and burst structure. Bilayer thin film was used as a sample, and
structure changed by the film formation. For example, Burst structure could be generated in the case of Au on Ag film
structure. In the case of Ag on Au structure, only nanobump and bead-on-bump structure could be generated, and which
is similar to single layer case.
We report total-reflection active-mirror laser experiments by using a cryogenic Yb:YAG composite ceramic. The
composite ceramic has no high reflection coating on the bottom surface, and is cooled with liquid nitrogen directly. We
obtained 273 W output power with optical efficiency of 65% and slope efficiency of 72% against the absorbed pump
power. The laser power and optical efficiency will be improved more when the pump power increases further. To
investigate thermal effects of the laser material in more detail, we have measured the thermal lens focal length and the
temperature of Yb:YAG. We observed thermal lensing effect of f ~ 2000 mm, and the maximum temperature of 150 K
for 400 μm-thick Yb:YAG sample. We have also studied the theoretical analysis of thermal distribution in the composite
ceramic.
In vacuum ultraviolet (VUV) spectral region, coherent light sources are being thus in high demand for advanced precise
and microscopic processing. A sub-picosecond VUV light source at 126 nm has been produced by the nonlinear
wavelength up-conversion of a near infrared femtosecond Ti:Sapphire laser at 882 nm in rare gases. We obtained the
maximum output of the 7th harmonic at 126 nm in Xe at the pressure of around 2 Torr. The 126 nm beam will be
amplified by an optical-field-ionization produced Ar2 medium and then high-power sub-picosecond VUV pulses will be
obtained.
We have demonstrated an OFI Ar2* excimer VUV amplifier at 126 nm pumped by a high-intensity laser in the table top
size. We observed the Ar2* excimer emission centered at 126 nm with the spectral bandwidth of 10 nm (FWHM), which
was produced in the OFI plasma. Significant amplification was observed inside the OFI Ar2* excimer as a result of the
optical feedback provided by a VUV reflector. The gain-length product of 5.6 was observed at the Ar pressure of 11 atm.
The population inversion density on the order of 1017 cm-3 was evaluated inside the OFI plasma, which would be
sufficient for the amplification of a subpicosecond VUV pulse at 126 nm produced by the harmonic generation.
We have developed a novel debris-free in-air laser dicing technology, which is expected to give less failure of MEMS
devices and hence improves yields. Our technology combines two processes: a dicing guide fabrication and a wafer
separation process. The first process is internal transformation using a nanosecond Nd:YVO4 laser with high repetition
rate and/or a pulsed fiber laser with 200ns pulsewidth. The laser pulses are focused inside the MEMS wafer without surface ablation. In order to make cross-sectional internal transformation, the laser beam is scanned several times with defocusing. The laser scanning speed per each scanning is 100-700 mm/sec depending on the layer material, the machining time is much faster than the conventional blade dicing. The second process is non-contact separation by thermally-induced crack propagation using a CO2 laser or mechanical separation by bending stress. In the each separation process, the internal transformation fabricated in the first process worked well as the guide of separation, and the processed wafer was diced with low stress. This dicing technology was applied for 4-inch MEMS wafers, e.g. pressure sensors, etc., and the sensor chips were separated without mechanical damages.
Top down technology of ultra-short pulse laser processing was applied to induce liquidly process and generate new
nanostructures such as nano-waterdrop, nanocrown, and fine web structure. For example, a nano-waterdrop was
generated by a single shot ps laser irradiation and had the narrow dieter of about 50 nm. In the case of nanocrown,
whiskers were standing at the edge of a nanohole, and the diameter of the whiskers was around 100 nm. In addition,
wavelength-sized web structure was generated in a single shot of femtosecond laser irradiation.
We have demonstrated an argon excimer vacuum ultraviolet (VUV) amplifier at 126 nm by using the optical-field induced ionization (OFI) of argon. The gain-length product of 5.6 was achieved as a result of the optical feedback inside the amplifier with a VUV mirror. Plasma self-channeling caused by the high-intensity pump laser was simultaneously observed when the maximum gain-length product was observed. We have also optimized the output power of a subpicosecond VUV seed beam at 126 nm produced in low-pressure
rare-gases as a result of the seventh harmonic nonlinear wavelength conversion of a Ti:Sapphire laser at 882 nm.
In the present paper, we overview fabrication methods to produce density-controlled tin and xenon targets for generating
extreme ultraviolet (EUV) light. The target can be classified as a mass-limited target. In the case of tin, EUV was
relatively monochromatic, and its conversion efficiency was higher than bulk tin. Using the nano-template method, the
cellular foam size was controlled by the template size. The density was 0.5 ~1.5 g/cm3. In the case of the 0.5 g/cm3
foam, its morphology was controlled by changing the ethanol content of the precursor tin solution. The morphology
difference was useful to control the angular distribution of EUV radiation. SnO2 nanofiber, which is oriented low-density
material, was fabricated continuously using a electrospinning method. The width and the shape of the fiber were
controlled by optimizing precursor solution. A transparent film with tin and SnO2 elliptic spheres were prepared using
liquid crystalline cellulose derivative. Low density xenon was prepared from liquid xenon using a swirl atomizer to
produce a density of 0.2 g/cm3.
EUV emission from spherical and planer targets were precisely characterized as an experimental database for use in
EUV source generation at high repetition rates. In the single-shot base experiments, conversion efficiency as high as
those for the plasma geometry has been demonstrated. The integrated experiment was made with 10 Hz plasma
generation.
We propose a new scheme for high conversion efficiency from laser energy to 13.5 nm extreme ultra violet emission
within 2 % band width, a double pulse laser irradiation scheme with a tin droplet target. We consider two-color lasers, a
Nd:YAG laser with 1.06 µm in wavelength as a prepulse and a carbon dioxide laser with 10.6 µm in wavelength for a
main pulse. We show the possibility of obtaining a CE of 5 - 7 % using a benchmarked radiation hydro code. We have
experimentally tested the new scheme and observed increase of CE greater than 4 %. We show many additional
advantages of the new scheme, such as reduction of neutral debris, energy reduction of debris ions, and decrease of out
of band emission. We also discuss debris problems, such as ion sputtering using newly developed MD simulations, ion
mitigation by a newly designed magnetic coil using 3-PIC simulations and tin cleaning experiments.
We have been developing an ultrashort-pulse high-intensity vacuum ultraviolet (VUV) laser. Ultrashort VUV pulses at 126 nm have been produced in rare-gases by nonlinear wavelength conversion of an infrared Ti:sapphire laser at 882 nm. This pulse will be amplified inside an Ar2* amplifier excited by optical-field-induced ionization electrons. The amplification characteristics of the Ar2* amplifier has been improved by plasma channeling induced by a high-intensity plasma-initiating laser.
We have been developing the vacuum ultraviolet (VUV) light sources and novel applications using such short
wavelength emission sources. High quality amorphous Si thin films were successfully produced at room temperature as
a result of photo-dissociation of SiH4 gas by using an Ar2* excimer lamp irradiation at 126 nm. To enhance such novel
VUV processing applications, a compact VUV amplifier at 126 nm was developed by use of the optical-field-ionization
(OFI) electrons. The gain-length product around 5 was obtained as a result of the optical feedback by using a VUV
mirror. This amplifier was operated in a table-top size with a high repetition rate up to several kHz, which should be
appropriate for any process applications. We also describe the schematic concept of the ultrashort pulse high-intensity
VUV laser system at 126 nm with a pulse width of 100 fs.
Laser-produced Sn plasma is an efficient extreme ultraviolet (EUV) light source, however the highest risk in the Sn-based EUV light source is contamination of the first EUV collection mirror caused by debris emitted from the Sn plasma. Minimum mass target is a key term associated with relaxation of the mirror contamination problem. For design of the optimum minimum mass Sn target, opacity effects on the EUV emission from the laser-produced Sn plasma should be considered. Optically thinner plasma produced by shorter laser pulse emits 13.5 nm light more efficiently; 2.0% of conversion efficiency was experimentally attained with drive laser of 2.2 ns in pulse duration, 1.0 × 1011 W/cm2 in intensity, and 1.064 μm in wavelength. Under the optimum laser conditions, the minimum mass required for sufficient EUV emission, which is also affected by the opacity, is equal to the product of the ablation thickness and the required laser spot size. Emission properties of ionized and neutral debris from laser-produced minimum mass Sn plasmas have been measured with particle diagnostics and spectroscopic method. The higher energy ions have higher charge states, and those are emitted from outer region of expanding plasmas. Feasibility of the minimum mass target has been demonstrated to reduce neutral particle generation for the first time. In the proof-of-principle experiments, EUV emission from a punch-out target is found to be comparable to that from a static target, and expansion energy of ion debris was drastically reduced with the use of the punch-out target.
For EUV lithography the generation of clean and efficient light source and the high-power laser technology are key issues. Theoretical understanding with modeling and simulation of laser-produced EUV source based on detailed experimental database gives us the prediction of optimal plasma conditions and their suitable laser conditions for different target materials (tin, xenon and lithium). With keeping etendue limit the optimal plasma size is determined by an appropriate optical depth which can be controlled by the combination of laser wavelength and pulse width. The most promising candidate is tin (Sn) plasma heated by Nd:YAG laser with a pulse width of a few ns. Therefore the generation technology of clean Sn plasma is a current important subject to be resolved for practical use. For this purpose we have examined the feasibility of laser-driven rocket-like injection of extremely mass-limited Sn or SnO2 (punched-out target) with a speed exceeding 100m/s. Such a mass-limited low-density target is most preferable for substantial reduction of ion energy compared with usual bulk target. For high average power EUV generation we are developing a laser system which is CW laser diode pumped Nd:YAG ceramic laser (master oscillator and power amplifier system) operating at 5-10 kHz repetition rate. The design of practical laser for EUV source is being carried out based on the recent performance of >1 kW output power.
We have been developing a high average-power laser system for science and industry applications that can generate an output of 20 J per pulse at 10-Hz operation. Water-cooled Nd:glass zig-zag slab is pumped with 803-nm AlGaAs laser-diode modules. To efficiently extract energy from the laser medium, the laser beam alternately passes through dual zig-zag slab amplifier modules. Twin LD modules equipped on each slab amplifier module pump the laser medium with a peak power density of 2.5 kW/cm2. In high power laser system, thermal load in the laser medium causes serious thermal effects. We arranged cladding glasses on the top and bottom of the laser slab to reduce thermal effects.
It is very effective for mass-limited tin-foil targets to adapt for the EUV source. Tin-foil targets in account of formation, size, and thickness have been developed for debris mitigation. The amount of ions from targets is 40 % decreased tin-foil targets of 1μm or 5μm thickness than tin-bulk targets. The ion velocity is one order of magnitude less than bulk targets. The EUV emission spectra of tin-foil are more narrowing than bulk targets. The targets supply for high repetition rate of 10 kHz is applied for a novel method. It is called "Punch-out" method. The flight of graphite foil that it is a test targets was succeed to observe by using a gated ICCD camera. The target velocity is achieved to be about 120 m/s. This value can be applied for targets supply with high repetition rate of 10 kHz.
Properties of laser-produced tin (Sn) plasmas were experimentally investigated for application to the Extreme Ultra-Violet (EUV) lithography. Optical thickness of the Sn plasmas affects strongly to EUV energy, efficiency, and spectrum. Opacity structure of uniform Sn plasma was measured with a temporally resolved EUV spectrograph coupled with EUV backlighting technique. Dependence of the EUV conversion efficiency and spectra on Sn target thickness were studied, and the experimental results indicate that control of optical thickness of the Sn plasma is essential to obtain high EUV conversion efficiency and narrow spectrum. The optical thickness is able to be controlled by changing initial density of targets: EUV emission from low-density targets has narrow spectrum peaked at 13.5 nm. The narrowing is attributed to reduction of satellite emission and opacity broadening in the plasma. Furthermore, ion debris emitted from the Sn plasma were measured using a charge collector and a Thomson parabola ion analyzer. Measured ablation thickness of the Sn target is between 30 and 50 nm for the laser intensity of 1.0 x 1011 W/cm2 (1.064 μm of wavelength and 10 ns of pulse duration), and the required minimum thickness for sufficient EUV emission is found to be about 30 nm under the same condition. Thus almost all debris emitted from the 30 nm-thick mass-limited Sn targets are ions, which can be screened out by an electro-magnetic shield. It is found that not only the EUV generation but also ion debris are affected by the Sn target thickness.
We have developed an integrated Laser Ablation Fluid Radiation simulation Code (LAFRAC) to estimate the behaviors of highly energetic ions and neutral particles from LPP EUV light sources, and estimated recombination and charge transfer processes between the particles from laser-produced Xe EUV light sources. We clarify that charge transfer effects greatly affect on the number density of neutral particles, especially high energy (more than roughly 500 eV) neutral particles.
Our institute has been investigating laser-produced tin plasma and EUV emission, and found the highest conversion efficiency of 3% at 13.5 nm in 2% bandwidth. In the present paper, we introduce fabrication methods of density-controlled tin targets to generate relatively monochromatic EUV with keeping similar conversion efficiency.
The first method is the nano-template method, where liquid tin solution was immersed into a polymer film with monodispersed size nanoparticle. The density can be controlled by tin concentration of the solution. The target can be classified into mass-limited target. We have shown a tendency of monochromatic EUV emission around 13.5 nm with decreasing of tin density. The intensity was higher than tin foil with bulk density. The tendency has a merit to mitigate heat effect of the first EUV mirror. The fabrication method has another merit to control not only density but also the poresize of tin oxide to be 100 nm ~ 10 mm. Recent experiments exhibited an EUV character depending on the poresize.
The second is liquid crystalline template method to obtain porous tin oxide. The precursor with tin oxide and cellulose provides mechanically stable and transparent film. The film has wavy sub-microstructure derived from microscopic liquid crystal domain structure. The method is simple and short duration for the hydrolysis reaction to solidify tin compound. This material has a merit of feasibility of fabrication, and was applied for rotation target for 10 Hz and 5 kHz laser repetition.
Extreme ultraviolet (EUV) emission from laser produced tin plasma was investigated for 1064, 532 and 266 nm laser wavelengths. The EUV conversion with tin target tends to be high for shorter laser wavelength and is optimized at 4-5x1010 W/cm2 for 1064 and 532 nm. The EUV emission exhibits laser wavelength dependence in terms of angular distribution and structures of emission spectra. It is found that spectra for 532 nm and 266 nm showed spectral dips at around 13.5 nm and these dips are well replicated in computer simulations. Both the angular distribution together with the spectral dips may suggest existence of opaque plasmas surrounding the EUV emission region.
Extremely ultraviolet (EUV) light at around 13.5 nm of wavelength is the most probable candidate of the light source for lithography for semiconductors of next generation. We have been studying about the EUV light source from laser-produced plasma. Detailed understanding of the EUV plasma is required for developments of modeling with simulation codes. Several parameters should be experimentally measured to develop the important issues in the simulation codes. We focused on density profile, properties of EUV emission, and opacity of the laser-produced plasmas. We present re-cent experimental results on these basic properties of the laser-produced EUV plasmas.
Extreme ultraviolet (EUV) emission from laser produced plasma attracts much attention as a next generation lithography
source. The characterization of EUV emission has been carried out using GEKKO XII laser system. The twelve beams
irradiated tin or tin-oxide coated spherical targets uniformly and dependence of EUV spectra on laser intensity were
obtained with a transmission grating spectrometer and two grazing incidence spectrometers. The EUV Conversion
Efficiency (CE, the ratio of EUV energy at the wavelength of 13.5 nm with 2 % bandwidth to incident laser energy) was
measured using an absolutely calibrated EUV calorimeter. Optimum laser intensities for the highest conversion were
found to be 0.5- 1x1011 W/cm2 with CE of 3 %. The spectroscopic data indicate that shorter wavelength emission
increases at higher laser intensities due to excessive heating beyond optimum temperatures (20- 40 eV). The CE was
almost independent on the initial coating thickness down to 25 nm.
Extreme Ultra Violet (EUV) light source produced by laser irradiation emits not only the desired EUV light of
13 ~ 14 nm (about 90 eV) but also shorter x-rays. For example, emissions around 4 ~ 8 nm (about 150 ~ 300 eV)
and 1 ~ 2.5 nm (about 0.5 ~ 1.2 keV) are experimentally observed from Sn and/or SnO2 plasmas. These
emissions are correspond to the N-shell and M-shell transitions, respectively. From the view point of energy
balance and efficiency, these transitions should be suppressed. However, they may, to some extent, contribute
to provide the 5p and 4f levels with electrons which eventually emit the EUV light and enhance the intensity.
To know well about radiative properties and kinematic of the whole plasma, atomic population kinetics and
spectral synthesis codes have been developed. These codes can estimate the atomic population with nl-scheme
and spectral shapes of the EUV light. Radiation hydrodynamic simulation have been proceeding in this analysis.
Finally, the laser intensity dependence of the conversion efficiency calculated by these codes agrees with that of
the corresponding experimental results.
A new research project on extreme ultraviolet (EUV) source development has just been started at the Institute of Laser Engineering, Osaka University. The main task of this project is to find a scientific basis for generating efficient, high-quality, high power EUV plasma source for semiconductor industry. A set of experimental data is to be provided to develop a detailed atomic model included in computer code through experiments using GEKKO-XII high power laser and smaller but high-repetitive lasers. Optimum conditions for efficient EUV generation will be investigated by changing properties of lasers and targets. As the first step of the experiments, spherical solid tin and tin-oxide targets were illuminated uniformly with twelve beams from the GEKKO XII. It has been confirmed that maximum conversion efficiency into 13.5 nm EUV light is achieved at illumination intensity less than 2 x 1011 W/cm2. No significant difference is found between laser wavelengths of one μm and a half μm. Density structure of the laser-irradiated surface of a planar tin target has beem measured experimentally at 1012 W/cm2 to show formation of double ablation structure with density plateau by thermal radiation transport. An opacity experiment has just been initiated.
An imploded plasma core is irradiated by a 100 ps laser pulse in a model experiments of fast ignition. Additional laser pulses for drilling and heating are introduced co- axially with the laser beams for the implosion. The preformed imploded core is created by the 12 beams of 0.53 micrometers laser with the total energy of 800 J. The additional heating pluses contain 100 ps pulses separated by 300 ps at the wavelength of 1.06 micrometers with the total energy of 320J. The first pulse is intended for drilling the coronal pulses surrounding the core and the second is for addition heating of the core. We measured the imploded core additionally heated with 100 ps pulses.
Ultra-fast x-ray imaging is of great importance for diagnosing laser-driven inertial confinement fusion (ICF) plasmas. Typical required spatial and temporal resolutions are 10 micrometer and 10 ps, respectively. We have developed variety of one- (1D) and two-dimensional (2D) image sampling technique for ultrafast time-resolved x-ray imaging with x-ray streak cameras. Moire imaging of an x-ray-backlit target has been developed as 1D image sampling of an objective with 1D repetitive structure with a spatial resolution of 5 micrometer for use in experiments on hydrodynamic instabilities in laser- accelerated targets. With 1D sampling of repetitive 2D images, a multi-imaging x-ray streak camera (MIXS) with temporal- and spatial-resolutions of 10 ps and 15 micrometer, respectively, has been developed and successfully utilized for diagnosing uniformity and heating process of the imploded core plasmas. Two types of spectroscopic applications of the MIXS have been developed. One is multi-channel MIXS (McMIXS) which has three MIXS channels with various spectral responses for time- resolved 2D temperature measurement. Another is monochromatic MIXS (M-MIXS) for temperature, density and material mixing measurement, in which monochromatic images with Bragg crystals are coupled to MIXS. Finally, 2D image sampling of a 2D image on an x-ray streak camera (2D-SIXS) was also developed.
Propagation and amplification of partial coherent light (PCL) is simulated with a 3-D propagation code and fast Fourier transformation. The PCL is useful to realize highly uniform irradiation on a target in internal confinement fusion. On the other hand, the PCL has speckle structure in space and time owing to partial coherence. The intensity modulation due to speckles is enhanced by self-focusing in laser glasses due to nonlinear refractive index. Calculations on the laser propagation for the PCL show that the speckles of cm scale self-focus to 10 mm scale. As a result, average fluence of the PCL is limited at lower level comparing with that of the coherent laser. Experimental results are also coincident with the simulation results.
Indirect/direct-hybrid drive scheme to suppress the initial imprint of the laser irradiation nonuniformities has been proposed and investigated as a new drive scheme for inertial fusion. In direct drive inertial confinement fusion, initial imprinting of laser irradiation nonuniformity is considered to cause seeding of the perturbation on target surface in the very beginning of the irradiation which may be amplified by Rayleigh-Taylor instability in the acceleration phase of the implosion and be deleterious to efficient heating of the hot spark at the center of the compressed fuel core plasma. In indirect/direct-hybrid drive scheme, the target is first irradiated very uniformly with low-intensity soft x-ray prepulse from external sources apart from the target. Indirect x-ray pre-irradiation of the surface causes a pre- expansion layer of the plasma before the irradiation of the direct drive laser beam. When the drive beam comes later, the target has a substantial stand-off distance between the ablation front and the beam absorption region. Thus the thermal smoothing effect is expected to occur in this transport layer, and the initial imprint can be significantly reduced. We have demonstrated planar target experiments on the indirect/direct hybrid scheme and observed reduction of the initial imprint. Implosion experiments the indirect/direct hybrid drive spherical capsules with external x-ray sources has been started. Overall implosion was performed successfully.
We have newly developed a 2D space-resolved spectrometer for an ultra high intense laser plasma experiment. Output of this data, we can obtain 4D information: space, spectrum, and intensity. An optical fiber bundle is combined with a Cross Czerny-Turner type spectrometer to acquire 2D space- resolved spectrum. This has the spectral resolution 0.8 nm and the space 8 micrometers . Back-scattered light was measured from plasmas irradiated with a n ultra-intense laser pulse at laser intensity 1019 W/cm2. The obtained spectra show distinctive difference depending on both spatial and intensity profiles.
In the direct-drive scheme implosion of the inertial confinement fusion, the hot spark formation is critically affected by laser irradiation non-uniformities and subsequent hydrodynamic instabilities. Influence of the low- modal irradiation non-uniformities on the hot spark formation was investigated by means of the time- and space- resolved x-ray spectroscopic measurements. Experimental results were compared with post-processed hydro-code simulations by the aid of x-ray spectrum analysis code.
IN order to directly observe low-mode implosion nonuniformities, especially of l equals 1, which prevents stable formation of a hot spark in the compressed core plasma at the final stage of the implosion, a series of direct-drive implosion experiments has been performed at the Gekko-XII glass laser facility by using gas-filled plastic- shell targets. Partially coherent light (PCL) was used as a drive laser to suppress middle- to higher-modes of the irradiation nonuniformity down to approximately 1 percent. A clear shift from the center of the chamber and a slight crosswise structure were seen in the time- and space- resolved shape of the shell in the accelerating phase with an x-ray framing camera and absorbed laser intensity were also estimated by using a rocket equation. In order to confirm the property of these results, separate experiments under similar laser conditions were performed by using Au- coated sapphire spheres as surrogate target. The x-ray intensity distribution on the circumference of the target in the XFCs image, which is strongly dependent on the drive nonuniformity in the accelerating shell resulted from the l equals 1 drive nonuniformity. In our experiments, the l equals 1 drive nonuniformity due to some reproducible factors was found to be of the order of approximately 10-20 percent.
Reviewed are the progress in direct drive implosion researches with Gekko XII laser system. Precise observation of the growth rate of Rayleigh-Taylor instability and the suppression of imprints using indirect-direct hybrid implosion have been investigated. Theoretical and experimental researchers on the fast ignition scheme are also studied. Relativistic laser plasma interaction experiments with Peta-Watt Module and Gekko XII are also described. Finally, the future direction of the research including the development of solid state laser for fusion reactor is discussed.
We estimate the adaptation of the kinoform phase plate (KPP) for direct irradiation of the spherical target. The KPP has the focusing depth of about 200 micrometers in Gekko XII irradiation system. This corresponds to the radius of typical target. The optimum KPP is designed by using the direct-search method. The KPP removes the nonuniformity of 6th spherical mode.
The progress of implosion physics research and relevant technologies enable us to examine technical and economical feasibility, and to plan the realistic strategy to the commercial power plant. The most important key issue for IFE is the driver technology. The development of the laser fusion driver is opening new industrial technologies based on the photon processes and new fields of high energy physics.
The laser irradiation uniformity on the target surface has been improved using a 2D scheme of the smoothing by spectral dispersion. The laser beam was dispersed in both orthogonal directions by installing the series of pair of phase modulator and diffraction gratings. A polarization- preserving single mode fiber was utilized as the first phase modulator by which the modulation frequency and depth are easily controlled by using the cross-phase modulation. The second phase modulator was a quasi-velocity matched LiTaO3 modulator driven by 9.4-GHz microwave. Using these modulators, the combination of modulation frequencies was varied in a range from 5/9.4 GHz to 14/9.4 GHz. The advanced phase modulation was obtained by adopting the self-phase modulation in conjunction with the cross-phase modulation in the optical fiber. It has been confirmed that the temporally changing modulation frequency (the chirped phase modulation) is favorable for better smoothing in comparison with the ordinary scheme with fixed modulation frequencies.
We demonstrate the beam smoothing by polarization control plate (PCP) produced by liquid crystal. The liquid crystal PCP using photoisomerization of the azo-dye was developed. The PCP is constructed by 14 X 14 array of 2 mm X 2 mm square element. The damage threshold is about 12 J/cm2 (532 nm, 0.9 ns). The PCP suppressed the speckles produced by a random phase plate. The contrast of the speckles is reduced at 30% by a PCP.
We have developed a pulse-shaping system of the partially coherent light with angular spectral dispersion for the application to inertial confinement fusion experiment. The angular spectral dispersion is essential not only to the efficient frequency conversion but also to the reduction of the medium mode irradiation nonuniformity. The pulse-shaping system is based on the temporal stacking of pulses using optical fiber couplers and variable attenuators. This system enables us to generate arbitrarily pulse-shaped partially coherent light with a fast rise time of approximately 50 ps. Using this system we have demonstrated a 1.6-ns flat-top pulse with a 200-ps picket pulse under the operation condition of approximately 1000 J/beam at the fundamental wavelength, and this pulse is being supplied in the current implosion experiment with the isentrope control.
Several key technologies for uniform laser irradiation are reported. This paper includes the uniformity performance as a result of the introduction of the random phase plate, the partially coherent light, and the beam smoothing by spectral dispersion into the New Gekko XII glass laser system. Finally we summarize the overall irradiation uniformity on the spherical target surface by considering the power imbalance effect. The technologies developed for the beam smoothing and the power balance control enable us to achieve the irradiation nonuniformities of around 1% level for a foot pulse and of a few percent for a main drive pulse, respectively.
In this paper we report the characteristics of the partially coherent light sources used in the high-power Nd-phosphate glass laser system, Gekko XII. One is an amplified spontaneous emission (ASE) from Nd:glass rod and the other is a spatial mode mixing by an optical multimode fiber of a frequency chirped and pulse compressed laser light. We also introduced the angular dispersion of spectrum (AD) with a large beam divergence which is very effective for the beam smoothing without sacrificing harmonic conversion efficiency. The effect of AD gave an additional spatial and also temporal speckle smoothing at a farfield due to movement of the interference pattern. The measured temporal evolution of the speckle structure showed the rapid smoothing of the speckle with a larger beam divergence. The focused beam patterns at a target surface at Gekko XII were measured at the equivalent target plane by changing the beam divergence. A statistical model is applied for analyzing the speckle pattern in order to treat a beam smoothing effect.
In order to improve an irradiation uniformity on a spherical target, an amplified spontaneous emission (ASE) with an angular dispersion of spectrum is introduced in Gekko XII laser system. The irradiation nonuniformity was estimated from a measured beam pattern at the equivalent target plane. The ASE light substantially reduces the irradiation nonuniformity (rms value) from 19.7% to 3.8%. We also theoretically analyzed smoothing characteristics of the ASE light, and compared them with the experimental results. The Legendre-mode-dependent reduction rates of irradiation nonuniformities which are determined by the ratio of mode amplitudes with and without introduction of ASE, were very consistent with theoretical predictions.
The glass laser fusion driver, GEKKO 12 beam system, at ILE, Osaka, has operated since 1983 to deliver 20 kJ output in 1 ns pulse with the second and third harmonic frequency converters and the pulse tailoring optics. The uniform irradiation of the spherical target in the implosion experiments is a key issue to get the higher density of final compressed core. The pulse power to drive 2000 flash lamps exciting the rod and disk amplifiers is a moderate system with the total energy of 21 MJ in 300 microsecond(s) duration. High reliability and reproducibility are required to keep the laser energy balance of 12 beams within 3%, and no misfiring of 88 high-voltage switches of the laser amplifiers and 14 Pockels optical switches with the total probability of less than 0.1% per shot. Power balance technology involves not only the power conditioning of the flash power supply but also the gain control of the amplifiers and the alignment of a phase angle of the frequency converters, both working very nonlinearly. The precise control is sustained by the precise measurements of the beam energies, power waveforms, and the optical components characteristics such as reflectivity and transmittance. This report includes the optical beam control to realize the uniform illumination with an incoherent light source and the optical phase control in a focusing optics.
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