Wide-bandgap semiconductors are fundamental components in many optoelectronic and power devices. The free-carrier properties, i.e., carrier density and mobility, are crucial parameters that determine device performance. This paper presents the characterization of a gallium nitride (GaN) wafer with ~1016 cm-3 carrier concentration using terahertz (THz) time- domain ellipsometry. In addition, THz time-domain spectroscopy of monoclinic beta-gallium oxide (β-Ga2O3) semi- insulating bulk and n-type homoepitaxial film are presented. The free-carrier properties are extracted by theoretically fitting the complex refractive index to the Drude and Drude-Lorentz models. THz time-domain techniques are a practical and powerful tool to nondestructively characterize the free-carrier properties of wide-bandgap semiconductors for device development.
The developments of ultra-high purity material which resist the damage by DUV laser are strongly required. According to the general mechanism of laser-induced damage, some kinds of defects and contaminations on the optical material are very important factors for DUV laser-induced damage. The borate crystal SrB4O7 (SBO) was reported to be a nonlinear optical material with a wide transparency down to 130 nm. In this study, we grew a high-quality SBO single crystal and measured the surface DUV laser-induced damage threshold (LIDT) in several polished conditions.
The SBO crystals grown over 13 days was 60 x 6 x 30 mm3 (a x b x c) without cracks or other defects. Two (020) plates were cut from the SBO crystal, and the both faces of the plates were optically polished. After that, we introduced catalyst-referred etching (CARE) to the one plate in order to atomically produce flat and damage-free SBO surfaces. As a result of the CARE treatment at a removal rate of 364 nm/h, the surface condition changed drastically, and a linear step-and-terrace structure was grew with a step height of 0.2 nm.
The surface LIDT in several polished conditions were measured with a 1-on-1 method at 266 nm (5 ns pulse width). The polarization direction was parallel to the c-axis of the (020) sample. Synthetic silica was also evaluated for comparison. The surface LIDT (17.3 J/cm2) of SBO after optical polishing is 4.3 times that of synthetic silica (4.0 J/cm2). In addition, the surface LIDT (24.1 J/cm2) of CARE-treated SBO is 6.0 times that of synthetic silica. This suggests that CARE-treated SBO crystals are a promising material for optical components in high-power DUV laser systems.
With current technologies, dislocation-free bulk shape GaN crystals with centimeter-size can be grown from small seed crystals. This technique can be realized by putting a sapphire plate with a small hole (0.5~1.5 mm in diameter) on a GaN plate seed. Centimeter-sized bulk GaN single crystals with large dislocation-free areas could be fabricated by this technique. Cathodoluminescence measurement at the interface between the seed and the grown crystal has revealed that almost all dislocations propagated from the GaN seed were bent and terminated at the initial growth stage. To enlarge the diameter of bulk shape GaN crystals, we have developed the coalescence of GaN crystals from many isolated small seeds. As a first step, we grew two GaN point seeds and coalesced them. Two GaN point seeds were established by mounting a sapphire plate with two small holes. We have found the two GaN crystals grown from two separate seed area coalesced without generating dislocations at a coalescence boundary. The grown GaN crystal can remove from substrate easily during the growth. This phenomenon is effective to reduce the stress in the grown GaN crystal. 2-inch GaN crystals by the coalescence technique. Some of the crystals have very large curvature radius (~100 m).
Single human hairs using a scanning laser terahertz (THz) imaging system are evaluated. The system features near-field THz emission and far-field THz detection. A sample is set in the vicinity of a two-dimensional THz emitter, and an excitation laser beam is scanned over the emitter via a galvanometer. By detecting the transmitted THz wave pulses that are locally generated at the irradiation spots of the excitation laser, we can obtain the THz transmission image and the spectrum of the sample with imaging time of 47 s for 512×512 pixels and maximum resolution of ∼27 μm . Using the system, we succeeded in observing the specific features of single human hairs in both the THz transmittance spectra and transmission images; it was found that the THz transmittance spectrum of gray hair shows a tendency of increase while that of black hair shows a decrease with increasing frequency above 1.2 THz. We could also observe the change of the moisture retention in the hair, and it is found that cuticles play one of the important roles in keeping moisture inside the hair. Those obtained data indicate that our system can be useful for evaluating single human hairs and those kinds of microscale samples.
Protein crystals are required for X-ray crystallography to determine three-dimensional structures of proteins at atomic resolution. The conventional microscopy is currently used for observation and screening of protein crystals. However, the three-dimensional imaging, which is important for automated treatment of protein crystals, is generally difficult by light microscopy. In addition, the protein crystals in the media are frequently difficult to identify by conventional light microscopy owing to the appearance of salt crystals or amorphous materials. In this work, we successfully demonstrated micro-scale, non-invasive, three-dimensional cross-sectional imaging of protein crystals using ultrahigh resolution optical coherence tomography (UHR-OCT). A low noise, Gaussian like, high power supercontinuum at wavelength of 800 nm was used as the light source. The axial resolution of 2 um in sample and the sensitivity of 95 dB were achieved. Since the protein crystal has homogeneous nano-structure, the optical scattering is negligibly small. Therefore, we used gel-inclusion technique to enhance the intensity of scattered signals, and clear, sharp 3D cross-sectional images of protein crystals were successfully observed. As the gel concentration was increased, the OCT signal intensity was increased. Using this method, the protein crystals surrounded by substantial amount of precipitates could be visualized, which is difficult by conventional light microscopy. The discrimination of protein and salt crystals was also demonstrated by the OCT signal intensity. The wavelength dependence of OCT imaging for protein crystal was examined at wavelength of 800-1700 nm regions. It was confirmed that the finest images were observed using 800 nm wavelength system.
GaN substrates are desirable for fabricating ultra-violet LEDs and LDs, and high-power and high-frequency transistors.
High-quality GaN single crystals can be obtained by using Na flux method, but the growth habit of bulk crystals must be
controlled. In this study, we investigated the effects of additives (Ca, Ba) on the growth habit and impurity concentration
in the crystals. The aspect ratio (c/a) of the crystals was increased by increasing the amount of additives, showing that the
growth habit could be changed from the pyramidal shape to the prism shape. Ba concentration was below the detection
limit (1x1015 atoms/cm3).
In this paper, we reported the recent advances in the growth of GaN crystals on GaN templates and spontaneously
nucleated GaN seeds by Na flux method. In the growth on GaN templates, it was clarified that the growth mode could be
controlled by changing the flux composition. Based on the changes in the growth mode under different flux compositions,
a growth sequence that is effective for the growth of thick GaN substrates with a low dislocation density was proposed.
In the growth on pyramidal GaN seeds, we investigated the dependence of the growth rate, crystallinity and the growth
habit on the flux composition. Results showed that a low Ga composition was preferred to grow high-crystallinity
prismatic GaN crystals with a high growth rate. When a spontaneously nucleated GaN seed was used, a bulk GaN crystal
with a hexagonal pillar consisting of six m-facets, and its length and diameter were 10 mm and 8 mm, respectively, was
obtained. Furthermore, we found that the addition of Ca and Li to Ga-Na melt improved transparency of GaN crystals
grown on pyramidal GaN seeds.
A frequency-converted optically pumped semiconductor laser (OPSL) is described. The 976-nm OPSL is frequency
doubled intracavity and is forced to operate in single longitudinal mode. An external resonator, containing a cesium
lithium borate crystal is locked to the 488-nm fundamental, generating the second harmonic at 244 nm. Continuous
wave output in excess of 200 mW is generated.
We have been developing the technology that enables us to grow high quality and large GaN single crystals for
producing high quality GaN single crystal substrates using Na flux method. In 2005, we have succeeded in the growth of
2-inch GaN substrates with low dislocation density (< 105 cm-2) by applying the Liquid Phase Epitaxy to the Na flux
method. Recently, we reported that dislocation density lower than the order of 104 cm-2 and the growth rate more than 20 um/h is
achievable in the Na flux LPE. This achievement was due to introducing some new techniques; such as thermal
convection, mechanical stirring, addition of carbon additive, development of new growth apparatus and so on, to the Na
flux LPE.
We have developed novel techniques of protein crystallization and processing using femtosecond laser, all solid-state
193 nm laser, and solution stirring. Femtosecond laser technique enables us to extremely increase the crystallization
probability, and to trigger the nucleation at low supersaturation of solution where spontaneous nucleation dose not occur.
Since the laser-induced nuclei grow slowly in the low-range supersaturated solution, the crystals exhibit high
crystallinity. The processing techniques using femtosecond laser and all solid-state 193 nm laser are effective for
processing and manipulation of protein crystals without significant damage. Solution stirring technique also contributes
the improvement of the crystal quality. We have succeeded in obtaining high-quality crystals of various proteins using
the techniques, and revealing the precise structure of the protein molecules from the X-ray analysis. These techniques
can accelerate structural biology and subsequent structure-based drug discoveries, resulting in important revelations in
these fields.
We propose new application of a pulsed deep-UV laser to processing biological macromolecule crystals. Single crystals of model protein, hen egg white lysozyme, were processed by deep-UV laser irradiation at 193 nm and 266 nm. Desired laser ablation was achieved using the 193-nm light source. A crystallinity of the processed crystals was evaluated by X-ray diffraction pattern recording. The crystal diffracted beyond a 1.9 Å resolution, which was the same as that of the as-grown crystal obtained under identical growth conditions. In addition, we observed that a diffraction pattern from a cracked protein crystal was considerably improved by eliminating the damaged section with the 193-nm laser ablation. A processing tool for protein crystals currently employed, such as a micro needle or knife, suffers from low reproducibility and poor accuracy. The technique using deep-UV laser pulses will be a powerful tool for processing very fragile protein crystals.
High-power solid-state ultraviolet (UV) lasers by using a have been in high demand because of their convenient operation procedure. An effective technique for UV generation is cascaded sum-frequency generation pumped by the output of near-IR solids-state lasers. The performance of such solid-state UV lasers appears to depend on the ability and reliability of nonlinear optical (NLO) crystals that are employed for laser frequency conversion. Discovery of CsLiB6O10(CLBO) crystals have enabled the production of such practical high-power all solid-state UV lasers. In 2001, UV output power up to 23.0 W by fourth harmonic generation of Nd:YAG laser was achieved. It is fact that laser-induced damage of NLO crystal is a limiting factor on reliable operation of high-power solid-state UV lasers. Bulk laser-induced damage of NLO crystal is related to the crystal's quality. In this paper, we have investigated the relationship among the bulk laser-induced damage threshold (LIDT), dislocation density and absorption of laser light in CLBO crystals with various crystallinity. The bulk LIDT of CLBO increased with decreasing dislocation density. High-quality crystals with a higher LIDT (15 - 18 GW/cm2) have a lower dislocation density of 6.6 x 103/cm2 than that of conventional CLBO (~15.0 x 103/cm2). The relationships between crystal quality and absorption of laser light will be presented.
There has been great interest in high-repetition, high-power ultraviolet (UV) source for various applications in semiconductor processing, micro machining, and other fields. Discovery of CsLiB6O10(CLBO) crystals have enabled the production of such practical high-power all solid-state UV lasers. In 2001, UV output power up to 23.0 W by fourth harmonic generation of Nd:YAG laser was achieved.
In general, one of the limiting factors for the development of high-power solid-state UV lasers is laser-induce damage of NLO crystal due to some kinds of defects inside the materials. Recently, we have succeeded to grow the high crystallinity CLBO with an enhanced bulk laser damage resistance. On these samples, an increase in the surface damage resistance could be expected. Measurement of the surface laser-induced damage threshold (LIDT) on CLBO crystals with various crystallinity was performed by using a 266 nm laser. For the crystal with high damage resistance (15-18 GW/cm2), LIDT of as-polished surface was 1.3 times higher than that of crystal with conventional damage resistance (9-12 GW/cm2). In addition, polishing compound embedded inside the crystal surface was removed by using an ion-beam etching process. We have observed 1.5 times improved surface LIDT by ion beam etching for both high damage resistance and conventional damage resistance samples. The relationships between vickers hardness and crystal quality will be presented.
We obtained the 266nm UV power of 23W by fourth-harmonic generation using a high-brightness high-power all-solid- state green laser and a high-quality CLBO crystal. This value is, to our knowledge, the highest UV power below 300nm wavelength. Moreover, the UV-power variation was negligibly small by using high-crystal CLBO crystals.
We have developed GdxY1-xCa4O(BO3)3 (GdYCOB) crystal in order to control birefringence. As a result, GdYCOB is noncritically phase matchable for third- harmonic generation of a 1064 nm light by type-I mixing (1064 + 532 yields 355 nm). However, during high-power operation, degradation of output power and distortion of beam pattern occurred due to photo-induced damages and thermal dephasing. In this paper, we report on nonlinear optical properties and improved the performance of GdYCOB by suppression of photo- induced damages and thermal dephasing.
Effect of ion beam etching on surface damage resistance was investigated in CsLiB6O10(CLBO) crystal. In high-power UV operation, an as-polished CLBO surface was damaged due to absorption of the polishing compound embedded inside the crystal surface. In the as-polished surface of CLBO, polishing compound ZrO2 (absorption edge is about 300 nm) was detected to a depth of 60 nm. We have removed polishing compound with ion beam etching without degrading the surface quality. The effects of polishing compound removal on surface damage were characterized for the surface laser-induced damage threshold (LIDT) at 355 nm (pulse width 0.85 ns) as a function of etching depth and surface lifetime for the generation of fourth-harmonic of ND:YAG laser (266 nm, 20 ns, 4 kHz). We found an improvement of the surface damage resistance. LIDT of etched surface increased up to 15 J/cm2 as compared with that of the as-polished surface of 11 J/cm2. Etched CLBO surface also exhibits an improvement lifetime 4 times longer than that of as-polished surface.
KEYWORDS: Deep ultraviolet, Crystals, Atomic force microscopy, Ultraviolet radiation, Sapphire lasers, Solid state lasers, Neodymium lasers, Laser crystals, Nonlinear crystals, High power lasers
We describe an all solid-state, high power, deep-UV (DUV) source based on sum-frequency mixing (SFM) of two single- frequency laser outputs. The system consists of a CW diode- pumped, Q-switched Nd:YLF laser operating at 1047 nm, a Ti:sapphire laser at 785-nm, and cascading SFM stages. Both laser sources are configured with an injection-seeded oscillator followed by amplifier to produce high power, single-frequency, TEM00 outputs. The third harmonic of Nd:YLF MOPA is mixed with the output from Ti-sapphire MOPA to generate the first UV, which is used for the second mixing with the residual fundamental output to generate the DUV radiation. CLBO crystal is employed for each SFM process. The system produced UV pulses at 241.6 nm with 3.4 W, and also DUV at 196.3 nm with 1.5 W of average powers at a 5-kHz pulse- repetition rate. The linewidth of the DUV output was measured to be less than 0.05 pm.
High-power all-solid-state UV lasers are used for precise material processing applications in industrial fields with the potential advantage in the maintenance cost compared with other UV lasers. In this paper we report on the evaluation of nonlinear crystals by high-power intracavity second harmonic generation, the evaluation of nonlinear crystals for UV generation by the transmission loss and the surface damage threshold measurements, and high-power UV beam generation up to 20 W with the repetition rate of 10 kHz.
High-power all solid-state UV lasers are highly demanding for many applications because of their compactness and ease of operation. An effective technique for UV generation is cascaded sum-frequency generation pumped by the output of near-IR solid-state lasers. The performance of these laser systems is limited by the laser power handling capability of the nonlinear optical crystals that employed for frequency up-conversion.
The laser-damage resistance of CLBO and fused silica surfaces was successfully improved after removing polishing compound by ion beam etching. The polishing compound embedded in the CLBO and fused silica surfaces was to a depth of less than 100 nm. We were able to remove polishing compound without degrading the surface condition when the applied ion beam voltage was less than 200 V. After surface etching, the effects of polishing compound removal on surface damage were characterized for the surface laser- induced damage threshold at 355 nm and surface lifetime at 266 nm as a function of etching depth. In the fused silica surface, we found improvement of the surface LIDT up to 15 J/cm2 as compared with that of the as-polished surface of 7.5 J/cm2. For the irradiation of a 266 nm high- intensity and high-repetition laser light, the exit surface lifetime of CLBO and fused silica could be more doubled compared with that of the as-polished surface.
Bulk laser damage in CsLiB6O10 was measured using a single-shot Q-switched Nd:YAG laser in a transverse and longitudinal single model. The bulk laser damage thresholds of CLBO, with laser irradiation direction (parallel) < 001 > and polarization (parallel) < 001 >, were determined to be 29 GW/cm2 at 1.064 micrometers , and 6.4 GW/cm2 at 0.266 micrometers . The value at 1.064 micrometers is higher than that of fused quartz, (beta) - BaB2O4 and KH2PO4. The morphology of bulk damage in CLBO crystal was also observed. THe damage pattern suggests that the < 001 > direction is mechanically weak, which is consistent with the result of the mechanical strength tests.
This paper investigates ion etching process to the surface of CsLiB6O10 (CLBO) crystal. Laser-induced surface damage was reduced and surface durability of CLBO crystal was improved by removing the subsurface embedded polishing compound. There was no surface degradation as a result of the ion etching. The effects of ion etching on surface damage were measured by a 1-on-1 test at a laser wavelength of 266 nm. Durability of the CLBO crystal was tested by the approximately 7 W fourth harmonic generated by a Nd:YAG laser. The durability of the ion etched surface was improved more than 10 times as compared with the as-polished surface.
The various properties of CsLiB6O10 crystal are reviewed. CLBO has substantial advantages for growth and UV generation. On the other hand, there have been found some problems on CLBO crystal, i.e. hygroscopic and change of crystal refractive index. The effect of thermal annealing on CLBO will be discussed, which can be a clue to overcome these problems on CLBO.
We report on the growth and various properties of CsLiB6O10 (CLBO). The fourth and fifth harmonic generations of Nd:YAG laser radiation were realized in CLBO. The values of 500 mJ and 180 mJ at 266 nm and 213 nm were obtained from 2200 mJ of fundamental energy. The Al doping was found to give rise to an enhancement of mechanical and chemical properties of CLBO.
A new nonlinear optical crystal, CsLiB6O10 (CLBO), is described, which is a congruently melting crystal and can realize fourth harmonic and fifth harmonic generations of the 1.064 micrometer Nd:YAG laser radiation with type-I phase matching. A large and high quality single crystal with dimensions of 14 by 11 by 11 cm3 was obtained by the top- seeded Kyropoulos method. CLBO showed higher fourth harmonic generation efficiency from the second harmonic of Nd:YAG laser output compared to beta-BaB2O4.
KEYWORDS: Diamond, Silicon, Silicon carbide, Ions, Carbon, Chemical species, Ion implantation, Chemical vapor deposition, Scanning electron microscopy, Annealing
The defect structures of Si surface layer modified by high dose carbon ion implantation have been studied by ESR (electron spin resonance) method. The ESR analysis revealed the presence of three paramagnetic defects, that is, Si-dangling bonds (g=2.0060, Hpp =6.3 Oe) in Si amorphous region caused by the implantation, Si-dangling bonds with C atom neighbors (g=2.0035, LHpp =6.8 Oe) and C-dangling bond with C atoms neighbors. Moreover, it was found that the tight diamond-like C-C bonds are formed in the implanted Si surface layer. The diamond-like C-C bonds have an important role in obtaining high quality synthesized CVD diamonds.
Effect of carbon implantation prior to diamond growth on diamond nucleations has been investigated for various substrates such as Si, Cu, Ti, Ni and Fe. The nucleation density is increased by one order of magnitude for carbonimplanted Si substrate via microwave plasma CVD. The diamond particles grown show well-defined (100) and (111) facets without twins or secondary nucleations. An enhancement of diamond nucleations by two orders of magnitude for carbon-implanted Cu and Ti substrates has been observed via magnet-active plasma CVD. The implanted carbon atoms seem to act as nuclei for diamond growth.
Nitrogen ions were implanted into diamond films formed by microwave plasma CVD. A color cathodoluminescence (CL) system were used to investigate the emission centers of as implanted and the subsequent annealed films. A zero-phono-line (ZPL) from the implanted N+ was observed at 3.19 eV. After annealing, a ZPL at 2.16 eV and a strong emission center in the violet region with a ZPL at 3.19 eV and phonon replicas were observed. The color CL images of the annealed films show that an orange-red color emission comes only from the {100} sectors because of the difference in crystal quality between the {100} and {111} sectors.
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