Effect of polishing contamination and surface defects for the LIDT has already been extensively studied in fused silica based transparent optics. The presence of contamination and damage layers on the surface of polished fused silica contributes to a large reduction in LIDT at ultraviolet wavelengths. The magnetic field-assisted finishing (MAF) technique has been shown to be successful in the fine finishing of optical components such as transparent Nd:YAG ceramics. Magnetic field-assisted finishing (MAF) with several tools has previously been applied to fused silica and was shown to improved surface LIDT at 266 nm.
In this paper, the damaged surface of fused silica with enhanced damage resistance after MAF was analyzed to classify the MAF processed condition. Irradiated energy density and damaged volume calculated from depth geometry were measured with a white color interference microscope (Zygo: Zegage). Fused silica substrates polished with CeO2 compounds were prepared as workpieces, and the surface roughness was about 0.3 nm Sq after optical polishing. Material removal over 100 nm occurred with the MAF process, however the final surface roughness did not change. The LIDT at a laser wavelength of 266 nm of processed surfaces was about 1.4 times higher than for the as-polished surface. The damaged volume of as-polished surface was linearly increased as increase in the irradiated energy density. In contrast, MAF processed surface showed little change for the damaged volume. The damage morphology will be also discussed.
Degradation and resulting photo luminescence (PL) are sometimes induced by irradiation with UV lasers. The mechanisms of the degradation are dependent on the grades of silica glass and their production conditions. In this study, relationship between degradation phenomena and photo luminescence during the laser irradiation of pulsed 266 nm was evaluated with several glades of silica glass.
Synthetic fused silica containing ca. 1000 ppm of OH (ES), silica glass produced from synthetic silica powder melted with oxy-hydrogen flame (S), and plasma torch (SD) were used for this measurement. Pulsed 266 nm laser with 80 kHz was irradiated at the inside of silica glass by using lens with a focal length of 50 mm. Degradation at the focused area was observed from the transmitted laser power. PL from the focused area was also detected with a spectrometer. SD needs nine times longer time for degradation than ES. PL around 640 nm related to the NBOHC (SiO) was gradually increased with degradation change, and rapidly increased when the laser damage occurred at the focused area. The mechanism of the degradation during the laser irradiation of pulsed 266 nm will be presented.
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.
For the semiconductor and liquid crystal display manufacturing process, resist removal by using laser irradiation has been investigated instead of conventional processes such as oxygen plasma and chemical method. An advanced laser resist stripping method for the positive-tone diazonaphthoquinone (DNQ) / novolak resist was successfully developed without causing the laser damage to the Si wafer. The pulsed laser irradiation in water can improve the resist stripping effect when compared with that of conventional atmosphere irradiation, however, the mechanism has yet to be clarified.
In this study, we investigated the analysis of resist stripping phenomenon by using a high-speed laser imaging system. A pulsed laser at 640 nm (pulse duration: 40 ns) was used as an illumination laser and a CCD camera detected the reflectance image on the sample. Time resolution of this system depended on the pulse duration of illumination laser. Time-resolved images were acquired based on the “1-on-1” method. Time-resolved images were acquired from 40 ns to 10 us after the laser irradiation. At the laser irradiated spot, changes of the resist were observed after 40 ns from the laser irradiation. The resist was completely stripped from the Si wafer surface after 10 us. The duration of resist removal phenomenon in the water condition was longer than that in the normal atmosphere condition. A resist stripping mechanism could be elucidated by combining experimental high-speed laser imaging and a finite element (FE) analysis. The mechanism of the resist stripping in the water condition will be presented.
Novolak resists which are implanted with B, P, and As ions, respectively, were irradiated with a
pulsed 532nm laser. Regardless of the implanted ion species and density, more than 74 % of the laser
power was found to absorb into the Si wafer surface. For the laser irradiation of 1 pulse, the ion-implanted
resist with a density of 5.0x1013 atoms/cm2 was completely stripped in the same way as that of a non-implanted resist. The optical absorption of the resist surface increased as the density of the ion-implantation
increased. In case of the ion-implanted resist with a density of 5.0x1015 atoms/cm2, the resist
was stripped by 20 pulses irradiation without occurring laser-induced surface damage. A scanning removal
of the highly ion-implanted resist was also successfully stripped by using an optimized irradiation
condition. A highly ion-implanted resist was continuously stripped by the scanning laser irradiation with 20 pulses.
Positive-tone diazonaphthoquinone / novolak (DNQ / novolak) resist was stripped from Si wafer by using a pulsed
laser beam from visible to near infrared. Silicon wafer with resist was sunk in water to utilize irradiated laser energy
effectively. When the resist was irradiated with the fundamental wavelength of the Nd:YAG laser, the resist was stripped
from the Si wafer. No damage could be detected from the processed silicon wafer surface. The resist stripping effect in
water condition was improved due to both the thermal expansion of the Si wafer and pressure from water. And also, laser
irradiation of wavelength 532 nm, having large photon energy, was found to have a higher resist stripping effect than that
of wavelength 1064 nm.
We have investigated a relationship among the bulk laser-induced damage threshold (LIDT) and YAG ceramics
with various structural defects. The correlation of scattering defect density and laser damage resistance was clearly
observed. A high-quality YAG ceramic having a low-scattering density showed a higher LIDT than that of a low-quality
YAG ceramic. Laser damage threshold (LIDT) of high-quality YAG ceramic was almost the same as that of a single
crystal. In addition, the high-quality Nd:YAG ceramics with
low-defect density showed an excellent oscillation
efficiency which was comparable to that of a single crystal. Thus, high-quality YAG ceramic with low-defect density is
more reliable as a material which is highly resistant to laser damage.
We report the first demonstration of polycrystalline Nd-doped YAG ceramics with almost perfect pore-free structure and Nd-doped YAG single crystal by advanced ceramic processing. The laser conversion efficiency of pore-free polycrystalline Nd:YAG ceramics is extremely high and its optical quality is comparable to that of commercial high quality Nd:YAG single crystal. Moreover, we have succeeded also in fabrication of Nd:YAG single crystal, which enables laser oscillation, by solid-state reaction method. Laser oscillation efficiency was very low when the pores were remained inside single crystal, however the laser oscillation efficiency of pore-free Nd:YAG single crystal was slightly higher than that of polycrystalline Nd:YAG ceramics having grain boundaries. From this fact, it was found that the optical scattering inside the Nd:YAG ceramics occurs mainly at the residual pores and the scattering at the grain boundary is very little. In addition, we confirmed that high concentration Nd:YAG single crystal can be fabricated by sintering method. Applying the above single crystallization technology by sintering method, we have demonstrated the fabrication of layer-by-layer single crystal composite and micron size spherical single crystal.
The laser-induced damage threshold of polished fused silica surfaces is much lower than the damage threshod of its bulk. It is well known that contaminations of polished surface are one of the causes of low threshold of laser-induced surface damage. Particularly, polishing contamination such as cerium dioxide (CeO2) compound used in optical polishing process is embedded inside the surface layer, and cannot be removed by conventional cleaning. For the enhancement of surface damage resistance, various surface treatments have been applied to the removal of embedded polishing compound. In this paper, we propose a new method using slective chemical removal with high-temperature sulfuric acid (H2SO4). Sulfuric acid could dissolve only CeO2 from the fused silica surface. The surface roughness of fused silica treated H2SO4 was kept through the treatment process. At the wavelength of 355 nm, the surface damage threshold was drastically improved to the nearly same as bulk quality. However, the effect of our treatment was not observed at the wavelength of 1064 nm. The comparison with our previous results obtained from other surface treatments will be discussed.
Since the composite laser media using single crystal such as Nd:YAG and undoped YAG was reported in 1998[1], the composite with various structures provided to experimental or industrial field. However, conventional bonding condition is not enough for laser application, and fabrication process is extremely complex and long delivery owing to necessity of polishing and diffusion bonding. Ceramic composite laser gain media having layer by layer and clad-core structure were fabricated successfully for the first time by advanced ceramic processing. Advanced ceramic technology provides a direct formation of composite having complex structure without polishing and diffusion bonding. The bonding condition of ceramic composite was confirmed to almost optically perfect, so we could oscillate successfully using composite laser media.
Silica glasses can be used as optical material in various applications such as deep-ultraviolet lithography and nuclear fusion, because they have no internal absorption and extremely small defects. In these applications, laser-induced bulk damage is an important factor in practical use. The laser-induced damage threshold (LIDT) is expected to depend on the types and production conditions of silica glasses. In this paper, the LIDT of synthetic fused silica which contains 4~1220ppm of OH, 1.7x1018~2x1019 molecules/cm3 of H2, and 100~3700 ppm of fluorine were studied by irradiating the higher harmonics of Nd:YAG laser at 355 and 266 nm with pulse width of 4 ns. Current experimental results show that the improvement of the LIDT with impurity contained silica glass is possible.
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.
The surface-damage resistance of fused silica was enhanced 2.8-fold by removing a subsurface damage. For the conventionally polished fused silica surface, μm-scale subsurface damage and a shallow (20 nm to 100 nm) structurally modified zone produced during grinding and polishing were formed on the top of surface. Several surface etching techniques and super-precise polishing process were used to remove subsurface damage from a fused silica surface. First the conventionally polished surfaces were chemically etched in a buffered HF solution to remove 300μm of surface material, and then super-precise polishing was performed to obtain an optical surface. After that, the polishing compound was removed by using ion-beam etching. The effect of subsurface damage on laser damage resistance was characterized by the measuring of the laser-induced damage threshold (LIDT) for the laser radiations of 1064 nm and 266 nm respectively. For the wavelength of 1064 nm, the effect of the removal of subsurface damage wasn't clearly seen, although the enhancement of surface-damage resistance by the ion-beam etching could be confirmed. However, in the case of 266 nm, enhanced LIDT of 28 J/cm2 was obtained from the subsurface damage removed surface. The surface LIDT increased by 2.8 times compared to that of conventionally polished fused silica surfaces.
Optical characteristics of a new commercially available perfluorinated organic polymer have been measured to prepare a quarterwave AR coating for Nd3+:YAG crystal and other optical materials. Coatings are carried out by dip method from solution at room temperature, they have a refractive index of 1.364. The laser-induced damage threshold at 1064nm, 532nm, 355nm, and 266nm is approximately same values with Teflon AR 2400.
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.
Laser-induced damage of thin films coated on optical components used for UV and DUV lasers are serious problems. Gradient refractive index antireflection films are produced by the combination of evaporation and leaching process on fused silica substrate. It has a high laser-induced damage threshold of 7.4-8.7 J/cm2 at 266 nm. The reflectance is about 0.35%.
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.
UV and DUV lasers are very important for many applications such as micro-machining, new material creation, medical care, and photo-lithography. Laser-induced damage of thin films coated on optical components used for UV and DUV lasers are serious problem. We have developed a new technique for producing anti- reflection (AR) coating on various substrate materials including fused silica glass and CaF2 crystal. In this technique, porous magnesium fluoride (MgF2) AR coating was developed. The reflectance of the porous AR coating on fused silica substrate was 0.3% at 500 nm.
High-power solid-state UV lasers are in high demand because of the convenient operation procedure. An effective technique for UV generation is cascaded sum-frequency generation pumped by the output of near-IR solid state lasers. The major challenge to such a UV laser seems to depend on the ability and reliability of the nonlinear optical (NLO) crystals that employed for frequency up conversion. The recent development of borate materials such as CsLiB6O10(CLBO) and (open square)-BaB2O4(BBO) have permitted an improved UV generation. In this work, we have investigated the bulk laser damage and the light scatter from bulk defects in CLBO and BBO crystals.
The development of high power lasers requires highly damage- resistant optical coatings. Present multilayer dielectric coatings do not have sufficient laser-induced damage thresholds (LIDTs) to pulsed lasers, particularly in the short wavelength region. The LIDT strongly depends on the absorption coefficient of optical coatings and the absorbing contaminants on the optical substrate. The absorption of optical coatings can be minimized by optimizing the deposition condition. However, polishing compound embedded inside subsurface among the absorbing contaminants cannot be completely removed by standard optical cleaning techniques. In this paper, the significant improvement of LIDT of optical coatings on subsurface-damage removed fused silica glass due to ion etching is presented.
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.
Nonlinear-optical crystals are attractive materials for the high-power frequency converter with high damage threshold, phase matching characteristics, wide transparency range, and large effective nonlinear coefficients. Especially as to the power laser applications, its laser-induced damage threshold determines the limit of performance in the optical system. The threshold depends not only on the intrinsic material parameters but also on the laser beam parameters is use. We have investigated the bulk damage threshold of several crystals at a single-shot operation for frequency converter depending on the laser irradiation direction and its polarization. For KDP and CLBO crystals, the damage threshold in the direction of c-axis is about tow times higher than that in the a- or b-axis at 1.064 micrometers of wavelength. This result is consistent with the molecular bonding structure in different directions of the crystal. The relation between the bulk damage pattern and the crystal structure is also discussed.
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.
Exfoliation of optical thin films at high humidity in the air is serious problem. Deterioration of three kind of optical thin films deposited on three types of substrate with different water-resistance at high humidity condition was studied. Thin films deposited on fused silica glass with highest water-resistance showed a best result for the exfoliation problem. Silica film seamed to be best for the surface observation at high humidity, but the laser -induced damage threshold was considerably lowered.
Laser induced damage threshold (LIDT) of various types of vitreous silica at 1064, 532, 355 and 266 nm were investigated. At 1064 nm no difference of LIDT were observed for all samples. At 1064-355 nm, wavelength dependence of LIDT of synthetic fused silica (SFS) can be well described by a relation Ith equals 1.45 (lambda) 0.43 where Ith is LIDT in J/cm2 and (lambda) is wavelength in nm. At 266 nm, however, LIDTs were smaller than a half of the calculated value from above relation. This difference can be explained by the damage mechanism; at 266 nm, two-photon absorption-induced defects lowered the LIDT same as in the case of KrF-excimer-laser induced defects, whereas at longer wavelength two-photon processed is not occur. LIDTs of fused quartz (FQ) at 532 and 355 nm, and that of a SFS containing about 1000 ppm of Cl and no OH at 355 nm were a little lower than those of the other SFS. This may be related to the absorption of metallic impurities in FQ and dissolved Cl2 molecules in SFS. At 266 nm, on the other hand, LIDTs of FQs were lower than those of most SFSs.
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.
Optical coatings deposited by conventional evaporation processes have a number of shortcomings including weak adhesion which depends upon the subsurface condition of substrate and the change of spectral characteristics in the relative humidity. Thin films deposited by reactive-rf- magnetron sputtering improve these problems. Laser-induced damage thresholds at 355nm, measured by 1-on-1 and R-on-1 methods, of substrate and single high index zirconia, hafnia and aluminum oxide, and low-index silicon dioxide coated by magnetron sputtering, are reported.
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.
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.
Laser-induced damage threshold (LIDT) of various types of vitreous silica at 1064, 532, 355 and 266 nm was investigated. At 1064 nm no difference of LIDT was observed for all samples. At 1064-355 nm, wavelength-dependence of LIDT of synthetic fused silica (SFS) can be well described by a relation Ith equals 1.45 (lambda) 0.43, where Ith is LIDT in J/cm2 and (lambda) is wavelength in nm. At 266 nm, however, LIDTs were smaller than a half of the calculated value from above relation. This difference can be explained by the damage mechanism; at 266 nm, two-photon absorption-induced defects lowered the LIDT same as a KrF-excimer-laser induced defects, whereas at longer wavelengths two photon process does not occur. LIDTs of fused quartz (FQ) at 532 and 355 nm, and that of a SFS containing about 1000 ppm of Cl and no OH at 355 nm were a little lower than that of the other SFSs. This may be related to the absorption of metallic impurities contained in FQ and dissolved Cl2 molecule in SFS.
Optical coatings deposited by conventional evaporation processes have a number of shortcomings including weak adhesion which depends upon the subsurface condition of substrate, and the change of spectral characteristics and film stress in the relative humidity. Thin films coated by reactive-rf-magnetron sputtering improve these problems. In this paper, optical properties of thin films deposited by reactive-rf-magnetron sputtering are presented.
The development of high power lasers requires highly damage-resistant optical coatings. Present multilayer dielectric coatings, do not have sufficient laser-induced damage thresholds (LIDTs) to pulsed lasers, particularly in the short wavelength region. LIDT strongly depends on the absorption coefficient of optical coatings and the impurities on the optical substrate. The absorption coefficient of optical coatings can be minimized by optimizing the deposition conditions. However, the impurities on the optical substrate cannot be completely removed by standard optical cleaning techniques. In this paper, the significant improvement of LIDT of optical coatings on subsurface-damage removed fused silica glass due to ion beam etching is presented.
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