We used a combination of dip-pen nanolithography and scanning optical confocal microscopy to fabricate and visualize luminescent nanoscale patterns of various materials on glass substrates. We show that this method can be used successfully to push the limits of dip-pen nanolithography down to controlled deposition of single molecules. We also demonstrate that this method is able to create and visualize protein patterns on surfaces. Finally, we show that our method can be used to fabricate polymer nanowires of controlled size using conductive polymers. We also present a kinetic model that accurately describes the deposition process.
Severe scattering losses from KDP crystals have been correlated with the exposure of porous sol AR coated crystal to ambient humidity. The scattering is attributed to formation of etch pits which develop under the coating on the KDP surface along crystallographic axes. This high angle scattering can in turn produce laser damage of downstream optics either through modulation of the beam or by optic contamination from ablation of adjacent metal structures. We have developed a simple tool to characterize the evolution of scatter from sol-coated KDP surface.s We have measured the rate of etch pit formation as a function of relative humidity and surface treatment using both microscopy and scattering. We will discuss various surface treatments which can be utilized to retrad or eliminate the environmental degradation of KDP crystals.
KEYWORDS: Crystals, National Ignition Facility, Error analysis, Laser crystals, Data modeling, Frequency conversion, Tolerancing, Frequency converters, Performance modeling, Nonlinear crystals
The design of the NIF incorporates a type I/type II third harmonic generator to convert the 1.053-micrometers fundamental wavelength of the laser amplifier to a wavelength of 0.351 micrometers for target irradiation. To understand and control the tolerances in the converter design, we have developed a comprehensive error budget that accounts for effects that are known to influence conversion efficiency, including variations in amplitude and phase of the incident laser pulse, temporal bandwidth of the incident laser pulse, crystal surface figure and bulk non-uniformities, angular alignment errors, Fresnel losses, polarization errors and crystal temperature variations. The error budget provides specifications for the detailed design of the NIF final optics assembly and the fabrication of optical components. Validation is accomplished through both modeling and measurement, including full-scale Beamlet tests of a 37-cm aperture frequency converter in a NIF prototype final optics cell. The prototype cell incorporates full-perimeter clamping to support the crystals, and resides in a vacuum environment as per the NIF design.
Accurate predictions of the performance of frequency conversion requires knowledge of the spatial variation of departures from the phase-matching condition in the converter crystals. This variation is caused by processes such as crystal growth and crystal surface finishing. Gravitational sag and mounting configurations also lead to deformation and stresses which cause spatially varying departures from the phase-matching condition. We have modeled the effect of gravitational forces on conversion efficiency performance of horizontal converter crystals and have shown for the NIF mounting configurations that gravity has very little effect on conversion efficiency.
Considerable attention has been paid over the years to the problem of growing high purity KDP and KD*P to meet damage threshold requirements of inertial confinement fusion lasers at LLNL. The maximum fluence requirement for KD*P triplers on the NIF is 14.3 J/cm2 at 351 nm in a 3 ns pulse. Currently KD*P cannot meet this requirement without laser (pre)conditioning. In this overview, recent experiments to understand laser conditioning and damage phenomena in KDP and KD*P are discussed. These experiments have led to a fundamental revision of damage test methods and test result interpretation. In particular, the concept of a damage threshold has given way to measuring performance by damage distributions using beams of millimeter size. Automated R/1 damage test have shown that the best rapidly grown KDP crystals exhibit the same damage distributions as the best conventionally grown KD*P. Continuous filtration of the growth solution and post-growth thermal annealing are shown to increase the damage performance as well.
Microscopic fluorescence imaging and time-resolved Raman scattering are employed to investigate the effect of high power 355 nm laser irradiation on preexisting and transient defect populations in KH2PO4. Defect clusters in the bulk of KDP crystals are imaged with 1 micron spatial resolution using their NIR emission. The intensity of the emission clusters varies widely within the image field. The exposure of the crystal at high power 355 nm, 3 ns laser irradiation leads to a reduction of the number of observed optically active centers. In addition, time resolved Raman scattering was employed to study the transient generation of defects during high power 355 nm laser irradiation.
Recently reported experiments have investigated the statistics of laser damage in KDP and KD*P. Automated damage tests have allowed cumulative failure and damage probability distributions to be constructed. Large area tests have investigated the feasibility of on-line laser conditioning and damage evolution for tripler harmonic generation crystal on the NIF. These test have shown that there is a nonzero probability of damage at NIF redline fluence and that the damage pinpoint density evolves exponentially with fluence.
Defect clusters in the bulk of large KDP crystals are revealed using a microscopic fluorescence imaging system and CW laser illumination. Exposure of the crystal to high power 355-nm, 3-ns laser irradiation leads to a significant reduction of the number of observed optically active centers. The initially observed defect cluster concentration is approximately 104-106 per mm3 depending on the crystal growth method and sector of the crystal. The number of defect clusters can be reduced by a factor of 102 or more under exposure to 355-nm laser irradiation while their average intensities also decreases. Spectroscopic measurements provide information on the electronic structure of the defects.
KEYWORDS: Laser irradiation, Crystals, High temperature raman spectroscopy, Laser crystals, Raman spectroscopy, Temperature metrology, Visible radiation, Near infrared
A spectral and temperature investigation of fast-grown KDP crystals under high fluence, 355 nm laser irradiation is discussed. Pump-and-probe Raman spectroscopy indicate transient changes of the vibrational spectrum. Photothermal deflection experiments provide information on the temporal behavior of the temperature changes. The presence of emission in the visible and NIR spectral regions is attributed to the presence of impurities and/or defects in the crystal.
Interest in producing high-damage-threshold KH2PO4 (KDP) and DKDP for frequency conversion and optical switching applications is driven by the requirements of the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL). At present only the best crystals meet the NIF system requirements at the third harmonic and only after a laser conditioning process. Neither the mechanism for damage in bulk DKP nor the mechanism for conditioning is understood. As part of a development effort to increase the damage thresholds of DKP and DKDP, we have been developing techniques to pinpoint the locations where damage will initiate in the bulk material. After successfully developing a diagnostic tool that will find these locations, we will use other measurement techniques to determine how these locations differ form the surrounding materials and why they cause damage. This will allow crystal growers to focus their efforts during the growth process in improving damage thresholds. Previously we reported that there was a low correlation between defects in crystals located using light scatter and the sites where damage would initiate. Damage, when it occurs, is almost certainly associated with a localized heating of the crystal which results in mechanical damage to or chemical decomposition of the crystal lattice. If this heating occurs at fluences below the damage threshold, we should be able to measure this heating and predict the locations where damage will initiate prior to damaging the crystal. As a result, we are developing photothermal techniques to probe the bulk material to look for heating due to localized absorption and to determine the correlation between localized heating and the initiation sites for damage.
Considerable attention has been paid over the years to the problem of growing high purity KDP and KD*P to meet damage threshold requirements on succeeding generations of inertial confinement fusion lasers at LLNL. While damage thresholds for these materials have increased over time, the current National Ignition Facility (NIP) maximum fluence requirement (redline) for KD*P frequency triplers of 14.3 J/cm2 at 351 nm, 3 ns has not been reached without laser (pre)conditioning. It is reasonable to assume that. despite the rapid increase in damage thresholds for rapidly grown crystals, ·a program of large scale conditioning of the 192 NIF triplers will be required. Small area ramp (R/1) tests on single sites indicate that KDP damage thresholds can be raised on average up to 1.5X the unconditioned values. Unpublished LLNL 3m raster conditioning studies on KDP, however, have not conclusively shown that off-line conditioning is feasible for KD*P. Consequently, investigating the feasibility of on-line conditioning of NIF triplers at 3m has become a high priority for the KDP damage group at LLNL. To investigate the feasibility of on-line conditioning we performed a series of experiments using the Optical Sciences Laser (OSL) on numerous samples of conventional and rapid growth KDP and KD*P. The experiment entailed exposing sites on each sample to a range of ramped shot (Nil) sequences starting at average fluences of -2 J/cm2 (in a 7 mm "top hat" beam @ 351 nm, 3 ns) up to peak fluences of approximately 13 J/cm2• Test results indicated that the most effective conditioning procedure entailed a 7-8 shot ramp starting at 2 J/cm2 and ending at 12-13 J/cm2• The pinpoint onset fluence for the 8/1 tests was 1.4 times that of the unconditioned site. Damage evolution appears to be exponential as a function of increasing fluence. When damage occurs after conditioning however, pinpoint density evolution exhibits a greater slope than less conditioned sites. The overall reduction in the total pinpoint number can be as high as 300X. Despite laser conditioning , the pinpoint onset for the samples considered is below the NIF redline fluence of 14.3 J/cm2• In addition, the exponential pinpoint evolution curves indicate that damage levels at NIF redline fluences will be on the order of 1 Q4 pinpoints/mm3• 1bis suggests that there will be significant damage in NIF triplers, however, substantial damage has not been observed in the large Beamlet tripler (conventionally grown KD*P) under similar exposure conditions. By applying the OSL damage evolution curves to model NIF THG output spatial profiles it is possible to show damage in NIF triplers will be slight, consisting of isolated clusters with a few pinpoints at high fluence portions of the beam. This prediction has been verified by scatter mapping the 37 cm Beamlet tripler crystal. These results will be discussed in a future memo. These results indicate the feasibility of on-line conditioning for the NIF laser. Keywords: KDP, DKDP, KD*P, bulk laser damage, laser conditioning
Development of high damage threshold, 50 cm, rapidly grown KD*P frequency triplers for operation on the National Ignition Facility (NIF) in the 14 J/cm2, 351 nm, 3 ns regime requires a thorough understanding of how the crystal growth parameters and technologies affect laser induced damage. Of particular importance is determining the effect of ionic impurities which may be introduced in widely varying concentrations via the starting salts. In addition, organic particulates can contaminate the solution as leachants from growth platforms or via mechanical ablation. Mechanical stresses in the crystals may also play a strong role in the laser-induced damage distribution (LIDD), particularly in the case of large boules where hydrodynamic forces in the growth tank may be quite high. In order to investigate the effects of various impurities and stresses on laser damage we have developed a dedicated, automated damage test system with diagnostic capabilities specifically designed or measuring time resolved bulk damage onset and evolution. The data obtained makes it possible to construct characteristic damage threshold distributions for each samples. Test results obtained for a variety of DKP samples grown form high purity starting salts and individually doped with Lucite and Teflon, iron, chromium and aluminum show that the LIDD drops with increasing contamination content. The results also show that solution filtration leads to increased damage performance for undoped crystals but is not solely responsible for producing the high LIDDs required by the NIF. The highest LIDD measured on a rapidly grown sample indicate that it is possible to produce high damage threshold material using ultrahigh purity, recrystallized starting salts, continuous filtration and a platform designed to minimize internal stress during growth.
KDP (KH2PO4) single crystals up to 47 cm in size have been grown by the rapid growth technique on the point seed in glass crystallizers of 1000 L in volume at growth rates of 10 to 25 mm/day in both the [001] and [100] directions. Measurement of the optical quality of 41 X 41 cm single crystal plates are presented.
Interest in producing high damage threshold KH2PO4 (KDP) and (DxH1-x)2PO4 (DKDP) for frequency conversion and optical switching applications is driven by the requirements of the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory. At present only the best crystals meet the NIF system requirements at the third harmonic and only after a laser conditioning process. Neither the mechanism for damage in bulk KDP nor the mechanism for conditioning ins understood. As part of ta development effort to increase the damage thresholds of KDP and DKDP, we have been developing techniques to pinpoint the locations where damage will initiate in the bulk material. After we find these locations we will use other measurement techniques to determine how these locations differ from the other surrounding materials and why they cause damage. This will allow crystal growers to focus their efforts to improve damage thresholds. Historically, damage thresholds have increased it is believed as a consequence of increased purity of the growth solution and through the use of constant filtration during the growth process. As a result we believe that damage is caused by defects in the crystals and have conducted a series of experiments using light scatter to locate these defects and to determine when and where damage occurs. In this paper we present results which show a low correlation between light scatter from bulk defects in KDP and the initiation sites for damage. We have also studied the effects of thermal conditioning on light scatter, strain induced birefringence and damage threshold. We have seen evidence that regions of high strain also exhibit lower damage threshold than the surrounding lower strain material. When thermally conditioned, these crystals show a decrease in some of the strong linear scattering features and a decrease in the strain birefringence while the damage threshold in these regions increased to that of the surrounding bulk material.
We report the experimental results of impurity contamination and laser-induced damage investigations on rapidly grown potassium dihydrogen phosphate (KDP) crystals. Using absorption spectroscopy and chemical analysis, we determined the impurity distribution in the different growing sectors of KDP single crystals. The level of impurity was dependent on the starting materials and growth rate. We also studied the influence of impurities on the laser-induced damage in fast grown KDP. The laser damage threshold in the impurity- rich prismatic sector is same as in the high purity pyramidal sector within the experimental error. Meanwhile, the laser damage threshold (LDT) at the boundary of the prismatic and pyramidal sectors is less than half of that in the bulk. Furthermore, we found that the thermal annealing of the crystal eliminated the weakness of this sector boundary and increased its LDT to the same level as in the bulk of the crystal. Our result suggests that laser damage occurred in the vicinity of a high, localized strain field.
Interest in producing high damage threshold KH2PO4 (KDP) and (DxH1-x)2PO4 (KD*P, DKDP) for optical switching and frequency conversion applications is being driven by the system requirements for the National Ignition Facility (NIF) at Lawrence Livermore National Lab (LLNL). Historically, the path to achieving higher damage thresholds has been to improve the purity of crystal growth solutions. Application of advanced filtration technology has increased the damage threshold, but gives little insight into the actual mechanisms of laser damage. We have developed a laser scatter diagnostic to better study bulk defects and laser damage mechanisms in KDP and KD*P crystals. This diagnostic consists of a cavity doubled, kilohertz class, Nd:YLF laser (527 nm) and high dynamic range CCD camera which allows imaging of bulk scatter signals. With it, we have performed damage tests at 355 nm on four different `vintages' of KDP crystals, concentrating on crystals produced via fast growth methods. We compare the diagnostic's resolution to LLNL's standard damage detection method of 100X darkfield microscopy and discuss its impact on damage threshold determination. We have observed the disappearance of scatter sites upon exposure to subthreshold irradiation. In contrast, we have seen scatterers appear where none previously excited. This includes isolated, large (high signal) sites as well as multiple small scatter sites which appear at fluences above 7 J/cm2 (fine tracking). However, we have not observed a strong correlation of preexisting scatter sites and laser damage sites. We speculate on the connection between the laser-induced disappearance of scatter sites and the observed increase in damage threshold with laser conditioning.
Potassium dihydrogen phosphate (KDP) has traditionally been used as a nonlinear optical material for frequency conversion to produce second and third harmonic radiation. A high laser induced damage threshold for KDP crystals is required for high power laser applications, such as laser fusion. High quality KDP crystals for such applications can be produced by a recently developed rapid crystal growth method. We report the results of an impurity contamination study in raidlygrown KDP crystals. Using absorption spectroscopy, we identified the impurity contamination in the different growth sectors of the crystals. We show that the level of contamination depends on the growth rate achieved during the rapid growth. The impurities observed by absorption spectroscopy are identified as the origin of lattice distortion and optical birefringence in the KDP crystals. The study of impurity incorporation during crystal growth is important for understanding the damage mechanism of KDP.
Single crystals of KH2PO4 (KDP) and (DxHlx)2PO4 (DKDP) will be used for frequency conversion and as part of a large aperture optical switch in the proposed National Ignition Facility (NW) at the Lawrence Livermore National Laboratory (LLNL). These crystals must have good optical properties and high laser damage thresholds. Currently these crystals have a lower laser damage threshold than other optical materials in the laser chain which has forced designers to limit the output fluence of the NIF in order to avoid damaging the crystals. Furthermore, while more efficient frequency conversion schemes are being explored both theoretically and experimentally, the advantages of these schemes can not be fully realized unless the damage thresholds of the conversion crystals are increased. Over the past decade, LLNL has generated an extensive data base on the laser damage in KDP and DKDP crystals both at the first and third harmonics of Nd-YAG.1 While the damage thresholds of these crystals have increased over this time period due, in part, to better filtration of the growth solution,2 the damage thresholds of the best crystals are still far below what is expected from theoretical limits calculated from the band structure of perfect crystals. Thus damage in KDP and DKDP is caused by defects in the crystals. We also rely on a process called laser conditioning to improve the damage thresholds of the crystals. Unfortunately, little is understood about the mechanism of laser induced damage, the conditioning process in the crystals, or the defects which are responsible for damage. We have recently implemented a scatter diagnostic for locating and studying defects in crystals and as a tool for studying the mechanism of laser damage and laser conditioning.
Large solid state lasers such as Beamlet and the proposed National Ignition Facility require optical materials with extremely high damage thresholds. Potassium dihydrogen phosphate (KDP) and its deuterated analog (KD*P) both require some form of conditioning to reach the design fluence of these lasers. Both the bulk material and the crystal surfaces must have damage thresholds in excess of 16 J/cm2 at 1053 nm and 11 J/cm2 at 351 nm for 3- ns pulselengths. The use of ultrafiltration techniques has been demonstrated to produce bulk material with damage thresholds exceeding these requirements with the use of R:1 laser conditioning. More recent results at LLNL using large-area laser conditioning and thermal annealing are described for a variety of state-of-the-art KDP and KD*P crystals. Results on thermally annealed KD*P with a deuteration range of 60% to 80% are also presented, and compared to those of ordinary KDP.
We present the results of laser damage measurements conducted on potassium dihydrogen phosphate (KDP) and deuterated potassium dihydrogen phosphate (KD*P) crystals that were grown recently for both production and research applications by several sources. We have measured extrinsic damage thresholds that cover wavelengths from 1064 nm to 266 nm at pulse durations in the 3 to 10 ns regime. Many of the samples were extracted from boules grown specifically to yield large-area crystals, up to 32 cm square, for laser fusion applications. These crystals were the result of efforts, both by the Lawrence Livermore National Laboratory (LLNL) and commercial crystal-growth companies, to yield high- threshold KDP. In particular we have established that such crystals can reliably survive fluences exceeding 15 J/cm2 at 355 nm and 20 J/cm2 at 1064 nm when irradiated with 3 ns pulses. We present details of how bulk and surface damage to these crystals scale with pulse duration and wavelength as well as of morphological effects due to laser conditioning.
When 001 plates of KD2PO4 (KD*P) are used in Pockels cells, strain induced refractive index variations result in beam depolarization and transmitted wavefront distortion. The depolarization is determined by the induced birefringence while the wavefront distortion is controlled by the average index shift. Here we show that the birefringence is determined by the shear stress in the xy-plane of the crystal while the average index shift depends only on the normal stresses. Furthermore, for depolarization losses of 0.1 to 1.0% and wavefront distortion of 0.1 to 1.0 (lambda) , the critical range of stress is 105 to 106 Pa. We also present measured depolarization loss and wavefront distortion profiles for 5, 16, and 27 cm, 95% deuterated, KD*P crystals. Using the analysis described above we show that the maximum internal stresses in these crystals are within the critical range, but that the area averaged stresses are substantially lower. We find that crystals from different locations along the length of a boule have similar strain birefringence and wavefront distortion profiles indicating that the growth conditions which generate the internal strain persist throughout much of the growth history of the boule. Finally, we discuss potential sources of strain in KD*P.
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