193-nm Radiation Research Articles

Pulsed-laser-induced dark traces in LiNbO_3:Zn and LiNbO_3:Mg

Laser-induced dark traces in LiNbO(3):Zn (7.5 mol.%) and LiNbO(3):Mg (6.5 mol.%) crystals have been studied with the crystals as the component for 1064-532-nm second-harmonic generation of Nd:YAG radiation operating in a pulsed mode. The results show that, at high input power density, both crystals suffer from dark traces, which reduce the second-harmonic-generation efficiency. The dark traces are induced by the 532-nm radiation and can be recovered after the laser beam is shut off. The transmission loss spectra show a broad absorption peak near 400 nm.

New Transition Metal Phosphates Related to KTiOPO4. Synthesis of K0.5M0.5M′0.5OPO4 (M = Nb, Ta; M′ = Ti, V) and K1-xTi1-xVxOPO4 Exhibiting Nonlinear Optical Behavior

Several transition metal phosphates of the general formula KM0.5M′0.5OPO4, where M = NbV or TaV and M′ = TiIII, VIII, CrIII, FeIII, possessing the KTiOPO4 (KTP) structure have been synthesized. Unlike KTP, none of these phosphates exhibit second harmonic generation (SHG) of the 1064-nm radiation. Soft-chemical oxidation of the TiIII and VIII phosphates using chlorine in CHCl3, however, restores the SHG property to the oxidized materials, K0.5M0.5M′0.5OPO4 (M = NbV, TaV; M′ = TiIV, VIV). K1-x Ti1-xVxOPO4 (x ≤ 0.25) prepared similarly from KTi1-xVxOPO4 also exhibit SHG. The results reveal that exclusive presence of d0 cations at the TiIV site preserves the SHG response of KTP derivatives.

Soft-chemical routes to synthesis of solid oxide materials

We describe three different families of metal oxides, viz., (i) protonated layered perovskites, (ii) framework phosphates of NASICON and KTiOPO4 (KTP) structures and (iii) layered and three-dimensional oxides in the H-V-W-O system, synthesized by ‘soft-chemical’ routes involving respectively ion-exchange, redox deintercalation and acid-leaching from appropriate parent oxides. Oxides of the first family, HyA2B3O10(A = La/Ca; B = Ti/Nb), exhibit variable Bronsted acidity and intercalation behaviour that depend on the interlayer structure. V2(PO4)3 prepared by oxidative deintercalation from Na3 V2(PO4)3 is a new host material exhibiting reductive insertion of lithium/hydrogen, while K0.5Nb0.5 M0.5OPO4 (M = Ti, V) are novel KTP-like materials exhibiting second harmonic generation of 1064nm radiation. HxVxW1-xO3 for x = 0–125 and 0.33 possessing α-MoO3 and hexagonal WO3 structures, prepared by acid-leaching of LiVWO6, represent functionalized oxide materials exhibiting redox and acid-base intercalation reactivity.

Intense-field molecular spectroscopy: Vibrational and rotational effects in harmonic generation by H2+

We present results of a complete treatment of electronic, vibrational, and rotational motion in numerical calculation of harmonic generation (HG) of 1064-nm laser radiation by the ${\mathrm{H}}_{2}^{+}$ molecular ion for intensities ${10}^{13}$\ensuremath{\le}I\ensuremath{\le}${10}^{14}$ W/${\mathrm{cm}}^{2}$. We show that efficient HG can be enhanced by suppression of photodissociation, a phenomenon which results from vibrational trapping in laser-field-induced potential wells. The HG spectra exhibit peaks clustered around even and odd harmonic orders. All peaks can be assigned to Raman-like transitions between dressed eigenstates of the field-molecule system. Rotational excitation is shown to compete with HG. Thus harmonic generation and photon scattering in molecules holds the promise of a potential diagnostic for molecular stabilization by intense laser fields.

19-W cw ring-cavity KTP singly resonant optical parametric oscillator

We report high-power and single-frequency operation of a continuous-wave singly resonant potassium titanyl phosphate (KTiOPO(4)) optical parametric oscillator in a ring-cavity configuration. The ring singly resonant optical parametric oscillator threshold is 4.3 W. When the oscillator is pumped by 6.7 W of 532-nm radiation from an 11.2-W single-frequency resonantly doubled Nd:YAG laser, 1.9 W of single-axial-mode output is generated at the idler wavelength of 1039 nm.

Unimolecular photochemistry of alkanethiols studied by photodissociation-photoionization mass spectrometry

Unimolecular photodissociation of isomeric alkanethiols in the C 3 to C 8 size range was studied by using photodissociation-photoionization mass spectrometry. Photodissociation was performed with 193- and 248-nm radiation. The primary products were photoionized with coherent vacuum ultraviolet radiation and detected in a time-of-flight mass spectrometer. Three basic reaction channels were observed: C-S, C-C, and S-H bond dissociation. The branching ratios for fragmentation through these channels were found to be strongly dependent upon molecular structure and photodissociation wavelength. All compounds gave intense products of C-S bond dissociation. Photoionization and secondary fragmentation characteristics of the primary products suggested that complete statistical partitioning of the excess internal energy did not occur

Scanning-force microscope based on an optical trap

An optically trapped prolate glass stylus is the force-sensing element of a novel scanning-force microscope. Stylus displacement is detected with the use of the forward scatter of the trapping laser beam. Radiation forces owing to the three-dimensional intensity distribution near the focus permit the stylus to be both positioned with fine control and oriented along the z (optic) axis. Details of 20-nm size appear in traces recorded with a crude stylus in a trap formed with 1064-nm radiation. The spring constant of the optical-force transducer is below 10−4 N/m, which is to be compared with ∼0.1 N/m for typical mechanical cantilevers used in atomic-force microscopy. This gentler technique should improve the sensitivity of scanning-force microscopy for the imaging of soft samples in aqueous media.

Characterization and Application of an Infrared Linear Array Spectrometer for Time-Resolved Infrared Spectroscopy

An infrared spectrometer is described which uses a 32-element InSb linear array detector. The infrared linear array (IRLA) spectrometer collects infrared spectra with a spectral width of 0.50 µm every 16 µs. The performance of the IRLA spectrometer is characterized with respect to signal-to-noise ratio, spectral resolution, time resolution, and instrumental line shape. The IRLA spectrometer is used to collect the time-resolved infrared spectra following 193-nm irradiation of a solution of Mo(CO)6 in hexane.

Optical spectroscopy of Cr^4+:Y_2SiO_5

Spectroscopic data on the absorption and the fluorescence spectra and fluorescence lifetimes are presented for Cr4+:Y2SiO5, the newest Cr4+-doped laser crystal. The fluorescence measurements were conducted at temperatures ranging from 10 to 310 K, and the absorption spectra were measured at room temperature and 77 K. Spectroscopic analysis of the absorption and the emission spectra is performed on the basis of a distorted tetrahedral symmetry for the Cr4+ site, namely C3V. The tetravalent chromium substitutes for Si4+ in this crystal, and there is no evidence for the presence of chromium in any other valence state. The Cr4+ site is characterized by strong crystal field parameters. We have tentatively assigned the sharp line at 1146 nm in the low-temperature fluorescence spectrum (excited by 1064-nm radiation) to the spin-forbidden singlet-to-triplet transition 1E–3A2, and the broad band with the peak at 1225 nm to the spin-allowed 3T2–3A2 transition. Excitation at 532 nm results in an additional type of emission in the near infrared. A similar dependence of the lasing wavelength on the pump wavelength was observed in the laser operation of the crystal at 77 K [ Chai , in Advanced Solid-State Lasers, ChaseL. L.PintoA. A., eds., Vol. 13 of OSA Proceedings Series ( Optical Society of America, Washington, D.C., 1992), pp. 28– 30].

X-ray emission from a 650-fs laser-produced barium plasma.

We have used x rays in the 9--15-\AA{} band emitted from a solid target of ${\mathrm{BaF}}_{2}$ irradiated by \ensuremath{\sim}120 mJ of 248-nm radiation in a 650-fs full width at half maximum Gaussian laser pulse to characterize spectroscopically the resulting ultrashort-pulse laser-produced plasma. The emission was spectrally resolved but space and time integrated. By comparing the spectrum with unresolved-transition-array calculations and measurements of plasma emission from longer-pulse experiments, it is clear that ions as highly stripped as titaniumlike barium are present. We have successfully modeled the observed spectrum by assuming an optically thin source in local thermodynamic equilibrium (LTE) and using the super-transition-array theory [A. Bar-Shalom et al., Phys. Rev. A 40, 3183 (1989)] for emission from a hot, dense plasma. The model indicates that the emitting region is at comparatively low temperature (200--300 eV) and high electron density (${10}^{23--}$${10}^{24}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$). The degree of agreement between the model and the measured spectrum also suggests that the emitting plasma is near LTE. We conclude that the emission in this band arises from a solid-density plasma formed early in time, and is thus localized in both space and time. This interpretation is reinforced by lasnex [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 11, 51 (1975)] simulations that indicate that emission in this band closely tracks the laser pulse.

Highly nonlinear near-resonant photodarkening in a thulium-doped aluminosilicate glass fiber

Permanent photoinduced optical attenuation has been observed in Tm(3+)-doped aluminosilicate glass fibers on exposure to near-resonance mode-locked 1064-nm radiation at 300 K. The rate of this darkening was observed to follow a 4.7 +/- 0.4 power dependence on the 1064-nm intensity. The result shows that absorption of infrared light of moderate intensity can lead to photoionization in rare-earth-doped glass, provided that a multiphoton stepwise excitation channel exists.

Ultraviolet luminescence from latent fingerprints

Illumination of latent fingerprints on white paper using 266-nm radiation from a Nd:YAG laser and photographic detection of their ultraviolet fluorescence, produces images with good ridge detail. The detection rate was 69% in a survey of fingerprints from 34 people compared with only 23% using an argon-ion laser at 514 nm. Prolonged exposure to UV light decreased the inherent UV fluorescence intensity but no adverse effects were observed on subsequent treatment with 1,8-diazafluoren-9-one or ninhydrin.

Laser desorption laser photoionization time-of-flight mass spectrometry of dyes

The mass spectra of a number of dye species have been investigated using two-step desorption laser photoionization time-of-flight mass spectrometry (L 2 TOFMS). A pulsed CO 2 laser, wavelength 10.6 μm, was used to vaporize the samples as intact neutral molecules which were photoionized using either 193- or 266-nm laser radiation. Four classes of dyestuffs have been examined: azo, antraquinone, phthalocyanine, and coumarin. Significant differences were found in the fragmentation patterns and ionization efficiencies at these two laser wavelengths particularly for the azo dyes

Photoionization of small silicon clusters: ionization potentials for Si2 to Si40

Photoionization thresholds of Si n (n=2−40) were examined by vacuum UV radiation (5.7–8.5 eV) generated by stimulated Raman scattering of narrow-bandwidth 193-nm radiation in high pressure hydrogen and deuterium gases. A strong threshold energy dependence on cluster size is observed, featuring major maxima at 10 and 20. The magic behavior atn=10 is consistent with the results of the photofragmentation and CID experiments reported previously.

Acid-catalyzed single-layer resists for ArF lithography

A positive-tone single-layer resist for use with 193-nm radiation has been developed. The system contains a terpolymer of methyl methacrylate, methacrylic acid, and t -butyl methacrylate, along with a photoacid generator. The chemically amplified deprotection of the t -butyl methacrylate into methacrylic acid increases the polarity of the resist and allows selective dissolution in metal-ion-free aqueous-base solutions. The resist sensitivity is less than 10 mJ/cm 2 , and its inherent resolution is better than 0.1 μm. These acrylate-based systems have the potential to be both lower in cost and have better environmental stability than the deep ultraviolet chemically amplified resists that use phenolic resins.

Changes in Gene Expression by 193- and 248-nm Excimer Laser Radiation in Cultured Human Fibroblasts

Tissue ablation by ultraviolet excimer lasers results in exposure of viable cells to subablative doses of radiation. To understand the potential biological consequences better, we have studied changes in gene expression in cultured human skin fibroblasts exposed to either 193- or 248-nm laser light. Northern blot analyses revealed that both treatments up-regulate a common set of genes, including interstitial collagenase, tissue inhibitor of metalloprotease, metallothionein, and the proto-oncogene c-fos. Dose-response and kinetic studies of collagenase induction by 193-nm radiation showed a maximal effect with 60 J/m2 and at approximately 24 h. The induction was still persistent 96 h later. In addition to the commonly affected genes, known to be activated also by conventional UV light (254 nm) and tumor-promoting phorbol esters, other genes were found to be selectively induced by the 193-nm radiation. The heat-shock hsp70 mRNA, undetectable in controls and in cultures irradiated at 248 nm, was transiently induced 8 h after exposure to 193-nm radiation. Furthermore, a selective up-regulation of collagen type I expression was observed. The results indicate that the 193- and 248-nm radiations by excimer lasers elicit specific and different cellular responses, in addition to an overlapping pathway of gene activation common also to UV radiation by germicidal lamps. The laser-induced genes could serve as molecular markers in evaluating cell injury in situ.

Reactions of hydrogenated defects in fused silica caused by thermal treatment and deep ultraviolet irradiation

Nuclear magnetic resonance (NMR) was used to study hydrogen in fused silica for the first time. The 1H NMR spin-lattice relaxation time constant (T1) in synthetic fused silica was found to be strongly affected by thermal treatments, correlating with the concentration of E′ centers produced during subsequent 193-nm excimer-laser irradiation. In addition, the relative proportion of the two components in the NMR spectrum were affected by thermal treatment, although no change in the total hydrogen content was observed. The broad NMR component, because of its linewidth and dependence upon thermal processing, is postulated to be a direct signature of the NMR relaxation centers. These results are interpreted within the framework of a model in which pairs of silanol groups are consumed during thermal treatment, forming a strained bond which is susceptible to E′ center formation.

Characterization of thiol self-assembled films by laser desorption Fourier transform mass spectrometry

Self-assembled alkanethiol monolayers on gold films have been studied using the technique of laser desorption coupled with Fourier transform mass spectrometry. The authors demonstrate that thiol molecules can be desorbed intact using 193-nm laser radiation. Information on the self-assembled films at a molecular level is achieved. In addition to the well-known thiolate species, alkane sulfonates resulting from air exposure of the self-assembled film are also detected. No phase segregation {ge}30 {mu}m is observed from the laser desorption of self-assembled films made of alkanethiol mixtures. 37 refs., 8 figs.

High-energy, high-efficiency second-harmonic generation of 1064-nm radiation in KTP

An energy conversion efficiency of 80% for second-harmonic generation of 532-nm radiation from 1064-nm radiation has been achieved with a single KTP crystal and a commerical injection-seeded Q-switched Nd:YAG laser. A second-harmonic output of 720 mJ was generated with 900 mJ of incident 1064-nm radiation at 10 Hz.

Generation of vacuum-ultraviolet radiation inH2by nonlinear optical processes near theEF- andB-state resonances

We report on the observation of several nonlinear optical processes in connection with two-photon excitation of ${\mathrm{H}}_{2}$ by 193-nm radiation generated by a narrow-bandwidth ArF laser system: stimulated electronic hyper-Raman scattering, parametric four-wave mixing (PFWM), and stimulated resonant Raman scattering (SRRS). These processes lead to the generation of several intense lines in the ir and vacuum ultraviolet (vuv). vuv generation occurs by two interconnected schemes: first, a vuv wave at \ensuremath{\lambda}\ensuremath{\sim}109 nm is generated by PFWM (power up to 16 kW). This wave subsequently generates Stokes lines around \ensuremath{\lambda}\ensuremath{\sim}150 nm by SRRS. The frequencies of these waves are tunable over spectral intervals of \ensuremath{\sim}20 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ by phase matching with admixture of various gases (e.g., noble gases, ${\mathrm{N}}_{2}$, ${\mathrm{D}}_{2}$). With additional irradiation of ir laser radiation, four-wave mixing (FWM) takes place. In this case, continuously tunable vuv radiation (tuning range \ensuremath{\sim}600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) can be generated with power in the 10- and 100-kW range. The tuning ranges are localized around the rovibronic resonances X-B. FWM is not possible on resonance. As in the case of PFWM, Stokes lines are generated by SRRS, tunable over intervals up to \ensuremath{\sim}60 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The susceptibilities governing the processes involved are calculated. The agreement between the theoretical expectations and the experimental results is very good with respect to the frequency dependence. The calculated absolute values of the vuv intensities, however, turn out to be by far too high. This is due to the neglect of several limiting processes which will be discussed together with the experimental results. In addition to the processes mentioned above, several B-X resonance transitions exhibit a limited tunability (\ensuremath{\sim}10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). It is proposed to explain this tunability in terms of stimulated two-photon emission.