Tunable Laser Radiation Research Articles

Imaging of electronic structure of lead phthalocyanine films studied by combined use of PEEM and Micro-UPS

The thin film growth of lead phthalocyanine (PbPc) on graphite substrate has been studied by photoemission electron microscopy (PEEM) operated with tunable UV laser radiation. Origin of the contrast of PEEM images was elucidated by comparison with microspot ultraviolet photoemission spectroscopy (Micro-UPS). The photoelectron yield curves measured with PEEM for annealed films of different coverages were in agreement with the shifts of the vacuum levels determined by Micro-UPS. Below 1 monolayer (ML) coverage, PbPc molecules did not form islands/domains but were sparsely distributed and formed uniform films. Above 1 ML, PbPc molecules formed stable islands of bilayer in a well-ordered 1 ML film. On the other hand, for as-deposited films with thickness equivalent to annealed 0.3 ML coverage, PbPc molecules formed metastable bilayer islands of a few μm-size in the sparsely packed uniform film. These results demonstrate the capability of combined use of the complementary methods PEEM and Micro-UPS to resolve inhomogeneity of electronic structure.

Measurements of Collisional Broadening Coefficients by Infrared Polarization Spectroscopy

We present measurements of collisional broadening coefficients, obtained at atmospheric pressure, by polarization spectroscopy. Using tunable single mode laser radiation at approximately 2 microm, high-resolution infrared polarization spectra were recorded for CO2-Ar and CO2-He binary mixtures. The recorded polarization spectra were fitted with a Lorentzian cubed function form to obtain the broadening coefficients. The full-width at half-maxima (FWHM) collisional broadening rates of CO2 by Ar and He, for the R14 (12 degrees1<--00 degrees0) line, have been determined to be 0.161+/-0.018 cm-1 atm-1 and 0.1823+/-0.0032 cm-1 atm-1, respectively.

Light-controlled change in the helical pitch and broadband tunable cholesteric liquid-crystal lasers

This paper presents an overview of the results obtained in recent investigations of the control over the helical pitch under irradiation and the use of these data for the design of compact broadband tunable lasers based on dye-doped cholesteric liquid crystals. It is demonstrated that a reversible change in the lasing frequency can be achieved upon exposure to two low-power light-emitting diodes. Another alternative approach to the generation of tunable laser radiation in the visible and ultraviolet spectral regions (370–680 nm) in a specially designed cell with a cholesteric liquid crystal that is doped with several dyes and possesses a helical pitch gradient is considered.

AgGaGeS4 Crystals for Nonlinear Laser Device Applications

A mid-infrared frequency converter with a nonlinear crystal, AgxGaxGe(1-x)S4 (AgGaGeS4), for x=0.5 has been reported by the second harmonic generation of tunable CO2 laser radiation. Two pole coefficients of Sellmeier equations are also presented.

Photoacoustic spectroscopy of formaldehyde with tunable laser radiation at the parts per billion level

Selective and sensitive detection of formaldehyde (CH2O) was achieved using a grazing-incidence optical parametric oscillator (GIOPO), pumped at high repetition rate, as a light source for photoacoustic spectroscopy. The photoacoustic spectrum of formaldehyde was measured in the range from 2785 to 2840 cm-1 and an absorption line at 2805.0 cm-1 was selected for detection. Concentrations down to 20 ppbv (parts per billion by volume) formaldehyde in nitrogen were recorded. The detection limit determined by background fluctuations was 3 ppbv (S/N=1) for 3 s lock-in time constant and 3 min acquisition time. The 2805.0 cm-1 absorption line of the ν1 vibrational mode was chosen because of the absence of interference with water and carbon-dioxide bands. This allowed the direct detection of formaldehyde in laboratory air without filtering.

Tunable middle infrared radiation with HgGa2S4 crystal

Optical properties of orange phased HgGa2S4 crystal is investigated. Generation of tunable middle infrared radiation by second harmonic of tunable CO2 laser radiation has been demonstrated. Second harmonic conversion efficiency with respect to other infrared crystal has also been discussed.

Modified GaSe crystal as a parametric frequency converter

Physical and optical properties of GaSe, doped with indium and sulphur has been discussed. Generation of tunable mid-infrared radiation by the technique of second harmonic generation (SHG) of tunable CO2 laser radiation has been demonstrated. Conversion efficiency of SHG in GaSe:In and GaSe:S has been compared.

Energy partitioning in the femtosecond-laser-induced associativeD2desorption from Ru(0001)

Energy transfer to different degrees of freedom during the femtosecond-laser-induced recombinative desorption of ${\mathrm{D}}_{2}$ from a deuterium-covered Ru(0001) surface $({\mathrm{D}}_{\mathrm{ads}}+{\mathrm{D}}_{\mathrm{ads}}∕\mathrm{Ru}\ensuremath{\rightarrow}{\mathrm{D}}_{2,\text{gas}}+\mathrm{Ru})$ has been investigated. $(1+{1}^{\ensuremath{'}})$-resonance-enhanced multiphoton photoionization (REMPI) and time-of-flight (TOF) measurements are utilized to provide information on the internal and external (translational) energy content, respectively. Rovibrational population distributions of the reaction product are detected via various $B\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{u}^{+}\ensuremath{\leftarrow}X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{g}^{+}$ Lyman bands using tunable vacuum ultraviolet laser radiation in the resonant excitation step. Rotational quantum state populations in the vibrational ground state and the first excited state are measured yielding average rotational energies of $⟨{E}_{\mathrm{rot}}⟩∕{k}_{\mathrm{B}}=800$ and $1500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively, for an absorbed laser fluence $⟨F⟩$ of $85\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$. In addition, a mean vibrational energy of the desorbing molecules is extracted which amounts to $⟨{E}_{\mathrm{vib}}⟩∕{k}_{\mathrm{B}}=1200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Extensive TOF measurements enable complete energy balancing with $⟨{E}_{\mathrm{trans}}⟩∕2{k}_{\mathrm{B}}=2500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at $⟨F⟩=85\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$ and underline the nonthermal and unequal energy partitioning between the different degrees of freedom within the reaction product. The effects of multidimensional electronic friction between substrate and adsorbate layer and peculiarities of the potential energy landscape governing the ${\mathrm{D}}_{2}$ recombination are discussed.

Noncollinear noncritically phase-matched generation of tunable far-UV radiation in an LB4 crystal

Tunable far-UV laser radiation is generated in the 204.7–205.5-nm region in a 3.4-cm long LB4 crystal by using noncritical noncollinear sum frequency mixing of the Nd:YAG laser radiation and the second harmonic of a tunable dye laser. A large value of phase-matching angular acceptance of 0.50 mrad is measured at noncritical phase-matching, which is 1.8 times larger than the critically phase-matched value. A conversion efficiency of 2.1% is obtained for the generation of far-UV at 205 nm. Tunable UV radiation is also generated in the range from 246 to 255 nm using collinear second harmonic of tunable dye laser radiation in an LB4 crystal.

A new grating tuning compound unstable resonator

A proposal of new grating tuning compound unstable resonator is presented. This new type of unstable resonator is profitable for generating single transverse mode, high energy, high-average power tunable laser radiation from TEA CO 2 laser and other gas lasers.

Coherent Transverse Speckle Dynamics in the Case of Probing of Stationary Bulk Scattering Media by a Laser with Frequency Deviation

A coherent transverse dynamics of scattered speckle fields that arises upon propagation of tunable laser radiation through stationary optically inhomogeneous bulk media has been discovered. The dynamic photochromic speckle effect can be observed experimentally if the deviation of the laser radiation frequency is comparable with or greater than the effective phase delays arising upon scattering of waves in a random or partly organized medium. It was found experimentally that, with an increase in the frequency tuning range of a probing laser diode (λ = 650 nm) or a compact YAG:Nd laser (λ = 532 μm), which can reach hundreds of gigahertz for multiply scattering bulk media from several millimeters to two centimeters thick (fluoroplastic phantoms), the two-dimensional correlation coefficient of speckle structures decreased monotonically, and the characteristic drop for thicker structures was observed at smaller frequency detunings.

Nonlinear optical properties of Li 2 B 4 O 7 (LB4) crystal for the generation of tunable ultra-fast laser radiation by optical parametric amplification

Using a very simple and straightforward approach we derive the condition to be satisfied for achieving wavelength-insensitive broadband phase matching in a type-I noncollinear optical parametric amplifier (NOPA), required for the generation of ultra-fast laser radiation. Nonlinear optical properties of a relatively newly grown crystal Li2B4O7 (LB4) have also been studied and we found that this crystal satisfies the condition required to realize the broadband phase matching and is suited for the generation of tunable visible–near-infrared ultra-fast laser radiation employed in a 395-nm-pumped type-I NOPA. The phase-matching bandwidths of type-I NOPAs in different borate-group crystals, such as BBO, CLBO, and KABO, are also numerically estimated. The values are ∼157, 164, 152, and 174 THz for 1-mm-thick BBO, CLBO, KABO, and LB4 crystals, with the noncollinear angles between the input pump and the signal beams 3.7°, 3.0°, 3.4°, and 2.9°, respectively, for the signal wavelengths centered at 630 nm. In addition to the largest bandwidth, LB4 crystal has several other attractive properties to be used in optical parametric applications, such as high laser damage threshold, wide optical transmission, easy crystal growth to excellent optical quality with large sizes, easy treatment of cutting and polishing, and nonhygroscopicity.

Quantum yield for N(4S) production in the ultraviolet photolysis of N2O

Direct detection of N(4S) atom formation in the 193 nm photolysis of N2O by a technique of vacuum ultraviolet (vuv) laser‐induced fluorescence spectroscopy has been reported. Tunable vuv laser radiation around 120.071 nm that is resonant to the one‐photon N(2p23s 4P1/2 − 2p3 4S3/2) transition has been generated by two‐photon resonant four‐wave sum frequency mixing in Hg vapor. The quantum yield value for N(4S) formation in the N2O photolysis at 193 nm has been determined to be 2.1 (±0.9) × 10−3. The N(4S) detection technique, which is developed in this study, is very sensitive, and the minimum detection limit is estimated to be 2 × 109 atoms cm−3. Impact of the photolytic N(4S) and NO(X2Π) production from N2O photolysis on stratospheric chemistry has been explored using a one‐dimensional photochemical model, while the fragmentation was not considered in former model calculations. When the N(4S) + NO dissociation channel is considered in the photochemical model, an enhancement of the NOx production rate (up to 3%) is observed, which is followed by a decrease of the steady state O3 concentration throughout the stratosphere.

Ultrabroad-band phase matching in two recently grown nonlinear optical crystals for the generation of tunable ultrafast laser radiation by type-I noncollinear optical parametric amplification

It was found that the generation of ultrabroad-band femtosecond pulsed laser radiation is possible in two recently grown nonlinear optical crystals, K2Al2B2O7 (KABO) and CsLiB6O10 (CLBO), by employing a type-I noncollinear optical parametric amplification (NOPA) scheme, pumped by Ti:sapphire second harmonic (395 nm) radiation. The phase-matching bandwidths of the process are as large as 152 and 164 THz in KABO and CLBO crystals, with the values of the noncollinear angles (α) between pump input (considered monochromatic) and seed pulses input are 3.3° and 3.0°, respectively. The phase-matching bandwidth calculated for the same type-I NOPA in BBO crystal is 157 THz with α=3.7°. In addition to the large phase-matching bandwidth in CLBO, the major advantages of this crystal are that it can easily be grown to large sizes and it also has a higher threshold against laser damage. The most attractive properties of KABO crystal are that it is nonhygroscopic as well as mechanically robust and easy to cut and polish. Having moderate birefringence the angular and spectral bandwidths of both CLBO and KABO crystals are larger than that of BBO crystal and the deleterious walk-off angles are smaller than that of BBO. The walk-off angles at the pump wavelength of NOPA with CLBO, KABO, and BBO crystals are about 2.1°, 2.7°, and 4.0°, respectively. Here we present the nonlinear optical parametric properties of CLBO and KABO crystals for the generation of ultrabroad band femtosecond laser radiation by the NOPA technique, since we know of no such report so far in the literature.

Atomic spectroscopy studies of short-lived isotopes and nuclear isomer separation with the ISOLDE RILIS

The resonance ionization laser ion source (RILIS) at the ISOLDE on-line isotope separator is based on the selective excitation of atomic transitions by tunable laser radiation. Ion beams of isotopes of 20 elements have been produced using the RILIS setup. Together with the mass separator and a particle detection system it represents a tool for high-sensitive laser spectroscopy of short-lived isotopes. By applying narrow-bandwidth lasers for the RILIS one can study isotope shifts and hyperfine structure (HFS) of atomic optical transitions. Such measurements are capable of providing data on nuclear charge radii, spins and magnetic moments of exotic nuclides far from stability. Although the Doppler broadening of the optical absorption lines limits the resolution of the technique, the accuracy of the HFS measurements examined in experiments with stable Tl isotopes approaches a value of 100 MHz. Due to the hyperfine splitting of atomic lines the RILIS gives an opportunity to separate nuclear isomers. Isomer selectivity of the RILIS has been used in studies of short-lived Ag, Cu and Pb isotopes.

Transition-metal-doped chalcogenide lasers

Broadly tunable mid-infrared lasers are desirable for a number of scientific, remote-sensing, and military applications. Recently, Cr2+-doped chalcogenide lasers have emerged as an attractive source of tunable laser radiation in the 2-3.4-µm region. This paper provides a brief introduction to this field, summarizes key laser work to date, and provides examples of transmission-metal-doped lasers that operate in this region. In particular, recent results with the Cr2+:ZnSe laser material are highlighted.

Control of laser desorption using tunable single pulses and pulse pairs

We desorb ground state Br and spin–orbit excited Br* atoms from KBr single crystals using single pulses and sequential pulse pairs of tunable nanosecond laser radiation. Irradiation of cleaved KBr crystals near the bulk absorption threshold produces hyperthermal Br emission without a significant thermal component, and with little spin–orbit excited Br* emission. The Br kinetic energy distribution may be controlled either by choice of photon energy or by excitation of transient defect centers created within the crystal. In this latter scheme, a first laser pulse generates transient centers within the bulk crystal and in the vicinity of the surface, and a second delayed laser pulse then excites the transient centers leading to atomic desorption. The Br* to Br yield ratio is significantly enhanced using two-pulse excitation as compared to resonant single-pulse desorption. Single and multiple pulse excitation of KBr produces Br and Br* in controllable quantities, velocities, and spin state distributions.

Laser frequency converter for continuous-wave tunable Ti:sapphire lasers based on aperiodically poled LiNbO3:Nd3+

We present a laser device that shifts the frequencies contained in the 750–850 nm emission band interval of a continuous wave Ti:sapphire tunable laser and produces continuous-wave tunable laser radiation between 440 and 475 and 485 and 505 nm. The device is based on self-sum-frequency mixing of the tunable emission of a Ti:sapphire laser injected in the cavity of a diode-pumped aperiodically poled Nd3+:LiNbO3 laser oscillating in continuous-wave at 1084 or 1372 nm.

Selective ultra-trace detection of NO and NO2 in complex gas mixtures using broad-bandwidth REMPI mass spectrometry.

In this paper we demonstrate the feasibility of ultra-trace resonance enhanced multiphoton ionization (REMPI) detection employing a small broad-bandwidth solid state laser system. The results reported here are compared with measurements carried out with a conventional excimer pumped dye laser combination. Mass selected broad-bandwidth REMPI spectra for the environmentally relevant nitrogen oxides NO and NO2 are presented. Tunable broad-bandwidth laser radiation with a spectral resolution of > 10 cm(-1) in the wavelength range 560-400 nm was employed for the detection of NO2. For NO detection, the range 230-224 nm was covered. Laser radiation was generated using an optical parametric oscillator pumped by an unseeded Nd:YAG laser. A mobile time-of-flight mass spectrometer equipped with an atmospheric pressure laser ionization source allowed for mass selective parent ion detection at m/z 30 for NO and m/z 46 for NO2. The limit of detection was 10 pptV for NO and 20 pptV for NO2. A selectivity of > 2000 for both compounds with respect to N2O5, organic nitrates and NO2 in the case of NO is reported. An improved laser system currently under construction is expected to provide detection limits below pptv mixing ratios for both nitrogen oxides in a 20 s integration interval.

Theoretical modelling of CW perturbations of excited states, axialfield and electron density for laser diagnostics of positive columnplasmas

We present a theoretical treatment of the fundamental perturbationscaused when continuous wave tunable laser radiation is absorbed by thelow-temperature plasma of a noble gas positive column. For the first time wepresent a full theoretical description of the contributions due to the inducedchanges in the electric field, electron density and radiation trapping, aswell as the primary laser perturbations. Comparison of our theory withexperimental observation shows that the primary perturbations are generallydominant but in some instances it is the field perturbation which has thegreatest effect.