Visible Dye Laser Research Articles

Strong-field ionization and Coulomb explosion of small neodymium and europium oxide clusters

A femtosecond visible ring dye laser centered at 624nm is used to probe the ionization dynamics of small neodymium and europium oxide clusters. Semi-classical atomic tunneling theory using sequential ionization potential values is compared to experimental ion signal and the gaps between the saturation intensities of each charge state are quantified. Differences in saturation intensities for the onset of ion signal between atomic charge states were found to increase as the metal–oxygen bond strength of the cluster increases. Neodymium showed larger enhancements in ionization whereas larger gaps in ion signal between europium charge states indicate more sequential-like ionization.

Resonance-enhanced multiphoton ionization of jet-cooled 2-methylfuran

Mass-resolved, resonance-enhanced multiphoton ionization (REMPI) spectra of jet-cooled 2-methylfuran have been measured in various spectroscopic regions (visible and UV ns dye laser). The results have been interpreted as two-photon resonances and as three-photon resonances to the 3d Rydberg manifold chiefly. The 3d 1 A 1 ← ( X ∼ ) A 1 transition, located at 56,360 cm −1, is the strongest one in the (2 + 1) REMPI spectrum and has a discrete vibrational structure. The vibrational analysis has been performed. The three-photon resonance spectrum tracks the 3d VUV absorption spectrum. The strongest transition in this case is the 3d 1 B 2 ← ( X ∼ ) A 1 transition located at 56,013 cm −1. The transition to the 3s A 2 Rydberg state has been detected at 44,018 cm −1 (3 + 2 REMPI spectrum).

Processing of transparent materials using visible nanosecond laser pulses

Laser micromachining of transparent materials is an intensively studied research area from the point of view of microoptical element fabrication. One of the most promising indirect processing methods is the laser-induced back-side dry etching (LIBDE). During this method, transparent targets are contacted with solid thin layers, which absorb and transform the pulse energy resulting in etching. The applicability of LIBDE technology for processing of fused silica using a visible nanosecond dye laser (λ=500 nm, FWHM=11 ns) and a 100-nm-thick aluminium absorbing layer was investigated. The applied fluence was varied in the range of 0–3050 mJ/cm2; the illuminated area was 0.1 mm2. The threshold fluence of the LIBDE etching of fused silica was found to be approximately 540 mJ/cm2. The chemical composition of the surface layers on and around the etched holes was investigated by field-emission scanning electron microscopy and energy-dispersive X-ray spectrometry. It was found that on average 0.4±0.3 at. % aluminium is built into the upper ∼1-μm-thick volume of the illuminated fused silica, while the aluminium content fell below the detection limit in the case of the original surface. Our experiments proved that the LIBDE procedure is suitable for microprocessing of transparent materials using visible nanosecond laser light.

Pyrromethene doped silica laser glasses by sol–gel processes

A new laser dye pyrromethene impregnated solid-state material has been developed for visible wavelength solid-state dye laser by means of a sol–gel process. The use of N,N′-dimethylformamide (DMF) as a drying control chemical additive was found to be effective in order to obtain the best quality silica and organically modified silicate (Ormosil) in terms of monolithicity, transparency and density. The addition of γ-glycidyl-propyl-trimethoxisilane (GPTMS) in Ormosil can efficiently increase the luminescent emission intensity. Using these new materials as laser gain media, an all-solid-state, efficient, compact, long-lifetime and tunable dye laser has been demonstrated.

New measurements of appearance potentials and optical absorption of Na2F and Na3O

Oxides and fluorides of sodium clusters were produced by reacting a beam of metal clusters with N2O or SF6 in the region between nozzle and skimmer. The threshold for single photon ionization was measured by using a pulsed, frequency doubled visible dye laser or a UV dye laser and detecting ions with a quadrupole mass spectrometer. Resonant two-photon ionization was employed to record optical spectra. Comparisons with previous measurements and theoretical predictions are discussed.

Trace Analysis of NO2 in the Presence of NO by Laser Photofragmentation/Fragment Photoionization Spectrometry at Visible Wavelengths

Trace concentrations of NO and NO2 molecules are differentiated spectrally by using a visible dye laser and a simple flow cell with a pair of miniature electrodes for ion detection. NO is detected near 452 nm by (2+2) resonance-enhanced multiphoton ionization via its A2∑+-X2II (0,0) transitions, while NO2 is detected by laser photofragmentation with subsequent fragment NO ionization via the A2∑+-X2II (0,0) and (1,1) transitions. Spectral differentiation is possible since the internal energy of the NO photofragment differs from that of “ambient” NO. Measurement of vibrationally excited NO via its A2∑+-X2II (0,3) band is also demonstrated at 517 nm. Rotationally resolved spectra of NO and fragment NO are analyzed by using a multiparameter computer program based on two-photon energy level expressions and line strengths for A2∑+-X2II transitions. Boltzmann analysis of the P12 + O22 branch of the (0,0) band reveals that the rotational temperature of fragment NO is approximately 500 K compared to room-temperature NO. Limits of detection [signal-to-noise (S/N) = 3] of NO and NO2 are in the 20–40-ppbv range at 449.2, 450.7, and 452.6 nm for a 10-s integration time. The limit of detection of NO2 at 517.5 nm is 75 ppbv. The analytical utility of the technique for ambient air analysis is evaluated and discussed.

A Terahertz Photomixing Spectrometer: Application to SO2Self Broadening

A new tunable cw far-infrared difference-frequency spectrometer has been developed by mixing two single-mode visible dye lasers separated by terahertz frequencies in an ultrahigh-speed photoconductor. The photomixer device is fabricated with submicrometer interdigital electrodes driving a broadband self-complementary spiral antenna on epitaxial low-temperature-grown (LTG) GaAs material. The LTG-GaAs material has subpicosecond recombination lifetimes, facilitating the ultrafast response of the mixer. The coherent far-infrared radiation is coupled out of the rear surface of the GaAs substrate using an aplanatic Si lens and is detected by a conventional Si-composite bolometer atT= 4.2 K. We have obtained usable signal-to-noise ratios for radiation from 0.1 to 1.0 THz using 584-nm dye laser pumps with the prospect of reaching 2 or 3 THz with pump wavelengths closer to the GaAs bandgap. The instrumental linewidth is currently about 2 MHz, limited by the jitter of the dye lasers. Baseline normalization is necessary to compensate for amplitude fluctuations of the lasers and for the strong fringes arising from standing waves between the source and detector and within the detector dewar. The instrument has been used to obtain SO2self-broadening coefficients for pure rotational transitions in severalQbranches.

Synchronous pumping of a visible dye laser by a frequency doubled mode-locked Ti:sapphire laser and its application for difference frequency generation in the near infrared

The output of a dye laser that is synchronously pumped by the second harmonic of a self mode-locked femtosecond Ti:sapphire laser is mixed in a BBO crystal with the fundamental of the pump laser to achieve femtosecond pulses tunable between 1230 nm and 1760 nm at ∼ 80 MHz repetition rate.

Time-resolved identification of modes and measurement of intermodal dispersion in optical fibers

Picosecond pulses from a tunable mode-locked visible dye laser are injected into a 603-m few-mode optical fiber. Measurements of the output power temporal profile with 35-ps resolution show six clearly resolved modes. Time-profile measurements over a two-dimensional array of points in the fiber output far field yield a picture of each mode that identifies it. Measurements as a function of wavelength and fitting to waveguide dispersion calculations are used to model the fiber core refractive-index profile. As a result for the nominal step-index fiber used in this work, the presence of an index dip at the center of the core was detected.

Removal rates of CHF (Ã 1A″ (0, 0, 0)) by alkenes

Absolute removal rates of CHF (à 1A″ (0, 0, 0)) by ethene (C 2H 4), propene (C 3H 6), 1-butene (1-C 4H 8), isobutene( i-C 4H 8), 1,3-butadiene (C 4H 6), difluoromethane (CH 2F 2), nitric oxide (NO) and argon (Ar) have been measured at room temperature. CHF in the à 1A″ state was produced by infrared multiphoton dissociation of CH 2F 2 forming the CHF (X̃ 1A′) state and further pumping to the à 1A″ state by absorption of a visible dye laser pulse. Removal processes were found to be second order with the following rate constants in units of 10 −10 cm 3 molecule −1 s −1: k(C 2H 4) = 0.9 ± 0.2. k(C 3H 6) = 1.0 ± 0.2; k (1−1C 4H 8) = 1.1 ± 0.2; k( i-C 4H 8) = 1.1 ± 0.2; k(C 4H 6) = 1.0 ± 0.2; k(Ar) = 0.27 ± 0.02; k(NO) = 0.8 ± 0.1; k(CH 2F 2) = 1.3 ± 0.1. The Parmenter—Seaver correlation for collisional removal of à 1A″ CHF is discussed.

Local modes of HOOH probed by optical-infrared double resonance

We have used an optical-infrared double resonance technique to probe the nature of the eigenstates prepared by 4νOH vibrational overtone excitation in hydrogen peroxide. A visible dye laser excites the 4←0 OH stretch transition and an optical parametric oscillator promotes the molecules above the dissociation threshold by a ΔvOH=2 transition from the 4νOH level. Fixing the overtone excitation laser wavelength and scanning the wavelength of the infrared photon while monitoring the dissociation fragments by laser-induced fluorescence generates an infrared predissociation spectrum of the vibrationally excited molecule that contains information about vibrational state mixing at the 4νOH level. This spectrum indicates that the zeroth-order state that gives oscillator strength to the 4←0 OH stretch transition (i.e., the 4νOH bright state) is almost entirely comprised of a single vibrational eigenstate. Since the bright state is predominantly an OH stretch, the vibrational eigenstate prepared by 4νOH vibrational overtone excitation is well localized on the OH bond. This localization allows us to perform sequential local mode–local mode excitation of the two equivalent OH oscillators in HOOH.

Infrared spectroscopy of vibrationally excited HONO2: Shedding light on the dark states of intramolecular vibrational energy redistribution

Infrared predissociation spectroscopy of nitric acid subsequent to vibrational overtone excitation reveals vibrational state mixing of the highly excited levels and probes the character of the coupled dark states. A visible dye laser excites the 4←0 or 5←0 OH stretch transition and an optical parametric oscillator promotes the molecules above the dissociation threshold by a ΔvOH=1 transition from the excited level. Scanning the optical parametric oscillator frequency while monitoring the predissociation products via laser-induced florescence produces an infrared spectrum of the excited molecules. Although the 4νOH vibrational overtone band consists of a single clean rotational contour that falls directly on a Birge–Sponer plot, the infrared transitions from this level indicate that the zeroth-order bright state is extensively mixed. On the average, the zeroth-order bright state is only a minor component of the eigenstates at this energy. The largest collective contribution is from zeroth-order states that have zero quanta of OH stretch.

Rotationally resolved vibrational overtone spectroscopy of hydrogen peroxide at chemically significant energies

An infrared–optical double resonance scheme simplifies the room temperature 6νOH vibrational overtone spectrum of hydrogen peroxide and prepares highly excited reactant molecules in single rotational states for unimolecular reaction studies. First, an optical parametric oscillator excites the OH asymmetric stretch (ν5) and selects a single or small subset of rotational states. A visible dye laser pulse then promotes molecules from vOH=1 to vOH=6 where they subsequently dissociate to produce two OH fragments. A third laser detects the dissociation products via laser induced fluorescence. The rotationally resolved vibrational overtone spectra of hydrogen peroxide generated by scanning the visible dye laser frequency are assignable to a parallel band of a near prolate symmetric top. Linewidths of the individual rovibrational features range from 1–3 cm−1 but show no systematic dependence upon the rotational quantum numbers and are attributed predominantly to anharmonic coupling of the zeroth-order bright state to dark background states. The assignability of the double-resonance vibrational overtone spectra to J and K quantum numbers implies that K is conserved for at least a time determined by the linewidth of a single zeroth-order rovibrational feature.

Tunable UV generation to 0.2325 mu m in LiB/sub 3/O/sub 5/

LiB/sub 3/O/sub 5/ has been found to be phase matchable for sum-frequency generation down to 0.2325 mu m at 20.0 degrees C by mixing the Nd:YAG laser wavelength at 1.0642 mu m with the second harmonic of a visible dye laser. Sellmeier's equations, which are highly accurate from 0.22 to 1.32 mu m, are reported. It is demonstrated that a much shorter wavelength of 0.218 mu m could be generated in this compound by mixing the fourth harmonic of a Nd:YAG laser with the idler of the KNbO/sub 3/ parametric oscillator tuned to 1.208 mu m.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An efficient Raman shifter for the generation of tunable infrared radiation in the range 3.0–3.5 μm

Tunable infrared radiation with pulse energies of more than 1 mJ has been generated in the spectral range between 3.0 to 3.5 μm via third order stimulated Raman scattering of visible dye laser light (⩽ 75 mJ pulse energy) in an extended cell filled with H 2-gas without using a waveguide. A systematic investigation of the extended cell design provided the following interesting features: (i) the energy of the infrared light pulse ( E s3) rises nearly exponential with the focal length of the entrance lens and the length of the cell, (ii) E s3 depends very strongly on the H 2-pressure within the cell and exhibits a sharp maximum at a surprisingly low value of around 5 bar, (iii) the S3-radiation is emitted in a cone centered around the axis of the pump beam with virtually no axial intensity and (iv) the apex angle of the cone increases with the cell pressure but is independent of the pump energy. Results (ii) to (iv) can be rationalized on the basis of a four-wave mixing process involving the pump beam and the three possible orders of Stokes waves. The reasons for result (i) are probably more involved and unknown as yet.

High-irradiance 248-nm laser system

A laser system that routinely generates irradiances of 8 x 10(17) W cm(-2) is described. Pulses from a mode-locked visible dye laser are heterodyned to 248 nm and amplified in two KrF amplifiers. Output is 25-30 mJ in a 700-fsec pulse that can be focused to 5 microm(2).

Comparative photon statistics of several ultraviolet laser systems determined by transient two-photon absorption

The photon statistics of several different laser systems operating at 226 nm are probed by measuring absolutely the fluorescence following two-photon absorption in atomic oxygen. The integrated fluorescence, obtained by scanning the laser frequency through the two-photon resonance, is theoretically shown to be proportional to G(2) (0) σ0(2). The second-order intensity-correlation function at time zero, G(2)(0), characterizes the laser, and σ0(2) is a fundamental atomic property. Measurement of the integrated fluorescence permits determination of the relative value of G(2) (0) for each laser. The assumption that G(2)(0) = 1 for one of the lasers, a single-frequency source, allows G(2)(0) for the other lasers to be placed on an absolute scale. The values thereby obtained range from 0.8 ± 0.2 to 3.2 ± 0.4. Different values are obtained when the radiation is generated by frequency mixing or Raman shifting of the same frequency-doubled visible dye laser, suggesting that frequency-conversion processes have a significant effect on G(2)(0).

Picosecond infrared generation from Nd: YAG and a visible, short cavity dye laser

We report efficient infrared conversion of picosecond, visible dye laser pulses and picosecond 1.064 μm by a sequence of difference frequency generations in LiIO 3 and AgGaS 2 crystals. The dye laser was a short cavity design and was pumped by the second harmonic of a mode-locked Nd: YAG laser to give 23 ps pulses. The design can span 4–11 μm in the infrared, and we tested the efficiencies between 5.2–6.4 μm with a single dye. Conversion efficiencies of the dye laser were 16%, while the resulting near infrared was converted to infrared at 35% efficiency.

IR spectrometer based on upconversion with intracavity nonlinear frequency converter

The authors investigate the operation of a nonlinear IR spectrometer in which the probing IR radiation is obtained by frequency mixing the radiation of a pump laser with the radiation of a visible tunable dye laser. In so doing the generation of the difference frequency radiation and subsequent upconversion are realized using an intracavity method in the same nonlinear crystal. The spectral information is extracted by analyzing the dye laser radiation. The dye laser and pump laser radiation are combined with the help of a polarization prism or a selective mirror in the nonlinear crystal. Results for increases in spectral density are assessed.

High-resolution coherent extreme-ultraviolet spectroscopy

Classical high-resolution spectroscopy is limited in the XUV by finite grating size and the absence of interferometer elements. Parametric techniques now permit the generation of tunable, coherent XUV radiation with absolute wavelengths determined interferometrically in the visible. The linewidth limit set by pulse duration is generally less than inherent XUV transition widths. A high-resolution XUV spectroscopic system operating below 1000 A is described. Wavelengths are scanned by pressure scanning a visible dye laser, and absolute wavelengths are measured on a single-shot basis to 1 part in 106 with a pulsed Fizeau interferometer. The autoionizing Rydberg series in Xe going to the second limit has been scanned in the 923- to 942-A spectral region. New line profiles are obtained, and absolute wavelengths have been compared with the best available classical values.