Optical Parametric Oscillator Laser Research Articles

Temperature-Jump Apparatus with Raman Detection Based on a Solid-State Tunable (1.80–2.05 μm) kHz Optical Parametric Oscillator Laser

The operating characteristics of a pulsed (10 ns) tunable near-infrared (NIR) laser source are described for temperature-jump (T-jump) applications. A Q-switched Nd:YLF laser (approximately 10 ns pulses) with a 1 kHz repetition rate is used to pump a potassium titanyl arsenate (KTA) crystal-based optical parametric oscillator (OPO), producing approximately 1 mJ NIR pulses that are tunable (1.80-2.05 microm) across the 1.9 microm vibrational overtone band of water. This T-jump source has been coupled to a deep ultraviolet (UV) probe laser for Raman studies of protein dynamics. T-jumps of up to 30 degrees C, as measured via the O-H stretching Raman band of water, are readily achieved. Application to cytochrome c unfolding is demonstrated.

Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry for Detection of Nitrogen Containing Aliphatic and Aromatic Compounds: Resonance-Enhanced Multiphoton Ionization Spectroscopic Investigation and On-Line Analytical Application

Resonance-enhanced multiphoton ionization (REMPI) combined with time-of-flight mass spectrometry (TOFMS) is an analytical method capable of on-line monitoring of trace compounds in complex matrices. A necessary prerequisite for substance selective detection is spectroscopic investigation of the target molecules. Several organic nitrogen compounds comprising aliphatic and aromatic amines, nitrogen heterocyclic compounds, and aromatic nitriles are spectroscopically investigated with a tunable narrow bandwidth optical parametric oscillator (OPO) laser system providing a scannable wavelength range between 220 and 340 nm. These species are known as possible precursors in fuel-NO formation from combustion of solid fuels such as biomass and waste. A newly conceived double inlet system was used in this study, which allows rapid change between effusive and supersonic molecular beams. The resulting REMPI spectra of the compounds are discussed with respect to electronic transitions that could be utilized for a selective ionization of these compounds in complex mixtures such as combustion and process gases. The practicability of this approach is demonstrated by wavelength selected on-line REMPI-TOFMS detection of aniline and cyanonaphthalene in the burning chamber of a waste incineration plant. REMPI mass spectra recorded at different excitation wavelengths as well as variations in time show the utilization of species-selective REMPI-TOFMS detection for on-line monitoring of crucial substances in pollutant formation.

GaP THz wave generator and THz spectrometer using Cr:Forsterite lasers

We have developed a type of THz wave generator that uses Cr:Forsterite lasers as the pump and signal sources for difference frequency generation in GaP (Cr:F source system). We confirmed the generation of THz waves in the frequency range from 0.3to7.5THz, which is just similar to that obtained using the THz wave generator previously developed utilizing yttrium aluminum garnet and optical parametric oscillator (OPO) lasers (OPO source system). A peak output power of 100mW was obtained from 1.2to5THz when the power of the two input beams was 3mJ each, similar to the OPO source system. A wide measurable frequency range from below 0.6THz to over 6THz was obtained by using the Cr:F source system as the light source of a spectrometer, which has the merits of simple structure, easy maintenance, and low cost compared with the OPO source system. Although the linewidth of the Cr:F source system is greater than that of the OPO source system, the THz spectrometer still has sufficient resolution for measuring solids or liquids at room temperature.

Detection of sputtered metals with cavity ring-down spectroscopy

We report on use of cavity ring-down spectroscopy (CRDS) as a means to detect and quantify ion sputtering of refractory metal species. CRDS measurements are made with a neodymium:YAG-pumped optical parametric oscillator laser system in the 375-400 nm region. CRDS sputtering measurements are presented for argon ions incident on iron, aluminum, molybdenum, and titanium. The measurements are based on absorption from fine-structure levels of the electronic ground-state multiplets. For each species, characteristic spectra are provided, the dependence of sputtered particle number density on the beam current is examined, measured densities are compared with a sputter model, and detection limits are determined. For iron, aluminum, and titanium we probe multiple fine-structure levels within the ground-state multiplet and obtain information on their relative populations.

In vitro measurements of absolute blood oxygen saturation using pulsed near-infrared photoacoustic spectroscopy: accuracy and resolution

Pulsed photoacoustic spectroscopy was used to measure blood oxygen saturation in vitro. An optical parametric oscillator laser system provided nanosecond excitation pulses over the wavelength range 740–1040 nm which were used to generate photoacoustic signals in a cuvette through which a saline suspension of red blood cells was circulated. The signal amplitude and the effective attenuation coefficient were extracted from the photoacoustic signals as a function of wavelength to provide photoacoustic spectra of the blood. From these, the relative concentrations of oxy- and deoxyhaemoglobin, and therefore blood oxygen saturation (SO2), were determined using forward models of the absorbed energy distribution based on diffusion theory. A standard linear model of the dependence of absorbance on the concentration of chromophores was also used to calculate the blood oxygen saturation from the signal amplitude spectra. The diffusion approximation model was shown to produce the highest accuracy in blood SO2. The photoacoustically determined oxygen saturation was found to have an accuracy of ±4% SO2 for signal amplitude data and ±2.5% SO2 for effective attenuation spectra. The smallest change in oxygen saturation that can be measured using this technique was ±1% SO2.

Photothermal imaging of nanoparticles and cells

This review summarizes the findings of recent applications of time-domain far-field photothermal (PT) technique to the detection and imaging of nanoscale absorbing particles. This two-beam (pump-probe) technique is based on time-resolved PT visualization of laser-induced thermal effects around nanoparticles. Imaging is accomplished, after an adjustable time delay after the pump laser pulse, with a second probe beam that senses the nanotarget. Using a tunable optical parametric oscillator laser (wavelength, 420 to 570 nm; energy, 0.1-300 /spl mu/J; pulse width, 8 ns) as the pump laser and a Raman shifter (639 nm, 10 nJ, 13 ns) as the probe laser, with a tunable delay of 0 to 5 000 ns of the probe pulse relative to the pump pulse, this approach has demonstrated the capability to visualize nanoscale gold particles (2 to 250 nm) alone and in cells, liposomes (30 to 90 nm), neutral red-stained particles (30 to 500 nm), and polystyrene beads. Different applications of the time-resolved PT technique are discussed, including imaging of absorbing cellular nanostructures and optimization of selective killing of cancer cells and bacteria.

Strategies for Formaldehyde Detection in Flames and Engines Using a Single-Mode Nd:YAG/OPO Laser System

This paper presents technical developments for the detection of formaldehyde (CH2O) using laser-induced fluorescence. The easily accessible third harmonic of the Nd:YAG laser at 355 nm was used for excitation of formaldehyde. In order to investigate potential background fluorescence, e.g., from large molecules such as polyaromatic hydrocarbons, special attention was paid to investigating the possibility of scanning the wavelength of a single-mode Nd:YAG laser under the gain profile, approximately 3 cm(-1), on and off resonance. Furthermore, a technique for simultaneous detection of formaldehyde and OH using one laser system is presented. The single-mode Nd: YAG laser at 355 nm in combination with an optical parametric oscillator (OPO) laser tuned to 283 nm was used for simultaneous two-dimensional imaging of both species using one charge-coupled device (CCD) detector equipped with a dual filter image separator. The techniques are demonstrated with measurements in laboratory flames and the combined measurements are also demonstrated in an engine.

Time-Resolved Imaging of the Plume Dynamics in Infrared Matrix-Assisted Laser Desorption/Ionization with a Glycerol Matrix

The dynamics of the expanding material plume after irradiation of a matrix sample with two different infrared (IR) lasers, an Er:YAG laser of ca. 100 ns and an optical parametric oscillator (OPO) laser system of 6 ns pulse duration, were investigated by imaging the plumes with nanosecond time resolution. Both lasers emitted at an identical wavelength of 2.94 microm. Laser exposure parameters were typical for infrared matrix-assisted laser desorption/ionization mass spectrometry (IR-MALDI-MS); glycerol was employed as a liquid matrix to provide a homogeneous sample and reproducible plume formation. A Nd:YAG laser (532 nm; 8 ns) was used as the illumination source and a CMOS camera with a ten-bit dynamic range served for recording of the images. Dark-field as well as scattered light illumination was employed to preferentially image the gaseous and particulate components of the plume, respectively. During the initial phase of its expansion (ca. 1 micros) the plume appears to consist of a continuous cloud of material of varying density. At later times after exposure, individual particles of several micrometers in size dominate the images. For both laser pulse durations material ejection was observed for times as long as 100 micros postexposure. Subtle but distinct differences in the plume dynamics are observed for the two different pulse durations. They are related to a transition between the regimes below and above acoustic confinement. The experimental findings are compared to results obtained in two previous studies by photoacoustic analysis of the desorption process and IR-laser postionization of the plume.

Photodissociation spectroscopy of trapped protonated tryptophan

We have coupled a quadrupole ion trap with a frequency doubled optical parametric oscillator laser. The photodissociation spectrum of the protonated tryptophan ion from 215 to 320 nm is reported. The yields of fragmentation on each mass channel as a function of the laser wavelength were obtained. We also report experiments involving multiple stages of laser induced dissociation and discuss possible structures for the fragmentation products.

High repetition rate pulsed laser of twin wavelengths from KTiOPO 4 optical parametric oscillation

In this paper, some key techniques of KTiOPO4 optical parametric oscillation (OPO) laser with repetitive frequencies and twin wavelengths output have been analysed theoretically and studied experimentally in detail. An intracavity optical parametric oscillator (IOPO) has been applied. Operating under 20Hz, laser output energy of more than 38mJ with twin wavelengths 1.57μm and 1.06μm has been achieved. Results of the experiments agree well with the theoretical discussion.

Sputter erosion measurements of titanium and molybdenum by cavity ring-down spectroscopy

We report cavity ring-down spectroscopy measurements of the gas-phase number density of titanium and molybdenum sputtered by argon ions. A neodymium: yttrium–aluminum–garnet pumped optical parametric oscillator laser system is used to probe optical absorption features of titanium and molybdenum in the 375–400 nm region. For an 18 mA ion beam current, and 750 eV ions, we find that the spatially averaged number density of sputtered atoms is 6.4±0.6×108 and 5.1±0.5×108 cm−3, for titanium and molybdenum, respectively. The measured number densities exhibit the expected linear behavior versus beam current, and are in reasonable agreement with values found from a simple sputtering model. Our current configuration yields number density detection limits of ∼9×106 and ∼7×105 cm−3, for titanium and molybdenum, respectively. The technique may ultimately provide a means to measure sputter erosion rates, which are of particular importance for the electric propulsion field.

High resolution multiphoton ionization and dissociation of acetone via $\mathsf{3s\leftarrow n}$ Rydberg transitions

Measurements of multiphoton ionization and dissociation of acetone are reported in the wavelength range 582.60-585.80 nm at photon energy resolution of 0.3 cm-1. To our knowledge there are not available results of (3 + 2) REMPI on acetone at 0.01 nm in this region. The experiments were performed using an Nd: YAG-OPO (optical parametric oscillator) laser system coupled to time-of-flight mass spectrometer. The ion yield and the single ion channel are presented. The high-resolution three-photon resonance multiphoton spectrum of the acetone \(3s\leftarrow n\) Rydberg transition is also reported. The experimental results show three dissociation channels of the acetone ion, leading to the products: (CH3CO + ), (CH3 + ) and (COH + ); the channel CH3COCH \(_{3}^{ + } \to \rm CH\) 3CO + + CH3 being the most favored. The acetone and acetyl ions are observed in all wavelength range investigated. In addition, we have measured the origin of the \(3s\leftarrow n\) and \(4s\leftarrow n\) transitions, and vibrational bands of the 3s state.

Atomic mercury flux monitoring using an optical parametric oscillator based lidar system

A newly developed optical parametric oscillator (OPO) based differential absorption lidar (DIAL) system has been applied to the monitoring of atomic mercury emissions at several chlor-alkali plants in Europe. The versatility of the system is illustrated by measured time series of mercury flux and movies of vertical and horizontal concentration distributions, which yield important input parameters for the environmental community. Long term measurements of the resonance absorption of mercury at 253.65 nm poses special demands, i.e. long term stability, on the light source that often have been hard to fulfill, in different respects, for standard OPO and dye laser based systems. Here, approaches to meet these demands are presented.

Development of a fluorescence detection system using optical parametric oscillator (OPO) laser excitation for in vivo diagnosis.

In this work, the development and applications of a fluorescence detection system using optical parametric oscillator (OPO) laser excitation for in vivo disease diagnosis including oral carcinoma are described. The optical diagnosis system was based on an OPO laser for multi-wavelength excitation and time-resolved detection. The pulsed Nd-YAG-pumped OPO laser system (6 ns, 20 Hz) is compact and has a rapid, broad, and uniform tuning range. Time-gated detection of intensified charge-coupled device (ICCD) making use of external triggering was used to effectively eliminate the laser scattering and contribute to the highly sensitive in vivo measurements. Artificial tissue-simulating phantoms consisting of polystyrene microspheres and tissue fluorophores were tested to optimize the gating parameters. 51-ns gate width and 39-ns gate delays were determined to be the optimal parameters for sensitive detection. In vivo measurements with the optical diagnosis system were applied to esophagus, stomach, and small intestine using an endoscope in canine animal studies. The rapid tuning capability of the optical diagnosis system contributed greatly to the optimization of wavelength for the observation of porphyrin in the small intestine. When the small intestine was thoroughly washed with water, the emission band which corresponds to porphyrin disappeared. Based on this observation, it was concluded that the detected signal was yielded by porphyrin-containing bile secretion. Also, multispectral analyses using multiple excitations from 415 to 480 nm at 5 nm intervals confirmed the porphyrin detection in the small intestine. The optical diagnosis system was also applied to the detection of human xenograft of oral carcinoma in mice using 5-aminolevulinic acid (5-ALA) which is a photodynamic therapy (PDT) drug. Significant differences in protoporphyrin IX fluorescence intensity between normal and tumor tissue could be obtained 2 hours after the injection of 5-ALA into mice due to the preferential accumulation of 5-ALA in tumors. Results reported herein demonstrate potential capabilities of the LIF-OPO system for in vivo disease diagnosis.

Photoacoustic Analysis of Matrix-Assisted Laser Desorption/Ionization Processes with Pulsed Infrared Lasers

A fast piezoelectric detection system was utilized to record time-resolved photoacoustic signals reflecting the thermal expansion and the rate of material ablation in infrared matrix-assisted laser desorption/ionization (IR-MALDI). Glycerol was employed as a liquid and 2,5-dihydroxybenzoic acid (2,5-DHB) as a crystalline matrix. An Er:YAG laser (λ = 2.94 μm; τL ∼ 100 ns) and a wavelength-tunable optical parametric oscillator (OPO) laser system (λ = 1.4−4.0 μm; τL = 6 ns) were used for desorption/ionization. Material ejection and MALDI ion signals were recorded as a function of laser fluence, pulse duration, and wavelength. For glycerol as matrix, material ejection was found to be temporally confined to about the laser pulse duration when the OPO was employed. For excitation with the Er:YAG laser, a prolonged material ejection with an approximately exponential decay was observed with a characteristic time constant of ∼1 μs. For both lasers, material ejection was observed already at fluences substantially below the MALDI ion threshold. For excitation with the OPO, a correlation between a change in the desorption/ablation mechanism and the onset of ion generation was found. Crystalline 2,5-DHB preparations were investigated with the OPO only. Material ejection from this matrix was found to exhibit a significantly different dependence of the overall ejected material on laser fluence. The article also provides an introduction to the underlying photoacoustic theory adapted to the MALDI process.

Development and testing of a frequency-agile optical parametric oscillator system for differential absorption lidar

An all-solid-state fast-tuning lidar transmitter for range- and temporally resolved atmospheric gas concentration measurements has been developed and thoroughly tested. The instrument is based on a commercial optical parametric oscillator (OPO) laser system, which has been redesigned with piezoelectric transducers mounted on the wavelength-tuning mirror and on the crystal angle tuning element in the OPO. Piezoelectric transducers similarly control a frequency-mixing stage and doubling stage, which have been incorporated to extend system capabilities to the mid-IR and UV regions. The construction allows the system to be tuned to any wavelength, in any order, in the range of the piezoelectric transducers on a shot-to-shot basis. This extends the measurement capabilities far beyond the two-wavelength differential absorption lidar method and enables simultaneous measurements of several gases. The system performance in terms of wavelength, linewidth, and power stability is monitored in real time by an étalon-based wave meter and gas cells. The tests showed that the system was able to produce radiation in the 220–4300-nm-wavelength region, with an average linewidth better than 0.2 cm−1 and a shot-to-shot tunability up to 160 cm−1 within 20 ms. The utility of real-time linewidth and wavelength measurements is demonstrated by the ability to identify occasional poor quality laser shots and disregard these measurements. Also, absorption cell measurements of methane and mercury demonstrate the performance in obtaining stable wavelength and linewidth during rapid scans in the mid-IR and UV regions.

Wavelength dependence of laser-induced damage: determining the damage initiation mechanisms.

A novel experimental approach is employed to understand the mechanisms of laser induced damage. Using an OPO (optical parametric oscillator) laser, we have measured the damage thresholds of deuterated potassium dihydrogen phosphate (DKDP) from the near ultraviolet into the visible. Distinct steps, whose width is of the order of k(B)T, are observed in the damage threshold at photon energies associated with the number of photons (3-->2 or 4-->3) needed to promote a ground state electron across the energy gap. The wavelength dependence of the damage threshold suggests that a primary mechanism for damage initiation in DKDP is a multiphoton process in which the order is reduced through excited defect state absorption.

Two-color photoionization spectroscopy using vacuum ultraviolet synchrotron radiation and infrared optical parametric oscillator laser

A two-color, vacuum ultraviolet (VUV)-infrared (IR), photoionization study using monochromatized undulator VUV synchrotron radiation and high-repetition rate IR optical parametric oscillator (OPO) laser source has been demonstrated at the Chemical Dynamics Beamline of the Advanced Light Source. The OPO setup, optical alignment, and ion detection arrangement are described. The VUV-IR ionization spectrum of Ar has been recorded in the energy region between the ionization thresholds for Ar+(2P3/2) and Ar+(2P1/2) to illustrate the feasibility and efficacy of this scheme as a general approach for high-resolution two-color photoionization studies. The autoionization resonances of the Ar(np′ and nf′) Rydberg series are resolved up to the principal quantum number of n=57, showing that the optical resolution of 0.3 cm−1 (full width at half maximum) achieved in this two-color photoionization study is mostly limited by the bandwidth of the IR OPO laser and the Doppler width due to the random motion of the Ar sample.

On the First Overtone Spectra of Protonated Water Clusters [H+(H2O)3–5] in the Free‐OH Stretch Region

AbstractThe gas‐phase near‐infrared spectra of protonated water clusters [H+(H2O)n, n = 3–5] have been obtained using a vibrational predissociation ion trap spectrometer coupled with a tunable optical parametric oscillator laser. The spectral bands in the frequency range from 7000 to 7500 cm−1 are assigned to the first overtone vibrations of non‐hydrogen‐bonded (free) OH stretches. Assignments of the spectra are made with reference to the data for H2O vapor, (H2O)2 in Ar matrices, and considering the presence of the excess proton in the clusters. Blue shifts of the bands are observed to increase with cluster size and are interpreted in terms of a charge screening effect. The observations have allowed determination of the anharmonicity coefficients of these cluster ions on the basis of the calculations using the term value expression of vibrational energy.

The role of the laser pulse duration in infrared matrix-assisted laser desorption/ionization mass spectrometry

The role of the laser pulse duration in matrix-assisted laser desorption/ionization mass spectrometry with infrared lasers (IR-MALDI-MS) emitting in the 3 μm wavelength range has been evaluated. Mass spectrometric performance and characteristics of the IR-MALDI process were examined by comparing a wavelength-tuneable mid-infrared optical parametric oscillator (OPO) laser of 6 ns pulse duration, tuned to wavelengths of 2.79 and 2.94 μm, with an Er:YAG laser (λ = 2.94 μm) with two pulse durations of 100 and 185 ns, and an Er:YSGG laser (λ = 2.79 μm) with a pulse duration of 75 ns. Threshold fluences for the desorption of cytochrome C ions were determined as a function of the laser pulse duration for various common IR-MALDI matrices. For the majority of these matrices a reduction in threshold fluence by a factor of 1.2–1.9 was found by going from the 75–100 ns long pulses of the Erbium lasers to the short 6 ns OPO pulse. Within the experimental accuracy threshold fluences were equal for the 100 and the 185 ns pulse duration of the Er:YAG laser. Some pronounced pulse duration effects related to the ion formation from a glycerol matrix were also observed. The effect of the laser pulse length on the duration of ion emission was furthermore investigated.