Steady-state and transient ultraviolet resonance Raman spectrometer for the 193-270 nm spectral region

We describe a state-of-the-art tunable ultraviolet (UV) Raman spectrometer for the 193-270 nm spectral region. This instrument allows for steady-state and transient UV Raman measurements. We utilize a 5 kHz Ti-sapphire continuously tunable laser (approximately 20 ns pulse width) between 193 nm and 240 nm for steady-state measurements. For transient Raman measurements we utilize one Coherent Infinity YAG laser to generate nanosecond infrared (IR) pump laser pulses to generate a temperature jump (T-jump) and a second Coherent Infinity YAG laser that is frequency tripled and Raman shifted into the deep UV (204 nm) for transient UV Raman excitation. Numerous other UV excitation frequencies can be utilized for selective excitation of chromophoric groups for transient Raman measurements. We constructed a subtractive dispersion double monochromator to minimize stray light. We utilize a new charge-coupled device (CCD) camera that responds efficiently to UV light, as opposed to the previous CCD and photodiode detectors, which required intensifiers for detecting UV light. For the T-jump measurements we use a second camera to simultaneously acquire the Raman spectra of the water stretching bands (2500-4000 cm(-1)) whose band-shape and frequency report the sample temperature.     

Simple nanosecond to minutes transient absorption spectrophotometer

We built a transient absorption spectrophotometer that can determine transient absorption spectral changes that occur at times as fast as approximately 200 ns and as slow as a minute. The transient absorption can be induced by a temperature-jump (T-jump) or by optical pumping from the deep ultraviolet (UV) to the infrared (IR) by use of single ns Nd:YAG laser pulses. Our use of a fiber-optic spectrometer coupled to a XeF flashlamp makes the collection of transient spectra easy and convenient in the spectral range from the near IR (1700 nm) down to the deep UV (200 nm), with high signal-to-noise (S/N) ratios. The spectral resolution is determined by the specific configuration of the fiber-optic spectrometer (grating groove density, fiber diameter, slit width) and varies between 0.3 and 10 nm. The utility of this spectrometer was demonstrated by measuring the rate at which a polymerized crystalline colloidal array (PCCA) of poly(N-isopropylacrylamide) nanogel particles optically switch light due to a T-jump induced by nanosecond 1.9 microm laser pulses. In addition, we measured the rate of optical switching induced by a 3 ns 355 nm pump pulse in PCCA functionalized with azobenzene.     

Tunable ultrafast infrared/visible laser to probe vibrational dynamics

A tunable, ultrafast (approximately 100 fs-approximately 1 ps) laser system generating mid-IR (3-10 microm) and UV/visible (392-417 nm, 785-835 nm) radiation is described and its output characterized. The system is designed to explore vibrational dynamics in the condensed phase in a direct, two-pulse, time-resolved manner, using Raman spectroscopy as the probe. To produce vibrational resolution, probe pulses are spectrally narrowed by use of a long doubling crystal. Frequency-resolved optical gating is used to evaluate beam characteristics. An effective method for determining the temporal overlap of the pump and probe pulses for a one-color, 400 nm configuration is illustrated. Representative results from studies of heme and paranitroaniline vibrational dynamics illustrate the effectiveness of the visible pump-visible probe portion of the system in illuminating fast structure and energy dynamics.     

Optical Studies of Nd-doped benzil, a potential luminescent and laser material

Neodymium-doped benzil crystals have been synthesized and characterized for their absorption, emission, and kinetics properties. From Judd-Ofelt analysis, the radiative decay time of Nd emission (peaking at 1055 nm) is estimated to be equal to 441 mus. The experimental Nd lifetime (under Ar+ laser excitation) is equal to 19 mus. The broad emission band centered at approximately 700 nm (tau(decay) approximately 15 ns) and the Raman scattering with characteristic frequency shift of 1600 cm(-1) have been observed at excitation of benzil with 532-nm Q-switched laser pulses. We show that rare-earth-doped benzil can be considered as a potential candidate for luminescent and solid-state laser material.     

Solid hydrogen Raman shifter for the mid-infrared range (4.4-8 microm)

We developed a pulsed, continuously tunable laboratory laser source for the mid-infrared spectral range of 4.4-8 microm, which is characterized by the spectral linewidth of 0.4 cm(-1). The device is based on the stimulated backward Raman scattering in solid para-hydrogen at T = 4 K. It is pumped by a focused beam obtained from a commercial near-infrared optical parametric oscillator with output energy of approximately 20 mJ (7-ns pulse). Output energies range from 1.7 mJ at 4.4 microm to 120 microJ at 8 microm, which correspond to quantum efficiencies of 0.53 and 0.08, respectively. Spectra of NO, H2O, and CH4 molecules in the mid-infrared were recorded. The operation of the Raman cell pumped with 532-nm radiation was also studied.     

Saturable absorber Co2+:MgAl2O4 crystal for Q switching of 1.34-microm Nd3+:YAlO 3 and 1.54-microm Er3+:glass Lasers

Saturable-absorber Q switching of a neodymium-doped YAlO(3) laser at 1.34 microm and an erbium-doped glass laser at 1.54 microm with a Co(2+):MgAl(2)O(4) crystal was demonstrated. Q-switched 1.34-microm pulses of 19-mJ energy and 60-ns duration and Q-switched 1.54-microm pulses of 2.7-mJ energy and 75-ns width were obtained. The ground-state absorption cross sections of the Co(2+):MgAl(2)O(4) crystal were estimated to be (2.8 +/- 0.4) x 10(-19) and (3.5 +/- 0.6) x 10(-19) cm(2) at 1.34 and 1.54 microm, respectively. The (4)T(1)((4)F)-->(4)A(2) relaxation time of the Co(2+) ion in the MgAl(2)O(4) crystal was measured to be approximately 350 ns.     

Tunable excitation source for coherent Raman spectroscopy based on a single fiber laser

We demonstrate a wavelength tunable optical excitation source for coherent Raman scattering (CRS) spectroscopy based on a single femtosecond fiber laser. Electrically controlled wavelength tuning of Stokes optical pulses was achieved with soliton self frequency shift in an optical fiber, and linear frequency chirping was applied to both the pump and the Stokes waves to significantly improve the spectral resolution. The coherent anti-Stokes Raman scattering (CARS) spectrum of cyclohexane was measured and vibrational resonant Raman peaks separated by 70?cm(-1) were clearly resolved. Single laser-based tunable excitation may greatly simplify CRS measurements and extend the practicality of CRS microscopy.     

Programmable select multiwavelength gigahertz Raman soliton pulse generation

The generation of programmable multiwavelength pulses based on the self-frequency shift of a Raman soliton is demonstrated. The approach produces tunable multiwavelength picosecond pulses. Only select multiwavelength signals with a tuning range of approximately 50 nm are generated with a repetition rate of 9.95 GHz at each wavelength channel. A bit error rate (BER) of better than 1 x 10(-9) was successfully obtained for all the measured multiwavelength Raman soliton pulses. Furthermore, it was found that the signal has an excellent relative intensity noise (RIN) of better than -135.5 dBc/Hz. The BER and RIN measurements show that the frequency-shifted Raman soliton pulses are promising for use in measurement systems, optical gating, signal processing, and wavelength routing optical packet networks with the ability to provide 1:1 communication and 1:N multicasting.     

Transmission fourier transform Raman spectroscopy of pharmaceutical tablet cores

Transmission Fourier transform (FT) Raman spectroscopy of pharmaceutical tablet cores is demonstrated using traditional, unmodified commercial instrumentation. The benefits of improved precision over backscattering Raman spectroscopy due to increased sample volume are demonstrated. Self-absorption effects on analyte band ratios and sample probe volume are apparent, however. A survey of near-infrared (NIR) absorption spectra in the FT-Raman spectral range (approximately 0 to 3500 wavenumber shift from 1064 nm, or 1064 to 1700 nm) of molecules with a wide range of NIR-active functional groups shows that although absorption at the laser wavelength (1064 nm) is relatively small, some regions of the Raman spectrum coincide with NIR absorbances of 0.5 per cm or greater. Fortunately, the pharmaceutically important regions of the Raman shift spectrum from 0 to 600 cm(-1) and from 1400 to 1900 cm(-1) exhibit low self-absorption for most organic materials. A statistical analysis of transmission FT-Raman noise in spectra collected from different regions of a pharmaceutical tablet provides insight into both spectral distortion and reduced sampling volume caused by self-absorption.     

Raman shifter optimized for lidar at a 1.5 microm wavelength

A Raman shifter is optimized for generating high-energy laser pulses at a 1.54 microm wavelength. A forward-scattering design is described, including details of the multiple pass and nonfocused optical design, Stokes injection seeding, and internal gas recirculation. First-Stokes conversion efficiencies up to 43%--equivalent to 62% photon conversion efficiency--were measured. Experimental results show output average power in excess of 17.5 W, pulse energies of 350 mJ at 50 Hz, with good beam quality (M2<6). Narrow bandwidth and tunable output is produced when pumping with a single longitudinal mode Nd:YAG laser and seeding the process with a Stokes wavelength narrowband laser diode.     

Near-infrared spectroscopic determination of salinity and internal pressure of fluid inclusions in minerals

A near-infrared (NIR) spectroscopic method is proposed to achieve the simultaneous determination of salinity and internal pressure of fluid inclusions in natural minerals. A combination band between the anti-symmetric stretching and bending vibrations of molecular water at approximately 5180 cm-1 was observed for standard salt solutions and natural minerals containing fluid inclusions with known salinities. A curve-fitting procedure was used to analyze the change in the band shape of the combination. Justification of the calibration was confirmed by observation of fluid inclusions in natural minerals whose salinities had already been determined using microthermometry. The detection limit of the present method is 1 NaCl-eq wt. %. The minimum size of fluid inclusions that produced well-resolved spectra was approximately 30 microm. This method was applied to assess micro fluid inclusions in a natural diamond with cubic growth habit (cuboid). The salinity and residual pressure of those fluid inclusions were estimated respectively as 4.4 wt. % NaCl-eq and 0.6-0.8 GPa. The present method is complementary to Raman microscopy and microthermometry for the determination of salinity in fluid inclusions of geological samples.     

Raman microspectroscopy: a comparison of point, line, and wide-field imaging methodologies

Three different Raman microspectroscopic imaging methodologies using a single experimental configuration are compared; namely, point and line mapping, as representatives of serial imaging approaches, and direct or wide-field Raman imaging employing liquid-crystalline tunable filters are surveyed. Raman imaging data acquired with equivalent low-power 514.5-nm laser excitation and a cooled CCD camera are analyzed with respect to acquisition times, image quality, spatial resolution, intensity profiles along spatial coordinates, and spectral signal-to-noise ratios (SNRs). Point and line mapping techniques provide similar SNRs and reconstructed Raman images at spatial resolutions of approximately 1.1 microm. In contrast, higher spatial resolution is obtained by direct, global imaging (approximately 313 nm), allowing subtle morphological features on test samples to be resolved.     

Mars laser hygrometer

We have designed and built a miniature near-IR tunable diode laser (TDL) spectrometer for measuring in situ the water vapor mixing ratio either in the Martian atmosphere or thermally evolved from Martian soil or ice samples. The laser hygrometer uses a thermoelectrically cooled single-mode distributed-feedback TDL at 1.87 microm to scan over a selected vibration-rotation line of both H2O and CO2 near 5327.3 cm(-1). A working prototype that weighs only 230 g has been built and used to generate spectra whose analysis demonstrates precision sensitivities as fine as 1 part in 10(6) by volume in 1 s or 0.1 part in 10(6) in 10 s at Martian pressures and temperatures. Absolute uncertainties of approximately 5% are calculated.     

Doppler width limited near-infrared Raman spectrometer

A new ultra high resolution spontaneous Raman spectrometer with a single mode tunable laser diode as an excitation source and a 0.275 m, f/#4 spectrograph is presented. The spectrometer is expanded by an atomic vapor (Rb) absorption filter. One of the rubidium resonance lines serves as the frequency marker by absorbing selected Raman lines during the frequency scan of the excitation laser. High resolution was achieved while preserving the speed of the spectrometer. The extended spectrometer's capability was tested by differentiating overlapping Raman lines from molecular hydrogen isotopomers: H2, D2, and HD. Experiments were carried out at 300 K and pressures near 10(4) Pa. Spectral lines separated by approximately 10 Ghz (0.3 cm-1) can be resolved with this instrument with data collection times of minutes.     

Multipass acoustically open photoacoustic detector for trace gas measurements

What we believe to be a novel multipass, acoustically open photoacoustic detector designed for fast-response, high-sensitivity detection of trace gases and pollutants in the atmosphere is demonstrated. The acoustic pulses generated by the absorption of the light pulses of a tunable optical parametric oscillator by target molecules are detected by an ultrasonic sensor at 40 kHz. The photoacoustic signal is enhanced by an optical multipass arrangement and by concentration of the acoustic energy to the surface of the ultrasonic sensor. The detection sensitivity, estimated from CO2 measurements around a 2 microm wavelength, is approximately 3.3 x 10(-9) W cm(-1).     

On-line multicomponent trace-gas analysis with a broadly tunable pulsed difference-frequency laser spectrometer

The design and application of a novel automated room-temperature laser spectrometer are reported. The compact instrument is based on difference-frequency generation in bulk LiNbO(3). The instrument employs a tunable cw external-cavity diode laser (795-825 nm) and a pulsed diode-pumped Nd:YAG laser (1064 nm). The generated mid-IR nanosecond pulses of 50-muW peak power and 6.5-kHz repetition rate, continuously tunable from 3.16 to 3.67 mum, are coupled into a 36-m multipass cell for spectroscopic studies. On-line measurements of methane are performed at concentrations between 200 ppb (parts in 10(9) by mole fraction) and approximately 1%, demonstrating a large dynamic range of 7 orders of magnitude. Furthermore computer-controlled multicomponent analysis of a mixture containing five trace gases and water vapor with an overall response time of 90 s at an averaging time of only approximately 30 s is reported. A minimum detectable absorption coefficient of 1.1 x 10(-7) cm(-1) has been achieved in an averaging time of 60 s, enabling detection limits in the ppb range for many important trace gases, such as CH(4), C(2)H(6), H(2)CO, NO(2), N(2)O, HCl, HBr, CO, and OCS.     

Development of a Tunable, Narrow-Linewidth, CW 2.066-mum Ho:YLF Laser for Remote Sensing of Atmospheric CO(2) and H(2)O

A smoothly tunable, narrow-linewidth, cw, 32-mW, 2.066-mum Ho:YLF laser was constructed and used for the first time in preliminary spectroscopic measurements of atmospheric CO(2) and H(2)O. The laser was constructed with a 4.5-mm-long, TE-cooled, codoped 5% Tm and 0.5% Ho yttrium lithium fluoride crystal (cut at Brewster's angle) pumped by an Ar(+)-pumped 500-mW Ti:sapphire laser operating at 792 nm. Intracavity etalons were used to reduce the laser linewidth to approximately 0.025 cm(-1) (0.75 GHz), and the laser wavelength was continuously and smoothly tunable over approximately 6 cm(-1) (180 GHz). The Ho:YLF laser was used to perform spectroscopic measurements on molecular CO(2) in a laboratory absorption cell and to measure the concentration of CO(2) and water vapor in the atmosphere with an initial accuracy of approximately 5-10%. The measurement uncertainty was found to be due to several noise sources, including the effect of asymmetric intensity of the laser modes within the laser linewidth, fluctuations caused by atmospheric turbulence and laser beam/target movement, and background spectral shifts.     

Development of the 2.8 microm emission doubly shifted Raman laser using stimulated Brillouin scattering in a cascaded cavity

A doubly shifted Raman laser using CH(4) gas has been developed for 2.8 microm generation, pumped by a Nd:YAG laser with 65.5 mJ at 17 ns. A dichroically coated meniscus-type lens is modified to utilize the backward stimulated Brillouin scattering and backward Stokes beams from a previous laser design [Appl. Opt.46, 5516-5521 (2007)APOPAI0003-693510.1364/AO.46.005516]. A maximum output energy of 4.76 mJ at 2.80 microm wavelength has been achieved in the cascaded resonator. A maximum conversion efficiency of 8.9% has been achieved at a CH(4) gas pressure of 600 psi. The obtained spatial beam profile is quite smooth, and its output pulse width is 10 ns.     

Tunable kHz deep ultraviolet (193-210 nm) laser for Raman application

The performance characteristics of a kilohertz solid-state laser source for ultraviolet Raman spectroscopy are described. Deep ultraviolet (UV) excitation in the 193-210 nm region is provided by mixing of the fundamental and third harmonics of a Ti-sapphire laser, which is pumped by the second harmonic of a Q-Switched Nd-YLF laser. The combination of tunability, narrow linewidth, high average power, good stability, and kilohertz repetition rate makes this laser suitable for deep UV resonance Raman applications. The short pulse duration (approximately 20 ns) permits nanosecond time resolution in pump-probe applications. The low peak power and high data rate provide artifact-free spectra with a high signal-to-noise ratio. UV Raman cross-section and Raman excitation profiles are reported for gaseous O2 (relative to N), aqueous ClO4-, tyrosine, phenylalanine, tryptophan, histidine, and hemoglobin excited between 193 nm and 210 nm to illustrate laser performance.     

Construction of a subnanosecond time-resolved, high-resolution ultraviolet resonance Raman measurement system and its application to reveal the dynamic structures of proteins

A subnanosecond time-resolved ultraviolet (UV) resonance Raman system has been developed to study protein structural dynamics. The system is based on a 1 kHz Nd:YLF-pumped Ti:Sapphire regenerative amplifier with harmonic generation that can deliver visible (412, 440, 458, and 488 nm) and UV (206, 220, 229, and 244 nm) pulses. A subnanosecond (0.2 ns) tunable near-infrared pulse from a custom-made Ti:Sapphire oscillator is used to seed the regenerative amplifier. A narrow linewidth of the subnanosecond pulse offers the advantage of high resolution of UV resonance Raman spectra, which is critical to obtain site-specific information on protein structures. By combination with a 1 m single spectrograph equipped with a 3600 grooves/mm holographic grating and a custom-made prism prefilter, the present system achieves excellent spectral (<10 cm(-1)) and frequency (approximately 1 cm(-1)) resolutions with a relatively high temporal resolution (<0.5 ns). We also report the application of this system to two heme proteins, hemoglobin A and CooA, with the 440 nm pump and 220 nm probe wavelengths. For hemoglobin A, a structural change during the transition to the earliest intermediate upon CO photodissociation is successfully observed, specifically, nanosecond cleavage of the A-E interhelical hydrogen bonds within each subunit at Trpalpha14 and Trpbeta15 residues. For CooA, on the other hand, rapid structural distortion (<0.5 ns) by CO photodissociation and nanosecond structural relaxation following CO geminate recombination are observed through the Raman bands of Phe and Trp residues located near the heme. These results demonstrate the high potential of this instrument to detect local protein motions subsequent to photoreactions in their active sites.