Picosecond Probe Pulse Research Articles

Broadband Coherent Raman Scattering Spectroscopy at 50,000,000 Spectra per Second

Raman scattering spectroscopy is widely used as an analytical technique in various fields, but its measurement process tends to be slow due to the low scattering cross-section. In the last decade, various broadband coherent Raman scattering spectroscopy techniques have been developed to address this limitation, achieving a measurement rate of 500 kSpectra/s. Here, we present a substantially increased measurement rate of 50 MSpectra/s, which is 100 times higher than the previous state-of-the-art, by developing time-stretch coherent Raman scattering spectroscopy. Our newly developed system, based on a mode-locked Yb fiber laser, enables highly efficient broadband excitation of molecular vibrations via impulsive stimulated Raman scattering with an ultrashort femtosecond pulse and sensitive time-stretch detection with a picosecond probe pulse at a high repetition rate of the laser. As a proof-of-concept demonstration, we measure broadband coherent Stokes Raman scattering spectra of organic compounds covering the molecular fingerprint region from 200 to 1,200 cm −1 . This high-speed broadband vibrational spectroscopy technique holds promise for unprecedented measurements of sub-microsecond dynamics of irreversible phenomena and extremely high-throughput measurements.

Analysis of picosecond coherent anti-Stokes Raman spectra for gas-phase diagnostics

We present a hybrid frequency- and time-domain solution, applicable to the case of picosecond coherent anti-Stokes Raman scattering (CARS), for gas-phase diagnostics. A solution has been derived based on both physical arguments and four-wave mixing equations for picosecond CARS, with pulse durations that are comparable to the dephasing time scale for gas-phase Raman coherence—a regime where commonly employed solutions for impulsive (femtosecond) or cw (nanosecond) pump/Stokes forcing are not strictly valid. We present the ps-CARS spectrum in the form of incoherent sums of CARS intensity spectra, calculated from the fundamental solution for impulsive pump/Stokes Raman preparation. The solution was examined for temperatures from 1000–3000 K, for four plausible experimental configurations, with laser pulse durations of 50–150 ps, and probe pulse delays from −20 to 240 ps. Approximations based on cw and impulsive pump/Stokes preparation to fit picosecond CARS spectra at atmospheric pressure were examined and the relative thermometric accuracy and computational cost of these approximations were quantified for the case of a zero nonresonant CARS contribution, and a nonresonant susceptibility equal to 10% of the Raman-resonant value at the N2 bandhead. The nanosecond CARS approximation can result in large fitting errors when the probe pulse time delay is less than the probe pulse duration. Errors as large as 10–20% are observed in the fit temperatures for a zero picosecond probe pulse delay, when the nonresonant background is neglected, largely due to an inability of the time-independent cw model to capture transient frequency spread dephasing effects at the Q-branch bandhead. The inclusion of a nonresonant background results in 40–60% thermometry errors with a nanosecond model at a zero-probe delay. Time-dependent impulsive calculations used for femtosecond CARS better approximate the structure of the N2 bandhead, reducing temperature fitting errors to 5–10% at a short probe pulse delay. The impulsive approximation results in errors up to 10% at intermediate probe pulse delays, where the coherence of the pump and probe pulses leads to multiple terms in the picosecond CARS solution. Both approximations improve as the probe pulse delay exceeds the probe duration. The nanosecond approximation results in a 2–3% error, while the impulsive model results in differences of less than 1% in some cases. Fits to experimental data obtained using short, ∼60ps pulses at a zero probe time delay and longer 100 ps pulses at a substantial 200 ps delay are presented with accuracies of 1–3% in the fit temperature.

Discriminating Congested Vibrational Peaks of Condensed Organic Materials with Time- and Frequency-Resolved Coherent Anti-Stokes Raman Scattering Spectroscopy

The spectral congestion of highly overlapping vibrational peaks of molecules in condensed phases is a persistent challenge in conventional linear vibrational spectroscopy, making it difficult to accurately determine the spectroscopic parameters. This study demonstrates the utility of time- and frequency-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy with a time-delayed picosecond probe pulse in resolving congested C-H stretching vibrational peaks of condensed organic matters. The results show that the overlapping vibrational peaks of polymeric films and oily liquids, which are not easily distinguishable in spontaneous Raman spectroscopy, can be separated in the time-resolved CARS (tr-CARS) spectra. To understand the physical basis of the enhanced spectral resolution, we examine the time series of CARS spectra obtained by varying the delay time between the pump and probe pulses. The global fit analysis indicates that the effective suppression of faster Raman free-induction-decay components and instantaneous nonresonant background signals contributes to improved spectral resolution. Additionally, the present study reveals that the CARS spectra at a sufficient probe delay time are highly sensitive to the incident and detection polarizations, further improving vibrational peak distinguishability through polarization-controlled tr-CARS.

Rotational hydrogen thermometry by hybrid fs/ps coherent anti-Stokes Raman scattering in the plume of a burning metalized propellant

We employed a hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) instrument to probe rotational temperatures of molecular hydrogen in the multiphase reaction zone of an aluminized ammonium perchlorate (AP) propellant flame. Significant concentrations of hydrogen, present in the plume due to the decomposition of the propellant binder material and subsequent reactions with AP oxidizers, allowed for single-shot thermometry at the laser repetition rate of 1 kHz. A time-asymmetric picosecond probe pulse time-gated the impulsively generated Raman coherence at a delay of 2.66 ps from the pump and Stokes pulses, before any appreciable coherence dephasing occurred in the atmospheric pressure flames and mitigating uncertainties in the Raman transition frequencies and dephasing processes. Measurements in near-adiabatic H2-air flames at equivalence ratios of $$\phi$$ = 1–1.8 demonstrated measurement accuracy to near 5% of equilibrium predictions with a precision approaching 3% for high signal-to-noise ratio spectra. Introduction of a time-delayed probe pulse provided Raman-resonant spectra from the plumes of burning propellants with ample signal above broadband background emission, which were fit to libraries of synthetic spectra to infer the gas rotational temperature 0–15 mm from the burning surface. The mean fitted temperature of 2494 K from three propellant burns compares favorably to other measurements of gas and particle temperatures in similar propellant studies.

Inline measurement of modal phase differences for the characterization of intermodal cross-phase modulation

We present an inline measurement technique for the phase difference between two transverse fiber modes. This technique proves to be well suited for direct measurement of an all-optically changed phase difference between two transverse modes of picosecond probe pulses, induced via intermodal cross-phase modulation by co-propagating picosecond control pulses. We achieved unprecedented agreement with the corresponding analytical description for time-independent changes of the phase difference as well as for time-dependent changes on an intra-pulse timescale. The simple and accurate predictability combined with the ability to measure modal phase differences inline poses a key step towards full all-optical control over transverse modes in fibers. This may lead the way to a variety of novel functionalities for all-fiber applications, like a spatial degree of freedom in fiber-sensing technology by using transverse fiber modes or all-optical spatial and temporal pulse shaping inside a fiber.

Rovibrational hybrid fs/ps CARS using a volume Bragg grating for N₂ thermometry.

Coherent anti-Stokes Raman scattering (CARS) spectra of N2 in the hybrid femtosecond/picosecond regime have been recorded with 0.7 cm(-1) resolution. The Q-branch rovibrational structure has been resolved, making it suitable for gas-phase simultaneous rotational and vibrational thermometry applications. Resolving this spectral structure requires synchronization of a narrowband picosecond probe pulse with a broadband femtosecond pair of pump and Stokes pulses. It is achieved using a single femtosecond ytterbium-laser source and a volume Bragg grating in a compact experimental arrangement.

Ultrafast two-color all-optical transverse mode conversion in a graded-index fiber.

We report on experimental conversion of picosecond probe pulses from the fundamental mode (LP01) to the next higher-order mode (LP11) by optically induced transient long-period gratings in a graded-index fiber using subpicosecond control pulses. In contrast to former experiments, the temporally synchronized control (1030 nm) and probe pulses (1250 nm) were easily distinguishable by their wavelengths, allowing for the first direct observation of such an ultrafast transverse mode conversion. Despite a nonperfect pulse duration ratio between control and probe pulses as well as an unavoidable excitation of unwanted higher-order modes, a clear energy conversion of 18% between the LP01-mode and the LP11-mode was observed at 2.3 times lower control pulse energies compared to previous experiments.

Biological imaging with nonlinear photothermal microscopy using a compact supercontinuum fiber laser source.

Nonlinear photothermal microscopy is applied in the imaging of biological tissues stained with chlorophyll and hematoxylin. Experimental results show that this type of organic molecules, which absorb light but transform dominant part of the absorbed energy into heat, may be ideal probes for photothermal imaging without photochemical toxicity. Picosecond pump and probe pulses, with central wavelengths of 488 and 632 nm, respectively, are spectrally filtered from a compact supercontinuum fiber laser source. Based on the light source, a compact and sensitive super-resolution imaging system is constructed. Further more, the imaging system is much less affected by thermal blurring than photothermal microscopes with continuous-wave light sources. The spatial resolution of nonlinear photothermal microscopy is ~ 188 nm. It is ~ 23% higher than commonly utilized linear photothermal microscopy experimentally and ~43% than conventional optical microscopy theoretically. The nonlinear photothermal imaging technology can be used in the evaluation of biological tissues with high-resolution and contrast.

Direct Coherent Raman Temperature Imaging and Wideband Chemical Detection in a Hydrocarbon Flat Flame.

A single-shot coherent Raman imaging technique has been developed for spatially correlated one-dimensional high-fidelity gas-phase thermometry and multiplex chemical detection in flames. The technique utilizes two-beam phase matching, operating a single ultrashort pump/Stokes excitation pulse (7 fs) and a narrowband picosecond probe pulse (70 ps), interrogating a Raman active window of ∼4200 cm(-1) with ∼0.3 cm(-1) spectral resolution. The measurement geometry is formed intersecting the two beams shaped as laser-sheets and the one-coordinate spatial information is retrieved with a linespread function of <40 μm. The advance provides the possibility for the multiplexed measurement of all combustion relevant major species simultaneously with gaseous temperature monitored over a several millimeter field of view. The current technique is optimized in a premixed hydrocarbon flat-flame. At the flame-front, it is shown that direct imaging renders the temperature profile within ∼1% inaccuracy, whereas typical point-wise raster scanning may have relative systematic deviations up to 15% due to spatial averaging effects.

Numerical modeling of evolution of nano- and picosecond probe pulses of electromagnetically induced transparency

Results of numerical analysis of the influence of probe pulse duration on the character of its evolution under the conditions of electromagnetically induced transparency in a Λ-scheme involving degenerate quantum transitions are presented. The cases of weak and strong input probe pulses linearly polarized in the same direction as the polarized coupling radiation, along with the case of linearly polarized probe radiation and circularly polarized coupling radiation, are considered. Calculations showed that the regime of weak probe radiation is most favorable for formation of a short probe pulse with a duration of about 1 ns and pulse shape that does not change substantially upon pulse propagation. However, even in this case, picosecond probe pulses experience considerable deformation of their shape and loss of power. Considerable changes of the pulse shape in the case of a strong probe pulse occur already when the pulse duration approaches 1 ns. This circumstance can limit the response time of prospective devices based on the effect of electromagnetically induced transparency.

Generation of vacuum-ultraviolet pulses with a Doppler-broadened gas utilizing high atomic coherence

We carry out the theoretical study for the generation of vacuum-ultraviolet pulses with a Doppler-broadened gas utilizing high atomic coherence. It is essentially a difference-frequency generation scheme induced by the two-photon near-resonant pump and probe pulses, where the key point is to generate high atomic coherence between the ground and two-photon near-resonant states through a variant of stimulated Raman adiabatic passage with a time-dependent detuning. The advantage of our scheme is that the degree of coherence is sensitive to neither the exact amount and even sign of the detuning, nor the exact timing between the pump, auxiliary, and probe pulses. Hence our scheme is practically insensitive to Doppler broadening. As a specific example, we consider the generation of picosecond Lyman-α pulses with a Kr gas, and quantitatively study the influence of Doppler broadening as well as the intensity and incident timing of the picosecond probe pulse with respect to the pump pulse. The numerical results indicate that our scheme has a certain advantage over the conventional scheme which utilizes two-photon resonant excitation.

Coherent anti-Stokes Raman scattering with broadband excitation and narrowband probe

An improved CARS setup based on noncollinear optical parametric amplifiers (NOPAs) is presented which combines broad tunability and a wide excitation bandwidth with good spectral and temporal resolution. Picosecond Raman pump and probe pulses are generated by a modified narrowband NOPA. Combining them with sub-50 fs Stokes pulses results in highly time resolved CARS spectra with line widths down to 20 cm-1. The determination of a vibrational decoherence time is demonstrated for chloroform. Beating phenomena in case of overlapping Raman bands and an increase of spectral structure for coalescing bands are observed for cyclohexane and an ionic liquid respectively.

Methods for studying the coherent 4D structural dynamics of free molecules and condensed state of matter

Studies in the coupled 4D spatial and temporal continuum are necessary for understanding the dynamic features of molecular systems with a complex profile of the potential energy surface. The introduction of time sweep into diffraction methods and the development of principles for studying coherent processes have revealed new approaches to the analysis of the dynamics of wave packets, the intermediate products and the transition state of the reaction center, and short-lived compounds in gaseous and condensed media. The use of picosecond and femtosecond electron probe pulses, synchronized with excitation laser pulses, determined the development of ultrafast electron crystallography, time-resolved X-ray diffraction, and dynamic transmission electron microscopy (DTEM). One of the most promising applications of the developed diffraction methods is the characterization and visualization of the processes occurring upon the photoexcitation of free molecules and biological objects and the analysis of surface and thin films. The whole set of spectral and diffraction methods based on different physical principles, which are complementary and make it possible to perform the photoexcitation of nuclei and electrons and carry out diagnostics of their dynamics at ultrashort time sequences, reveal new possibilities for studies with the necessary integration of the “structure-dynamics-function” triad in chemistry, biology, and materials science.

Spatial–spectral distribution of Rabi radiation generated in plasma

We report observation and control of the spatial-spectral distributions of coherent, dynamic Rabi sideband radiation. The Rabi sidebands result from the interaction of a shaped picosecond probe laser of intensity 10(10) W cm(-2), with neutral excited atomic oxygen generated in a laser-induced microplasma. The spatial-spectral distribution is measured and compared for picosecond laser pulses having either an asymmetric temporal shape or a Gaussian temporal shape. The resulting spatial-spectral distributions are in quantitative agreement with theoretical predictions that account for the radial intensity distribution of the picosecond probe pulse.

Observation of parametric gain due to four-wave mixing in dispersion engineered GaInP photonic crystal waveguides

We investigate four-wave mixing (FWM) in GaInP 1.5 mm long dispersion engineered photonic crystal waveguides. We demonstrate an 11 nm FWM bandwidth in the CW mode and a conversion efficiency of -24 dB in the quasi-CW mode. For picosecond pump and probe pulses, we report a 3 dB parametric gain and nearly a -5 dB conversion efficiency at watt-level peak pump powers.

Dynamic Rabi sidebands in laser-generated microplasmas: Tunability and control

Broadband, coherent radiation in the optical-frequency range is generated using microplasma channels in atmospheric gases in a pump-probe experiment. A microplasma medium is created in a gas by a focused intense femtosecond pump pulse. A picosecond probe pulse then interacts with this microplasma channel, producing broad, coherent sidebands that are associated with luminescence lines and are redshifted and blueshifted with respect to the laser carrier frequency. These sidebands originate from the induced Rabi oscillations between pairs of excited states that are coupled by the probe pulse. Thus the sideband radiation intensity tracks the microplasma evolution. The sidebands arise from broad and tunable Rabi shifts corresponding to varying values of the electric-field magnitude in the probe pulse. The \ensuremath{\sim}10${}^{10}$ W cm${}^{\ensuremath{-}2}$ probe beam creates a maximum sideband shift of g90 meV from the carrier frequency, resulting in an effective bandwidth of 200 meV. The sidebands can be tuned and controlled by the intensity and temporal profile of the probe pulse. The fact that the coherence is observed in a microplasma demonstrates that Rabi cycling is possible at high temperature with moderately high laser intensities as long as transitions close to the driving frequency ($\ensuremath{\Delta}$\ensuremath{\sim}2% ${\ensuremath{\omega}}_{c}$) are available. Plasma excitation combined with Rabi-shifting measurements also serves as a means to simultaneously extract quantitative ratios for the transition-dipole moments between multiple sets of highly excited states with transitions in the optical regime.

Suppression of Raman-resonant interferences in rotational coherent anti-Stokes Raman spectroscopy using time-delayed picosecond probe pulses

We measure time-dependent pure-rotational coherent anti-Stokes Raman spectroscopy (CARS) spectra for room-temperature N(2), O(2), CO(2), C(2)H(4), and C(3)H(8), as well as in a C(3)H(8) diffusion flame, using picosecond lasers. Because Raman coherences for N(2) and O(2) persist significantly longer than those for the other species, delayed probing can significantly reduce unwanted resonant contributions to rotational coherent anti-Stokes Raman spectroscopy spectra, enabling temperature and relative O(2)/N(2) concentration determination in fuel-rich gas mixtures. Delayed probing also eliminates interference from smeared vibrational CARS. Probe delay affects both the temperature and relative O(2)/N(2) concentrations inferred from rotational spectra when using a standard frequency-domain analysis.

Comprehensive Finite-Difference Time-Dependent Beam Propagation Model of Counterpropagating Picosecond Pulses in a Semiconductor Optical Amplifier

In this paper, we present a numerical model to study counter pulse propagation in semiconductor optical amplifiers. An improved finite-difference beam propagation method for solving the modified nonlinear Schrodinger equation is applied for the first time in the counterpropagation regime. In our model, group velocity dispersion, two-photon absorption, ultrafast nonlinear refraction, and the change in the gain peak wavelength with carrier density are included, which have not been considered simultaneously in previous counterpropagation models. The model is applied to demonstrate how a subpicosecond and picosecond probe pulse shape and spectrum can be modified by a counterpropagating pump pulse. Based on the results obtained by this model, while subpicosecond probe pulses can be compressed by in this scheme, their time-bandwidth product are also improved significantly. Furthermore, the effects of several parameters are analyzed to obtain the proper probe spectral peak shift using counterpropagating probe pulses. The accuracy and computational efficiency of the new scheme are assessed through numerical examples and are shown to be superior to previously published approaches.

Time-resolved study of Raman gain in highly confined silicon-on-insulator waveguides

We show time-resolved measurement of Raman gain in Silicon submicron-size planar waveguide using picosecond pump and probe pulses. A net nonlinear gain of 6 dB is obtained in a 7-mm long waveguide with 20.7-W peak pump power. We demonstrate an ultrafast all-optical switch based on the free-carrier dispersion effect in the silicon waveguide, whose transmission is enhanced by more than 13 dB due to the Raman effect.

Picosecond time-resolved Raman system for studying photochemical reaction dynamics: application to the primary events in vision

A time-resolved Raman system was developed to study the picosecond reaction dynamics of energy and information transducing proteins. This system is based on a 1 kHz Nd : YLF-pumped Ti-sapphire regenerative amplifier that delivers picosecond pump and probe pulses with high temporal and spectral resolution. Using second-harmonic generation and Raman shifting, near transform-limited picosecond pulses are generated at wavelengths ranging from the UV to the near-infrared (190–850 nm) with ∼2 ps temporal width. This laser system, together with a CCD-detected spectrograph, facilitates the performance of picosecond Raman experiments over a much wider range of pump and probe wavelengths than previously possible with low repetition rate amplified dye laser systems. The construction and performance of the laser system is presented first, then single-pulse and two-color, pump–probe experiments examining the primary reaction dynamics of the visual protein rhodopsin are presented to illustrate the system performance. The strong characteristic hydrogen out-of-plane modes of the bathorhodopsin photoproduct are observed at 850, 873 and 920 cm−1 in the shortest time resolution data. This demonstrates that the trans-bathorhodopsin retinal chromophore structure is completely formed in <2 ps after photoexcitation. Copyright © 1999 John Wiley & Sons, Ltd.