Raman Probe Pulses Research Articles

Time-Resolved Impulsive Stimulated Raman Spectroscopy with Synchronized Triple Mode-Locked Lasers.

A complete understanding of a photochemical reaction dynamics begins with real-time measurements of both electronic and vibrational structures of photoexcited molecules. Time-resolved impulsive stimulated Raman spectroscopy (TR-ISRS) with femtosecond actinic pump, Raman pump, and Raman probe pulses is one of the incisive techniques enabling one to investigate the structural changes of photoexcited molecules. Herein, we demonstrate that such femtosecond TR-ISRS is feasible with synchronized triple mode-locked lasers without using any time-delay devices. Taking advantage of precise control of the three repetition rates independently, we could achieve automatic scanning of two delay times between the three pulses, which makes both rapid data acquisition and wide dynamic range measurement of the fifth-order TR-ISRS signal achievable. We thus anticipate that the present triple mode-locked laser-based TR-ISRS technique will be of critical use for long-term monitoring of photochemical reaction dynamics in condensed phases and biological systems.

A tunable time-resolved spontaneous Raman spectroscopy setup for probing ultrafast collective excitation and quasiparticle dynamics in quantum materials.

We present a flexible and efficient ultrafast time-resolved spontaneous Raman spectroscopy setup to study collective excitation and quasi-particle dynamics in quantum materials. The setup has a broad energy tuning range extending from the visible to near infrared spectral regions for both the pump excitation and Raman probe pulses. Additionally, the balance between energy and time-resolution can be controlled. A high light collecting efficiency is realized by high numerical aperture collection optics and a high-throughput flexible spectrometer. We demonstrate the functionality of the setup with a study of the zone-center longitudinal optical phonon and hole continuum dynamics in silicon and discuss the role of the Raman tensor in time-resolved Raman scattering. In addition, we show an evidence for unequal phonon softening rates at different high symmetry points in the Brillouin zone of silicon by means of detecting pump-induced changes in the two-phonon overtone spectrum. Demagnetization dynamics in the helimagnet Cu2OSeO3 is studied by observing softening and broadening of a magnon after photo-excitation, underlining the unique power of measuring transient dynamics in the frequency domain, and the feasibility to study phase transitions in quantum materials.

Femtosecond stimulated Raman spectroscopy as a tool to detect molecular vibrations in ground and excited electronic states.

We give a detailed theoretical analysis of the simplest variant of femtosecond stimulated Raman spectroscopy, where a picosecond Raman pump pulse and a femtosecond Raman probe pulse are applied resonantly to a chromophore in thermal equilibrium in the ground electronic state. We demonstrate that this technique is capable of the detection of dephasing-free Raman-like lines revealing vibrational modes not only in the electronic ground state but also in the excited electronic state of the chromophore. The analytical results obtained with simplifying assumptions for the shape of the laser pulses are substantiated by numerical simulations with realistic laser pulses, employing the equation-of-motion phase-matching approach.

A versatile femtosecond stimulated Raman spectroscopy setup with tunable pulses in the visible to near infrared

We demonstrate a versatile and efficient setup to perform femtosecond stimulated Raman spectroscopy (FSRS). Technical innovations are implemented to achieve the wavelength tunability for both the picosecond narrowband Raman pump pulse and femtosecond broadband Raman probe pulse. Using a simplified one-grating scheme in a home-built second harmonic bandwidth compressor followed by a two-stage noncollinear optical parametric amplifier, we tune the Raman pump pulse from ca. 480 to 750 nm. To generate the suitable Raman probe pulse in tandem, we rely on our recently demonstrated broadband up-converted multicolor array technique that readily provides tunable broadband laser sidebands across the visible to near-infrared range. This unique setup has unparalleled flexibility for conducting FSRS. We measure the ground-state Raman spectra of a cyclohexane standard using tunable pump-probe pairs at various wavelengths across the visible region. The best spectral resolution is ∼12 cm−1. By tuning the pump wavelength closer to the electronic absorption band of a photoacid pyranine in water, we observe the pre-resonantly enhanced Raman signal. The stimulated Raman gain of the 1627 cm−1 mode is increased by over 15 times.

Femtosecond Stimulated Raman Study of Energy Transfer in Allophycocyanin

Allophycocyanin (APC) is a light-harvesting protein found in cyanobacteria and red algae. APC along with other phycobiliproteins such as phycocyanin (PC) and phycoerythrin (PE) constitutes phycobilisome and plays an important role in collecting light and cascading the excitation energy into a reaction center where the light energy is converted into chemical potential. Thus it is important to understand how excitation energy is transferred between the phycobiliproteins and inside them. APC is a trimer of αβ subunit, each of which contains a linear tetrapyrrole chromophore, phycocyanobilin (PCB), covalently bound to a cystein residue (Fig. 1). Close proximity (~20 A) between α-84 and nearby β-84 in APC can cause the electronic states of the chromophores to interact to form split exciton states. In this case, interexciton dynamics mediates the energy transfer. On the other hand, it is also possible that energy transfer from α-84 to β-84 occurs via dipole-dipole coupling between the two chromophores (Forster model). The intermediate range of distance between the PCBs in APC makes both models quite feasible for an energy transfer mechanism and has generated much controversy. Here we report on the excited state dynamics of APC measured by femtosecond stimulated Raman spectroscopy. Femtosecond stimulated Raman spectroscopy (FSRS) reveals the structure of fast-evolving molecules by providing vibrational spectra with excellent spectral resolution on a femtosecond time scale. We prepare the electronic excited state of APC by 40 nJ, 30 fs actinic pump pulse at 1 kHz, centered at 610 nm. The femtosecond stimulated Raman spectra of the excited state APC are obtained at a series of time delays by coupling 0.5 μJ/pulse, 2 ps, 800 nm Raman pump pulse and 4 nJ/pulse, 20 fs, broadband continuum Raman probe pulse. The detailed description of the detection system and how to generate the laser pulses has been published elsewhere. APC was isolated from the filamentous cyanobacterium Anabaena variabilis and used in phosphate buffer solution. Figure 2 presents the FSRS spectra of the ground state and the excited state APC at selected time delays. The excited state spectra were obtained by subtracting the spectra of buffer solution and the ground state APC which accounts for the unexcited 85% of APC. The ground state FSRS spectrum is in excellent agreement with the resonance Raman spectra previously reported at UV wavelengths. The excited state spectra show broad and dispersive features without any well-defined peaks. The amplitude of the dispersive features increases up to ~80 fs and decreases gradually, completely gone by 5 ps (not shown). McCamant et al. found that dispersive lineshapes appear when the Raman pump and probe pulses drive resonant stimulated emission from the excited state and create vibrational coherence on the electronic ground state surface, which they termed the Raman initiated by nonlinear emission (RINE). The RINE spectra contain the information on the vibrational frequencies of the electronic ground state whose dynamics is determined by the evolution of the excited state population. From this model, McCamant et al. successfully extracted the vibrational spectra of the ground state bacteriorhodopsin, which evolve with the population changes in the excited state. Adopting a similar method described in ref. 14, we fit the excited state FSRS spectra of APC to the RINE model. From

Wave packet theory of dynamic stimulated Raman spectra in femtosecond pump-probe spectroscopy

The quantum theory for stimulated Raman spectroscopy from a moving wave packet using the third-order density matrix and polarization is derived. The theory applies, in particular, to the new technique of femtosecond broadband stimulated Raman spectroscopy (FSRS). In the general case, a femtosecond actinic pump pulse first prepares a moving wave packet on an excited state surface which is then interrogated with a coupled pair of picosecond Raman pump pulse and a femtosecond Raman probe pulse and the Raman gain in the direction of the probe pulse is measured. It is shown that the third-order polarization in the time domain, whose Fourier transform governs the Raman gain, is given simply by the overlap of a first-order wave packet created by the Raman pump on the upper electronic state with a second-order wave packet on the initial electronic state that is created by the coupling of the Raman pump and probe fields acting on the molecule. Calculations are performed on model potentials to illustrate and interpret the FSRS spectra.

Time-resolved resonance Raman spectrum of chrysene in the S1 and T1 states

The time-resolved Sn–S1 and Tn–T1 resonance Raman spectra of highly purified chrysene in tetrahydrofuran were observed at room temperature using the third harmonic of a Q-switched Nd:YAG laser as an electronic excitation pulse and a dye laser pumped by the second harmonic of another Q-switched Nd:YAG laser as a Raman probe pulse. We observed the Raman lines attributed to the S1 state at 1549, 1369, 1344, 1110, and 990 cm−1, and to the T1 state at 1528, 1494, 1353, 1199, 1110, 990, 678, 554, 472, and 286 cm−1. The decay time of 1369 cm−1 (S1) line, the formation time of 1528 cm−1 (T1) line, that of 1199 cm−1 (T1) line, and the fluorescence lifetime were obtained to be 35+6−5, 51+19−12, 41+10−7, and 41+− 1 ns, respectively. The observed Sn–S1 and Tn–T1 resonance Raman spectra were compared with the previously reported fluorescence and phosphorescence spectra in Shpolskii matrices, Sn–S1 and Tn–T1 resonance CARS spectra, and Tn–T1 spontaneous resonance Raman spectra. Discussion was made on the geometrical changes in the excited states on the assumption that the changes were not very large for the rigid molecular structure. We came to the following conclusions: (1) The resonance-coupled excited singlet state is different from the ground state in geometry mainly along the normal coordinates of the vibrational modes which have the Sn–S1 resonance Raman peaks at 1549 and 1344 cm−1. (2) The geometry of the resonance-coupled excited triplet state differs from that of the ground state mainly along five coordinates. Four of them correspond to the vibrational bands observed in the Tn–T1 resonance Raman spectrum at 1528, 1353, 1110, and 990 cm−1. The other corresponds to the vibrational bands observed in the Shpolskii spectrum of phosphorescence at 1385 cm−1 in literature. (3) The S1 and T1 states differ along the normal coordinates of the modes which have the Tn–T1 resonance Raman bands at 678, 554, 472, and 286 cm−1.