Pump-probe Spectrometer Research Articles

In situ generation and reaction of a sub-picosecond electron pulse: ultrafast broadband spectroscopy.

A sub-picosecond resolved broadband transient absorption spectrometer for in situ generation and study of ultrafast reaction of short pulse electrons in water has been reported. A femtosecond near infrared (NIR) laser driven multiphoton ionization of water was used for the in situ generation of an ultra-short electron pulse in aqueous media. The dynamics of such electrons have been investigated using a broadband pump-probe spectrometer with a temporal resolution of sub-picosecond. The formation of the pre-hydrated electron having absorption in the NIR region and its subsequent solvation process leading to the formation of the hydrated electron has been monitored in real time using pump-probe spectroscopy. As a proof of concept, a reaction of pre-hydrated electrons and hydrated electrons with a highly oxidizing agent, methyl viologen, has been carried out. The competition of the reaction with methyl viologen and solvent relaxation of the pre-hydrated electron has been demonstrated by broadband ultrafast spectroscopy. Our results show that the reactivity of pre-hydrated electrons with methyl viologen is almost two orders of magnitude higher as compared to the hydrated electrons. Unlike complicated laser driven accelerators, the present table-top setup for the generation of an in situ ultrashort electron pulse and its reaction with different classes of molecules, especially biomolecules, will help us understand the early events of radiation-induced processes under reductive stress.

Interfacial Effect on the Transient Dielectric Function and Charge Transfer in a Monolayer WS2/Si Heterojunction.

Monolayer tungsten disulfide (WS2) is a highly promising material for silicon photonics. Thus, the WS2/Si interface plays a very important role due to the interfacial complex effects and abundant states. Among them, the effect of charge transfer on exciton dynamics and the optoelectronic property is determined by the dielectric function, which is very crucial for the performance of optoelectronic devices. However, research on the exciton dynamics or the transient dielectric function of WS2 in such WS2/Si junctions is still rare. In this work, both the transient dielectric function and charge transfer of WS2/Si heterojunctions are analyzed based on the transient reflectance spectra measured by the pump-probe spectrometer. The dynamic processes of the A exciton, affected by charge transfer within the WS2/Si heterojunction, are interpreted. Moreover, the transient dielectric function of WS2 is quantitatively analyzed. The dielectric function of WS2 exhibits a notable 19% change, persisting for more than 180 ps within the WS2/Si heterojunction. These findings can pave the way for the advancement of silicon photonic devices based on WS2.

Rapid scan white light pump–probe spectroscopy with 100 kHz shot-to-shot detection

We demonstrate a femtosecond pump–probe spectrometer that utilizes a white light supercontinuum as input and relies on mutual synchronization of the laser repetition rate, acousto-optical chopper, pump–probe delay stage, and the CCD camera to record shot-to-shot pump–probe spectra while the pump–probe delay is scanned synchronously with the laser repetition rate. The unique combination of technologies implemented here allows for electronically controllable and repetition-rate scalable detection throughput that is only limited by the camera frame rate. Despite high probe RMS fluctuations due to sample scatter (from ∼1.8% with solvent to 7.9% with sample scatter), a combination of fast and slow averaging with a fine sampling of pump–probe delay leads to reduction of RMS noise without multichannel referencing down to ∼0.4 mOD for a scattering nanotube sample. Throughput and limitations of the rapid versus stepwise scanning approaches are analyzed. Experimental comparison with stepwise scan shows ∼1.9x noise reduction in a significantly faster experiment, suggesting an additional suppression of 1/f noise enabled by rapid scan data collection. The particular combination of technologies implemented here makes our approach especially suitable for high throughput impulsive pump–probe micro-spectroscopy of highly scattering samples, without added cost and complexity of light sources, multichannel detection, or long sample exposure.

A Wide-Band Femtosecond Pump–Probe Spectrometer Based on a Laser with an Active Medium Based on Chromium-Doped Forsterite Crystal

An instrument for recording photoinduced-absorption spectra using the pump–probe method with femtosecond time resolution is described, in which a regenerative amplifier based on a chromium-doped forsterite crystal with a fundamental harmonic of 1240 nm is used as the laser system. Two optical schemes with pumping at a wavelength of 620 nm and a wavelength of 413 nm that are described in detail allow measurements in the spectral range of 280–1000 nm. The time resolution of the system is 250 fs.

Near shot-noise limited time-resolved circular dichroism pump-probe spectrometer.

We describe an optical near shot-noise limited time-resolved circular dichroism (TRCD) pump-probe spectrometer capable of reliably measuring circular dichroism signals in the order of μdeg with nanosecond time resolution. Such sensitivity is achieved through a modification of existing TRCD designs and introduction of a new data processing protocol that eliminates approximations that have caused substantial nonlinearities in past measurements and allows the measurement of absorption and circular dichroism transients simultaneously with a single pump pulse. The exceptional signal-to-noise ratio of the described setup makes the TRCD technique applicable to a large range of non-biological and biological systems. The spectrometer was used to record, for the first time, weak TRCD kinetics associated with the triplet state energy transfer in the photosynthetic Fenna-Matthews-Olson antenna pigment-protein complex.

Pump-probe spectrometer for measuring x-ray induced strain.

A hard x-ray pump-probe spectrometer using a multi-crystal Bragg reflector is demonstrated at a third generation synchrotron source. This device derives both broadband pump and monochromatic probe pulses directly from a single intense, broadband x-ray pulse centered at 8.767 keV. We present a proof-of-concept experiment which directly measures x-ray induced crystalline lattice strain.

A femtosecond pump–probe spectrometer for dynamics in transmissive polymer films

An experimental setup and data collection strategy for femtosecond transient absorption spectroscopy on thin (1 ) solid polymer film samples is described. The experiment allows for parallel detection of the changes in optical density via broadband supercontinuum probing in the VIS/UV range and single-color detection at an independently selected wavelength from the deep UV to the IR with a sensitivity of per laser shot (r.m.s. standard deviation) and a time resolution below 40 fs. A fast and reproducible bi-directional translation of a two-dimensional film sample of in size is used to measure fresh sample spots at each detection interval. Signal readout at a 1 kHz rate enables single-shot analysis and automated signal discrimination, as well as detailed statistics on sample homogeneity, signal evolution with increasing number of pump pulses, and reproducibility. The technique was employed to study the photoisomerization of Disperse Red 1 in films of polymethylmethacrylate after photoexcitation at nm. The results revealed excited-state dynamics characterized by time constants of and , almost identical as in solution, but evidently enhanced vibrational excitation and slower vibrational cooling (time constant ) after return to the electronic ground state due to the constraining polymer environment.

Reaction path dependent coherent wavepacket dynamics in excited state intramolecular double proton transfer

The primary steps and the wavepacket dynamics of the ultrafast photoinduced double proton transfer in (2,2′-bipyridyl)-3,3′-diol are observed. A two-color pump probe spectrometer with 30 fs time resolution allows the variation of the vibrational excess energy. We find that the first step of the sequential proton transfer and the concerted double proton transfer both proceed in about 50 fs. We observe two types of coherently excited vibrational modes: (i) two symmetric stretching modes are closely associated with the concerted double proton transfer indicating that a simultaneous and symmetric reduction of the donor acceptor distances in both chelate rings takes place during the transfer; (ii) a nonsymmetric in-plane-bending mode is associated with the sequential proton transfer and provides evidence that the transient reduction of the donor acceptor distance occurs only in one chelate ring. This mode is exclusively excited by the reaction and cannot be excited optically according to symmetry selection rules. This constitutes a direct proof that a vibrational wavepacket can be excited by an ultrafast reaction itself.

Ultrasensitive ultraviolet-visible 20fs absorption spectroscopy of low vapor pressure molecules in the gas phase

We describe an ultrasensitive pump-probe spectrometer for transient absorption measurements in the gas phase and in solution. The tunable UV pump and the visible (450-740 nm) probe pulses are generated by two independently tunable noncollinear optical parametric amplifiers, providing a temporal resolution of 20 fs. A homebuilt low gain photodetector is used to accommodate strong probe pulses with a shot noise significantly lower than the overall measurement noise. A matched digitizing scheme for single shot analysis of the light pulses at kilohertz repetition rates that minimizes the electronic noise contributions to the transient absorption signal is developed. The data processing scheme is optimized to yield best suppression of the laser excess noise and thereby transient absorbance changes down to 1.1 x 10(-6) can be resolved. A collinear focusing geometry optimized for a 50 mm interaction length combined with a heatable gas cell allows us to perform measurements on substances with low vapor pressures, e.g., on medium sized molecules which are crystalline at room temperature. As an application example highlighting the capability of this instrument, we present the direct time-domain observation of the ultrafast excited state intramolecular proton transfer of 2-(2(')-hydroxyphenyl)benzothiazole in the gas phase. We are able to compare the resulting dynamics in the gas phase and in solution with a temporal precision of better than 5 fs.

Sensitive femtosecond wave-packet spectrometer

We developed a highly sensitive femtosecond pump–probe spectrometer suitable for observation of ultrafast phenomena such as wave-packet dynamics and phase relaxation. Unlike the conventional pump–probe spectroscopy, our method does not require lock-in amplifiers. Selective detection of nonlinear signal is possible by combination of a rapid-scanning delay stage and multichannel DC-cut amplifiers, which is the core of our new method. Simultaneous measurement of spectra is also realized with short acquisition time less than 50min. The detection limit is up to 1×10−5. This new method enables us to trace detailed dynamics of vibrational wave packet in condensed matters, and provides information of the potential energy surface and dephasing process.

Subpicosecond optical sampling spectrometer using asynchronous tunable mode-locked lasers

We have developed a subpicosecond nondegenerate pump-probe spectrometer based on the optical sampling technique using a combination of asynchronous tunable femtosecond lasers. Owing to its simple instrumentation, time evolution of subpicosecond to nanosecond excited molecular processes in solution and of exciton relaxation processes in semiconductor multiple quantum wells has been detected on an oscilloscope by an effective time base magnification of 760 000 times. Fast acquisition of the temporal profiles and ease of wavelength scanning allowed compilation of time-resolved absorption and excitation spectra. A possible extention of the spectrometer with one of the light sources being a synchrotron radiation instead of the femtosecond laser is discussed.

The origins of nonphotochemical quenching of chlorophyll fluorescence in photosynthesis. Direct quenching by P680+ in photosystem II enriched membranes at low pH.

In most plants and algae, a down-regulation of photosynthesis under "excess" light conditions occurs which is associated with a quenching of chlorophyll a fluorescence. This nonphotochemical quenching of chlorophyll a fluorescence most likely arises from a mechanism which protects photosystem II from excessive excitation and resulting photoinhibition. In this report, nonphotochemical quenching of variable chlorophyll a fluorescence was induced by low pH in photosystem II enriched spinach thylakoid membranes. The origin of quenching was investigated with picosecond fluorescence decay spectroscopy in samples suspended in buffers ranging from pH 6.5 to pH 4.0. The yield of a relatively slow (approximately 1.5 ns) fluorescence decay process associated with the photosystem II reaction center decreased with decreasing pH. There were no significant changes in the yield of faster decay components associated with photosystem II antenna chlorophyll a processes. These results suggest a reaction center based rather than antenna chlorophyll based mechanism for nonphotochemical quenching in these preparations. Measurements of the photosystem II absorption cross section revealed no decrease in the functional antenna size at low pH which also supports a reaction center quenching mechanism. The kinetics of electron transfer in photosystem II were investigated using a pump probe spectrometer which measured simultaneously the flash-induced absorbance change at 820 nm (formation of oxidized photosystem II reaction center pigment, P680+) and the variable fluorescence yield (formation of reduced photosystem II, electron acceptor, QA-). A large increase in the lifetime of P680+ at low pH was correlated with fluorescence quenching. After flash excitation of photosystem II the loss of fluorescence quenching occurred with the same kinetics as the reduction of P680+. In conflict with reaction center based quenching mechanisms based on charge recombination between P680+ and QA-, the oxidation rate of QA- was unaffected by low pH and under all conditions occurred at a slower rate than the reduction of P680+. Our data are discussed in terms of a model for low pH dependent nonphotochemical quenching in photosystem II based on direct quenching by P680+.

Transient mid-ir picosecond spectroscopy of indium arsenide at room temperature: Evidence of spectral hole burning due to nonthermalized carriers.

Spectral hole burning in the interband absorption continuum of InAs has been studied using pump radiation (with photon energy 32--57 meV above the band edge and intensity \ensuremath{\approxeq}${10}^{7}$ W/${\mathrm{cm}}^{2}$) generated by a picosecond midinfrared double-resonance pump-probe spectrometer. The quasi-steady-state spectral holes observed had depths which followed the instantaneous pump intensity and typical widths of \ensuremath{\approxeq}300 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The evolution of the shape of the spectral holes is correlated with the primary carrier scattering processes and an analysis of their widths and depths gives values of the carrier-carrier thermalization and dephasing times ${\mathit{T}}_{1}$\ensuremath{\approxeq}65 fs and ${\mathit{T}}_{2}$\ensuremath{\approxeq}40 fs, respectively.

Mid-infrared picosecond spectroscopy of MBE indium arsenide epilayers at 300 K

Using a newly developed picosecond mid-infrared double-resonance pump-probe spectrometer carrier recombination and intraband scattering have been studied in a range of high-quality InAs epilayers grown by MBE. In as-grown samples recombination rates are typically dominated by recombination at the InAs/GaAs heterointerface with an interfacial recombination velocity of approximately=2.7*104 cm s-1, but Auger recombination (characterized by an Auger coefficient of 1.1*10-26 cm6 s-1) dominates in the thicker samples at higher excitation levels. At short pump-probe delays dynamic spectral hole burning in the absorption continuum has been seen for the first time. Spectral holes (with typical widths of approximately=300 cm-1) are created at pump pulse intensities of approximately=107 W cm-2 and photon energies 32-57 meV above the band edge. The spectral holes track with the pump intensity and photon energy, and an analysis of their widths and depths gives an estimate of 30-50 fs for the carrier intraband scattering time.