Femtosecond Time-resolved Optical Spectroscopy Research Articles

Structure and Photophysics of N-Tolanyl-phenochalcogenazines and their Radical Cations.

The study focuses on the structural and photophysical characteristics of neutral and oxidized forms of N-tolanyl-phenochalcogenazines PZX-tolan with X=O, S, Se, and Te. X-ray crystal structure analyses show a pseudo-equatorial (pe) structure of the tolan substituent in the O, S, and Se dyads, while the Te dyad possesses a pseudo-axial (pa) structure. DFT calculations suggest the pe structure for O and S, and the pa structure for Se and Te as stable forms. Steady-state and femtosecond-time resolved optical spectroscopy in toluene solution indicate that the O and S dyads emit from a CT state, whereas the Se and Te dyads emit from a tolan-localized state. The T1 state is tolan-localized in all cases, showing phosphorescence at 77 K. The heavy atom effect of chalcogens induces intersystem crossing from S1 to Tx, resulting in a decreasing S1 lifetime from 2.1 ns to 0.42 ps. The T1 states possess potential for singlet oxygen sensitization with a high quantum yield (ca. 40 %) for the O, S, and Se dyads. Radical cations exhibit spin density primarily localized at the heterocycle. EPR measurements and quasirelativistic DFT calculations reveal a very strong g-tensor anisotropy, supporting the pe structure for the S and Se derivatives.

Search for the origin of synergistic solvation in methanol/chloroform mixture using optical Kerr effect spectroscopy

The synergistic effect of methanol/chloroform mixture on solute solvation was earlier attributed to the formation of an extended hydrogen-bonding network in the mixture. Such a network was proposed to be weak through the solute dipole-moment dependent experiment. In this study, we search for signatures of such interactions using the femtosecond time-resolved optical Kerr effect spectroscopy, which is sensitive to the ultrafast intermolecular dynamics. We observed a 2-3-fold retardation of the orientational diffusion time of chloroform molecules that is attributed to the formation of hydrogen-bonds with methanol. Our frequency domain analysis in terms of the excess reduced spectral density and partial reduced spectral density allowed us to detect a hydrogen-bond stretching band around 90 cm−1 (with 107 cm−1 natural frequency) associated with methanol molecules simultaneously accepting a hydrogen-bond from chloroform and donating a hydrogen-bond to another methanol molecule. Additionally, in an auxiliary mixture used in this study, where chloroform is replaced with carbon tetrachloride, we found evidence of carbon tetrachloride-methanol halogen-bond formation; however, its signature in the spectra is much weaker than in the case of chloroform-methanol hydrogen-bonding.

Femtosecond time-resolved nonlinear optical spectroscopy of charge transfer at the buried GaP/Si(001) interface

The ultrafast charge-carrier dynamics at the buried heterointerface of gallium phosphide on silicon(001) are investigated by means of time-resolved optical second-harmonic generation. Photon energy dependent measurements reveal the existence of electronic interface states in the bandgap of both materials. Charge carriers excited via these interface states are efficiently injected within a few hundred femtoseconds from the GaP/Si interface into the Si substrate, resulting in the build-up of an electric field perpendicular to the interface on a picosecond time scale.

Lasing dynamics study by femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy**Project supported by the National Natural Science Foundation of China (Grant Nos. 20925313 and 21503066), the Innovation Program of Chinese Academy of Sciences (Grant No. KJCX2-YW-W25), the Postdoctoral Project of Hebei University, China, and the Project of Science and Technology Bureau of Baoding City, China (Grant No. 15ZG029).

Femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy (FNOPAS) is a versatile technique with advantages of high sensitivity, broad detection bandwidth, and intrinsic spectrum correction function. These advantages should benefit the study of coherent emission, such as measurement of lasing dynamics. In this letter, the FNOPAS was used to trace the lasing process in Rhodamine 6G (R6G) solution and organic semiconductor nano-wires. High-quality transient emission spectra and lasing dynamic traces were acquired, which demonstrates the applicability of FNOPAS in the study of lasing dynamics. Our work extends the application scope of the FNOPAS technique.

Spectrum Correction in Study of Solvation Dynamics by Fluorescence Non-collinear Optical Parametric Amplification Spectroscopy

Femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy can extract the curve of spectral gain from its parametric superfluorescence. This unique spectrum correction method enables fluorescence non-collinear optical parametric amplification spectroscopy acquiring the genuine transient fluorescence spectrum of the studied system. In this work we employ fluorescence non-collinear optical parametric amplification spectroscopy technique to study the solvation dynamics of DCM dye in ethanol solution, and confirm that genuine solvation correlation function and shift of peak frequency can be derived from transient fluorescence spectra after the spectral gain correction. It demonstrates that fluorescence non-collinear optical parametric amplification spectroscopy can benefit the research fields, which focuses on both fluorescence intensity dynamics and fluorescence spectral shape evolution.

Multi-channel lock-in amplifier assisted femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy with efficient rejection of superfluorescence background.

Superfluorescence appears as an intense background in femtosecond time-resolved fluorescence noncollinear optical parametric amplification spectroscopy, which severely interferes the reliable acquisition of the time-resolved fluorescence spectra especially for an optically dilute sample. Superfluorescence originates from the optical amplification of the vacuum quantum noise, which would be inevitably concomitant with the amplified fluorescence photons during the optical parametric amplification process. Here, we report the development of a femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectrometer assisted with a 32-channel lock-in amplifier for efficient rejection of the superfluorescence background. With this spectrometer, the superfluorescence background signal can be significantly reduced to 1/300-1/100 when the seeding fluorescence is modulated. An integrated 32-bundle optical fiber is used as a linear array light receiver connected to 32 photodiodes in one-to-one mode, and the photodiodes are further coupled to a home-built 32-channel synchronous digital lock-in amplifier. As an implementation, time-resolved fluorescence spectra for rhodamine 6G dye in ethanol solution at an optically dilute concentration of 10(-5)M excited at 510 nm with an excitation intensity of 70 nJ/pulse have been successfully recorded, and the detection limit at a pump intensity of 60 μJ/pulse was determined as about 13 photons/pulse. Concentration dependent redshift starting at 30 ps after the excitation in time-resolved fluorescence spectra of this dye has also been observed, which can be attributed to the formation of the excimer at a higher concentration, while the blueshift in the earlier time within 10 ps is attributed to the solvation process.

Ultrafast excited-state dynamics of copper(I) complexes.

Bis-diimine Cu(I) complexes exhibit strong absorption in the visible region owing to the metal-to-ligand charge transfer (MLCT) transitions, and the triplet MLCT ((3)MLCT) states have long lifetimes. Because these characteristics are highly suitable for photosensitizers and photocatalysts, bis-diimine Cu(I) complexes have been attracting much interest. An intriguing feature of the Cu(I) complexes is the photoinduced structural change called "flattening". Bis-diimine Cu(I) complexes usually have tetrahedron-like D2d structures in the ground (S0) state, in which two ligands are perpendicularly attached to the Cu(I) ion. With MLCT excitation, the central Cu(I) ion is formally oxidized to Cu(II), which induces the structural change to the "flattened" square-planar-like structure that is seen for usual Cu(II) complexes. In this Account, we review our recent studies on ultrafast excited-state dynamics of bis-diimine Cu(I) complexes carried out using femtosecond time-resolved optical spectroscopy. Focusing on three prototypical bis-diimine Cu(I) complexes that have 1,10-phenanthroline ligands with different substituents at the 2,9-positions, i.e., [Cu(phen)2](+) (phen = 1,10-phenanthroline), [Cu(dmphen)2](+) (dmphen = 2,9-dimethyl-1,10-phenanthroline), and [Cu(dpphen)2](+) (dpphen = 2,9-diphenyl-1,10-phenanthroline), we examined their excited-state dynamics by time-resolved emission and absorption spectroscopies with 200 fs time resolution, observed the excited-state coherent nuclear motion with 30 fs time resolution and performed complementary theoretical calculations. This combined approach vividly visualizes excited-state processes in the MLCT state of bis-diimine Cu(I) complexes. It was demonstrated that flattening distortion, internal conversion, and intersystem crossing occur on the femtosecond-early picosecond time scale, and their dynamics is clearly identified separately. The flattening distortion predominantly occurs in the S1 state on the subpicosecond time-scale, and the precursor S1 state retaining the initial undistorted structure appears as a metastable state before the structural change. This observation indicates that the traditional understanding based on the Jahn-Teller effect appears irrelevant for realistically discussing the photoinduced structural change of bis-diimine Cu(I) complexes. The lifetime of the precursor S1 state significantly depends on the substituents in the three complexes, indicating that the flattering distortion requires a longer time as the substituents at 2,9-positions of the ligands become bulkier. It is suggested that the substituents are rotated to avoid steric repulsions to achieve the flattened structure at the global minimum of the S1 state, implying the necessity of discussion based on a multidimensional potential energy surface to properly consider this excited-state structural change. After the flattening distortion, the S1 states of [Cu(dmphen)2](+) and [Cu(dpphen)2](+), which have bulky substituents, relax to the T1 state by intersystem crossing on the ∼10 ps time scale, while the flattened S1 state of [Cu(phen)2](+) relaxes directly to the S0 state on the ∼2 ps time scale. This difference is rationalized in terms of the different magnitude of the flattening distortion and relevant changes in the potential energy surfaces. Clear understanding of the ultrafast excited-state process provides a solid basis for designing and using Cu(I) complexes, such as controlling the structural change to efficiently utilize the energy of the MLCT state in solar energy conversion.

Ultrafast free-carrier dynamics in Cu2ZnSnS4 single crystals studied using femtosecond time-resolved optical spectroscopy

We studied the dynamics of photogenerated carriers in Cu2ZnSnS4 (CZTS) single crystals using femtosecond transient reflectivity (TR) and optical pump-THz probe transient absorption (THz-TA) spectroscopy. The TR and THz-TA decay dynamics consistently showed that free carriers have long lifetimes of up to a few nanoseconds. The excitation-photon-energy-dependent TR measurements revealed a slow picosecond energy relaxation of free carriers to the band edge in CZTS. The relaxation and recombination dynamics of free carriers were affected by nonradiative recombinations at the surface. Our results revealed a global feature of energy relaxation and recombination processes of free carriers in CZTS single crystals.

Coherent photon interference elimination and spectral correction in femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy

We report an improved setup of femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy (FNOPAS) with a 210 fs temporal response. The system employs a Cassegrain objective to collect and focus fluorescence photons, which eliminates the interference from the coherent photons in the fluorescence amplification by temporal separation of the coherent photons and the fluorescence photons. The gain factor of the Cassegrain objective-assisted FNOPAS is characterized as 1.24 × 10(5) for Rhodamine 6G. Spectral corrections have been performed on the transient fluorescence spectra of Rhodamine 6G and Rhodamine 640 in ethanol by using an intrinsic calibration curve derived from the spectrum of superfluorescence, which is generated from the amplification of the vacuum quantum noise. The validity of spectral correction is illustrated by comparisons of spectral shape and peak wavelength between the corrected transient fluorescence spectra of these two dyes acquired by FNOPAS and their corresponding standard reference spectra collected by the commercial streak camera. The transient fluorescence spectra of the Rhodamine 6G were acquired in an optimized phase match condition, which gives a deviation in the peak wavelength between the retrieved spectrum and the reference spectrum of 1.0 nm, while those of Rhodamine 640 were collected in a non-optimized phase match condition, leading to a deviation in a range of 1.0-3.0 nm. Our results indicate that the improved FNOPAS can be a reliable tool in the measurement of transient fluorescence spectrum for its high temporal resolution and faithfully corrected spectrum.

Acoustic and optical phonon dynamics from femtosecond time-resolved optical spectroscopy of the superconducting iron pnictide Ca(Fe0.944Co0.056)2As2

We report the temperature evolution of coherently excited acoustic and optical phonon dynamics in the superconducting iron pnictide single crystal Ca(Fe0.944Co0.056)2As2 across the spin density wave transition at TSDW ∼ 85 K and the superconducting transition at TSC ∼20 K. The strain pulse propagation model applied to the generation of the acoustic phonons yields the temperature dependence of the optical constants, and longitudinal and transverse sound velocities in the temperature range from 3.1 K to 300 K. The frequency and dephasing times of the phonons show anomalous temperature dependence below TSC indicating a coupling of these low-energy excitations with the Cooper-pair quasiparticles. A maximum in the amplitude of the acoustic modes at T ∼ 170 is seen, attributed to spin fluctuations and strong spin-lattice coupling before TSDW.

Hot electron relaxation in the heavy-fermionYb1−xLuxAl3compound using femtosecond optical pump-probe spectroscopy

Femtosecond time-resolved optical spectroscopy was used to systematically study photoexcited carrier re­ laxation dynamics in the intermediate-valence heavy-fermion system Yb 1_xLuxAl 3 (0 oSx oS I). Given the dem­ onstrated sensitivity of this experimental technique to the presence of the low-energy gaps in the charge excitation spectrum, the aim of this work was to study the effect of dilution of the Kondo lattice on its low-energy electronic structure. The results imply that in Yb 1_xLuxAl3 the hybridization gap, resulting from hybridization of local moments and conduction electrons, persists up to 30% doping. Interestingly, below some characteristic, doping dependent temperature T(x) the relaxation-time divergence, governed by the relaxation bottleneck due to the presence of the indirect hybridization gap, is truncated. This observation is attributed to the competing ballistic transport of hot electrons out of the probed volume at low temperatures. The derived theoretical model accounts for both the functional form of relaxation dynamics below T'(x), as well as the doping dependence of the low-temperature relaxation rate in Yb J._xLuxAI3.

Femtosecond Time-Resolved Optical and Raman Spectroscopy of Photoinduced Spin Crossover: Temporal Resolution of Low-to-High Spin Optical Switching

A combination of femtosecond electronic absorption and stimulated Raman spectroscopies has been employed to determine the kinetics associated with low-spin to high-spin conversion following charge-transfer excitation of a FeII spin-crossover system in solution. A time constant of tau = 190 +/- 50 fs for the formation of the 5T2 ligand-field state was assigned based on the establishment of two isosbestic points in the ultraviolet in conjunction with changes in ligand stretching frequencies and Raman scattering amplitudes; additional dynamics observed in both the electronic and vibrational spectra further indicate that vibrational relaxation in the high-spin state occurs with a time constant of ca. 10 ps. The results set an important precedent for extremely rapid, formally forbidden (DeltaS = 2) nonradiative relaxation as well as defining the time scale for intramolecular optical switching between two electronic states possessing vastly different spectroscopic, geometric, and magnetic properties.

Nonequilibrium electronic and structural Jahn-Teller dynamics in(NbSe4)3I

Femtosecond time-resolved optical spectroscopy reveals complex nonequilibrium electronic and structural relaxation dynamics associated with an electronically driven Jahn-Teller (JT) structural phase transition at $274\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the quasi-one-dimensional semiconductor ${(\mathrm{Nb}{\mathrm{Se}}_{4})}_{3}\mathrm{I}$. The relaxation dynamics of the electronic order parameter $\ensuremath{\eta}$ (associated with the JT transition) and accompanying structural relaxation are described using the Landau-Khalatnikov equation, and equations of motion for coupled phonons, respectively.

Femtosecond time-resolved optical pump-probe spectroscopy at kilohertz-scan-rates over nanosecond-time-delays without mechanical delay line

We demonstrate a technique for femtosecond time-resolved optical pump-probe spectroscopy that allows to scan over a nanosecond time delay at a kilohertz scan rate without mechanical delay line. Two mode-locked femtosecond lasers with approximately 1 GHz repetition rate are linked at a fixed difference frequency of ΔfR=11kHz. One laser delivers the pump pulses, the other provides the probe pulses. The relative time delay is linearly ramped between zero and the inverse laser repetition frequency at a rate ΔfR, enabling high-speed scanning over a 1 ns time delay. The advantages of this method for all-optical pump-probe experiments become evident in an observation of coherent acoustic phonons in a semiconductor superlattice via transient reflectivity changes. A detection shot-noise limited signal resolution of 7×10−8 is obtained with a total measurement time of 250 s. The time resolution is 230 fs.

Electron Lifetime in Armchair Carbon Nanotubes

The excited conduction electrons in metallic armchair carbon nanotubes can decay by inelastic electron–electron scattering. The deexcitation channels include the single-particle and collective excitations of different angular momenta. The higher conduction subbands have more deexcitation channels, or shorter electron lifetimes. Each conduction subband has certain states with very long lifetimes, and a simple relationship between the inverse electron lifetime and the state energy is absent. Such results directly reflect the characteristics of the 1D excitation spectra. The inverse electron lifetime contrasts sharply with those of semiconducting carbon nanotubes, graphite, and 2D and 3D metallic systems. Dimensionalities play an important role in the many-body effects. The predicted results could be verified by femtosecond time-resolved optical spectroscopy.

Ultrafast quasiparticle relaxation dynamics in normal metals and heavy-fermion materials

We present a detailed theoretical study of the ultrafast quasiparticle relaxation dynamics observed in normal metals and heavy fermion materials with femtosecond time-resolved optical pump-probe spectroscopy. For normal metals, a nonthermal electron distribution gives rise to a temperature (T) independent electron-phonon relaxation time at low temperatures, in contrast to the T^{-3}-divergent behavior predicted by the two-temperature model. For heavy fermion compounds, we find that the blocking of electron-phonon scattering for heavy electrons within the density-of-states peak near the Fermi energy is crucial to explain the rapid increase of the electron-phonon relaxation time below the Kondo temperature. We propose the hypothesis that the slower Fermi velocity compared to the sound velocity provides a natural blocking mechanism due to energy and momentum conservation laws.

Quasiparticle dynamics and gap structure inHgBa2Ca2Cu3O8+δinvestigated with femtosecond spectroscopy

Measurements of the temperature dependence of the quasiparticle (QP) dynamics in Hg1223 with femtosecond time-resolved optical spectroscopy are reported. From the temperature dependence of the amplitude of the photoinduced reflection, the existence of two gaps is deduced, one temperature dependent Dc that closes at Tc, and another temperature independent ''pseudogap'' Dp. The zero-temperature magnitudes of the two gaps are Dc/kTc = 6 +/- 0.5 and Dp/kTc = 6.4 +/- 0.5 respectively. The quasiparticle lifetime is found to exhibit a divergence as T -> Tc from below, which is attributed to the existence of a superconducting gap which closes at Tc. Above Tc the relaxation time is longer than expected for metallic relaxation, which is attributed to the presence of the ''pseudogap''. The QP relaxation time is found to increase significantly at low temperatures. This behavior is explained assuming that at low temperatures the relaxation of photoexcited quasiparticles is governed by a bi-particle recombination process.

Quasiparticle relaxation dynamics in Hg-1223 studied by femtosecond time-resolved optical spectroscopy.

Following recent progress in understanding the relaxation dynamics of photoexcited carriers in materials exhibiting a small gap in the low-energy excitation spectrum we have performed pump-probe measurements on near optimally doped Hg-1223. We show that the behavior is very similar as in optimally-doped YBCO, where the data can be interpreted with the coexisting presence of two energy gaps: normal state T-independent pseudogap and a mean-field-like collective gap, associated with intrinsic spatially inhomogeneous ground state. An important difference between the two compounds is found in the low temperature relaxation time, which in Hg-1223 is found to be strongly temperature dependent.

The lifetimes and energies of the first excited singlet states of diadinoxanthin and diatoxanthin: the role of these molecules in excess energy dissipation in algae

The lifetimes of the first excited singlet states (2 1A g) of diadinoxanthin and diatoxanthin, carotenoids involved in the xanthophyll cycle in some genera of algae, have been measured by femtosecond time-resolved optical spectroscopy to be 22.8 ± 0.1 ps and 13.3 ± 0.1 ps, respectively. Using the energy gap law for radiationless transitions set forth by Englman and Jortner (Mol. Phys. 18 (1970) 145–164), these lifetimes correspond to S 1 excited state energies of 15 210 cm −1 for diadinoxanthin and 14 620 cm −1 for diatoxanthin. The lowest excited singlet state energy of Chl a has an energy of 14 700 cm −1. The fact that the S 1 state energy of diadinoxanthin lies above that of Chl a, whereas the S 1 state energy of diatoxanthin lies below that of Chl a, suggests that the xanthophyll cycle involving the enzymatic interconversion of diadinoxanthin and diatoxanthin may play a role in regulating energy flow between these molecules and Chl a in many species of algae, essentially fulfilling a role identical to that proposed for violaxanthin and zeaxanthin in higher plants and green algae (Frank et al. (1994) Photosyn. Res. 41, 389–395).

Ultrafast laser-induced order-disorder transitions in semiconductors.

Laser-induced ultrafast order-disorder transitions in silicon and gallium arsenide are studied by means of femtosecond time-resolved linear and nonlinear optical spectroscopy. Detailed measurements of the reflectivity and of the reflected second harmonic over a wide range of fluences reveal a complex picture of the phase transformation. We show that during the first 100 fs the changes of the optical constants and of the nonlinear optical susceptibility ${\mathrm{\ensuremath{\chi}}}^{(2)}$ are determined by the various electronic excitation processes and only to a lesser extent by the process of disordering. On the other hand, time-resolved measurements of reflectivity spectra indicate that the development of a Drude-like metallic spectrum takes a few hundred femtoseconds. Our data show that the laser-induced structural changes develop slower than previously believed, occurring on a time scale of a few hundred femtoseconds.