Picosecond Relaxation Times Research Articles

Ultrafast dynamics in the high-symmetry and in the charge density wave phase of 2H−NbSe2

We investigate carrier and collective mode dynamics in 2H-NbSe$_2$ using time-resolved optical pump-probe spectroscopy and compare the results with first-principle calculations. Broadband ultrafast reflectivity studies of 2H-NbSe$_2$ in a wide temperature interval covering the normal, charge density wave (CDW) and superconducting phase were performed. Spectral features observed in the transient reflectivity experiment were associated with specific optical transitions obtained from band structure calculations. Displacive excitation of coherent phonons showed CDW-associated coherent oscillations of the soft phonon mode across the whole spectral range. Temperature evolution of this coherent phonon mode in the low-excitation linear regime shows softening of the mode down to the CDW transition temperature T$_{CDW}$ with subsequent hardening below T$_{CDW}$. The global fit of the broadband probe data reveals four different relaxation times associated with characteristic electron-electron, electron-phonon and phonon-phonon relaxation processes. From first principle calculations of electron-phonon coupling we associate the few picosecond electron-phonon relaxation time $\tau_2$ with a specific group of phonons with frequencies around 20 meV. On the other hand, the anomalously long relaxation time of $\tau_3$~100 ps is associated with anharmonicity-driven phonon-phonon scattering. All relaxation processes result from anomalies near the second order CDW phase transition that are reflected in the temperature dependencies of the characteristic relaxation times and amplitudes of optical densities. At highest fluences we observe electronic melting of the CDW and disappearance of the mode hardening below T$_{CDW}$.

Hot-spot relaxation time current dependence in niobium nitride waveguide-integrated superconducting nanowire single-photon detectors.

We investigate how the bias current affects the hot-spot relaxation dynamics in niobium nitride. We use for this purpose a near-infrared pump-probe technique on a waveguide-integrated superconducting nanowire single-photon detector driven in the two-photon regime. We observe a strong increase in the picosecond relaxation time for higher bias currents. A minimum relaxation time of (22 ± 1) ps is obtained when applying a bias current of 50% of the switching current at 1.7 K bath temperature. We also propose a practical approach to accurately estimate the photon detection regimes based on the reconstruction of the measured detector tomography at different bias currents and for different illumination conditions.

Quantitative characterization of hydration state and destructuring effect of monosaccharides and disaccharides on water hydrogen bond network

Terahertz time-domain attenuated total reflection measurements of monosaccharide (glucose and fructose) and disaccharide (sucrose and trehalose) solutions from 0.146 M to 1.462 M were performed to evaluate (1) the hydration state and (2) the destructuring effect of saccharide solutes on the hydrogen bond (HB) network. Firstly, the extent of hydration water was determined by the decreased amount of bulk water with picosecond relaxation time that was replaced by that with much longer orientational relaxation time. As a result, we found glucose and trehalose exhibits stronger hydration capacity than fructose and sucrose, respectively, despite of the same number of the hydroxyl groups. For each saccharide, the hydration number tended to decrease with solute concentration. Secondly, the destructuring effect of these saccharide solutes on the HB network of the surrounding bulk water was discussed from the perspective of the fraction of non-hydrogen-bonded (NHB) water isolated from the HB network. We found the fraction of NHB water molecules that are not engaged in the HB network monotonously increased with saccharide concentration, indicating saccharide solutes promote the disruption of the water HB network. However, no noticeable differences were confirmed in the fraction of NHB water between glucose and fructose or between sucrose and trehalose. In contrast to hydration number, the number of NHB water produced by a single saccharide solute was less dependent on solute concentration, and three monosaccharide/disaccharide solutes were found to produce one/two NHB water molecules.

Ultrafast Relaxation Dynamics via Acoustic Phonons in Carbon Nanotubes

Carbon nanotubes as one-dimensional nanostructures are ideal model systems to study relaxation channels of excited charged carriers. The understanding of the ultrafast scattering processes is the key for exploiting the huge application potential that nanotubes offer, e.g., for light-emitting and detecting nanoscale electronic devices. In a joint study of two-color pump-probe experiments and microscopic calculations based on the density matrix formalism, we extract, both experimentally and theoretically, a picosecond carrier relaxation dynamics, and ascribe it to the intraband scattering of excited carriers with acoustic phonons. The calculated picosecond relaxation times show a decrease for smaller tube diameters. The best agreement between experiment and theory is obtained for the (8,7) nanotubes with the largest investigated diameter and chiral angle for which the applied zone-folded tight-binding wave functions are a good approximation.

Single-wavelength functional photoacoustic microscopy in biological tissue

Recently, we developed a reflection-mode relaxation photoacoustic microscope, based on saturation intensity, to measure picosecond relaxation times using a nanosecond laser. Here, using the different relaxation times of oxygenated and deoxygenated hemoglobin molecules, both possessing extremely low fluorescence quantum yields, the oxygen saturation was quantified in vivo with single-wavelength photoacoustic microscopy. All previous functional photoacoustic microscopy measurements required imaging with multiple-laser-wavelength measurements to quantify oxygen saturation. Eliminating the need for multiwavelength measurements removes the influence of spectral properties on oxygenation calculations and improves the portability and cost-effectiveness of functional or molecular photoacoustic microscopy.

THz Spectra and Dynamics of Aqueous Solutions Studied by the Ultrafast Optical Kerr Effect

The nature and extent of the effects that hydrophilic and hydrophobic solutes have on the dynamics of water molecules continues to be an area of intense experimental and theoretical investigation. In this work, we use the ultrafast optical Kerr effect to measure the picosecond dynamics and THz Raman spectral densities of a series of aqueous solutions. The solutes studied are the hydrophilic urea and formamide and the hydrophobic trimethylamine N-oxide and tetramethylurea. Measurements are made as a function of concentration between <0.1 M and >4 M. At low concentrations (<0.5 M), the THz spectrum resembles that of bulk water, but the picosecond relaxation time, reflecting dynamics in the water H-bonded network, is increased relative to bulk water for all four solutes. The extent to which water relaxation is slowed down depends on the nature of the solute, and is more pronounced for hydrophilic than for hydrophobic solutes. At concentrations above 1 M, a range of solute-solvent and solute-solute interactions gives rise to diverse solute dependent changes in the THz spectral density and to a further slowing down of the picosecond relaxation. The hydrophobic trimethylamine N-oxide has remarkably little effect on the spectral density of water, which may indicate solute self-association and the formation of water pools in more concentrated solutions. For hydrophilic urea and formamide, the THz spectral density suggests that water structure is disrupted at concentrations where most water molecules are part of a solvation shell. At such high concentrations, modes associated with the H-bonded solute make a significant contribution to the spectral density at around 100 cm(-1). The hydrophobic tetramethylurea solute makes a substantial contribution to the spectral density, complicating the interpretation, but a line shape analysis suggests that it also does not strongly perturb the water structure.

Optical excitation and detection of picosecond acoustic pulses in liquid mercury

Ultrashort optical pulses are used to excite and interferometrically detect picosecond longitudinal-acoustic pulses in thin films of liquid mercury sandwiched between sapphire plates. We show that the shape of the strain pulses in the mercury can be directly measured through ultrafast changes in optical reflectivity. By analyzing consecutive acoustic echoes, we derive the dispersion of the ultrasonic attenuation and sound velocity for this liquid at frequencies up to 10 GHz. Significant effects of structural relaxation are observed and are compared to a simple model that indicates the presence of picosecond relaxation times in mercury.

Measurement of the velocity dispersion and attenuation in a liquid metal at GHz frequencies

Ultrashort optical pulses are used to excite and interferometrically detect picosecond longitudinal acoustic pulses in thin films of liquid mercury sandwiched between sapphire plates. By analysing consecutive acoustic echoes we derive the dispersion of the ultrasonic attenuation and sound velocity for this liquid at frequencies up to 10 GHz. Two types of optical detection, from the same side of the film as the excitation light and from the opposite side to the excitation light, are presented. Significant effects of structural relaxation are observed and are compared to a simple model that indicates the presence of picosecond relaxation times in mercury.

State of Water in Perfluorosulfonic Ionomer (Nafion 117) Proton Exchange Membranes

The nature of water in acid-form Nafion 117 was quantified at several hydration levels by dielectric relaxation spectroscopy. Two independent experimental setups were used to collect complex dielectric permittivity spectra at low frequencies ( at to ) and in the microwave region ( at ). We directly observed the states of water, manifested through three population averages with distinctly resolved dynamical behaviors, and their changes with temperature and hydration level. The fastest process observed was identified as the cooperative picosecond relaxation of free (isotropic, bulklike) water, whereas the slowest process (microsecond relaxation times) corresponded to water molecules strongly bound to the charged sulfonic groups. A third type of water was also observed, also characterized by picosecond relaxation times, close to and about three times slower than those of bulk water, which was attributed to loosely bound water and may contain substantial dynamical heterogeneities.

Femtosecond absorption dynamics in glass-metal nanocomposites

We report strong optical nonlinearity of glasses embedded with copper and silver nanoparticles. In pump-probe experiments with copper-doped glasses, the observed absorption bleaching with picosecond relaxation time is as high as 22%. Transmission femtosecond measurementsreveal the reverse saturable absorption with nonlinear absorption coefficient of 10(-10)cm/W in both copper- and silver-doped nanocomposites.

A New Trans-to-Cis Photoisomerization Mechanism of Azobenzene on the S1(n,π*) Surface

The rotation-inversion controversy for the photoisomerization mechanism of azobenzene has been addressed on the basis of the CASSCF calculations for the excited-state relaxed surface scan along the CNN bending (inversion), the CNNC torsion (rotation), and the concerted CNN bending (concerted inversion) reaction pathways. According to our calculated results, the inversion channel involves a substantial energy barrier and a large S0−S1 energy gap so that it is a highly unfavorable channel to be considered for an efficient electronic relaxation. The rotation channel is essentially barrierless with a conical intersection close to the midpoint of the pathway to reasonably account for the subpicoseond to picosecond relaxation times observed in recent ultrafast experiments. Along the concerted inversion reaction path, a sloped conical intersection (S0/S1 CI_inv) was found to have the geometry close to a linear CNNC configuration. When the photoexcitation occurs in the S2 state, the concerted inversion channel may be open and the S0/S1 CI_inv would become energetically accessible to produce more trans-S0 isomers. A new mechanism is proposed to rationalize the early experimental results for the following two cases: (1) the trans-to-cis photoisomerization quantum yields on S2 excitation are much lower than the yields on S1 excitation; (2) the yields from both the S2 and the S1 excitations become essentially equal when the rotation channel is blocked by chemical modification or in an inclusion complex.

Formation and optical properties of intermetallic nanoclusters formed by sequential ion implantation

Recent demonstrations that large third-order non-linear responses can be achieved in metal nanocluster glass composites are of significant interest because of their potential for use in all optical switching networks. These composite materials exhibit picosecond switching and relaxation times, thermal and chemical stability, high laser damage thresholds, and low two-photon absorption. Ion implantation has been shown to be a useful fabrication method to form these nanoclusters in silica because of its ability to produce thin films in waveguide configurations containing a high volume fraction (>1%) of metal colloids with well-defined vertical and horizontal dimensional control. Using sequential ion implantation of more than one element, we can modify the composition and microstructure of the composites by forming intermetallic metal colloids. In this work, we report on the improved optical response of metallic nanocluster composites formed by sequential implantation of Cd and Ag and Sb and Ag. Characterization of the samples by transmission electron microscopy (TEM) reveals that approximately spherical metallic colloids are formed for all implanted species during the implantation process. Selected area diffraction patterns indicate that the colloids formed are intermetallic in composition. Linear optical absorption measurements made at room temperature in air from 900 to 200 nm show significant changes in both the magnitude and wavelength of the surface plasmon resonance. The formation of intermetallic nanoclusters results in changes in both the linear and non-linear optical properties of the composite material that are not possible with single element colloids alone. The results are explained in terms of effective medium theory.

Ultrafast nonlinear optics due to electron-hole plasma in bulk semiconductors

It is theoretically demonstrated that, in the presence of an electric potential landscape, the ground-state energy of the electron-hole plasma can be substantially lowered against the ground-state energy of the excitations. The electron-hole plasma is found to be unstable at lower carrier densities. The critical carrier density for the transition between these two states can be reduced. This reduction provides a new mechanism of nonlinear optics for bulk direct band-gap semiconductors, with picosecond relaxation time, large nonlinearity, and requiring low pump intensity.

Measurement of the picosecond population relaxation time of cresyl violet by time-delayed four-wave mixing with incoherent light

Measurements of the picosecond population relaxation time of cresyl violet in methanol were performed by phase-conjugation time-delayed four-wave mixing using incoherent light. This time is about 157 ps. The result was verified with the correlation technique of mode-locked picosecond pulses with different intensities.

Free-Carrier Induced Optical Nonlinearities in Narrow Bandgap Semiconductors

ABSTRACTFree-carrier induced third-order optical nonlinearities can be both large and fast in narrow-gap semiconductors. We have studied a variety of mechanisms in bulk semiconductors and heterostructures using CO2 lasers and found third-order susceptibilities as large as 2×10−3 esu with picosecond relaxation times. These mechanisms saturate at much higher intensities than do slower mechanisms and hence induce huge modulations of the dielectric function. In addition, most of these processes are nonresonant so they do not require the exact matching of material parameters and are relatively insensitive to temperature.

Nonlinear optical studies of relaxation times of carriers in MBE layers of PbSe and PbEuSe

Four wave mixing has been observed as a function of the difference frequency of two laser beams. Intraband energy relaxation rates were derived from a fit to the experimental data. This results in picosecond relaxation times for all samples at 77 K. The relaxation time of epitaxial material is comparable to that of bulk material at 300 K, but shorter at 77 K. PbEuSe shows smaller scattering times than PbSe, as expected.

Subpicosecond molecular dynamics studied by degenerate four-wave mixing with incoherent light.

A time-resolved study of the relaxation times of the third-order susceptibility for the optical Kerr effect in ${\mathrm{CS}}_{2}$ and nitrobenzene was made by degenerate four-wave mixing using 5-ns pulses from a broadband laser. In the case of ${\mathrm{CS}}_{2}$ we observed both subpicosecond (\ensuremath{\sim}0.2 ps) and picosecond (\ensuremath{\sim}2 ps) relaxation times for the susceptibility tensor element of ${\ensuremath{\chi}}_{\mathrm{xyyx}}^{(3)}$ and ${\ensuremath{\chi}}_{\mathrm{xxxx}}^{(3)}$ both at 553 and 623 nm. The observed relaxation times agree with those obtained utilizing subpicosecond pulses. The relaxation times of nitrobenzene could not be measured because the shorter relaxation time is shorter than the resolution time and the longer one is longer than that of ${\mathrm{CS}}_{2}$.

Ultrafast molecular relaxation of isolated stilbene: measurements by picosecond pump-probe techniques

In this Letter, we report measurements of the picosecond relaxation time of trans-stilbene in S1 under collisionless conditions. By acurate determination of the zero-time in our picosecond pump-probe experiments we obtained r = 3 ± 0.5 ps, which is entirely consistent with our earlier measurements of IVR rates of stilbene in bulbs and beams. This establishes the molecular nature of the decay in these picosecond pump-probe multiphoton ionization (or mass spectrometry) experiments.

Fast relaxing absorptive nonlinear refraction in superlattices

Two nonlinear optical processes in superlattices are studied. The first involves free-carrier absorption and the second involves direct intersubband absorption. The optical field modulates the energy-dependent effective mass or the distribution of electrons among the subbands, giving rise to nonlinear refraction. These processes have picosecond relaxation times and nonlinearities up to four orders of magnitude larger than the nonlinear refractive effects in bulk crystals with comparable relaxation times.

Degenerate four-wave mixing due to intervalance band transition in p-type mercury cadmium telluride

Degenerate four-wave mixing in p-type HgCdTe has been observed with CO2 lasers. A room temperature, third order nonlinear susceptibility χ(3) of 8×10−6 esu has been measured. This is the largest χ(3) with picosecond relaxation time reported. The observed nonlinearity is interpreted in terms of the effective mass change induced by the transition from the heavy to light hole band. This process is shown to be stronger than the nonlinearity caused by the saturation of the intervalence band absorption.