Coherence Relaxation Research Articles

Coherent defect-assisted multiphonon intraband carrier relaxation in semiconductor quantum dots

A defect-assisted mechanism of multiphonon intraband carrier relaxation in semiconductor quantum dots, where the carrier is found in a coherent superposition of the initial, final, and defect states, is proposed. It is shown that this mechanism is capable of explaining the observed trends in temperature dependences of the intraband relaxation rates for PbSe and CdSe colloidal nanocrystal quantum dots.

CONTROLLING SPIN COHERENCE WITH SEMICONDUCTOR NANOSTRUCTURES

We present two emerging opportunities for manipulating and communicating coherent spin states in semiconductors. First, we show that semiconductor microcavities offer unique means of controlling light-matter interactions in confined geometries, resulting in a wide range of applications in optical communications and inspiring proposals for quantum information processing and computational schemes. Studies of spin dynamics in microcavities — a new and promising research field — have revealed novel effects such as polarization beats, stimulated spin scattering, and giant Faraday rotation. Here, we study the electron spin dynamics in optically-pumped GaAs microdisk lasers with quantum wells and interface-fluctuation quantum dots in the active region. In particular, we examine how the electron spin dynamics are modified by the stimulated emission in the disks, and observe an enhancement of the spin coherence time when the optical excitation is in resonance with a high quality ( Q ~ 5000) lasing mode.1 This resonant enhancement, contrary to expectations from the observed trend in the carrier recombination time, is then manipulated by altering the cavity design and dimensions. In analogy to devices based on excitonic coherence, this ability to engineer coherent interactions between electron spins and photons may provide novel pathways towards spin dependent quantum optoelectronics. In a second example, the nitrogen-vacancy (N-V) center in diamond has garnered interest as a room-temperature solid-state system not only for exploring electronic and nuclear spin phenomena but also as a candidate for spin-based quantum information processing. Spin coherence times of up to 50 microseconds have been reported for ensembles of N-V centers and a two-qubit gate utilizing the electron spin of a N-V center and the nuclear spin of a nearby C-13 atom has been demonstrated. Here, we present experiments using angle-resolved magneto-photoluminescence microscopy to investigate anisotropic spin interactions of single N-V centers in diamond at room temperature.2 Negative peaks in the photoluminescence intensity are observed as a function of both magnetic field magnitude and angle, and can be explained by coherent spin precession and anisotropic relaxation at spin-level anticrossings. Additionally, precise field alignment with the symmetry axis of a single N-V center reveals the resonant magnetic dipolar coupling of a single "bright" electron spin of an N-V center to small numbers of "dark" spins of nitrogen defects in its immediate vicinity, which are otherwise undetected by photoluminescence. Most recently, we are exploring the possibility of utilizing this magnetic dipole coupling between bright and dark spins to couple two spatially separated single N-V center spins by means of intermediate nitrogen spins. Note from Publisher: This article contains the abstract only.

Coherent Manipulation of Electron Spins up to Ambient Temperatures inCr5+(S=1/2)DopedK3NbO8

We report coherent spin manipulation on Cr(5+) (S = 1/2, I = 0) doped K(3)NbO(8), which constitutes a dilute two-level model relevant for use as a spin qubit. Rabi oscillations are observed for the first time in a spin system based on transition metal oxides up to room temperature. At liquid helium temperature the phase coherence relaxation time T2 reaches approximately 10 micros and, with a Rabi frequency of 20 MHz, yields a single-qubit figure of merit Q(M) of about 500. This shows that a diluted ensemble of Cr(5+) (S = 1/2) doped K(3)NbO(8) is a potential candidate for solid-state quantum information processing.

Elucidation of population and coherence dynamics using cross-peaks in two-dimensional electronic spectroscopy

In this work, we perform a theoretical study on the dynamics and two-dimensional electronic spectroscopy of a model trimer system and compare the results to experimental data on the Fenna–Matthews–Olson protein. We combine a time-nonlocal quantum master equation formalism and the recently developed method for the efficient calculation of third-order photon echo polarization [M.F. Gelin, D. Egorova, W.J. Domcke, J. Chem. Phys. 123 (2005) 164112] to simulate the 2D electronic spectra of the model system, and compare the time-evolution of the amplitude of cross-peaks to the coherent relaxation dynamics of the system following the excitation by a laser pulse. We show that beats of the upper diagonal peaks in the absolute value 2D spectra provide a direct probe for the coherence dynamics in the system, and the time-evolution of the amplitude of the lower diagonal cross-peaks in the real value 2D spectra can be used to reveal the population transfer among exciton states. Our results verify the intuitive description provided by response functions and demonstrate that the full coherent dynamics in a multichromophoric system can be elucidated using two-dimensional electronic spectroscopy.

Differential Line Broadening in MAS Solid-State NMR due to Dynamic Interference

Many MAS (magic angle spinning) solid-state NMR investigations of biologically relevant protein samples are hampered by poor resolution, particularly in the 15N chemical shift dimension. We show that dynamics in the nanosecond-microsecond time scale in solid-state samples can induce significant line broadening of 15N resonances in solid-state NMR experiments. Averaging of 15NH(alpha/beta) multiplet components due to 1H decoupling induces effective relaxation of the 15N coherence in case the N-H spin pair undergoes significant motion. High resolution solid-state NMR spectra can then only be recorded by application of TROSY (Transverse Relaxation Optimized Spectroscopy) type techniques which select the narrow component of the multiplet pattern. We speculate that this effect has been the major obstacle to the NMR spectroscopic characterization of many membrane proteins and fibrillar aggregates so far. Only in very favorable cases, where dynamics are either absent or very fast (picosecond), high-resolution spectra were obtained. We expect that this approach which requires intense deuteration will have a significant impact on the quality and the rate at which solid-state NMR spectroscopic investigations will emerge in the future.

Temperature Dependence of Electromagnetically Induced Transparency in a Cs Atomic Vapor Cell

The temperature dependence of electromagnetically induced transparency (EIT) in a Cs atomic vapor cell was investigated. A probe laser beam frequency was scanned around the resonance frequency of the transition line 6S1/2, F = 3 → 6P3/2, F′ = 4 while the coupling beam frequency was locked to the transition line 6S1/2, F = 4→ 6P3/2, F′ = 4. In order to analyze the temperature dependence of the EIT signal quantitatively, we calculated the dispersion slope of the EIT signal at the resonance frequency of the probe beam by taking the 2 derivative of the measured EIT signal. We found that there existed an optimum temperature at which the dispersion slope had a maximum value. This temperature dependence of the EIT signal was well explained by the calculated results considering the Doppler effect and the atomic coherence relaxation due to atomic collisions.

Laser-induced retardation of radiation pulses under arbitrary collisional relaxation of low-frequency coherence

The effect of thermal atomic motion in gaseous media on the features of electromagnetically induced transparency is studied. Relatively general and compact analytical expressions are derived for the absorption coefficient and refractive index of a probe wave in the presence of high-power driving radiation that is resonant with respect to the adjacent atomic transition allowing for the relaxation of low-frequency coherence. It is demonstrated that the shape of the spectrum is similar to laser-induced structures in a continuum. This representation of resonances allows the electromagnetically induced transparency to be studied on relatively general assumptions and a significant retardation of radiation pulses and their delays to be demonstrated.

Generation and relaxation of high rank coherences in AX3 systems in a selectively methionine labelled SH2 domain

The usefulness of selective isotope labelling patterns is demonstrated using the C-terminal SH2 domain of PLC-gamma1 selectively 13C labelled at methionine methyl groups. We demonstrate the generation and relaxation of coherences that are second rank in protons and first rank in carbons that derive from quadrupolar order in protons. The decay rates of second rank double quantum proton coherences are measured. These terms exhibit fewer channels for cross-correlated relaxation compared to single quantum coherences. Our results indicate the potential application of the measurement of high order proton coherences to the analysis of dynamics in methyl-bearing side chains.

Time‐resolved coherent Raman and sum‐frequency generation spectroscopy with wavelength‐tunable, short‐pulse, photonic‐crystal fiber light sources

AbstractPhotonic‐crystal fibers deliver frequency‐tunable radiation within the wavelength range stretching from the ultraviolet to the near‐infrared with output pulse widths from tens of femtoseconds to a few picoseconds and peak powers from a few watts to several megawatts, suggesting a convenient, efficient, and compact fiber format of short‐pulse sources for time‐resolved nonlinear spectroscopy and microscopy. Here, we examine the potential of photonic‐crystal fiber light sources for time‐resolved studies of ultrafast population and coherence relaxation dynamics in molecular systems using coherent anti‐Stokes Raman scattering and two‐photon‐resonant sum‐frequency generation. Copyright © 2007 John Wiley & Sons, Ltd.

Measurement of exciton spin coherence by nondegenerate four-wave mixing experiments in theχ(3)regime

We demonstrate that the spin coherence relaxation of excitons can be measured in a three-beam four-wave mixing experiment in the ${\ensuremath{\chi}}^{(3)}$ regime, using a sequence of contracircularly polarized pulses. In contrast to other techniques like Faraday rotation measurement, our method allows one to access spin coherence without applying an external magnetic field. The superposition of opposite spin states (exciton-polariton spin coherence) is then probed in a nondegenerate configuration at the biexciton-exciton transition. Measurements are performed on a bulk CuCl platelet. The polariton effect is taken into account by modeling the propagation of the pulses. The exciton-polariton spin coherence time is demonstrated to be mainly determined by their spin lifetime.

Temperature dependence of surface photovoltage spectroscopy in vertically coupled self-organized InAs/GaAs quantum dots

The surface photovoltage (SPV) spectra of a series of vertically stacked self-organized InAs/GaAs quantum dot (QD)-based laser structures with different spacer layer (SL) thickness were obtained as a function of temperature (77 K </= T </= 300 K). A decrease of the compressive stress for thinner SL samples arising from coherent relaxation enables us to designate the effect of material intermixing as the most probable mechanism of the energetic blueshift of the observed structures. The turnaround characteristic of the temperature-dependent spectral intensity shows that the reduced SPV signal at higher temperature is limited by the carrier scattering and at lower temperature it is governed by the magnitude of built-in electric field and the escape efficiency of the photogenerated carriers. The dot states to be blueshifted by material intermixing are expected to have higher escape rate for carriers out of QDs, thus resulting in lower measurable temperature for the detected SPV signal. The relatively higher signal at low temperature for the 10 nm SL sample provides a direct evidence of the tunneling process of carriers in the stacked QD layers.

Golden rules for coherent transport relaxation rates in semiconductor superlattices in the field‐induced localization regime

AbstractCoherence decay rates are derived from first principles in electronic systems the quantum states of which are localized by an electric field. The decay rates are written down as golden rules in which the transition matrix elements and the energy conservation relations involve different quantum states. A numerical evaluation is carried out in GaAs/Al0.3Ga0.7As superlattices for the electron‐LO phonon interaction. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Will Spin-Relaxation Times in Molecular Magnets Permit Quantum Information Processing?

Using X-band pulsed electron-spin resonance, we report the intrinsic spin-lattice (T1) and phase-coherence (T2) relaxation times in molecular nanomagnets for the first time. In Cr7M heterometallic wheels, with M=Ni and Mn, phase-coherence relaxation is dominated by the coupling of the electron spin to protons within the molecule. In deuterated samples T2 reaches 3 micros at low temperatures, which is several orders of magnitude longer than the duration of spin manipulations, satisfying a prerequisite for the deployment of molecular nanomagnets in quantum information applications.

The determination of an inhomogeneous linewidth for a strongly coupled adsorbate system

We present a set of experiments that provide a complete mapping of coherent and incoherent vibrational relaxation times for a molecule on a metal surface, CO/Ir{111}. Included is the first detection of a midinfrared photon echo from a metallic surface, some 15 years after the analogous measurement on a semiconductor surface, which sets a precedent for the ability to manipulate and rephase polarization on a subpicosecond time scale on surfaces. For the C-O stretch in a strongly dipole-coupled CO layer we obtain a total linewidth of 5.6 cm-1, composed of a homogeneous width of 2.7 cm-1 and an inhomogeneous contribution of 3.0 cm-1. Pure dephasing is negligible at liquid nitrogen temperatures, making CO/Ir{111} an attractive model system for quantum computing.

Influence of magnetic-field inhomogeneity on nonlinear magneto-optical resonances

In this work, a sensitivity of the rate of relaxation of ground-state atomic coherences to magnetic-field inhomogeneities is studied. Such coherences give rise to many interesting phenomena in light-atom interactions, and their lifetimes are a limiting factor for achieving better sensitivity, resolution or contrast in many applications. For atoms contained in a vapor cell, some of the coherence-relaxation mechanisms are related to magnetic-field inhomogeneities. We present a simple model describing relaxation due to such inhomogeneities in a buffer-gas-free anti-relaxation coated cell. A relation is given between relaxation rate and magnetic-field inhomogeneities including the dependence on cell size and atomic spices. Experimental results, which confirm predictions of the model, are presented. Different regimes, in which the relaxation rate is equally sensitive to the gradients in any direction and in which it is insensitive to gradients transverse to the bias magnetic field, are predicted and demonstrated experimentally.

Wavelength-tunable ultrashort-pulse output of a photonic-crystal fiber designed to resolve ultrafast molecular dynamics

Wavelength-tunable 100 fs pulses generated through the soliton self-frequency shift in a photonic-crystal fiber are employed to visualize femtosecond coherence and population relaxation dynamics in molecular aggregates by means of time-resolved sum-frequency generation. This technique reveals an ultrafast dephasing of coherent molecular excitations with a phase relaxation time of about 120 fs and resolves an ultrafast switching of the nonlinear-optical response of molecular aggregates.

Solid−Solid Phase Transformation via Internal Stress-induced Virtual Melting, Significantly below the Melting Temperature. Application to HMX Energetic Crystal

We theoretically predict a new phenomenon, namely, that a solid-solid phase transformation (PT) with a large transformation strain can occur via internal stress-induced virtual melting along the interface at temperatures significantly (more than 100 K) below the melting temperature. We show that the energy of elastic stresses, induced by transformation strain, increases the driving force for melting and reduces the melting temperature. Immediately after melting, stresses relax and the unstable melt solidifies. Fast solidification in a thin layer leads to nanoscale cracking which does not affect the thermodynamics or kinetics of the solid-solid transformation. Thus, virtual melting represents a new mechanism of solid-solid PT, stress relaxation, and loss of coherence at a moving solid-solid interface. It also removes the athermal interface friction and deletes the thermomechanical memory of preceding cycles of the direct-reverse transformation. It is also found that nonhydrostatic compressive internal stresses promote melting in contrast to hydrostatic pressure. Sixteen theoretical predictions are in qualitative and quantitative agreement with experiments conducted on the PTs in the energetic crystal HMX. In particular, (a) the energy of internal stresses is sufficient to reduce the melting temperature from 551 to 430 K for the delta phase during the beta --> delta PT and from 520 to 400 K for the beta phase during the delta --> beta PT; (b) predicted activation energies for direct and reverse PTs coincide with corresponding melting energies of the beta and delta phases and with the experimental values; (c) the temperature dependence of the rate constant is determined by the heat of fusion, for both direct and reverse PTs; results b and c are obtained both for overall kinetics and for interface propagation; (d) considerable nanocracking, homogeneously distributed in the transformed material, accompanies the PT, as predicted by theory; (e) the nanocracking does not change the PT thermodynamics or kinetics appreciably for the first and the second PT beta <--> delta cycles, as predicted by theory; (f) beta <--> delta PTs start at a very small driving force (in contrast to all known solid-solid transformations with large transformation strain), that is, elastic energy and athermal interface friction must be negligible; (g) beta --> alpha and alpha --> beta PTs, which are thermodynamically possible in the temperature range 382.4 < theta < 430 K and below 382.4 K, respectively, do not occur.

Mixed Frequency/Time Domain Optical Analogues of Heteronuclear Multidimensional NMR

Ultrafast spectroscopy is dominated by time domain methods such as pump-probe and, more recently, 2D-IR spectroscopies. In this paper, we demonstrate that a mixed frequency/time domain ultrafast four wave mixing (FWM) approach not only provides similar capabilities, but it also provides optical analogues of multiple- and zero-quantum heteronuclear nuclear magnetic resonance (NMR). The method requires phase coherence between the excitation pulses only over the dephasing time of the coherences. It uses twelve coherence pathways that include four with populations, four with zero-quantum coherences, and four with double-quantum coherences. Each pathway provides different capabilities. The population pathways correspond to those of two-dimensional (2D) time domain spectroscopies, while the double- and zero-quantum coherence pathways access the coherent dynamics of coupled quantum states. The three spectral and two temporal dimensions enable the isolation and characterization of the spectral correlations between different vibrational and/or electronic states, coherence and population relaxation rates, and coupling strengths. Quantum-level interference between the direct and free-induction decay components gives a spectral resolution that exceeds that of the excitation pulses. Appropriate parameter choices allow isolation of individual coherence pathways. The mixed frequency/time domain approach allows one to access any set of quantum states with coherent multidimensional spectroscopy.

Femtosecond pump–probe studies of phonons and carriers in bismuth under high pressure

We investigate the high-pressure phase of Bi under hydrostatic pressure using pump–probe spectroscopy at pressures up to 3.0 GPa, and we observe coherent phonons signal and relaxation signal of photo-excited carriers at Bi(II) and Bi(III) phases. The pressure dependence of the coherent phonons shows that the amplitude of coherent phonons is extremely small and the frequency of coherent phonons changes at high-pressure phases. As results from our experiment, we obtain its frequencies are 2.5 and 2.2 THz at Bi(II) and Bi(III), respectively. Furthermore, photo-excited carrier relaxation indicates drastic changes near 2.5 GPa. Bismuth transforms from semimetal to semiconductor near 2.5 GPa, and band-overlapping between at L-point and at T-point disappears. We consider that the drastic changes of the photo-excited carrier relaxation are strongly correlated with the band-overlapping disappearing.

Femtosecond CARS of methanol–water mixtures

AbstractWe utilize femtosecond time‐resolved coherent anti‐Stokes Raman spectroscopy (CARS) to study the vibrational dynamics of methanol–water solutions. We measure the beat wavenumbers between Raman modes and coherence decay rates of CH stretch modes (at 2835 cm−1 and 2943 cm−1) in methanol. We observe a qualitative dependence of the coherence relaxation rate on the concentration of methanol in its water solution. This may be linked to the formation of the recently observed supramolecular structures in methanol–water mixtures. Copyright © 2006 John Wiley &amp; Sons, Ltd.