Dephasing Time T2 Research Articles

Dynamics of excitons in CuBr nanocrystals: Spectral-hole burning and transient four-wave-mixing measurements

CuBr nanocrystals ~NC’s ! embedded in a borosilicate glass matrix have been studied by transmission pump-and-probe experiments with both spectral and temporal resolution. Transient as well as persistent spectral hole-burning phenomena are observed in the excitonic absorption of small NC’s ~mean radius less than 4 nm! when picosecond and nanosecond dye-laser pulses excite selectively NC’s of a specific size at low temperatures. The holes consist of a zero-phonon line ~at the photon energy of the excitation! and marked phonon sidebands. This structure is explained by a strong exciton‐optical-phonon interaction. A value of about 1 of the Huang-Rhys factor S is obtained from the Stokes shifts between absorption and photoluminescence spectra. From the width of zero-phonon holes, a lower limit of 2.3 ps of the exciton dephasing time T2 is deduced. This value is compared to the one obtained by transient four-wave-mixing measurement ~6.4 ps! at low excitation intensities. When the radii of NC’s decrease, both types of spectral holes become broader and the persistent spectral-hole burning is more efficient. @S0163-1829~98!07903-X#

Excitonic Effects in Cd1?xMnxSe Quantum Dots and Cd1?xMnxTe/ZnTe Quantum Wells

Excitonic properties and dynamics are studied in nanostructure diluted magnetic semiconductors by transient luminescence and nonlinear optical spectroscopies. Quantum dots (QDs) of Cd1—xMnxSe show enhanced excitonic magnetic polaron effects with an increased binding energy of 20 meV. Quantum wells (QWs) of the Cd1—xMnxTe/ZnTe (x = 0 to 0.2) system display fast dephasing relaxation of the QW exciton with the dephasing time T2 of 360 to 450 fs, which indicates the fast dephasing process of free excitons in the strained QWs. In the asymmetric double QWs, the tunneling time of carriers through the intermediate barrier is determined as 0.01 to 106 ps for barrier thicknesses of 40 to 320 A. An enhanced excitonic magnetic polaron binding energy, the rapid dephasing of free excitons and tunneling of carriers are the characteristic properties of the QW excitons in the diluted magnetic semiconductor nanostructures.

Thermal depinning and motions of the flux lines inYBa 2 Cu 4 O 8 from NMR experiments

The spin-lattice relaxation time T1, the spin echo dephasing time T2 and the NMR linewidth δv are used to study the spontaneous fluctuations under thermal excitation of the flux lines (FL) in YBa2Cu4O8. It is shown how the relaxation rate T1-1 is sensitive to the fluctuations of the transverse components of the local field at a frequency around ω = γH0, while T2-1 and δv probe the longitudinal components at frequencies smaller by a factor 103-105. Thus, from a combination of T1 T2 andδv measurements one derives microscopic insights on the degree of correlation in the motion of 2D pancakes belonging to adjacent superconducting planes, on the barriers for the pseudo-diffusion of the FL’s and on their field and temperature dependence. 2D NMR spectroscopy can also be used to evidence the diffusion of the FL’s and a comparison between YBa2Cu3O7 (YBCO123) and YBa2Cu48 (YBCO124) is briefly discussed. Finally for temperatures below Tirr(H the evolution of the amplitude of the63Cu NQR echo after sudden application and/or rotation of a magnetic field is used to investigate the FL flow in the critical state.

Sub-picosecond resonant third-order nonlinear optical response of azobenzene-doped polymer film

The dynamics and the sub-picosecond resonant third-order nonlinearity of azobenzene-doped polymer films are investigated by means of transient degenerate four-wave mixing and z-scan techniques. The third-order susceptibility χ(3) and the dephasing time T2 are measured to be (6.3±0.6)×10−12 esu and (1.0±0.1)×10−12 psec, respectively. The creation of a new intermediate state related to a conformational deformation mechanism is supported by experimental observations.

Femtosecond dephasing in porous silicon

We determine dephasing times T2 in free standing porous silicon samples using nondegenerate four-wave mixing. Within the framework of an idealized inhomogeneously broadened two-level system, values of 20 fs for different porosities are obtained. Fast energy relaxation times are found in the range of 1.5–3.5 ps for 64%–73% porosities, respectively. They are attributed to carrier thermalization within the band tails, originating from porous silicon nanocrystallites and their surface states.

Dephasing in the gain region of II–VI semiconductor nanocrystals

We investigate the dephasing times in highly excited CdSe and CdS0.3Se0.7 nanocrystals by spectral hole burning throughout the gain region. The energy dependence of the dephasing time T2 is compared between quantum dots in the strong-confinement regime and bulklike microcrystals. T2 in strongly confined quantum dots remains rather constant, whereas the bulklike sample shows a continuous increase of T2 toward the transparency point. This observation is attributed to the different gain mechanisms in the strong and the weak quantum-confinement regimes.

Time-domain CARS study of dephasing kinetics of molecular hydrogen rotational transitions

Pure rotational transitions of molecular hydrogen were studied by means of time-domain coherent anti-Stokes Raman spectroscopy (time-domain CARS) in a wide pressure range from 26 down to 0.003 atm at room temperature. Dicke effect manifestation was observed for S0(0) and S0(1) transitions in the forward scattering geometry. The longest dephasing time T2 measured was 8.7 and 6.2 ns, corresponding to spectral line bandwidth 0.0012 and 0.0017 cm−1.

Degenerate four-wave mixing in picosecond pulses: study of the temporal shape of the diffracted signal as a function of the material response time

We present a time-resolved study of the diffracted pulse in degenerate four-wave mixing experiments with short-response-time materials. The first step is the validation of a theoretical model that takes into account the temporal characteristics of the energy level (relaxation time T1 and dephasing time T2) and of the incident pulse (duration and spectral phase) in a simple particular case. Two methods are used for characterizing the diffracted beam: the amplitude correlation with the reference pulse and the spectrum intensity measurements. We discuss the experimental results, which show good agreement with the theoretical model and evidence the strong influence of the incident pulse characteristics.

^7F_0-^5D_1 Transition in Eu^3+:Y_2SiO_5

We report a spectroscopic study of the 7F0–5D1 transition of europium ions in crystal yttrium orthosilicate (Eu3+:Y2SiO5) with a focus on low-temperature properties relevant to its use in time-domain optical data storage. The optical dephasing time T2 for this green transition was found to be approximately 50 μs for Eu3+ in one optical site (site 1) and 56 μs for Eu3+ in the other site (site 2), both of which exceeded their respective excited-state lifetimes (32 μs for site 1 and 42 μs for site 2). Of the two optical sites, the transition on site 2 has a slightly larger inhomogeneous linewidth (~2.7 GHz) but a significantly larger oscillator strength (~6 × 10−8), making it a good candidate for time-domain optical storage. The hyperfine structures of the transition for both sites were also measured by spectral hole burning.

Dephasing of three- and two-dimensional magnetoexcitons in InxGa1−xAs

We investigate the dephasing of three- and two-dimensional magnetoexcitons in InxGa1−xAs at low temperatures by femtosecond four-wave mixing. In both cases the dephasing time T2 increases substantially with magnetic field. For a cyclotron energy one order of magnitude larger than the three-dimensional exciton binding energy, T2 rises by a factor of about 4. This is interpreted in terms of a considerably reduced scattering cross section of the 1s magnetoexciton due to the magnetic confinement.

Femtosecond dephasing times in semiconductor microcrystals measured with incoherent light

Incoherent laser pulses are used to measure the dephasing time of photogenerated excitons in semiconductor-doped glasses. The coherent polarization dephasing time T2 is determined for several commercial glasses as well as for a semiconductor thin film. It is found that T2 varies systematically from less than 5 fs to longer than 70 fs, depending on the degree of excess excitation beyond the band edge. The temperature dependence of T2 is also measured. A systematic behavior of smaller T2 at higher temperature is observed in all samples. These T2 values are believed to result from the loss of orientational coherence of the excitons generated by laser excitation.

Suppression of vibron state formation in Arx(N2)1−x mixed crystals

The dephasing of the k≂0 states of the vibrons in Arx(N2)1−x crystals was studied by picosecond time-resolved coherent anti-Stokes Raman scattering as a function of temperature (0<T<40 K) and Ar concentration (0<x<0.15). Homogeneous (exponential) and inhomogeneous (Gaussian) contributions to the observed dephasing signal were separated. In the α phase, an increase of the inhomogeneous linewidth was obtained starting from small concentrations of Ar atoms. For larger Ar concentrations, a decrease of the exponential dephasing time T2 and a temperature dependence of the increasingly faster Gaussian decay is observed. Substitution of N2 molecules by Ar atoms increases the librational amplitude. Through anharmonic terms in the crystal Hamiltonian the vibron state formation is reduced and as a result, the T2 time decreases. This suppression of vibron state formation, which counteracts the reduction of the inhomogeneous broadening, is responsible for the increasing and temperature dependent inhomogeneous linewidth. The Ag−Tg factor group splitting increases almost linearly with Ar concentration. Surprisingly, no inhomogeneous line broadening could be resolved for β-Arx(N2)1−x crystals and the observed dephasing time T2 remains long compared to the α phase, even for high Ar concentrations.

Optimal control of unimolecular reactions in the collisional regime

The possibility of controlling unimolecular–dissociation processes with multiple laser fields in the collisional regime is examined. Employing the Bloch equations to describe optical excitation and decay processes, optimal control theory is used to design amplitude modulated fields which produce the desired excited-state products. The selectivity of the product distribution of a simple four-state photodissociation system is shown to have a square-root dependence on the relative value of the mean dephasing time T2 to the pulse length τ, i.e, (T2/τ)1/2. The equivalence between T2 decay and phase disruptions occurring in a random-walk fashion is also examined. In the Appendix it is shown that the essential effect of the system temperature is to introduce a Boltzmann population factor on the product selectivity without affecting the nature of the optimal field.

Photochemical hole burning of tetraphenylporphin in epoxy resin: Effect of crosslinked structure

Photochemical hole burning (PHB) of free-base tetraphenylporphin (TPP) in epoxy resin was performed at 4.2–80 K. Effect of crosslinked structure of the epoxy resin on the dephasing time T2, low-energy excitation modes, and the temperature dependence of Debye–Waller factor were studied through the PHB measurements. The excellent thermal stability of TPP in epoxy resin including the hole burning above liquid-nitrogen temperature (80 K) was demonstrated.

248-Bit optical data storage in Eu^3+:YAlO_3 by accumulated photon echoes

The successful storage and retrieval of 248-bit temporal optical data in Eu(3+):YAlO(3) by accumulated photon echoes is reported. Owing to the long storage time of this crystal, the input data can be written bit by bit by quickly varying the separation of two laser pulses, while the stored data are recalled simultaneously after some storage period, which extends up to a few hours. The maximum data length, previously thought to be limited by the optical dephasing time T(2), has been found to be four times shorter than T(2) and attributable to spectral diffusion.

The Pressure Dependence of T2- Relaxation Rates for Rotational Transitions of Sulfur Dioxide in Mixtures with Oxygen and Nitrogen

Abstract The transient emission technique in the time domain has been used to determine the pressure dependence of the coherent dephasing time T2 of sulfur dioxide in mixtures with O2 and N2 for several rotational transitions in the vibrational ground state. From these data line broadening parameters Δv/p and collisional cross-sections are calculated.

Spectral hole burning properties of rotationally localized CN- deffects in cesium halides

Persistent spectral hole and antihole burning has been used to study the homogeneous linewidth and dynamical behavior of rotationally aligned CN- defects in cesium halides. Rotational alignment occurs by statistically associating the CN- defect to a neighboring K+ or Rb+ impurity. The resulting defect pair produces sharp background-free stretching absorption lines v1 and v2 near 2000 cm-1, corresponding to two possible orientations of the CN- electric dipole moment with respect to the neighboring alkaline-ion impurity. Measurements were performed on the v1 and v2 2nd harmonic transitions near 4000 cm-1 using a single-mode tunable color center laser. Burning polarized persistent holes into the v2 line produces equally polarized antiholes in the v1 line and vice versa, indicating that 180° CN- reorientations among the 〈111〉 pair axes are optically induced. At T<40 K, each configuration is stable due to a high rotational barrier and can be fully populated. Thermal hole refilling occurs with a sharp temperature dependence near 40 K. For dilute CN-:K+ defects in CsCl minimum homogeneous linewidths of ≈10 MHz were measured, corresponding to a dephasing time T2≈15 ns. Since the radiative lifetime T1 is in the millisecond range, fast dephasing processes are thus dominating the homogeneous broadening.

Analysis of dephasing signal in picosecond stimulated-Raman-gain experiments

A detailed theoretical analysis is presented of the dephasing signal measured in four-beam picosecond time-resolved stimulated-Raman-gain experiments. The analytical expressions derived are successful in simulating observations on the CS2 model system under various experimental conditions. It is established that the tail of the curve for sufficiently long delay times is uniquely determined by the vibrational Raman term that yields the vibrational dephasing time T2. It is also found that the rotational dephasing time cannot be fundamentally determined from the rotational Raman peak centered around zero delay. Furthermore, it is shown that the coherence properties of the stimulating laser fields, as distinguished from their pure pulse-envelope properties, are crucially important in determining the time resolution of the experiments and for explaining several experimental phenomena observed under mismatched excitation-pulse conditions. In this respect, the domain model for mode-locked pulses has proved to be useful. It is found that the time resolution of the dephasing measurements is determined by the combined pulse-coherence time of the driving fields and not by the cross-correlation width of the pulse envelopes.

Measurement of ultrafast dephasing time of ruby by transient grating with incoherent light

Dephasing time T2 of ruby in the absorption band 4T2 has been measured for the first time using the method of transient grating with incoherent light. The population grating was induced in the material by two broad-band nanosecond pulses of a dye laser and probed by a Continuous Wave (CW) Ar+ laser. At room temperature, T2=0.04 similar 0.08 ps was obtained when the sample was excited at λ = 5640Å.

Pure dephasing and nonradiative decay processes in the excited electronic states of NaF:Cu+

We report the first optical dephasing study of an inorganic impurity system possessing sharp, low frequency mode structure in its ground and excited state spectra. The total dephasing times T2 and population relaxation times T1 for the 1T2g and 1Eg d9s excited states of NaF:Cu+ are determined at a series of temperatures between 1.8 and 296 K. The T2 values are determined by extracting the Lorentzian components from one- and two-photon excitation line shapes. The T1 values caused by nonradiative decay rates are obtained by detecting very low quantum yield emission from the fast-relaxing excited states and applying the formula tnr=Qtr, where t’s are radiative and nonradiative lifetimes and Q is the quantum yield. T1(1T2g)=4.6 ps and T1(1Eg )=2.0 ps at 8 K. Significantly, these are very similar to the T1 values calculated from lowest temperature Lorentzian linewidths by the relationship 1/T2=1/T′2 +1/(2T1). The T1 values stay approximately constant over the temperature range 1.8–45 K, while the linewidths rapidly increase indicating that pure dephasing dominates. Using ground and excited state information on the low frequency modes, we test optical Redfield theory and the nonperturbative harmonic theory for pure dephasing by pseudolocal phonons against data for this system which displays strongly anhamonic progressions. The nonperturbative theory fits the line broadening data to higher temperatures than optical Redfield theory for the least anharmonic excited state potential, 1T2g. Both theories underpredict the broadening with temperature of the extremely anharmonic 1Eg state. A simple anharmonic theory including scattering to overtone levels also fails to predict the observed linewidth temperature behavior, although it is demonstrated these processes should be occurring.