Degenerate Four-wave Mixing Spectroscopy Research Articles

Phase relaxation control in heterostructures featuring charge-transfer excitons

Type-II quantum well heterostructures are considered high-potential candidates for next-generation active semiconductor devices. They promise low fundamental transition energies and suppressed Auger scattering as well as temperature stability in device performance. Understanding their fundamental properties, such as scattering and diffusion, and revealing intricate differences from type-I quantum structures are important steps towards optimized structure design. Using degenerate four-wave mixing spectroscopy, we investigate the phase coherence of excitonic polarizations in a (Ga,In)As/GaAs/Ga(As,Sb) type-II double-quantum-well structure. It is designed to exhibit spectrally isolated resonances in its linear absorption spectrum including a well-resolved charge-transfer exciton resonance. This allows us to study the coherent dynamics of the unperturbed charge-transfer exciton polarization. In addition, the effects of many-body interactions with free charge carriers as well as excitons that are injected by an optical prepulse are revealed. Scattering of charge-transfer excitons with free charge carriers is three times more efficient than scattering of charge-transfer excitons with each other. The comparison with Wannier excitons in a type-I quantum well structure shows that the interaction strength between charge-transfer excitons is about twice as large as for excitons in a type-I quantum well structure.

Single-scan polarization-resolved degenerate four-wave mixing spectroscopy using a vector optical field

We report on a new method to achieve the single-scan polarization-resolved degenerate four-wave mixing (DFWM) spectroscopy in a Rb atomic medium using a vector optical field, in which two pump beams are kept linearly polarized and a vector beam is employed as the probe beam. As the polarization and intensity of the DFWM signal are closely dependent on the polarization state of the probe beam, a vector probe beam with space-variant states of polarization is able to generate a DFWM signal with space-variant states of polarization and intensity across the DFWM image. Accordingly, the polarization-resolved spectra can be retrieved from a single DFWM image. To the best of our knowledge, this is the first time that the single-scan polarization-resolved spectrum detection has been realized experimentally with a vector beam. This work provides a simple but efficient single-scan polarization-resolved spectroscopic method, which would be of great utility for the samples of poor light stability and fast optical processes.

Snap-shot optical polarization spectroscopy using radially polarized pulses

By using radially polarized pulses for excitation in degenerate four-wave mixing (DFWM) spectroscopy, we realize highly sensitive snap-shot measurement of optical anisotropies in semiconductor films. Because the radially polarized pulses exhibit spatially varying linear polarizations, the optical anisotropies in samples can be evaluated from the spatial distributions of DFWM signals without rotating the polarization of the excitation pulses. We measure the excitons in GaN layers and evaluate the energies of uniaxial strain and the spin-exchange interaction constant. Our results prove both the accuracy and sensitivity of the snap-shot measurement.

Polarization‐ and time‐resolved DFWM spectroscopy of the A 2Σ+− X 2Π (0,0) band transitions of nascent OH radicals generated by 266 nm laser photolysis of H2O2

The potential of a non‐linear spectroscopic technique – degenerate four‐wave mixing (DFWM) spectroscopy – for photo‐dissociation studies is investigated. By applying a carefully considered control of the input beam polarizations, the technique is used to measure the line shapes of specific rovibrational transitions in the A 2Σ+ − X 2Π (0,0) band of nascent OH radicals upon pulsed photo‐dissociation of H2O2. The photofragmentation is performed by linearly polarized radiation at 266 nm, and DFWM spectra are observed at various time delays. In contrast to linear spectroscopic methods, isotropic and anisotropic components of the non‐linear susceptibility tensor are separately accessible by using appropriate polarization geometries. In particular, four‐wave mixing signals can be generated that are exclusively due to the occurrence of the transient anisotropy of the recoil velocity and angular momentum vector distributions of the fragments emerging from the dissociation reaction. The observed line shapes in isotropic and anisotropic DFWM spectra are governed by the Doppler effect and vectorial correlations between the parent H2O2 molecule's transition dipole moment, the OH fragments’ recoil velocity and angular momentum, as well as by the processes of collision‐induced rotational and translational relaxation. Copyright © 2013 John Wiley & Sons, Ltd.

Isotopic Dependence of Excited-State Proton-Tunneling Dynamics in Tropolone Probed by Polarization-Resolved Degenerate Four-Wave Mixing Spectroscopy

The origin band of the Ã1B2-X1A1 (π* ← π) absorption system in monodeuterated tropolone (TrOD) has been probed with near-rotational resolution by applying the frequency-domain techniques of polarization-resolved degenerate four-wave mixing (DFWM) spectroscopy under ambient, bulk-gas conditions. Judicious selection of polarization geometries for the incident and detected electromagnetic waves alleviated intrinsic spectral congestion and facilitated dissection of overlapping transitions, thereby enabling refined rotational-tunneling parameters to be extracted for the Ã1B2(π*π) manifold. A tunneling-induced bifurcation of Δ0Ã = 2.241(14) cm(-1) was measured for the zero-point level of electronically excited TrOD, reflecting the presence of a substantial barrier along the O-D···O ↔ O···D-O reaction coordinate and representing nearly a 10-fold decrease in magnitude over the analogous quantity in the parent (TrOH) isotopologue. Observed trends in hydron-migration rates are discussed in light of the changes in the potential-surface topology sustained from the π* ← π electron promotion and the dynamical effects incurred by selective isotopic modification of the nuclear framework, with similar considerations being applied to interpret rotational constants and inertial defects. Simultaneous analyses performed on an interloping sequence band built upon ν38(b1) gave an excited-state tunneling splitting of Δ(ν38)Ã = 1.217(61) cm(-1), highlighting the ability of this symmetric, out-of-plane normal mode to inhibit the unimolecular tautomerization process.

Large Amplitude Motions in Cyclopentene and 1-Butene: Quantum Chemical Insights into the Ground- and Excited State Potential Energy Surfaces

Abstract The ring-puckering motion in cyclopentene as well as the hindered internal rotation around the central C–C bond in 1-butene have been studied by high-level quantum chemical calculations. Relevant potential energy surfaces of these molecules along these large-amplitude motions are provided as well as rotational constants are given allowing for thorough comparison with recent results from time-resolved femtosecond degenerate four-wave mixing (fs DFWM) spectroscopy. Emphasis is put on the performance of various post-Hartree Fock methods, the required level of electron correlation as well as the basis set quality.

The conformational stability of gaseous 1-butene studied by femtosecond nonlinear spectroscopy and ab initio calculations

1-Butene was investigated by rotational femtosecond degenerate four-wave mixing spectroscopy (fs DFWM) under supersonic expansion conditions as well as by quantum chemical calculations. Fs DFWM is a time-resolved rotational Raman spectroscopy, which allows for precise determination of rotational constants. The experimental fs DFWM spectrum was successfully reproduced by a fitted simulation using a single structure ascribed to the gauche conformer of 1-butene. The obtained rotational constants A = 22.6 ± 1.7 GHz, B = 4.1554 ± 0.0004 GHz and C = 4.0550 ± 0.0004 GHz are in excellent agreement with the ones from microwave spectroscopy. The cis conformer was not observed in the fs DFWM spectrum recorded under the low-temperature conditions of a supersonic expansion. The absence of the cis form along with simulations of a fs DFWM spectrum indicate that the cis conformer is at most equally but most likely less stable than the gauche rotamer. The experimental findings were supplemented by high level quantum chemical calculations, which predict the gauche form to have lower total energy than the cis structure.

Ring-puckering motion in cyclopentene studied by time-resolved rotational coherence spectroscopy and ab initio calculations

The ring-puckering vibration in cyclopentene was studied by rotational time-resolved femtosecond degenerate four-wave mixing (fs DFWM) spectroscopy. The fs DFWM spectra of cyclopentene were measured both in a supersonic expansion and in a gas cell at room temperature. The room temperature fs DFWM spectrum has been satisfactorily reproduced by a fitted simulation based on a one-dimensional model for the ring-puckering vibration. This has allowed for the determination of energetic parameters of the ring-puckering motion such as the energy barrier to ring inversion of 274(+12/-20) cm(-1) and the equilibrium ring-puckering angle of 24.3 degrees . The derived dependences of the rotational constants A and B on the puckering angle resemble very closely those obtained by microwave spectroscopy. In addition, previous theoretical estimates of the ring inversion barrier of cyclopentene were improved by performing high level ab initio calculations. Zero-point vibrational energy correction was found to be essential for an accurate evaluation of the puckering potential. Altogether, this study provides a proof-of-principle of the applicability of the fs DFWM technique for investigating large amplitude intramolecular motions.

Pseudorotation in pyrrolidine: rotational coherence spectroscopy and ab initio calculations of a large amplitude intramolecular motion

Pseudorotation in the pyrrolidine molecule was studied by means of femtosecond degenerate four-wave mixing spectroscopy both in the gas cell at room temperature and under supersonic expansion. The experimental observations were reproduced by a fitted simulation based on a one-dimensional model for pseudorotation. Of the two conformers, axial and equatorial, the latter was found to be stabilized by about 29 +/- 10 cm(-1) relative to the former one. The barrier for pseudorotation was determined to be 220 +/- 20 cm(-1). In addition, quantum chemical calculations of the pseudorotational path of pyrrolidine were performed using the synchronous transit-guided quasi-Newton method at the MP2 and B3LYP levels of theory. Subsequent CCSD(T) calculations yield the energy preference of the equatorial conformer and the barrier for pseudorotation to be 17 and 284 cm(-1), respectively.

Silver aggregates and twofold-coordinated tin centers in phosphate glass: A photoluminescence study

The optical properties of silver species in various oxidation and aggregation states and of tin centers in melt-quenched phosphate glasses have been assessed by optical absorption and photoluminescence (PL) spectroscopy. Glasses containing silver and tin, or either dopant, were studied. Emission and excitation spectra along with time-resolved and temperature-dependent PL measurements were employed in elucidating the different emitting centers observed and investigating on their interactions. In regard to silver, the data suggests the presence of luminescent single Ag + ions, Ag +–Ag + and Ag +–Ag 0 pairs, and nonluminescent Ag nanoparticles (NPs), where Ag +–Ag 0→Ag +–Ag + energy transfer is indicated. Tin optical centers appear as twofold-coordinated Sn centers displaying PL around 400 nm ascribed to triplet-to-singlet electronic transitions. The optically active silver centers were observed in glasses where 8 mol% of both Ag 2O and SnO, and 4 mol% of Ag 2O were added. Heat treatment (HT) of the glass with the high concentration of silver and tin leads to chemical reduction of ionic silver species resulting in a large volume fraction of silver NPs and the vanishing of silver PL features. Further characterization of such heat-treated glass by transmission electron microscopy and X-ray photoelectron spectroscopy appears consistent with silver being present mainly in nonoxidized form after HT. On the other hand, HT of the glass containing only silver results in the quenching of Ag +–Ag 0 pairs emission that is ascribed to nonradiative energy transfer to Ag NPs due to the positioning of the pairs near the surface of NPs during HT. In this context, an important finding is that a faster relaxation was observed for this nanocomposite in relation to a heat-treated glass containing both silver and tin (no silver pairs) as revealed by degenerate four-wave mixing spectroscopy. Such result is attributed to Ag NP→Ag +–Ag 0 plasmon resonance energy transfer. The data thus indicates that energy transfer between Ag +–Ag 0 pairs and NPs is bi-directional.

Ultrafast Exciton Dephasing in Semiconducting Single-Walled Carbon Nanotubes

Femtosecond two-pulse degenerate four-wave mixing spectroscopy was applied to study the exciton dephasing in a broad range of excitation intensities and lattice temperatures. We find that both exciton-exciton and exciton-phonon scattering have profound effects on the dephasing process. The dominant phonon mode involved in the dephasing is identified as the out-of-plane, transverse optical mode with a frequency of 847 cm(-1). The extracted homogeneous linewidths at all measured temperatures are in excellent agreement with the results of a single-tube photoluminescence experiment.

Exciton spin relaxation in GaN observed by spin grating experiment

The authors studied the exciton spin relaxation of bulk GaN by creating spin polarization gratings using degenerate four-wave mixing spectroscopy. The spectrally resolved analysis achieved with this technique facilitated the direct evaluation of spin polarizations in the individual excitons (A and B excitons). The spin polarizations for each exciton decay very quickly (τs∼1ps) at low temperatures. Moreover the τs is faster than the dephasing time T2 throughout the measured temperature range, suggesting the existence of fast intrinsic spin relaxation processes, which can be attributed to a large exchange constant characterized in GaN.

Degenerate four-wave mixing spectroscopy of GaN films on various substrates

We have carried out time-integrated and spectrally-resolved four-wave mixing (FWM) measurements to study the effects of built-in strain within GaN films on their exciton structures and on their coherent dynamics. Precise values for exciton energies, linewidths, and oscillation periods of quantum beats were presented. Remarkable phase shifts of the quantum beats were observed in the GaN films on a-sapphire, suggesting the exciton–exciton interaction caused by the reduction of the crystal symmetry. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Slow dephasing and local field effects of exciton–biexciton transition in [formula omitted] detected by femtosecond degenerate four-wave mixing

In femtosecond degenerate four-wave mixing (FWM) spectroscopy for the exciton and biexciton system in a semiconductor β - ZnP 2 , we have observed that the spectrally resolved FWM signals from the exciton–biexciton transition exhibit a specific time profile consisting of a growing up and decay for negative and positive delay between two excitation pulses, respectively. The decay profile indicates unexpectedly much slower dephasing of the exciton–biexciton transition compared to 1s exciton polariton. The time profile is interpreted as the manifestation of local field renormalization due to many-body Coulomb correlation among carriers in β - ZnP 2 , which is made observable due to the narrow homogeneous linewidth of the exciton–biexciton transition.

Comparative study of degenerate four‐wave mixing and cavity ringdown signal intensities of formaldehyde in a molecular beam

AbstractA comparison of degenerate four‐wave mixing (DFWM) and cavity ring‐down (CRD) spectroscopy is made on jet‐cooled formaldehyde under identical expansion conditions. We show that both methods are applicable to the low‐density environment of a molecular beam. However, a slightly superior signal‐to‐noise ratio for the background‐free DFWM method is observed for our experimental configurations. Absorption spectra simulated by applying an asymmetric rotor model are used to deduce Boltzmann temperatures of the rotational population distribution by a nonlinear least‐squares fit to the experimental data. In addition, partial interconversion of the nuclear spin upon supersonic expansion is observed. Consequently, a second fit parameter, describing the para/ortho concentration ratio in the molecular beam, is introduced. The resulting simulated spectra are in favorable agreement with the relative CRD intensities. A simple model for the reduction of the DFWM signal intensities to population is applied yielding absorption‐like spectra in accordance with both the simulations and the CRD measurements. We conclude that DFWM spectroscopy is applicable to deduce population distributions of an asymmetric rotor molecule under free‐jet conditions. Copyright © 2006 John Wiley & Sons, Ltd.

Ultrafast Exciton Dynamics in (AlAs)0.3/(GaAs)0.7 Ultrashort-Pperiod Superlattice

We investigated the optical properties of a direct-transition-type ultrashort-period (AlAs)0.3/(GaAs)0.7 superlattice. The exciton dephasing time of this superlattice measured by transient degenerate four-wave mixing spectroscopy was shorter than that of normal mixed crystal Al0.3Ga0.7As. This is attributed to the structural difference between both structures due to the growth procedure difference.

Calculations of Stark-broadened line shapes in phase-conjugate degenerate four-wave mixing laser spectroscopy

The Stark-broadened line shapes for phase-conjugate degenerate four-wave mixing (PCDFWM) laser spectroscopy are studied. The line profiles are calculated for high-density (N(e) > 10(21) m(-3) plasma conditions and for different contributions of the Doppler broadening. Calculations are performed in the limit of low laser intensities using the perturbation approach. The theoretical model takes into account the Stark effect together with the ion dynamics, the electron collisions, and the Doppler effect. The resultant PCDFWM spectral profiles are significantly less broadened and less asymmetric than the emission profiles but shifted by similar magnitudes. Moreover, the results of our calculations show the sub-Doppler character of the PCDFWM profiles and also their strong dependence on the geometric configuration of the pump and the probe laser beams.

Optical diffraction spectroscopy of excitons in uniaxially strained GaN films

The degenerate four-wave mixing (FWM) spectroscopy of uniaxially strained GaN layers is demonstrated using colinearly polarized laser pulses. The nonlinear response of FWM signal on exciton oscillator strength enhances the sensitivity for polarized exciton, allowing for mapping out the in-plane anisotropy of the strain field. The observed high-contrast spectral polarization clearly shows fine structure splittings of excitons, which are also confirmed in the change of quantum beating periods of time.

Observation of all- trans-β-carotene wavepacket motion on the electronic ground and excited dark state using degenerate four-wave mixing (DFWM) and pump–DFWM

Wavepacket dynamics on the ground and on the optically dark, first electronic state of all- trans-β-carotene is presented. Ground state dynamics are studied using resonant and non-resonant degenerate four-wave mixing spectroscopy with 16 fs time resolution. Wavepacket motion on the electronic dark state was monitored using pump-degenerate four-wave mixing spectroscopy.

Degenerate four-wave mixing spectroscopy based on two-dimensional femtosecond pulse shaping.

We report noncollinear, degenerate four-wave mixing experiments that employ a new device based on two-dimensional femtosecond pulse shaping that delays and modulates all incident fields. Heterodyne detection is easily implemented due to the full phase stability of the device.