Nonlinear Optical Response Of Excitons Research Articles

Simultaneous Existence of Confined and Delocalized Vibrational Modes in Colloidal Quantum Dots

Coupling to phonon modes is a primary mechanism of excitonic dephasing and energy loss in semiconductors. However, low-energy phonons in colloidal quantum dots and their coupling to excitons are poorly understood because their experimental signatures are weak and usually obscured by the unavoidable inhomogeneous broadening of colloidal dot ensembles. We use multidimensional coherent spectroscopy at cryogenic temperatures to extract the homogeneous nonlinear optical response of excitons in a CdSe/CdZnS core/shell colloidal quantum dot ensemble. A comparison to the simulation provides evidence that the observed lineshapes arise from the coexistence of confined and delocalized vibrational modes, both of which couple strongly to excitons in CdSe/CdZnS colloidal quantum dots.

Excitation power dependence of nonlinear optical response of excitons in GaAs/AlAs superlattices

AbstractWe studied effects of the miniband structure to control the nonlinear optical response of excitons in GaAs/AlAs superlattices. The excitation‐power dependence of the degenerate four‐wave‐mixing signal shows different saturation power in the samples with the thinner and thicker barriers, while the dependence of dephasing time is almost independent of the barrier width. The analysis of the excitation‐power dependence of the photoluminescence spectrum indicates the biexciton generation at lower excitation power in the sample with the thicker barrier width. These results demonstrate that the saturation of the four‐wave‐mixing signal intensity of the exciton comes from the contribution of the biexcitons. Our results support a possibility that the nonlinear optical response of excitons can be controlled by controlling the barrier width. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Sum-over-states versus quasiparticle pictures of coherent correlation spectroscopy of excitons in semiconductors: Femtosecond analogs of multidimensional NMR

Two-dimensional correlation spectroscopy 2DCS, based on the nonlinear optical response of excitons to sequences of ultrafast pulses may provide some unique insights into carrier dynamics in semiconductors. The most prominent feature of 2DCS, cross peaks, can best be understood using a sum-over-states picture involving the many-body eigenstates. However, the optical response of semiconductors is usually calculated by solving truncated equations of motion for dynamical variables, which result in a quasiparticle picture. In this work we derive Green’s function expressions for the four wave mixing signals generated in various phase-matching directions and use them to establish the connection between the two pictures. The formal connection with Frenkel excitons hard-core bosons and vibrational excitons soft-core bosons is pointed out.

Coherent nonlinear optical response of excitons and biexcitons in quantum dots coupled to phonons

AbstractFour wave mixing signals from single dots and dot ensembles involving excitonic and biexcitonic transitions are discussed. The results have been obtained analytically from a four level dot model coupled to longitudinal acoustic phonons. Time integrated four wave mixing amplitudes for quantum dot ensembles exhibit polarization dependent quantum beats that have already been found in experimental data. In four wave mixing signals of single quantum dots we find that the ground state to exciton transition has two contributions. One is identical to the known solution of the two level quantum dot model and leads to symmetric spectra for longer delay times, while the other comprises phase information from the biexciton and results in asymmetric spectra. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Theory of ultrafast exciton dynamics in the Quantum Hall system

We discuss a theory of the ultrafast non-linear optical response of excitons in the Quantum Hall system. Our theory focusses on the role of the low energy collective electronic excitations of the cold, strongly correlated, two-dimensional electron gas (2DEG) present in the ground state. It takes into account ground state electron correlations and Pauli exchange and interaction effects between the photoexcited excitons and the collective excitations. Our formulation addresses both the initial coherent regime, where the dynamics is determined by exciton and 2DEG polarizations, and the subsequent incoherent regime, dominated by population dynamics. We describe non-Markovian memory, dephasing, and correlation effects and a non-linear exciton hybridization due to the non-instantaneous interactions. We identify the signature of the coherent inter-Landau level magnetoplasmon (MP) in the temporal profile of the three-pulse time-integrated four-wave-mixing signal in the initial coherent regime.

Electronic four-particle correlations in semiconductors: Renormalization of coherent pump-probe oscillations

The nonlinear optical response of excitons in a single ZnSe quantum well is investigated by pump-probe measurements. In the counter-circular polarization configuration at negative delay, $\ensuremath{\Delta}t,$ spectral oscillations are observed the period ${E}_{\mathrm{PP}}$ of which is energy dependent. Whereas at energies above the $1S$ exciton the usual ${E}_{\mathrm{PP}}=h/|\ensuremath{\Delta}t|$ dependence is found, the period strongly deviates from that value at energies below the exciton. Model calculations reveal that this behavior is caused by the bound biexciton state in the four-particle correlations.

Signatures of the electromagnetic field quantization in the nonlinear optical response of excitons

We present a fully quantum mechanical theory for an interacting system of photons and Coulomb correlated electrons and holes in a semiconductor, using a pertubation series in the exciting laser field. The theory provides microscopic descriptions of nonlinear optical processes in semiconductors related to the electromagnetic field quantization. As an application of the theory we show that it is possible to transfer the exciton-exciton Coulomb correlation to photons, thus producing pairs of near-gap photons with a high degree of quantum entanglement. The photon pairs emerge from the spontaneous optical decay of biexcitons into two polaritons. The pair intensity correlations, calculated in the low density limit for a CuCl slab and for semiconductor microcavities, exhibit quantum features which can be observed by coincidence detection.

Strong coherent gain from semiconductor microcavities in the regime of excitonic saturation

The nonlinear optical response of excitons strongly and coherently driven by intense laser pulses is a topic raising much interest. 1 So far, femtosecond ~fs! pump-probe experiments in free-space semiconductors, with the pump pulses tuned to the transparency region below the band gap, showed the so-called optical Stark shift of the exciton resonance. 2 Similar experimental investigations have recently reported a hyper-Raman gainon the low-energy side of the nonresonant

Ultrafast nonlinear optical response of excitons inZnxCd1−xSe/ZnSySe1−ymultiple-quantum-well structures studied by femtosecond pump and probe spectroscopy

Femtosecond pump and probe spectroscopy has been applied to II-VI semiconductor multiple-quantum-well structures. The nonlinear optical response of excitons has been investigated by measuring differential absorption spectra with various time delays for different pumping intensities. The spectral shape analysis has shown that the spectral change induced by the pumping pulses is composed of the broadening, the peak shift, and the decrease of the absorption area of the exciton band. From the intensity dependence of the absorption area under the low pumping intensity limit, the saturation exciton density has been determined to be 5\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, which agrees with the theoretical value taking into account the phase-space filling and the exchange effect due to the excitons generated from free electron-hole pairs excited by the pumping light. The intensity dependences of the peak shift and the absorption area are also discussed. The peak shift and the absorption area exhibit the same dependence, and the origin of both spectral changes is found to be the exciton-exciton exchange effect. The saturation density and the peak shift depending on the width of the well layers are also discussed. The dependences of their parameters are explained by the confinement effect of excitons in thin well layers.

Theory of nonlinear optical response of excitons in solid C60.

We present a theoretical study of the dispersion of Frenkel-type excitons in the low-temperature Pa3\ifmmode\bar\else\textasciimacron\fi{} phase of solid ${\mathrm{C}}_{60}$. Exciton propagation is accomplished by an exchangelike two-step process, where an electron-hole pair on a given ${\mathrm{C}}_{60}$ molecule is split by the intermolecular kinetic energy and recombines on a neighboring molecule. We apply our results to the theoretical calculation of the nonlinear optical susceptibility for second-harmonic generation, for which we find reasonable agreement with experiment. \textcopyright{} 1996 The American Physical Society.

Nonlinear optical response of excitons in small ring clusters

The static second hyperpolarizability, γ(N) of interacting excitons on rings of up toN = 19 unit cells is investigated as the system evolves from a Frenkel to a photoconducting ring. Coulomb interactions are treated exactly in a complete basis set of one and two electron-hole pairs.

Coherent Nonlinear Response of Excitons in Semiconductors

AbstractThe nonlinear optical response of excitons in semiconductors is studied theoretically, particularly the so‐called nonresonant optical Stark effect. Results from a two‐band density matrix formalism and its reduction to a two‐level approximation are compared. It is found that the two models give similar results except at very high pumping levels. The calculated results agree well with experiments on quantum wells in (AlGa)As–GaAs.

Nonlinear optical response of excitons confined to one dimension.

We present femtosecond time-resolved transient reflectivity measurements on polydiacetylene performed at the peak of the exciton resonance. The magnitude of the nonlinear refractive index at 1.97 eV was found to be 3.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ ${\mathrm{cm}}^{2}$/W. We discuss a model which accounts quantitatively for the large resonant and nonresonant optical nonlinearity observed in polydiacetylene in terms of phase-space filling by one-dimensional excitons.

Nonlinear Optical Response of Excitons Confined to one Dimension

ABSTRACTWe present femtosecond time-resolved transient absorption data taken on polydiacetylene-pTS single crystals over the entire wavelength region of exciton resonance. Observed optical changes are accounted for by a model which predicts quantitatively the large resonant and nonresonant optical nonlinearities.