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Compared to pristine Cu2O nanoparticles (NPs), Ag@Cu2O core-shell NPs exhibit photocatalytic activity over an extended wavelength range because of the presence of localized surface plasmon resonance (LSPR) in the Ag core. The photocatalysis action spectra and transient absorption measurements show that the plasmonic energy is transferred from the metal to the semiconductor via plasmon-induced resonant energy transfer (PIRET) and direct electron transfer (DET) simultaneously, generating electron–hole pairs in the semiconductor. The LSPR band of Ag@Cu2O core-shell NPs shows a red-shift with an increase in the Cu2O shell thickness, extending the light absorption of Ag@Cu2O heterostructures to longer wavelengths. As a result, the photocatalytic activity of the Ag@Cu2O core-shell NPs is varied by modulation of the shell thickness on the nanometer scale. This work has demonstrated that the Ag@Cu2O core-shell heterostructure is an efficient visible-light plasmonic photocatalyst, which allows for tunable light ab...

Semiconductors systems exhibiting excitonic properties are discussed in terms of their coherent response, which is extracted using two-dimensional coherent spectroscopy. This control method allows for separation of quantum pathways that comprise the optical response, such as interactions between excitons, their dephasing rates, the effects of many-body interactions and the role of structure on the microscopic electronic environment. Additional controls, such as polarization can be used to further distinguish biexcitons and suppress many-body interactions. These result are compared and contrasted with those from a semiconductor microcavity where the excitons form polaritonic modes due to normal-mode splitting. Rephrasing spectra map the detuning dependence of the exciton-polariton branches. Increasing the detuning shifts all features to higher energy and the expected anti-crossing is observed. An isolated biexciton is seen only at negative detuning, separated by a binding energy. For positive detuning, the spectral weight of the off-diagonal features swap, as the lower polariton branch and biexciton come into resonance. This indicates that the off-diagonal features are sensitive to the interactions of the exciton-polaritons and other resonances in the system.

Transient four-wave mixing studies of bulk GaAs under conditions of broad bandwidth excitation of primarily interband transitions have enabled four-particle correlations tied to degenerate (exciton-exciton) and nondegenerate (exciton-carrier) interactions to be studied. Real two-dimensional Fourier-transform spectroscopy (2DFTS) spectra reveal a complex response at the heavy-hole exciton emission energy that varies with the absorption energy, ranging from dispersive on the diagonal, through absorptive for low-energy interband transitions to dispersive with the opposite sign for interband transitions high above band gap. Simulations using a multilevel model augmented by many-body effects provide excellent agreement with the 2DFTS experiments and indicate that excitation-induced dephasing (EID) and excitation-induced shift (EIS) affect degenerate and nondegenerate interactions equivalently, with stronger exciton-carrier coupling relative to exciton-exciton coupling by approximately an order of magnitude. These simulations also indicate that EID effects are three times stronger than EIS in contributing to the coherent response of the semiconductor.

Polarization-dependent two-dimensional Fourier-transform spectroscopy (2DFTS) is performed on excitons in strained bulk GaAs layers probing the coherent response for differing amounts of strain. Biaxial tensile strain lifts the degeneracy of heavy-hole (HH) and light-hole (LH) valence states, leading to an observed splitting of the associated excitons at low temperature. Increasing the strain increases the magnitude of the HH/LH exciton peak splitting, induces an asymmetry in the off-diagonal coherences, increases the difference in the HH and LH exciton homogenous linewidths, and increases the inhomogeneous broadening of both exciton species. All results arise from strain-induced variations in the local electronic environment, which is not uniform along the growth direction of the thin layers. For cross-linear polarized excitation, wherein excitonic signals give way to biexcitonic signals, the high-strain sample shows evidence of bound LH, HH, and mixed biexcitons., Comment: 10 pages, 4 figures, 2 tables, journal article

Optical 2-D Fourier transform spectroscopy is a powerful technique for studying resonant light-matter interactions, determining the transition structure and monitoring dynamics of optically created excitations. The ability to separate homogeneous and inhomogeneous broadening is one important capability. In this paper, we discuss the use of this technique to study excitonic transitions in semiconductor nanostructures. In quantum wells, the effects of structural disorder is observed as inhomogeneous broadening of the exciton resonances. In quantum dots, the temperature dependence of the homogeneous width gives insight into the nature of the dephasing processes.

Optical two-dimensional Fourier transform spectroscopy (2DFTS) is used to measure thermal broadening of the ground state exciton homogeneous linewidth and energy relaxation in a single layer of interfacial GaAs quantum dots (QDs). We observe a nonlinear increase in the homogeneous linewidth with temperature from 6 to 50 K and find that a phonon activation term + offset fit the data well. The absence of an activation peak in the 2D spectra indicates that elastic exciton–phonon scattering via virtual transitions between the ground and excited states significantly contributes to the thermal broadening. Measurements of the linewidth across the inhomogeneous distribution show that excitons localized in smaller QDs experience stronger thermal broadening. We record 2D spectra for a population time up to 500 ps and temperature up to 50 K to reveal excitonic relaxation from quantum well to QD states. As the temperature increases from 6 to 50 K, the relaxation time decreases from 180 to 30 ps, respectively. This behavior is consistent with a phonon-mediated scattering process.

While two-photon emission processes are firmly established in atomic physics, their observation and use in semiconductor physics remains elusive. Here, we experimentally investigate stimulated two-photon emission in photoexcited bulk CdSe and identify requirements for the observation of stimulated two-photon emission. In particular, this process requires population inversion as well as two-photon transition energies close to the bandgap energy. In any regime investigated in the present study, net optical gain is not achieved, as the free-carrier absorption intrinsically linked to the photoexcitation completely masks the two-photon gain. The results are well in line with a recent study on nondegenerate versions of two-photon emission in GaAs and place clear limits for the practical use of two-photon emission in optically excited semiconductors.

Understanding and controlling the spin degree of freedom in two-dimensional transition metal dichalcogenides offers the potential for designing functional quantum materials. This work investigates the dynamics of photo- and resident carrier spins in an encapsulated MoSe2 monolayer using non-degenerate time-resolved Kerr-rotation microscopy. The lightly doped monolayer exhibits clear exciton and trion resonances with spin-polarizations that are characterized by a fast (~20 ps) decay attributed to photocarrier relaxation and recombination, followed by a slow (~690 ps) decay associated with resident carrier depolarization. Dual-frequency Kerr-rotation spectra directly reveal exciton-trion coupling on ultrashort timescales and within the spin coherence time of the system. Moreover, the distribution of the exciton-trion coupling features exposes inhomogeneous broadening likely arising from different domains within the excitation spot.

Coherent light-matter interactions of direct-gap semiconductor nanostructures provide a great test system for fundamental research into quantum electronics and many-body physics. The understanding gained from studying these interactions can facilitate the design of optoelectronic devices. Recently, we have used optical two-dimensional Fourier-transform spectroscopy to explore coherent light-matter interactions in semiconductor quantum wells. Using three laser pulses to generate a four-wave-mixing signal, we acquire spectra by tracking the phase of the signal with respect to two time axes and then Fourier transforming them. In this Account, we show several two-dimensional projections and demonstrate techniques to isolate different contributions to the coherent response of semiconductors. The low-temperature spectrum of semiconductor quantum wells is dominated by excitons, which are electron-hole pairs bound through Coulombic interactions. Excitons are sensitive to their electronic and structural environment, which influences their optical resonance energies and line widths. In near perfect quantum wells, a small fluctuation of the quantum well thickness leads to spatial localization of the center-of-mass wave function of the excitons and inhomogeneous broadening of the optical resonance. The inhomogeneous broadening often masks the homogeneous line widths associated with the scattering of the excitons. In addition to forming excitons, Coulombic correlations also form excitonic molecules, called biexcitons. Therefore, the coherent response of the quantum wells encompasses the intra-action and interaction of both excitons and biexcitons in the presence of inhomogeneous broadening. Transient four-wave-mixing studies combined with microscopic theories have determined that many-body interactions dominate the strong coherent response from quantum wells. Although the numerous competing interactions cannot be easily separated in either the spectral or temporal domains, they can be separated using two-dimensional Fourier transform spectroscopy. The most common two-dimensional Fourier spectra are S(I)(omega(tau),T,omega(t)) in which the second time period is held fixed. The result is a spectrum that unfolds congested one-dimensional spectra, separates excitonic pathways, and shows which excitons are coherently coupled. This method also separates the biexciton contributions and isolates the homogeneous and inhomogeneous line widths. For semiconductor excitons, the line shape in the real part of the spectrum is sensitive to the many-body interactions, which we can suppress by exploiting polarization selection rules. In an alternative two-dimensional projection, S(I)(tau,omega(Tau),omega(t)), the nonradiative Raman coherent interactions are isolated. Finally, we show S(III)(tau,omega(Tau),omega(t)) spectra that isolate the two-quantum coherences associated with the biexciton. These spectra reveal previously unobserved many-body correlations.

Silicon has been the dominant material in electronics since the invention of the integrated transistor. In contrast, silicon’s indirect bandgap and vanishing second-order optical nonlinearity limit its applications in optoelectronics1. Although all-optical components such as Raman lasers2, parametric amplifiers3 and electro-optic modulators4,5 have recently been reported, control over charge motion in silicon has only ever been achieved electronically. Here, we report all-optical generation of ultrafast ballistic electrical currents in clean, unbiased, bulk silicon at room temperature. This current injection, which provides new insights into optical processes in silicon, results from quantum interference between one- and two-photon absorption pathways across the indirect bandgap despite phonon participation and the multi-valley conduction band. The transient currents induced by 150 fs pulses are detected via the emitted THz radiation. The efficiency of this third-order optical process is surprisingly large for fundamental wavelengths in the 1,420–1,800 nm range.

Plasmonics can enhance solar energy conversion in semiconductors by light trapping, hot electron transfer, and plasmon-induced resonance energy transfer (PIRET). The multifaceted response of the plasmon and multiple interaction pathways with the semiconductor makes optimization challenging, hindering design of efficient plasmonic architectures. Therefore, in this paper we use a density matrix model to capture the interplay between scattering, hot electrons, and dipole–dipole coupling through the plasmon's dephasing, including both the coherent and incoherent dynamics necessary for interactions on the plasmon's timescale. The model is extended to Shockley–Queisser limit calculations for both photovoltaics and solar-to-chemical conversion, revealing the optimal application of each enhancement mechanism based on plasmon energy, semiconductor energy, and plasmon dephasing. The results guide application of plasmonic solar-energy harvesting, showing which enhancement mechanism is most appropriate for a given semiconductor's weakness, and what nanostructures can achieve the maximum enhancement.

Efficient solar energy conversion in photovoltaics and solar to chemical conversion is hindered by large band gaps and poor absorption in thin films. The easily tunable absorption and scattering cross section of localized surface plasmon resonance (LSPR) make it an ideal solution to capturing lost light. For above band edge light, scattering and light trapping can be used to increase absorption in thin semiconductor films, improving photoconversion without sacrificing recombination times. Below the band edge, plasmonic hot electrons can transfer to the semiconductor directly or resonant energy transfer can non-radiatively induce charge separation, allowing photoconversion where the semiconductor cannot absorb. In this brief review, we explore the mechanisms and efficiency of light recovery in plasmonics. Surface plasmon polaritons are used to increase light trapping in semiconductor nanowires using a metal nanohole array. Metal-semiconductor nanostructures with varying energy alignment, insulating barrier thickness, and spectral overlap are systematically varied to differentiate hot electron and resonant energy transfer. Transient absorption spectroscopy and action spectrum analysis are applied to track plasmonic charge creation and transfer, linking short and long time scale behavior. Guidelines are given for achieving optimal plasmonic light capturing and enhancement across the solar spectrum.

Rephasing and non-rephasing two-dimensional coherent spectra map the anti-crossing associated with normal-mode splitting in a semiconductor microcavity. For a 12-meV detuning range near zero detuning, it is observed that there are two diagonal features related to the intra-action of exciton-polariton branches and two off-diagonal features related to coherent interaction between the polaritons. At negative detuning, the lineshape properties of the diagonal intra-action features are distinguishable and can be associated with cavity-like and exciton-like modes. A biexcitonic companion feature is observed, shifted from the exciton feature by the biexciton binding energy. Closer to zero detuning, all features are enhanced and the diagonal intra-action features become nearly equal in amplitude and linewidth. At positive detuning the exciton- and cavity-like characteristics return to the diagonal intra-action features. Off-diagonal interaction features exhibit asymmetry in their amplitudes throughout the detuning range. The amplitudes are strongly modulated (and invert) at small positive detuning, as the lower polariton branch crosses the bound biexciton energy determined from negative detuning spectra., Comment: 13 pages, 4 figures

Charge currents in thin films of Bi 2 Se 3 are controlled using a two-color scheme that exploits quantum interference of single- and two-photon absorption pathways. Injection and shift currents are observed and linked to surface state excitations.

Epitaxial NbO2 (110) films, 20 nm thick, were grown by pulsed laser deposition on Al2O3 (0001) substrates. The Ar/O2 total pressure during growth was varied to demonstrate the gradual transformation between NbO2 and Nb2O5 phases, which was verified using x-ray diffraction, x-ray photoelectron spectroscopy, and optical absorption measurements. Electric resistance threshold switching characteristics were studied in a lateral geometry using interdigitated Pt top electrodes in order to preserve the epitaxial crystalline quality of the films. Volatile and reversible transitions between high and low resistance states were observed in epitaxial NbO2 films, while irreversible transitions were found in case of Nb2O5 phase. Electric field pulsed current measurements confirmed thermally-induced threshold switching., Comment: This is an author-created, un-copyedited version of an article accepted for publication in Journal of Physics D: Applied Physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at http://dx.doi.org/10.1088/0022-3727/48/33/335308

Films of the topological insulator Bi2Se3 are grown by molecular beam epitaxy with in-situ reflection high-energy electron diffraction. The films are shown to be high-quality by X-ray reflectivity and diffraction and atomic-force microscopy. Quantum interference control of photocurrents is observed by excitation with harmonically related pulses and detected by terahertz radiation. The injection current obeys the expected excitation irradiance dependence, showing linear dependence on the fundamental pulse irradiance and square-root irradiance dependence of the frequency-doubled optical pulses. The injection current also follows a sinusoidal relative-phase dependence between the two excitation pulses. These results confirm the third-order nonlinear optical origins of the coherently controlled injection current. Experiments are compared to a tight-binding band structure to illustrate the possible optical transitions that occur in creating the injection current., 11 pages, 3 figure, journal article

A one-dimensional photonic crystal is patterned into an active planar waveguide containing multiple InGaAs quantum wells. External coupling reflectivity is used to map out the photonic band structure, revealing a clear anti-crossing at ~1.37 eV in the 45° TM polarized spectrum. The band structure is compared to and confirmed by scattering-matrix calculations. Photoluminescence measurements are performed in the same geometry on the patterned and unpatterned regions of the sample. The latter shows conventional quantum-well emission and GaAs emission with increasing pump power. In contrast, the spectra from the patterned sample contain a sharp mode from the dielectric-like band of the anti-crossing, corresponding to the Brillouin zone edge. Power-dependent excitation shows this mode to be strongly super-linear, indicative of band edge lasing.

We have studied the power-dependent wavelength shift of photonic coupling resonances of a two-dimensional photonic crystal waveguide by reflection geometry pump-probe measurements. The quadratic response is indicative of two-photon induced carrier creation, which alters the refractive index of the semiconductor core of the photonic lattice. A free-carrier model is used to simulate the phenomenon, giving values of the change in refractive index per unit carrier density that satisfactorily compare to values calculated for bulk AlGaAs under similar conditions. Time-resolved spectra are also presented, showing relaxation times of

Strong resonant polarization conversion is observed in the reflectivity properties of one-dimensional (1-D) lattices of air trenches deeply etched in AlGaAs surface waveguides. The symmetry properties and the magnitudes of the observed effects are found to be in good agreement with the results of scattering matrix calculations.

We present compelling experimental results of the optical characteristics of transparent oxide CuGaO2 and related CuGa1-xFexO2 (with 0.00≤x≤0.05) alloys, whereby the forbidden electronic transitions for CuGaO2 become permissible in the presence of B-site (Ga sites) alloying with Fe. Our computational structural results imply a correlation between the global strain on the system and a decreased optical absorption edge. However, herein, we show that the relatively ordered CuGa1-xFexO2 (for 0.00≤x≤0.04) structures exhibit much weaker vis-absorption compared to the relatively disordered CuGa0.95Fe0.05O2.

Photoluminescence spectra of YVO$_{4}$:Eu$^{3+}$ nanoparticles are presented,with and without the attachment of of organic linker molecules that are proposed for linking to biomolecules. YVO$_{4}$:Eu$^{3+}$ nanoparticles with 5% dopant concentration were synthesized by wet chemical synthesis. X-ray diffraction and transmission electron microscopy show the expected wakefieldite structure of tetragonal particles with an average size of 17 nm. Fourier-transform infrared spectroscopy determines that metal-carboxylate coordination is successful in replacing the native metal-hydroxyl bonds with three organic linkers, namely benzoic acid, 3-nitro 4-chloro-benzoic acid and 3,4-dihydroxybenzoic acid, in separate treatments. UV-excitation photoluminescence spectra show that the position and intensity of dominant $^{5}D_{0}-^{7}F_{2}$ electric-dipole transition at 619 nm is unaffected by the benzoic acid and 3-nitro 4-chloro-benzoic acid treatments. Attachment of the 3,4-dihydroxybenzoic acid produces an order-of-magnitude quenching of the photoluminescence, due to the presence of high-frequency modes in the linker. Ratios of the dominant electric- and magnetic-dipole transitions confirm infrared measurements, which indicate that the bulk crystal of the nanoparticle is unchanged by all three treatments., Comment: 9 pages, 5 figures, journal article

The absorption of phase-related near-infrared fundamental (ω) and second harmonic (2ω) femtosecond pulses results in ballistic electrical currents in clean bulk silicon at room temperature. The ultrafast charge motion is directly monitored via a time-resolved analysis of the emitted THz bursts of radiation. The vector direction of the current is precisely controlled by the polarization of the two-color pump field and the relative phase ΔΦ = 2Φω – Φ2ω. The current generation process relies on a third order optical nonlinearity. Microscopically, current injection can be understood as arising from the quantum interference of one- and two-photon absorption processes. Surprisingly, the efficiency of the current injection is relatively high despite the multitude of possible phonon-assisted absorption channels in the multi-valley band structure. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Monolayer fluctuations in the thickness of a semiconductor quantum well (QW) lead to the formation of spectrally resolved excitons located in the narrower, average, and thicker regions of the QW. Whether or not these excitons are coherently coupled via Coulomb interaction is a long-standing debate. We demonstrate that different types of disorder potential govern coherent coupling among excitons, and the coupling strength can be quantitatively measured using optical two-dimensional Fourier transform spectroscopy. Strong coherent coupling occurs between certain types of excitons but is missing between other types of excitons because the distinctive nature of excitons results in different spatial overlap. Our finding may be applicable to other disordered systems, such as photosynthesis and conjugated polymers, where exciton coupling plays a critical role in determining charge and energy transfer.

Optical single- and double-quantum three-dimensional Fourier-transform spectra are obtained for atomic vapors. We show that three-dimensional spectra can be used to identify the Hamiltonian of complex systems and to reveal the nature of many-body interactions.

Predicting and controlling quantum mechanical phenomena require knowledge of the system Hamiltonian. A detailed understanding of the quantum pathways used to construct the Hamiltonian is essential for deterministic control and improved performance of coherent control schemes. In complex systems, parameters characterizing the pathways, especially those associated with inter-particle interactions and coupling to the environment, can only be identified experimentally. Quantitative insight can be obtained provided the quantum pathways are isolated and independently analysed. Here we demonstrate this possibility in an atomic vapour using optical three-dimensional Fourier-transform spectroscopy. By unfolding the system’s nonlinear response onto three frequency dimensions, three-dimensional spectra unambiguously reveal transition energies, relaxation rates and dipole moments of each pathway. The results demonstrate the unique capacity of this technique as a powerful tool for resolving the complex nature of quantum systems. This experiment is a critical step in the pursuit of complete experimental characterization of a system’s Hamiltonian., Knowledge of the Hamiltonian of a quantum system is essential for predicting and controlling its behaviour. Li et al. use optical three-dimensional Fourier-transform spectroscopy to separate and study each pathway, gaining quantitative insight into the quantum pathways of an atomic vapour Hamiltonian.

We present the use of three-dimensional coherent spectroscopy to isolate quantum pathways. Isolation of individual quantum pathways allows the parameters of the Hamiltonian to be determined. We demonstrate this possibility using an atomic vapor.

Collinear phase-matched optical rectification is studied in ZnGeP$_{2}$ pumped with near-infrared light. The pump-intensity dependence is presented for three crystal lengths (0.3, 1.0 and 3.0 mm) to determine the effects of linear optical absorption, nonlinear optical absorption and terahertz free-carrier absorption on the generation. Critical parameters such as the coherence length (for velocity matching), dispersion length (for linear pulse broadening) and nonlinear length (for self-phase modulation) are determined for this material. These parameters provide insight into the upper limit of pulse intensity and crystal length required to generate intense terahertz pulse without detriment to the pulse shape. It is found that for 1-mm thick ZnGeP$_{2}$(012), pumped at 1.28 micron with intensity of ~15 GW/cm2 will produce intense undistorted pulses, whereas longer crystals or larger intensities modify the pulse shape to varying degrees. Moreover, phase-matching dispersion maps are presented for the terahertz generation over a large tuning range (1.1-2.4 micron) in longer (3 mm) crystal, demonstrating the phase-matching bandwidth and phase mismatch that leads to fringing associated with multi-pulse interference. All observed results are simulated numerically showing good qualitative agreement., Comment: 7 pages, 4 figures, journal submission, version 2

We have used femtosecond pump-probe spectroscopy to study the ultrafast nonlinear response of AlGaAs two-dimensional photonic crystal waveguides in the near-infrared spectral region. The modulation of the reflectivity spectra due to the refractive index change produced by photogenerated carriers was measured. We observed an instantaneous pump-induced shift in the wavelength of a photonic resonance at 882 nm with a fast decay time of ≈8 ps. The magnitude of the reflectivity change was very large at wavelengths close to the photonic resonance, with a maximum value of ΔR/R>30% at 877 nm. These results confirm the excellent potential of photonic crystal waveguides in ultrafast nonlinear switching applications.

Plasmonic metal nanostructures have been incorporated into semiconductors to enhance the solar-light harvesting and the energy-conversion efficiency. So far the mechanism of energy transfer from the plasmonic metal to semiconductors remains unclear. Herein the underlying plasmonic energy-transfer mechanism is unambiguously determined in Au@SiO(2)@Cu(2)O sandwich nanostructures by transient-absorption and photocatalysis action spectrum measurement. The gold core converts the energy of incident photons into localized surface plasmon resonance oscillations and transfers the plasmonic energy to the Cu(2)O semiconductor shell via resonant energy transfer (RET). RET generates electron-hole pairs in the semiconductor by the dipole-dipole interaction between the plasmonic metal (donor) and semiconductor (acceptor), which greatly enhances the visible-light photocatalytic activity as compared to the semiconductor alone. RET from a plasmonic metal to a semiconductor is a viable and efficient mechanism that can be used to guide the design of photocatalysts, photovoltaics, and other optoelectronic devices.

Exciton dephasing and relaxation dynamics are studied in a GaAs quantum dot ensemble using optical twodimensional Fourier transform spectroscopy. We measure the temperature and excitation-density dependence of the exciton ground-state homogeneous lineshape of quantum dots within the ensemble and show that acoustic phonon sidebands are absent. The linewidth increases nonlinearly with temperature from 6 to 50 K and the behavior is well-described by an Arrhenius equation with an offset. The absence of a phonon-activation peak in the spectra reveals that elastic exciton-phonon scattering is the primary dephasing mechanism and the results can be explained qualitatively using an extension of the independent Boson model that includes quadratic coupling in the phonon displacement coordinates. At temperatures ≥ 35 K, spectral features associated with phonon-assisted population transfer of excitons out of the quantum dots and into quantum wells states begin to appear.

Generation of terahertz pulses by optical rectification is demonstrated in chalcopyrite ZnGeP 2 (ZGP) as a function of pump intensity, wavelength and crystal orientation. For infrared excitation, ZGP emission exceeds that of GaAs and GaP.

Exciton inter- and intra-actions in a GaAs/AlGaAs quantum well (QW) and quantum dot (QD) ensemble are studied using optical two-dimensional Fourier transform spectroscopy. We measure population dynamics for times up to 300 ps and temperatures up to 50 K and observe biexponential decay for both QW and QD excitons, strong QW $\ensuremath{\rightarrow}$ QD relaxation, and weak QD $\ensuremath{\rightarrow}$ QW activation. The population dynamics are modeled using a system of rate equations that incorporate radiative and nonradiative decay, coupling between bright and dark exciton states, and QW $\ensuremath{\leftrightarrow}$ QD coupling. The fast decay rates are attributed to exciton-bound hole spin flips between optically active and inactive states and are similar for the QW and QDs, indicating excitons are weakly localized in the QDs. The QW $\ensuremath{\rightarrow}$ QD relaxation rate increases with temperature, and QD $\ensuremath{\rightarrow}$ QW excitation is observed at temperatures $\ensuremath{\geqslant}$35 K.

Using optical two-dimensional Fourier transform spectroscopy, we report temperature- and excitation-density-dependent measurements of the homogeneous linewidth of the exciton ground-state transition in a single layer of interfacial GaAs quantum dots (QDs). We show that the homogeneous linewidth increases nonlinearly with temperature from 6 to 50 K and that the thermal broadening is well described by an activation term and offset. The absence of a phonon-activation peak in the two-dimensional spectra reveals that elastic scattering of excitons with acoustic phonons via virtual transitions between the ground and excited states significantly contributes to the thermal broadening. We find that the combination of increasing virtual activation energy and exciton-phonon coupling strength with decreasing QD size results in greater thermal broadening for excitons localized in smaller QDs. The homogeneous linewidth also exhibits a strong excitation-density dependence and is shown to increase linearly as the photon density increases from $2\ifmmode\times\else\texttimes\fi{}{10}^{11}$ to $1\ifmmode\times\else\texttimes\fi{}{10}^{12}$ photons pulse${}^{\ensuremath{-}1}$ cm${}^{\ensuremath{-}2}$ at 6 K. This trend is attributed to strong coupling of excitons within the same QD and is independent of the quantum-well exciton population density.

Optical two-dimensional Fourier-transform spectroscopy is used to study the heavy- and light-hole excitonic resonances in weakly disordered GaAs quantum wells. Homogeneous and inhomogeneous broadening contribute differently to the two-dimensional resonance line shapes, allowing separation of homogeneous and inhomogeneous line widths. The heavy-hole exciton exhibits more inhomogeneous than homogeneous broadening, whereas the light-hole exciton shows the reverse. This situation occurs because of the interplay between the length scale of the disorder and the exciton Bohr radius, which affects the exciton localization and scattering. Utilizing this separation of line widths, excitation-density-dependent measurements reveal that many-body interactions alter the homogeneous dephasing, while disorder-induced dephasing is unchanged.

Optical rectification is demonstrated in (110)-cut ZnGeP_2 (ZGP) providing broadband terahertz (THz) generation. The source is compared to both GaP and GaAs over a wavelength range of 1150 nm to 1600 nm and peak intensity range of 0.5 GW/cm^2 to 40 GW/cm^2. ZGP peak-to-peak field amplitude is larger than in the other materials due to either lower nonlinear absorption or larger second order nonlinearity. This material is well suited for broadband THz generation across a wide range of infrared excitation wavelengths., Comment: 4 pages, 3 figures, journal article

We report the observation of double-quantum coherence signals in a gas of potassium atoms at twice the frequency of the one-quantum coherences. Since a single atom does not have a state at the corresponding energy, this observation must be attributed to a collective resonance involving multiple atoms. These resonances are induced by weak interatomic dipole-dipole interactions, which means that the atoms cannot be treated in isolation, even at a low density of ${10}^{12}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$.

Optical multidimensional Fourier transform spectroscopy excites a sample with a sequence of ultrafast pulses. A spectrum is constructed by taking Fourier transforms with respect to pulse delays, which are interferometrically controlled.

Optical two-dimensional Fourier-transformed (2DFT) spectroscopy is used to study the coherent optical response of potassium vapor in a thin transmission cell. Rephasing and non-rephasing spectra of the D1 and D2 transitions are obtained and compared to numerical simulations. Calculations using the optical Bloch equations gives very good agreement with the experimental peak strengths and line shapes. Non-radiative Raman-like coherences are isolated using a different 2DFT projection. Density-dependent measurements show distortion of 2DFT spectra due to pulse propagation effects.

Electronic structure and dynamics are captured by optical 2D-Fourier-transform (2DFT) spectroscopy, which tracks the phase of the nonlinear signal during two time delays of a multi-pulse excitation sequence. These Fourier-transformed spectra separate and isolate overlapping and competing contributions to the coherent response. We have developed an ultra-stable platform consisting of nested interferometers with active phase control, allowing for exploration of single- and two-quantum coherences. Phase-resolved spectra are retrieved by all-optical determination of experimental phase ambiguities. GaAs quantum wells show suppression of many-body interactions in cross-linear polarized 2DFT spectra and many-body two-quantum coherences. Potassium vapor also shows unexpected two-quantum coherences.

We demonstrate optical two-dimensional Fourier transform spectroscopy of heavy- and light-hole excitons in GaAs quantum wells. This is enabled by active interferometric stabilization of the excitation pulse separation and of a reference pulse.

Optical 2D Fourier-transform spectra reveal time and temperature dependent relaxation from GaAs quantum well states into the interfacial quantum dot ensemble. We attribute the increased rate of relaxation at higher temperature to stimulated phonon emission.

Optical two-dimensional Fourier-transform spectroscopy is used to study interfacial GaAs quantum dots (QDs). We extract the temperature dependence of the QD homogeneous linewidth and energy relaxation from quantum well excitons to the lower energy QDs.

We present optical two-dimensional Fourier transform spectra for the pulse sequence sensitive to two-quantum coherences. In semiconductors, two-quantum coherences occur due to biexcitons and many-body effects, in a potassium vapor, they arise from atomic interactions.

Optical two-dimensional Fourier-transform spectroscopy is used to study interfacial GaAs quantum dots (QDs). We extract the temperature dependence of the QD homogeneous linewidth and energy relaxation from quantum well excitons to the lower energy QDs.

We present experimental coherent two-dimensional Fourier-transform spectra of Wannier exciton resonances in semiconductor quantum wells generated by a pulse sequence that isolates two-quantum coherences. By measuring the real part of the signals, we determine that the spectra are dominated by two-quantum coherences due to mean-field many-body interactions, rather than bound biexcitons. Simulations performed using dynamics controlled truncation agree with the experiments.

Two-quantum coherences in two-dimensional Fourier-transform (2DFT) spectra are attributed to many-body interactions. 2DFT spectroscopy allows two-quantum coherences in semiconductors to be isolated. As a result, many-body coherences can be separated from bound biexciton coherences.

Two-dimensional Fourier-transform spectroscopy shows a strong variation in the biexciton binding energy across the inhomogeneous absorption width. This effect is observed for both co- and cross-linearly polarized excitation, where the latter suppresses many-body interactions.