Katherine W. Stone

Two-Quantum 2D FT Electronic Spectroscopy of Biexcitons in GaAs Quantum Wells

Seeing Double Whereas electrons roam free in a metal, their motion in a semiconductor often correlates with that of the positively charged holes their excitation has left behind. Various spectroscopic techniques have quantified the energetics associated with such electron-hole pairs, termed excitons. However, the higher-order correlations that ensue from exciton-exciton interactions are harder to probe. Now, Stone et al. (p. 1169) have characterized the dynamics of such exciton pairs, or biexcitons, directly in a gallium arsenide quantum well structure using a sequence of four ultrashort optical pulses with precisely controlled mutual phase relationships. Controlling the phase and timing of four optical pulses enables measurement of correlations between the electronic semiconductor carriers. The motions of electrons in solids may be highly correlated by strong, long-range Coulomb interactions. Correlated electron-hole pairs (excitons) are accessed spectroscopically through their allowed single-quantum transitions, but higher-order correlations that may strongly influence electronic and optical properties have been far more elusive to study. Here we report direct observation of bound exciton pairs (biexcitons) that provide incisive signatures of four-body correlations among electrons and holes in gallium arsenide (GaAs) quantum wells. Four distinct, mutually coherent, ultrashort optical pulses were used to create coherent exciton states, transform these successively into coherent biexciton states and then new radiative exciton states, and finally to read out the radiated signals, yielding biexciton binding energies through a technique closely analogous to multiple-quantum two-dimensional Fourier transform (2D FT) nuclear magnetic resonance spectroscopy. A measured variation of the biexciton dephasing rate indicated still higher-order correlations.

Three-dimensional electronic spectroscopy of excitons in GaAs quantum wells

We demonstrate three-dimensional (3D) electronic Fourier transform spectroscopy of GaAs quantum wells using four fully phase-coherent, noncollinear optical fields. Since the full complex signal field is measured as a function of all three time intervals, nearly every peak in the resulting 3D spectral solid arises from a distinguishable sequence of transitions represented by a single Feynman pathway. We use the 3D spectral peaks to separate two pathways involving weakly bound mixed biexcitons generated in different time orders. In the process, we reveal a peak that was previously obscured by a correlated but unbound exciton pair coherence. We also demonstrate a calibration procedure for the carrier frequency which yields biexciton binding energy values with high accuracy.

Invited Article: The coherent optical laser beam recombination technique (COLBERT) spectrometer: Coherent multidimensional spectroscopy made easier

We have developed an efficient spectrometer capable of performing a wide variety of coherent multidimensional measurements at optical wavelengths. The two major components of the largely automated device are a spatial beam shaper which controls the beam geometry and a spatiotemporal pulse shaper which controls the temporal waveform of the femtosecond pulse in each beam. We describe how to construct, calibrate, and operate the device, and we discuss its limitations. We use the exciton states of a semiconductor nanostructure as a working example. A series of complex multidimensional spectra-displayed in amplitude and real parts-reveals increasingly intricate correlations among the excitons.

Analysis of replica pulses in femtosecond pulse shaping with pixelated devices.

We present a detailed analysis of commonly encountered waveform distortions in femtosecond pulse shaping with pixelated devices, including the effects of discrete sampling, pixel gaps, smooth pixel boundaries, and nonlinear dispersion of the laser spectrum. Experimental and simulated measurements are used to illustrate the effects. The results suggest strategies for reduction of some classes of distortions.

Thermally-Limited Exciton Delocalization in Superradiant Molecular Aggregates

We present two-dimensional Fourier transform optical spectroscopy measurements of two types of molecular J-aggregate thin films and show that temperature-dependent dynamical effects govern exciton delocalization at all temperatures, even in the presence of significant inhomogeneity. Our results indicate that in the tested molecular aggregates, even when the static structure disorder dominates exciton dephasing dynamics, the extent of exciton delocalization may be limited by dynamical fluctuations, mainly exciton-phonon coupling. Thus inhomogeneous dephasing may mediate the exciton coherence time whereas dynamical fluctuations mediate the exciton coherence length.

Two-quantum Two-dimensional Fourier Transform Electronic Spectroscopy of Biexcitons in GaAs Quantum Wells

Using two-dimensional Fourier transform electronic spectroscopy, we directly observe two-quantum biexciton-ground state coherences which are typically temporally and spectrally convolved with one-quantum excitonic coherences.

Multiple-quantum 2D spectroscopy of many-body correlations in GaAs quantum wells

Multiple-quantum two-dimensional Fourier transform optical (2D FTOPT) spectroscopy was developed and conducted on GaAs quantum wells. Spatiotemporal femtosecond pulse shaping was used to control the optical phases and time delays of ultrashort pulses in multiple non-collinear beams so that fully coherent four-wave and higher-order mixing measurements could be conducted without delay stages, multiple interferometers, or any active phase control. Coherences of biexcitons, unbound but correlated exciton pairs, and excitons undergoing rephasing were observed directly.

Probing biexcitons in quantum dots using femtosecond pump/probe and two dimensional electronic spectroscopy

We report on the electronic structure of biexcitons in CdSe quantum dots using state-selective femtosecond pump/probe spectroscopy. The pump/probe experiments are compared to direct probing of biexcitons via two-dimensional electronic spectroscopy.

Three-Dimensional Electronic Four Wave-Mixing Spectroscopy in GaAs Quantum Wells

Three-dimensional electronic Fourier transform four wave-mixing spectroscopy of GaAs quantum wells is demonstrated for the first time using wave vector beam shaping and femtosecond spatiotemporal pulse shaping to create four fully phase-coherent, non-collinear optical fields. This technique is an optical analogue of multidimensional and multi-quantum NMR. From a study of GaAs quantum wells, an internally calibrated, yet previously unmeasured two-dimensional projection correlating events between the first two time periods is presented.

Coherently Controlled Multidimensional Optical Spectroscopy

Multidimensional optical spectroscopy of potassium dimer is demonstrated using a 2D femtosecond pulse shaper with excellent phase stability. Coherent control of intramolecular dynamics is achieved by introducing specific pulse sequences and chirps.