The JILA multidimensional optical nonlinear spectrometer (JILA-MONSTR) is a robust, ultrastable platform consisting of nested and folded Michelson interferometers that can be actively phase stabilized. This platform generates a square of identical laser pulses that can be adjusted to have arbitrary time delay between them while maintaining phase stability. The JILA-MONSTR provides output pulses for nonlinear excitation of materials and phase-stabilized reference pulses for heterodyne detection of the induced signal. This arrangement is ideal for performing coherent optical experiments, such as multidimensional Fourier-transform spectroscopy, which records the phase of the nonlinear signal as a function of the time delay between several of the excitation pulses. The resulting multidimensional spectrum is obtained from a Fourier transform. This spectrum can resolve, separate, and isolate coherent contributions to the light-matter interactions associated with electronic excitation at optical frequencies. To show the versatility of the JILA-MONSTR, several demonstrations of two-dimensional Fourier-transform spectroscopy are presented, including an example of a phase-cycling scheme that reduces noise. Also shown is a spectrum that accesses two-quantum coherences, where all excitation pulses require phase locking for detection of the signal.

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.

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 1012 cm−3.

We study the coherent light-matter interactions associated with excitons, biexcitons and manybody effects in GaAs quantum wells. For most polarization configurations the phase-resolved twodimensional Fourier-transform (2DFT) spectra are dominated by excitonic features, where their strength and dispersive lineshapes is due to many-body interactions. Cross-linear excitation suppresses many-body interactions, changing the lineshape and strength of the 2DFT features.PACS numbers: 78.47. Fg, 78.47.nj, 78.67.De The coherent response of excitons in semiconductor quantum wells (QWs) is strongly dependent on the excitation conditions and material properties, such as polarization configuration and inhomogeneous broadening (due to well-width fluctuations). Contributions to the light-matter interactions include the excitons themselves, the formation of excitonic "molecules," or biexcitons, and the many-body interactions of these states. (See for example, the recent reviews 1,2 and references therein.) The interplay of these contributions has been explored though intensity-and polarization-dependent transient four-wave mixing (TFWM) studies.3-20 The latter result in changes of the dephasing time, 3-6 the temporal profile of the emission, 3,5,7 and a phase shift of the beats.5,8 Some experiments have also characterized the Stokes parameters of the emission with detailed polarimetry.9,10 Explanations of these results vary and include inhomogeneity 3,5 or exciton-exciton interactions, 7,11 such as exciton-exciton exchange, 5,10 excitation-induced dephasing (EID), 9,12-14 local-field corrections, 9,13 and excitation-induced shift (EIS).15 Many authors have attributed the polarization dependence to biexcitons and their subsequent interactions. 4,6,[16][17][18][19][20] TFWM measurements have not resulted in a completely satisfactory explanation of the polarizationdependent coherent response, because of ambiguities associated with competing processes in the coherent response. Additional information has been gained by recording the time evolution of the emission.3,21 However, great enhancements are obtained by explicitly tracking the evolving phase of the TFWM signal using either a coherent-control scheme 22,23 or two-dimensional Fouriertransform (2DFT) spectroscopy.24-26 The latter results in a two-dimensional spectrum from the Fourier-transform of the phase evolution of the signal along two time dimensions, and has separated the population from coupling contributions, 24,25 confirmed EID and EIS, 24 and shown that agreement with theory requires the inclusion of terms beyond the Hartree-Fock approximation. 26In this paper, 2DFT spectroscopy is used to separate and isolate the competing intraactions and interactions of the excitons and biexcitons, which are strongly polarization dependent. Through a quantitative comparison of the magnitude of 2DFT data and the lineshape in the phase-resolved spectra, the selection rules are exploited to demonstrate the suppression of either many-body or biexcitonic effects in the coherent respons...