D. Karaiskaj

A versatile ultrastable platform for optical multidimensional Fourier-transform spectroscopy

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.

Two-Quantum Many-Body Coherences in Two-Dimensional Fourier-Transform Spectra of Exciton Resonances in Semiconductor Quantum Wells

Denis Karaiskaj, ∗ Alan D. Bristow, Lijun Yang, Xingcan Dai, Richard P. Mirin, Shaul Mukamel, and Steven T. Cundiff † JILA, National Institute of Standards and Technology and University of Colorado, Boulder, CO 80309-0440, USA Chemistry Department, University of California, Irvine, California, 92697-2025, USA National Institute of Standards and Technology, Boulder, CO 80305, USA (Dated: December 21, 2009)

Ultrafast (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser for the 1.3 μm wavelength regime

(GaIn)(NAs) vertical-cavity surface-emitting lasers for room-temperature emission at 1.3 μm wavelength are designed and grown by metal-organic vapor-phase epitaxy using dimethylhydrazine and tertiarybutylarsine. Room-temperature operation at wavelengths up to 1.285 μm is achieved with low optical pumping thresholds between 1.6 and 2.0 kW/cm2. Stimulated emission dynamics after femtosecond optical pumping are measured and compare favorably with results on (GaIn)As/Ga(PAs)-based structures.

Optical Two-Dimensional Fourier Transform Spectroscopy of Semiconductor Quantum Wells

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.

Polarization dependence of semiconductor exciton and biexciton contributions to phase-resolved optical two-dimensional Fourier-transform spectra

We study the coherent light-matter interactions associated with excitons, biexcitons, and many-body effects in GaAs quantum wells. For most polarization configurations the phase-resolved two-dimensional Fourier-transform (2DFT) spectra are dominated by excitonic features, where their strength and dispersive line shapes are due to many-body interactions. Cross-linear excitation suppresses many-body interactions, changing the line shape and strength of the 2DFT features.

Direct observation of the donor nuclear spin in a near-gap bound exciton transition: P31 in highly enriched S28ia)

We report on ultrahigh resolution studies of the bound exciton states associated with the shallow acceptor B and the shallow donor P in highly enriched S28i using a tuneable single frequency laser to perform photoluminescence excitation spectroscopy. The linewidths and fine structure of the transitions, which were too narrow to be resolved previously using an available photoluminescence apparatus, are now fully revealed. The P bound exciton transition shows a complicated additional structure, which the Zeeman spectroscopy demonstrates to be a result of the splitting of the donor ground state by the hyperfine interaction between the spin of the donor electron and that of the P31 nucleus. The P31 nuclear spin populations can thus be determined, and hopefully modified, by optical means. The predominant Auger recombination channel of these bound excitons is used to observe the same resolved hyperfine transitions in the photocurrent spectrum. This demonstrates that donors in specific electronic and nuclear spi...

Optical 2-D Fourier Transform Spectroscopy of Excitons in Semiconductor Nanostructures

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.

Photoluminescence studies of isotopically enriched silicon: isotopic effects on the indirect electronic band gap and phonon energies

We have performed high-resolution photoluminescence spectroscopy on silicon crystals with different isotopic composition and investigated the effects of this composition on the indirect electronic band gap and phonon energies. From the relative energy shift of the indirect electronic band gap between two crystals of different isotopic composition, the zero-point renormalization energy of that electronic band gap was estimated. The experimentally determined phonon frequency shifts between crystals of different isotopic composition are compared with theoretically predicted frequency shifts. We further present a detailed analysis of the different contributions leading to the observed phonon frequency shifts.

Optically pumped (GaIn)As/Ga(PAs) vertical-cavity surface-emitting lasers with optimized dynamics

We present a vertical-cavity surface-emitting laser structure optimized for fast intrinsic emission dynamics, using the strain-compensated (GaIn)As/Ga(PAs) material system with a 2λ sin-type cavity. The high quality of the epitaxial growth is revealed by the large normal mode splitting of 10.5 meV found in reflectivity measurements. The fast dynamical response of our structure after femtosecond optical excitation at 30 K yields a pulse width of 3.2 ps and a peak delay of only 4.8 ps. A structure designed for laser emission at higher temperatures exhibits picosecond dynamics at room temperature.

Continuously tunable optical multidimensional Fourier-transform spectrometer

A multidimensional optical nonlinear spectrometer (MONSTR) is a robust, ultrastable platform consisting of nested and folded Michelson interferometers that can be actively phase stabilized. The MONSTR provides output pulses for nonlinear excitation of materials and phase-stabilized reference pulses for heterodyne detection of the induced signal. This platform generates a square of identical laser pulses that can be adjusted to have arbitrary time delays between them while maintaining phase stability. This arrangement is ideal for performing coherent optical experiments, such as multidimensional Fourier-transform spectroscopy. The present work reports on overcoming some important limitations on the original design of the MONSTR apparatus. One important advantage of the MONSTR is the fact that it is a closed platform, which provides the high stability. Once the optical alignment is performed, it is desirable to maintain the alignment over long periods of time. The previous design of the MONSTR was limited to a narrow spectral range defined by the optical coating of the beam splitters. In order to achieve tunability over a broad spectral range the internal optics needed to be changed. By using broadband coated and wedged beam splitters and compensator plates, combined with modifications of the beam paths, continuous tunability can be achieved from 520 nm to 1100 nm without changing any optics or performing alignment of the internal components of the MONSTR. Furthermore, in order to achieve continuous tunability in the spectral region between 520 nm and 720 nm, crucially important for studies on numerous biological molecules, a single longitudinal mode laser at 488.5 nm was identified and used as a metrology laser. The shorter wavelength of the metrology laser as compared to the usual HeNe laser has also increased the phase stability of the system. Finally, in order to perform experiments in the reflection geometry, a simple method to achieve active phase stabilization between the signal and the reference beams has been developed.