Session P28: Superlattices and Nanostructures (Wires, Dots, etc.): Optical Properties I

Enhancement of optical sensitivity of quantum dots near metal-dielectric interface

We study theoretically the enhancement of the incident light transmitted through the diffraction grating. We are interested in the mid-infrared frequency range, corresponding to the intraband absorption by quantum dots. We show that for the s-polarized light the enhancement is much stronger than for p-polarized light. By tuning the parameters of the diffraction grating the enhancement of the light can be increased by a few orders of magnitude. The distribution of the transmitted light is highly nonuniform with very sharp peaks with the spatial width about 10 nm. Due to strongly inhomogeneous distribution of electromagnetic field the quantum dots should be placed at special points, i.e. hot spots, with large intensity of the field. We discuss the application of this effect to the quantum dot infrared photodetectors.   

Surface Enhanced Infrared Absorption Spectroscopy (SEIRA) using Infrared resonant Au-nanoshell based substrates

Enhancements of the molecular signals are known to occur in the mid-IR region when the molecules are in close proximity to rough metal surfaces. This phenomenon, known as surface enhanced infrared absorption (SEIRA), is complementary to surface enhanced Raman scattering (SERS) and can be used for biochemical sensing. However, designing substrates for SEIRA that are resonant in the mid-IR has proven challenging. One solution is to use metal nanoshells, plasmonic nanoparticles with a wide plasmon tunability range from the visible to the mid-IR. Here, we exploit this tunability property of nanoshells to fabricate nanoshell aggregates and nanoshell arrays as SEIRA substrates. Para-aminothiophenol (pMA) is used as a test molecule for studying SEIRA activity of these substrates. SEIRA enhancement factors are evaluated to be in the 10000 range for these substrates. These strong enhancements allow for sensing of biologically relevant molecules such as adenine. Spectral interpretation using SEIRA surface selection rule allows for insight into the molecule's preferred orientation on the nanoparticle surface.   

Polarization memory of charged excitons in vertically coupled InAs/GaAs quantum dots

Polarized photoluminescence of the InAs/GaAs coupled quantum dot system was studied, and circular polarization memory signatures of the neutral exciton, the positive trion and the negative trion are reported. Our samples are Stranski-Krastanow dots, vertically separated by a GaAs barrier. We obtain results for circular polarization memory that are consistent with previous polarization studies on single quantum dots, indicating that coupled dot systems have polarization signatures very similar to single dot systems. Due to their structure, our samples display hole level anticrossings. As the system shifts from one positive trion configuration to the other, a continuous change in circular polarization memory is observed. This change in polarization memory is explained by hole tunneling and exchange interactions. Identifying the two positive trion configurations as neutral exciton-like and positive trion-like provides a theoretical basis for understanding the circular polarization memory signature.   

Spin interactions in a coupled InAs/GaAs quantum dot studied by polarization dependent photoluminescence

Spins in a quantum dot molecule are of interest for possible quantum information and spintronic applications. By studying in detail the polarization dependent photoluminescence in the region where the ground state energy levels are in resonance and therefore behaving molecular-like we can gain insight into the various relevant interactions. Vertically stacked, self-assembled, InAs QDs grown by molecular beam epitaxy (MBE) on GaAs substrate were used in this study where the relative dot heights were controlled using a GaAs capping layer and the indium flush technique. The QDMs were embedded in a Schottky diode to control the electric field and selectively charge them. QDMs are brought into resonance which results in anticrossings at the positive trion states. The positive trion states demonstrated a high and low polarization for positive trion like, neutral exciton like, configurations respectively. Detailed polarization results show spin fine structure along with a continuous variation between the high and low values indicating an electric field tunable exchange interaction.   

THz Charge Oscillations in a Modulation Doped Parabolic Quantum Well

We used ultrafast terahertz (THz) spectroscopy to observe THz-frequency electron oscillations in a modulation-doped InGaAs/AlGaAs parabolic quantum well. THz emission and absorption measurements yielded an electron subband spacing of 0.01eV, in agreement with sample design. Our study examined how the extrinsic electron density in the well influences THz emission efficiency, and we found no strong dependence. This indicates that THz emission in this structure arises from quantum beating of the photogenerated electrons. In contrast to a previously published report [1], we find that THz emission from the cold extrinsic electrons due to ultra-fast field screening plays at most a secondary role, even when the density of extrinsic electrons [$\sim $10$^{11}$ cm$^{-2}$] exceeds the density of photogenerated charge. This work was funded by the National Science Foundation under the NSF-RUI Program (DMR-0606181). \newline [1] R. Bratschitsch, et. al., APL 76, 3501 (2000).   

Scalable Single Photon Detector for Terahertz and Infrared Applications

Recent advancements in the research areas of quantum dot (QD) and single electron transistors (SET) open up an opportunity for the development of quantum dot detector, which can respond to a single photon over microwave to infrared (IR) frequencies. Currently, single photon detection is possible by means of the photomultiplier tube, but only for photons with wavelengths shorter than 1.5 $\mu$m. For the detection of photons with wavelengths longer than the IR, a bolometer is typically used. The sensitivity of a state-of-the-art bolometer, in terms of noise equivalent power (NEP), is 10$^{-17}$ W/Hz$^{1/2}$, which requires 10$^{5}$ photons to yield a detectable signal. In our research, similarly to Komiyama's work, we use QD and SET structures to develop single photon detectors, for which the NEP will be about 10$^{-22}$ W/Hz$^{1/2}$. This is five orders of magnitude more sensitive than the state-of-the-art bolometer, and sensitive enough to detect a single photon at microwave and IR frequencies. The small diameter of the QD and SET, about 200 - 250 nm, can increase the charging energy and therefore the operating temperature. In this presentation we will discuss the fabrication process of small quantum dots, which includes new lithographic methods in combination with e-beam lithography, and experimental problems and promises of our single photon detectors.   

Terahertz Absorption of (In,Ga)As Quantum Post Nanostructures

Quantum posts (QPs) are a new kind of self-assembled semiconductor nanostructure created by vertical stacking of self-assembled InAs quantum dots into roughly cylindrical In rich regions embedded in a GaAs matrix.$^{2}$ These structures have potential applications for THz quantum information processing,$^{1}$ THz generation, and THz detection. For a single electron trapped in a 40 nm high QP, the orbital transition between the ground and first excited state is predicted to occur near 1 THz.$^{2}$ Voltage controlled electron loading of QPs is measured by capacitance-voltage spectroscopy. Terahertz absorption spectroscopy of electrons in quantum post samples is demonstrated as a function of electron loading. $^{1}$ M. S. Sherwin, A. Imamoglu and C. Montroy, PRA 60, 3508 (1999) $^{2}$ J. He et al, Nanoletters 7, 802 (2007)   

Intrinsic optical bistability: the mechanism and features

Intrinsic optical bistability (IOB) in two-state quantum systems is investigated theoretically within the framework of density matrix formalism. The analytical relations governing the IOB are presented. The mechanism of the IOB is discovered. The IOB in the realm of time is considered within the framework of phase transitions. It is shown that the IOB in the realm of time represents cyclic process, and, thus, may be applied for description of evolution process. Numerical simulations are performed for two-state quantum dot (QD) systems. The obtained results may find applications for designing and exploiting all-optical components of QD-based optical switches and optical transistors.   

Measurement of the separation dependence of the resonant energy transfer between CdSe nanocrystals

An apparatus has been developed to study the separation dependence of interaction between two resonant groups of CdSe/ZnS quantum dots. A near-field scanning optical microscope (NSOM) is used to bring a group of mono-disperse 5.5 nm dots close (near-field range) to an 8.5 nm group of dots which are deposited on a solid immersion lens. The size of the small and large nanocrystals dots have been selected to make the excitonic ground state of the small dots coincide with the excited state of the large dots, as determined by photoluminescence and photoluminescence excitation experiments. Combination of spectral and positional filtering allows us to measure the interaction between a few quantum dots (with the ultimate goal of identifying the interaction between individual dots). The analysis of the separation-dependent photoluminescence signal from the two groups of quantum dots, yields the dipole-dipole and higher order (dipole-quadrupole) interaction terms. We expect that our results will improve the knowledge of the quantum states and decoherence processes in quantum dots.   

Coherent coupling and energy transfer enhancement via multi-exciton levels in semiconductor nanocrystals

The theory of coherent energy transfer (ET) in nanocrystal (NC) systems [1] is generalized for multiexciton levels. The relevant excitonic states in an isolated NC can be described by an effective four-level system, consisting of the ground level, two degenerate single exciton levels, and the biexciton level. We study the dynamics of a single donor-acceptor pair via the equation of motion for the density matrix of the system and consider analytical limits as well as numerical solutions. We show that the enhancement of the ET efficiency introduced by the biexciton levels is limited due to the coherent coupling of the exciton-biexciton levels in the donor-acceptor pair. The saturation of the ET rate in the donor-acceptor pair suggests a new mechanism to control the dipole-dipole coupling strength in NC systems, and we present here its dependence on structure parameters. \newline [1] A. N. Al-Ahmadi and S. E. Ulloa, Phys. Rev. B \textbf{70}, 201302(R) (2004).   

Luminescence excitation of InAs/GaAs coupled quantum dots

An understanding of the excited states in coupled quantum dots is a necessary step in the road towards a coherent control of this system. Photoluminescence excitation studies were performed on an InAs/GaAs coupled quantum dot system embedded in a Schottky diode structure. The ground states of the positive trion, negative trion and neutral exciton are first clearly identified by their photoluminescence spectra in bias maps. Preliminary results are reported on the luminescence excitation spectra of these charge configurations; both near and far away from the region where molecule-like behavior is observed.   

Record Extinction of a Laser Beam by a Single Quantum Dot

The ability to efficiently couple far-field light to subwavelength light emitters is of critical importance for the rapidly growing field of nanophotonics. In this talk we present our recent work on the use of both index matched GaAs solid immersion lenses (SIL) and numerical aperture increasing lenses (NAIL) to improve far-field light coupling to and from single InAs$\backslash $GaAs quantum dots. The SIL$\backslash $NAIL leads to significant improvements in both non-resonant and resonant spectroscopic studies of single QDs. By incorporating a SIL$\backslash $NAIL in resonant scattering measurements we find that a single InAs QD can extinguish nearly 12{\%} of the exciting laser beam; a seven-fold improvement in extinction when compared to measurements made without a SIL$\backslash $NAIL. The strong extinction makes it possible to measure a typical QD extinction using a dc power-meter without the need for phase sensitive lock-in detection.   

Quantum-mechanical description of Faraday rotation in a single quantum dot

Faraday rotation is one way to realize quantum non-demolition (QND) measurement of electron spin in a quantum dot. In the literature, it has been semiclassically modeled based on quantized electron spin states and classical electromagnetic fields. We have developed a fully quantum- mechanical model to describe Faraday rotation in single quantum dots, using an extension of the Jaynes-Cumming model which includes quantum Stokes operators. The intrinsic noise of Faraday rotation that results from the interaction between photon and electron is quantified under this model. Some effects, such as hyperfine interactions and transitions between off-resonant states such as light hole and conduction band electron states, and have not been included in our calculation. It is believed that these effects will affect the dynamics of spin and based on the current model, our calculation could be extended to examine the behavior of Faraday rotation with these effects included. This work was supported by NSF-DMR-0602846.   

Optical Aharanov-Bohm oscillations in DMS type-II ZnMnTe/ZnSe quantum dots

Low temperature magneto-photo luminescence studies of diluted magnetic semiconductor Zn(Mn)Te quantum dots (QDs) will be presented. As expected, the exchange interaction between the Mn spin and electric charge carriers results in a strong optical polarization of the luminescence at low temperature. However, in addition, the sample geometry for the structure, which consists of five Zn(Mn)Te QD layers separated by narrow ZnSe spacer layers, will be shown to be particularly suitable for the observation of the optical Aharanov-Bohm effect. This is illustrated by the presence of strong Aharanov-Bohm oscillations in the photoluminescence intensity. Finally, although the (ZnMn)Te system is known to be paramagnetic, at low temperatures the QD structures described display evidence of spontaneous magnetization at zero applied magnetic field both in the optical circular polarization degree and the magnetization. The origin of this behavior will be discussed.   

Robust Aharanov-Bohm oscillations at elevated temperatures in type-II ZnTe/ZnSe quantum dots

The Zn(Te)Se material system is remarkable in that it is possible to study both Te-bound isoelectronic excitons and type-II ZnTe quantum dots (QDs) in the same sample. This is possible since with increasing tellurium deposition there is a clustering of the Te-atoms resulting in an evolution of Te isoelectronic centers, formed by Te-Se substitution, into ZnTe QD structures. The formation of columns of such QDs in multilayer superlattice structures has recently been shown to be particularly suitable for the observation of the optical Aharanov-Bohm effect. Here we present magneto-photoluminescence from such type-II ZnTe/ZnSe QDs that demonstrate large and persistent oscillations in \textit{both} the exciton energy \textit{and} intensity at high temperature indicating the formation of coherently rotating states. Furthermore, this high temperature Aharanov-Bohm effect is remarkably robust persisting until 180K despite significant quenching of the luminescence due to ionization of the type-II excitons.