Alper Kiraz

Quantum-dot single-photon sources: Prospects for applications in linear optics quantum-information processing

An optical source that produces single-photon pulses on demand has potential applications in linear optics quantum computation, provided that stringent requirements on indistinguishability and collection efficiency of the generated photons are met. We show that these are conflicting requirements for anharmonic emitters that are incoherently pumped via reservoirs. As a model for a coherently pumped single photon source, we propose cavity-assisted spin-flip Raman transitions in a single electron charged quantum dot embedded in a microcavity. We demonstrate that using such a source, arbitrarily high collection efficiency and indistinguishability of the generated photons can be obtained simultaneously with increased cavity coupling. We analyze the role of errors that arise from distinguishability of the single-photon pulses in linear optics quantum gates by relating the gate fidelity to the strength of the two-photon interference dip in photon cross-correlation measurements. We find that performing controlled phase operations with error

Cavity-quantum electrodynamics using a single InAs quantum dot in a microdisk structure

l1\phantom{\rule{0.2em}{0ex}}%

Laser emission from quantum dots in microdisk structures

requires nanocavities with Purcell factors

Photon correlation spectroscopy of a single quantum dot

{F}_{P}\ensuremath{\geqslant}40

Parallel atomic force microscopy with optical interferometric detection

in the absence of dephasing, without necessitating the strong coupling limit.

Simple largely tunable optical microcavity

We investigate cavity-quantum electrodynamics (QED) effects in an all-semiconductor nanostructure by tuning a single self-assembled InAs quantum dot (QD) into resonance with a high quality factor microdisk whispering gallery mode (WGM). The stronger temperature dependence of the QD single-exciton (1X) resonance allows us to change the relative energy of the WGM and the 1X transitions by varying the sample temperature. The two coupled resonances exhibit crossing behavior due to the weak coupling cavity-QED regime. We demonstrate exciton lifetime reduction by 6 due to the Purcell effect by tuning the QD into resonance with the WGM. Our experiments also show that single-exciton lifetime is independent of temperature up to 50 K.

Low temperature single molecule spectroscopy using vibronic excitation and dispersed fluorescence detection

We report optically pumped continuous-wave lasing from self-assembled InAs and InGaAs quantum dots (QDs) embedded in high-quality-factor microdisk laser structures. The microdisk emission spectra show lasing on 1–5 well separated modes in the wavelength range between 900 and 990 nm. The estimated threshold pump densities are between 20 and 200 W/cm2. The lasing characteristics are discussed in terms of both inhomogeneously and homogeneously broadened QD transitions.

Preparation and characterization of superhydrophobic surfaces based on hexamethyldisilazane-modified nanoporous alumina

It is by now widely accepted that various quantum dot ~QD! structures exhibit features in transport 1,2 or optical spectroscopy 3‐5 that indicate full three-dimensional confinement of carriers. Identification of QD’s as artificial atoms has been strengthened by the recent observation of strong photon antibunching in single-exciton emission, 6,7 which is the typical signature of an anharmonic quantum system: after a photon is emitted from a single two-level ~anharmonic! emitter, the system is necessarily in the radiatively inactive ground state and a second photon cannot be emitted immediately after the first one. Even though the coherence properties of QD single-exciton emission closely follow those of atoms, the overall spectral features of single QD’s are significantly more complicated. Since the size of QD’s is roughly two orders of magnitude larger than those of atoms, multiparticle excitations give rise to emission peaks with energies comparable to that of a single exciton. Of primary importance in QD spectroscopy is the biexciton state, which corresponds to a doubly-excited QD with completely filled lowest electronand hole-energy levels. When the biexciton state decays by radiative recombination, the final state is a single-exciton state and the generated photon is shifted as compared to the single exciton emission due to Coulomb interaction between the carriers. Biexciton emission in QD spectroscopy has been traditionally identified using the ~quadratic! pump-power dependence of the corresponding peak. In this paper, we demonstrate that photon-correlation measurements provide a powerful tool for characterizing the multiexciton spectral features of QD’s. Our measurements provide a strong support for the identification of a biexciton emission peak, by demonstrating its strong correlations with the subsequent single-exciton emission. We observe that biexciton intensity autocorrelation exhibits bunching together with antibunching or only antibunching under continuous-wave ~cw! excitation depending on the excitation level. In contrast, we find strong antibunching under pulsed excitation. The large difference between the levels of antibunching under continuous wave and pulsed excitations points out to the importance of excitation mechanism and the role of free carriers in QD physics. The lack of polarization correlation between biexciton and single-exciton emissions indicates that spin dephasing is likely to play a key role under nonresonant excitation. We also observe that a third emission peak in QD spectra exhibits strong correlations with both exciton and biexciton fluorescence: we argue that these correlation signatures suggest the identification of this additional line as a charged-exciton emission. Our self-assembled InAs QD’s were grown by molecularbeam epitaxy using the partially covered island technique. 8 Growth resulted in typically lens-shaped QD’s with a base diameter of 40‐50 nm and a height of 3 nm, having their single-excitonic emissions between 925 nm and 975 nm in the spectrum. In our sample, the QD’s were embedded in the center of a 200-nm-thick GaAs microdisk structure located above a 0.5-mm-thick Al0.65Ga0.35As post. The diameter of the disks was 5-mm and the average number of QD’s within the disks was less than one. Details of the microdisk processing can be found elsewhere. 9 Our experimental setup consisted of a combination of a low-temperature diffractionlimited scanning optical microscope and a Hanbury Brown

Indistinguishable photons from a single molecule.

We have developed an atomic force microscope that uses interferometry for parallel readout of a cantilever array. Each cantilever contains a phase sensitive diffraction grating consisting of a reference and movable set of interdigitated fingers. As a force is applied to the tip, the movable set is displaced and the intensity of the diffracted orders is altered. The order intensity from each cantilever is measured with a custom array of silicon photodiodes with integrated complementary metal–oxide–semiconductor amplifiers. We present images from five cantilevers acquired in the constant height mode that reveal surface features 2 nm in height. The interdigital method for cantilever array readout is scalable, provides angstrom resolution, and is potentially simpler to implement than other methods.

Haptic guidance for improved task performance in steering microparticles with optical tweezers.

The authors demonstrate more than 9nm tunability of the whispering gallery modes (WGMs) of rhodamine B doped water microdroplets resting on a superhydrophobic surface. Tunability was achieved by controlling the size of the microdroplets in a current controlled mini humidity chamber. WGMs were observed with quality factors of more than 8000 when kept stable. The sensitivity of the resonances to the size and shape of the microdroplet reveals opportunities for the use of this technique as a probe to characterize superhydrophobic surfaces and investigate liquid-solid surfaces.