A. A. Mikhailovsky

From amplified spontaneous emission to microring lasing using nanocrystal quantum dot solids

We study different emission regimes in close-packed films of chemically synthesized CdSe nanoparticles [nanocrystal quantum dots (NQDs)]. We observe that the NQD photoluminescence is dominated by excitons and biexcitons, respectively, before and after the threshold for stimulated emission. Furthermore, we demonstrate the regime of microring lasing into sharp, whispering-gallery modes using NQD solids incorporated into microcapillary tubes. This result indicates a feasibility of miniature, solid-state laser devices based on chemically synthesized NQDs.

Multiparticle interactions and stimulated emission in chemically synthesized quantum dots

We study the effect of multiparticle interactions on optical gain and stimulated emission in close-packed solids of chemically synthesized CdSe nanocrystals (nanocrystal quantum dots). An analysis of pump-dependent nonlinear absorption signals indicates that the band-edge optical gain is due to multiparticle states with a dominant contribution from doubly excited nanocrystals (quantum-confined biexcitons). We observe that optical gain dynamics are due to the competition between ultrafast hole surface trapping and multiparticle Auger decay. We analyze the effect of intrinsic Auger recombination on optical gain lifetimes and gain pump intensity thresholds.

Broadband near-field interference spectroscopy of metal nanoparticles using a femtosecond white-light continuum

We demonstrate a new nanoscale spectroscopic technique that combines subwavelength near-field imaging with broadband interference spectroscopy. We apply this technique to study phase spectra of surface plasmons in individual gold nanoparticles and nanoparticle dimers. Collective plasmon oscillations in selected nanostructures are excited by a femtosecond white-light continuum transmitted through a subwavelength aperture. The interference spectra detected in the far field result from the coherent superposition of the aperture field and the secondary field re-emitted by the nanostructure. The analysis of these spectra allows us to accurately measure the positions and damping constants of single-nanostructure plasmon resonances.

Optical gain and lasing in colloidal quantum dots

Summary form only given. Semiconductor quantum dots (QDs) promise the lowest lasing threshold for semiconductor media. Additionally, QDs in the strong confinement regime have an emission wavelength that is a pronounced function of size, adding the advantage of continuous spectral tunability simply by changing the dot radius. Lasing has previously been demonstrated for epitaxially grown III-V QDs. Large lateral dimensions and difficulties in size control limit their spectral tunability using quantum confinement effects. An alternative approach to fabricating QDs is through chemical synthesis which can produce semiconductor nanoparticles (colloidal QDs) with radii from 1 to 6 nm and with size dispersions as small as 5%. Such dots show strong quantum confinement and permit size-controlled spectral tunability over an energy range as wide as 1 eV. The combination of tunable electronic energies and chemical flexibility make colloidal QDs ideal building blocks for the bottom-up assembly of optical device structures, including optical amplifiers and lasers. However, despite more than a decade of effort, lasing in small-size colloidal nanoparticles has not been realized. To determine what hinders lasing action, we performed extensive dynamical studies of radiative and nonradiative processes in CdSe colloidal QDs.

Femtosecond intraband modulation spectroscopy

We demonstrate several femtosecond spectroscopic techniques utilizing intraband carrier reexcitation for studies of relaxation processes in semiconductor materials. By introducing an additional infrared post-pump pulse into a conventional transient absorption experiment, we are able to visualize carrier intraband dynamics and to detect weak conduction-to-valence band transitions unresolved in the traditional transient absorption configuration. Intraband reexcitation spectroscopy also allows us to study dynamics of radiative transitions by monitoring a post-pump-induced modulation of time-integrated photoluminescence.

Carrier relaxation dynamics and ultrafast optical nonlinearities in semiconductor quantum dots

We use a variety of femtosecond nonlinear optical techniques to study carrier relaxation and mechanisms for resonant optical nonlinearities in semiconductor quantum dots. Combining femtosecond inter- and intra-band probes we are able to separate electron and hole relaxation paths and to resolve single exponential dynamics of different multiparticle states.

Near-field, phase-sensitive spectroscopy of metal nanoassemblies

We study amplitude and phase responses of individual gold nanoparticles and nanoparticle dimers by applying near-field illumination with a bright, broad-band, femtosecond, white-light continuum.

Molecular control of charge and energy transfer in self-assembled polymer films: toward improved ultrafast holographic materials

We report on the optical characterization of a series of multi-layered organic superlattices made by polyelectrolyte self-assembly. Using a high molecular weight, water-soluble, anionic form of poly-phenylene vinylene, self-assembled films can be formed which show high photoluminescence quantum efficiency (QE). A dramatic red shift of the luminescence and increase in QE is observed as additional PPV layers are added. We attribute the red shift and increasing QE to a changing conformation of the polymer chains as the superstructure is assembled, together with efficient Frster energy transfer in a preferred direction away from the substrate toward layers with longer effective conjugation length. Upon adding a C60 top layer, the luminescence spectrum is strongly quenched. We attribute this to charge transfer of from the top-most polymer layer to the C60 layer. We discuss the possibilities of exploiting this directional charge transfer in an ultrafast holographic device along with other optics for increasing the temporal diffraction efficiency of polymer-based mediums.