Alexey Yamilov

Fabrication of inverted opal ZnO photonic crystals by atomic layer deposition

We have fabricated three-dimensional optically active ZnO photonic crystals by infiltrating polystyrene opal templates using a low-temperature atomic layer deposition process. The polystyrene is removed by firing the samples at elevated temperatures, and reactive ion etching is used to remove the top layer of ZnO and expose the (111) photonic crystal surface. The resulting structures have high filling fractions, possess photonic band gaps in the near-UV to visible spectrum, and exhibit efficient photoluminescence.

Ultraviolet photonic crystal laser

We fabricated two-dimensional photonic crystalstructures in zinc oxide films with focused-ion-beam etching. Lasing is realized in the near-ultraviolet frequency at room temperature under optical pumping. From the measurement of lasing frequency and spatial profile of the lasing modes, as well as the photonicband structure calculation, we conclude that lasing occurs in the strongly localized defect modes near the edges of photonic band gap. These defect modes originate from the structure disorder unintentionally introduced during the fabrication process.

Random Lasing in Weakly Scattering Systems

We present detailed experimental and numerical studies of random lasing in weakly scattering systems. The interference of scattered light, which is weak in the passive systems, is greatly enhanced in the presence of high gain, providing coherent and resonant feedback for lasing. The lasing modes are confined in the vicinity of the pumped volume due to absorption of emitted light outside it. In the ballistic regime where the size of the gain volume is less than the scattering mean free path, lasing oscillation occurs along the direction in which the gain volume is most extended, producing directional laser output. The feedback for lasing originates mainly from backscattering of particles near the boundaries of the pumped region. It results in nearly constant frequency spacing of lasing modes, which scales inversely with the maximum dimension of the gain volume.

Random lasing in closely packed resonant scatterers

We report experimental and theoretical studies of the random lasing threshold and its fluctuation in an ensemble of highly packed spherical dielectric scatterers. The ratio of the sphere diameter to the lasing wavelength was varied in a wide range, which covered the transition from the weak Rayleigh scattering regime to the strong Mie scattering regime. Experimentally, when the diameters of monodispersed ZnO spherical particles changed from less than 100 to more than 600 nm we observed a drastic decrease of the lasing threshold at small-particle size followed by a plateau at large particle size. We attribute this effect to the particle-size dependence of transport mean free path lt, which was deduced from coherent backscattering measurements. Theoretical calculation of lt reproduced experimental behavior. Using the finite-difference time domain method, we obtained the lasing threshold and its standard deviation as functions of particle size in two-dimensional systems. The results of our numerical simulations are in qualitative agreement with the experimental data.

Ultraviolet lasing in high-order bands of three-dimensional ZnO photonic crystals

UV lasing in three-dimensional ZnO photonic crystals is demonstrated at room temperature. The photonic crystals are inverse opals with high refractive index contrast that simultaneously confine light and provide optical gain. Highly directional lasing with tunable wavelength is obtained by optical pumping. Comparison of the experimental results to the calculated band structure shows that lasing occurs in high-order bands with abnormally low group velocity. This demonstrates that the high-order band structure of three-dimensional photonic crystals can be used to effectively confine light and enhance emission. Our findings may also impact other applications of photonic crystal devices.

Large enhancement of spontaneous emission rates of InAs quantum dots in GaAs microdisks.

We have studied the enhancement of spontaneous emission rates for InAs quantum dots embedded in GaAs microdisks in a time-resolved photoluminescence experiment. Inhomogeneous broadening of the quantum dot energy levels and random spatial distribution of the quantum dots in a microdisk lead to a broad distribution of the spontaneous emission rates. Using a nonnegative least-norm algorithm, we extract the distribution of spontaneous emission rates from the temporal decay of emission intensity. The maximum spontaneous emission enhancement factor exceeds 10.

Quantum dots by ultraviolet and x-ray lithography

Highly luminescent semiconductor quantum dots have been synthesized in porous materials with ultraviolet and x-ray lithography. For this, the pore-filling solvent of silica hydrogels is exchanged with an aqueous solution of a group II metal ion together with a chalcogenide precursor such as 2-mercaptoethanol, thioacetamide or selenourea. The chalcogenide precursor is photodissociated in the exposed regions, yielding metal chalcogenide nanoparticles. Patterns are obtained by using masks appropriate to the type of radiation employed. The mean size of the quantum dots is controlled by adding capping agents such as citrate or thioglycerol to the precursor solution, and the quantum yield of the composites can be increased to up to about 30% by photoactivation. Our technique is water-based, uses readily available reagents, and highly luminescent patterned composites are obtained in a few simple processing steps. Polydispersity, however, is high (around 50%), preventing large-scale usage of the technique for the time being. Future developments that aim at a reduction of the polydispersity are presented.

Effect of local pumping on random laser modes in one dimension

We have developed a numerical method based on the transfer matrix to calculate the quasi modes and lasing modes in one-dimensional random systems. Depending on the relative magnitude of the localization length versus the system size, there are two regimes in which the quasi modes are distinct in spatial profile and frequency distribution. In the presence of uniform gain, the lasing modes have one-to-one correspondence to the quasi modes in both regimes. Local excitation may enhance the weight of a mode within the gain region due to local amplification, especially in a weakly scattering system.

Absorption-induced confinement of lasing modes in diffusive random media.

We present a numerical study of lasing modes in diffusive random media with local pumping. The reabsorption of emitted light suppresses the feedback from the unpumped part of the sample and effectively reduces the system size. The lasing modes are dramatically different from the quasi modes of the passive system (without gain or absorption). Even if all the quasi modes of a passive diffusive system are extended across the entire sample, the lasing modes are still confined in the pumped volume with an exponential tail outside it. The reduction of effective system volume by absorption broadens the distribution of decay rates of quasi modes and facilitates the occurrence of discrete lasing peaks.

Position-dependent diffusion of light in disordered waveguides

Summary form only given. Diffusion is a statistical description of random walk of a classical particle, and the diffusion constant D0 is the only parameter in the diffusion equation. For light as well as for other kinds of waves, this is an approximation, because the interference of partial waves is ignored [1]. Such interference is essential to Anderson localization. Proper account of the interference effects in random samples of finite size [2] and/or with absorption [3] results in spatial variation of the diffusion coefficient D(r) in the self consistent theory (SCT) of localization.To observe position-dependent diffusion, disordered waveguide structures were fabricated with the silicon on insulator wafer (see Fig. 1a). The patterns were written by electron beam lithography and etched in an inductive coupled reactive ion etcher. The waveguides contained 2D random arrays of air holes that scattered light, and the scattering length was varied by the hole size and filling fraction. The waveguide walls were made of photonic crystals that had complete bandgap in 2D, so that light could not escape laterally. However, light will leak out of the plane while being scattered by the air holes. This vertical leakage can be described by an effective absorption or dissipation. The relevant parameters are the diffusive absorption length ȟa0 and the transport mean free path . The localization length ȟ is determined by and the waveguide width W. Light from a CW laser source was injected into the waveguide from one end, and transported through the random medium. Spatial distribution of light intensity on the sample surface was imaged onto a camera by an objective lens. After entering the random medium, light is attenuated due to competing effects of backscattering and dissipation. I(y, z) was integrated along the transverse y-direction to determine the variation of intensity along the axial z-direction (parallel to the waveguide axis).Fig. lb shows the measured light intensity I(z) inside the ensembles of random waveguides of different width W (blue). The values of ξ and ξασ are obtained by fitting the most diffusive sample (W = 60 μm, longest ξ) with SCT (red dashed line) [2,3]. Using these values, SCT successfully predicts I(z) for all other samples. D(z) corresponding to red curves in Fig. lb are plotted in Fig. lc, showing a suppression of diffusion in the middle of the sample with increase ξασ/ξ (decrease of W) as predicted by SCT.