We report our recent development in pursuing high Quality-Factor (high-Q factor) plasmonic resonances, with vertically
aligned two dimensional (2-D) periodic nanorod arrays. The 2-D vertically aligned nano-antenna array can have high-Q
resonances varying arbitrarily from near infrared to terahertz regime, as the antenna resonances of the nanorod are highly
tunable through material properties, the length of the nanorod, and the orthogonal polarization direction with respect to
the lattice surface,. The high-Q in combination with the small optical mode volume gives a very high Purcell factor,
which could potentially be applied to various enhanced nonlinear photonics or optoelectronic devices. The 'hot spots'
around the nanorods can be easily harvested as no index-matching is necessary. The resonances maintain their high-Q
factor with the change of the environmental refractive index, which is of great interest for molecular sensing.
We fabricated large-area stacked complementary plasmonic crystals (SC PlCs) by employing ultraviolet nanoimprint lithography. The SC PlCs were made on silicon-on-insulator substrates consisting of three layers: the top layer contacting air was a perforated Au film, the bottom layer contacting the buried oxide layer included an Au disk array corresponding to the holes in the top layer, and the middle layer was a Si photonic crystal slab. The SC PlCs have prominent resonances in optical wavelengths. It is shown that the fabricated PlCs were precise in structure and uniform in their optical properties. We examined the photoluminescence (PL) enhancement of monolayer dye molecules on the SC PlC substrates in the visible range and found large PL enhancements of up to a 100-fold in comparison with dye molecules on nonprocessed Si wafers.
We fabricated large-area stacked complementary plasmonic crystals (SC PlCs) by employing ultra-violet (UV) nanoimprint lithography (NIL). The SC PlCs was made on silicon on insulator (SOI) substrates, consisting of three layers: the top layer contacting air was perforated Au film, the bottom layer contacting buried oxide (BOX) layer included Au disk array corresponding to the holes in the top layer, and the middle layer was Si photonic crystal slab. The SC PlCs have prominent resonances in the optical wavelengths. It is shown that the fabricated PlCs were precisely made in structure and well uniform in the optical properties. We have examined photoluminescence (PL) enhancement of dye molecules on the SC PlC substrates in the visible range and found large enhancement up to 100-fold in comparison with the dye molecules on non-processed Si wafers.
Optical and spin properties of individual GaAs droplet dots in AlGaAs barriers are studied in photoluminescence
experiments at 4K. First we report strong mixing of heavy hole-light hole states. Using the neutral and charged
exciton emission as a monitor we observe the direct consequence of quantum dot symmetry reduction in this strain free system. By fitting the polar diagram of the emission with simple analytical expressions obtained from k•p theory we are able to extract the mixing that arises from the heavy-light hole coupling due to the geometrical asymmetry of the quantum dot. Second we report optical orientation experiments. Circularly polarized optical excitation yields strong circular polarization of the resulting photoluminescence. Optical injection of spin polarized electrons into a GaAs dot gives rise to dynamical nuclear polarization that considerably changes the exciton Zeeman splitting (Overhauser shift). We show that the created nuclear polarization is bistable and present a direct measurement of the build-up time of the nuclear polarization in a single GaAs dot in the order of one second.
Excitons of CdTe quantum tetrapods are theoretically analyzed. Individual electron and hole states are calculated by solving one-particle Schrödinger equation by the finite element method with the single-band effective-mass approximation and exciton states are obtained by exact diagonalization of the configuration interaction Hamiltonian. Spatial symmetries of the exciton states are related to those of the one-particle states by group theory and verified by numerical calculation. Then optical transition spectra are calculated and compared with available experimental data.
Properties of propagating wave in a composite right/left-handed (CRLH) leaky wave antenna rely heavily on the
constituent stripe patches. In this paper, we conducted full-wave simulations of structural resonances in a CRLH
leaky wave antenna by using finite difference time domain (FDTD) method. Ez field distributions for several
lowest-order modes in individual stripe patch were examined, which are named as structural resonances. The
Bloch modes in a transmission line were also calculated at high symmetric points in k-space. Study shows that
Bloch mode properties are closely related to those in an isolated structure. The mode power densities are well
confined below or around the metallic strip patches, which enables tight binding approximation to estimate band
properties in the vicinity of Γ point. The calculated dispersion diagram by FDTD tells us many new features,
such as accidental degeneracy, a third band arisen from slab mode and the many anti-crossing phenomena. These
features are of great importance to analyze the performance of CRLH leaky wave antennae.
Developments in the self-assembly by droplet epitaxy in our research group have enabled us to fabricate various GaAs quantum nanostructures of high optical quality such as quantum dots, single quantum rings, and concentric double quantum rings. We clarified their electronic states and relaxation processes by micro photoluminescence experiments. We achieved lasing of lattice-matched GaAs quantum dots and their excitonic Rabi oscillation by resonant excitation. We succeeded in the control of their photon emission rate by photonic crystal micro cavities.
We report on observation of spontaneous emissions from a single quantum dot after resonant excitation. To capture
weak emissions from a single quantum dot, reflection of an excitation laser was reduced by applying obliquely incident
geometry and crossed polarization configuration. In addition, collected emissions were temporally resolved to be
separated from residual reflection. These allowed us to realize a simple manipulation and read out of an exciton qubit.
We observed an exciton Rabi oscillation in excitation amplitude dependence of the emission intensity. We determined an
exciton dipole moment from the Rabi oscillation and from the emission decay time, independently. Comparison between
the two values shows reasonable agreement.
Photoluminescence from a single semiconductor quantum dot after perfect resonant excitation has been temporally resolved with picosecond time resolution. Making use of non-confocal geometry for the micro photoluminescence setup, we can greatly remove the elastic scattering component, and successfully capture the resonantly-excited spontaneous emission from a single quantum dot. The emission intensity as a function of coherent photoinjection shows peculiar oscillatory dependence, i.e., Rabi oscillations of ground-state excitons. The transition strength of the groundstate exciton is precisely determined through the analysis. The value indicates a good agreement with that expected by the radiative decay rate of the quantum dots.
We fabricated original probes for the near-field scanning optical microscopy (NSOM) with polarization-preserving optical fibers, and succeed polarization observation in guide-collection-mode NSOM. The polarized guide-collection-mode NSOM technique revealed the polarized properties of propagation light within a polymeric optical waveguide by separating independent polarization components. Using the polarized NSOM technique, we characterized the influences of defects and weak stresses within the waveguide. For the characterization, we intentionally printed an indentation in the vicinity of the waveguide, then evaluated the resulting influences from the NSOM images taken around the indentation. When transverse magnetic (TM) polarized light enters a waveguide, the light intensity becomes greater on the near side of the indentation than on the far side, as measured by a linearly polarized component perpendicular to the direction of light propagation. Under the polarization conditions of incident light and collection, it is expected that only the polarization-independent component will be observed and the electric field therefore does not become large. However, if scattering phenomena are present, the electric field should have a non-zero value. The most likely origin of this scattering is microdefects in the polymer generated by the stress-strain field at the interface between the waveguide and the cladding region around the indentation. These microdefects, which are generally a disorder in molecular chains, cause Rayleigh scattering. Finally, it is important to keep in mind that the polarized guide-collection-mode NSOM technique is capable of detecting defects or weak stresses in the nanoscale range within an optical waveguide.
An analytical expression is presented for light amplification by stimulated emission in arbitrary photonic crystals, which shows an enhancement due to small group velocity. This enhancement was evaluated quantitatively for a 2D crystal with a finite thickness composed of a square array of circular air-rods formed in a dielectric material. In addition to the enhancement at photonic band edges where the group velocity is equal to zero, an extremely large enhancement due to the group-velocity anomaly peculiar to 2- and 3-D crystals was found even for quite a thin structure.