Shih-Hui Chang

Surface plasmon generation and light transmission by isolated nanoholes and arrays of nanoholes in thin metal films

Extensive 3-D finite-difference time-domain simulations are carried out to elucidate the nature of surface plasmon polaritons (SPPs) and localized surface plasmon polaritons (LSPs) generated by nanoscale holes in thin metallic films interacting with light. Both isolated nanoholes and square arrays of nanoholes in gold films are considered. For isolated nanoholes, we expand on an earlier discussion of Yin et al. [Appl. Phys. Lett. 85, 467-469 (2004)] on the origins of fringe patterns in the film and the role of nearfield scanning optical microscope probe interactions. The associated light transmission of a single nanohole is enhanced when a LSP excitation of the nanohole itself is excited. Periodic arrays of nanoholes exhibit more complex behavior, with light transmission peaks exhibiting distinct minima and maxima that can be very well described with Fano lineshape models. This behavior is correlated with the coupling of SPP Bloch waves and more directly transmitted waves through the holes.

Optically pumped InAs quantum dot microdisk lasers

We have achieved lasing in InAs quantum dot embedded GaAs microdisks under optical pumping. Above the lasing threshold, a drastic increase of emission intensity is accompanied by a decrease of the spectral linewidth of the whispering gallery modes. The laser light is linearly polarized. The polarization direction is parallel to the disk plane. The wide gain spectrum of quantum dots allows simultaneous lasing in several whispering gallery modes of a microdisk.

Gap structure effects on surface-enhanced raman scattering intensities for gold gapped rods

Gapped rods provide a unique platform for elucidating structure/function relationships, both for single-molecule electrochemical techniques and for surface-enhanced Raman scattering (SERS). This paper attempts to elucidate the dependence of SERS intensities on gap topography and gap distance for gold gapped rods with segment lengths varying over a wide range (40-2000 nm). Significantly, we have determined that rough gaps lead to a smaller SERS enhancement than smooth gaps for these structures even though the rough gaps have a larger total surface area. Both theory and experiment show periodic variation of SERS intensity with segment length as determined by odd-symmetry plasmon multipoles. Excitation of even-symmetry modes is dipole forbidden (for polarization along the rod axis), but this selection rule can be relaxed by roughness or, for smooth gaps, by near-field coupling between the rod segments.

Finite-difference time-domain model of lasing action in a four-level two-electron atomic system

We report a new finite-difference time-domain (FDTD) computational model of the lasing dynamics of a four-level two-electron atomic system. Transitions between the energy levels are governed by coupled rate equations and the Pauli Exclusion Principle. This approach is an advance relative to earlier FDTD models that did not include the pumping dynamics, or the Pauli Exclusion Principle. Further, the method proposed in this paper is more versatile than the conventional modal expansion of the electromagnetic field for complex inhomogeneous laser geometries constructed in photonic crystals or light-localizing random media. For such complex geometries, the lasing modes are either difficult or impossible to calculate. The present work aims at the self-consistent treatment of the dynamics of the 4-level atomic system and the instantaneous ambient optical electromagnetic field. This permits in principle a much more robust treatment of the overall lasing dynamics of four-level gain systems integrated into virtually arbitrary electromagnetic field confinement geometries.

Apertureless scanning near-field optical microscopy: a comparison between homodyne and heterodyne approaches

In coherent homodyne apertureless scanning near-field optical microscopy (ASNOM) the background field cannot be fully suppressed because of the interference between the different collected fields, making the images difficult to interpret. We show that implementing the heterodyne version of ASNOM allows one to overcome this issue. We present a comparison between homodyne and heterodyne ASNOM through near-field analysis of gold nanowells, integrated waveguides, and a single evanescent wave generated by total internal reflection. The heterodyne approach allows for the control of the interferometric effect with the background light. In particular, the undesirable background is shown to be replaced by a controlled reference field. As a result, near-field information undetectable by a homodyne ASNOM is extracted by use of the heterodyne approach. Additionally, it is shown that field amplitude and field phase can be detected separately.

Cavity formation and light propagation in partially ordered and completely random one-dimensional systems

We study light transport in ordered, partially ordered, and completely random one-dimensional (1-D) systems. In a periodic structure, there are three types of passbands with different origins. When disorder is introduced to a periodic system, the passbands change differently, depending on their origins. The transmissivity and decay length in the passbands near the band edges decrease drastically. The stopbands are widened. The introduction of randomness to a periodic structure enhances light localization in frequency regions in which it is delocalized in a periodic structure. In a completely random system, a resonant cavity is formed by two stacks of multiple layers which serve as two highly reflective broadband mirrors. We calculate the size and the quality factor of 1-D random cavities. With an increase in the degree of disorder, the lasing threshold in such a cavity first decreases, then increases. The lasing frequency spreads from the band edge toward the stopband center.

Ultrafast dynamics of InAs/GaAs quantum-dot microdisk lasers

The dynamical response of InAs/GaAs quantum-dot microdisk lasers has been experimentally investigated using femtosecond optical pumping. Because surface recombination and carrier diffusion are suppressed in the quantum dots, the response speed of a quantum-dot microdisk laser is much faster than that of a quantum-well microdisk laser. A turn-on time as short as 7.8 ps has been achieved in a quantum-dot microdisk laser at 5 K. The temperature dependence of the dynamical response of the quantum-dot microdisk lasers has also been studied over a wide temperature range. At the same pumping level, the turn-on time of the laser decreases as the temperature increases from 5 to 120 K. Such behavior may be due to a faster carrier relaxation process at higher temperature.

Dynamics of GaAs/AlGaAs microdisk lasers

Dynamic response of a GaAs/AlGaAs microdisk laser has been experimentally investigated using femtosecond optical pumping. Below the lasing threshold, the delay time of the emission pulse from the microdisk hardly changes with the pump power. Above the lasing threshold, the delay time is shortened dramatically, and it decreases with increasing the pump power. The theoretical simulation based on the rate equations reproduces the experimental observation after the effect of carrier diffusion is taken into account. The simulation result illustrates that the speed of a microdisk laser is limited mainly by the carrier diffusion in the disk plane.

Heterodyne apertureless near-field scanning optical microscopy on periodic gold nanowells

Heterodyne detection for apertureless near-field scanning optical microscopy was used to study periodic gold nanowell arrays. Optical near-field amplitude and phase signals were obtained simultaneously with the topography of the gold nanowells and with different polarizations. Theoretical calculations of the near-fields were consistent with the experiments; in particular, the calculated amplitudes were in especially good agreement. The heterodyne method is shown to be particularly effective for these types of periodic photonic structures and other highly scattering media, which can overwhelm the near-field scattered signal when conventional apertureless near-field scanning optical microscopy is used.

Numerical study of light correlations in a random medium close to the Anderson localization threshold

We applied a finite-difference time domain algorithm to the study of field and intensity correlations in random media. Close to the onset of Anderson localization, we observe deviations of the correlation functions, in both shape and magnitude, from those predicted by the diffusion theory. Physical implications of the observed phenomena are discussed.