Yish-Hann Liau

Femtosecond-pulse cavity-dumped solid-state oscillator design and application to ultrafast microscopy

The construction, modeling, and performance characteristics of a new resonator design for ultrafast cavity-dumped oscillators are presented. An acousto-optic Bragg cell was incorporated at the end of the longer arm of a Ti:sapphire oscillator rather than in the shorter arm as in several recent studies. The new arrangement improves the pulse intensity stability of the oscillator and significantly reduces the effort required in construction. The experimental findings are supported by comparison of the stability regions of the laser cavities based on the two different designs. To demonstrate the potential of cavity-dumped oscillators for spatially resolved ultrafast spectroscopy studies, the pulse duration is characterized at the focal plane of two achromatic high-N.A. oil-immersion objectives with different amounts of flat-field correction. Transform-limited pulse widths as short as 15 fs are obtained. To our knowledge, this is the shortest pulse duration measured with true high-N.A. (N.A. > 1) focusing conditions.

Imaging scanning tunneling microscope-induced electroluminescence in plasmonic corrals

An approach to image localized and propagating surface plasmon (SP) modes is introduced. It is shown that scanning tunneling microscope (STM)-electroluminescence, the radiative decay of SPs induced by inelastically tunneling electrons, observed in Fourier space yields distinct features that reflect the degree of delocalization and spatial distribution of SP modes. The propagating SP is isolated from the localized mode by way of this Fourier space imaging approach. Furthermore, a cylindrically symmetric spatial interference pattern is obtained when the STM-induced plasmon is created within a circular “corral” boundary condition.

Ultrafast interferometric measurements of plasmonic transport in photonic crystals

We present a novel time-domain experimental approach to the study of the dynamics of surface electromagnetic wave propagation in a two-dimensional photonic crystal. A surface plasmon polariton is launched by ultrafast laser pulses and propagates into a photonic crystal, the dynamics of which are measured by an interferometric cross-correlation method. Plasmon photonic stopgaps are characterized by a single measurement. The dispersion around the stopgaps is determined with a series of angle-resolved measurements.

Mechanism for photon emission from Au nano-hemispheres induced by scanning tunneling microscopy

The photon emission yield observed in scanning tunneling microscopy (STM) measurements of Au hemispheroid-decorated thin films is used to elucidate the interaction of tunneling electrons with local surface plasmon modes. The photon emission probability is found to depend on the surface feature size. The agreement of a model calculation with the experimental results demonstrates that inelastic electron tunneling is the dominant mechanism of STM-induced plasmon excitation for 10–60 nm size metallic features.

Time-resolved Carrier Dynamics Near The Insulator-metal Transition

Carrier dynamics in polyaniline and colloidal Au films have been examined using a combined approach of time resolved laser spectroscopy and atomic force microscopy (AFM). These systems exhibit insulator-metal transitions (IMT) in conjunction with synthetic modification of their structure. The relationship between structure and reactivity, in terms of hot carrier lifetimes and transport, has been identified by correlati ng changes in the dynamics with the directly measured morphology. Physical insight into the processes affecting carrier lifetimes and localization and the nature of the IMT has been derived from an analysis of the experimental data. The results and conclusions presented herein have implications for directing research and development in device applications based on thin film technologies.

Ultrafast STM-tip Localized Responses from Nanostructured Surfaces

Spatially correlated reactivity and femtosecond time-scale responses from mesoscopic rough films are reported. The STM-tip spatially-localized transient SHG dynamics of nanometer scale metallic structures are used to establish localized surface plasmon dynamics.

Ultrafast Interferometric Studies of Multiple Scattering of Light in Photonic Structures

We report the first observation of pronounced recurrent signals in the pump-probe interferometry measurements of structured metallic interfaces. The coherent multiple scattering of surface plasmons from the microsphere overlayer causes light localization in the photonic structure. Photonic structures have attracted extensive attention in the past decade because of their unprecedented potential for manipulating light.(1) Previous research on photonic structures almost exclusively employed CW light, thereby limiting the studies to static and linear properties.(2) Elucidation of the dynamics of wave packet propagation in the photonic structure requires ultrafast spectroscopic techniques.(3) Further, the use of high peak-intensity femtosecond pulses opens possibilities to exploit the nonlinearity of photonic materials.(4) Scattering of light plays a critical role in many important phenomena including multiple scattering as a precursor for light localization in a random medium.(5) Interference of scattered light in the photonic structure is responsible for the formation of photonic band-gaps. In our earlier studies, coherent multiple scattering of surface plasmon polaritons on rough metal surfaces was observed in pump-probe measurements(6). In the present study, surface waves associated with the surface plasmon that propagates on a decorated metal film are used as a 2-D model system. Surface plasmons can be launched by light by satisfying the dispersion relation at the interface.(7) Plasmon scattering can be introduced by spatially modulating the interfacial dielectric properties. In this paper, we report the first observation of pronounced recurrent interferometric signals resulting from multiple scattering of surface plasmons propagating in photonic structures. The ultrafast laser system and the experimental setup have been described elsewhere.(8) The surface plasmon was launched in the Kreschmann geometry.(7) The pump-probe laser pulses with controlled interferometric delay were sent to a prism coupler and focused onto a thin metallic film sample. A CCD camera was used to acquire the plasmon field distribution. The sample, a 46-nm thickness Ag film, thermally evaporated on a glass slip, was spin - coated with a monolayer of latex spheres (diameter = 2.1 μm). Optical contact between the glass slip and the prism hypotenuse was made by index matching oil. The interferometric signal resulting from the surface plasmon created by the pump beam scattering into the direction of the probe beam was detected by the lock-in