Shi-Hao Guol

Two-photon fluorescence microscope with a hollow-core photonic crystal fiber

Self-phase-modulation and group velocity dispersion of near IR femtosecond pulses in fibers restrict their use in two-photon fluorescence microscopy (TPFM). Here we demonstrate a hollow-core photonic crystal fiber based two-photon fluorescence microscope with low nonlinearity and dispersion effects. We use this fiber-based TPFM system to take two-photon fluorescence (chlorophyll) images of mesophyll tissue in the leaf of Rhaphidophora aurea. With less than 2mW average power exposure on the leaf at 755nm, the near zero-dispersion wavelength, chloroplasts distribution inside the mesophyll cells can be identified with a sub-micron spatial resolution. The acquired image quality is comparable to that acquired by the conventional fiber-free TPFM system, due to the negligible temporal pulse broadening effect.

Observation of femtosecond carrier thermalization time in indium nitride

Ultrafast carrier thermalization in n-type indium nitride (InN) with an electron concentration of 3.8×1018 cm−3 was investigated by femtosecond transient transmission measurements at room temperature with different wavelengths. An extremely fast carrier external thermalization time on the order of 400 fs was observed, which is much faster than all previous reports. This observed femtosecond thermalization time is consistent with a prediction based on a Coulomb screening effect. Through wavelength dependent and power dependent studies, even with a 400 fs thermalization time, we did not observe any evidence of the existence of the hot phonon effect, which agrees with a recent report that a longitudinal optical phonon lifetime could be shorter than 300 fs in specific InN samples.

Femtosecond ultrasonic spectroscopy using a piezoelectric nanolayer: Hypersound attenuation in vitreous silica films

We report ultra-broadband ultrasonic spectroscopy with an impedance-matched piezoelectric nanolayer, which enables optical generation and detection of a 730-fs acoustic pulse (the width of ten lattice constants). The bandwidth improvement facilitates THz laser ultrasonics to bridge the spectral gap between inelastic light and x-ray scatterings (0.1-1 THz) in the studies of lattice dynamics. As a demonstration, this method is applied to measure sound attenuation α in a vitreous SiO2 thin film. Our results extend the existing low-frequency data obtained by ultrasonic-based and light scattering methods and also show a α∝ f2 behavior for frequencies f up to 650 GHz.

The Structure of GaN-Based Transverse Junction Blue LED Array for Uniform Distribution of Injected Current/Carriers

In this study, we demonstrate a GaN-based transverse junction blue LED array. This device was realized by the regrowth of n-type GaN layers on the sidewall of p-type GaN and undoped multiple quantum wells (MQWs). Due to the transverse flow of injection carriers, problems related to nonuniform current distribution, nonuniform carrier distribution among different MQWs, and bias-dependent shape of the electroluminescence spectra such as that occurring in traditional GaN-based blue LEDs with vertical p-n junctions and large active area (>1 mm2) are all greatly minimized in our structure.

Transverse-junction superluminescent diodes at the 1.1µm wavelength regime

We demonstrate, for the first time to our knowledge, GaAs-based transverse-junction (TJ) superluminescent diodes (SLDs) that operate at a wavelength of 1.1 µm. Due to lateral current injection by use of TJ, specified as transverse carrier flow spread in each quantum well horizontally instead of vertical well-by-well injection, nonuniform carrier distribution can be minimized among different multiple quantum wells (MQWs), which is a problem in vertical-junction (VJ) SLDs whose electroluminescent (EL) spectrum is governed by the center wavelength of QWs near the p side. In contrast with a VJ SLD, the EL spectrum of our device is determined by QWs that have a larger differential gain than the positions of QWs neighbored with a p side layer.

Minimization of Damping in the Electrooptic Frequency Response of High-Speed Zn-Diffusion Single-Mode Vertical-Cavity Surface-Emitting Lasers

We utilize the Zn-diffusion technique to fabricate a single-mode high-speed 850-nm vertical-cavity surface-emitting laser. With this technique, we are able to minimize the thermal effect without greatly scaling down the diameter of the oxide-confined aperture. The demonstrated device has a 9-mum active diameter with which we can attain a bandwidth of 8 GHz, a small differential resistance (~47 Omega), and a maximum output power of 3 mW. The single-mode characteristics can be sustained under dynamic operation for the whole bias current range. The dynamic measurement results indicate that with this single-mode device the damping-limited bandwidth of our multimode control can be eliminated without a Zn-diffusion aperture. A larger intrinsic bandwidth (32 versus 21 GHz) is also obtained due to the minimization of damping. The narrower divergence angle (8deg versus 20deg) means that the device exhibits a larger alignment tolerance and much lower coupling loss (9 dB) when used with the standard multimode fiber than those of the control sample.

Bipolar cascade superluminescent diodes at the 1.04μm wavelength regime

In this study, we investigate the performance of GaAs-based bipolar cascade superluminescent diodes with different cavity lengths. The device operates around the important bio-optical therapeutic 1.04-mum wavelength window. The introduction of tunnel junctions tends to minimize the nonuniform carrier distribution between distinct multiple quantum-wells (QWs), which is a problem in conventional SLDs, whose electroluminescent spectra are governed by the center wavelength of QWs near the p-side. Our devices exhibit nice electrical characteristics of low leakage current and overcome the limitation of nonuniform carrier distribution, thereby presenting a promising prospect for the near infrared white-light sources.

Electrically manipulating the optical sensitivity function in quantum wells for nanoacoustic wave detection

We demonstrate electrical control of the optical sensitivity function in multiple quantum wells (MQWs) for nanoacoustic wave detection. This is realized by bias controlling the quantized level and the quasi-Fermi level of carrier-populated InGaN/GaN MQWs. Experimentally, a strongly bias-dependent optical sensitivity was observed when the optical probe transition was near the quasi-Fermi level, which agrees well with the theoretical prediction.

GaAs-based bipolar cascade light-emitting-diodes and superluminescent-diodes at the 1.04-μm wavelength regime

We first demonstrate GaAs-based bipolar cascade superluminescent-diodes by use of chirped multiple-quantum-wells in series by a tunnel junction, effectively minimizing the problem of non-uniform carrier-distribution among MQWs and operating around important bio-optical window of 1.04-mum wavelength.

GaAs-Based Transverse Junction Superluminescent Diodes With Strain-Compensated InGaAs–GaAsP Multiple-Quantum-Wells at 1.1-

In this study, we demonstrate a transverse junction superluminescent diode (TJ-SLD) with an engagement of chirped InxGa1-xAs-GaAs0.9P0.1 strain-compensated multiple-quantum-wells (SC MQWs) at 1.1-μm wavelength. The problem relative to inhomogeneous carrier distribution in each QW, which is a problem in traditional vertical junction SLDs (VJ-SLDs), can be effectively minimized by utilizing the benefit of lateral carrier injection in TJ devices. Our demonstrated device offers significant improvements in threshold current, output power, and optical bandwidths compared to TJ-SLD without SC MQWs. Furthermore, compared with the high-performance ~1-μm VJ-SLDs, this novel device exhibits a comparable output power and 3-dB bandwidth performance with a more stable electroluminescence spectrum, which varies only negligibly under a wide range of bias current.