C.-M. Lai

Spatial manipulation of nanoacoustic waves with nanoscale spot sizes

Coherent acoustic phonons are generated at terahertz frequencies when semiconductor quantum-well nanostructures are illuminated by femtosecond laser pulses. These phonons-also known as nanoacoustic waves-typically have wavelengths of tens of nanometres, which could prove useful in applications such as non-invasive ultrasonic imaging and sound amplification by the stimulated emission of radiation. However, optical diffraction effects mean that the nanoacoustic waves are produced with spot sizes on the micrometre scale. Near-field optical techniques can produce waves with smaller spot sizes, but they only work near surfaces. Here, we show that a far-field optical technique--which suffers no such restrictions--can be used to spatially manipulate the phonon generation process so that nanoacoustic waves are emitted with lateral dimensions that are much smaller than the laser wavelength. We demonstrate that nanoacoustic waves with wavelengths and spot sizes of the order of 10 nm and 100 nm, respectively, can be generated and detected.

Optical response from lenslike semiconductor nipple arrays

The authors reported the use of recessive size reduction in self-assembled polystyrene sphere mask with anisotropic etching to form lenslike nipple arrays onto the surface of silicon and gallium nitride. These devices are shown to exhibit a filling factor near to an ideal close-packed condition and paraboloidlike etch profile with slope increased proportionally to the device aspect ratio. Specular reflectivity of less than 3% was observed over the visible spectral range for the 0.35-μm-period nipple-lens arrays. Using two-dimensional rigorous coupled-wave analysis, the latter phenomenon can be ascribed to a gradual index matching mechanism accessed by a high surface-coverage semiconductor nipple array structure.

Single mode stimulated emission from prismlike gallium nitride submicron cavities

The authors report single mode stimulated emission from optical pumping of prismlike gallium nitride (GaN) with a side length of 0.75μm. The cavities were formed by reaction-rate-limited photoetching that preserved the nonpolar {101¯0} or {112¯0} facets. They were characterized by an average quality factor above 103 and an equivalent facet reflectivity exceeding 98%, which allowed field amplification by repeated internal reflections in the transverse plane and field polarization along the c axis. Slight spectral blueshift (0.35nm) and narrowing in linewidth (∼0.4nm) were observed with increase of pump intensity. These observations manifested resonant coupling of the band edge emission to a single mode of the prismlike GaN cavity.

Experimental and theoretical study of polarized photoluminescence caused by anisotropic strain relaxation in nonpolar a-plane textured ZnO grown by a low-pressure chemical vapor deposition

Anisotropic strain relaxation and the resulting degree of polarization of photoluminescence (PL) in nonpolar a-plane textured ZnO are experimentally and theoretically studied. A thicker nonpolar a-plane textured ZnO film enhances the anisotropic in-plane strain relaxation, resulting in a larger degree of polarization of PL and better sample quality. Anisotropic in-plane strains, sample quality, and degree of polarization of PL in nonpolar a-plane ZnO are consequences of the degree of anisotropic in-plane strain relaxation. By the k·p perturbation approach, simulation results of the variation of the degree of polarization for the electronic transition upon anisotropic in-plane strain relaxation agree with experimental results.

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.

Generation of green frequency comb from chirped χ(2) nonlinear photonic crystals

Spectrally broad frequency comb generation over 510–555 nm range was reported on chirped quasi-phase-matching (QPM) χ(2) nonlinear photonic crystals of 12 mm length with periodicity stepwise increased from 5.9 μm to 7.1 μm. When pumped with nanosecond infrared (IR) frequency comb derived from a QPM optical parametric oscillator (OPO) and spanned over 1040 nm to 1090 nm wavelength range, the 520 nm to 545 nm up-converted green spectra were shown to consist of contributions from (a) second-harmonic generation among the signal or the idler modes, and (b) sum-frequency generation (SFG) from the neighboring pairs of the signal or the idler modes. These mechanisms led the up-converted green frequency comb to have the same mode spacing of 450 GHz as that in the IR-OPO pump comb. As the pump was further detuned from the aforementioned near-degeneracy point and moved toward the signal (1020–1040 nm) and the idler (1090–1110 nm) spectral range, the above QPM parametric processes were preserved in the chirped QPM de...

Broad green generation using adiabatically chirped χ (2) nonlinear photonic crystals

Broad green generation from 500 to 565nm was observed on chirped χ(2) nonlinear photonic crystals made of periodically-poled lithium tantalate (PPLT) with single pass up-conversion efficiency ~10%. This was ascribed to simultaneous multi-wavelength SHG and SFG due to nonlinear interaction of (signal, idler) waves in the adiabatically chirped PPLT.

Simultaneous multi-optical parametric oscillations and broad up-conversion on χ (2) nonlinear photonic crystals

Parallel excitation of optical parametric oscillations was observed on PPLT dispersed with tri-QPM structures in the transverse direction to a nano-second pump beam. It led to simultaneous IR generation from 1000 to 1150nm with slope efficiency > 30%. When a nonlinearly-chirped PPLT was subject to such IR pump, it exhibits simultaneous broad green generation from 500 to 560nm with single pass efficiency ∼10%. This was due to broad spectral coverage of QPM-SHG and SFG in the chirp structures.

Bias-controlled coherent acoustic phonon generation in InGaN/GaN multiple-quantum-wells light emitting diodes

We demonstrate the control of coherent acoustic phonon generation by applying voltage across InGaN/GaN multiple-quantum-wells light emitting diodes (LEDs). The acoustic phonons oscillation can be switched off by increasing the reverse bias up to −25V.

High Quality Factor with Fundamental Resonant Mode near the Bandedge of GaN Triangular Submicron Laser Cavity

Optically-pumped, single-mode stimulated emission was observed on GaN triangular submicron-cavity bounded by {10-10} facets. FDTD analysis indicates a high-Q factor (103) resultant from materials dispersion effect near the bandedge.