Seng Tiong Ho

Ultraviolet lasing in resonators formed by scattering in semiconductor polycrystalline films

A semiconductor laser whose cavities are “self-formed” due to strong optical scattering in highly disordered gain media is demonstrated. The lasers are made of zinc oxide polycrystalline films grown on amorphous fused silica substrates. Lasing occurs at an ultraviolet wavelength of ∼380 nm under optical pumping. Actual images of the microscopic laser cavities formed by multiple scattering have been captured. These results suggest the possibility of using disordered semiconductor microstructures as alternative sources of coherent light emission.

FDTD microcavity simulations: design and experimental realization of waveguide-coupled single-mode ring and whispering-gallery-mode disk resonators

We investigate the properties of high-Q, wide free-spectral-range semiconductor microcavity ring and disk resonators coupled to submicron-width waveguides. Key optical design parameters are characterized using finite-difference time-domain (FDTD) solutions of the full-wave Maxwells equations. We report coupling efficiencies and resonant frequencies that include the effects of waveguide dispersion and bending and scattering losses. For diameters of 5 /spl mu/m, the microcavity resonators can have Qs in the several thousands and a free spectral range of 6 THz (50 nm) in the 1.55 /spl mu/m, wavelength range. Studies of the transmittance characteristics illustrate the transition from single-mode resonances to whispering-gallery-mode resonances as the waveguide width of the microring is increased to form a solid microdisk. We present nanofabrication results and experimentally measured transmission resonances of AlGaAs/GaAs microcavity resonators designed in part with this method.

Waveguide-coupled AlGaAs/GaAs microcavity ring and disk resonators with high finesse and 21.6-nm free spectral range

We report the realization and demonstration of novel semiconductor waveguide-coupled microcavity ring and disk resonators. For a 10.5-microm-diameter disk resonator, we measure a finesse of 120, a resonant linewidth of 0.18 nm, and a free-spectral range of 21.6 nm in the 1.55-mum-wavelength region. We present the nanofabrication methods and the experimental results for 10.5- and 20.5-mum-diameter ring and disk resonators to show the feasibility of such devices.

Thin-film channel waveguide electro-optic modulator in epitaxial BaTiO3

We report on a thin-film channel waveguide electro-optic modulator fabricated in epitaxial BaTiO3 on MgO. Films had an effective dc electro-optic coefficient of reff∼50±5 pm/V and reff∼18±2 pm/V at 5 MHz for λ∼1.55 μm light. Extinction ratios of 14 dB were obtained. The electro-optic effect decreases to ∼60% of the dc value at 1 Hz, 50% of the dc value at 20 kHz, and ∼37% of the dc value at 5 MHz.

THIN FILM CHANNEL WAVEGUIDES FABRICATED IN METALORGANIC CHEMICAL VAPOR DEPOSITION GROWN BATIO3 ON MGO

We report on the fabrication of channel waveguides in epitaxial grown BaTiO3 layers on MgO. Layers were prepared by metalorganic chemical vapor deposition. Ridge waveguides with ridge heights ranging from 15 to 200 nm were fabricated in a 0.2‐μm‐thick film. Single mode waveguide throughput, scattering loss, and mode profiles are reported. Coating waveguides with spin on glass significantly increase waveguide throughput. Throughputs of up to 10.4% were measured in 15 nm ridge waveguides which were 2.85 mm long and coated with spin on glass. Waveguide throughput is found to increase significantly with an increase in wavelength from 1.06 to 1.55 μm.

Effect of a microstructure on the formation of self-assembled laser cavities in polycrystalline ZnO

The optical properties of polycrystalline ZnO have been studied to elucidate the occurrence of random laser action. The spatially-resolved refractive index has been mapped out by using the scanning electron energy loss spectroscopy across the grain boundary. It is observed that the refractive index decreases gradually when the probe beam is approaching to the grain boundary. A thin reflective layer of ∼10 nm is found to form in the vicinity of the grain boundary, which assists the optical scattering. The photon scattering factor of the reflective layer has been determined and is shown to correlate well with the results of the coherent backscattering method. Together with the cathodoluminescence studies, it is suggested that the overall structure, which includes the grain and grain boundary, determines the laser action in ZnO.

GaAs microcavity channel-dropping filter based on a race-track resonator

We have demonstrated add-drop filters using racetrack-shaped resonators coupled to straight waveguides across gaps which are larger compared with the conventional microcavity ring resonators. The finesse and the maximum transmission are characterized and are shown to be determined uniquely by the round-trip loss and the coupling factor of the resonator.

Polymer waveguides useful over a very wide wavelength range from the ultraviolet to infrared

We have designed and fabricated polymer waveguides using the glassy polymers Cytop™ (a fluorinated polyether), PMMA C6 [poly(methyl methacrylate)], and Cyclotene™ 3022-35 (bisbenzocyclobutane). Since these materials exhibit excellent transparency over a wide wavelength range, and since the refractive index difference of Cytop™ and Cyclotene™ or Cytop™ and PMMA is greater than 0.19, both Cytop™/Cyclotene™/Cytop™ and Cytop™/PMMA/Cytop™ waveguide structures can be employed over a very wide wavelength range from the ultraviolet to the infrared. Efficient waveguiding is achieved for different light sources with 390, 633, 1064, 1310, and 1550 nm wavelengths.

Propagation loss measurements in semiconductor microcavity ring and disk resonators

We report the measurement of cavity propagation losses in nearly single-mode semiconductor waveguide-coupled ring and disk microcavity optical resonators. Using a novel 10.5-/spl mu/m-diameter ring resonator, we measure transverse electric (TE) and transverse magnetic (TM) field intensity losses in 0.35-/spl mu/m-wide ring waveguide cavities in the 1.55-/spl mu/m-wavelength region. We present the experimental results for nanofabricated AlGaAs-GaAs 10.5-/spl mu/m-diameter ring and disk resonators to quantify cavity losses and to show the feasibility of these promising and robust submicron-scale devices.

Growth of InxGa1−xAs/GaAs heterostructures using Bi as a surfactant

The effects of a bismuth surfactant layer on the molecular beam epitaxy of GaAs and InxGa1−xAs layers on GaAs (001) were studied. The InxGa1−xAs surface reconstruction changed from arsenic stabilized 2×4 to bismuth stabilized 1×3 for high enough bismuth fluxes and low enough substrate temperatures. Maintaining a bismuth stabilized surface during InxGa1−xAs growth resulted in a larger number of reflection high-energy electron diffraction (RHEED) oscillations. RHEED patterns were also streakier after InxGa1−xAs growth with Bi. Roughness measurements using atomic force microscopy showed reduced root mean square roughness with Bi, e.g., from 3.8 to 2.8 nm, for 4 nm thick In0.3Ga0.7As layers. Simulations of x-ray diffraction results from 10 period In0.5Ga0.5As/GaAs superlattices showed that Bi reduced interface roughness from 1.1 to 0.5 nm and reduced interfacial broadening from 2.8 to 2.1 nm. The latter was attributed to reduced In segregation. InxGa1−xAs/GaAs (x=0.2–0.4) multiple quantum wells grown with Bi ...