Yuan-Yao Lin

GaN nanowire ultraviolet photodetector with a graphene transparent contact

We report on the fabrication of graphene contact to GaN nanowire ensemble and on the demonstration of photodetectors using chemical vapor deposition-grown few-layered graphene as a transparent electrode. The optimization of the transfer method allowed to form a continuous contact to the nanowires over a large area. The adhesion energy of the graphene sheet to the nanowire ensemble is estimated to be 0.3–0.7 J/m2. Ultraviolet photodetectors with a room-temperature responsivity of ∼25 A/W at 357 nm were fabricated. The photocurrent spectrum shows that the device has a strong response up to 4.15 eV confirming a good transparency of the top graphene contact.

Above room-temperature ferromagnetism of Mn delta-doped GaN nanorods

One-dimensional nitride based diluted magnetic semiconductors were grown by plasma-assisted molecular beam epitaxy. Delta-doping technique was adopted to dope GaN nanorods with Mn. The structural and magnetic properties were investigated. The GaMnN nanorods with a single crystalline structure and with Ga sites substituted by Mn atoms were verified by high-resolution x-ray diffraction and Raman scattering, respectively. Secondary phases were not observed by high-resolution x-ray diffraction and high-resolution transmission electron microscopy. In addition, the magnetic hysteresis curves show that the Mn delta-doped GaN nanorods are ferromagnetic above room temperature. The magnetization with magnetic field perpendicular to GaN c-axis saturates easier than the one with field parallel to GaN c-axis.

Carrier recombination dynamics in Si doped InN thin films

Time-integrated and time-resolved photoluminescence (PL) of InN thin films of different background carrier concentrations are investigated. The PL formation mechanism is attributed to the “free-to-bound” transition by analyzing the time-integrated PL spectra at different pump fluences. The dependence of the PL decay time with emission energy is investigated using a theoretical model which speculates upon the carrier localization in InN thin films. The radiative lifetime, mobility edge, and carrier localization energy are obtained from the dependence of the PL decay time on emission energy and are studied at different background carrier concentrations. The effect of intervalley scattering between the Γ1 and Γ3 valley on the radiative lifetime, mobility edge, and carrier localization energy is discussed. The longer radiative lifetime and smaller values of the mobility edge and localization energy for 3.06 eV excitation are observed than that for the 1.53 eV excitation due to the intervalley scattering process.

Photogated transistor of III-nitride nanorods

A III-nitride-based photogated transistor using photons to control the channel width of an otherwise gateless field effect transistor (FET) is investigated. This is accomplished by stacking sequential layers of p-GaN/InGaN/n-GaN on a Si substrate in an array of nanorods. The nitride p-i-n diode can be activated by light, whereupon the nanorod device shows phototransistor characteristics in forward bias but behaves like a photoconductor when in reverse bias. An optically pumped FET model, as justified by the low-dimensional nanogeometry, is used in analysis of the device. The resulting photogate efficiency and photocarrier mobilities are estimated to be ∼0.04 V/(W/cm2) and, ∼2000–3000 cm2/V s, respectively.

Four-wave-mixing in the loss low submicrometer Ta₂O₅ channel waveguide.

A degenerate four-wave-mixing (FWM) operation in the Ta2O5 submicrometer channel waveguide has been successfully demonstrated. The propagation loss of 1.5  dB/cm and total insertion loss of 5.1 dB are realized in a 12.6 mm long waveguide with inverse taper structure. The wavelength and quadratic pumping power-dependent measurements on optical transmission confirm FWM performance and characterize the nonlinearity of waveguide. The conversion efficiency of -50  dB at coupled pump power of 40 mW is observed, suggesting that the nonlinear refractive index of Ta2O5 waveguide at 1550 nm is estimated to be 1×10(-14)  cm2/W. Our primary results indicate that the Ta2O5 submicrometer channel waveguide has great potential in developing nonlinear waveguide applications.

Low-loss and high-Q Ta(2)O(5) based micro-ring resonator with inverse taper structure.

A low-loss and high-Q Ta(2)O(5) based micro-ring resonator is presented. The micro-ring resonator and channel waveguide with core area of the 700 by 400 nm(2) were fabricated on amorphous Ta(2)O(5) thin films prepared by reactive sputtering at 300°C and post annealing at 650°C for 3 hours. The Ta(2)O(5) micro-ring resonator with a diameter of 200 μm was coupled to the channel waveguide with a coupled Q up to 38,000 at a 0.9 μm coupling gap. By fitting the transmission spectrum of the resonator, the extracted loss coefficient inside the ring cavity and transmission coefficient of TE mode were 8.1dB/cm and 0.9923, leading to the estimated unloaded Q of higher than 44,000. In addition, based on the cut-back method, the propagation loss and the coupling loss of Ta(2)O(5) channel waveguide with an inverse taper were 1.5dB/cm and 3.2 dB, respectively. The proposed Ta(2)O(5) technology offers an unique alternative for fabricating high performance guided wave devices, and may well lead to novel applications in photonic integrated circuits.

p-GaN/InGaN/n-GaN pedestal nanorods: Effect of postgrowth annealing on the electrical performance

Pedestal p-GaN/InGaN/n-GaN nanorods have been fabricated on n-type Si (111) substrates by properly reducing the growth temperature of the p-GaN surface layer. Continuous p-GaN layers were formed on the top region by accelerated lateral growth, while keeping the underlying nanostructures and physical properties of InGaN and n-GaN intact, making it feasible for large-scale vertical integration. Growth of the p-GaN layer at 500 °C followed by annealing at 600–800 °C improved crystal structures and the overall electrical and luminescence properties of pedestal nanorods.

High-Pulse-Energy Topological Insulator Bi 2 Te 3 -Based Passive Q-Switched Solid-State Laser

Due to the ultralow saturation properties of the topological insulator (TI), stable and high-power TI-based solid state has become an important issue. In this paper, using a folding V-shaped laser cavity to minimize the residual pump to the broadband saturable absorber, passive Q-switched operations with high power not only at 1.06 μm but at 1.34 μm as well were demonstrated. The 1.06-μm laser of 150 mW and the corresponding repetition rate and a pulse duration of 28.57 kHz and 576 ns, respectively, were performed. The pulse energy of 5.24 μJ and the corresponding peak power of over 9 W were also estimated. Additionally, the laser at 1.3-μm exhibited a passive Q-switched operation with an average power of as high as 326 mW and a pulse duration of 673 ns. A tunable repetition rate of 75.5 to 116.6 kHz, a pulse energy of 2.8 μJ, and the corresponding peak power of 4.2 W were obtained. In addition, the stability of Q-switched pulses was analyzed. The results experimentally extend the promising application of the 2-D material, i.e., exfoliated TI Bi2Te3, in solid-state lasers.

Chirped-pulse manipulated carrier dynamics in low-temperature molecular-beam-epitaxy grown GaAs

Chirped pulse controlled carrier dynamics in low-temperature molecular-beam-epitaxy grown GaAs are investigated by degenerate pump-probe technique. Varying the chirped condition of excited pulse from negative to positive increases the carrier relaxation time so as to modify the dispersion and reshape current pulse in time domain. The spectral dependence of carrier dynamics is analytically derived and explained by Shockley-Read Hall model. This observation enables the new feasibility of controlling carrier dynamics in ultrafast optical devices via the chirped pulse excitations.

Self-phase modulation in highly confined submicron Ta 2 O 5 channel waveguides

Optical spectra broadening as a result self-phase modulation in a channel waveguide fabricated on a high quality tantalum pentoxide (Ta₂O₅) film by using RF sputtering is measured. The full-width at half maximum of the optical spectra for transverse electric (TE)/transverse magnetic (TM) polarizations of 42.5/31.7 nm is obtained using pulses of 10 nm at a wavelength of 800 nm with a peak-coupled power of 43.77 W. The nonlinear Kerr coefficients of 2.14 × 10^-14 cm²/W and 1.92 × 10^-14 cm²/W for TE and TM polarizations, respectively, are then extracted from the experiments using a theoretical model based on the method of moments. The obtained results on the nonlinearity further suggest that Ta₂O₅ is a promising material to develop nonlinear waveguide devices for integrated photonics.