Lung-Han Peng

Synthesis and measurement of ultrafast waveforms from five discrete optical harmonics.

Optical fields have been manipulated like radio frequencies into sawtooth and square-wave patterns. Achieving the control of light fields in a manner similar in sophistication to the control of electromagnetic fields in the microwave and radiofrequency regimes has been a major challenge in optical physics research. We manipulated the phase and amplitude of five discrete harmonics spanning the blue to mid-infrared frequencies to produce instantaneous optical fields in the shape of square, sawtooth, and subcycle sine and cosine pulses at a repetition rate of 125 terahertz. Furthermore, we developed an all-optical shaper-assisted linear cross-correlation technique to retrieve these fields and thereby verified their shapes and confirmed the critical role of carrier-envelope phase in Fourier synthesis of optical waveforms.

GaN nanorod light emitting diode arrays with a nearly constant electroluminescent peak wavelength

A practical process to fabricate InGaN/GaN multiple quantum well light emitting diodes (LEDs) with a self-organized nanorod structure is demonstrated. The nanorod array is realized by using nature lithography of surface patterned silica spheres followed by dry etching. A layer of spin-on-glass (SOG), which intervening the rod spacing, serves the purpose of electric isolation to each of the parallel nanorod LED units. The electroluminescence peak wavelengths of the nanorod LEDs nearly remain as constant for an injection current level between 25mA and 100mA, which indicates that the quantum confined stark effect is suppressed in the nanorod devices. Furthermore, from the Raman light scattering analysis we identify a strain relaxation mechanism for lattice mismatch layers in the nanostructure.

Characterizations of Amorphous IGZO Thin-Film Transistors With Low Subthreshold Swing

Subthreshold swing (SS) is a key parameter in evaluating the power consumption and material properties of thin-film transistors (TFTs). In this letter, we report an amorphous indium gallium zinc oxide (a-IGZO) TFT with a high-κ SiO₂/HfO₂ gate insulator. The device shows a SS of 96 mV/decade and an on-to-off current ratio of 1.5 × 10¹⁰. The low SS was attributed to the fully depleted channel state, low interface defects, and efficient modulation of the device. With low defect states, the device demonstrates only 2.71% change of operating currents after 1.5 × 10⁴ s stress.

Enhanced light collection of GaN light emitting devices by redirecting the lateral emission using nanorod reflectors

We demonstrate a method of utilizing self-assembled nanorod array reflectors to collect the laterally propagating guided modes from a light emitting diode (LED). We measure an enhancement factor of 12.2% and 18.4%, respectively, from the sidewall emission of GaN-based LEDs encompassed with 10 and 20 microm thick nanorod array reflectors. Such enhancement is found to be omnidirectional due to a broken symmetry from a randomized distribution of the nanorod array placed along the periphery of the LEDs mesa. These observations indicate that the use of nanorod reflectors can efficiently redirect the propagation of the laterally guided modes to the surface normal direction.

Speckle noise reduction on a laser projection display via a broadband green light source

A broadband green light source was demonstrated using a tandem-poled lithium niobate (TPLN) crystal. The measured wavelength and temperature bandwidth were 6.5 nm and 100 °C, respectively, spectral bandwidth was 36 times broader than the periodically poled case. Although the conversion efficiency was smaller than in the periodic case, the TPLN device had a good figure of merit owing to the extremely large bandwidth for wavelength and temperature. The developed broadband green light source exhibited speckle noise approximately one-seventh of that in the conventional approach for a laser projection display.

Short channel effects on gallium nitride/gallium oxide nanowire transistors

Gallium nitride/gallium oxide GaN/Ga2O3 nanowire metal-oxide-semiconductor field-effect-transistors are shown to operate at an average electron velocity of ∼1.24u2009×u2009107u2009cm/s and threshold-voltage roll-off of −0.2u2009V as the transistor gate length Lg reduced from 500 to 50u2009nm. Improvement of saturation current to 120u2009μA and unity current/power-gain cut-off frequency to 150/180 GHz is observed on Lgu2009=u200950u2009nm devices. Our study reveals the advantages of using (i) polarization-induced positive charges and high-k dielectric at the {11¯01¯}GaN/{002}Ga2O3 interface to provide carrier confinement and to shield the drain field, and (ii) polarization-induced negative charges at the (0001)GaN/sapphire interface to form a back-barrier to suppress leakage and improve the short-channel transport properties.

Broad multiwavelength second-harmonic generation from two-dimensional /spl chi//sup (2)/ nonlinear photonic crystals of tetragonal lattice structure

We investigate the effects of lattice spacing and domain-filling ratio on the wavelength dependence in second-harmonic generation (SHG) from two-dimensional /spl chi//sup (2)/ nonlinear photonic crystals (2-D NPC) on lithium niobate. Spectral coverage of 150 and 200 nm around 1.5 and 1.9 /spl mu/m in quasi-phase-matched (QPM) SHG is obtained by angular rotation of the NPC with a lattice periodicity of 20/spl times/20 and 29.5/spl times/29.5 /spl mu/m/sup 2/, respectively. The SHG signals are shown to have off-axial propagation directions determined by the incident wavelength and the QPM vector. The intensity of the high-order reciprocal-lattice-vector (G/sub mn/)-assisted SHG signal is shown to depend on the domain-filling ratio. These observations are attributed to the unique dispersion of G/sub mn/ together with the nonzero /spl chi//sup (2)/(G/sub mn/) nonlinearity in the 2-D QPM-NPC having a tetragonal lattice structure.

Experimental study of multiwavelength parametric generation in a two-dimensional periodically poled lithium tantalate crystal

In this Letter, we experimentally investigate multiwavelength parametric generation in two-dimensional second-order nonlinear photonic crystals. For this purpose, a 2D periodically poled lithium tantalate crystal with rectangular lattice was fabricated and characterized. We demonstrate multiple and simultaneous wavelength generation due to the contribution of different lattice vectors. Numerical simulations emphasize the agreement of our phase matching scheme with the experimental results and made it possible to assign the observed wavelengths to the reciprocal lattice vectors involved in the parametric generation process. Moreover, our results indicate that some signals are the result of the joint contribution of more than one lattice vector.

High mobility thin film transistors with indium oxide/gallium oxide bi-layer structures

We investigate the transport properties of thin-film transistors using indium oxide (In2O3)/gallium oxide (Ga2O3) bi-layer stacks as the channel material. At low gate bias, we observe the transistor field-effect mobility increases with the film resistivity to μFEu2009=u200951.3 cm2/Vs and ON/OFF current ratio to 108 due to combinatorial layer thickness modulation. With the Ga2O3 layer thickness ratio increase to Ru2009=u200914.35%, these observations are accompanied with one-order-of-magnitude reduction in the transistor subthreshold swing to 0.38 V/decade and suggest a trap-limited conduction mechanism upon which the reduced scattering centers due to annihilation of subgap states improve the device electric characteristics without post-growth annealing.

Scaling performance of Ga2O3/GaN nanowire field effect transistor

A three-dimensional finite element solver is applied to investigate the performance of Ga2O3/GaN nanowire transistors. Experimental nanowire results of 50u2009nm gate length are provided to compare with the simulation, and they show good agreement. The performance of a shorter gate length (<50u2009nm) is studied and scaling issues of the short-channel effect are analyzed. With a better surrounding gate design and a recessed gate approach, the optimal conditions for a 20u2009nm gate length are explored in this paper.