Photonic miniaturization requires seamless integration of linear and nonlinear optical components to achieve passive and active functions simultaneously. Among the available material systems, silicon photonics holds immense promise for optical signal processing and on-chip optical networks. However, silicon is limited to wavelengths above 1.1 mu m and does not provide the desired lowest order optical nonlinearity for active signal processing. Here we report the integration of aluminum nitride (AlN) films on silicon substrates to bring active functionalities to chip-scale photonics. Using CMOS-compatible sputtered thin films we fabricate AlN-on-insulator waveguides that exhibit low propagation loss (0.6 dB/cm). Exploiting AlN's inherent Pockels effect we demonstrate electro-optic modulation up to 4.5 Gb/s with very low energy consumption (down to 10 fJ/bit). The ultrawide transparency window of AlN devices also enables high speed modulation at visible wavelengths. Our low cost, wideband, carrier-free photonic circuits hold promise for ultralow power and high-speed signal processing at the microprocessor chip level.
We demonstrate a new optomechanical device system which allows highly efficient transduction of femtogram nanobeam resonators. Doubly clamped nanomechanical resonators with mass as small as 25 fg are embedded in a high-finesse two-dimensional photonic crystal nanocavity. Optical transduction of the fundamental flexural mode around 1 GHz was performed at room temperature and ambient conditions, with an observed displacement sensitivity of 0.94 fm/Hz(1/2). Comparison of measurements from symmetric and asymmetric double-beam devices reveals hybridization of the mechanical modes where the structural symmetry is shown to be the key to obtain a high mechanical quality factor. Our novel configuration opens the way for a new category of "NEMS-in-cavity" devices based on optomechanical interaction at the nanoscale.
Jung, Hojoong
Guo, Xiang
Zhu, Na
Papp, Scott B.
Diddams, Scott A.
Tang, Hong X.
Nonlinear optical conversion with frequency combs is important for self-referencing and for generating shorter wavelength combs. Here we demonstrate efficient frequency comb doubling through the combination of second-harmonic generation (SHG) and sum-frequency generation (SFG) of an input comb with a high Q, phase-matched chi((2)) microring resonator. Phase coherence of the SHG and SFG nonlinear conversion processes is confirmed by sinusoidal phase-dependent interference between frequency doubled comb lines. (C) 2016 Optical Society of America
Bhaskaran, Harish
Li, Mo
Garcia-Sanchez, Daniel
Zhao, Peng
Takeuchi, Ichiro
Tang, Hong X.
We report the piezoresistivity in magnetic thin films of Fe(0.7)Ga(0.3) and their use for fabricating self-transducing microcantilevers. The actuation occurs as a consequence of both the ferromagnetic and magnetostrictive properties of Fe(0.7)Ga(0.3) thin films, while the deflection readout is achieved by exploiting the piezoresistivity of these films. This self-sensing self-actuating micromechanical system involves a very simple bilayer structure, which eliminates the need for the more complex piezoelectric stack that is commonly used in active cantilevers. Thus, it potentially opens opportunities for remotely actuated cantilever-based sensors. (c) 2011 American Institute of Physics. [doi:10.1063/1.3533390]
Henry, Tania
Kim, Kyungkon
Ren, Zaiyuan
Yerino, Christopher
Han, Jung
Tang, Hong X.
We report the growth of horizontally aligned arrays and networks of GaN nanowires (NWs) as resonant components in nanoelectromechanical systems (NEMS). A combination of top-down selective area growth (SAG) and bottom-up vapor-liquid-solid (VLS) synthesis enables flexible fabrication of highly ordered nanowire arrays in situ with no postgrowth dispersion. Mechanical resonance of free-standing nanowires are measured, with quality factors (Q) ranging from 400 to 1000. We obtained a Young's modulus (E) of similar to 338 GPa from an array of NWs with varying diameters and lengths. The measurement allows detection of nanowire motion with a rotating frame and reveals dual fundamental resonant modes in two orthogonal planes. A universal ratio between the resonant frequencies of these two fundamental modes, irrespective of their dimensions, is observed and attributed to an isosceles cross section of GaN NWs.
Surya, Joshua B.
Guo, Xiang
Zou, Chang-Ling
Tang, Hong X.
Nonlinear optical effects have been studied extensively in microresonators as more photonics applications transitions to integrated on-chip platforms. Due to low optical losses and small mode volumes, microresonators are demonstrably the state-of-the-art platform for second-harmonic generation (SHG). However, the working bandwidth of such microresonator-based devices is relatively small, presenting a challenge for applications where a specifically targeted wavelength needs to be addressed. In this Letter, we analyze the phase-matching window and resonance wavelength with respect to varying microring widths, radii, and temperatures. A chip with precise design parameters was fabricated with phase matching realized at the exact wavelength of a twophoton transition of Rb-85. This procedure can be generalized to any target pump wavelength in the telecom band with picometer precision. (C) 2018 Optical Society of America
A new species of Hemiboea, H. roseoalba, from Guangdong, China, is described and illustrated. It is similar to H. gracilis and H. angustifolia in leaf and flower shape, but can be distinguished by leaf blade with serrulate from the middle to the apex, lateral veins 7-9 on each side of midrib; peduncle 2.5-3.0 cm long, glabrous; cymes 4-6-flowered; triangular calyx base connate for c. 0.3 cm; pink corolla 4.0-4.3 cm long and margin of lobes curled or retroflexed; staminodes 3; pistil 3.7-4.0 cm long; disc 2 mm high and truncate stigma slightly swollen.
Differences in ozone (O 3), nitrogen oxides (NO x), and respirable suspended particulate (RSP) weekday and weekend concentrations in Hong Kong were investigated. Data were collected hourly by the Hong Kong Environmental Protection Department from January 1999 to October 2007 at 14 sites: 9 urban, 4 newly developed, and 1 rural station. Weekend O 3 concentrations were 14% and 22% higher for the afternoon peak O 3 time and 8-h O 3 average concentrations, respectively, at six urban stations, whereas NO 2 and NO x concentrations were 19% and 25% lower, respectively, during the afternoon O 3 peak time on weekends. No significant differences in peak afternoon O 3 concentrations were found at the newly developed or rural stations. Weekend RSP concentrations were lower at most urban stations. Urban RSP concentrations were lower on weekends than weekdays, by 14% for the morning rush hour and 8% for the daily average RSP. O 3 formation sensitivity to volatile organic compounds and lower weekend NO x emissions could explain the O 3 weekend effect in urban Hong Kong. Lower weekend RSP concentrations are predominantly the result of fewer anthropogenic emissions, primarily due to decreased traffic flow.
Leilei Fei
L.Y. Chan
Xinhui Bi
Hai Guo
Yonglin Liu
Qinhao Lin
Xinming Wang
Ping'an Peng
Guoying Sheng
Abstract Cloud-to-ground (CG) lightning, meteorological conditions and corresponding surface nitrogen oxides (NO x ) and ozone (O 3 ) variations in relation to thunderstorm and lightning activities over Hong Kong at Kwai Chung (urban), Tung Chung (new town) and Tap Mun (background) during active lightning seasons from 2009 to 2013 were studied by analyzing respective air quality monitoring station data along with CG lightning and meteorological data. We observed NO x enhancement and significant O 3 decline on lightning days. Influences of land use types, lightning activities and meteorological conditions on surface NO x and O 3 were examined. NO x and O 3 concentrations shifted towards higher and lower levels, respectively, during lightning days especially in the dominant wind directions. Principal component analysis/absolute principal component scores (PCA/APCS) method and stepwise multiple linear regression (MLR) analysis were employed to examine the influence of thunderstorm related lightning and meteorological parameters on surface NO x and O 3 . Wind speed was supposed to be the most important meteorological parameter affecting the concentration of NO x , and lightning activities were observed to make a positive contribution to NO x . Negative contribution of hot, cloudy and wet weather and positive contribution of wind speed were found to affect the concentration of O 3 . Lightning parameters were also found to make a small positive contribution to O 3 concentration at Tap Mun and Tung Chung, but the net effect of lightning activities and corresponding meteorological conditions was the decrease of O 3 on lightning days. Reasonably good agreement between the predicted and observed NO x and O 3 values indicates that PCA/APCS-MLR is a valuable method to study the thunderstorm induced NO x and O 3 variations. Highlights • Surface NO x enhancement and O 3 decline were observed on lightning days in Hong Kong. • Effects of land use types, lightning and meteorological conditions were examined. • PCA/APCS-MLR was applied to the study of thunderstorm induced NO x and O 3 variations.