Xinming Ji

A nano-opto-mechanical pressure sensor via ring resonator

This paper reports a nano-opto-mechanical pressure sensor based on nano-scaled ring resonator. The pressure is measured through the output spectrum shift which is induced via mechanical deformation of the ring resonator. The sensitivity as high as 1.47 pm/kPa has been experimentally achieved which agrees with numerical prediction. Due to the strong variation of sensitivity with different ring radius and thickness of the diaphragm, the pressure sensor can be used to form an array structure to detect the pressure distribution in highly accurate measurement with low-cost advantages. The nano-opto-mechanical pressure sensor has potential applications such as shear stress displacement detection, pressure wave detector and pressure mapping etc.

A MEMS IR thermal source for NDIR gas sensors

A MEMS thermal emitter based on heating thin film resistors has been studied for the MEMS infrared gas sensor as an infrared (IR) source. The IR source, with an effective emitting area of 1.6 times 1.6 mm2, is fabricated using heated platinum thin film resistors deposited on a Si3N4/Si02 membrane. IR radiation power of the emitter is up to 60mW with the effective blackbody temperature ranged from 300 to 850K. Moreover, it has very good dynamic parameters a modulation frequency as high as 100Hz and a response time of 23ms. The properties of the source demonstrate the potential applications to meet the performance, size and low-cost requirements of the mass market

Numerical and experimental investigation of kerf depth effect on high-frequency phased array transducer.

BACKGROUND High-frequency ultrasonic transducer arrays are essential for high resolution imaging in clinical analysis and Non-Destructive Evaluation (NDE). However, the structure design and fabrication of the kerfed ultrasonic array is quite challenging when very high frequency (≥100MHz) is required. OBJECTIVE AND METHOD Here we investigate the effect of kerf depth on the performances of array transducers. A finite element tool, COMSOL, is employed to simulate the properties of acoustic field and to calculate the electrical properties of the arrays, including crosstalk effect and electrical impedance. Furthermore, Inductively Coupled Plasma (ICP) deep etching process is used to etch 36°/Y-cut lithium niobate (LiNbO(3)) crystals and the limitation of etching aspect ratio is studied. Several arrays with different profiles are realized under optimized processes. At last, arrays with a pitch of 25μm and 40μm are fabricated and characterized by a network analyzer. RESULTS Kerf depth plays an important role in the performance of the transducer array. The crosstalk is proportional to kerf depth. When kerf depth is more than 13μm, the array with crosstalk less than -20dB, which is acceptable for the real application, could provide a desired resolution. Compared to beam focusing, kerf depth exhibits more effect on the beam steering/focusing. The lateral pressure distribution is quantitatively summarized for four types of arrays with different kerf depth. The results of half-cut array are similar to those of the full-cut one in both cases of focusing and steering/focusing. The Full-Width-at-Half-Maximum (FWHM) is 55μm for the half-cut array, and is 42μm for the full-cut one. The 5-μm-cut array, suffering from severe undesired lobes, demonstrates similar behaviors with the no-cut one. ICP process is used to etch the 36°/Y-cut LiNbO(3) film. The aspect ratio of etching profile increases with the kerf width decreasing till it stops by forming a V-shaped groove, and the positive tapered profile angle ranges between 62° and 80°. If the mask selectivity does not limit the process in terms of achievable depth, the aspect ratio is limited to values around 1.3. The measurement shows the electrical impedance and crosstalk are consistent with the numerical calculation. CONCLUSION The numerical results indicate that half-cut array is a promising alternative for the fabrication of high-frequency ultrasonic linear arrays. In fact, the minimum pitch that could be obtained is around 25μm, equivalent to a pitch of 1.6λ, with a kerf depth of 16μm under the optimized ICP parameters.

Modelling and simulation of high-frequency (100 MHz) ultrasonic linear arrays based on single crystal LiNbO3

BACKGROUND High-frequency ultrasonic transducer arrays are essential for high resolution imaging in clinical analysis and Non-Destructive Evaluation (NDE). However, the fabrication of conventional backing-layer structure, which requires a pitch (distance between the centers of two adjacent elements) of half wavelength in medium, is really a great challenge. OBJECTIVE AND METHOD Here we present an alternative buffer-layer structure with a silicon lens for volumetric imaging. The requirement for the size of the pitch is less critical for this structure, making it possible to fabricate high-frequency (100MHz) ultrasonic linear array transducers. Using silicon substrate also makes it possible to integrate the arrays with IC (Integrated Circuit). To compare with the conventional backing-layer structure, a finite element tool, COMSOL, is employed to investigate the performances of acoustic beam focusing, the influence of pitch size for the buffer-layer configuration, and to calculate the electrical properties of the arrays, including crosstalk effect and electrical impedance. RESULTS For a 100MHz 10-element array of buffer-layer structure, the ultrasound beam in azimuth plane in water could be electronically focused to obtain a spatial resolution (a half-amplitude width) of 86μm at the focal depth. When decreasing from half wavelength in silicon (42μm) to half wavelength in water (7.5μm), the pitch sizes weakly affect the focal resolution. The lateral spatial resolution is increased by 4.65% when the pitch size decreases from 42μm to 7.5μm. The crosstalk between adjacent elements at the central frequency is, respectively, -95dB, -39.4dB, and -60.5dB for the 10-element buffer, 49-element buffer and 49-element backing arrays. Additionally, the electrical impedance magnitudes for each structure are, respectively, 4kΩ, 26.4kΩ, and 24.2kΩ, which is consistent with calculation results using Krimholtz, Leedom, and Matthaei (KLM) model. CONCLUSION These results show that the buffer-layer configuration is a promising alternative for the fabrication of high-frequency ultrasonic linear arrays dedicated to volumetric imaging.

Transmission enhancement properties of double-layered metallic hole arrays

We report experimental results on enhanced light transmission through double-layered (Ag/Au) metallic hole arrays within a skin-depth. Zero-order transmission spectrums are characterized as a function of Ag films thickness, which extends from \delta/15, \delta/6 to approximately \delta, where \delta is a skin-depth. In contrast with other reported results (Refs.[11-13]) in single-layered metallic hole arrays, our experimental results show much more dramatic properties of transmission process dependent on sub- thickness. It is shown that there is no negligible transmission enhancement at \delta/16. At \delta/6, much higher transmission efficiency can be achieved. With films thickness being close to \delta, the transmission efficiency declines contrarily. Simultaneously, the corresponding resonant peak also slightly moves toward the shorter wavelength. It is proposed that the coupling of surface plasmon polaritons (SPPs) at Ag/Au interface within is involved in the process.

Narrow-band midinfrared thermal emitter based on photonic crystal for NDIR gas sensor

A high performance plasmonic thermal emitter with narrow bandwidth emission property were proposed and fabricated. This emitter is based on a metal coated two-dimensional (2D) photonic crystal of air holes in a silicon substrate. The PC structure is found to be highly effective in suppressing the thermal emission, which can yield 2 orders of magnitude more efficient that conventional IR sources. Thermally isolated, uniformly heated emitters, with photonic crystal structure modified surfaces are exploited for narrow-band spectral tuning in the infrared wavelength regime. The results show that narrow band spectral tuning is accomplished through control of the lattice constant of the PC patterns.

A self-aligned mask-free fabrication process for high-frequency ZnO array transducer

High-frequency ultrasonic array transducers are essential for high resolution imaging in clinical analysis and Non Destructive Evaluation (NDE). However, the fabrication of piezoelectric array transducers is a great challenge due to the small features in elaborating piezoelectric array films. This paper describes a MEMS based self-aligned mask-free process for fabrication of ZnO linear array transducers of more than 100MHz. A four-step-rotation deposition approach is proposed and investigated, that improves the lateral growth in ZnO array deposition. The ratio of vertical to lateral growth is improved by 40% compared to one-step deposition method. The results prove that the reduction of the lateral growth helps to achieve full-kerfed ZnO array with smaller pitch.

Nano-opto-mechanical actuator driven by optical radiation force

This paper presents a Nano-opto-mechanical actuator, which is driven by optical radiation force. The actuator consists of two waveguides, two identical ring resonators, and an actuator with Bragg reflector. Light is injected into the waveguides and coupled to the Bragg reflector via the ring resonator. The actuator is displaced by the optical force. The achieved maximum displacement of the actuator is 500.2 nm with the optical power up to 200 mW. The optical actuator has merits of high resolution (2.501 nm/mW), approximately perfect linear displacement and contact-free optical drive, which results in potential applications such as precise distance control, tunable laser, and weak force detection.

A Nano-opto-mechanical pressure sensor

A Nano-opto-mechanical pressure sensor using optical force is reported in this paper. The pressure sensor consists of a square diaphragm, a ring resonator and four waveguides. By applying a pressure ranging from 900 kPa to 990 kPa, the output intensity ratio sensitivity of −2.272 /kPa is achieved. Compared with traditional MEMS pressure sensor, the proposed ring pressure sensor has advantages such as higher sensitivity and resolution, which could be applied to acoustic pressure sensors and microphones etc.

Photonic crystal biosensors based on surface plasmons

Surface plasmon photonic crystals can couple the incident light with surface plasmons by the periodic array of holes on the metal film. In this paper, surface plasmon photonic crystals with periodic square array of hexagon holes were fabricated. The infrared optical properties in transmittance and reflectance of the devices were tested. A sharp peak of surface plasmon resonance was observed at the incident angles of 30° and 90°. The optical wavenumber shift at 30° incident angle was used to detect the reaction of antigen and antibody, and the line correlation between protein concentrations and wavenumber shifts were investigated. By the comparison of the results between 4 ¿m and 8 ¿m periodic arrays, the 4 ¿m periodic structure has better performance. Based on the results, the surface plasmon photonic crystals can be used as biosensors.