J. F. Tao

Polarization dependent state to polarization independent state change in THz metamaterials

We experimentally demonstrated a polarization dependent state to polarization independent state change in terahertz (THz) metamaterials. This is accomplished by reconfiguring the lattice structure of metamaterials from 2-fold to 4-fold rotational symmetry by using micromachined actuators. In experiment, it measures resonance frequency shift of 25.8% and 12.1% for TE and TM polarized incidence, respectively. Furthermore, single-band to dual-band switching is also demonstrated. Compared with the previous reported tunable metamaterials, lattice reconfiguration promises not only large tuning range but also changing of polarization dependent states, which can be used in photonic devices such as sensors, optical switches, and filters.

A nanomachined optical logic gate driven by gradient optical force.

In this letter, a nanomachined optical logic gate using optical gradient force is demonstrated. The device consists of a partially free-hanging silicon double-ring resonator developed by the nano-electro-mechanical system technology. The logic NOR gate function is demonstrated at 20u2009Mb/s with a high extinction ratio of about 21.3u2009dB. This proposed NOR gate has the advantages of low power consumption (∼0.5 mW), highly compacted size (40u2009μmu2009×u200945u2009μm), and easy batch fabrication which has potential applications in silicon-photonic integration for digital signal processing.

A MEMS tunable metamaterial filter

This paper presents a terahertz tunable metamaterial filter using microelectromechanical systems (MEMS) structures. The metamaterial unit cells are tuned by micromachined comb-drive. The metamaterial slab works as a notch filter of THz region, which can lower the transmission of interest frequency down to −70 dB. In the experiment, it measures the tuning range of filter frequency from 3.32 to 3.80 THz. The tunable metamaterial filter has better tunability compared with traditional active metamaterial because the optical property of metamaterial is more sensitive to the change of the unit cell structure.

Ultra-high coupling efficiency of MEMS tunable laser via 3-dimensional micro-optical coupling system

This paper reports a 3-dimensional (3D) micro-optical coupling system for improving coupling efficiency in the Littrow configured micro-electro-mechanical system (MEMS) tunable lasers. In the coupling system, an optical fiber acts as a rod lens for light convergence in the vertical plane, while a deep-etched silicon parabolic mirror confines the light in the horizontal plane. Compared with previous MEMS lasers without any light focusing or only one-directional focusing mechanism, the proposed 3D micro-optical system allows longer external cavity length and provides higher coupling efficiency. A prototype is fabricated on a SOI-wafer with an etching depth of 100 µm. The laser obtains a coupling efficiency as high as 76.5%, which is much higher than typical value of 3% – 50% in previous designs. The laser has dimensions as small as 3 mm × 3.2 mm in single-chip integration. It achieves large tuning range of 48.3 nm within 1 ms tuning speed.

Optical wavelength signal detector via tunable micro-ring resonator for sensor applications

This paper reports a compact optical wavelength tracker, which consists of an electrically controlled tunable micro-ring resonator and a flip-chip bonded photodiode. The input optical wavelength is measured and the wavelength change is tracked by analyzing the photocurrent variation of the photodiode. This wavelength tracker has compact size (0.5 mm × 1.5 mm), high resolution (3 pm), cost-effective and can be easily integrated with other devices on a single chip. It offers a simple chip-level optical wavelength tracking and detection approach for low-cost batch production, which has potential applications in optical sensors and fiber communications.

On-chip optical power measurement by optical force

This paper reports a silicon-based micromachined optical power detector with on-chip measurement ability. The optical power is detected by an integrated electron-tunneling displacement transducer, in which optical force is employed as the bridge between optical energy and mechanical energy transition. Compared with the traditional optical power detectors, which are based on photo absorption, the proposed optical power detector has advantages of small size (0.08 mm × 0.3 mm), low thermal noise (0.03 V/°C), large measurement range (> 20 mW) and wavelength independence. Therefore, it has potential applications as high-speed detecting element in silicon-based photonic chips and lab-on-chip systems.

A nano-optical switch driven by optical force using a laterally coupled double-ring resonator

This paper reports a nano-optical switch driven by optical force in a laterally coupled double-ring resonator. The nano-switch consists of two bus waveguides and a double-ring resonator, with one ring suspended. The circulating power in the double-ring resonator generates strong optical force, leading to deformation of the suspended ring. As a result, a resonance shift at the output leads to the switching operation. In experiment, a switching contrast of more than 25 dB is achieved, with a switching time of nano-seconds. Compared with other reported free-carrier effect and/or silicon-based high speed switches, the proposed switch allows switching operation at low pumping power levels in planar nano-scaled structures.

A THz dual mode switch using MEMS switchable metamaterial

This paper presents a THz dual mode switch realized by a proper designed MEMS switchable metamaterial. The switch simultaneously controls the reflection intensity of two optical resonant modes and thus switches the ON/OFF state at two resonant frequencies. The switchable output is defined as ON state when its reflection intensity is above 65%, and OFF state if below. Furthermore, the switching between dual mode output and single mode output is also realized by suppressing one resonance mode. The proposed metamaterial dual mode switch has merits as dual mode switching and working on THz band which enable the potential applications in multi-signal control.

Tunable Optical Filter by Thermal Effect Based on MEMS Technology

In the broadband communication network, the wavelength-division-multiplexed (WDM) system is widely used to maximize the information that the signals can carry. As a result, the number of channels which are carried by different optical wavelengths in the WDM optical fiber network also keeps increasing. To separate the huge number of different wavelength signals, optical filter is required. The optical filter based on semiconductor has been widely studied due to the maturation of semiconductor fabrication technology and that it is possible to integrate the filter with the stable semiconductor devices such as laser diodes and MOSFETS. The tunable optical filter is basically a selective optical resonator that only allows the resonant modes passing through. Various mechanical methods are studied to achieve the tunable effect by tuning the physical structure of the filter; however, there is not much research on how the semiconductor material will affect the tuning function. In this paper, the author studied the influence of refractive index of the multi-silicon-slabs on the filter, whereby the tuning of refractive index is reached by thermal effect. It is found by simulation that when heating the silicon slabs, the increasing refractive index of silicon will lead to a shift of the resonant mode wavelength. This shift is almost linear with the change of the temperature, which is about 1nm with every 20K temperature increase. For certain devices, the result of the simulation showed it is possible to tune the resonant mode from C band to L band in the Fiber Optical Communication.

A micromachined optical delay line by switchable metamaterials

This paper presents a MEMS switchable optical delay line, which works at terahertz (THz) region. The geometry of the unit cell is tunable by MEMS actuators [1]. In the experiment, it measures that the round trip time of the 5-µm silicon-metal cavity with the metamaterial slab is delayed by a factor of 100 when the gap of the metamaterial unit cell is shifted from 4 µm to 8 µm. Compared with the traditional tunable optical delay line [2], the metamaterial optical delay line has merits of fine tuning resolution, high tuning speed and feasible for high density integration.