Ke Liu

Thermo-optically tunable silicon photonic crystal light modulator

We designed, fabricated, and characterized a thermo-optically tunable compact (10 μm × 10 μm) silicon photonic crystal (PhC) light modulator that operates at around 1.55 µm for TE polarization. The operational principle of the device is the modulation of the cutoff frequency in a silicon-based line defect PhC. The cutoff frequency is shifted because of the thermo-optic tuning of the silicon refractive index, which is realized by localized heating on the PhC. The modulator is formed by a triangular lattice array of cylindrical air holes on a silicon-on-insulator wafer. Optical characterization was carried out, and the result clearly showed thermo-optic tuning of the cutoff frequency at around 1.55 µm.

Silicon-Based Thermo-Optically Tunable Photonic Crystal Lens

We report an extremely compact (30 ¿m × 7 ¿m) silicon-based 2-D thermo-optically tunable photonic crystal (PhC) lens operated at around telecommunication wavelength (1.55 ¿m ) with transverse-magnetic-like polarization light. A honeycomb lattice array of high index silicon rods with 340 nm in thickness, 234 nm in diameter, and 338 nm in lattice constant were embedded in 3-¿m-thick low index silicon dioxide. A 150-nm-thick NiCr micro-heater was placed directly on top of the PhC structure to provide localized heating to the silicon rod array. The localized heating causes refractive index change in silicon due to thermo-optic effect which results in change of the focal length of the PhC lens. The device was characterized with a tunable laser light source in the wavelength range of 1500 ~ 1580 nm. Tuning of focal length in this device was experimentally demonstrated by applying different current through the heater. Such experimental results showed good agreement with the simulation results.

Four-Port Nanophotonic Frustrated Total Internal Reflection Coupler

Four-port frustrated total internal reflection couplers in InP-based GalnAsP quantum-well substrates are realized and characterized. Each coupler forms an X at the perpendicular intersection of two ridge waveguides and is aligned 45° to the optical path. The 180-nm-wide couplers are fabricated by dry etching deep trenches through the quantum wells and backfilling with alumina (n = 1.71) by atomic layer deposition. Coupling coefficients for the fabricated coupler are in good agreement with a three-dimensional finite-difference time-domain theory, and an 82% coupler efficiency is estimated.

High-sensitivity microfluidic pressure sensor using a membrane-embedded resonant optical grating

We present the design, fabrication, and characterization of a sensor capable of detecting minute changes in pressure within a polydimethylsiloxane (PDMS) microchannel. The device consists of a guided-mode resonance (GMR) grating with a resonant wavelength of 731 nm embedded in an 85 µm thick PDMS membrane which acts as one wall of the microchannel. As pressure within the channel is increased, the membrane expands, straining the PDMS as well as the embedded grating. This strain results in an increase in the pitch of the grating, which in turn produces a shift in the resonance of the device. By measuring the resulting change in the reflectivity spectrum of the device, changes in relative pressure as small as 50 pascal may be detected over a range of over 5 kilopascal.

Active Integrated Filters for RF-Photonic Channelizers

A theoretical study of RF-photonic channelizers using four architectures formed by active integrated filters with tunable gains is presented. The integrated filters are enabled by two- and four-port nano-photonic couplers (NPCs). Lossless and three individual manufacturing cases with high transmission, high reflection, and symmetric couplers are assumed in the work. NPCs behavior is dependent upon the phenomenon of frustrated total internal reflection. Experimentally, photonic channelizers are fabricated in one single semiconductor chip on multi-quantum well epitaxial InP wafers using conventional microelectronics processing techniques. A state space modeling approach is used to derive the transfer functions and analyze the stability of these filters. The ability of adapting using the gains is demonstrated. Our simulation results indicate that the characteristic bandpass and notch filter responses of each structure are the basis of channelizer architectures, and optical gain may be used to adjust filter parameters to obtain a desired frequency magnitude response, especially in the range of 1–5 GHz for the chip with a coupler separation of ∼9 mm. Preliminarily, the measurement of spectral response shows enhancement of quality factor by using higher optical gains. The present compact active filters on an InP-based integrated photonic circuit hold the potential for a variety of channelizer applications. Compared to a pure RF channelizer, photonic channelizers may perform both channelization and down-conversion in an optical domain.

Electro-thermally tunable silicon photonic crystal lens

We report extremely compact (30 µm × 7 µm) silicon based two-dimensional (2D) thermo-optically tunable photonic crystal lens which operates at around telecommunication wavelength (1.55 µm) with transverse magnetic (TM) like polarized light. A 150 nm thick NiCr micro-heater was placed directly on top of the photonic crystal lens to provide localized heating. The focal length of the photonic crystal lens is modulated through refractive index change caused by thermo-optic effect of silicon. Tuning of the focal length was experimentally demonstrated by applying various current through the heater and the results showed good agreed with the simulation results.

Experiment and Theory of an Active Optical Filter

The role of gain in an optical filter is advanced by good agreement between theory and experiment presented herein. The particular integrated photonic filter is composed of four semiconductor optical amplifiers and one four-port coupler located at the intersection of the amplifiers. The four-port coupler is realized using frustrated total internal reflection off a very thin slab of alumina embedded in the substrate. The delta function response of the filter is measured using an ultra-fast laser and cross-correlator, and the measured transfer functions agree well with a z-transform-based description of the device.

A PDMS-based pressure-tunable nanograting

We report the design, fabrication, and characterization of a pressure-tunable guided-mode resonance grating. A thin, flexible polydimethylsiloxane (PDMS) membrane was fabricated with an embedded titanium dioxide (TiO2) nanograting. When pressure is applied to the device, the membrane expands, and the pitch of the grating increases; this causes a shift in its resonant wavelength. By varying the pressure by only 3500 Pa, a tunable range of over 35 nm can be observed.

Ultra-compact electro-thermally tunable photonic crystal prism for on-chip optical router application

We report design, fabrication and characterization of MEMS enabled electro-thermally tunable 2D photonic crystal prism. The tunable photonic crystal prism was realized by utilizing thermo-optic effect of silicon. A nickel chromium heater was used to heat up the photonic crystal prism. The localized heating on the prism (to 222°C) modifies the photonic band structure, leading to an angle shift (4°) of the propagation direction of light. The tunable prism was characterized using a far field imaging infrared camera. The profiles of the scattered light spot at the end tip of the output waveguides clearly showed angle shift of the propagation of the light as expected. Both electromagnetic simulation and experiment results were in good agreement.

Extended active optical lattice filters: filter synthesis

In this paper, we study the synthesis of asymptotically stable filters from a unit cell of a two-dimensional tunable lattice filter architecture consisting of four four-port couplers and four waveguides containing semiconductor optical amplifiers. Upper bounds on the number of gains that will produce a filter with a priori prescribed poles, for a specific system, are obtained. We also provide sufficient conditions on the reflection-type coefficients, characterizing each four-port coupler, which ensure that real-valued gains, taking values in [0,1], exist so that the filter is asymptotically stable. Finally, we motivate the notion of a transmission zero of a filter and discuss the possibility of simultaneously placing both poles and transmission zeros for the unit cell.