Il Woong Jung

Highly Sensitive Monolithic Silicon Photonic Crystal Fiber Tip Sensor for Simultaneous Measurement of Refractive Index and Temperature

Fiber optic sensors have applications in the measurement of a wide range of physical properties such as temperature, pressure, and refractive index. These sensors are immune to electromagnetic interference, made of high temperature dielectric materials and hence can be deployed in harsh environments where conventional electronics would fail. Photonic crystal (PC) fiber tip sensors are highly sensitive to changes in the refractive index and temperature while remaining compact and robust. In comparison to conventional fiber sensors such as fiber Bragg gratings (FBG) or long period fiber gratings (LPFG), they are attractive in several aspects. PC fiber tip sensors have better sensitivity to refractive index and temperature than FBG sensors and are have much smaller sensing volumes than FBGs and LPFGs. Their small size allows them to combine high sensitivity and structural robustness. The most attractive feature may be that PC fiber tip sensors also return a spectrally rich signal with independently shifting resonances that can be used to extract multiple physical properties of the measurand and distinguish between them. In this paper, we show that the PC fiber tip sensor is highly sensitive to the refractive index and temperature of the environment and that both parameters can be simultaneously determined using multiple wavelengths.

Single-Crystal-Silicon Continuous Membrane Deformable Mirror Array for Adaptive Optics in Space-Based Telescopes

In this paper, we present a single-crystal-silicon (SCS) continuous membrane deformable mirror (DM) as a corrective adaptive-optics (AO) element for space-based telescopes. In order to correct the polishing errors in large aperture (~8 m) primary mirrors, a separate high-quality surface DM array must be used. Up to 400000 elements and a mirror stroke of ~100 nm are required for the correction of these polishing errors. A continuous membrane mirror formed by the the SCS device layer of a silicon-on-insulator (SOI) wafer is used to achieve a high-quality optical surface and to minimize the additional diffractive effects in the optical system. To achieve substantial local deformation needed to correct high-order errors, we use a highly deformable silicon membrane of 300-nm thickness. This thin membrane is able to deform locally by 125 nm at an operating voltage of 100 V with a pixel pitch of 200 mum. The resonance frequency of a pixel is 25 kHz with a low Q-factor of 1.7 due to squeeze-film damping. The device is fabricated by processing the microelectromechanical system (MEMS) and electronic chips separately and then combining them by flip-chip bonding. This allows optimization of the MEMS and electronics separately and also allows the use of an SOI layer for the mirror by building the MEMS bottom up. A small prototype array of 5times5 pixels with 200-mum pitch is fabricated, and we demonstrate single pixel and multiple pixel actuation

Heterogeneous silicon mesostructures for lipid-supported bioelectric interfaces

Silicon-based materials have widespread application as biophysical tools and biomedical devices. Here we introduce a biocompatible and degradable mesostructured form of silicon with multiscale structural and chemical heterogeneities. The material was synthesized using mesoporous silica as a template through a chemical-vapor-deposition process. It has an amorphous atomic structure, an ordered nanowire-based framework, and random submicrometre voids, and shows an average Young’s modulus that is 2–3 orders of magnitude smaller than that of single crystalline silicon. In addition, we used the heterogeneous silicon mesostructures to design a lipid-bilayer-supported bioelectric interface that is remotely controlled and temporally transient, and that permits non-genetic and subcellular optical modulation of the electrophysiology dynamics in single dorsal root ganglia neurons. Our findings suggest that the biomimetic expansion of silicon into heterogeneous and deformable forms can open up opportunities in extracellular biomaterial or bioelectric systems.

Photonic Crystal Fiber Tip Sensor for High-Temperature Measurement

We demonstrate a temperature sensor consisting of a 2-D, Silicon (Si), Photonic Crystal (PC) attached to the facet of a standard single-mode optical fiber. The 2-D PC sensors are fabricated on standard Si wafers, using a single mask and a combination of isotropic and anisotropic etching, and microassembled onto the facets of the optical fibers by Si-welding. The temperature of the Si-PC sensor is monitored by measuring its reflectance spectrum in the 1250 to 1650 nm wavelength range. The measured reflectivity peak shift is 0.11 nm°C in the 100°C to 700°C temperature range. The observed spectral shift and temperature sensitivity are significantly higher than high-temperature fiber Bragg grating sensors, and comparable to long-period fiber gratings sensors. The high sensitivity, combined with compactness and robust structure, give these sensors strong potential for use in harsh environments.

High-Reflectivity Broadband Photonic Crystal Mirror MEMS Scanner With Low Dependence on Incident Angle and Polarization

In this paper, we introduce a single-axis resonant comb drive highly reflective broadband photonic crystal (PC) mirror microelectromechanical systems (MEMS) scanner with low dependence of reflectivity on incident angle and polarization. Two-dimensional PC mirrors have several advantages over metal coatings and dielectric stacks (1-D PCs) as mirrors. They can tolerate much higher processing temperatures and higher incident powers, as well as operate in more corrosive environments than metals. Compared to multilayer dielectric stacks, 2-D PC mirrors are compact and allow for simpler process integration, making them highly compatible with CMOS and MEMS processing. Our PC mirrors show broadband high reflectivity of > 90% in the wavelength range from 1345 to 1490 nm (> 95% for 1380 to 1450 nm) and very low angular and polarization dependence over this same range. The single-axis MEMS scanners are fabricated on silicon-on-insulator wafers with the PC mirrors fabricated in polysilicon films on oxide. Although the scanner presented in this paper is single axis, the low angular and polarization dependence properties of the 2-D-PC mirror also make it useful for dual-axis scanners. The scanners are actuated by in-chip plane electrostatic combdrives on resonance. Dynamic deflection measurements show that the scanners can achieve 20deg total scan angle with an input square wave of 40 V and up to 120deg total scan angle with 190 V at a resonance frequency of 2.03 kHz. Although the achievable scan ranges are very large, the useful scan angles will be limited by the angular and polarization dependence properties of the PC reflector and its application. For the 2-D PC scanner presented in this paper, applications that can tolerate 10% decrease in reflectivity will have a scan range of 20deg.

Imaging Ferroelectric Domains and Domain Walls Using Charge Gradient Microscopy: Role of Screening Charges

Advanced scanning probe microscopies (SPMs) open up the possibilities of the next-generation ferroic devices that utilize both domains and domain walls as active elements. However, current SPMs lack the capability of dynamically monitoring the motion of domains and domain walls in conjunction with the transport of the screening charges that lower the total electrostatic energy of both domains and domain walls. Charge gradient microscopy (CGM) is a strong candidate to overcome these shortcomings because it can map domains and domain walls at high speed and mechanically remove the screening charges. Yet the underlying mechanism of the CGM signals is not fully understood due to the complexity of the electrostatic interactions. Here, we designed a semiconductor-metal CGM tip, which can separate and quantify the ferroelectric domain and domain wall signals by simply changing its scanning direction. Our investigation reveals that the domain wall signals are due to the spatial change of polarization charges, while the domain signals are due to continuous removal and supply of screening charges at the CGM tip. In addition, we observed asymmetric CGM domain currents from the up and down domains, which are originated from the different debonding energies and the amount of the screening charges on positive and negative bound charges. We believe that our findings can help design CGM with high spatial resolution and lead to breakthroughs in information storage and energy-harvesting devices.

Photonic crystal fiber tip sensor for precision temperature sensing

We demonstrate that monolithic 2-dimensional photonic crystals (PCs) confined to the facet, or tip, of single-mode optical fibers can be designed as highly sensitive temperature sensors. Monolithic 2-D photonic-crystals are fabricated on silicon wafers and subsequently released and micro-assembled onto the tip of optical fibers. The PCs reflection spectrum is modulated by the temperature of the sensor and its environment and shows sensitivity comparable to Fiber Bragg Gratings (FBGs). In contrast to FBGs, the compact sensing element is localized at the tip of the fiber and the active sensor element is a disk of 10 mum diameter and 480 nm thickness. The sensor system is made of robust, high-temperature dielectric materials and therefore has the potential to be used for many applications including measurements in harsh environments.

A Large-Area High-Reflectivity Broadband Monolithic Single-Crystal-Silicon Photonic Crystal Mirror MEMS Scanner With Low Dependence on Incident Angle and Polarization

In this paper, we introduce a single-axis resonant combdrive microelectromechanical systems (MEMS) scanner with a large-area highly reflective broadband monolithic single-crystal-silicon (SCS) photonic crystal (PC) mirror. PC mirrors can be made from a single monolithic piece of silicon through alternate steps of etching and oxidation. This process allows the fabrication of a stress-free PC reflector in SCS with better optical flatness than deposited films such as polysilicon slabs on low-index oxide. PC mirrors can be made in IR transparent dielectric material and can achieve high reflectivity over a broad wavelength range. PC reflectors have several advantages over other mirror technologies. They can tolerate much higher processing temperatures and higher incident optical powers as well as operate in more corrosive environments than metals. Compared to multilayer dielectric stacks, PC mirrors allow for simpler process integration, thus making them highly compatible with CMOS and MEMS processing. In this paper, we fabricate a PC mirror MEMS scanner in SCS without any deposited films. Our PC mirrors show broadband high reflectivity in the wavelength range from 1550 to 1600 nm, and very low angular and polarization dependence over this same range. The single-axis MEMS scanners are fabricated on silicon-on-insulator (SOI) wafers with the PC mirrors also fabricated in the SOI device layer. The scanners are actuated by electrostatic comb drives on resonance. Dynamic deflection measurements show that the scanners achieve 22deg total scan angle with an input square wave of 67 V and have a resonance frequency of 2.13 kHz.

Monolithic silicon photonic crystal slab fiber tip sensor

We demonstrate that monolithic photonic crystals (PCs) confined to the facet, or tip, of single-mode optical fibers can be designed as highly sensitive refractive index point sensors. Monolithic photonic-crystal slabs are fabricated on silicon wafers and subsequently released and micro-assembled on the tip of optical fibers. The PC slabs reflection spectrum is modulated by the refractive index of the environment and shows sensitivity comparable to Fiber Bragg Gratings (FBGs). The compact sensing element is located at the tip of a fiber and hence is a highly localized sensor. The sensor is made of robust, high-temperature dielectric materials and therefore has the potential to be used for many applications including measurements in harsh environments.

Optical Pattern Generation Using a Spatial Light Modulator for Maskless Lithography

In this paper, we present a piston-type spatial light modulator (SLM) as a programmable optical pattern generator for maskless lithography. The SLM is a dual-lever-type actuator with pixel-overlapping springs to achieve low voltage actuation with a small capacitor area. Large arrays of up to ~40000 mirrors with 20-mum pixels and ~17000 mirrors with 30-mum pixels have been fabricated. The arrays have various prewired configurations to alleviate the need to control all the pixels individually. The mirrors have high resonance frequencies for fast response and achieve deflections of lambda/2 (lambda=193 nm for 20-mum pixels and lambda=633 nm for 30-mum pixels) at voltages less than 100 V. The mirrors are fabricated using thin-film processing and chemical mechanical polishing of the reflector layer. Using the SLM in an aerial imaging system, we demonstrate the generation of dark lines, dark areas, linewidth modulation, and subgrid positioning. We also demonstrate compensation of image shifts due to through-focus wafer drift using a double exposure (or phase-compensated exposure) technique