A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

Effective manipulation of cavity resonant modes is crucial for emission control in laser physics and applications. Using the concept of parity-time symmetry to exploit the interplay between gain and loss (i.e., light amplification and absorption), we demonstrate a parity-time symmetry-breaking laser with resonant modes that can be controlled at will. In contrast to conventional ring cavity lasers with multiple competing modes, our parity-time microring laser exhibits intrinsic single-mode lasing regardless of the gain spectral bandwidth. Thresholdless parity-time symmetry breaking due to the rotationally symmetric structure leads to stable single-mode operation with the selective whispering-gallery mode order. Exploration of parity-time symmetry in laser physics may open a door to next-generation optoelectronic devices for optical communications and computing.

http://science.sciencemag.org/content/sci/346/6212/972.full.pdf

Single-mode laser by parity-time symmetry breaking.

Sensing technology has been widely investigated and utilized for gas detection. Due to the different applicability and inherent limitations of different gas sensing technologies, researchers have been working on different scenarios with enhanced gas sensor calibration. This paper reviews the descriptions, evaluation, comparison and recent developments in existing gas sensing technologies. A classification of sensing technologies is given, based on the variation of electrical and other properties. Detailed introduction to sensing methods based on electrical variation is discussed through further classification according to sensing materials, including metal oxide semiconductors, polymers, carbon nanotubes, and moisture absorbing materials. Methods based on other kinds of variations such as optical, calorimetric, acoustic and gas-chromatographic, are presented in a general way. Several suggestions related to future development are also discussed. Furthermore, this paper focuses on sensitivity and selectivity for performance indicators to compare different sensing technologies, analyzes the factors that influence these two indicators, and lists several corresponding improved approaches.

A Survey on Gas Sensing Technology

Quantitative phase imaging (QPI) has emerged as a valuable method for investigating cells and tissues. QPI operates on unlabelled specimens and, as such, is complementary to established fluorescence microscopy, exhibiting lower phototoxicity and no photobleaching. As the images represent quantitative maps of optical path length delays introduced by the specimen, QPI provides an objective measure of morphology and dynamics, free of variability due to contrast agents. Owing to the tremendous progress witnessed especially in the past 10–15 years, a number of technologies have become sufficiently reliable and translated to biomedical laboratories. Commercialization efforts are under way and, as a result, the QPI field is now transitioning from a technology-development-driven to an application-focused field. In this Review, we aim to provide a critical and objective overview of this dynamic research field by presenting the scientific context, main principles of operation and current biomedical applications. Over the past 10–15 years, quantitative phase imaging has moved from a research-driven to an application-focused field. This Review presents the main principles of operation and representative basic and clinical science applications.

Quantitative phase imaging in biomedicine

Many procedures in modern clinical medicine rely on the use of electronic implants in treating conditions that range from acute coronary events to traumatic injury. However, standard permanent electronic hardware acts as a nidus for infection: bacteria form biofilms along percutaneous wires, or seed haematogenously, with the potential to migrate within the body and to provoke immune-mediated pathological tissue reactions. The associated surgical retrieval procedures, meanwhile, subject patients to the distress associated with re-operation and expose them to additional complications. Here, we report materials, device architectures, integration strategies, and in vivo demonstrations in rats of implantable, multifunctional silicon sensors for the brain, for which all of the constituent materials naturally resorb via hydrolysis and/or metabolic action, eliminating the need for extraction. Continuous monitoring of intracranial pressure and temperature illustrates functionality essential to the treatment of traumatic brain injury; the measurement performance of our resorbable devices compares favourably with that of non-resorbable clinical standards. In our experiments, insulated percutaneous wires connect to an externally mounted, miniaturized wireless potentiostat for data transmission. In a separate set-up, we connect a sensor to an implanted (but only partially resorbable) data-communication system, proving the principle that there is no need for any percutaneous wiring. The devices can be adapted to sense fluid flow, motion, pH or thermal characteristics, in formats that are compatible with the body's abdomen and extremities, as well as the deep brain, suggesting that the sensors might meet many needs in clinical medicine.

Bioresorbable silicon electronic sensors for the brain

Digital projection photochemical etching is a novel single-step process for fabricating customized gray-scale semiconductor structures. Several features including a variable height pyramid array are fabricated, demonstrating the resolution, range, accuracy, and dynamics of the technique.

Fabrication of gray-scale semiconductor structures with dynamic digital projection photochemical etching

We combine conventional planar semiconductor processing with gray-scale topography created by digital projection photochemical etching to fabricate adiabatic waveguide mode converters. Applications include efficient coupling to fiber or between planes of multi-layer photonic integrated circuits.

Vertically Tapered Adiabatic Waveguide Mode Converters Fabricated with Digital Projection Photochemical Etching

We studied the nanoscale thermal expansion of a suspended resistor both theoretically and experimentally and obtained consistent results. In the theoretical analysis, we used a three-dimensional coupled electrical-thermal-mechanical simulation and obtained the temperature and displacement field of the suspended resistor under a direct current (DC) input voltage. In the experiment, we recorded a sequence of images of the axial thermal expansion of the central bridge region of the suspended resistor at a rate of 1.8 frames/s by using epi-illumination diffraction phase microscopy (epi-DPM). This method accurately measured nanometer level relative height changes of the resistor in a temporally and spatially resolved manner. Upon application of a 2 V step in voltage, the resistor exhibited a steady-state increase in resistance of 1.14 Ω and in relative height of 3.5 nm, which agreed reasonably well with the predicted values of 1.08 Ω and 4.4 nm, respectively.

/pdf/diffraction-phase-microscopy-imaging-and-multi-physics-1bo9vf0wct.pdf

Diffraction phase microscopy imaging and multi-physics modeling of the nanoscale thermal expansion of a suspended resistor.

We present a reflection-based broadband illumination quantitative phase imaging technique that provides halo-free images of opaque samples with sub-nanometer spatial and temporal noise.

Reflection-based diffraction phase microscopy using broadband illumination

The next generation of optical communication will depend heavily on all optical signal processing. Because digital clock pulses are needed for driving functional gates and synchronizing optical analog to digital (A/D) converters and other optical components, great effort has been placed on generating optical pulses. Approaches include non-linear polarization rotation, photonic crystal based methods, and cross gain modulation of semiconductor optical amplifiers. Although these methods share the advantage of high optical bandwidth, they each suffer from complicated fabrication and poor scalability. In this work, we propose using optical self-heating in silicon nitride microring resonators and thermal nonlinearity for creating all optical signal pulses. They key factor in our design is the passive framework we have chosen which result in scalability of our design. The resonances in the reflection spectrum of a plain microring are caused by coherent optical backscattering from the natural sidewall roughness created during dry etching. Light absorbed through the ring resonator will be converted to heat resulting in a temperature rise inside the ring. This temperature increase will alter the material refractive index which in turn will result in a red shifting of the resonance wavelength. Shifting the resonance wavelength changes the optical energy stored in the resonator as well as the absorption of the ring. Hence, the thermal and optical dynamics of the ring are coupled through the self-heating and thermo-optic effect [1]. Considering these dynamics, if we tune to a wavelength that is slightly on the blue side of the resonance and we increase the input signal power, the resonance wavelength will move away from our set wavelength and consequently the reflected power will decrease. Conversely, reducing the input power creates an increase in reflected power. Thus, by altering the input power, it is possible to switch between two states of the ring resonator and more importantly to realize an inverter function. By feeding back the reflection signal to the input of the ring resonator (Fig. 1(a)), we were able to generate all optical pulses (Fig. 1(b)) with a controllable frequency.

Lynford L. Goddard

Papers

Fabrication of gray-scale semiconductor structures with dynamic digital projection photochemical etching

Vertically Tapered Adiabatic Waveguide Mode Converters Fabricated with Digital Projection Photochemical Etching

Diffraction phase microscopy imaging and multi-physics modeling of the nanoscale thermal expansion of a suspended resistor.

Reflection-based diffraction phase microscopy using broadband illumination

Thermal nonlinearity based optical pulse generation in microrings