Analytical Instrumentation : Optics


Optimized Nanoresonator Design Signals Breakthroughs in Spectrometry and Device Efficiency

UW–Madison researchers have developed a new method and structure for increasing the cross section of nanoresonators, thereby improving the concentration ratio of light (or other electromagnetic radiation) and device performance. The key to their approach is that the nanoresonator is surrounded by a material that provides increased light concentration.

Compact Spectrometer

A UW–Madison researcher has developed a more compact and easily manufactured optical spectrometer. Specifically, the new design includes a filter system that receives and modifies light beams according to frequency. It generates a set of uncorrelated and varying filter spectra over an extremely short optical path. Although the filter spectra are complex and appear largely random, they can be ordered into an absorption spectrum using compressed sensing techniques.

Tunable, Silicon-Based Fresnel Lenses

UW–Madison researchers have developed a new, silicon-based Fresnel lens that is simple to manufacture and has an adjustable focal length.

The lens is made of transparent and opaque rings. The set of opaque rings is etched with tightly spaced silicon nanowires. The structure is set on an elastic plate that can stretch to change the focal length of the lens.

Strain-Tunable Light Emitting Diodes Using Germanium

UW–Madison researchers have developed new tunable LEDs with germanium PIN heterojunctions. The diodes are made of an undoped (intrinsic) Ge layer between p-type and n-type doped Ge layers. The nano-thin structure can be epitaxially grown and then transferred to a flexible substrate.

Once bonded to the flexible substrate, the whole structure is stretched, causing biaxial tensile strain. Given sufficient strain, the Ge is transformed into a direct-bandgap semiconductor. When voltage is applied, radiation is emitted via electroluminescence. The wavelengths of the emitted radiation can be tuned by adjusting the amount of stretch (i.e., the amount of tensile strain) that is applied.

More Flexible Microlens Assembly

UW–Madison researchers have developed an electrowetting liquid lens assembly that can be wrapped onto a curved surface.

The lens is made of two immiscible fluids, such as water and silicone oil, and is contained within a chamber. This chamber sits on a flexible polymer base that takes stress off the lens and allows it to be mounted onto a non-flat surface.

Voltage can be applied to electrodes set within the chamber. This causes the curvature of the water-oil interface to change, thereby adjusting focal length.

Improved Infrared-Responsive Hydrogel for Use in Microfluidics and Optics

UW–Madison researchers have developed an improved infrared-responsive hydrogel by incorporating graphene oxide flakes into a thermo-responsive hydrogel polymer. These composite hydrogels have an intrinsically higher surface area and absorbance band than conventional metal nanoparticles, resulting in a larger volumetric change in response to infrared light. The researchers also have provided a microfluidic device and a lens structure that incorporate these composite hydrogels as actuators. Both devices can be operated by heating the composite hydrogel in its swollen state to a temperature sufficient enough to shrink its volume. The hydrogel can be restored to its original volume by allowing it to cool and re-swell. In the microfluidic device volume reduction of the hydrogel allows fluid to flow through a channel and in the lens structure volume change relates to a change in focal length. 

Flexible Germanium Lateral PIN Diodes and 3-D Arrays for Photodetector Applications

UW-Madison researchers have developed a flexible lateral PIN diode made of germanium for photodetector applications.  The flexibility of the diode gives it the unique ability to be formed into flexible two- or three-dimensional arrays.  Each diode is composed of a flexible layer of single-crystalline germanium on top of a flexible substrate.  The middle of the single-crystalline semiconductor is the intrinsic (I) semiconductor layer, which is disposed laterally between an N- and a P-type doped region.  These individual diodes are shaped like irregular polygons and can be formed into hemisphere shaped arrays. 

The arrays can be used in photodetector applications such as digital cameras, solar cells or LADAR devices, and the individual diodes can be used for electronic switch applications.  Digital cameras can use the arrays to capture light at its natural incident angle instead of bulky and expensive lenses that focus the light.  Omnidirectional, three-dimensional LADAR systems for military surveillance applications can be formed by creating a hemispherical array made up of numerous vertical cavity light emitting sources, each surrounded by the photodetector diodes.  The individual flexible, lightweight diodes also can be used in high-frequency switching applications where their flexibility is desired.  These diodes have the advantage of utilizing high-performance active components while still incorporating low-cost plastic substrates.

Nested Waveguides for Generating or Detecting Radiation, Including Terahertz Radiation

UW Madison researchers have recently demonstrated a room temperature, tunable THz source operating at 1.3 THz with an extremely narrow linewidth (< 200 kHz; < 7 x 10-6 cm-1) and record conversion efficiency.  This source takes advantage of a new nested waveguide structure to produce continuously phase-matched difference frequency mixing between spectrally pure, amplified diode laser pumps.  The thin film active medium for this source (LiNbO3 ) is interchangeable with other nonlinear materials operating at other frequency ranges (e.g., AlGaAs for 3.5 THz).  It can be designed to generate ultra-narrow band radiation across a range of frequencies. The device also may be designed to detect THz radiation.

The nested waveguides are fabricated using well established lithography and semiconductor fabrication techniques, such as chemical vapor deposition. A smaller waveguide can be embedded within a larger waveguide.  The smaller waveguide provides guidance for radiation of a shorter wavelength, while the larger waveguide provides a transition to radiation of a longer wavelength. The waveguides enhance the efficiency at which the nonlinear process converts the radiation to the desired frequency by providing strong optical confinement of the input and output radiation, reducing diffraction and improving phase matching. Nested waveguides also have a small footprint, making them ideal for creating small THz-based systems.

Multilayer Si/SiO2 Semiconductors for Photoelectric Device Fabrication

UW-Madison researchers have developed an improved method for manufacturing quantum-well photoelectric devices from monocrystalline semiconductor layers.  The proposed invention aims to address the limitation of current technology with the use of Si charge carrying layers in between silicon dioxide (SiO2) barrier layers, allowing operation at room temperature and with wavelengths in the visible region.  The method used to fabricate these layers is similar to the previously developed technology except a different barrier material is used to promote electron confinement.

This method uses standard integrated-circuit deposition techniques and strain-symmetrization to produce superior virtual substrates upon which multiple layers are grown.  The process starts with the SOI substrate, a well characterized and developed technology commonly used in the integrated-circuit industry to fabricate Si computer chips.  The SOI base is thinned and smoothed using chemical or mechanical processes.  The alternating silicon/silicon dioxide layers then are grown on the SOI using standard deposition techniques.  The strain produced by the lattice constants of the two different materials is relieved by physically removing the layers from the SOI, creating a virtual substrate. 

The multiple growth and removal of the multilayer structure reduces the amount of defects in the crystal lattice, resulting in increased device power.  More layers can be grown on the virtual substrate to produce a component layer, but the number of grown layers is limited to reduce imperfections in the crystal lattice.  The multilayer structures then can be annealed to produce stacks of Si semiconductor of the desired thickness.  These semiconductors can be used to produce inexpensive and easily integrated quantum-well photoelectric devices, which have potential applications in chemical sensors, LEDs and all digital devices.

Robust Substrates Expand the Utility of Surface Plasmon Resonance Imaging for Analysis of Biomolecular Interactions

UW–Madison researchers have developed robust, SPR-compatible substrates. The key to these substrates is a rugged, chemically versatile carbon thin film overlayer placed on an SPR-active metal thin film.

Specifically, the substrates include a support surface capable of transmitting light, a metallic layer adhered to the support surface and a carbonaceous layer deposited on the metallic layer. The substrates also may include biomolecules attached to the carbonaceous, or carbon-rich, layer. These biomolecules may include oligonucleotides, DNA, RNA, proteins, amino acids, peptides or other small biomolecules that can be configured in one or more arrays.

The new substrates are more robust than conventional gold substrates, allowing assays to be performed under higher temperatures and harsher chemical conditions than currently is possible. Additionally, the carbon thin film overlayer is not susceptible to damage from UV irradiation.

“Super” Artificial Compound Eyes Formed from Microlenses

UW-Madison researchers have now combined many liquid-liquid microlenses on a planar or domed array to form a “super” artificial compound eye (SACE) with a large field of view and high resolution. By coupling the benefits of microlenses with those of compound eyes, this technology could provide low-cost, high-resolution imaging for medical, industrial and military applications. It could be used to develop medical devices, such as fiber endoscopes and laparoscopes, that make procedures like colonoscopy or appendectomy safer and easier. The SACE could perform image scanning without bulky control systems that can be cumbersome and costly. This technology could also improve current monitoring and surveillance instruments for the military, as well as consumer products, such as miniaturized digital cameras.

Multispectral Laser with Improved Time Division Multiplexing

UW-Madison researchers now have developed an improved, multi-wavelength, time-division multiplexed (TDM) laser capable of individually controlling multiple narrow wavelengths in separate time-division windows. Unlike the previous laser, this device uses a cavity that provides the same cavity length for each wavelength. To separate light into multiple wavelengths, it introduces a fixed time shift in the arrival of each wavelength at the output coupler as a function of the wavelength. It continuously cycles through the spectrally narrow wavelengths, spending a predetermined amount of time on each one. Specifically, the laser cavity holds the optical amplifier between a wavelength-dependent delay element (WDE) that temporally separates a multi-wavelength light pulse into discrete constituent monochromatic components, and a complementary wavelength-dependent delay element (CWDE) that temporally collects the monochromatic components after separation. This design reduces gain competition, simplifies the circuitry for controlling the amplifier and distinguishing the output pulses, and improves the consistency and control of the multiple monochromatic wavelengths.

Nanoelectromechanical Switch in Co-Planar Waveguide

UW-Madison researchers have developed a co-planar waveguide that uses a NEMSET electromechanical signal switch for signal modulation. The NEMSET switch acts as a natural communication pathway between co-planar elements of co-planar waveguides. The device operates in the frequency range of 1MHz to 2GHz to filter, modulate, mix and rectify microwave signals. This waveguide could be used in the same way as a traditional waveguide in electrical engineering, but with the benefits of NEMSET and a co-planar structure.

Method of Producing Short-Wavelength Quantum-Entangled Light Beams

UW-Madison researchers have developed a new approach to generating quantum-entangled light beams using second harmonic generation to provide entanglement at short wavelengths. The science is in its infancy, but as it develops, entanglement generated at shorter wavelengths may be important for applications in quantum measurements, quantum-enhanced lithography and ultra-secure quantum communication. Entangled light could also lead to more sensitive medical diagnoses, more powerful computer chips and scalable quantum computing.

Variable-Focus Lens Assembly

UW-Madison researchers have developed an alternative means of making microlenses with adjustable focal lengths. In this method, many fluids may be used for the lens. Responsive hydrogel structures create the tube in which the lens fluid sits. Alternatively, the hydrogel can be coupled to a transparent thin film that will act as the lens. When an environmental parameter, such as temperature or pH, changes, it causes the hydrogel to swell or contract. This in turn causes a change in the focal length of the lens.

Versatile Substrate for SPR Detection

UW-Madison researchers have developed a superior SPR chip surface structure that enhances the imaging signal. The new chip consists of a glass/dielectric material/gold-layered structure. The glass support surface and the dielectric layer are transparent to analyzing light. The gold layer consists of metallic islands on top of the hydrophobic dielectric layer. Probe molecules may be attached to the metallic islands. The dielectric layer surrounding the islands promotes “long-range” SPR and increases the sensitivity of imaging.

Optical Imaging of Nanostructured Substrates

UW-Madison researchers have developed devices and methods for imaging phenomena that occur on a nanostructured substrate surface and detecting the presence of a target analyte. Their invention starts with a substrate, such as a molded polymer, that has nanoscale grooves or other topography on its surface. The polymer substrate may include a support, such as a glass slide or plate, and also may be coated with a thin layer of a metal, such as gold or silver. The substrate may include a blocking layer, such as bovine serum albumin, to prevent non-specific adsorption, and may contain surface-bound receptor molecules for the target species.

To image phenomena that occur on the surface, a fluid is placed on top of the substrate, and the surface is observed with polarizing light. To detect an analyte of interest, the substrate is viewed with polarizing light, contacted with a sample and then imaged again. The target analyte is present if the surface appears different after the sample is added.

Machining of Lithium Niobate by Laser Fracturing

UW-Madison researchers have developed a method for rapidly dicing lithium niobate wafers into a variety of shapes, including curved shapes. The edges cut with this method are nearly atomically smooth, allowing direct attachment of fiber optic pigtails without further polishing.

The process uses a commercially available laser to create and guide a fracture through the wafer to cut it. Under different beam conditions, the same laser can also ablate features on the wafer surface, such as alignment marks, gratings and microwave and optical cavities. Model calculations have shown that ablated features can significantly improve the performance of devices, such as the traveling wave modulator.

Micromachined Shock Sensor

UW–Madison researchers have developed a micromachined shock sensor that has an array of acceleration sensing units formed to make contact at different levels of acceleration. This allows the shock sensor to measure a wide range of accelerations from 10g to 150g. The system also contains a built-in redundancy that circumvents challenges with the closing and opening of electrical contacts.

Micromechanical Phase-Shifting Gate Optical Modulator

UW–Madison researchers have developed a phase shifting gate (PSG) that, instead of using micromirrors, exploits interference effects to create a highly reflective surface. This device eliminates the alignment problems present in the micromirror system.

Signal Enhancement for Fluorescence Microscopy

UW-Madison researchers have developed a method for significantly improving photon collection efficiency and image intensity by placing a second dichroic mirror on the opposite side of the specimen as the objective lens. Specifically, an excitation beam is focused by an objective lens onto a specimen. The fluorescence emitted from the specimen is collected by the objective lens. This light is also collected by a condenser lens on the opposite side of the specimen and is directed to a dichroic mirror, which reflects the light photons through the specimen and then to the objective lens. This adds to the photons directly emitted from the specimen, thereby enhancing signal intensity.