Vala Fathipour

Deep-UV microsphere projection lithography.

In this Letter, we present a single-exposure deep-UV projection lithography at 254-nm wavelength that produces nanopatterns in a scalable area with a feature size of 80 nm. In this method, a macroscopic lens projects a pixelated optical mask on a monolayer of hexagonally arranged microspheres that reside on the Fourier plane and image the masks pattern into a photoresist film. Our macroscopic lens shrinks the size of the mask by providing an imaging magnification of ∼1.86×10(4), while enhancing the exposure power. On the other hand, microsphere lens produces a sub-diffraction limit focal point-a so-called photonic nanojet-based on the near-surface focusing effect, which ensures an excellent patterning accuracy against the presence of surface roughness. Ray-optics simulation is utilized to design the bulk optics part of the lithography system, while a wave-optics simulation is implemented to simulate the optical properties of the exposed regions beneath the microspheres. We characterize the lithography performance in terms of the proximity effect, lens aberration, and interference effect due to refractive index mismatch between photoresist and substrate.

Isolated electron injection detectors with high gain and record low dark current at telecom wavelength

We report on recent performance breakthroughs in a novel short-wave infrared linear-mode electron-injection-based detector. Detectors consist of InP material system with a type-II band alignment and provide high internal avalanche-free amplification mechanism. Measurements on devices with 10-μm injector diameter and 30-μm absorber diameter show internal dark current density of about 0.1 nA/cm2 at 160 K. Compared with our previous reported results, dark current is reduced by two orders of magnitude with no sign of surface leakage limitation down to the lowest measured temperature. Compared with the best-reported linear-mode avalanche photodetector, which is based on HgCdTe, the electron-injection detector shows over three orders of magnitude lower internal dark current density at all measured temperatures. Using a detailed simulation with experimentally measured parameters, dark count rate of 1 Hz at 90% photon detection efficiency at 210 K is anticipated. This is a significantly higher operating temperature compared with superconducting detectors with a similar performance.

Integrated all-optical infrared switchable plasmonic quantum cascade laser.

We report a type of infrared switchable plasmonic quantum cascade laser, in which far field light in the midwave infrared (MWIR, 6.1 μm) is modulated by a near field interaction of light in the telecommunications wavelength (1.55 μm). To achieve this all-optical switch, we used cross-polarized bowtie antennas and a centrally located germanium nanoslab. The bowtie antenna squeezes the short wavelength light into the gap region, where the germanium is placed. The perturbation of refractive index of the germanium due to the free carrier absorption produced by short wavelength light changes the optical response of the antenna and the entire laser intensity at 6.1 μm significantly. This device shows a viable method to modulate the far field of a laser through a near field interaction.

Isolated nanoinjection photo detectors for high-speed and high-sensitivity single-photon detection

Our group has designed and developed a new SWIR single photon detector called the nano-injection detector that is conceptually designed with biological inspirations taken from the rod cells in human eye. The detector couples a nanoscale sensory region with a large absorption volume to provide avalanche free internal amplification while operating at linear regime with low bias voltages. The low voltage operation makes the detector to be fully compatible with available CMOS technologies. Because there is no photon reemission, detectors can be formed into high-density single-photon detector arrays. As such, the nano injection detectors are viable candidates for SPD and imaging at the short-wave infrared band. Our measurements in 2007 proved a high SNR and a stable excess noise factor of near unity. We are reporting on a high speed version of the detector with 4 orders of magnitude enhancement in speed as well as 2 orders of magnitude reduction in dark current (30nA vs. 10 uA at 1.5V).

Impact of three-dimensional geometry on the performance of isolated electron-injection infrared detectors

We present a quantitative study of the influence of three-dimensional geometry of the isolated electron–injection detectors on their characteristics. Significant improvements in the device performance are obtained as a result of scaling the injector diameter with respect to the trapping/absorbing layer diameters. Devices with about ten times smaller injector area with respect to the trapping/absorbing layer areas show more than an order of magnitude lower dark current, as well as an order of magnitude higher optical gain compared with devices of same size injector and trapping/absorbing layer areas. Devices with 10 μm injector diameter and 30 μm trapping/absorbing layer diameter show an optical gain of ∼2000 at bias voltage of −3 V with a cutoff wavelength of 1700 nm. Analytical expressions are derived for the electron-injection detector optical gain to qualitatively explain the significance of scaling the injector with respect to the absorber.

Analytical modeling and numerical simulation of the short-wave infrared electron-injection detectors

This paper describes comprehensive analytical and simulation models for the design and optimization of the electron-injection based detectors. The electron-injection detectors evaluated here operate in the short-wave infrared range and utilize a type-II band alignment in InP/GaAsSb/InGaAs material system. The unique geometry of detectors along with an inherent negative-feedback mechanism in the device allows for achieving high internal avalanche-free amplifications without any excess noise. Physics-based closed-form analytical models are derived for the detector rise time and dark current. Our optical gain model takes into account the drop in the optical gain at high optical power levels. Furthermore, numerical simulation studies of the electrical characteristics of the device show good agreement with our analytical models as well experimental data. Performance comparison between devices with different injector sizes shows that enhancement in the gain and speed is anticipated by reducing the injector size...

On the Sensitivity of Electron-Injection Detectors at Low Light Level

We present the signal-to-noise performance of a short-wave infrared detector, which offers an internal avalanche-free gain. The detector is based on a similar mechanism as the heterojunction phototransistor and takes advantage of a type-II band alignment. Current devices demonstrate a noise-equivalent sensitivity of ~670 photons at 260 K and over a linear dynamic range of 20 dB. While this level of sensitivity is about an order of magnitude better than an ideal p-i-n detector attached to the same low-noise amplifier, it was still limited by the amplifier noise (~2600 electrons root mean square) due to the insufficient device gain. Performance comparison with other SWIR detector technologies demonstrates that the so-called electron-injection detectors offer more than three orders of magnitude better noise-equivalent sensitivity compared with state-of-the-art phototransistors operating at similar temperature.

Approaching high temperature photon counting with electron-injection detectors

Our group has designed and developed a novel telecom band photon detector called the electron-injection detector. The detector provides a high avalanche-free internal-amplification and a stable excess noise factor of near unity while operating at linear-mode with low bias voltages. In our previous reports on un-isolated detectors, the large dark current of the detectors prevented long integration times in the camera. Furthermore, the bandwidth of the un-isolated detectors was in the KHz range. Recently, by changing the 3D geometry and isolating the detectors from each other, we have achieved 3 orders of magnitude reduction in dark current at same bias voltage and temperature compared to our previous results. Isolated detectors have internal dark current densities of 0.1nA/cm2 at 160 K. Furthermore, they have a bandwidth that is 4 orders of magnitude higher than the un-isolated devices. In this paper we report room temperature and low temperature characteristics of the isolated electron-injection detectors. We show that the measured optical gain displays a small dependence on temperature over our measured range down to 220 K.

Demonstration of Shot-noise-limited Swept Source OCT Without Balanced Detection

Optical coherence tomography (OCT) has been utilized in a rapidly growing number of clinical and scientific applications. In particular, swept source OCT (SS-OCT) has attracted many attentions due to its excellent performance. So far however, the limitations of existing photon detectors have prevented achieving shot-noise-limited sensitivity without using balanced-detection scheme in SS-OCT, even when superconducting single-photon detectors were used. Unfortunately, balanced-detection increases OCT system size and cost, as it requires many additional components to boost the laser power and maintain near ideal balanced performance across the whole optical bandwidth. Here we show for the first time that a photon detector is capable of achieving shot noise limited performance without using the balanced-detection technique in SS-OCT. We built a system using a so-called electron-injection photodetector, with a cutoff-wavelength of 1700 nm. Our system achieves a shot-noise-limited sensitivity of about −105 dB at a reference laser power of ~350 nW, which is more than 30 times lower laser power compared with the best-reported results. The high sensitivity of the electron-injection detector allows utilization of micron-scale tunable laser sources (e.g. VCSEL) and eliminates the need for fiber amplifiers and highly precise couplers, which are an essential part of the conventional SS-OCT systems.

New generation of isolated electron-injection imagers

This paper describes a novel electron-injection based short-wave infrared imager. The first generation of electron-injection imager achieved two orders of magnitude better signal to noise ratio compared with a commercial high-end SWIR camera. In the second generation, detectors are isolated and achieve extremely low dark current, record low noise levels and fast rise times while maintaining the very large internal amplification. Furthermore, electron-injection imager shows superior noise performance compared with imagers made with the state-of-the-art InGaAs PIN and MCT eAPDs.