Sung Jun Jang

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.

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.

Novel high-throughput and maskless photolithography to fabricate plasmonic molecules

Fabrication of nanostructures for applications such as plasmonics and metamaterials is typically low throughput, due to the required submicron feature sizes. Therefore, rapid production of optically engineered structures with low cost and large area is an enabling technology for many applications, such as light harvesting, solid state lighting, disposable biosensing, and metamaterials. Here, the authors propose a simple technique, based on microsphere nanolithography, to fabricate arrays of optical elements, or so-called plasmonic molecules, at about one third of exposure wavelength. This method is capable of producing many symmetric/asymmetric array of submicron arrangement of circles and is compatible with high-throughput nanomanufacturing schemes such as roll-to-roll production. The gap size between disks is precisely controllable by the angle of exposure. Here, the authors demonstrate the capabilities of this method in producing an array of complex plasmonic molecules over a large area. The periodicit...

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.

Deep UV microsphere nanolithography to achieve sub-100 nm feature size

Nano-fabrication technologies are usually associated with complication, high cost, and limited area of coverage. However, advances in optics and nanophotonics constantly demand novel fabrications for nano-manufacturing systems with extraordinary optical, electrical, mechanical, or thermal responses. While, these properties are vital for health, energy, and information technology applications, proposing new methods of fabricating nanostructures that can be compatible with high throughput and large scale manufacturing is quite desirable. Here, we propose a deep ultra-violet (DUV) photolithography technique that can produce a variety of periodic nanostructure clusters with sub-100 nm feature sizes. The method is based on microsphere nanolithography, which focuses DUV field into a socalled photonic nano-jet – a propagative intensive field underneath the sphere. The position of a photonic nano-jet can be moved by changing the angle of exposure. The DUV microsphere nanolithography is inherently self-aligned, mask-less and optics-less (the bulky optical element such as lens is not required), which makes this method attractive for low-cost and high-throughput nano-manufacturing schemes, such as roll-to-roll production. Here, we present fabricated arrays of nanoscale complex structures to demonstrate the capabilities of this nanolithography method.

Tilted exposure microsphere nanolithography for high-throughput and mask-less fabrication of plasmonic molecules

Fabrication of nanostructures for applications such as plasmonics and metamaterials are typically accompanied by a slow production and limited area due to the required sub-micron feature sizes. In these applications, periodic array of metal/dielectric features can produce optical resonance responses such as optical field enhancement response, Fano response, chiral response, and negative refractive index. Here, we propose a mask-less photolithography technique that can produce a variety of periodic nanostructure clusters. The method is based on microsphere nanolithography, which focuses UV field into the so-called photonic jet which is a propagative intensive field underneath the sphere. The position of photonic jet can be moved by changing the angle of exposure. The method introduces a controllable scheme to realize nano-gap size by controlling the angle of exposure. The feature sizes generated by this method are about one third of exposure wavelength. The method is compatible with highthroughput nano-manufacturing schemes, such as roll-to-roll production. Here we present some examples to demonstrate the capabilities of this method in producing an array of complex plasmonic molecules over a large area. The periodicity of array and element’s diameter can be tuned by microsphere size and exposure/developing time, respectively. Tilted exposure lithography inherently is self-aligned and readily extendible to deep UV lithography due to absent of mask and optical elements. FDTD simulation agrees well with our experimental results, and suggests that much smaller feature sizes can be achieved at shorter wavelengths.

Optomechanical nanoantenna: Far-field control of near-field through mechanical reconfiguration

We have introduce optomechanical nanoantennae, which showed dramatic changes in scattering properties by minuscule changes in geometry. These structures are very compact, with a volume 500 times smaller than free space optical wavelength volume. Through these optical elements, far-field can directly control the near-field of antenna by mechanical reconfiguration. Here we present the functionality of the optomechanical nanoantenna and challenges in fabricating and measuring these devices.

Approaching single-photon sensitivity at high temperatures

Photon number resolving (PNR) detectors are extremely sensitive devices capable of registering and counting quanta of light, or photons. Existing PNR detectors are far from ideal, requiring cooling to near absolute zero to avoid very large false detection rates, and are the performance bottleneck in systems such as quantum computers, quantum key distribution, optical communication with high number of bits per photon, and optical tomography. Many applications at the frontiers of science and engineering, including scalable quantum computing, detection and visualization of remote stars and nebulae, non-invasive diffused optical imaging, and optical coherence tomography (OCT) could benefit from high-performance imaging systems with PNR ability at short-wave IR (SWIR) wavelengths. For instance, the speed and depth of OCT, which produces 3D images of human tissue from minute reflections of photons, could be significantly enhanced. The best existing SWIR PNR detectors are transition edge sensor (TES) superconducting detectors and semiconductor avalanche photodiodes (APDs).1–3 A TES is in principle a supersensitive thermometer that can identify the heat signature of a single photon. An APD uses carrier avalanche multiplication to produce millions of carriers from the initial electron-hole pair generated by a single photon. Although there have been dramatic improvements in TES rates of false detection per second (dark count rates, DCRs) and detection efficiency, and also APD response speed over the past 10 years, neither can produce large imaging arrays. Their practical applications are limited by inherent properties of their constituent materials. Despite having excellent PNR capabilities and the lowest dark count rates ever reported, the dead time (the time right after a photon detection, during which the detector cannot detect further photons) of TES detectors is limited by the thermal time constant of the detector element ( 1 s), and so their timing properties are relatively poor.1 Furthermore, they require Figure 1. (a) Schematic and (b) scanning electron microscopy image of an isolated nano-injection detector with 10 m-diameter injector and 30 m-diameter absorber showing the top metal contact, injector, and absorber regions. p+ GaAsSb: Gallium arsenide antimonide (doped for positive ‘hole’ charge carriers, h+). n– InGaAs: Indium gallium arsenide (doped for negative charge carriers, e–).