John Montoya

High operating temperature interband cascade focal plane arrays

In this paper, we report the initial demonstration of mid-infrared interband cascade (IC) photodetector focal plane arrays with multiple-stage/junction design. The merits of IC photodetectors include low noise and efficient photocarrier extraction, even for zero-bias operation. By adopting enhanced electron barrier design and a total absorber thickness of 0.7 μm, the 5-stage IC detectors show very low dark current (1.10 × 10−7 A/cm2 at −5 mV and 150 K). Even with un-optimized fabrication and standard commercial (mis-matched) read-out circuit technology, infrared images are obtained by the 320 × 256 IC focal plane array up to 180 K with f/2.3 optics. The minimum noise equivalent temperature difference of 28 mK is obtained at 120 K. These initial results indicate great potential of IC photodetectors, particularly for high operating temperature applications.

Multifunctional metamaterial pyroelectric infrared detectors

Pyroelectric materials enable the construction of high-performance yet low-cost and uncooled detectors throughout the infrared spectrum. These devices have been used as broadband sensors and, when combined with an interferometric element or filter, can provide spectral selectivity. Here we propose the concept of and demonstrate a new architecture that uses a multifunctional metamaterial absorber to directly absorb the incident longwave IR (8–12 μm) energy in a thin-film lithium niobate layer and also to function as the contacts for the two-terminal detector. Our device achieves a narrowband (560 nm FWHM at 10.73 μm), yet highly efficient (86%) absorption. The metamaterial creates high field concentration, reducing temperature fluctuation noise, and lowering device capacitance and loss tangent noise. The metamaterial design paradigm applied to detectors thus results in a very fast planar device with a thermal time constant of 28.9 ms with a room temperature detectivity, D*, of 107  cm W/Hz.

Barrier Selection Rules for Quantum Dots-in-a-Well Infrared Photodetector

We report on a systematic study of the effect of barriers on quantum dots-in-a-well infrared photodetectors. Four devices are fabricated and characterized with varying composition for barriers adjacent to quantum dots and away from quantum dots. Effects of these “proximity” and “remote” barriers are studied by comparing photoluminescence, responsivity, dark current, background-limited operating temperature, activation energy, and detectivity. The growth mechanism for a conformal coverage of quantum dots with proximity barriers is described and supported with reflection high-energy electron diffraction and transmission electron microscopy images. It is shown that proximity barriers and remote barriers influence the characteristics of the detector very differently, with increases in proximity barrier energy leading to higher responsivity and lower dark current, while remote barriers reduce the responsivity and dark currents simultaneously. It is demonstrated that confinement enhancing barriers as proximity barriers optimize the SNR at low bias range, suitable for focal plane array applications.

Metamaterial-based imaging for potential security applications

In this paper, we present two different types of THz spatial light modulators (SLMs) that use dynamic metamaterials (MMs) to enable multiplex imaging. One imaging setup consists of a doped semiconducting MM as the SLM, with multi-color super-pixels composed of arrays of electronically controlled metamaterial absorbers (MMAs). Our device is capable of modulation of THz radiation at frequencies up to 12 MHz with maximum modulation depths over 50%. We have also implemented a different system enabling high resolution, high-fidelity, multiplex single pixel THz imaging. We use optical photoexcitation of semiconductors to dynamically tune the electromagnetic properties of MMs. By copropagating a THz and collimated optical laser beam through a high-resistivity silicon (Si) wafer with a MM patterned on the surface, we may modify the THz transmission in real-time by modifying the optical power. By further encoding a spatial pattern on the optical beam, with a digital micro-mirror device (DMD), we may write masks for THz radiation.

Backside Illuminated Infrared Detectors with Plasmonic Resonators

Next generation infrared photodetector technology will require focal plane array (FPA) systems that have multi-spectral imaging capabilities. One proposed approach to realizing these multicolor devices is to use plasmonic resonators. However, device development and characterization are commonly addressed with large front side illuminated single pixel detectors on a supporting epitaxial substrate. The focal plane arrays on the other hand are backside illuminated. Moreover, in a front side illuminated device, there is significant substrate scattering of the incident light. Here, we propose a method for the accurate measurement of device performance by using a hybridized chip design (hybrid chip) that is similar to the fabrication of an FPA system, with the substrate completely removed through a combination of mechanical polishing and subsequent wet etching techniques. The hybrid chip was also designed to precisely characterize the effects of varying mesa size by incorporating square mesa structures that range from 25 to 200 μm in width. This approach offers an advantage over conventional device characterization because it incorporates mesas that are on the same scale as those normally used in FPA systems, which should therefore provide a fast transition of new photodetector technology into camera based systems. The photodetector technology chosen for this work is a multi-stack quantum dots-in-a-well (DWELL) structure designed to absorb electromagnetic radiation in the mid-infrared spectral range.

Mid-wave infrared interband cascade photodetectors and focal plane arrays

In recent years, type-II InAs/GaSb superlattices (T2-SLs) have demonstrated dramatic advances and are a serious contender for the high performance infrared (IR) imaging market. The improved understanding of the material properties, as well as the implantation of advanced device architectures, has substantially improved the device performance. Here we will report our efforts to develop mid-IR type-II T2-SL photodetectors and focal plane arrays based on interband cascade structure. The interband cascade photodetector exploits the energy band alignment in the nearly lattice-matched “6.1-Å-family” (InAs, GaSb, AlSb, and their alloys) material system. The InAs/GaSb T2-SL is adopted as the absorber, and two unipolar barriers are placed at each side of the absorber.

Coded and compressive THz imaging with metamaterials

Imaging in long wavelength regimes holds huge potential in many fields, from security to skin cancer detection. However, it is often difficult to image at these frequencies – the so called ‘THz gap1’ is no exception. Current techniques generally involve mechanically raster scanning a single detector to gain spatial information2, or utilization of a THz focal plane array (FPA)3. However, raster scanning results in slow image acquisition times and FPAs are relatively insensitive to THz radiation, requiring the use of high powered sources. In a different approach, a single pixel detector can be used in which radiation from an object is spatially modulated with a coded aperture to gain spatial information. This multiplexing technique has not fully taken off in the THz regime due to the lack of efficient coded apertures, or spatial light modulators (SLMs), that operate in this regime. Here we present the implementation of a single pixel THz camera using an active SLM. We use metamaterials to create an electronically controllable SLM, permitting the acquisition of high-fidelity THz images. We gain a signal-to-noise advantage over raster scanning schemes through a multiplexing technique4. We also use a source that is orders of magnitude lower in power than most THz FPA implementations3,5. We are able to utilize compressive sensing algorithms to reduce the number of measurements needed to reconstruct an image, and hence increase our frame rate to 1 Hz. This first generation device represents a significant step towards the realization of a single pixel THz camera.

Ultra-thin infrared metamaterial detector for multicolor imaging applications

The next generation of infrared imaging systems requires control of fundamental electromagnetic processes - absorption, polarization, spectral bandwidth - at the pixel level to acquire desirable information about the environment with low system latency. Metamaterial absorbers have sparked interest in the infrared imaging community for their ability to enhance absorption of incoming radiation with color, polarization and/or phase information. However, most metamaterial-based sensors fail to focus incoming radiation into the active region of a ultra-thin detecting element, thus achieving poor detection metrics. Here our multifunctional metamaterial absorber is directly integrated with a novel mid-wave infrared (MWIR) and long-wave infrared (LWIR) detector with an ultra-thin (~λ/15) InAs/GaSb Type-II superlattice (T2SL) interband cascade detector. The deep sub-wavelength metamaterial detector architecture proposed and demonstrated here, thus significantly improves the detection quantum efficiency (QE) and absorption of incoming radiation in a regime typically dominated by Fabry-Perot etalons. Our work evinces the ability of multifunctional metamaterials to realize efficient wavelength selective detection across the infrared spectrum for enhanced multispectral infrared imaging applications.

Electronic and Thermally Tunable Infrared Metamaterial Absorbers

In this paper, we report a computational and experimental study using tunable infrared (IR) metamaterial absorbers (MMAs) to demonstrate frequency tunable (7%) and amplitude modulation (61%) designs. The dynamic tuning of each structure was achieved through the addition of an active material—liquid crystals (LC) or vanadium dioxide (VO2)--within the unit cell of the MMA architecture. In both systems, an applied stimulus (electric field or temperature) induced a dielectric change in the active material and subsequent variation in the absorption and reflection properties of the MMA in the mid- to long-wavelength region of the IR (MWIR and LWIR, respectively). These changes were observed to be reversible for both systems and dynamic in the LC-based structure.

Metamaterial-based single pixel imaging system (Presentation Recording)

Single pixel cameras are useful imaging devices where it is difficult or infeasible to fashion focal plan arrays. For example in the Far Infrared (FIR) it is difficult to perform imaging by conventional detector arrays, owing to the cost and size of such an array. The typical single pixel camera uses a spatial light modulator (SLM) - placed in the conjugate image plane – and is used to sample various portions of the image. The spatially modulated light emerging from the SLM is then sent to a single detector where the light is condensed with suitable optics for detection. Conventional SLMs are either based on liquid crystals or digital mirror devices. As such these devices are limited in modulation speeds of order 30 kHz. Further there is little control over the type of light that is modulated. We present metamaterial based spatial light modulators which provide the ability to digitally encode images – with various measurement matrix coefficients – thus permitting high speed and fidelity imaging capability. In particular we use the Hadamard matrix and related S-matrix to encode images for single pixel imaging. Metamaterials thus permit imaging in regimes of the electromagnetic spectrum where conventional SLMs are not available. Additionally, metamaterials offer several salient features that are not available with commercial SLMs. For example, metamaterials may be used to enable hyperspectral, polarimetric, and phase sensitive imaging. We present the theory and experimental results of single pixel imaging with digital metamaterials in the far infrared and highlight the future of this exciting field.