Sungho Kang

Graphene–aluminum nitride NEMS resonant infrared detector

The use of micro-/nanoelectromechanical resonators for the room temperature detection of electromagnetic radiation at infrared frequencies has recently been investigated, showing thermal detection capabilities that could potentially outperform conventional microbolometers. The scaling of the device thickness in the nanometer range and the achievement of high infrared absorption in such a subwavelength thickness, without sacrificing the electromechanical performance, are the two key challenges for the implementation of fast, high-resolution micro-/nanoelectromechanical resonant infrared detectors. In this paper, we show that by using a virtually massless, high-electrical-conductivity, and transparent graphene electrode, floating at the van der Waals separation of a few angstroms from a piezoelectric aluminum nitride nanoplate, it is possible to implement ultrathin (460 nm) piezoelectric nanomechanical resonant structures with improved electromechanical performance (>50% improved frequency×quality factor) and infrared detection capabilities (>100× improved infrared absorptance) compared with metal-electrode counterparts, despite their reduced volumes. The intrinsic infrared absorption capabilities of a submicron thin graphene–aluminum nitride plate backed with a metal electrode are investigated for the first time and exploited for the first experimental demonstration of a piezoelectric nanoelectromechanical resonant thermal detector with enhanced infrared absorptance in a reduced volume. Moreover, the combination of electromagnetic and piezoelectric resonances provided by the same graphene–aluminum nitride-metal stack allows the proposed device to selectively detect short-wavelength infrared radiation (by tailoring the thickness of aluminum nitride) with unprecedented electromechanical performance and thermal capabilities. These attributes potentially lead to the development of uncooled infrared detectors suitable for the implementation of high performance, miniaturized and power-efficient multispectral infrared imaging systems.

Zero-power infrared digitizers based on plasmonically enhanced micromechanical photoswitches

State-of-the-art sensors use active electronics to detect and discriminate light, sound, vibration and other signals. They consume power constantly, even when there is no relevant data to be detected, which limits their lifetime and results in high costs of deployment and maintenance for unattended sensor networks. Here we propose a device concept that fundamentally breaks this paradigm-the sensors remain dormant with near-zero power consumption until awakened by a specific physical signature associated with an event of interest. In particular, we demonstrate infrared digitizing sensors that consist of plasmonically enhanced micromechanical photoswitches (PMPs) that selectively harvest the impinging electromagnetic energy in design-defined spectral bands of interest, and use it to create mechanically a conducting channel between two electrical contacts, without the need for any additional power source. Our zero-power digitizing sensor prototypes produce a digitized output bit (that is, a large and sharp off-to-on state transition with an on/off conductance ratio >1012 and subthreshold slope >9 dec nW-1) when exposed to infrared radiation in a specific narrow spectral band (∼900 nm bandwidth in the mid-infrared) with the intensity above a power threshold of only ∼500 nW, which is not achievable with any existing photoswitch technologies.

Narrowband MEMS resonant infrared detectors based on ultrathin perfect plasmonic absorbers

This paper reports on the demonstration of narrowband uncooled microelectromechanical resonant infrared detectors based on ultrathin perfect plasmonic absorbers. The integration of a deeply subwavelength thin (230 nm) plasmonic absorber on a piezoelectric nano plate (480 nm) enables a near perfect and narrowband absorption of mid-wavelength infrared radiation (∼92% at 4.7 μm with a full width at half maximum ∼800 nm) in an ultralow volume resonant thermal detector. The excellent thermal isolation (∼2.4×105 K/W) and the high electromechanical performance of the resonant thermal detector (quality factor, Q∼1527 and electromechanical coupling coefficient kt2 ∼1.74%) guarantee the achievement of high sensitivity and a low noise performance, resulting in the demonstration of a high resolution (noise equivalent power ∼633 pW/Hz12) narrowband infrared detectors suitable for infrared spectroscopy and multispectral imaging system.

NEMS infrared detectors based on high quality factor 50 nm thick AlN nano-plate resonators

This paper reports on the first demonstration of ultra-fast (thermal time constant, τ ∼80 μs) and high resolution (noise equivalent power, NEP ∼656 pW/Hz^1/2) infrared detectors based on high quality factor 50-nm thick aluminum nitride (AlN) piezoelectric resonant nano-plates. For the first time, we show that by employing nanoscale (30 nm) aluminum anchors, both high thermal resistance (R<inf>th</inf> ∼9.2×10⁵ K/W) and high quality factor (Q ∼1000) can be achieved in greatly scaled AlN nano-plate resonators. Furthermore, we experimentally demonstrate that the resonant structures are capable of efficiently absorbing short-to mid- wavelength infrared (absorption, η ∼47%, for blackbody radiation at 625 °C) without any additional absorber. These unique features are exploited for the first experimental demonstration of AlN NEMS resonant infrared detectors with highly reduced area (down to 20×22 μm², comparable to the ones of state-of-the-art microbolometers) and over 4x improved figure of merit (FoM=1/(NEP·τ)) compared to what previously achieved by the same technology.

Microelectromechanical detector of infrared spectral signatures with near-zero standby power consumption

This paper reports the first experimental demonstration of a near-zero power (<1 pW standby power consumption) detector of infrared (IR) spectral signatures. The proposed passive digitizer of IR spectral signatures is composed of 2 Plasmonically-enhanced MEMS Relays (PMRs) implementing a logic circuit. Unlike solid-state photodetectors, the PMRs exploit a plasmonically-enhanced thermomechanical coupling to selectively harvest the energy contained in an IR spectral band of interest itself and use it to mechanically create a conducting channel between the battery and the load without consuming any power in standby (i.e. while in the OFF state). In this work, we demonstrate for the first time, that by connecting in series 2 PMRs, tuned to different spectral bands (i.e. 4.1 μm and 5.6 μm), it is possible to implement a passive logic circuit capable of producing an output voltage (i.e. a quantized output bit that wakes up the active wireless sensor) only when exposed to the IR spectral signature associated to a target of interest (i.e. the exhaust plume of a vehicle). The event-driven sensing capability enabled by such a micro-scale IR digitizer can dramatically extend the battery life of wireless sensor nodes remotely deployed to detect infrequent but time critical events.