Ultrasensitive Transmissive Infrared Spectroscopy via Loss Engineering of Metallic Nanoantennas for Compact Devices.

Miniaturized infrared spectroscopy is highly desired for widespread applications, including environment monitoring, chemical analysis, and biosensing. Nanoantennas, as a promising approach, feature strong field enhancement and provide opportunities for ultrasensitive molecule detection even in the nanoscale range. However, current efforts for higher sensitivities by nanogaps usually suffer a trade-off between the performance and fabrication cost. Here, novel crooked nanoantennas are designed with a different paradigm based on loss engineering to overcome the above bottleneck. Compared to the commonly used straight nanoantennas, the crooked nanoantennas feature higher sensitivity and a better fabrication tolerance. Molecule signals are increased by 25 times, reaching an experimental enhancement factor of 2.8 × 104. The optimized structure enables a transmissive CO2 sensor with sensitivities up to 0.067% ppm-1. More importantly, such a performance is achieved without sub-100 nm structures, which are common in previous works, enabling compatibility with commercial optical lithography. The mechanism of our design can be explained by the interplay of radiative and absorptive losses of nanoantennas that obeys the coupled-mode theory. Leveraging the advantage of the transmission mode in an optical system, our work paves the way toward cheap, compact, and ultrasensitive infrared spectroscopy.

A Black Phosphorus Carbide Infrared Phototransistor.

Photodetectors with broadband detection capability are desirable for sensing applications in the coming age of the internet-of-things. Although 2D layered materials (2DMs) have been actively pursued due to their unique optical properties, by far only graphene and black arsenic phosphorus have the wide absorption spectrum that covers most molecular vibrational fingerprints. However, their reported responsivity and response time are falling short of the requirements needed for enabling simultaneous weak-signal and high-speed detections. Here, a novel 2DM, black phosphorous carbide (b-PC) with a wide absorption spectrum up to 8000 nm is synthesized and a b-PC phototransistor with a tunable responsivity and response time at an excitation wavelength of 2004 nm is demonstrated. The b-PC phototransistor achieves a peak responsivity of 2163 A W-1 and a shot noise equivalent power of 1.3 fW Hz-1/2 at 2004 nm. In addition, it is shown that a response time of 0.7 ns is tunable by the gating effect, which renders it versatile for high-speed applications. Under the same signal strength (i.e., excitation power), its performance in responsivity and detectivity in room temperature condition is currently ahead of recent top-performing photodetectors based on 2DMs that operate with a small bias voltage of 0.2 V.