Raji Shankar

Electrically Tunable Metasurface Perfect Absorbers for Ultrathin Mid-Infrared Optical Modulators

Dynamically reconfigurable metasurfaces open up unprecedented opportunities in applications such as high capacity communications, dynamic beam shaping, hyperspectral imaging, and adaptive optics. The realization of high performance metasurface-based devices remains a great challenge due to very limited tuning ranges and modulation depths. Here we show that a widely tunable metasurface composed of optical antennas on graphene can be incorporated into a subwavelength-thick optical cavity to create an electrically tunable perfect absorber. By switching the absorber in and out of the critical coupling condition via the gate voltage applied on graphene, a modulation depth of up to 100% can be achieved. In particular, we demonstrated ultrathin (thickness < λ0/10) high speed (up to 20 GHz) optical modulators over a broad wavelength range (5-7 μm). The operating wavelength can be scaled from the near-infrared to the terahertz by simply tailoring the metasurface and cavity dimensions.

High-Responsivity Mid-Infrared Graphene Detectors with Antenna-Enhanced Photocarrier Generation and Collection

Graphene is an attractive photoconductive material for optical detection due to its broad absorption spectrum and ultrashort response time. However, it remains a great challenge to achieve high responsivity in graphene detectors because of graphenes weak optical absorption (only 2.3% in the monolayer graphene sheet) and short photocarrier lifetime (<1 ps). Here we show that metallic antenna structures can be designed to simultaneously improve both light absorption and photocarrier collection in graphene detectors. The coupled antennas concentrate free space light into the nanoscale deep-subwavelength antenna gaps, where the graphene light interaction is greatly enhanced as a result of the ultrahigh electric field intensity inside the gap. Meanwhile, the metallic antennas are designed to serve as electrodes that collect the generated photocarriers very efficiently. We also elucidate the mechanism of photoconductive gain in the graphene detectors and demonstrate mid-infrared (mid-IR) antenna-assisted graphene detectors at room temperature with more than 200 times enhancement of responsivity (∼0.4 V/W at λ0 = 4.45 μm) compared to devices without antennas (<2 mV/W).

Wide Wavelength Tuning of Optical Antennas on Graphene with Nanosecond Response Time

Graphene is emerging as a broadband optical material which can be dynamically tuned by electrostatic doping. However, the direct application of graphene sheets in optoelectronic devices is challenging due to graphenes small thickness and the resultant weak interaction with light. By combining metal and graphene in a hybrid plasmonic structure, it is possible to enhance graphene-light interaction and thus achieve in situ control of the optical response. We show that the effective mode index of the bonding plasmonic mode in metal-insulator-metal (MIM) waveguides is particularly sensitive to the change in the optical conductivity of a graphene layer in the gap. By incorporating such MIM structures in optic antenna designs, we demonstrate an electrically tunable coupled antenna array on graphene with a large tuning range (1100 nm, i.e., 250 cm(-1), nearly 20% of the resonance frequency) of the antenna resonance wavelength at the mid-infrared (MIR) region. Our device exhibits a 3 dB cutoff frequency of 30 MHz, which can be further increased into the gigahertz range. This study confirms that hybrid metal-graphene structures are promising elements for high-speed electrically controllable optical and optoelectronic devices.

Mid-infrared photonic crystal cavities in silicon

We demonstrate the design, fabrication, and characterization of silicon photonic crystal cavities realized in a silicon on insulator (SOI) platform, operating at a wavelength of 4.4 μm with a quality factor of 13,600. Cavity modes are imaged using the technique of scanning resonant scattering microscopy. To the best of our knowledge, this is the first demonstration of photonic devices fabricated in SOI and operating in the 4-5 μm wavelength range.

Integrated high-quality factor silicon-on-sapphire ring resonators for the mid-infrared

We demonstrate high-quality (Q) factor grating-coupled ring resonators in a silicon-on-sapphire platform, operating at wavelengths between 4.3 and 4.6 μm. Total Q-factors of 151 000 and intrinsic Q-factors of 278 000 are measured, representing the highest Q-factors measured at the mid-infrared in Si.

Study of thermally-induced optical bistability and the role of surface treatments in Si-based mid-infrared photonic crystal cavities

We report the observation of optical bistability in Si-based photonic crystal cavities operating around 4.5 µm. Time domain measurements indicate that the source of this optical bistability is thermal, with a time constant on the order of 5 µs. Quality (Q) factor improvement is shown by the use of surface treatments (wet processes and annealing), resulting in a significant increase in Q-factor, which in our best devices is on the order of ~45,000 at 4.48 µm. After annealing in a N(2) environment, optical bistability is no longer seen in our cavities.

Sensing nitrous oxide with QCL-coupled silicon-on-sapphire ring resonators.

We report the initial evaluation of a mid-infrared QCL-coupled silicon-on-sapphire ring resonator gas sensor. The device probes the N(2)O 2241.79 cm(-1) optical transition (R23 line) in the ν(3) vibrational band. N(2)O concentration is deduced using a non-linear least squares fit, based on coupled-mode theory, of the change in ring resonator Q due to gas absorption losses in the evanescent portion of the waveguide optical mode. These early experiments demonstrated response to 5000 ppmv N(2)O.

Silicon photonic devices for mid-infrared applications

Abstract The mid-infrared (IR) wavelength region (2–20 µm) is of great interest for a number of applications, including trace gas sensing, thermal imaging, and free-space communications. Recently, there has been significant progress in developing a mid-IR photonics platform in Si, which is highly transparent in the mid-IR, due to the ease of fabrication and CMOS compatibility provided by the Si platform. Here, we discuss our group’s recent contributions to the field of silicon-based mid-IR photonics, including photonic crystal cavities in a Si membrane platform and grating-coupled high-quality factor ring resonators in a silicon-on-sapphire (SOS) platform. Since experimental characterization of microphotonic devices is especially challenging at the mid-IR, we also review our mid-IR characterization techniques in some detail. Additionally, pre- and post-processing techniques for improving device performance, such as resist reflow, Piranha clean/HF dip cycling, and annealing are discussed.

Progress toward mid-IR chip-scale integrated-optic TDLAS gas sensors

We are building prototype chip-scale low-power integrated-optic gas-phase chemical sensors based on mid-infrared (3-5μm) Tunable Diode Laser Absorption Spectroscopy (TDLAS). TDLAS is able to sense many gas phase chemicals with high sensitivity and selectivity. Novel gas sensing elements using low-loss resonant photonic crystal cavities or waveguides will permit compact integration of a laser source, sampling elements, and detector in configurations suitable for inexpensive mass production. Recently developed Interband Cascade Lasers (ICLs) that operate at room temperature with low power consumption are expected to serve as monochromatic sources to probe the mid-IR molecular spectral transitions. Practical challenges to fabricating these sensors include: a) selecting and designing the high-Q microresonator sensing element appropriate for the selected analyte; b) coupling laser light into and out of the sensing element; and c) device thermal management, especially stabilizing laser temperature with the precision needed for sensitive spectroscopic detection. This paper describes solutions to these challenges.

Ultra-Compact Mid-IR Modulators Based on Electrically Tunable Optical Antennas

We demonstrated electrical tuning of optical antennas over a broad wavelength range (~1100 nm, ~20% of the resonance frequency) and achieved optical modulators at the mid-infrared wavelength range with nanosecond response time.