3 research outputs found
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 graphene’s 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 λ<sub>0</sub> = 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 graphene’s 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<sup>–1</sup>, 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
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 <
λ<sub>0</sub>/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