Abstract-Plasmonic heterodyne spectrometry for resolving the spectral signatures of ammonia over a 1-4.5 THz frequency range

Yen-Ju Lin, Semih Cakmakyapan, Ning Wang, Daniel Lee, Mitchell Spearrin, and Mona Jarrahi

 Schematic diagram of the terahertz spectrometry setup.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-25-36838

We present a heterodyne terahertz spectrometry platform based on plasmonic photomixing, which enables the resolution of narrow spectral signatures of gases over a broad terahertz frequency range. This plasmonic heterodyne spectrometer replaces the terahertz mixer and local oscillator of conventional heterodyne spectrometers with a plasmonic photomixer and a heterodyning optical pump beam, respectively. The heterodyning optical pump beam is formed by two continuous-wave, wavelength-tunable lasers with a broadly tunable terahertz beat frequency. This broadly tunable terahertz beat frequency enables spectrometry over a broad bandwidth, which is not restricted by the bandwidth limitations of conventional terahertz mixers and local oscillators. We use this plasmonic heterodyne spectrometry platform to resolve the spectral signatures of ammonia over a 1-4.5 THz frequency range.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Room-temperature heterodyne terahertz detection with quantum-level sensitivity

Ning Wang, Semih Cakmakyapan, Yen-Ju Lin, Hamid Javadi,  Mona Jarrahi,

         Fig. 1: Principles of heterodyne terahertz detection through plasmonic photomixing.

https://www.nature.com/articles/s41550-019-0828-6

Our Universe is most radiant at terahertz frequencies (0.1–10.0 THz), providing critical information on the formation of the planets, stars and galaxies, as well as the atmospheric constituents of the planets, their moons, comets and asteroids. The detection of faint fluxes of photons at terahertz frequencies is crucial for many planetary, cosmological and astrophysical studies. For example, understanding the physics and molecular chemistry of the life cycle of stars and their relationship with the interstellar medium in galaxies requires heterodyne detectors with noise temperatures close to the quantum limit. Near-quantum-limited heterodyne terahertz detection has so far been possible only through the use of cryogenically cooled superconducting mixers as frequency downconverters. Here we introduce a heterodyne terahertz detection scheme that uses plasmonic photomixing for frequency downconversion to offer quantum-level sensitivities at room temperature. Frequency downconversion is achieved by mixing terahertz radiation and a heterodyning optical beam with a terahertz beat frequency in a plasmonics-enhanced semiconductor active region. We demonstrate terahertz detection sensitivities down to three times the quantum limit at room temperature. With a versatile design capable of broadband operation over a 0.1–5.0 THz bandwidth, this plasmonic photomixer has broad applicability to astronomy, cosmology, atmospheric studies, gas sensing and quantum optics.

Abstract-Gold-patched graphene nano-stripes for high-responsivity and ultrafast photodetection from the visible to infrared regime

Semih Cakmakyapan, Ping Keng Lu, Aryan Navabi, Mona Jarrahi

https://www.nature.com/articles/s41377-018-0020-2

Graphene is a very attractive material for broadband photodetection in hyperspectral imaging and sensing systems. However, its potential use has been hindered by tradeoffs between the responsivity, bandwidth, and operation speed of existing graphene photodetectors. Here, we present engineered photoconductive nanostructures based on gold-patched graphene nano-stripes, which enable simultaneous broadband and ultrafast photodetection with high responsivity. These nanostructures merge the advantages of broadband optical absorption, ultrafast photocarrier transport, and carrier multiplication within graphene nano-stripes with the ultrafast transport of photocarriers to gold patches before recombination. Through this approach, high-responsivity operation is realized without the use of bandwidth-limiting and speed-limiting quantum dots, defect states, or tunneling barriers. We demonstrate high-responsivity photodetection from the visible to infrared regime (0.6 A/W at 0.8 μm and 11.5 A/W at 20 μm), with operation speeds exceeding 50 GHz. Our results demonstrate improvement of the response times by more than seven orders of magnitude and an increase in bandwidths of one order of magnitude compared to those of higher-responsivity graphene photodetectors based on quantum dots and tunneling barriers.

Abstract-A High-Power Broadband Terahertz Source Enabled by Three-Dimensional Light Confinement in a Plasmonic Nanocavity

Nezih Tolga Yardimci, Semih Cakmakyapan, Soroosh Hemmati,  Mona Jarrahi,

https://www.nature.com/articles/s41598-017-04553-4

The scope and potential uses of time-domain terahertz imaging and spectroscopy are mainly limited by the low optical-to-terahertz conversion efficiency of photoconductive terahertz sources. State-of-the-art photoconductive sources utilize short-carrier-lifetime semiconductors to recombine carriers that cannot contribute to efficient terahertz generation and cause additional thermal dissipation. Here, we present a novel photoconductive terahertz source that offers a significantly higher efficiency compared with terahertz sources fabricated on short-carrier-lifetime substrates. The key innovative feature of this source is the tight three-dimensional confinement of the optical pump beam around the terahertz nanoantennas that are used as radiating elements. This is achieved by means of a nanocavity formed by plasmonic structures and a distributed Bragg reflector. Consequently, almost all of the photo-generated carriers can be routed to the terahertz nanoantennas within a sub-picosecond time-scale. This results in a very strong, ultrafast current that drives the nanoantennas to produce broadband terahertz radiation. We experimentally demonstrate that this terahertz source can generate 4 mW pulsed terahertz radiation under an optical pump power of 720 mW over the 0.1–4 THz frequency range. This is the highest reported power level for terahertz radiation from a photoconductive terahertz source, representing more than an order of magnitude of enhancement in the optical-to-terahertz conversion efficiency compared with state-of-the-art photoconductive terahertz sources fabricated on short-carrier-lifetime substrates.

Abstract-High responsivity and bias-free graphene photodetector with nano-grating contact electrodes

Semih Cakmakyapan and Mona Jarrahi

https://www.osapublishing.org/abstract.cfm?uri=CLEO_QELS-2018-FTh4H.3

We present a high responsivity and bias-free graphene photodetector with responsivity levels as high as 225 mA/W at 800 nm wavelength, which uses nano-grating contact electrodes to enhance optical absorption and photocarrier extraction.

© 2018 The Author(s)

Abstract-Plasmonic Heterodyne Terahertz Spectrometry

Ning Wang, Semih Cakmakyapan, Mona Jarrahi,

https://www.osapublishing.org/abstract.cfm?uri=CLEO_SI-2018-SW4D.2

We demonstrate a new class of terahertz spectrometers that use plasmonic photomixers as frequency downconverters to offer quantum-level sensitivity at room temperature. We demonstrated double-sideband noise temperatures of 120–270 K (10hv/k -2hv/k) over 0.1–2 THz, equivalent to 10-2 photons.

© 2018 The Author(s)

Abstract-Reconfigurable metamaterials for terahertz wave manipulation

Mohammed R Hashemi, Semih Cakmakyapan and Mona Jarrahi

http://iopscience.iop.org/article/10.1088/1361-6633/aa77cb/meta

Reconfigurable metamaterials have emerged as promising platforms for manipulating the spectral and spatial properties of terahertz waves without being limited by the characteristics of naturally existing materials. Here, we present a comprehensive overview of various types of reconfigurable metamaterials that are utilized to manipulate the intensity, phase, polarization, and propagation direction of terahertz waves. We discuss various reconfiguration mechanisms based on optical, electrical, thermal, and mechanical stimuli while using semiconductors, superconductors, phase-change materials, graphene, and electromechanical structures. The advantages and disadvantages of different reconfigurable metamaterial designs in terms of modulation efficiency, modulation bandwidth, modulation speed, and system complexity are discussed in detail.

Abstract-Three-dimensional plasmonic light concentrators for efficient terahertz generation

Nezih Tolga Yardimci,  Semih Cakmakyapan, Soroosh Hemmati, Mona Jarrahi

http://ieeexplore.ieee.org/document/7999516/

Photoconductive antennas are extensively used in time-domain terahertz imaging and spectroscopy systems to generate terahertz radiation [1, 2]. These emitters consist of a terahertz antenna fabricated on a photoconductive semiconductor. When the semiconductor is pumped by a femtosecond laser and a bias voltage is applied to the antenna arms, an ultrafast photocurrent is generated. As this photocurrent drives the antenna, a pulsed terahertz radiation is generated. However, only the carriers that drift to the antenna arms in a sub-picosecond time scale can efficiently contribute to the generation of terahertz radiation. The rest of the photocarriers, namely the slow photocarriers, cause extra thermal dissipation and degrade device reliability. To improve device reliability, short carrier lifetime semiconductors are often used, which recombine the slow carriers and prevent early thermal breakdown. However, short carrier lifetime semiconductors cannot offer high carrier drift velocities. Therefore, the radiation efficiency of photoconductive emitters fabricated on short carrier lifetime substrates is limited. In this work, we present a highly reliable and efficient photoconductive terahertz emitter that circumvents the use of short carrier lifetime substrates by utilizing three-dimensional plasmonic light concentrators.