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-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.