High power telecommunication-compatible photoconductive terahertz emitters based on plasmonic nano-antenna arrays

Abstract

We present a high-power and broadband photoconductive terahertz emitter operating at telecommunication optical wavelengths, at which compact and high-performance fiber lasers are commercially available. The presented terahertz emitter utilizes an ErAs:InGaAs substrate to achieve high resistivity and short carrier lifetime characteristics required for robust operation at telecommunication optical wavelengths. It also uses a two-dimensional array of plasmonic nano-antennas to offer significantly higher optical-to-terahertz conversion efficiencies compared to the conventional photoconductive emitters, while maintaining broad operation bandwidths. We experimentally demonstrate pulsed terahertz radiation over 0.1-5 THz frequency range with the power levels as high as 300 lW. This is the highest-reported terahertz radiation power from a photoconductive emitter operating at telecommunication optical wavelengths. Published by AIP Publishing. Photoconductive emitters are widely used for generating sub-picosecond pulses in the time-domain terahertz imaging and spectroscopy systems.1-6 Telecommunication wavelengths (e.g., 1550 nm) are one of the most desired regimes for pumping photoconductive emitters due to the availability of high-performance, low-cost and compact fiber lasers. 7,8 However, the performance of previously demonstrated photoconductive terahertz emitters operating at telecommunication wavelengths has been limited by low resistivity of photo-absorbing semiconductor substrates at these wavelengths (e.g., InGaAs). This is because efficient operation of photoconductive terahertz emitters requires accelerating photo-generated carriers by high bias electric fields, which results in high dark currents and thermal breakdown when using low resistivity substrates.Various different techniques have been proposed to overcome the low resistivity limitation of photo-absorbing substrates at telecommunication wavelengths.9-12 One of the most promising solutions is the use of a superlattice of monolayers of ErAs in an InGaAs substrate. Since the ErAs layers act as deep recombination centers inside the superlattice, the resistivity and carrier lifetime of such superlattice structures can be controlled by the number and thickness of ErAs layers. [13][14][15][16] Although the high resistivity and short carrier lifetime characteristics of ErAs:InGaAs substrates can enable a more reliable device operation at telecommunication wavelengths, the radiation power of previously demonstrated ErAs:InGaAs photoconductive terahertz emitters operating at 1550 nm pump wavelengths has been limited by low quantum efficiency of conventional ultrafast photoconductors.In this work, we present a large-area photoconductive terahertz emitter based on a two-dimensional array of plasmonic nano-antennas fabricated on an ErAs:InGaAs substrate. The use of plasmonic nano-antennas allows absorption and concentration of the majority of incident pump photons in close proximity to the nano-antennas. As a result, most of the photo-generated carriers drift to t...

Self-triggered Asynchronous Optical Sampling Terahertz Spectroscopy using a Bidirectional Mode-locked Fiber Laser

“…Both the terahertz emitter and the detector have better performance under higher-power optical illumination 46 , 47 . Therefore, adding another amplification stage to the laser would help to increase the dynamic range and operation bandwidth of the ASOPS-THz-TDS system.…”

“…Each arm of the bidirectional mode-locked fiber laser generates femtosecond pulses with a 120 mW power at ~1556 nm center wavelength. One laser output is focused onto a plasmonic photoconductive nanoantenna emitter array fabricated on an ErAs:InGaAs substrate to generate pulsed terahertz radiation 46 . The generated terahertz radiation is routed and focused onto a plasmonic photoconductive nanoantenna detector array 47 fabricated on a low-temperature-grown GaAs substrate using two off-axis parabolic mirrors.…”

Metallic and dielectric metasurfaces in photoconductive terahertz devices: a review

“…The another approach is associated with the plasmonic nanoantenna arrays and can be used for generation 63,64 and detection 65 of the THz waves. The array of the plasmonic PCAs is able to significantly increase the emitted THz power by reducing thermal breakdown 66 and photocarrier screening 28 even at high energies of an optical pump.…”

“…Later in Ref. 64, a large-area PCA-emitter featuring an 0.5 × 0.5 mm 2 active area based on a 2-D array of plasmonic nanoantennas was proposed. The emitter utilizes an ErAs:InGaAs photoconductive substrate featuring very high-resistivity and ultrashort photocarrier lifetime.…”

Improved InGaAs and InGaAs/InAlAs Photoconductive Antennas Based on (111)-Oriented Substrates

“…The main development challenge with InGaAs-based photoconductors is relatively small bandgap resulting in low breakdown field strength and large dark background conductivity. Several methods for producing ultrafast InGaAs photoconductive structures have been proposed: ion implantation with heavy ions followed by thermal annealing treatment [8,9], epitaxial doping of InGaAs with impurities producing deep electronic traps in the bandgap [10,11], and growth of specially designed heterostructures such as ErAs inclusions in InGaAlAs [12]. The low-temperature (LT) InGaAs-based epitaxial structures are widely used in commercial THz systems.…”