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8The 30th International Conference on Advanced Laser Technologies N-I-14
ALT'23
Plasmonic-enhanced THz emission in high-aspect-ratio metal grating photoconductive antennas
D. Ponomarev1'2, D. Lavrukhin1,2, A. Yachmenev1, R. Khabibullin1 Yu. Goncharov2, I. Spektor2, K. Zaytsev2
1-Institute of Ultra-High Frequency Semiconductor Electronics of the Russian Academy of Sciences, 117105 Moscow, Nagorny proezd 7, Russian Federation 2- Prokhorov General Physics Institute of the Russian Academy of Sciences, 119991 Moscow, 38 Vavilov st., Russian Federation
email address: ponomarev_dmitr@mail.ru
Photoconductive antenna-emitters (PCAs) are intensively used in THz time-domain spectroscopic and imaging setups thanks to their reliability, cost-effectiveness, simplicity of fabrication, and their flexibility in the designing of antenna electrodes and topology, as well as the choice of photoconductive substrate. Compared with the existing THz devices [1,2], PCAs can efficiently work at room temperature demonstrating a broadband spectrum of 0.1-5.0 THz with a perfect dynamic range, i.e. signal-to-noise ratio exceeding even 100 dB [3]. Recently, the Ge-based PCA-emitters have demonstrated an unprecedented bandwidth reaching 70 THz thanks to an absence of polar phonons in Ge [4]. Moreover, an optical-to-THz conversion efficiency of the PCA-emitters is not limited by the Manley-Rowe relation [5-7], as the photoconductivity theoretically allows converting every single optical photon into one electron-hole pair. The drastic issue that limits the PCA-emitter performance is its low conversion efficiency, i.e. only the minor part of the laser radiation is transferred to the THz waves, limiting overall emitted THz power. Many approaches featuring seminal pho-toconductor designs and antenna topology have been predicted and demonstrated their efficiency, nevertheless the progress is still essential. The physical problem is due to low laser light confinement at the elec-trode/photoconductor interface, as only the photocarriers generated in vicinity to the electrodes can contribute to the THz emission.
We have proposed the design of the PCA-emitter with a plasmonic grating featuring a very high plasmonic Au electrode with a thickness of h = 170 nm. As we show numerically, the increase in h significantly changes the electric field distribution, owing to the excitation of higher-order plasmon guided modes in the Au slit waveguides, leading to an additional increase in the emitted THz power. We developed the plasmonic grating geometry with respect to maximal transmission of the incident optical light, so as to expect the excitation of higher-order plasmon guided Au modes. The bow-tie PCA was characterized via our laboratory THz-TDS [6,7], and compared to the same photoconductive emitter but featuring a 100 nm-thick grating [8]. The both PCAs were fabricated on LT-GaAs, while a wrapped-dipole PCA TERA-8 (by Menlo Systems) was used as a THz PCA-detector. We showed that the fabricated high aspect ratio plasmonic PCA efficiently work with low-power laser excitation, demonstrating an overall THz power of 5.3 ^W over an ~4.0 THz bandwidth, corresponding to a conversion efficiency of 0.2% [9].
The work was supported the Russian Science Foundation, project 18-79-10195.
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