THz-photomixer based on quasi-ballistic transport

Abstract

We report on a novel concept for THz photomixers with high conversion efficiency up to several THz. In contrast to the conventional pin photomixer we can overcome the trade-off between either optimizing transit-time or RC-roll-off. Using quasi-ballistic transport in nano-pin-diodes the transport path can be optimized regarding both path length and transit time. Independently, the capacitance can be kept small by using a sufficiently large number of optimized nano-pin-diodes in series. The concept is presented in detail and first experimental results are reported which corroborate our theoretical expectations.

Tunable, continuous-wave Terahertz photomixer sources and applications

“…II D 5 and II D 6, uni-traveling-carrier (UTC) photodiodes 90 and n-i-pn-i-p superlattice photomixers with ballistic electron transport. 91 In both cases only electrons contribute to the current [strictly speaking, the fraction ð1 À Dz=2d i Þ], and the hole contribution (strictly speaking, the fraction Dz=2d i ) becomes negligible. For this reason, this design is expected to provide higher THz power at intermediate frequencies for the case m h < m e .…”

Enhancing the photomixing efficiency of optoelectronic devices in the terahertz regime

“…When coupled with velocity overshoot phenomenon reported in many photoconductor materials [12,16,17,18,19,20], t tr of majority of carriers is reduced to < 1 ps, i.e., t tr ≤ f −1 . For efficient use of SPPs, it is desirable to have thin (∼10 nm) normal metal lines with subwavelength features distributed throughout the active volume (Fig 1a) in a manner that would collectively enhance the optical field intensity in the vicinity of the metal lines.…”

“…In particular, to be used as a local oscillator in communications systems the THz source should produce stable output power > 10 µW [8,11]. Among the various techniques being pursued [6,8,11,12,13] electrical down conversion of optical microwave sources in a suitable photomixer [6,11,12] are appealing due to the easy availability of tunable, narrow-linewidth, and stable solid state lasers. However, until now the output power from these photomixers is limited to < 10 µW in the cru- cial THz range.…”

“…where i p is the photocurrent generated in the external circuit, c is velocity of light in free space, C is the capacitance, R S is small internal resistance of the metal structure, and R L is the load resistance in the 72-200 range [6,12,14]. To accelerate carriers generated deep inside the semiconductor, high dc voltage (V b ≈ 40 V) is applied across the electrodes.…”

“…In addition, subwavelength features of the metal lines produce strong near field diffraction patterns that are not taken into account in conventional designs [6,12,14].…”