Unbiased continuous wave terahertz photomixer emitters with dis-similar Schottky barriers

Abstract

We are introducing a new bias free CW terahertz photomixer emitter array. Each emitter consists of an asymmetric metal-semiconductor-metal (MSM) that is made of two side by side dis-similar Schottky contacts, on a thin layer of low temperature grown (LTG) GaAs, with barrier heights of difference (ΔΦ(B)) and a finite lateral spacing (s). Simulations show that when an appropriately designed structure is irradiated by two coherent optical beams of different center wavelengths, whose frequency difference (∆f) falls in a desired THz band, the built-in field between the two dis-similar potential barriers can accelerate the photogenerated carriers that are modulated by ∆ω, making each pitch in the array to act as a CW THz emitter, effectively. We also show the permissible values of s and ΔΦ(B) pairs, for which the strengths of the built-in electric field maxima fall below that of the critical of 50 V/μm- i.e., the breakdown limit for the LTG-GaAs layer. Moreover, we calculate the THz radiation power per emitter in an array. Among many potential applications for these bias free THz emitters their use in endoscopic imaging without a need for hazardous external biasing circuitry that reduces the patient health risk, could be the most important one. A hybrid numerical simulation method is used to design an optimum emitter pitch, radiating at 0.5 THz.

Nanoslit cavity plasmonic modes and built-in fields enhance the CW THz radiation in an unbiased antennaless photomixers array

“…A bias-free THz photomixer is a potentially attractive THz radiation source for biomedical applications. As has already been demonstrated experimentally [3] and theoretically [4], unlike the large aperture emitters [5], unbiased emitters with dissimilar Schottky contacts can efficiently accelerate photogenerated carriers. Moreover, unlike scalable microstructured photoconductive emitters [6,7], an unbiased antenna-free THz photomixer does not suffer from the short circuiting defects that likely occur in the gaps between adjacent fingers in interdigitated emitters and that ruin the entire device operation.…”

“…The total radiated power can be scaled up by the squared number of cells in the array in one dimension [4]. Employing a unit cell similar to that shown in Fig.…”

“…1, in that work, we have already demonstrated that as the unit cell pitch (Λ) of the unbiased antennaless THz photomixer emitters array is reduced solely by reducing the slit (g) between two adjacent MSM strips, the THz dipole radiation also decreases. Details of the unit cell constituents and its electrostatics are given in [4].…”

“…The MSM is made of a pair of parallel but dissimilar metallic strips (M 1 and M 2 ) that make a pair of dissimilar Schottky barriers, Φ B1 > Φ B2 , on LTG-GaAs. The lateral spacing between the two dissimilar contacts is filled with a SiO 2 stripe of width s. Permissible values for both s and ΔΦ B Φ 1 − Φ 2 , for which the maximum strengths of the built-in electric fields are below the critical value of 50 V/μm for breaking down the LTG-GaAs [15], are given in [4]. As also seen in Fig.…”

“…In simulating the optical characteristics of the emitters, a periodic boundary condition is applied at both unit cell boundaries, along the x direction in the simulation domain. Moreover, employing the finite element method (FEM) in the time domain and following the same procedure as reported in [4], the THz photocurrent inside the photoconductor is modeled. In this numerical simulation, a drift-diffusion model, including various generation mechanisms, is considered, and appropriate boundary conditions for the Schottky metal-semiconductor interfaces and semiconductor-insulator interfaces along the outer boundaries are also applied.…”