Abstract

Transition radiation was first proposed in 1945.¹ Recently it was shown that superlattices consisting of e.g. Zn/Si alternating layers could be used for efficient generation of extreme-UV and soft X-ray waves based on transition radiation.² In this paper, for the first time to the best of our knowledge, we show that it is possible to use an electron beam to pump multilayers for efficiently generating narrow-Iinewidth THz waves based on transition radiation from each interface. There are a few advantages of using dielectric layers for THz generation First of all, in the THz domain, dielectric constants between GaAs and GaP or Si and Ge are quite different each other. Therefore, the emitted THz power can be large. Secondly, these layers can be diffusion-bonded together to form multilayers or epitaxially grown to form superlattices. However, design of an optimized multilayer structure is very complicated. Consider multilayers shown in Fig. 1. The resonant emission condition is $\sqrt{\overline{\epsilon }}\cos \theta =c/\nu -\lambda /l$, where $\overline{\epsilon }$ is the average dielectric constant, θ is the THz emission angle measured with respect to the direction of the electron velocity, λ is the emission wavelength, and l is the period of a multilayer structure. There is a critical value for the electron speed, β_cr = ν_cr/c, in order to maintain the resonant emission condition. Consider Ge/Si multilayers with λ_p ≈ 567 μm (the effective plasma wavelength), $\overline{\epsilon }\approx 17$, θ = 45°, and l = 546 μm, one obtains β_cr ≈ 0.34. Therefore, the kinetic energy is about 32 keV. This value is remarkably low. With increasing the period or decreasing emission wavelength, one has to increase the electron speed or kinetic energy to maintain the resonant emission condition. Due to interferences of THz beams from each interface, the output THz beam has a narrow linewidth (Fig. 2). Consider n-doped Ge/Si multilayers with their thickness of about 273 μm and designed output wavelength of 200 μm. The total number of pairs is 28 and electron doping density is 2.4×10¹⁶ cm⁻³. If the structure is pumped by an electron beam with a current of 1 mA and kinetic energy of 32 keV, the total THz power emitted is about 2.4 mW. This corresponds to a conversion efficiency of 0.0075%. Therefore, this new method is very efficient for generating narrow-Iinewidth THz waves. This work is supported by AFOSR.

© 2000 IEEE