A Coherent Tunable Smith-Purcell Radiator for the Far Infrared

Dartmouth College, Hanover NH

Investigators

Abstract

0070491 Walsh The purpose of the project is to develop a device which is defined by the term "grating coupled oscillator" (GCO) [1,2]. The GCO is a tunable source of coherent radiation that, we anticipate, can provide power for spectroscopic investigations and function as a local oscillator over the entire THz-FIR range of the spectrum (0.3-30 THz). For nearly a century the FIR spectral regime has been relatively under-exploited, largely due to the lack of tunable, coherent sources. Despite this difficulty, the importance of a broad range of scientific questions extending from biophysics and condensed matter to plasma physics and radio astronomy have motivated research in the FIR spectral region. The unique characteristics of the GCO will add a powerful new tool to the arsenal of techniques used in these investigations. The GCO is a novel adaptation of an old idea. The essential components are a very "bright" electron beam and a diffraction grating. Smith and Purcell [3] first described experiments with electron beams moving over a grating nearly fifty years ago. A single electron passing over a grating induces a surface current footprint which in turn produces a radiative wake. The response of the grating to the passage of the electron is coherent in the same sense that a dielectric material will respond coherently to the passage of a fast particle. A diffuse beam of electrons passing over a grating will generate a total signal that is an incoherent superposition of the contributions from each electron. In this limit, the radiated power scales only linearly with beam current and the total power produced is modest. When the beam current density exceeds a critical value, distributed feedback on the grating will cause individual electron contributions to also add coherently. The power available increases dramatically. In experiments to date, this "start-oscillation" threshold has been crossed at frequencies up to 1.5 THz. The GCO is superior to the other available FIR sources in several respects. First, the use of a free electron beam avoids the bulk material response which limits the spectral range of solid state devices. Second, contrary to conventional microwave tube engineering, the use of bright, low current electron beams with a suitably low loss open resonator (grating) structure surmounts all four impediments that limit the tuning range of conventional electron tubes to less than 1 THz. As early as fifty years ago [4-6], these impediments had been identified as: (1) limits set by the need for precision in fabrication, (2) thermal stability, (3) "circuit" losses, and (4) the rapid increase of the start current density with the operating frequency. Lastly, a complete GCO will be smaller than a briefcase, without cryogenics or intricate supporting hardware. Theoretical estimates of the GCO output power are 10's of mW (CW) with efficiency exceeding 0.01. The operating frequency range is limited only by the quality of the electron beam. Current GCO output power and efficiency below 1 THz are 100 nW and 10-7 respectively. The proposed course of research aims to lower the start current, thereby increasing the power and operating frequency, through improved electron beam quality and grating coupling efficiency. This goal requires extensive theoretical and experimental studies to achieve the anticipated >1000 increase in radiated and resolving powers. In current experiments, a modified scanning electron microscope (SEM) generates the driving beam. This SEM system will be improved to produce a much brighter electron beam. The signal collection optics and instrumentation will be developed in order to facilitate more precise monitoring of the experimental conditions. The grating resonator design will be investigated experimentally and theoretically to reduce losses, in order to lower the start oscillation threshold, and enhance the output coupling efficiency. The theory of GCO operation will be developed further to understand the dependence of output power on current. The final goal of the project is to operate the SEM with compact dc-dc converter based high voltage supplies and a single tip field emission cathode, thus providing proof of principle operation of a miniature GCO. ***