Imaging techniques for a high-power THz free electron laser

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Abstract

Two imaging techniques based on the thermal effect have been developed and implemented for recording images using radiation of a high-power terahertz free electron laser. The techniques were applied for the visualization of images in experiments on classical optics, as well as in a holographic experiment.

Introduction

Imaging in the terahertz spectral range is a subject of special interest for many applications such as medicine, biology, industry, custom control, and other applications. Recently developed methods for terahertz imaging are based on the employment of the low-power THz sources with a narrowband or wideband spectrum. Due to a low power of these sources, the visualization of images requires the employment of rather sophisticated detectors, and, in most cases, recording of an image requires plenty of time. High power of our laser enables application for the visualization of laser radiation the methods, which, probably, sometime were used for intense visible and NIR radiation, but were never previously employed in the terahertz range. We employed for the visualization, two methods based on the thermal effect of intense terahertz radiation. Now, the Novosibirsk high-power free-electron laser based on microtron-recuperator generates electromagnetic radiation in the wavelength range 120–180 μm as a continuous sequence of 50-ps pulses following with the repetition rate of 5.6 MHz at the average power up to 200 W (the peak power—0.6 MW), the measured relative line width is 0.003 [1].

Section snippets

Visualization with a FIR-NIR converter

This technique, further referred as FNC, employs temperature growing of a thin-film screen exposed to terahertz radiation (Fig. 1). Two-dimensional field of temperature at the screen surface is recorded with a thermograph SVIT, which have a 128×128 InAs focal plain array (FPA) sensitive to the radiation within the spectral range of 2.6–3.1 μm. Thermograph sensitivity at the room temperature is 0.03 °C at the frame frequency up to 40 Hz. In our case, time resolution was restricted by thermal

Visualization with the thermo-optical detector

The other technique used for the visualization of terahertz images was the Thermo-Optical Detector (TOD). The technique employs the change of the optical length of a medium, which is transparent for probe visible light but is opaque for the terahertz radiation, when the medium is exposed to terahertz radiation. The change of the optical length within the expose time τ isΔS≡∂S∂tτ=Δ[∫Ln(z,T(t))dz]where z is the coordinate across the medium slab and T is a local temperature. The difference occurs

Classical optic experiments

Photon energy of the submillimeter radiation is about 10 meV. Most of phenomena in this spectral range can be described with excellent precision by the classical optics theory. To demonstrate high transverse and longitudinal coherence of the FEL radiation, we carried out a number of experiments on classical optics.

Conclusion

First experiments on the visualization of high-power THz radiation, based on the thermal effects and carried out on the Novosibirsk high power free electron laser facility, have demonstrated its great potentiality for employment in many applications.

Acknowledgement

This work was supported in part by the Siberian Branch of Russian Academy of Science (Grant 174/03). Experimental verification of the thermo-optical detector has been done on the CATRION facility at NSU (reg. #06-06) supported by the Ministry of Education and Science of Russian Federation.

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