The interest in terahertz photometric and imaging measurements has motivated the development of bandpass resonant filters to be coupled to multiple-pixel devices such as bolometer arrays. Resonant grids are relatively simple to fabricate, exhibiting high transmission at the central frequency, a narrow bandpass, and good rejection of the side frequencies of the spectrum. We have fabricated filters centered at different frequencies between 0.4 and 10 THz, using photolithography and electroforming techniques. Transmission measurements have shown center frequencies and bandwidths close to the design predictions. The performance of the filters was found not to be critically dependent on small physical deformations in the mesh, becoming more noticeable at higher frequencies (i.e., for smaller physical sizes). Wider bandwidths, needed to attain higher sensitivities in the continuum, were obtained by changing the design parameters for filters at 2 and 3 THz.
The knowledge of THz continuum spectra is essential to investigate the emission mechanisms by high energy particle acceleration processes. Technical challenges appear for obtaining selective spectral sensing in the far infrared range to diagnose radiation produced by solar flare burst emissions measured from space as well as radiation produced by high energy electrons in laboratory accelerators. Efforts are been carried out intended for the development of solar flare high cadence radiometers at two THz frequencies to operate outside the terrestrial atmosphere (i.e. at 3 and 7 THz). One essential requirement is the efficient suppression of radiation in the visible and near infrared. Experimental setups have been assembled for testing (a) THz transmission of "low-pass" filters: rough surface mirrors; membranes Zitex G110G and TydexBlack; (b) a fabricated 2.4 THz resonant grid band-pass filter transmission response for polarization and angle of incidence; (c) radiation response from distinct detectors: adapted commercial microbolometer array using HRFZ-Si window, pyroelectric module and Golay cell; qualitative detection of solar radiation at a sub-THz frequency has been tested with a microbolometer array placed at the focus of the 1.5 m reflector for submillimeter waves (SST) at El Leoncito, Argentina Andes.
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