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Journal of Microwaves, Optoelectronics and Electromagnetic Applications

On-line version ISSN    2179-1074

P. Kaufmann^I;    R. Marcon^II; A.S. Kudaka^III; M. M. Cassiano^III;    L.O.T. Fernandes^III; A. Marun^IV; P. Pereyra^IV;    R. Godoy^IV; E. Bortolucci^V; M. Beny Zakia^V;    J.A. Diniz^V; A.M. Pereira Alves da Silva^V; A.V. Timofeevsky^VI;    V.A. Nikolaev^VI

^IUniversidade     Presbiteriana Mackenzie, CRAAM – Escola de Engenharia, São Paulo, SP,    Brazil, and Universidade Estadual de Campinas, CCS – Centro de Componentes Semicondutores,    Campinas, SP, Brazil, pierrekau@gmail.com       ^IIUniversidade Estadual de Campinas, IFGW, Campinas, SP, Brazil and    Observatório Solar “Bernard Lyot”, Campinas, SP, Brazil, rmarcon@mpcnet.com.br       ^IIIUniversidade Presbiteriana Mackenzie, CRAAM – Escola de Engenharia,    São Paulo, SP, Brazil, kudaka@mackenzie.br       ^IVComplejo Astronomico El Leoncito, CONICET San Juan, Argentina,    amarun@casleo.gov       ^VUniversidade Estadual de Campinas, CCS – Centro de Componentes Semicondutores,    Campinas, SP, Brazil, emilio@ccs.unicamp.br       ^VITydex J.S. Co, St. Petersburg, Russia, alexandertymofeevsky@tydex.ru

ABSTRACT

THz continuum spectral    photometry has new and unique applications in different civil and military areas    presenting a number of distinctive advantages on the well known microwaves or    mid- to near-infrared technologies. THz sensing is essential to investigate    the emission mechanisms by high energy particle acceleration processes. Technical    challenges appear to diagnose radiation produced by solar flare burst emissions    measured from space as well as radiation produced by high energy electrons in    laboratory accelerators. THz filters and detectors have been investigated for    the construction of solar flare high cadence radiometers to operate outside    the terrestrial atmosphere. Experimental setups have been assembled for testing    THz continuum radiation response from distinct detectors: adapted commercial     microbolometer array, pyroelectric module, and opto-acoustic (Golay cell). The    results permitted the final design of a THz double radiometer using Golay cells    to be flown in stratosphere balloon missions.

Index Terms-    Far IR continuum spectral photometry,THz radiometers, THz sensors, Solar THz    radiation

I.     INTRODUCTION

Technologies for    photometry and imaging in the THz range (arbitrarily 0.1 – 30 THz) are in full    expansion for a variety of new and unique applications in different civil and    military areas presenting a number of distinctive advantages on the well known    microwaves or mid- to near-infrared technologies. THz radiation propagates well    through cloth, dust and fog [1,2,3]. Sensing in this range is proving    to be particularly useful to determine internal characteristics of materials,    in the search for drugs, mines, and explosive materials. New biological and    medical THz imaging applications are far reaching. Aerospace THz remote sensing    applications include new approaches to determine atmospheric inhomogeneities    and cloud characteristics [4,5,6].

Photometry and    imaging at THz frequencies have important application in the diagnostics of    radiation produced by high energy electrons, observed in laboratory accelerators    [7] as well as by thermal and non-thermal space plasmas [8,9].    Solar flare accelerates electrons to high energies. Their radiation by synchrotron    mechanism predicts intense fluxes in the far IR or THz range of frequencies    [10]. The radiometry of temperature enhancements above a pre-existent    bright level – as it is the case of flare radiation excess over the solar disk    intense emission – requires the effective suppression of the incoming visible   and near-infrared (NIR) radiation. This has been accomplished with the use of    a number of THz low-pass filters [11], consisting in a combination of    rough surface mirrors [12,13,14] and commercially available membranes    [15,16]. We present the performance of distinct uncooled sensors in    response to black body THz radiation for different sensors: microbolometer array,    pyroelectric module, and Golay cell.

II. TEST OF    UNCOOLED THZ DETECTORS

A. Adapted microbolometer    array

A custom-made detector    consisted in a room-temperature vanadium oxide micro-bolometer focal plane array    (FPA) camera IRM 160A with HRFZ-Si THz window provided by INO Company, Quebec,    Canada [17]. The camera total-power response for black body temperatures    ranging 300-1000 K was measured at El Leoncito laboratory. A nichrome resistor,    assumed as close to an ideal black body radiator, was placed at the focus of    150 mm concave reflector to produce an image occupying nearly 70 % of the FPA.    We selected the Region Of Interest (ROI) over the area in the frame filled by    the heated resistor image. All pixels readings on the ROI were added and averaged    for every frame reading, quoted in camera reading units. Several sets of measurements    were taken, for temperatures ranging from ambient (about 290 K) to about 900    K, without any low-pass filter, and using the two membrane low-pass filters     described elsewhere [11,15,16]. One set of measurements is summarized    in Fig. 1 (a). Figure    1(b) shows another expanded set of data showing the camera response using   the Zitex G110 [15] and the TydexBlack [16] low-pass membranes.

The fluctuation    of data points can be attributed to measurement uncertainties (of about ±    1 reading unit), since they were taken with high cadence (30 frames/s). It can    be noted that the camera readings with Zitex G110 low pass filter interposed    is about 20-40 reading units above the TydexBlack readings, for the whole range    of temperatures. This effect was repeatedly observed for all series of measurements.    It might correspond to the fraction of power in the visible-NIR transmitted    by Zitex G110 [11].

The substantial    reduction in the camera response to black body temperature changes when interposing    the low-pass membranes proves their effectiveness in the readings increase suppressing    the visible and NIR radiation. Indeed the predicted ratio of power increase    for a black body heated about 100 K, at the 700 K level, for the whole main    spectrum (λ    > 0.5 μm)    in comparison to the THz part of the spectrum (λ    > 15 μm     ) is close to 60. This might be compared to ratio of about 40 between the camera    ROI readings increase in that range compared to with the membrane low-pass filters.

The camera scale    of about 25 K per reading unit (± 1 reading unit) was too large to allow    any measurable differences when adding one resonant metal mesh band-pass filter    [18].

B. Pyroelectric    detector module

The pyroelectric    modular detector made by Spectrum Detector Inc. [19], model SPH65-THz,    was tested at the laboratory of the Center for Semiconductor Components (CCS),    State University of Campinas. The setup utilised a standard laboratory Newport    model 67030 black body source with a build-in wheel chopper, set at 20 Hz. The    detector response to black body temperatures is shown in Fig.    2 for open conditions (responding to the visible – THz range) and for low-pass    membranes interposed.

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