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On-line version ISSN 2179-1074
P. KaufmannI; R. MarconII; A.S. KudakaIII; M. M. CassianoIII; L.O.T. FernandesIII; A. MarunIV; P. PereyraIV; R. GodoyIV; E. BortolucciV; M. Beny ZakiaV; J.A. DinizV; A.M. Pereira Alves da SilvaV; A.V. TimofeevskyVI; V.A. NikolaevVI
IUniversidade Presbiteriana Mackenzie, CRAAM – Escola de Engenharia, São Paulo, SP, Brazil, and Universidade Estadual de Campinas, CCS – Centro de Componentes Semicondutores, Campinas, SP, Brazil, email@example.com IIUniversidade Estadual de Campinas, IFGW, Campinas, SP, Brazil and Observatório Solar “Bernard Lyot”, Campinas, SP, Brazil, firstname.lastname@example.org IIIUniversidade Presbiteriana Mackenzie, CRAAM – Escola de Engenharia, São Paulo, SP, Brazil, email@example.com IVComplejo Astronomico El Leoncito, CONICET San Juan, Argentina, firstname.lastname@example.org VUniversidade Estadual de Campinas, CCS – Centro de Componentes Semicondutores, Campinas, SP, Brazil, email@example.com VITydex J.S. Co, St. Petersburg, Russia, firstname.lastname@example.org
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
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  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 . 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 , 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 . 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  and the TydexBlack  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 .
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 .
B. Pyroelectric detector module
The pyroelectric modular detector made by Spectrum Detector Inc. , 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.