IR filters for high responsivity cryogenic detectors

Nuclear Instruments and Methods in Physics Research A 444 (2000) 457}460

IR "lters for high responsivity cryogenic detectors N. Rando *, P. Verhoeve , P. Gondoin , B. Collaudin
, J. Verveer , M. Bavdaz , A. Peacock
Astrophysics Division, Space Science Department, European Space Agency, ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
Thermal Control and Life Support Division, Mechanical Systems Department, European Space Agency, ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands

Abstract Modern cryogenic detectors, such as Superconducting Tunnel Junctions and Transition Edge Sensors, provide single photon counting performance, medium to high energy resolution, high count rates and good photon collection e$ciency over a wide wavelength range. In order to avoid background limited performance, it is necessary to shield the detectors from any thermal IR radiation originating from the surrounding warm surfaces. In this paper we analyse the contribution of the thermal radiation to the detector performance and describe the IR "lters used in the S-Cam camera and in other experimental con"gurations. Future detectors may require very severe attenuation of the IR #ux (j'1 lm). Solutions to this problem are proposed and their validity demonstrated with experimental results.  2000 Elsevier Science B.V. All rights reserved. Keywords: Electro-optical instrumentation; Cryogenic detectors; IR "lters

1. Introduction Modern cryogenic photon detectors provide single photon counting performance from NIR to X-ray energies [1,2]. In the case of a tantalumbased STJ, responsivities are of order 10 e\/eV, resolving power E/*E is of order 20 at j"300 nm and quantum e$ciency is about 70% in the visible range. Typical operating temperatures are about 300 mK for Ta and Nb-Al based STJs and of order 100 mK for Transition Edge Sensors (TES). In order to avoid background limited performance, it is necessary to reject any thermal radiation from warmer elements in the Field Of View (FOV) of the detectors. Such an issue is particularly challenging

* Corresponding author.

in the case of instruments targeting the lower photon energy range (e.g. visible range). While soft X-ray and UV applications can take advantage of very thin Al "lters [3], instruments operating in the visible range must provide high IR rejection combined with an adequate optical throughput. As predicted by the Planck distribution, the number of photons emitted by a grey-body is very large when compared to the low count rates expected from typical signal sources (e.g. faint astronomical objects). This is the case of S-Cam, an astronomical camera for ground-based astronomy in the visible range, recently installed at the Nasmyth focus of the William Herschel Telescope, in La Palma, Canary Islands (Spain) [4,5]. The e!ects of IR radiation applied to the cryogenic detectors are of twofold nature: (1) they may represent a heat load, increasing the detector temperature; (2) they

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produce a large number of photo-absorptions in the detector, thus generating pile-up e!ects and a corresponding degradation of the intrinsic energy resolution [6]. In the case of a linear detector, with responsivity R, the signal S measured over an integration time q is S"RE, with E the energy of the detected photon. The e!ects of an infrared photon absorption rate C are: (1) the continuous absorp'0 tion of IR photons determines an increase of the detector noise, p ; (2) #uctuations in the total "! amount of energy absorbed within the integration time produce an additional variance on the detector signal, p . In the case of a superconducting '0 tunnel junction (e"electron charge) we can write that [6] p [e\]"(eR1E 2q C "! '0 G '0

(1)

p [e\]"R(1E2q C . (2) '0 G '0 p and p are expressed in electrons and repres"! '0 ent the additional variance on the detector signal induced by the presence of thermal background radiation. It can be shown that p 'p if '0 "! 1E 2'e/R, condition which is generally met. In '0 the case of S-Cam (R"6000 e\/eV) the condition above is met for any thermal IR radiation emitted by grey bodies at temperatures larger than a few degree Kelvin. Large IR photon #uxes can induce a local heating of the detector substrate, thus in#uencing its performance. This e!ect is important for STJs, whose bias current I is strongly depen dent on temperature: to small temperature increases may correspond a large change in the shot noise over a given frequency bandwidth *f, as I (D, <, ¹)"I (<, D)(2pk¹De\DI2 (3)   p [e\]"(2eI (D, <, ¹)* f. (4)   In a tantalum STJ, a temperature variation from 400 to 500 mK quadruples the current, thus doubling the shot noise [7]. In the case of sapphire substrates, a heat input of order 1 lW due to IR radiation is enough to increase the detector temperature by more than 100 mK. It is evident how the noise increase related to heating e!ects can be as important as the energy degradation described by Eqs. (1) and (2).

2. IR 5lter solutions In order to attenuate the e!ects of the IR radiation two approaches are followed: (1) use of severe ba%ing, strictly limiting the detector Field Of View (FOV) to the application needs; (2) adoption of "ltering elements. A combination of the two techniques is often required. In the case of "lters, in addition to the band-pass, we must take into account their own grey-body thermal emission: thus, it is necessary to maintain the "ltering elements at cryogenic temperature. Di!erent "ltering con"gurations have been investigated by illuminating a Ta STJ detector (kept at 450 mK) through the sapphire substrate. In each case the "ltering element was clamped to the 4 K shield of the cryostat. The "lter attenuation factor is proportional to the ratio of the sub-gap current densities J (at 100 lV), measured with and with  out the "lter. The measured attenuation was compared with calculated values, under the assumption of 300 K black-body radiation reaching the detector through the "lters. The comparison showed that the measured attenuation was lower than predicted on the basis of the estimated IR #ux (Eqs. (1) (2). This suggested that heating of the substrate is likely to be present, enhancing the e!ect of the IR background (see Eqs. (3) and (4)). In addition to sapphire and quartz, we have also tested a 100 nm thick Al "lter, opaque to visible light, and a conductive micro-grid, a di!raction based IR blocking "lter. The main results are summarised in the Table 1. The thin Al "lters represent an ideal solution for soft X-ray applications, ensuring adequate transmittance to higher energy photons (typ. above 50% at 60 eV) and being opaque to visible and IR

Table 1 Con"guration

J attenuation 

No "lter Sapphire (at 4 K) Micro-grid on sapphire (at 4 K) SiO (at 4 K)  Thin Al "lter (at 4 K)

1 0.12 0.0069 0.0034 0.0017

N. Rando et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 457}460

Fig. 1. Metallic micro-grid on sapphire (IMM-D).

radiation (transmittance of order 10\). In our case the "lter was procured from Luxel (US): it is made of an Al:Si foil 101 nm thick, and supported by a Ni mesh [3]. A Cu #ange allows the installation on the thermal shield of the cryostat. The micro-grid (Fig. 1) was fabricated by IMM (D), by etching a WSi layer deposited on a sapphire substrate. The grid is 500 nm thick and has hexagonal shape, with an estimated open/masked area ratio of 50% and an average hole size of about 480 nm. Such metallic meshes have frequency selective properties: wavelengths shorter than the mesh hole size pass through, while longer wavelengths are severely re#ected out [8].

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¹"293 K corresponds to a heat input Q . Under  the assumption of an inner surface at a temperature ¹ "2 K (293 K), with an e!ective emissivity  eH+0.5, we can estimate Q "AeHp¹+  200 mW, representing a major heat input source for our cryogenic system and a correspondingly large IR load for our detector. In order to minimise such an IR load, we have used two di!erent "lters, both located inside the cryostat and in good thermal contact with the cold plate (at about 2 K). Both elements are based on Schott KG2 coloured glass, which is well known for its IR rejecting qualities. The "rst "lter (IRF ) is 9 mm thick and has a dia meter of 36 mm; it is mounted onto a dedicated copper holder. The second "lter (IRF ) is located  inside the former of the SC magnet required to operate the detector array and also connected to the cryostat cold plate. Fig. 2 shows the combined transmissivity of the two "lters. The "rst "lter is the most critical element, being subject to the intense radiation load produced by the window and by the warm part of the cryostat ba%e. Dedicated measurements have shown that the inner surface of such a "lter reaches a temperature of about 12 K. The second "lter is 3 mm thick and provides additional IR attenuation, absorbing the residual IR radiation transmitted and emitted by IRF . Due to the much lower thermal #ux, the  inner temperature of IRF coincides with the cold  plate temperature. The cold absorbing elements modify the spectrum of the thermal radiation originally produced by the cryostat window, with the

3. The S-Cam con5guration S-Cam is a cryogenic camera based on a 6;6 array of Ta-Al-AlO -Al-Ta STJs [4]. It has been V designed to operate at the Nasmyth focus of the William Herschel Telescope. A demagni"cation unit is required to achieve the desired plate scale on the camera focal plane (0.6 arcsec/pixel). Such a unit has an exit focal ratio f"2.06 (i.e. an opening angle h"arcsin (1/2f )"14.053) and a back focal length d"68 mm. At this distance, the area of the cryostat window in the FOV of the detector is A"p[d tan(arcsin(1/2f ))]"910 mm. The thermal radiation originated from this surface at

Fig. 2. Combined transmissivity of the two KG2 elements, IRF  and IRF . 

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emission of longer wavelength photons. Experimental tests based on this "ltering con"guration have demonstrated a resolving power j/*j of order 5 at j"500 nm, a factor 1.3 worse than the best experimental results obtained with the same detector and Front-End, but in absence of any signi"cant IR radiation. Such a result proves that the camera is still operating in background limited conditions. It should be noted that the intrinsic detector resolving power (in absence of any electronics and IR induced noise) at this wavelength corresponds to about 17 [9]. 4. Conclusions The suppression of any residual thermal IR radiation is a key issue for the full exploitation of the potential of modern cryogenic detectors. Thin Al "lters provide an excellent solution at UV and X-ray wavelengths, while applications in the visible are more problematic.

The IR "ltering solution adopted in S-Cam, while achieving single photon counting in the visible range does not allow to fully exploit the intrinsic energy resolution of the detector array. Alternative solutions, such as metallic meshes and di!erent absorbing glasses, are being investigated to overcome such limitations.

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