Ir TES electron-phonon thermal conductance and single photon detection D. Bagliani, F. Gatti, M. Ribeiro Gomes, L. Parodi, L. Ferrari and R. Valle I.N.F.N. of Genoa, Via Dodecaneso, 33, 16146 Genova, Italy University and I.N.F.N. of Genoa, Via Dodecaneso, 33, 16146 Genova, Italy Received July 22, 2007, Accepted September 15, 2007 We have operated an Ir TES as detector for single photon at 450 nm in a temperature range of 100-120 mk. The decoupling of the electron gas from the phonons in the film, caused by the fifth power dependence of the temperature, is measured from the pulse decay time. The detection of single photon generated by a laser diode with a pulse length of 500 ns in a 25x25 µm 2 detector area at a base temperature of 90 mk is shown. PACS numbers: 14.70.Bh, 85.25.Oj, 07.20.Mc, 44.10.+i 1. INTRODUCTION Development of cryogenic single photon detectors for application to Quantum Cryptography and Quantum Optics are under course being very promising due to the achievable low dark count and high quantum efficiency. In particular we foresee to apply such devices in test of quantum cryptography transmission system at ELSAG Finmeccanica Company- Genova, Italy 1 and to set a limit to the coupling constant of predicted axion like particles and visible light photons in the PVLAS experiment 2. 2.DETECTOR We have fabricated photon TES microcalorimeter with Ir films onto Si in our laboratory by pulsed laser deposition. The films was patterned with microlithographic techniques. These processes allow the realization of films with sharp superconducting to normal transition, that ranges typically from 0.13 to 0.7 mk. The normal resistance above Tc has typical values from 16 to 50 m_, depending on the film thickness, that is measured by means of AFM at the final stage of the fabrication process.
D. Bagliani et al. The detectors patterns were designed as in fig.1(a) below in which the small square film of Ir is between two aluminum contacts wires coming from the big soldering pads. A reference pattern for testing the film properties, a long strip with the same detector width, and a length of 2mm was built 1mm apart from three detectors (see fig1(b)). The strip is used to characterize the Ir film with a 4 wire measurement of R(T,I) in a well defined current flow. The comparison of the transition curves R(T,I) for the two geometries should allow to study the detector performance as function of the intrinsic film properties. Fig1. (color on line) (a) 25x25 µm 2 Ir TES with aluminum contact pads (left), (b) 3 detectors with 2mm long stripe for R(T,I) film characterization (right). 2.ELECTRON PHONON THERMAL CONDUCTANCE The TES microcalorimeter has been modeled as shown in fig.2. The contributions to the total thermal conductance are given by the electronphonon coupling in Ir and Al films, the electron-electron coupling between Ir and Al films, phonon-phonon coupling between the Ir, the Al films and the Si substrate. The Si substrate is glued to the heat sink with a silver epoxy that cover a 3x3 mm2 area. Because the film working temperature is below the Al Tc and the Ir doesn t become fully superconducting, the Andreev electron reflection between Ir and Al gives rise to a negligible conductance contribution. In the present analysis this term is neglected. Also the Kapitza resistance at Al-Ir surface is big, because of the small contact area.
Ir TES e-p thermal conductance and single photon detection Fig.2. Thermal model of Ir TES onto Si substrate and Al electrical contacts. to be Finally the main terms contributing to the total conductance turn out 111totIrIrSiepKGGG =+ Ir-Si The Kapitza term G K is proportional to the third power of the temperature, so that the total conductance is dominated by the electronphonon contribution when T is lowered. The TES microcalorimeters were excited with photons that are absorbed by the electron system of the Ir film. The base temperature and bias voltage of the film are chosen to operate within the transition from superconducting to normal state. The electron temperature transient, which time constant is reduced by the electro-thermal feedback, has been read as current signal by a SQUID electronics. A light pulse is sent to the detector using a multimode step index optical fibre which is inserted in the dilution refrigerator and thermalized at various stages from 4.2 K to the mixing chamber temperature. A vacuum feed-through with SMA connectors was used to couple the fibre into the cryostat to the optical system outside, at room temperature.
D. Bagliani et al. In order to achieve a good optical coupling between the fibre and the TES, we put on the sample holder a support that fixes the fibre end at about 0.6 mm from the detector surface, assuring normal incidence of the light. Fig.3. (color on line) Average thermal pulse of the 75x75 µm 2 microcalorimeter Ir The decay constant times of 75x75 µm 2 TES were measured in different conditions. An example of fitted averaged pulse is shown in fig.3 Data were taken in conditions of weak electro-thermal feedback in order to reduce the possible systematic error in calculating the effective α parameter at the operating point. In these approximation _ = C/G ETF = C/G tot, within 0.1%. We can consider this result as an evaluation of the thermal link between electrons and phonons in the Ir film because, as in the model discussed before, this term overrides the total conductance at low temperatures. Finally the electron-phonon thermal conductance is measured to be 1.2 10-11 W/K at 114 mk, that correspond to a conductivity g e-p = 5.4 10 4 W/Km 3. 1. SINGLE PHOTON DETECTION The 25x25 µm 2 detector shown in fig.1 is excited with a 450 nm laser diode with 500 ns pulse-width and at a repetition rate ranging from 1 to 1KHz. At very low laser excitation power, we reached the condition in which the detector response shows clearly null pulse with a probability of
Ir TES e-p thermal conductance and single photon detection about 97%. In this condition only a small fraction of laser pulses gives rise to a photon absorption. In order to observe the single and multiple photon peaks in the amplitude spectrum we operated at a slightly higher laser power. We have acquired the pulse amplitude spectrum gating a multichannel analyzer with the laser excitation trigger pulse and settling a threshold just after the noise to first peak valley. The pulses were shaped with a standard RC-CR filter at 10 khz. An example of shaped pulse is done in fig.4 (a). One of the amplitude spectra that have been acquired is shown in fig.4(b), in which the single ad the double photon peaks clearly appears. Fig.4. (color on line) (a) An example of 10Kz RC-CR shaped pulse of single photon (upper); (b) multichannel analyzer amplitude spectrum plotted against an arbitrary scale.
D. Bagliani et al. 3. CONCLUSION We have developed and tested a first Ir TES microcalorimeter as single photon detector for light at 450nm and measuring its electron-phonon thermal conductance. Further detector improvement are needed to work with a wider light spectral range that should extend to the interesting infrared region. REFERENCES 1. F.A.Bovino, et al. PRL 95, (2005) 240407. 2. E. Zavattini et al., Phys. Rev. Lett. 96, (2006) 110406.