WO2010081522A1

WO2010081522A1 - Thin-film radiation detector

Info

Publication number: WO2010081522A1
Application number: PCT/EP2009/009134
Authority: WO
Inventors: Eduard Frans Clemens Driessen; Michiel Jacob Andries De Dood
Original assignee: Universiteit Leiden; Stichting Voor Fundamenteel Onderzoek Der Materie
Priority date: 2009-01-14
Filing date: 2009-12-18
Publication date: 2010-07-22
Also published as: GB2466946A; GB0900534D0

Abstract

A method of operating a thin-film radiation detector such as a superconducting single photon detector comprising the steps of selecting a first polarization state of an incident beam of radiation; and directing the resulting first polarized beam of radiation at the thin- film detector surface such that the beam axis is oblique to the plane of the detector surface and the first polarized beam has an s-polarization state relative to the absorption surface of the detector. A thin-film radiation detector (1) comprises an absorbing thin film (3) material on a substrate (2); and coupling optics (4) having means for receiving an incident beam of radiation, a polarizer (16) configured to select a first polarization state of the incident beam of radiation to form a first polarized beam; and means (7) for directing the polarized beam of radiation at the absorbing thin-film material surface such that the beam axis is oblique to the plane of the detector surface and the first polarized beam has an s- polarization state relative to the absorption surface of the detector.

Description

THIN-FILM RADIATION DETECTOR

The present invention relates to thin-film radiation detectors and in particular, though not exclusively, to high efficiency superconducting single photon detectors (SSPDs).

Converting light into an electrical signal is an essential step for solar cells or photon detectors. The maximum achievable efficiency of these devices is determined by the maximum achievable absorption of light in the devices. Thin solar cells are desirable because they provide for efficient collection of charge carriers. Some optical detectors, like the superconducting single photon detector (SSPD) consist of a very thin layer of material in which the light is absorbed. This layer conventionally has a thickness of a few nanometres.

A SSPD device is described in patent application US2005/0051726. The SSPD comprises a sapphire or quartz substrate on which is deposited a thin film of superconducting material such as niobium nitride (NbN) which is formed into a narrow strip in a meandering or serpentine layout to maximise its surface area. Various arrangements for coupling light to the SSPD are shown in figure 8 of US 2005/0051726 in which an incident light beam is directed onto the SSPD by way of a hemispherical lens or hemispherical mirror.

Other examples of SSPDs are described in IPin et a Ultimate quantum efficiency of a superconducting hot-electron photodetector, Appl Phys Lett (1998) vol. 73 (26) pp. 3938- 3940 and in Gol'tsman et aϊ. Picosecond superconducting single-photon optical detector. Appl Phys Lett (2001 ) vol. 79 pp. 705-707.

A problem with SSPD devices having thin films with high absorption is that the incident light is not only absorbed but also reflected and/or transmitted. Typically, these detectors consist of a layer of NbN that is approximately 4 nm thick, deposited on a dielectric substrate (commonly sapphire). When illuminated from the air, under normal incidence, the absorption is limited to a maximum of between 30 and 35 % by reflection and transmission of the incident light. This can be improved to between 50 and 60 % by illuminating the detector from the substrate side. The maximum absorption is determined by the optical impedance mismatch between the substrate and the film, and it is impossible to increase the absorption above this maximum using existing methods. FR 2891400 describes a method to increase the absorption of an SSPD by coupling the incident light into a waveguide using a stack of uniform dielectric layers of different refractive index disposed on a substrate and including a planar dielectric layer immediately adjacent to the thin film superconducting layer forming the meander. Light is directed at an oblique angle to the stack using a prism, some light being reflected and some coupled into the waveguide in a leakage mode and re-radiated. The layers are configured so that the coupled then radiated wave and the Fresnel reflected wave have the same amplitude so that the light intensity resulting from their interference is zero which maximizes resonance absorption in the superconducting layer. The layer thicknesses of the detector described in FR '400 are designed for a specific light wavelength and polarization state and require a multiple layer configuration.

It is an object of the present invention to provide a method and apparatus for improving the absorption of thin-film radiation detectors.

According to one aspect, the present invention provides a method of operating a thin-film radiation detector having an absorbing thin film detector material defining a detector surface disposed at an interface between first and second materials of different refractive index, the method comprising the steps of: selecting a first polarization state of an incident beam of radiation; directing the resulting first polarized beam of radiation onto the thin-film detector surface such that the beam axis is oblique to the plane of the detector surface and the first polarized beam has an s-polarization state relative to the absorption surface of the detector.

The polarized incident beam may be directed onto the thin-film surface such that the beam axis is at or sufficiently close to the critical angle for total internal reflection such that ≥ 80 % of the polarized incident beam is absorbed by the thin-film detector. The polarized incident beam may be directed onto the thin-film surface such that the beam axis is at or within ten degrees of the critical angle. The polarized incident beam may be sufficient close to the critical angle such that substantially no transmission through the thin-film of the detector occurs, or such that the absorption of the polarized incident beam is at least 80 % of the intensity that would be absorbed at the critical angle. A second polarization state of the incident beam of radiation orthogonal to the first polarization state may be selected and a resulting second polarized beam may be directed at a second thin-film radiation detector such that the beam axis is oblique to the plane of the second detector surface and the second polarized beam has an s- polarization state relative to the absorption surface of the second detector.

According to another aspect, the present invention provides a thin-film radiation detector comprising: an absorbing thin film material defining a detector surface and disposed on a first surface of a substrate, the first surface of the substrate defining an interface between first and second materials of different refractive index; and coupling optics comprising: means for receiving an incident beam of radiation, a polarizer configured to select a first polarization state of the incident beam of radiation to form a first polarized beam; and means for directing the polarized beam of radiation onto the absorbing thin-film material detector surface such that the beam axis is oblique to the plane of the detector surface and the first polarized beam has an s-polarization state relative to the absorption surface of the detector.

The coupling optics may include the substrate on which the absorbing thin film is disposed. The means for directing the output polarized beam may comprise an optical element adapted to direct the beam onto the thin-film detector surface such that the beam axis is at or sufficiently close to the critical angle for total internal reflection such that > 80 % of the polarized incident beam is absorbed by the thin-film detector material. The means for directing the output polarized beam may comprise an optical element adapted to direct the beam onto the thin-film detector surface such that the beam axis is substantially at or within ten degrees of the critical angle. The means for directing the output polarized beam may direct the beam at the thin-film detector surface such that the beam axis is substantially at the critical angle such that substantially no transmission through the thin-film of the detector occurs, or sufficiently close to the critical angle such that the absorption of the polarized incident beam is at least 80 % of the intensity that would be absorbed at the critical angle. The absorbing thin film may comprise a meander of superconducting material disposed on the substrate. The meander may comprise a plurality of parallel wires. The coupling optics may be adapted to direct the polarized beam of radiation onto the absorbing thin-film material surface such that the s- polarization state is parallel to the direction of the plurality of superconducting material wires. The polarizer may be adapted to split the incident beam of radiation into two polarization states and to rotate one of said polarization states such that the first polarized beam comprises predominantly said s-polarization state. The detector may include a second thin-film radiation detector having an absorbing thin film detector material defining a second detector surface disposed at an interface between materials of different refractive index and means for selecting a second polarization state of the incident beam of radiation orthogonal to the first polarization state and means for directing the resulting second polarized beam onto said second detector surface such that the beam axis is oblique to the plane of the second detector surface and the second polarized beam has an s-polarization state relative to the absorption surface of the second detector.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which:

Figure 1 shows a cross-sectional side view of a thin-film radiation detector adapted for increased absorption in the thin-film;

Figure 2 shows a plan view of the meander patterned thin-film of the detector of figure 1 ;

Figures 3a and 3b respectively show schematic cross-sectional views of the thin film and substrate structure (a) for a continuous or unstructured thin film device and (b) for a grating type thin film device such as the meander pattern of the SSPD of figure 2;

Figure 4 shows the calculated absorption for s- and p-polarized light as a function of angle of incidence for the structure of figure 3a;

Figure 5 shows the calculated absorption for s- and p-polarized light as a function of angle of incidence for the structure of figure 3b;

Figure 6 shows a graph of the measured optical absorption at wavelength 775 nm of the detector arrangement of figure 3a having a 4.5 nm thick NbN film, as a function of angle of incidence for s-polarized light and p-polarized light;

Figure 7 shows a graph of the calculated optical absorption of the detector arrangement similar to figure 3b for detector geometry having a lattice period of 200 nm and a filling factor of 50 %, for s-polarized light and for p-polarized light; Figure 8 shows a graph of the calculated dependence of the optical absorption of the thin film as a function of the thickness of the film and the wavelength of incident light, for s- and p-polarized light; and

Figure 9 shows a schematic diagram of a two-stage detector adapted for capturing both polarization states of incident light on two detectors.

Throughout the present specification, reference to radiation detectors and optical detectors is intended to encompass all forms of photon detectors including those that operate in the visible optical spectrum, infra-red spectrum and ultra-violet spectrum. The principles described here may be applicable to any wavelength of light in which the two following conditions are fulfilled: a) the real part of the complex refractive index ε of the thin film of the detector is smaller (and preferably much smaller) than the imaginary part and b) the thin film of the detector is much thinner than an optical wavelength. In particular, NbN superconducting detectors discussed above have been shown to operate at least up to a wavelength of 5 micrometers.

Figure 1 shows a thin-film radiation detector 1 comprising a substrate 2 on which is deposited an absorbing thin film 3. The absorbing thin film 3 is, in preferred arrangements, a superconducting material such as NbN and in preferred embodiments is also formed into a meander pattern 20 as shown in the plan view of figure 2. The substrate 2 is preferably formed from a suitable optically transparent dielectric material providing structural support to the thin film. Preferred choices for the substrate material include sapphire or quartz.

Adjacent to the substrate 2 is an arrangement of coupling optics 4 for directing an incident beam 5 of radiation onto the detector surface. In the arrangement shown, the coupling optics includes a polarizer 6 and a prism 7. The polarizer 6 receives the incident beam 5 of radiation and generates an output beam 8 that is at least predominantly polarized to have an s-polarization state relative to the surface of the thin film. In other words, the electric field of the output beam 8 is transverse to the plane defined by the beam axis and a vector normal to the surface of the thin film. Preferably, the polarizer 6 converts all or most of the incident light in beam 5 into this s-polarization state. The polarizer 6 may use any method to provide a suitable s-polarized output, for example a beam splitter for separation of two orthogonal polarization states into separate paths and a polarization rotation device such as a Fresnel rhomb or quarter wavelength plate to rotate the polarization state of one of the beams so that as much energy as possible in the incident beam 5 is placed in to the s-polarization state.

The output beam 8 is directed to the prism 7. A first face 9 of the prism 7 is preferably substantially normal to the output beam 8 so that little or no reflection occurs at the prism face 9. The prism 7 has a second face 10 that forms a boundary with the substrate 2 and this second face 10 is substantially parallel to the interface 11 between the substrate 2 and the thin film 3. The materials of the prism 7 and that of the substrate 2 are preferably optically identical or at least optically very similar so that little or no reflection or refraction occurs at the interface 10 between the prism 7 and substrate 2. Preferably, an index matching fluid is disposed between the prism 7 and the substrate 2 at the second face 10. The first face 9 is disposed at an oblique angle θ relative to the normal to the second face 10 and the interface 11. This oblique angle θ is preferably at or as close as practicable to the critical angle for total internal reflection at the interface 11.

With an appropriate choice of film thickness for the thin film 3 and the polarization of the light, absorption can be increased to approximately 100 % by selecting the angle of incidence of beam 8 exactly equal to the critical angle for total internal reflection. This critical angle is determined by the refractive index of the substrate 2 and the refractive index of the medium 12 above the thin film, hereinafter referred to as the superstrate 12.

Thus, the substrate 2 effectively defines an interface 11 between first and second materials of different refractive index, the first material being the substrate 2 and the second material being the superstrate 12. The thin film absorbing material is disposed at this interface.

It will be recognised that the prism 7 could be any optical element configured to efficiently couple an incident beam of light in a first optical transmission medium (e.g. air) to a second optical transmission medium that is immediately adjacent to the thin film 3 (e.g. the material of the substrate 2) without significant losses in reflection and/or refraction. Thus, it can be understood that the prism 7 and the substrate 2 could comprise a unitary structure with a first face 7 and a second face corresponding to the interface 11. In other words, the coupling optics 4 can include the substrate 2 on which the absorbing thin film is disposed.

The prism 7 could be replaced with a suitable lens such as a hemispherical lens or half- cylinder lens providing a curved input face receiving the input beam 5 at an angle substantially normal to a tangent to the input face and a flat face forming the interface 10 or 11. An anti-reflection coating can be used at the first and/or second faces. An index- matching fluid may be used at any interface 10 and/or 11 , if required.

Thus, in a general aspect, the prism or lens exemplify a means for directing the polarized beam of radiation 8 at the absorbing thin-film material 3 surface such that the beam axis is oblique to the plane of the detector surface and the first polarized beam has a predominantly s-polarization state relative to the absorption surface of the detector. In this context it will be understood that the expression "detector surface" means the thin film material. For maximum benefit, the polarizer 6 is configured not merely to filter out p-polarization radiation, but to effect rotation of the light polarization state so that the energy is at least predominantly in the s-polarization state relative to the thin-film surface, e.g. substantially greater than 50 % of the energy of the incident beam.

To provide a critical angle of incidence, it is necessary that the transmission medium of the optical element coupling to the thin film 3 has a refractive index greater than that of the medium above the thin film, i.e. the superstrate 12. In these circumstances, total internal reflection occurs. The critical angle is given by θc = arcsin (n_superstrate / n_SUbstrate)- In one preferred example, shown in figure 3a, the thin film 3 has a thickness of d = 4.5 nm, the medium 12 above the thin film 3 is air with a refractive index of 1.00 and the substrate 2 is sapphire with a refractive index of 1.73. The critical angle for θ is where no transmission into the air superstrate 12 occurs. For a superconducting detector, it is possible that the superstrate could be liquid helium which would influence the critical angle required.

In figure 3b, the thin film 3 is a grating type structure equivalent to the meander 20 of figure 2, where the thin film 3 has a film thickness of 8 nm, the medium 12 above the thin film 3 is air with a refractive index of 1.00 and the substrate 2 is sapphire with a refractive index of 1.73. The grating period is 200 nm and the grating is 60 % metal. The reflection and transmission of the layered systems of figures 3a and 3b are given by the Fresnel equations. For the configuration in figure 3a the complex reflection and transmission coefficients are given by:

r = [r₁₂ + r₂₃ exp (2ik₂,_zd)] I [1 + r_nr_2Z exp (2//c_2iZd)] (1 )

t = [Ma exp (2ιK₂,cQJ / [1 + W₂₃ exp (2ik_2:Zd)} (2)

where r_tJ and r_tJ are the Fresnel reflection and transmission coefficients for an interface between medium / and medium / respectively, and k_z,_z is the perpendicular component of the wave vector in medium 2.

The Fresnel reflection and transmission coefficients are polarization dependent, and given by:

H_J = [P_J - P] I [PJ + P] (3)

where p, is given by p, = /c,_,z for s-polarization, and p, = k,,_z I ε, for p-polarization, respectively.

The absorption in the film is now given by:

A = 1 - I r I² - [Re k_3tZ

I [Re k_tz] (5)

Figure 4 shows a graph of the calculated absorption as a function of the angle of incidence, for the detector geometry of figure 3a, for the s-polarization parallel to the thin film (curve 41) and the p-polarization perpendicular to the thin film (curve 42). The dashed line 43 indicates the critical angle. At the critical angle, the absorption reaches > 99 % for s-polarized light. The higher absorption for p-polarized light at angles above the critical angle is a remnant of surface plasmons and/or other guided waves that propagate along the interface film / air. Overall absorption of unpolarized incident light may be significantly improved at an angle between the critical angle and the angle at which surface waves are maximally excited indicated by the peak in the curve 42. Figure 5 shows a graph of the calculated absorption as a function of the angle of incidence, for the geometry of figure 3b, for the s-polarization parallel to the thin film (curve 51) and p-polarization (curve 52) perpendicular to the thin film. The dashed line 53 indicates the critical angle and the curve 54 gives the average absorption. At the critical angle, the absorption reaches 98 % for s-polarized light.

Figure 6 shows the measured absorption of a 4.5 nm thick NbN film, at a wavelength of 775 nm, as a function of the angle of incidence. Curve 61 shows absorption for light polarized parallel to the interface (s-polarized, closed symbols) and curve 62 shows absorption for light polarized with an electric field component perpendicular to the interface (p-polarized, open symbols). The 4.5 nm thick NbN film was deposited on a double-polished R-plane sapphire substrate, on the long side of an isosceles BK7 (n = 1.51) prism, as shown in the insets of figure 6, using an n = 1.51 index-matching liquid between the sapphire substrate and the prism. This allows illumination of the film at angles larger than the critical angle for total internal reflection. See E. Kretschman and H. Raether, Z. Naturforsch. A 23, 2135 (1968).

The film was illuminated using a 775 nm continuous-wave diode laser that was collimated to an approximately 1 mm diameter beam. The polarization of the incoming beam was set using a Glan-Laser polarizer cube and the birefringent sapphire substrate was oriented such that the optical axis of the sapphire was either parallel or perpendicular to the incident polarization. In this way, the linear polarization of the incident radiation is unchanged. The transmitted (T) and reflected (R) fraction of the incident power were recorded as a function of angle of incidence using a silicon photodiode, and were corrected for the losses due to reflections at the prism-air interfaces. The absorption is defined as A = 1 - R - T.

The dash-dotted line 63 in figure 6 indicates the critical angle θ_c = arcsin(n_s ^"1), for a substrate refractive index n₅ = 1.75. For angles larger than this angle, light is not allowed to propagate in the air, and all incident light is either reflected or absorbed. At the critical angle, the absorption for s-polarized light goes to a maximum value of approximately 94%, for a film that is only 4.5 nm thick. At the same angle, the absorption for p-polarized light goes to a minimum of approximately 10%. The maximum and minimum in absorption can be understood as follows: at the critical angle θ_c, the amplitude of the evanescent wave extending into the air is zero. All the fields are contained in the half space bounded by the metal-air interface. The p- polarized light has its polarization perpendicular to the interface. At reflection at the metal-air interface the wave changes sign. This causes an antinode in the thin film, expelling the field from the absorbing medium. The s-polarized light does not change sign upon reflection, and therefore has a node at the boundary. A large part of the field is thus contained in the absorbing medium, and absorbed. We stress that the high absorption is not caused by coupling to a surface plasmon resonance or another polaritonic excitation on the metal-air interface. Such resonances only occur for p- polarized light (curve 62), at angles beyond the critical angle (F. Z. Yang, J. R. Sambles, and G. W. Bradberry, Phys. Rev. B 44, 5855 (1991)). In fact, the local maximum for p- polarized light at an angle of incidence θ = 55 degrees is a remnant of the surface plasmon. Since we are dealing with a very thin, very lossy metal film, there is no sharp resonance. The large absorption for s-polarization is a consequence of the total internal reflection only.

We compare the measured absorption to the absorption calculated using Fresnel's coefficients for the reflection and transmission of a system with two interfaces (M. Born and E. Wolf, Principles of optics (Cambridge University Press, 1999), 7th ed.). The curves in Fig. 6 show a fit to the measurements where we used the complex refractive index of the absorbing NbN layer as the only fit parameter, giving ε_fiι_m = -8.2 + 31.4i. We attribute the differences between the fit and the measurements to a small residual polarization rotation caused by the birefringence of the sapphire substrate.

The film 3 thickness and the dielectric constant of the film 3 determine the value of the absorption at the critical angle. For a film thickness much smaller than the wavelength, interference can be ignored. Furthermore, for a very lossy film material, we assume that I Re 8_fii_m I « I Im ε_fiι_m |. The absorption for s-polarization can then be approximated (K. E. Komelsen, M. Dressel, J. E. Eldridge, M. J. Brett, and K. L. Westra, Phys. Rev. B 44, 11882 (1991)) as:

A « 4 [ (n_s ² - 1 )° ⁵ kod Im ε_TΛm ] I [(π_s ² - 1 )^α5 + Jc₀CMm ε_fi,_m ]² (6) where k₀ is the wave vector of the light in vacuo, and d is the film thickness. The absorption reaches a maximum value at a film thickness given by:

C = (/7₅ ² - 1)^{0 5} / /c_o lm ε_film (7)

In the limit for very absorbing materials, the real part of the dielectric constant, and therefore also the sign of the real part, does not influence the maximum in absorption. This means that the maximum in absorption should occur both for lossy dielectrics and for lossy metals.

In the arrangement of figure 2, the detector thin film 3 comprises a non-continuous film comprising a meandering NbN wire 20. Absorption of a photon in this superconducting wire provides enough energy to give rise to a finite voltage pulse, which can be detected to count single photons. When a photon is absorbed in the meander, superconductivity is temporarily lost local to the absorption event, which leads to a finite voltage pulse over the detector. Light polarized parallel to the wires has a higher probability of being absorbed. (V. Anant, A. J. Kerman, E. A. Dauler, J. K. W. Yang, K. M. Rosfjord, and K. K. Berggren, Opt Express 16, 10750 (2008) and E. F. C. Driessen, F. R. Braakman, E. M. Reiger, S. N. Dorenbos, V. Zwiller, and M. J. A. de Dood, Preprint (2008).

Figure 7 shows calculated optical absorption as a function of the angle of incidence, for a detector geometry having a lattice period of 200 nm and a filling factor of 50% (as shown in the inset). The filling factor is the ratio of metal track to space. Curve 71 gives the absorption for s-polarized light, and curve 72 the absorption for p-polarized light. The lines of the detector are positioned such that they are parallel to the p-polarization. The thickness of the thin film detector layer was 11.3 nm and was set for optimal absorption. The dash-dotted line 73 indicates the critical angle.

Figure 7 shows the calculated absorption as a function of angle of incidence for a detector structure using the rigorous coupled-wave analysis developed in M G Moharam et al, J Opt Soc Am A 12 (1995), for light polarized parallel (TE) or perpendicular (TM) to the wires of the meander. The detector was oriented such, that the TE direction was parallel to the s-polarization of the incident light. This choice of orientation allows us to benefit from both the high absorption due to the polarization-dependence induced by the periodic grating structure, and from the maximum in absorption due to the illumination at the critical angle. The value of the film thickness for optimal absorption was obtained using equation (7) and taking an effective dielectric constant for the patterned layer such that ε_fiι_m = f + (1 - ή ε_NbN, with ε_NbN the dielectric constant of NbN that was fitted from the absorption measurements on the solid film.

At the critical angle, the calculated absorption reaches a maximum value of 94%, for s- polarized light. This confirms the fact, that the finite filling factor can be countered by increasing the film thickness accordingly (E. F. C. Driessen, F. R. Braakman, E. M. Reiger, S. N. Dorenbos, V. Zwiller, and M. J. A. de Dood, Preprint (2008)). At this angle, the absorption for p-polarized light reaches a local minimum. Moreover, it is important to notice that for an angular spread of approximately 10 degrees around the critical angle, the absorption is still well above 80%, making the proposed detector also efficient for absorbing light with a finite numerical aperture.

Figure 8 shows the calculated dependence of the absorption on the thickness of the NbN detector. For s-polarized light, the absorption has a maximum of 94.5% around 10.8 nm thickness. The inset shows the absorption as a function of the wavelength of the incident light. The wavelength dependence is small for s-polarized light.

From figure 8, we can conclude that for detectors with a thin film thickness between 7 nm and 20 nm, the absorption is still > 80%, much higher than the value obtained by other methods and comparable to the absorption of a Si avalanche photodiode at visible wavelengths. It also shows that the maximum in absorption is not very sensitive to the exact design of the detector. As a result, by changing the thickness significantly, the absorption is only compromised a small amount. Decreasing the thickness might give rise to an increased electronic conversion efficiency (A. Jukna, J. Kitaygorsky, D. Pan, A. S. Cross, A. J. Pearlman, I. Komissarov, O. Okunev, K. Smirnov, A. Korneev, G. Chulkova, et al., Acta Phys. Pol. A 113, 955 (2008)). Note however that increasing the filling factor of the detector immediately shifts the optimal thickness to lower values. This way the need for decreasing the thickness can be accommodated by increasing the filling factor accordingly. The minimal film thickness to achieve maximum absorption is however the thickness given by equation 7. In preferred embodiments, the film thickness of NbN would lie in the range 1 to 30 nm. The inset of figure 8 shows the calculated wavelength dependence of the absorption. For this calculation, we used literature values for the dispersion of the sapphire substrate (E. D. Palik, Handbook of Optical Constants of Solids, vol. Ill (Academic Press, 1998)) and a Drude model for the dielectric constant of the NbN material (K. Tanabe, H. Asano, Y. Katoh, and O. Michikami, J. Appl. Phys. 63, 1733 (1988)), where we changed the high-frequency dielectric constant and the loss parameter to fit the known NbN dielectric constant at 775 nm. The angle of incidence was set at the critical angle for a wavelength of 775 nm. The calculated absorption is almost constant. This is due to the fact that the maximum absorption is only dependent on the product k₀ Im ε, which is nearly constant for a Drude metal far away from resonance. Therefore, the optimal film thickness only varies marginally with the wavelength of the incident light. The decrease in absorption at wavelengths < 550 nm is caused by the fact that for these wavelengths, a diffraction order from the periodic structure exists. This diffraction order decreases the overall absorption. The dispersion of the sapphire substrate causes the small feature at 775 nm. Only at this wavelength, the angle of incidence is set exactly at the critical angle. For wavelengths below (above) this value, the angle is slightly below (above) the critical angle.

The effect generally is quite insensitive to variations in the wavelength and angle of incidence. The absorption of s-polarized radiation by a realistic SSPD, optimized for a wavelength of 780 nm, remains > 90% for a wavelength range of 700-1700 nm. However, a range of 450 nm to 5 microns may also be possible.

Thus it is generally apparent that by configuring the coupling optics to cause the incident beam to have its beam axis close to the critical angle at least 80 % of the polarized beam 8 is absorbed by the thin film 3. Preferably, the coupling optics direct the polarized beam such that its beam axis is within 10 degrees, or more preferably within 5 degrees of the critical angle. Preferably, the coupling optics direct the polarized beam sufficiently close to the critical angle such that at least 80 % of the intensity that would have been absorbed at the critical angle is absorbed by the detector.

The concept described here can be applied to all devices that rely on the absorption of light in a thin, strongly absorbing film. A detector can reach an absorption efficiency as high as 94% for a filling factor of only 50%. The calculated absorption of an NbN detector is shown to be almost wavelength-independent, and robust against changes in film thickness. At the critical angle, the light that is not absorbed is reflected and can be collected by a second detector. This way, a high-speed, broadband, near-100% absorbing single-photon detector is possible.

A particular advantage of the detector design discussed above is that the thin film superconducting material can be disposed directly onto the substrate 2 without intervening layers, and that the substrate 2 can also serve as the coupling optics prism 4. No complex layer processing is required such as the formation of high index and low index waveguide layers 34, 36 as found in the detector of FR 2891400 referenced above. The absorption of light by the thin film 3 in the arrangement described above does not rely on coupling the light into a waveguide parallel to the thin film surface, as is required in the detector of FR '400.

Figure 9 shows a detector in which the coupling optics provides for polarization and separation of the two polarization states to two different detectors. The coupling optics comprise a pair of prisms 91 and 92 in which the first prism 91 has a first detector thin film 93 disposed on the lower surface 95 as drawn. The left-and-upwardly facing oblique surface 94 defines the prism 91 input surface that receives an input beam 5, while the downwardly facing surface 95 provides the interface with the thin film 93. The right-and- upwardly facing oblique surface 96 (i.e. that which extends between the first prism baseline 96a and the first prism apex 96b) defines an output surface of the first prism, and also defines an input surface of the second prism 92. This interface may include an index matching fluid. The second prism 92 has a second surface 97 (i.e. that which extends between the second prism baselines 97a and 97b and opposite the second prism apex 97c) which second surface 97 forms the interface with a second detector thin film thereon (not shown).

If the input beam 5 arrives at the interface 95 at an angle of incidence θ at or close to the critical angle, then it will have any s-polarized component substantially completely absorbed by the thin film 93, and any p-polarized component will be substantially totally internally reflected as beam 98 toward the oblique surface 96 where at point T it is transmitted substantially without reflection or refraction into the second prism 92. The beam 98 then passes though the second prism 92 and is directed towards the thin film interface 97 at point R. It will be seen that the coupling optics 90 comprise a pair of identical prisms in which corresponding faces 96 of the prisms are brought together with the second prism being rotated through 90 degrees about the normal to the corresponding faces 96. Thus, the portion of the beam that was p-polarized with respect to the first prism surface 95 will be s-polarized with respect to the second prism surface 97. The first prism 91 effectively acts as a polarizing filter for the second prism and the two detectors in combination absorb substantially 100 % of incident unpolarized light. The two prisms may form a unitary structure.

Thus, in a general aspect, the second detector can be oriented such that it maximally absorbs light reflected from the first detector. More generally, the dual prism arrangement exemplifies a means for selecting a second polarization state of the incident beam of radiation orthogonal to the first polarization state and means for directing the resulting second polarized beam at said second detector such that the beam axis is oblique to the plane of the second detector surface and the second polarized beam has an s-polarization state relative to the absorption surface of the second detector. Thus, it can be seen that the first prism and detector arrangement in fact acts as a polarizer for the second detector.

Enhancing the absorption of a thin film is beneficial in a number of technology areas. In the prior art, many different methods are described. Mostly, these methods make use of a resonant effect by resonantly coupling to a waveguide, a cavity or a surface plasmon. The present invention does not require use of a resonant excitation and is much less sensitive to the choice of wavelength, and the high absorption is reached for a larger angular spread. Moreover, the principle works for all strongly absorbing films, i.e. for both metallic and dielectric layers.

Surface plasmons and other polaritons can only be excited at an angle of incidence that is larger than the critical angle. In the arrangements described here, the maximum of absorption is at the critical angle. Moreover, the polarization is perpendicular to the polarization used in prior art devices using surface waves. The method described here does not use a mode propagating along the surface and the propagation length along the device is much smaller, which opens possibilities to making smaller detectors. The method described here works for all devices that are based on the absorption of light in a thin absorbing film, particularly those with film thicknesses of only a few nanometres. Where the thin film provides a meandering wire this wire may comprise a plurality of parallel wires as shown in figure 2 or the meander may generally be any regular or irregular layout. The thickness of the thin film may be uniform or non-uniform.

Other embodiments are intentionally within the scope of the accompanying claims.

Claims

1. A method of operating a thin-film radiation detector having an absorbing thin film detector material defining a detector surface disposed at an interface between first and second materials of different refractive index, the method comprising the steps of: selecting a first polarization state of an incident beam of radiation; directing the resulting first polarized beam of radiation onto the thin-film detector surface such that the beam axis is oblique to the plane of the detector surface and the first polarized beam has an s-polarization state relative to the absorption surface of the detector.

2. The method of claim 1 in which the step of directing the polarized incident beam comprises directing the beam onto the thin-film detector surface such that the beam axis is at or sufficiently close to the critical angle for total internal reflection such that > 80 % of the polarized incident beam is absorbed by the thin-film detector.

3. The method of claim 1 in which the step of directing the polarized incident beam comprises directing the beam onto the thin-film detector surface such that the beam axis is within ten degrees of the critical angle for total internal reflection.

4. The method of claim 2 or claim 3 in which the step of directing the polarized incident beam comprises directing the beam onto the thin-film detector surface such that the beam axis is substantially at the critical angle such that substantially no transmission through the thin-film of the detector occurs.

5. The method of claim 1 in which the step of directing the polarized incident beam comprises directing the beam onto the thin-film detector surface such that the beam axis is at or sufficiently close to the critical angle for total internal reflection such that the absorption of the polarized incident beam is at least 80 % of the intensity that would be absorbed at the critical angle.

6. The method of claim 1 further including the step of selecting a second polarization state of the incident beam of radiation orthogonal to the first polarization state and directing the resulting second polarized beam at a second thin-film radiation detector having an absorbing thin film detector material defining a second detector surface disposed at an interface between materials of different refractive index such that the beam axis is oblique to the plane of the second detector surface and the second polarized beam has an s-polarization state relative to the absorption surface of the second detector.

7. A thin-film radiation detector comprising: an absorbing thin film material defining a detector surface and disposed on a first surface of a substrate, the first surface of the substrate defining an interface between first and second materials of different refractive index; and coupling optics comprising: means for receiving an incident beam of radiation, a polarizer configured to select a first polarization state of the incident beam of radiation to form a first polarized beam; and means for directing the polarized beam of radiation onto the absorbing thin-film material detector surface such that the beam axis is oblique to the plane of the detector surface and the first polarized beam has an s-polarization state relative to the absorption surface of the detector.

8. The detector of claim 7 in which the coupling optics includes the substrate on which the absorbing thin film is disposed.

9. The detector of claim 7 in which the means for directing the output polarized beam comprises an optical element adapted to direct the beam onto the thin-film detector surface such that the beam axis is at or sufficiently close to the critical angle for total internal reflection such that > 80 % of the polarized incident beam is absorbed by the thin-film detector material.

10. The detector of claim 7 in which the means for directing the output polarized beam comprises an optical element adapted to direct the beam onto the thin-film detector surface such that the beam axis is within ten degrees of the critical angle for total internal reflection.

11. The detector of claim 9 or claim 10 in which the means for directing the output polarized beam comprises an optical element adapted to direct the beam onto the thin- film detector surface such that the beam axis is substantially at the critical angle such that substantially no transmission through the thin-film of the detector occurs.

12. The detector of claim 7 in which the means for directing the output polarized beam comprises an optical element adapted to direct the beam onto the thin-film detector surface such that the beam axis is at or sufficiently close to the critical angle for total internal reflection such that the absorption of the polarized incident beam is at least 80 % of the intensity that would be absorbed at the critical angle.

13. The detector of claim 7 in which the absorbing thin film comprises a meander of superconducting material disposed on the substrate.

14. The detector of claim 13 in which the meander comprises a plurality of parallel wires.

15. The detector of claim 14 in which the coupling optics is adapted to direct the polarized beam of radiation onto the absorbing thin-film material surface such that the s- polarization state is parallel to the direction of the plurality of superconducting material wires.

16. The detector of claim 7 in which the polarizer is adapted to split the incident beam of radiation into two polarization states and to rotate one of said polarization states such that the first polarized beam comprises predominantly said s-polarization state.

17. The detector of claim 7 further including a second thin-film radiation detector having an absorbing thin film detector material defining a second detector surface disposed at an interface between materials of different refractive index and means for selecting a second polarization state of the incident beam of radiation orthogonal to the first polarization state and means for directing the resulting second polarized beam onto said second detector surface such that the beam axis is oblique to the plane of the second detector surface and the second polarized beam has an s-polarization state relative to the absorption surface of the second detector.