Abstract: A comparative study between theory and experiment is presented for transmission through lossy frequency selective surfaces (FSSs) on silicon in the 2 – 15 micrometer range. Important parameters controlling the resonance shape and location are identified: dipole length, spacing, impedance, and dielectric surroundings. Their separate influence is exhibited. The primary resonance mechanism of FSSs is the resonance of the individual metallic patches. There is no discernable resonance arising from a feed-coupled configuration. The real part of the element's impedance controls the minimum value of transmission, while scarcely affecting its location. Varying the imaginary part shifts the location of resonance, while only slightly changing the minimum value of transmission. With such fine-tuning, it is possible to make a good fit between theory and experiment near the dipole resonance on any sample. A fixed choice of impedance can provide a reasonable fit to all samples fabricated under the same conditions. The dielectric surroundings change the resonance wavelength of the FSS compared to its value in air. The presence of FSS on the substrate increases the absorptivity/emissivity of the surface in a resonant way. Such enhancement is shown for dipole and cross arrays at several wavelengths.

Abstract: Physical limitation of response time of a superconducting bolometer as well as the nature of non-equilibrium detection of radiation have been investigated for Al, Nb and NbN thin films in spectral range from submillimeter to near-infraredwavelengths [1,2]. In the case of ideal heat removal from the film with the f_‘. 100A thickness the detection mechanism is an electron heating effect that is not selective to radiation wavelength in a very broad range. The response time ofan electron heating bolometer is determined by an electron-phonon interaction time. This time is of about 10 ns, 0.5 ns and 20 ps for Al, Nb, and NbN correspondingly near the critical temperature of the superconducting film. Thesensitive area of the bolometer consists of a number of narrow strips (with awidth of 1µm) connected in parallel to contact pads; these pads together witha sapphire substrate and a ground plate represent the microstrip transmissionline with an impedance of 50 Q.

Abstract: We have developed a new class of superconducting single-photon detectors (SSPDs) for ultrafast counting of infrared (IR) photons for secure quantum communications. The devices are operated on the quantum detection mechanism, based on the photon-induced hotspot formation and subsequent appearance of a transient resistive barrier across an ultrathin and submicron-wide superconducting stripe. The detectors are fabricated from 3.5-nm-thick NbN films and they operate at 4.2 K inside a closed-cycle refrigerator or liquid helium cryostat. Various continuous and pulsed laser sources have been used in our experiments, enabling us to determine the detector experimental quantum efficiency (QE) in the photon-counting mode, response time, time jitter, and dark counts. Our 3.5-nm-thick SSPDs reached QE above 15% for visible light photons and 5% at 1.3 – 1.5 μm infrared range. The measured real-time counting rate was above 2 GHz and was limited by the read-out electronics (intrinsic response time is <30 ps). The measured jitter was <18 ps, and the dark counting rate was <0.01 per second. The measured noise equivalent power (NEP) is 2 x 10^-18 W/Hz^1/2 at λ = 1.3 μm. In near-infrared range, in terms of the counting rate, jitter, dark counts, and overall sensitivity, the NbN SSPDs significantly outperform their semiconductor counterparts. An ultrafast quantum cryptography communication technology based on SSPDs is proposed and discussed.