E. F. C. Driessen

The perfect absorber

We demonstrate that films of very lossy metal or dielectric, with a thickness of only a few nanometers, can absorb almost all incident radiation when illuminated from the substrate side at the critical angle for total internal reflection. The absorption for s-polarized light approaches 100%, while the absorption for p-polarized light vanishes. We demonstrate this effect by measuring the absorption as a function of the angle of incidence at a wavelength of 775 nm in a 4.5 nm thick NbN film with a dielectric constant ϵNbN=−8.2+31.4i. The measured absorption in this film reaches a maximum of 94%. We discuss the design of a near-unity efficiency single-photon detector for s-polarized light that has a broadband absorption coefficient of >90% for wavelengths from 700 to 1600 nm.We demonstrate that films of very lossy metal or dielectric, with a thickness of only a few nanometers, can absorb almost all incident radiation when illuminated from the substrate side at the critical angle for total internal reflection. The absorption for s-polarized light approaches 100%, while the absorption for p-polarized light vanishes. We demonstrate this effect by measuring the absorption as a function of the angle of incidence at a wavelength of 775 nm in a 4.5 nm thick NbN film with a dielectric constant ϵNbN=−8.2+31.4i. The measured absorption in this film reaches a maximum of 94%. We discuss the design of a near-unity efficiency single-photon detector for s-polarized light that has a broadband absorption coefficient of >90% for wavelengths from 700 to 1600 nm.

Impedance model for the polarization-dependent optical absorption of superconducting single-photon detectors

We measured the single-photon detection efficiency of NbN superconducting single-photon de- tectors as a function of the polarization state of the incident light for different wavelengths in the range from 488 nm to 1550 nm. The polarization contrast varies from ∼5% at 488 nm to ∼30% at 1550 nm, in good agreement with numerical calculations. We use an optical-impedance model to describe the absorption for polarization parallel to the wires of the detector. For the extremely lossy NbN material, the absorption can be kept constant by keeping the product of layer thickness and filling factor constant. As a consequence, the maximum possible absorption is independent of filling factor. By illuminating the detector through the substrate, an absorption efficiency of ∼ 70% can be reached for a detector on Si or GaAs, without the need for an optical cavity.

Asymmetry reversal in the reflection from a two-dimensional photonic crystal

The measured, angle-dependent, reflection spectra of a two-dimensional GaAs photonic crystal consist of an asymmetric peak on top of an oscillating background. At large angles of incidence (>70 degrees), the asymmetry of the peak is observed to flip for p-polarized light. We explain the observed spectra with a Fano model that includes loss and interference between a resonant waveguide component and direct Fresnel reflection of the layered structure. We show that the reversal of the asymmetry of the line is due to a change in sign of the direct reflection at Brewsters angle.

Enhanced coupling of plasmons in hole arrays with periodic dielectric antennas

We compare the angle-dependent transmission spectra of a metal hole array with dielectric pillars in each hole with that of a conventional metal hole array. The pillars enhance the optical transmission as well as the interaction between surface plasmon modes. This results in an observed splitting Delta omega/omega as large as 6%, at normal incidence, for the modes on the pillar side of the array.

The effect of magnetic field on the intrinsic detection efficiency of superconducting single-photon detectors

We experimentally investigate the effect of a magnetic field on photon detection in superconducting single-photon detectors (SSPDs). At low fields, the effect of a magnetic field is through the direct modification of the quasiparticle density of states of the superconductor, and magnetic field and bias current are interchangeable, as is expected for homogeneous dirty-limit superconductors. At the field where a first vortex enters the detector, the effect of the magnetic field is reduced, up until the point where the critical current of the detector starts to be determined by flux flow. From this field on, increasing the magnetic field does not alter the detection of photons anymore, whereas it does still change the rate of dark counts. This result points at an intrinsic difference in dark and photon counts, and also shows that no enhancement of the intrinsic detection efficiency of a straight SSPD wire is achievable in a magnetic field.

Transfer of photonic crystal membranes to a transparent gel substrate

We report a method of transferring 150 nm thick Al(0.35)Ga(0.65)As photonic crystal slabs to a transparent gel, without compromising their optical properties. We demonstrate successful transfer for membranes as large as ~ 425 × 425 μm(2). The transfer results in a 2.5% frequency red shift and increases the visibility of the resonances in reflection spectra. The avoided crossings between the modes show a subradiant mode with quality factors up to ~300. This suggests that the quality factor is only limited by the finite size of the crystal.

Polarization dependence of superconducting single photon detectors

We have measured the polarization dependence of the detection efficiency of NbN superconducting single photon detectors. This behavior is explained by the calculated absorption efficiency of a parallel set of metal lines.

Photonic bandstructure of a metal hole array with pillars

We have studied the angle and wavelength dependent extra-ordinary transmission of novel metal hole arrays that contain a dielectric pillar in each hole [1]. Similar to transmission spectra of conventional hole arrays, the measured transmission contains asymmetric resonances related to the excitation of surface plasmons on either side of the metal film. The pillars enhance the coupling of light to surface plasmons and introduce avoided crossings between the plasmon modes. We find that the dispersion of these plasmon modes can be described by the photonic bandstructure of a two-dimensional array of SiO2 pillars.

Direct imaging of the leaky modes in a 2-D photonic crystal slab

The reflection and transmission spectra of two-dimensional photonic crystal slabs show several asymmetric Fano resonances. The asymmetry is due to interference of a direct reflection from the slab with a resonant contribution due to coupling of light to a leaky waveguide mode [1]. In this work, we study the resonances in a photonic crystal slab by imaging the near-field of the propagating modes. This technique allows a separation of the direct and resonant contributions in the reflection and gives an unambiguous measure of the propagation length of the modes.

Metamaterials with 100% absorption over a large frequency range

Although very desirable for detector applications and photovoltaic cells, thin film materials that absorb all incoming light are extremely hard to find. In this work, we introduce a metamaterial consisting of a meandering wire of only a few nanometer thickness that can absorb all incident radiation when the film is illuminated at the critical angle for total internal reflection. The observed effect occurs for s-polarized radiation in very lossy metals or dielectrics for which the imaginary part of the dielectric constant is much larger than the real part.