Superconducting single photon detectors consist of a narrow and thin wire of superconductor and can detect single photons with high efficiency and high speed. Simultaneous optimization of the detection efficiency and detector response time is difficult because the response time is set by the dimensions of the wire (cross section and length) while the detection efficiency is determined by both the internal detection mechanism and the dimensions of the wire. Wider and shorter wires are easier to fabricate, are more robust and lead to shorter detector reset time, but are generally less efficient. Experiments employing detector tomography provide important insights into the photon detection mechanism and indicate a detection mechanism where the edges of the wire are more efficient. This leads to a position dependent local detection efficiency that can be explained in the context of a photon-assisted-vortex-entry model and predicts an optimum wire width ~70 nm. A 50 nm wide silicon nanowire deposited on top of a 150 nm wide NbN nanowire directs light at 1550 nm wavelength to the edges and improves both the total absorption efficiency and the internal detection efficiency of the wire. The total absorption efficiency can be enhanced by 30% while the internal detection efficiency is increased by 70%. Assuming that the wire covers a similar area the detector response time is reduced 4-fold compared to the standard design using a 70 nm wide wire.
We explore photonic crystals based on a triangular lattice of rods. Non-circular rods break the inversion symmetry of the lattice and removes degenerate modes at the K-point (Dirac point) that are protected by symmetry from the band structure. A sizable complete photonic band gap of 7.5% relative gap width ▵w/w can be created by maximal symmetry breaking. We achieve this maximal symmetry breaking by rotating equilateral triangles over 30° relative to the lattice directions. The gap width depends on the rotation angle and a near perfect sinusoidal dependence is found, hinting at a simple mechanism for gap formalism. The gap can be further maximized by tuning the size of the triangles and we report a photonic bandgap map for a structure with inversion symmetry and with maximally broken inversion symmetry. Once a gap is formed interesting edge modes can be created by joining two crystals rotated by 180°. This restores inversion symmetry at the edge and creates line defect modes that are different for the different edges. These defect modes offer interesting possibilities for future nanophotonic devices where the on-chip functionality and localization of light is protected by the symmetry of the edges.
A conical tip made out of good conductive metal can be used to efficiently localize the optical field at the apex of the tip. For a tip of finite length both a field singularity (lightning rod effect) and a surface plasmon resonance contribute to the E-field enhancement. A strongly absorbing superconducting nanodetector placed in the optical near-field of the tip shows enhanced optical absorption. The design of an optimal tip- detector system is non-trivial because the strong damping by the detector shifts the resonance wavelength of the tip and significantly lowers the quality factor of the resonance. We compare calculations of the field enhancement of a bare tip to the absorption enhancement in the detector in the presence of the tip as a function of tip length, apex radius and semi-angle of the cone. The resonance of a 225 nm long gold tip in the presence of a detector occurs at ̴1000 nm and is red-shifted by 150 nm compared to the resonance of a bare tip.
We experimentally characterize sources of frequency degenerate down-converted photons at 826.4 nm generated
in 2 mm, 5 mm and 10 mm long periodically-poled KTP crystals. The crystals are pumped by 413.2 nm laser
pulses with 2 ps duration. The dispersion D=1.3 ps/mm puts a limit to the length over which phase matching
is efficient for a 2 ps pulse and provides a lower limit for the angular width of SPDC in the far-field. We
investigate the far-field distribution of SPDC produced by periodically-poled KTP crystals and compare this
with the calculated intensity distribution and find good agreement with theory. We also discuss the performance
of PPKTP in terms of nonlinearity and group velocity walk-off compared to other available materials.
The measured reflection spectra of two-dimensional photonic crystal slabs consist of an asymmetric peak on
top of an oscillating background. For p-polarized light, the asymmetry of the peak flips for angles of incidence
beyond Brewster's angle. We explain the observed line shapes with a Fano model that includes loss and use a
waveguide model to predict the resonance frequencies of the photonic crystal slab. Finite-difference time domain
calculations support the model and show that the resonance due to a higher order mode disappears when the
substrate refractive index is increased beyond ns = 2.04. This is readily explained by the cut-off condition of
the modes given by the waveguide model.
We have measured the angle and wavelength dependent transmission of index matched metal hole arrays, and
of arrays with a dielectric pillar in each hole. Index matching enhances the transmission, but also broadens the
resonances due to an enhanced coupling between plasmon and radiation modes. Hole arrays that are covered
with glass or have a glass pillar in each hole are created using an imprinting technique. We observe additional
waveguide modes in the transmission spectra of these arrays and discuss the avoided crossing that we observe
for the hybrid structure with dielectric pillars in the holes.