Search

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 uw/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 silicon nitride ring resonator with implanted ytterbium ions offers a means for greatly enhanced ion–light interactions in an integrated optics platform. Rare-earth ions in solids are of particular interest for quantum information storage and processing because of the long coherence times of the 4f states1. In the past few years, substantial progress has been made by using ensembles of ions2,3,4,5,6 and single ions7,8,9,10. However, the weak optical transitions within the 4f manifold pose a great challenge to the optical interaction with a single rare-earth ion on a single-photon level. Here, we demonstrate a ninefold enhanced ion–light interaction (Purcell effect11) in an integrated-optics-based, fibre-coupled silicon nitride (Si3N4) ring resonator with implanted ytterbium ions (Yb3+). We unveil the one-, two- and three-dimensional contributions to the Purcell factor as well as the temperature-dependent decoherence and depolarization of the ions. The results indicate that this cavity quantum electrodynamics (QED) system has the potential of interfacing single rare-earth ions with single photons on a chip.

We use quantum detector tomography to investigate the detection mechanism in WSi nanowire superconducting single photon detectors (SSPDs). To this purpose, we fabricated a 250nm wide and 250nm long WSi nanowire and measured its response to impinging photons with wavelengths ranging from $\lambda$ = 900 nm to $\lambda$ = 1650 nm. Tomographic measurements show that the detector response depends on the total excitation energy only. Moreover, for energies Et > 0.8eV the current energy relation is linear, similar to what was observed in NbN nanowires, whereas the current-energy relation deviates from linear behaviour for total energies below 0.8eV.

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.

Si nanocrystals embedded in SiO 2 doped with P and Au at concentrations in the range of 1×10 18 –3×10 20 cm −3 exhibit photoluminescence quenching. Upon increasing the Au concentration, a gradual decrease in nanocrystal photoluminescence intensity is observed. Using a statistical model for luminescence quenching, we derive a typical radius of ∼3 nm for nanocrystals luminescing around 800 nm. Au doping also leads to a luminescence lifetime reduction, which is attributed to energy transfer between adjacent Si nanocrystals, possibly mediated by the presence of Au in the form of ions or nanocrystals. Doping with P at concentrations up to 3×10 19 cm −3 leads to a luminescence enhancement, most likely due to passivation of the nanocrystal–SiO 2 interfaces. Upon further P doping the nanocrystal luminescence gradually decreases, with little change in luminescence lifetime.

Poly(vinylpyrrolidone)−water solutions of 60−80 wt % PVP apparently do not freeze when cooled far below the expected freezing point. Such solutions remain highly viscous liquids and ultimately become glasses. It has been reported before that when small-angle X-ray scattering from these solutions is measured, occasionally very sharp peaks appear. The collection of those peaks, that have, in general, a short lifetime, constitutes a hexagonal planar reciprocal lattice. We present new, extensive small-angle scattering measurements, from which we are able to collect statistically relevant numbers of such peaks. This will allow us to infer more about their origin. The scattering results that are obtained on low concentration solutions show the existence of wavy layer structures, which may have a connection with a hexagonal structure at higher concentration. Most important is, however, that we found that the scattering spectra, time averaged in frames of 30 s or more, show a pronounced time dependence, be it ove...

We have studied surface-plasmons on metal hole arrays combined with gain and observe surface plasmon laser action. We will discuss the feedback mechanism, angular beam profile, and spatial emission profile of this surface plasmon laser.

Rare-earth-doped SiO2 colloids can be grown in a wet chemical procedure by adding the appropriate rare earth chloride to a solution of tetraethoxysilane, glacial acetic acid, and water. The size polydispersity of the particles can be controlled in a seeded growth process, and monodisperse Er3+-doped particles were grown by covering 183-nm-diameter seed particles with a 5−6-nm-thick (Er-doped) shell. After annealing, typical Er luminescence is observed with long luminescence lifetimes.

Silicon nitride (Si3N4) optical ring resonators provide exceptional opportunities for low-loss integrated optics. Here we study the transmission through a multimode waveguide coupled to a Si3N4 ring resonator. By coupling single-mode fibers to both input and output ports of the waveguide we selectively excite and probe combinations of modes in the waveguide. Strong asymmetric Fano resonances are observed and the degree of asymmetry can be tuned through the positions of the input and output fibers. The Fano resonance results from the interference between modes of the waveguide and light that couples resonantly to the ring resonator. We develop a theoretical model based on the coupled mode theory to describe the experimental results. The large extension of the optical modes out of the Si3N4 core makes this system promising for sensing applications.

Parametric down-conversion produces quantum states of photon pairs. We measure the relative importance of stimulated pair emission when all photons are emitted into the same mode and compare this to pair emission into different modes.

Surface plasmons in metal hole arrays have been studied extensively in the context of extraordinary optical transmission, but so far these arrays have not been studied as resonators for surface plasmon lasing at optical frequencies. We experimentally study a metal hole array with a semiconductor (InGaAs) gain layer placed in close (20 nm) proximity of the metal hole array. As a function of increasing pump power, we observe an intense and spectrally narrow peak, with a clear threshold. This laser emission is donut shaped and radially polarized. Three experimental observations support that the system shows surface plasmon lasing. First, the full wavelength dispersion of the observed resonances can be understood by using a single surface plasmon mode of the system. Second, the polarization of these resonances is as expected for surface plasmons. Third, the magnitude of the avoided crossing, which results from mode coupling at the holes, has a similar magnitude as found in simulations using surface plasmons.

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.

Spatial quantum correlations are an important resource for quantum imaging and future quantum technologies based on high-dimensional entanglement. We experimentally explore spatial correlations in four-photon states generated by a highly efficient parametric down-conversion process in a 2 mm long periodically poled KTP crystal. The four-photon states produced in the crystal contain a contribution due to independent double pairs produced via spontaneous pair emission and a highly correlated four-photon state produced by stimulated emission of a second photon pair. We separate this contribution and introduce a joint spatial density for stimulated pair emission. We observe a maximum visibility $\ensuremath{\chi}=0.25$ of the four-photon state for a 1.5 nm bandpass filter.

We explore four-photon spatial entanglement created by stimulated emission of photon pairs in a 2 mm long periodically poled KTP crystal. We vary the experimental conditions to explore and optimize the visiblity of stimulated pairs.

We demonstrate high fidelity quantum state reconstruction of coherent and thermal states of light using a single NbN superconducting single photon detector (SSPD). SSPDs outperform commonly used avalanche photodiodes for the task of quantum state reconstruction.

We present a method to transfer free standing photonic crystal membranes to a low refractive index substrate. These structures are optically flat and we compare the optical properties with the properties of free standing membranes.

Photonic graphene is a two-dimensional photonic crystal structure that is analogous to graphene. We use 5 mm diameter ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ rods placed on a triangular lattice with a lattice constant $a=8\text{ }\text{ }\mathrm{mm}$ to create an isolated conical singularity in the photonic band structure at a microwave frequency of 17.6 GHz. At this frequency, the measured transmission of microwaves through a perfectly ordered structure enters a pseudodiffusive regime where the transmission scales inversely with the thickness $L$ of the crystal ($L/a\ensuremath{\gtrsim}5$). The transmission depends critically on the configuration of the edges: distinct oscillations with an amplitude comparable to the transmission are observed for structures terminated with zigzag edges, while these oscillations are absent for samples with a straight edge configuration.

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 SiO 2 pillars.

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.

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.

We measured the transmission of a large array of holes in an optically thick gold film, immersed in liquids of different refractive indices. For a large difference in refractive index between the substrate and the liquid $(\ensuremath{\Delta}n\ensuremath{\gtrsim}0.05)$, the transmission spectra contain separate resonances, due to surface plasmons propagating on each of the metal-to-dielectric interfaces. When the index difference is reduced we observe an avoided crossing between a strong low-energy mode and a weak high-energy mode. The low-energy mode becomes broader and gains amplitude at the expense of the high-energy mode. For an index-matched array, a single broad resonance remains. These observations provide direct evidence that the two surface plasmon modes on both sides of the interface are coupled.

We propose an experiment to directly probe the local response of a superconducting single photon detector using a sharp metal tip in a scattering scanning near-field optical microscope. The optical absorption is obtained by simulating the tip-detector system, where the tip-detector is illuminated from the side, with the tip functioning as an optical antenna. The local detection efficiency is calculated by considering the recently introduced position-dependent threshold current in the detector. The calculated response for a 150 nm wide detector shows a peak close to the edge that can be spatially resolved with an estimated resolution of ∼ 20 nm, using a tip with parameters that are experimentally accessible.

We report on the demonstration of a high finesse micro-optomechanical system and identify potential applications ranging from optical cooling to weak force detection to massive quantum superpositions. The system consists of a high quality $30\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ diameter flat dielectric mirror cut from a larger substrate with a focused ion beam and attached to an atomic force microscope cantilever. Cavity ring-down measurements performed on a 25 mm long Fabry-P\'erot cavity with the $30\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ mirror at one end show an optical finesse of 2100. Numerical calculations show that the finesse is not diffraction limited and that orders of magnitude higher finesse should be possible. A mechanical quality factor of more than ${10}^{5}$ at pressures below ${10}^{\ensuremath{-}3}\text{ }\text{ }\mathrm{mbar}$ is demonstrated for the cantilever with a mirror attached.

We present an optical study of closely-spaced self-assembled InAs/GaAs quantum dots. The energy spectrum and correlations between photons subsequently emitted from a single pair provide not only clear evidence of coupling between the quantum dots but also insight into the coupling mechanism. Our results are in agreement with recent theories predicting that tunneling is largely suppressed between nonidentical quantum dots and that the interaction is instead dominated by dipole-dipole coupling and phonon-assisted energy transfer processes., Comment: 4 pages, 4 figures, to appear in Phys. Rev

We present a quantum mechanical description of parametric down-conversion and phase-matching of Bloch-waves in non-linear photonic crystals. We discuss the theory in one-dimensional Bragg structures giving a recipe for calculating the down-converted emission strength and direction. We exemplify the discussion by making explicit analytical predictions for the emission amplitude and direction from a one-dimensional structure that consists of alternating layers of Al0.4Ga0.6As and Air. We show that the emission is suitable for the extraction of polarization-entangled photons., 12 pages

The ultrafast response of a high-reflectivity GaAs/AlAs Bragg mirror to optical pumping is investigated for all-optical switching applications. Both Kerr and free carrier nonlinearities are induced with 100 fs, 780 nm pulses with a fluence of 0.64 kJ/m^2 and 0.8 kJ/m^2. The absolute transmission of the mirror at 931 nm increases by a factor of 27 from 0.0024% to 0.065% on a picosecond timescale. These results demonstrate the potential for a high-reflectivity ultrafast switchable mirror for quantum optics and optical communication applications. A design is proposed for a structure to be pumped below the bandgaps of the semiconductor mirror materials. Theoretical calculations on this structure show switching ratios up to 2200 corresponding to switching from 0.017% to 37.4% transmission., 4 pages, 4 figures, submitted to Applied Physics Letters

We demonstrate how a beam splitter in combination with different light sources can be used as an optimal universal 1 ! 2 quantum cloner and as an optimal universal quantum NOT machine for the polarization qubit of a single photon. For the cloning a source of single photons with maximally mixed polarization is required and for the NOT operation a source of maximally entangled photon pairs. We demonstrate both operations with near optimal fidelity. Our scheme can be generalized in a natural way to clone and NOT the spin state of electrons.

We investigate numerically the existence of photonic band gaps in woodpile crystals. We present a numerical method specifically developed to solve Maxwell's equations in such photonic structures. It is based upon a rigorous mathematical formulation and leads to a considerable improvement of the convergence speed as compared to other existing numerical methods. We tested our method by comparing the calculated reflectivity with measurements on an actual sample, i.e., a silicon woodpile photonic crystal designed for 1.5 microm wavelength. Excellent agreement is obtained, provided the main structural imperfections of the sample are taken into account. We show that the existence of photonic band gaps in woodpile crystals requires an index contrast higher than 2.05 +/- 0.01. The effects of imperfections of such structures with an index contrast equal to 2.25 are also investigated. Thus, the relative band gap width falls from 3.5% to 2.2% with structurals imperfection similar to those of the sample.

We investigate the use of a superconducting nano-detector as a novel near-field probe. In contrast to conventional scanning near-field optical microscopes, the nano-detector absorbs and detects photons in the near-field. We show that this absorption-based probe has a higher collection efficiency and investigate the details of the interaction between the nano detector and the dipole emitter. To this end, we introduce a multipole model to describe the interaction. Calculations of the local density of states show that the nano-detector does not strongly modify the emission rate of a dipole, especially when compared to traditional metal probes.

We observe a 104 times enhancement of the second harmonic generation of 1.535 μm laser light in a two-dimensional AlGaAs photonic crystal at normal incidence. The linear properties of the optical modes that are resonant with the fundamental frequency are well described by a Fano model and have a typical quality factor of 100. We present an extended version of the Fano interpretation to quantitatively analyze the second harmonic signal using the measured linear properties as input and find good agreement with the data.