T. Zijlstra

Fabrication of two-dimensional photonic crystal waveguides for 1.5 μm in silicon by deep anisotropic dry etching

Fabrication process for sharp waveguide bends in a two-dimensional photonic band gap structure in silicon is developed. The waveguide bend is defined by removing a row of pillars in a two-dimensional photonic crystal of 5 μm long, 205 nm diameter pillars placed on a square lattice with a pitch of 570 nm. To meet the severe nanotolerance requirements in such a device the SF6/O2 electron cyclotron resonance plasma process at reduced temperature is tailored to extreme profile control. The impact of main plasma parameters—i.e., temperature, oxygen/fluorine content, and ion energy—on the sidewall passivation process is unraveled in detail. Crystallographic orientation preference in the etch rate is observed.

Enhanced telecom wavelength single-photon detection with NbTiN superconducting nanowires on oxidized silicon

Superconducting nanowire single-photon detectors (SNSPDs) have emerged as a highly promising infrared single-photon detector technology. Next-generation devices are being developed with enhanced detection efficiency (DE) at key technological wavelengths via the use of optical cavities. Furthermore, new materials and substrates are being explored for improved fabrication versatility, higher DE, and lower dark counts. We report on the practical performance of packaged NbTiN SNSPDs fabricated on oxidized silicon substrates in the wavelength range from 830 to 1700 nm. We exploit constructive interference from the SiO2/Si interface in order to achieve enhanced front-side fiber-coupled DE of 23.2 % at 1310 nm, at 1 kHz dark count rate, with 60 ps full width half maximum timing jitter.

Balancing the etching and passivation in time-multiplexed deep dry etching of silicon

For the Bosch deep silicon dry etch process with SF6–C4F8 a quantitative approach is developed. Essential plasma surface interactions and the transport properties of ions and radicals in high aspect ratio structures are unravelled. Balancing the interactions during etching and passivation pulses is essential for maximal profile control. In the anisotropic regime the etch rate is aspect ratio dependent largely due to depletion of fluorine radicals and with some involvement of passivation polymer redeposition. The anisotropic process tends to stop at a limiting aspect ratio because of improper removal of polymer passivation at the trench bottom. Both higher ion flux and ion energy are found to be crucial to push the Bosch process to higher achievable aspect ratios. Practical process implications are discussed. In situ ellipsometry shows that the polymer passivation step is a complex process with an ion induced component. More efficient removal of the passivation layer at the trench bottom by adjusting the p...

Generation of correlated photon pairs in a chalcogenide As2S3 waveguide

We demonstrate a 1550 nm correlated photon-pair source in an integrated glass platform—a chalcogenide As2S3 waveguide. A measured pair coincidence rate of 80 s−1 was achieved using 57 mW of continuous-wave pump. The coincidence to accidental ratio was shown to be limited by spontaneous Raman scattering effects that are expected to be mitigated by using a pulsed pump source.

Contribution of dielectrics to frequency and noise of NbTiN superconducting resonators

We study NbTiN resonators by measurements of the temperature dependent resonance frequency and frequency noise. Additionally, resonators are studied covered with SiOx dielectric layers of various thicknesses. The resonance frequency develops a nonmonotonic temperature dependence with increasing SiOx layer thickness. The increase in the noise is independent of the SiOx thickness, demonstrating that the noise is not dominantly related to the low temperature resonance frequency deviations.

Amorphous silicon waveguides for microphotonics

Amorphous silicon a-Si was made by ion irradiation of crystalline silicon with 1×1015 Xe ions cm−2 at 77 K in the 1–4 MeV energy range. Thermal relaxation of the amorphous network at 500 °C for 1 h leads to an amorphous layer with a refractive index of n=3.73, significantly higher than that of crystalline silicon (n=3.45 at λ=1.55 μm). a-Si can thus serve as a waveguide core in Si based optical waveguides. Channel waveguides were made by anisotropic etching of a 1.5 μm silicon-on-insulator structure that was partly amorphized. Transmission measurements of these waveguides as function of the amorphous silicon length show that the a-Si part of the waveguides exhibit a modal propagation loss of 70 cm−1 (0.03 dB μm−1) and a bulk propagation loss of 115 cm−1 (0.05 dB μm−1). Losses due to sidewall roughness are estimated, and are negligible compared to the modal loss.

Kinetics and crystal orientation dependence in high aspect ratio silicon dry etching

A quantitative study of dry etch behavior in deep silicon trenches in high density plasmas (electron cyclotron resonance, inductively coupled plasma) at low temperatures (160–210 K) is presented. The quantitative approach implies etch behavior being studied in relation to the relevant particle fluxes (atomic F and O and ions) as measured by in situ diagnostics. Two etch modes are observed. In one mode faceting shows up as due to crystallographic orientation preference, i.e., Si〈111〉 being etched slower than Si〈100〉. In the other mode the normal anisotropic ion-induced behavior is observed. Controlled switch from one mode to the other is studied under influence of process parameters like pressure, ion energy, and substrate temperature. The second part of this study deals with aspect ratio dependent etching (ARDE). Both vertical and horizontal trenches have been taken into account as to distinguish between radical and ion-induced effects. The flux of radical species into the deep trench is governed by Knuds...

Generation of degenerate, factorizable, pulsed squeezed light at telecom wavelengths

We characterize a periodically poled KTP crystal that produces an entangled, two-mode, squeezed state with orthogonal polarizations, nearly identical, factorizable frequency modes, and few photons in unwanted frequency modes. We focus the pump beam to create a nearly circular joint spectral probability distribution between the two modes. After disentangling the two modes, we observe Hong-Ou-Mandel interference with a raw (background corrected) visibility of 86% (95%) when an 8.6 nm bandwidth spectral filter is applied. We measure second order photon correlations of the entangled and disentangled squeezed states with both superconducting nanowire single-photon detectors and photon-number-resolving transition-edge sensors. Both methods agree and verify that the detected modes contain the desired photon number distributions.

Position controlled nanowires for infrared single photon emission

We report the experimental demonstration of single-photon and cascaded photon pair emission in the infrared, originating from a single InAsP quantum dot embedded in a standing InP nanowire. A regular array of nanowires is fabricated by epitaxial growth on an electron-beam patterned substrate. Photoluminescence spectra taken on single quantum dots show narrow emission lines. Superconducting single photon detectors, which have a higher sensitivity than avalanche photodiodes in the infrared, enable us to measure auto and cross correlations. Clear antibunching is observed [g(2)(0) = 0.12] and we show a biexciton–exciton cascade, which can be used to create entangled photon pairs.

Reduced frequency noise in superconducting resonators

We report a reduction in the frequency noise in coplanar waveguide superconducting resonators. The reduction of 7 dB is achieved by removing the exposed dielectric substrate surface from the region with high electric fields and by using NbTiN. In a model-analysis the surface of NbTiN is found to be a negligible source of noise, experimentally supported by a comparison with NbTiN on SiOx resonators. The reduction is additive to decreasing the noise by widening the resonators.