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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Enhanced absorptance of infrared single-photon detectors comprising plasmonic structure integrated NbN pattern on silicon substrate

G. Szekeres, Áron Sipos, Mária Csete

Published May 1, 2013• 0 citations
Publisher

Abstract

Summary form only given. Integration of plasmonic structures into superconducting nanowire single-photon detectors (SNSPD) is capable of resulting in enhanced absorptance [1], and makes possible to realize single plasmon detection [2], i.e opens novel avenues also in secure communication and quantum-key computing. In our previous studies we have presented single optical-cavity (OC) and nano-cavity-array integrated (NCAI) SNSPDs, covered by noble metal reflector, and horizontal as well as vertical gold segments integrated into the meandered niobium-nitride (NbN) pattern on sapphire substrate, respectively [1]. Patterns with periodicity (~600-710 nm) approximating the plasmon wavelength were also inspected to realize parallel electric optimization [1].Our recent studies revealed that integration of longer vertical gold segments embedded into the substrate, namely nano-cavity-deflector integrated NCDAI-SNSPD designs can result in huge NbN absorptance enhancement. However the patterning of sapphire substrates is a challenge, so here we present the optimal illumination directions of integrated devices based on silica substrate, which can be etched more easily. The periodicity (792.5 nm) of the NbN pattern was set to match the condition of optimal cavity filling in NCAIand NCDAI-SNSPDs (corresponding to m=1, k=4 condition in ref. [1]), as well as to approximate the 0.75*λplasmon wavelength. Perpendicular incidence onto OC-SNSPD in P-orientation results in 27% absorptance, while S-orientation is more advantageous in NCAI-SNSPD and in NCDAI-SNSPD. For practical applications perpendicular incidence onto NCAI-SNSPDs, where 34% absorptance is attainable, is already advantageous, while strongly enhanced 75 % absorptance is achievable in NCDAI-SNSPDs at 19.35° tilting (Fig. 1a, b).The near-field study (Fig. 1c) revealed that in OC-SNSPD the E-field antinode at the silica substrate interface promotes the NbN absorptance. In NCAI-SNSPD the resonant transmission and the localized resonances on the nano-cavity-array result in enhanced absorptance at small polar angles. Although the polar angle dependent absorptance exhibits a plasmonic-band-gap characteristic [3], only a small local maximum (20.8°, 29 %) can be observed due to the field concentration below the NbN segments. This originates from backward propagating Brewster waves with enhanced wavelength according to ksurface wave=kphoton-kgrating relation. The advantage of the NCDAI-SNSPD is that the gold deflectors prevent the re-radiation of surface plasmon polaritons, as a result large absorptance maximum appears at the orientation where the grating couples into SPPs. In NCDAI-SNSPDs high detection efficiency is attainable in contempt of small fill-factor, i.e. parallel electric optimization is realizable as a result of coupled resonances of local and propagating plasmonic modes.

DOI
10.1109/cleoe-iqec.2013.6801984

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Summary form only given. Integration of plasmonic structures into superconducting nanowire single-photon detectors (SNSPD) is capable of resulting in enhanced absorptance [1], and makes possible to realize single plasmon detection [2], i.e opens novel avenues also in secure communication and quantum-key computing. In our previous studies we have presented single optical-cavity (OC) and nano-cavity-array integrated (NCAI) SNSPDs, covered by noble metal reflector, and horizontal as well as vertical gold segments integrated into the meandered niobium-nitride (NbN) pattern on sapphire substrate, respectively [1]. Patterns with periodicity (~600-710 nm) approximating the plasmon wavelength were also inspected to realize parallel electric optimization [1].Our recent studies revealed that integration of longer vertical gold segments embedded into the substrate, namely nano-cavity-deflector integrated NCDAI-SNSPD designs can result in huge NbN absorptance enhancement. However the patterning of sapphire substrates is a challenge, so here we present the optimal illumination directions of integrated devices based on silica substrate, which can be etched more easily. The periodicity (792.5 nm) of the NbN pattern was set to match the condition of optimal cavity filling in NCAIand NCDAI-SNSPDs (corresponding to m=1, k=4 condition in ref. [1]), as well as to approximate the 0.75*λplasmon wavelength. Perpendicular incidence onto OC-SNSPD in P-orientation results in 27% absorptance, while S-orientation is more advantageous in NCAI-SNSPD and in NCDAI-SNSPD. For practical applications perpendicular incidence onto NCAI-SNSPDs, where 34% absorptance is attainable, is already advantageous, while strongly enhanced 75 % absorptance is achievable in NCDAI-SNSPDs at 19.35° tilting (Fig. 1a, b).The near-field study (Fig. 1c) revealed that in OC-SNSPD the E-field antinode at the silica substrate interface promotes the NbN absorptance. In NCAI-SNSPD the resonant transmission and the localized resonances on the nano-cavity-array result in enhanced absorptance at small polar angles. Although the polar angle dependent absorptance exhibits a plasmonic-band-gap characteristic [3], only a small local maximum (20.8°, 29 %) can be observed due to the field concentration below the NbN segments. This originates from backward propagating Brewster waves with enhanced wavelength according to ksurface wave=kphoton-kgrating relation. The advantage of the NCDAI-SNSPD is that the gold deflectors prevent the re-radiation of surface plasmon polaritons, as a result large absorptance maximum appears at the orientation where the grating couples into SPPs. In NCDAI-SNSPDs high detection efficiency is attainable in contempt of small fill-factor, i.e. parallel electric optimization is realizable as a result of coupled resonances of local and propagating plasmonic modes.
Superconducting nanowire single photon detectors with an optical resonant cavity structure are fabricated directly on the single crystal substrate. In order to enhance the photons absorption of incident light and improve system detection efficiency (SDE) efficiently, the structure of SNSPD was designed and optimized using the finite-difference time-domain analysis for different kinds of substrates. The NbN-SNSPDs fabricated on different substrates are measured and compared. The SDE of NbN-SNSPD on MgO substrate reaches 66 % at the wavelength of 940nm at a dark count rate of 100 cps by illuminating from the back of the device through single mode optical fiber.
Superconducting nanowire single-photon detectors (SNSPDs) are the leading technology for low noise, high efficiency infrared single-photon detection. The basic SNSPD consists of a nanowire patterned in an ultrathin superconducting thin film, which is cooled below its critical temperature and biased close to its critical current. The absorption of a single-photon creates a resistive region, triggering a fast output voltage pulse which can be readily amplified and registered. The excellent performance of SNSPDs at near-infrared and telecommunications wavelengths has led to their adoption in important applications such as quantum secure communications, single-photon spectroscopy and single-photon LIDAR. A clear challenge for the SNSPD community is to extend the spectral range of SNSPDs into the mid infrared, and to improve material uniformity to enable the realization of large area arrays for multimode or free space coupling. The aim of this work is to evaluate potential materials for next generation mid-infrared SNSPD arrays. In this work, thin films of polycrystalline NbN and amorphous MoSi have been optimized to test the uniformity of a multipixel array configuration composed of 8 nanowire meander structures covering 10 um x 10 um area, 100 nm width and 50% filling factor. The 8-pixels SNSPD arrays have been patterned on 8 nm thickness NbN grown on high resistivity silicon (HR Si) substrate at room temperature and at 800 °C exhibiting respectively 4.4 K and 7.3 K as mean critical temperature across the pixels. The 8- pixels SNSPD array patterned on 8 nm thickness MoSi cooling the HR Si substrate to -180 °C has exhibited a mean critical temperature of 3.2 K across the pixels. Optical properties have been measured by an attenuated 1550 nm laser diode source delivered by single mode optical fibre at a controlled distance from the chip in order to broadly illuminate the array. The optical properties have been studied only for the 8 nm thickness NbN SNSPD array grown at room temperature has demonstrated uniform optical properties across pixels exhibiting similar saturation of the internal efficiency over a large bias, similar dark count rate and similar timing jitter (about 137 ps) across pixels. In the single photon regime at 1550 nm, pixels 4 and 6 of the 8 nm thickness NbN SNSPD array exhibit 28.4% and 4.7% pixel detection efficiency as measured at the bias current 95% of their respective critical current at 2.2 K.
This study proposed what we believe to be a novel method for fabricating superconducting nanowire single-photon detectors (SNSPDs) with high efficiency, polarization insensitivity, and ultrafast response. To achieve these properties in niobium nitride (NbN) SNSPDs, the periodic four-split rings (PFSR) were positioned above the nanowires. This design uses the localized surface plasmon resonance to enhance the electric field around nanowires. For an incident light with a wavelength of 1550 nm, the PFSR-SNSPD structure achieved a polarization extinction ratio of 1.0064 and absorptions of 88.94% and 88.37% under TE and TM polarizations, respectively. The nanowire length was reduced by 85% using a meandering nanowire arrangement with a fill factor of 0.074.
Abstract Superconducting nanowire single-photon detectors (SNSPDs) have become increasingly important for both classical and quantum applications requiring single-photon sensing. Most current SNSPDs based on superconducting niobium nitride (NbN) ultrathin films are fabricated utilizing polycrystalline or amorphous superconductor films, and film inhomogeneity and defectivity can cause significant issues. Here, we present an all-epitaxial approach for fabricating SNSPDs with integrated metal reflectors, semiconductor (dielectric) resonators, and superconductor nanowire detectors. A working example is realized by molecular-beam epitaxy growth of a tri-layered heterostructure, consisting of a titanium nitride (TiN) reflector with ~ 92% reflectivity at the telecommunication wavelength (1550 nm), an aluminum nitride (AlN) resonator with a broad optical transparency window, and a NbN nanowire on c -plane sapphire wafer. The all-epitaxial approach provides a manufacturable platform with superior material properties, as well as uniformity across the wafer, which manifest themselves in tunable material parameters of the superconducting nanowire, including normal-state resistance, superconducting critical temperature, superconducting critical current, and kinetic inductance. The optimized material properties can be utilized to achieve excellent SNSPD performance, including wafer-scale uniformity in device characteristics and broadband (~300 nm) system detection efficiency (>90%) centered at 1550 nm, suitable for large-scale detector arrays and wavelength-division multiplexing (WDM) applications.
Single photon detection advanced rapidly in the last decade. The superconducting nanowire single-photon detector (SNSPD) is an emerging technology, which offers low dark count rates, broadband sensitivity from visible to mid-infrared wavelengths and very high quantum efficiency. All these features are appealing for future emerging applications in remote gas sensing, quantum key distribution and quantum information processing. By enclosing the detector in an optical cavity, tuned to the operating wavelength, the total system detection of the SNSPD is improved. Based on the cavity design, the detector absorption is boosted at the desired experimental wavelength. Devices have been fabricated and tested, in this thesis, with enhanced detection efficiency at near-infrared, infrared and mid-infrared wavelengths. NbTiN detectors fabricated on Ta2O5 / SiO2 distributed Bragg reflectors (DBR) for front-side coupling exhibit a system detection efficiency of ≈ 90% at λ = 940 nm for the best device measured. SNSPDs were also fabricated in NbTiN on metallic mirrors for back-side coupling for 1550 and 2333 nm experimental wavelengths. The first design exhibited a system detection efficiency of ≈ 84.5%, at λ = 1550 nm for the best device measured. The second exhibited low system detection efficiency since the device suffered from unexpectedly high dark count rate mainly due to blackbody radiation at 2.2 K. However, it was possible to measure the timing jitter of this device at 350 mK indicating a FWHM of 84 ps. Another technique to improve the total system detection efficiency of the SNSPD, is by fabricating a hairpin-shaped nanowire atop a single mode waveguide. In this design, photons travel through the waveguide and via evanescent coupling are absorbed in the nanowire. These SNSPD devices have been designed to thread along single mode Si ridge waveguides using a hairpin-shaped nanowire structure, which offers enhanced coupling and absorption efficiency via evanescent coupling. These ultra-thin superconducting MoSi detectors are precisely patterned on top of a single mode Si waveguide (600 nm) with alignment error less than 40 nm. Low temperature electrical and nano-optical characterization was carried out using a two-stage pulse tube (PT) coldhead, using an additional 4He / 3He sorption pump, low vibration characterization of SNSPD devices at 350 mK was carried out. Characterised devices exhibit low dark count rate <5 counts per second and timing jitter of ≈ 83 ps full width half maximum (FWHM) at wavelength (λ) = 1550 nm and at critical current ≈ 30 µA.