Gábor Szekeres

Plasmonic Structure Integrated Single-Photon Detector Configurations to Improve Absorptance and Polarization Contrast

Configurations capable of maximizing both the absorption component of system detection efficiency and the achievable polarization contrast were determined for 1550 nm polarized light illumination of different plasmonic structure integrated superconducting nanowire single-photon detectors (SNSPDs) consisting of p = 264 nm and P = 792 nm periodic niobium nitride (NbN) patterns on silica substrate. Global effective NbN absorptance maxima appear in case of p/s-polarized light illumination in S/P-orientation (γ = 90°/0° azimuthal angle) and the highest polarization contrast is attained in S-orientation of all devices. Common nanophotonical origin of absorptance enhancement is collective resonance on nanocavity gratings with different profiles, which is promoted by coupling between localized modes in quarter-wavelength metal-insulator-metal nanocavities and laterally synchronized Brewster-Zenneck-type surface waves in integrated SNSPDs possessing a three-quarter-wavelength-scaled periodicity. The spectral sensitivity and dispersion characteristics reveal that device design specific optimal configurations exist.

Optimized Superconducting Nanowire Single Photon Detectors to Maximize Absorptance

Dispersion characteristics of four types of superconducting nanowire single photon de- tectors, nano-cavity-array-(NCA-), nano-cavity-deflector-array-(NCDA-), nano-cavity-double-deflector- array-(NCDDA-) and nano-cavity-trench-array-(NCTA-) integrated (A-SNSPD) devices were optimized in three periodicity intervals commensurate with half-, three-quarter- and one surface plasmon polariton wavelength. The optimal configurations capable of maximizing absorptance in niobium nitride corre- spond to periodicity-dependent tilting in S-orientation (90 ◦ azimuthal orientation). In NCAI-A-SNSPDs absorptance maxima are reached at the plasmonic Brewster angle due to light tunneling. The absorp- tance maximum is attained in a wide plasmonic-pass-band in NCDAI1/2∗λ-A, inside a flat-plasmonic- pass-band in NCDAI3/4∗λ-A and inside a narrower plasmonic-band in NCDAIλ-A. In NCDDAI1/2∗λ- A bands of strongly-coupled cavity and propagating plasmonic modes cross, in NCDDAI3/4∗λ-A an inverted-plasmonic-band-gap develops, while in NCDDAIλ-A a narrow plasmonic-pass-band appears inside an inverted-minigap. The absorptance maximum is achieved in NCTAI1/2∗λ-A inside a plasmonic- pass-band, in NCTAI3/4∗λ-A at an inverted-plasmonic-band-gap center, while in NCTAIλ-A inside an inverted-minigap. The highest 95.05% absorptance is attained at perpendicular incidence onto NCTAIλ- A. Quarter-wavelength type cavity modes contribute to the near-field enhancement around NbN seg- ments except in NCDAIλ-A and NCDDAI3/4∗λ-A. The polarization contrast is moderate in NCAI-A- SNSPDs (∼ 10 2 ). NCDAI- and NCDDAI-A-SNSPDs make possible to attain considerably large polar- ization contrast (∼ 10 2 − 10 3 and ∼ 10 3 − 10 4 ), while NCTAI-A-SNSPDs exhibit a weak polarization selectivity (∼ 10 − 10 2 ).

Rotated grating coupled surface plasmon resonance on wavelength-scaled shallow rectangular gratings

Theoretical investigation of rotated grating coupling phenomenon was performed on a multilayer comprising 416-nmperiodic shallow rectangular polymer grating on bimetal film made of gold and silver layers. During the multilayer illumination by 532 nm wavelength p-polarized light the polar and azimuthal angles were varied. In presence of 0-35 nm, 0-50 nm and 15-50 nm thick polymer-layers at the valleys and hills splitting was observed on the dual-angle dependent reflectance in two regions: (i) close to 0° azimuthal angle corresponding to incidence plane parallel to the periodic pattern (P-orientation); and (ii) around ~33.5°/29°/30° azimuthal angle (C-orientation), in agreement with our previous experimental studies. The near-field study revealed that in P-orientation the E-field is enhanced at the glass side with p/2 periodicity at the first minimum appearing at 49°/50°/52° polar angles, and comprises maxima below both the valleys and hills; while E-field enhancement is observable both at the glass and polymer side with p-periodicity at the second minimum developing at 55°/63/64° tilting, comprising maxima intermittently below the valleys or above the hills. In Corientation coupled plasmonic modes are observable, involving modes propagating along the valleys at the secondary maxima appearing at ~35°/32°/32° azimuthal and ~49°/51°/56° polar angles, while modes confined along the polymer hills are observable at the primary minima, which are coupled most strongly at the ~31.5°/25°/28° azimuthal and ~55°/63°/66° polar angles. The secondary peak observable in C-orientation is proposed for biosensing applications, since the supported modes are confined along the valleys, where biomolecules prefer to attach.

Visualisation of plasmonic fields at the nanoscale with single molecule localisation microscopy

Plasmonic coupling of light to free electrons on metallic surfaces allows the confinement of electric fields far below the optical diffraction limit. Scattering processes of molecules placed into these plasmonic ‘hotspots’ are dramatically enhanced[1] which is commonly used to increase the sensitivity of spectroscopic techniques for biological and chemical sensor applications [2, 3]. Strikingly, hardly any measurement technique exists for the direct visualisation and characterisation of the underlying nanoscopic electromagnetic field distributions that either do not perturb the field [3, 4] or require complex electron beam imaging [5]. In this paper we introduce surface enhanced localisation microscopy (SELM), demonstrating the direct visualisation of fields on patterned plasmonic substrates using optical super resolution microscopy [6]. The observed strong photo-blinking behaviour of single molecules in plasmonic fields is exploited in SELM to map electromagnetic field distributions at nanometer resolutions.

Optimization of plasmonic structure integrated single-photon detector designs to enhance absorptance

Plasmonic structure integrated SNSPD configurations were optimized for 1550 nm p-polarized light illumination to maximize absorptance. Orientation dependent NbN absorptance, spectral sensitivity and dispersion characteristics were investigated.

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

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.

Plasmon enhanced single-photon detection

Novel infrared superconducting nanowire single-photon detectors (SNSPD) were designed, which comprise a meandered pattern of niobium-nitride (NbN) stripes and different integrated plasmonic structures on silica substrate. To enhance absorptance of 1550 nm wavelength p-polarized light, patterns with p=264 nm periodicity were investigated, while to enhance detection efficiency, patterns with P=792.5 nm periodicity commensurate with the wavelength of surface plasmon polaritons at silica-gold interface were also designed. In OC-SNSPDs integrated with ~quarter-photonicwavelength nano-optical cavity closed by a gold reflector, the highest 63/27 % absorptance was attained in p/P-pitch design at perpendicular incidence onto NbN patterns in P-orientation corresponding to incidence plane parallel to the stripes, due to the E-field antinode at the NbN-silica interface. In NCAI-SNSPDs, where each NbN stripe is located at the entrance of a quarter-plasmon-wavelength MIM nano-cavity, enhanced 85.1/34 % absorptance is attainable in p/Ppitch design at perpendicular incidence in S-orientation, when the incidence plane is perpendicular to the integrated pattern, due to collective resonances. The maximal 95.3/70.3 % absorptances are attained at large tilting corresponding to plasmonic Brewster angles via ultra-broadband tunneling. In NCDAI-SNSPDs the longer vertical gold segments with P-pitch, which can be embedded into the silica substrate via two-step lithography, enable to attain large absorptance at small polar angles in S-orientation, due to efficient grating-coupling phenomenon. The highest 92.7/75 % absorptances are attained at 19.85°/19.35° polar angles in p/P-pitch design. P-pitch NCDAI-SNSPD supporting coupled surface waves capable of ensuring synchronous E-field enhancement below the NbN stripes is proposed for detection efficiency maximization in specific spectral-bands.