Nanoscale Gap-Plasmon-Enhanced Superconducting Photon Detectors at Single-Photon Level.

With a growing demand for detecting light at the single-photon level in various fields, researchers are focused on optimizing the performance of superconducting single-photon detectors (SSPDs) by using multiple approaches. However, input light coupling for visible light has remained a challenge in the development of efficient SSPDs. To overcome these limitations, we developed a novel system that integrates NbN superconducting microwire photon detectors (SMPDs) with gap-plasmon resonators to improve the photon detection efficiency to 98% while preserving all detector performance features, such as polarization insensitivity. The plasmonic SMPDs exhibit a hot-belt effect that generates a nonlinear photoresponse in the visible range operated at 9 K (∼0.64Tc), resulting in a 233-fold increase in phonon-electron interaction factor (γ) compared to pristine SMPDs at resonance under CW illumination. These findings open up new opportunities for ultrasensitive single-photon detection in areas like quantum information processing, quantum optics, imaging, and sensing at visible wavelengths.

Near-Field Generation and Control of Ultrafast, Multipartite Entanglement for Quantum Nanoplasmonic Networks.

For a quantum Internet, one needs reliable sources of entangled particles that are compatible with measurement techniques enabling time-dependent, quantum error correction. Ideally, they will be operable at room temperature with a manageable decoherence versus generation time. To accomplish this, we theoretically establish a scalable, plasmonically based archetype that uses quantum dots (QD) as quantum emitters, known for relatively low decoherence rates near room temperature, that are excited using subdiffracted light from a near-field transducer (NFT). NFTs are a developing technology that allow rasterization across arrays of qubits and remarkably generate enough power to strongly drive energy transitions on the nanoscale. This eases the fabrication of QD media, while efficiently controlling picosecond-scale dynamic entanglement of a multiqubit system that approaches maximum fidelity, along with fluctuation between tripartite and bipartite entanglement. Our strategy radically increases the scalability and accessibility of quantum information devices while permitting fault-tolerant quantum computing using time-repetition algorithms.

Flexing Piezoelectric Diphenylalanine-Plasmonic Metal Nanocomposites to Increase SERS Signal Strength.

Piezoelectric quasi-1D peptide nanotubes and plasmonic metal nanoparticles are combined to create a flexible and self-energized surface-enhanced Raman spectroscopy (SERS) substrate that strengthens SERS signal intensities by over an order of magnitude compared to an unflexed substrate. The platform is used to sense bovine serum albumin, lysozyme, glucose, and adenine. Finite-element electromagnetic modeling indicates that the signal enhancement results from piezoelectric-induced charge, which is mechanically activated via substrate bending. The results presented here open the possibility of using peptide nanotubes on conformal substrates for in situ SERS detection.

Controlled Cavity-Free, Single-Photon Emission and Bipartite Entanglement of Near-Field-Excited Quantum Emitters.

We report theoretical statistics of 1- and 2-qubit (bipartite) systems, namely, photon antibunching and entanglement, of near-field excited quantum emitters. The sub-diffraction focusing of a plasmonic waveguide is shown to generate enough power over a sufficiently small region (<50 × 50 nm2) to strongly drive quantum emitters. This enables ultrafast (10-14 s) single-photon emission as well as creates entangled states between two emitters when performing a controlled-NOT operation. A comparative analysis of silicon and near-zero index materials demonstrates advantages and uncovers challenges of embedding quantum emitters for single-photon emission and for bipartite entanglement. The use of a movable plasmonic waveguide, in lieu of stationary nanostructures, allows high-speed rasterization between sets of qubits and enables spatially flexible data storage and quantum information processing. Furthermore, the sub-diffraction focusing of the waveguide is shown to achieve cavity-free dynamic entanglement. This greatly reduces fabrication constraints and increases the speed and scalability of nanophotonic quantum devices.

Genetic algorithm optimization of high order surface etched grating tunable laser array.

A genetic algorithm is developed with a view to optimizing surface-etched grating tunable lasers over a large optimization space comprised of several variables. Using this approach, a new iteration of slotted lasers arrays are optimized showing significant improvements over previous designs. Output power, lower grating order, fabrication tolerance and performance at high temperatures are among key parameters improved. The new designs feature a much lower grating order (24-29) than used previously (37). The biggest improvement is a near doubling to slope efficiency to 0.1-0.13 mW/mA, with wavelengths from the array covering the C-band . The designs show a reduced sensitivity to etch depth variations. Designs with linewidths down to 100 kHz are also simulated. This algorithm can be readily applied to different wafer materials to efficiently generate slotted lasers designs at new wavelengths.

Comparison of Metal Adhesion Layers for Au Films in Thermoplasmonic Applications.

If thermoplasmonic applications such as heat-assisted magnetic recording are to be commercially viable, it is necessary to optimize both thermal stability and plasmonic performance of the devices involved. In this work, a variety of different adhesion layers were investigated for their ability to reduce dewetting of sputtered 50 nm Au films on SiO2 substrates. Traditional adhesion layer metals Ti and Cr were compared with alternative materials of Al, Ta, and W. Film dewetting was shown to increase when the adhesion material diffuses through the Au layer. An adhesion layer thickness of 0.5 nm resulted in superior thermomechanical stability for all adhesion metals, with an enhancement factor of up to 200× over 5 nm thick analogues. The metals were ranked by their effectiveness in inhibiting dewetting, starting with the most effective, in the order Ta > Ti > W > Cr > Al. Finally, the Au surface-plasmon polariton response was compared for each adhesion layer, and it was found that 0.5 nm adhesion layers produced the best response, with W being the optimal adhesion layer material for plasmonic performance.

Tuning behaviour of slotted vernier widely tunable lasers.

A detailed thermo-optic model combining the 1D transfer matrix method and a 3D finite element method is developed and used to simulate a widely tunable vernier laser based on surface etched slots. The model is used to investigate the experimentally observed tuning patterns. At low injection currents, carrier tuning dominates, while at high currents, thermal tuning is the dominant mechanism. These lasers are very simple to fabricate and have a wide tuning over 50 nm, but SMSR and linewidth performance is not yet optimised. Simulations give an insight into the observed tuning efficiency and linewidth performance of the lasers, with high carrier densities in the grating regions being identified as a key area, which is presently limiting these parameters.

Less is More: Improved Thermal Stability and Plasmonic Response in Au Films via the Use of SubNanometer Ti Adhesion Layers.

The use of a metallic adhesion layer is known to increase the thermo-mechanical stability of Au thin films against solid-state dewetting, but in turn results in damping of the plasmonic response, reducing their utility in applications such as heat-assisted magnetic recording (HAMR). In this work, 50 nm Au films with Ti adhesion layers ranging in thickness from 0 to 5 nm were fabricated, and their thermal stability, electrical resistivity, and plasmonic response were measured. Subnanometer adhesion layers are demonstrated to significantly increase the stability of the thin films against dewetting at elevated temperatures (>200 °C), compared to more commonly used adhesion layer thicknesses that are in the range of 2-5 nm. For adhesion layers thicker than 1 nm, the diffusion of excess Ti through Au grain boundaries and subsequent oxidation was determined to result in degradation of the film. This mechanism was confirmed using transmission electron microscopy and X-ray photoelectron spectroscopy on annealed 0.5 and 5 nm adhesion layer samples. The superiority of subnanometer adhesion layers was further demonstrated through measurements of the surface-plasmon polariton resonance; those with thinner adhesion layers possessed both a stronger and spectrally sharper resonance. These results have relevance beyond HAMR to all Ti/Au systems operating at elevated temperatures.

CMOS-compatible multi-band plasmonic TE-pass polarizer.

A CMOS-compatible plasmonic TE-pass polarizer capable of working in the O, E, S, C, L, and U bands is numerically analyzed. The device is based on an integrated hybrid plasmonic waveguide (HPW) with a segmented metal design. The segmented metal will avoid the propagation of the TM mode, confined in the slot of the HPW, while the TE fundamental mode will pass. The TE mode is not affected by the metal segmentation since it is confined in the core of the HPW. The concept of the segmented metal can be exploited in a plasmonic circuit with HPWs as the connecting waveguides between parts of the circuit and in a silicon photonics circuit with strip or slab waveguides connecting the different parts of the circuit. Using 3D FDTD simulations, it is shown that for a length of 5.5 µm the polarization extinction ratios are better than 20 dB and the insertion losses are less than 1.7 dB over all the optical communication bands.

Focusing element formed by scattering structures in a planar dielectric waveguide.

A design process for creating integrated diffractive focusing elements for use in planar waveguides is presented. The elements consist of a linear array of holes etched into the core layer of a planar dielectric waveguide. A complete element is a few micrometers in size, while the individual holes are sub-micrometer. The focusing element was designed using analytical Mie theory. The performance of the complete 3D structure was then evaluated using 3D finite difference time domain (FDTD) method. A focal spot width of 227 nm (full width at half maximum) was predicted by 3D FDTD simulations with a peak intensity more than 10x the incident intensity and back-reflections lower than 1%. The focusing elements were fabricated using electron beam lithography and plasma etching. Fluorescence imaging was used to map the intensity in the waveguide core. The experimentally measured intensity maps were in good agreement with the simulations when the finite spatial resolution of the imaging system was taken into account.

Effective heat dissipation in an adiabatic near-field transducer for HAMR.

To achieve a feasible heat-assisted magnetic recording (HAMR) system, a near-field transducer (NFT) is necessary to strongly focus the optical field to a lateral region measuring tens of nanometres in size. An NFT must deliver sufficient power to the recording medium as well as maintain its structural integrity. The self-heating problem in the NFT causes materials failure that leads to the degradation of the hard disk drive performance. The literature reports NFT structures with physical sizes well below 1 micron which were found to be thermo-mechanically unstable at an elevated temperature. In this paper, we demonstrate an adiabatic NFT to address the central challenge of thermal engineering for a HAMR system. The NFT is formed by an isosceles triangular gold taper plasmonic waveguide with a length of 6 µm and a height of 50 nm. Our study shows that in the full optically and thermally optimized system, the NFT efficiently extracts the incident light from the waveguide core and can improve the shape of the heating source profile for data recording. The most important insight of the thermal performance is that the recording medium can be heated up to 866 K with an input power of 8.5 mW which is above the Curie temperature of the FePt film while maintaining the temperature in the NFT at 390 K without a heat spreader. A very good thermal efficiency of 5.91 is achieved also. The proposed structure is easily fabricated and can potentially reduce the NFT deformation at a high recording temperature making it suitable for practical HAMR application.

Optical and thermal analysis of the light-heat conversion process employing an antenna-based hybrid plasmonic waveguide for HAMR.

We investigate a tapered, hybrid plasmonic waveguide which has previously been proposed as an optically efficient near-field transducer (NFT), or component thereof, in several devices which aim to exploit nanofocused light. We numerically analyze how light is transported through the waveguide and ultimately focused via effective-mode coupling and taper optimization. Crucial dimensional parameters in this optimization process are identified that are not only necessary to achieve maximum optical throughput, but also optimum thermal performance with specific application towards heat-assisted magnetic recording (HAMR). It is shown that existing devices constructed on similar waveguides may benefit from a heat spreader to avoid deformation of the plasmonic element which we achieve with no cost to the optical efficiency. For HAMR, our design is able to surpass many industry requirements in regard to both optical and thermal efficiency using pertinent figure of merits like 8.5% optical efficiency.

Combining ε-Near-Zero Behavior and Stopped Light Energy Bands for Ultra-Low Reflection and Reduced Dispersion of Slow Light.

We investigate media which exhibits epsilon-near-zero (ENZ) behavior while simultaneously sustaining stopped light energy bands which contain multiple points of zero group velocity (ZGV). This allows the merging of state-of-the-art phenomena that was hitherto attainable in media that demonstrated these traits separately. Specifically, we demonstrate the existence of Ferrell-Berreman (FB) modes within frequency bands bounded by points of ZGV with the goal to improve the coupling efficiency and localization of light in the media. The FB mode is formed within a double layer, thin-film stack where at subwavelength thicknesses the structure exhibits a very low reflection due to ENZ behavior. In addition, the structure is engineered to promote a flattened frequency dispersion with a negative permittivity able to induce multiple points of ZGV. For proof-of-concept, we propose an oxide-semiconductor-oxide-insulator stack and discuss the useful optical properties that arise from combining both phenomena. A transfer matrix (TM) treatment is used to derive the reflectivity profile and dispersion curves. Results show the ability to reduce reflection below 0.05% in accordance with recent experimental data while simultaneously exciting a polariton mode exhibiting both reduced group velocity and group velocity dispersion (GVD).

Improved performance of tunable single-mode laser array based on high-order slotted surface grating.

We present an improved design of a wavelength-tunable single-mode laser array based on a high order surface grating with non-uniformly spaced slots. The laser array consists of 12 slotted single-mode lasers. The fabricated device exhibits a quasi-continuous tuning range of more than 36 nm over the temperature range from 10°C - 45°C covering the full C-band. All lasers in the array have stable single-mode operation with side mode suppression ratio of 50 dB due to the modified slot design. A spectral linewidth of less than 500 kHz was obtained for all channels in the array.

Widely tunable six-section semiconductor laser based on etched slots.

A six section widely tunable laser based on slots etched into the waveguide is presented. This laser is re-growth free which makes it suitable for photonics integration. To improve the laser performance, the front and the back facets are anti- reflection (AR) coated and the laser is integrated with a semiconductor optical amplifier. A tuning range of 55nm covering 12 supermodes with side mode suppression ratio (SMSR) >30dB is reported for the fabricated device using the Vernier tuning effect. This laser platform requires very simple fabrication compared with more complex superstructure gratings.