High-Q and high finesse silicon microring resonator.

We demonstrate the design, fabrication, and experimental characterization of a single transverse mode adiabatic microring resonator (MRR) implemented using the silicon-on- insulator (SOI) platform using local oxidation of silicon (LOCOS) approach. Following its fabrication, the device was characterized experimentally and an ultrahigh intrinsic Q-factor of ∼2 million with a free spectral range (FSR) of 2 nm was achieved, giving rise to a finesse of ∼1100, the highest demonstrated so far in SOI platform at the telecom band. We have further studied our device to analyze the source of losses that occur in the MRR and to understand the limits of the achievable Q-factor. The surface roughness was quantified using AFM scans and the root mean square roughness was found to be ∼ 0.32±0.03 nm. The nonlinear losses were further examined by coupling different optical power levels into the MRR. Indeed, we could observe that the nonlinear losses become more pronounced at power levels in the range of hundreds of microwatts. The demonstrated approach for constructing high-Q and high finesse MRRs can play a major role in the implementation of devices such as modulators, sensors, filters, frequency combs and devices that are used for quantum applications, e.g., photon pair generation.

CMOS-compatible electro-optical SRAM cavity device based on negative differential resistance.

The impending collapse of Moore-like growth of computational power has spurred the development of alternative computing architectures, such as optical or electro-optical computing. However, many of the current demonstrations in literature are not compatible with the dominant complementary metal-oxide semiconductor (CMOS) technology used in large-scale manufacturing today. Here, inspired by the famous Esaki diode demonstrating negative differential resistance (NDR), we show a fully CMOS-compatible electro-optical memory device, based on a new type of NDR diode. This new diode is based on a horizontal PN junction in silicon with a unique layout providing the NDR feature, and we show how it can easily be implemented into a photonic micro-ring resonator to enable a bistable device with a fully optical readout in the telecom regime. Our result is an important stepping stone on the way to new nonlinear electro-optic and neuromorphic computing structures based on this new NDR diode.

MoSe2/WS2 heterojunction photodiode integrated with a silicon nitride waveguide for near infrared light detection with high responsivity.

We demonstrate experimentally the realization and the characterization of a chip-scale integrated photodetector for the near-infrared spectral regime based on the integration of a MoSe2/WS2 heterojunction on top of a silicon nitride waveguide. This configuration achieves high responsivity of ~1 A W-1 at the wavelength of 780 nm (indicating an internal gain mechanism) while suppressing the dark current to the level of ~50 pA, much lower as compared to a reference sample of just MoSe2 without WS2. We have measured the power spectral density of the dark current to be as low as ~1 × 10-12 A Hz-0.5, from which we extract the noise equivalent power (NEP) to be ~1 × 10-12 W Hz-0.5. To demonstrate the usefulness of the device, we use it for the characterization of the transfer function of a microring resonator that is integrated on the same chip as the photodetector. The ability to integrate local photodetectors on a chip and to operate such devices with high performance at the near-infrared regime is expected to play a critical role in future integrated devices in the field of optical communications, quantum photonics, biochemical sensing, and more.

Ultra-high-Q substrate-mode coupled resonances in complementary THz metamaterial.

Achieving high-Q resonances in the THz frequency range is significant for applications such as sensors, filters, and emitters. A promising approach for obtaining such resonances is by using metamaterials. However, high-Q resonances in THz metamaterials are usually limited by metallic radiation losses in the meta-atoms. In this Letter, we investigate both experimentally and numerically a complementary metallic disk-hole array (CMA) that uses the coupling between lattice resonances and Fabry-Pérot cavity resonances, and features in-substrate modes with experimentally obtained record breaking Q-factors of up to 750. To the best of our knowledge, this is the highest quality factor measured for free-space-coupled metallic metamaterial structure at THz frequencies.

Enhanced Chiral Sensing at the Few-Molecule Level Using Negative Index Metamaterial Plasmonic Nanocuvettes.

Chirality is a fundamental property of biological molecules and some pharmaceutical molecules. Chiral molecules have a pair of chiral isomers (enantiomers) with opposite handedness. Although both enantiomers of the same molecule have identical chemical and physical properties, one enantiomer may be toxic to living organisms while the other one is harmless. The detection of these enantiomers is done using their small differential absorption between right and left circularly polarized light, known as circular dichroism (CD). Considering the macroscopic size of these molecules, combined with their small differential absorption, the obtained CD signal is very small, imposing a severe limitation on the minimal concentration that can be detected. Chiral plasmonic and metamaterial structures have been used to enhance the sensitivity of CD measurements by orders of magnitude through chiral density hot spots (super chiral fields). However, the large background signal due to these structures' intrinsic chirality limits the effectiveness of these methods. Contrary to absorption-based chiral sensing measurements (CD), fluorescence detection circular dichroism (FDCD) sensing can greatly improve chiral measurement sensitivity, down to the ultimate limit of a few and even a single chiral molecule. Like differential absorption, differential fluorescence also produces a weak signal at the few-chiral-molecule limit. However, here we demonstrate a negative-index metamaterial (NIM) cavity that acts as a "plasmonic nanocuvette" with globally enhanced volume super chiral fields. Moreover, the achiral structure of the plasmonic nanocuvette allows for completely background-free chiral sensing. We show that with NIM-cavity-enhanced FDCD, we can detect as low as a few tens of chiral molecules, well within the zeptomole range.

MEMS cantilever-controlled plasmonic colors for sustainable optical displays.

Conventional optical displays using indium tin oxide and liquid crystal materials present challenges for long-term sustainability. We show here a cost-effective and complementary metal-oxide semiconductor (CMOS)-compatible fast and full-range electrically controlled RGB color display. This is achieved by combining transmission-based plasmonic metasurfaces with MEMS (microelectromechanical systems) technology, using only two common materials: aluminum and silicon oxide. White light is filtered into RGB components by plasmonic metasurfaces made of aluminum nanohole arrays. The transmission through each color filter is modulated by MEMS miniaturized cantilevers fabricated with aluminum and silicon oxide on top of the color filters. We show that the relative transmission of a color subpixel can be freely modulated from 35 to 100%. The pixels can also operate well above 800 Hz for future ultrafast displays. Our work provides a road to future circular economic goals by exploiting advances in structural colors and MEMS technologies to innovate optical displays.

Optical reconfiguration and polarization control in semi-continuous gold films close to the percolation threshold.

Controlling and confining light by exciting plasmons in resonant metallic nanostructures is an essential aspect of many new emerging optical technologies. Here we explore the possibility of controllably reconfiguring the intrinsic optical properties of semi-continuous gold films, by inducing permanent morphological changes with a femtosecond (fs)-pulsed laser above a critical power. Optical transmission spectroscopy measurements show a correlation between the spectra of the morphologically modified films and the wavelength, polarization, and the intensity of the laser used for alteration. In order to understand the modifications induced by the laser writing, we explore the near-field properties of these films with electron energy-loss spectroscopy (EELS). A comparison between our experimental data and full-wave simulations on the exact film morphologies hints toward a restructuring of the intrinsic plasmonic eigenmodes of the metallic film by photothermal effects. We explain these optical changes with a simple model and demonstrate experimentally that laser writing can be used to controllably modify the optical properties of these semi-continuous films. These metal films offer an easy-to-fabricate and scalable platform for technological applications such as molecular sensing and ultra-dense data storage.

Enhancement of two-photon photoluminescence and SERS for low-coverage gold films.

Electromagnetic field enhancement (FE) effects occurring in thin gold films 3-12-nm are investigated with two-photon photoluminescence (TPL) and Raman scanning optical microscopies. The samples are characterized using scanning electron microscopy images and linear optical spectroscopy. TPL images exhibit a strong increase in the level of TPL signals for films thicknesses 3-8-nm, near the percolation threshold. For some thicknesses, TPL measurements reveal super-cubic dependences on the incident power. We ascribe this feature to the occurrence of very strongly localized and enhanced electromagnetic fields due to multiple light scattering in random nanostructures that might eventually lead to white-light generation. Raman images exhibit increasing Raman signals when decreasing the film thickness from 12 to 6-nm and decreasing signal for the 3-nm-film. This feature correlates with the TPL observations indicating that highest FE is to be expected near the percolation threshold.

Surface-dominated transport on a bulk topological insulator.

Topological insulators are guaranteed to support metallic surface states on an insulating bulk, and one should thus expect that the electronic transport in these materials is dominated by the surfaces states. Alas, due to the high remaining bulk conductivity, it is challenging to achieve surface-dominated transport. Here we use nanoscale four-point setups with a variable contact distance on an atomically clean surface of bulk-insulating Bi2Te2Se. We show that the transport at 30 K is two-dimensional rather than three-dimensional, that is, surface-dominated, and we find a surface state mobility of 390(30) cm(2) V(-1) s(-1) at 30 K at a carrier concentration of 8.71(7) × 10(12) cm(-2).