Photonic crystals with split ring unit cells for subwavelength light confinement.

Here we report a photonic crystal with a split ring unit cell shape that demonstrates an order of magnitude larger peak electric field energy density compared with that of a traditional photonic crystal. Split ring photonic crystals possess several subwavelength tuning parameters, including split ring rotation angle and split width, which can be leveraged to modify light confinement for specific applications. Modifying the split ring's parameters allows for tuning of the peak electric field energy density in the split by over one order of magnitude and tuning of the air band edge wavelength by nearly 10 nm in the near infrared region. Designed to have highly focused optical energy in an accessible subwavelength gap, the split ring photonic crystal is well suited for applications including optical biosensing, optical trapping, and enhanced emission from a quantum dot or other nanoscale emitter that could be incorporated in the split.

Controlling optical return loss in production silicon photonic metamaterial fiber couplers.

A cost-efficient and low-complexity optical input/output (I/O) packaging solution is a substantial challenge for volume production of photonic integrated circuits. To address this, metamaterial fiber couplers are an attractive solution for integrated photonic devices especially for optical I/O, interfacing standard optical fibers to photonic chips. They offer the advantages of refractive index engineering to achieve better mode match as well as higher fabrication tolerances. Metamaterial waveguides, as a fundamental building block of these fiber couplers, have attracted tremendous attention in recent years. Here, we report on effective optical return loss control in Si metamaterial waveguide designs to achieve ultra-low reflection loss in CMOS-compatible silicon photonics implemented in a 300 mm production line. Low backscattering is a substantial consideration for a range of applications. Here, a return loss of better than -30dB is achieved.

Beam shaping for ultra-compact waveguide crossings on monolithic silicon photonics platform.

A beam shaping approach has been implemented to realize high-performance waveguide crossings based on cosine tapers. Devices with a compact footprint of 4.7µm×4.7µm were fabricated on the GLOBALFOUNDRIES 45 nm monolithic silicon photonics platform (45 CLO technology). Fabricated devices are found to be nearly wavelength independent (±0.035dB for 1260nm≤λ≤1360nm) with low insertion loss (∼0.2dB) and crosstalk (-35dB). The measured response of the devices is consistent with the three-dimensional finite-difference time-domain simulation results. The design stability is validated by measuring the device insertion loss on eight chips, which is found to be 0.197±0.017dB at the designed center wavelength of 1310 nm.

Realizing high transmission intensity in photonic crystal nanobeams using a side-coupling waveguide.

Side-coupled photonic crystal (PhC) nanobeam cavities were investigated to overcome challenges in measuring low-order resonances in traditional in-line PhC nanobeams that arise due to the trade-off between achieving high quality (Q)-factor and high transmission intensity resonances. On the same PhC nanobeam, we demonstrate that the side-coupling approach leads to measurable resonances even in cases in which high mirror strength unit cells severely limit the intensity of transmitted light through the in-line configuration. In addition, by coupling light directly into the cavity center, the design of side-coupled PhC nanobeams can be simplified such that high Q-factor PhC nanobeams can be achieved using only two different hole radii and uniform hole spacing.

Experimental realization of deep-subwavelength confinement in dielectric optical resonators.

The ability to highly localize light with strong electric field enhancement is critical for enabling higher-efficiency solar cells, light sources, and modulators. While deep-subwavelength modes can be realized with plasmonic resonators, large losses in these metal structures preclude most practical applications. We developed an alternative approach to achieving subwavelength localization of the electric and displacement fields that is not accompanied by inhibitive losses. We experimentally demonstrate a dielectric bowtie photonic crystal structure that supports mode volumes commensurate with plasmonic elements and quality factors that reveal ultralow losses. Our approach opens the door to the extremely strong light-matter interaction regime with, simultaneously incorporating both an ultralow mode volume and an ultrahigh quality factor, that had remained elusive in optical resonators.

Photonic crystal microring resonator for label-free biosensing.

A label-free optical biosensor based on a one-dimensional photonic crystal microring resonator with enhanced light-matter interaction is demonstrated. More than a 2-fold improvement in volumetric and surface sensing sensitivity is achieved compared to conventional microring sensors. The experimental bulk detection sensitivity is ~248nm/RIU and label-free detection of DNA and proteins is reported at the nanomolar scale. With a minimum feature size greater than 100nm, the photonic crystal microring resonator biosensor can be fabricated with the same standard lithographic techniques used to mass fabricate conventional microring resonators.

Visualizing Light Scattering in Silicon Waveguides with Black Phosphorus Photodetectors.

A black phosphorus photodetector is utilized to investigate the light-scattering patterns of a silicon waveguide through wavelength- and polarization-dependent scanning photocurrent measurements. The photocurrent signals exhibit similar patterns to the light-intensity distribution of the waveguide calculated by finite-difference time-domain simulations, suggesting that photoexcited electron-hole pairs in the silicon waveguide can be injected into phosphorene to induce its photoresponse.

Slow light Mach-Zehnder interferometer as label-free biosensor with scalable sensitivity.

The design, fabrication, and characterization of a label-free Mach-Zehnder interferometer (MZI) optical biosensor that incorporates a highly dispersive one-dimensional (1D) photonic crystal in one arm are presented. The sensitivity of this slow light MZI-based sensor scales with the length of the slow light photonic crystal region. The numerically simulated sensitivity of a MZI sensor with a 16 µm long slow light region is 115,000 rad/RIU-cm, which is sevenfold higher than traditional MZI biosensors with millimeter-length sensing regions. An experimental bulk refractive index detection sensitivity of 84,000 rad/RIU-cm is realized and nucleic acid detection is also demonstrated.

Porous silicon ring resonator for compact, high sensitivity biosensing applications.

A ring resonator is patterned on a porous silicon slab waveguide to produce a compact, high quality factor biosensor with a large internal surface area available for enhanced recognition of biological and chemical molecules. The porous nature of the ring resonator allows molecules to directly interact with the guided mode. Quality factors near 10,000 were measured for porous silicon ring resonators with a radius of 25 µm. A bulk detection sensitivity of 380 nm/RIU was measured upon exposure to salt water solutions. Specific detection of nucleic acid molecules was demonstrated with a surface detection sensitivity of 4 pm/nM.

Plasmonic Hot Electron Induced Photocurrent Response at MoS2-Metal Junctions.

We investigate the wavelength- and polarization-dependence of photocurrent signals generated at few-layer MoS2-metal junctions through spatially resolved photocurrent measurements. When incident photon energy is above the direct bandgap of few-layer MoS2, the maximum photocurrent response occurs for the light polarization direction parallel to the metal electrode edge, which can be attributed to photovoltaic effects. In contrast, if incident photon energy is below the direct bandgap of MoS2, the photocurrent response is maximized when the incident light is polarized in the direction perpendicular to the electrode edge, indicating different photocurrent generation mechanisms. Further studies show that this polarized photocurrent response can be interpreted in terms of the polarized absorption of light by the plasmonic metal electrode, its conversion into hot electron-hole pairs, and subsequent injection into MoS2. These fundamental studies shed light on the knowledge of photocurrent generation mechanisms in metal-semiconductor junctions, opening the door for engineering future two-dimensional materials based optoelectronics through surface plasmon resonances.