Breakthroughs in Photonics 2014: Relaxed Total Internal Reflection

A transverse-electric (TE)-pass polarization filter based on a slotted photonic crystal waveguide structure is proposed for application in photonic integrated circuits. The silicon-slab-based structure is fabrication-compatible with the standard silicon-on-insulator process technologies. Different structural parameters, such as the r/a ratio of the photonic crystal sections, slab height, slot width, and slot length, are optimized for improved performance. Moreover, the conversion efficiency of a mode of a W1 waveguide to that of a slot waveguide is analyzed for different starting positions of the slot in order to achieve a high extinction ratio of the TE wave over the transverse-magnetic wave. A 3D finite difference time domain simulation methodology, which has been verified against an earlier experimental study made by researchers on another structure, is adopted to evaluate the transmittance of the structure. It shows that the polarization filter, with a footprint of merely 6.28 μm×4.35 μm, offers an extinction ratio in the order of 22 dB; this corresponds to an effective extinction ratio of 3.5 dB μm−1 considering the total length (or 6.5 dB μm−1 considering only the slot length). TE transmittance of the device is calculated as ≈−3 dB at a wavelength of 1550 nm.

We show the existence of an inherent property of evanescent electromagnetic waves: spin-momentum locking, where the direction of momentum fundamentally locks the polarization of the wave. We trace the ultimate origin of this phenomenon to complex dispersion and causality requirements on evanescent waves. We demonstrate that every case of evanescent waves in total internal reflection, surface states and optical fibers/waveguides possesses this intrinsic spin-momentum locking. We also introduce a universal right-handed triplet consisting of momentum, decay and spin for evanescent waves. We derive the Stokes parameters for evanescent waves which reveal an intriguing result - every fast decaying evanescent wave is inherently circularly polarized with its handedness tied to the direction of propagation. We also show the existence of a fundamental angle associated with total internal reflection (TIR) such that propagating waves locally inherit perfect circular polarized characteristics from the evanescent wave. This circular TIR condition occurs if and only if the ratio of permittivities of the two dielectric media exceeds the golden ratio. Our work leads to a unified understanding of this spin-momentum locking in various nanophotonic experiments and sheds light on the electromagnetic analogy with the quantum spin hall state for electrons.

In an on-chip silicon-organic hybrid electro-optic (EO) modulator, the mode overlap with EO materials, in-device effective r33, and propagation loss are among the most critical factors that determine the performance of the modulator. Various waveguide structures have been proposed to optimize these factors, yet there is a lack of comprehensive consideration on all of them. In this Letter, a one-dimensional (1D) photonic crystal (PC) slot waveguide structure is proposed that takes all these factors into consideration. The proposed structure takes advantage of the strong mode confinement within a low-index region in a conventional slot waveguide and the slow-light enhancement from the 1D PC structure. Its simple geometry makes it robust to resist fabrication imperfections and helps reduce the propagation loss. Using it as a phase shifter in a Mach-Zehnder interferometer structure, an integrated silicon-organic hybrid EO modulator was experimentally demonstrated. The observed effective EO coefficient is as high as 490 pm/V. The measured half-wave voltage and length product is less than 1 V·cm and can be further improved. A potential bandwidth of 61 GHz can be achieved and further improved by tailoring the doping profile. The proposed structure offers a competitive novel phase-shifter design, which is simple, highly efficient, and with low optical loss, for on-chip silicon-organic hybrid EO modulators.

Mid infrared silicon photonic integrated components, namely grating couplers, strip and slot waveguides, slotted and un-slotted photonic crystal waveguides, photonic crystal microcavities are experimentally demonstrated in silicon-on-sapphire. Application in trace gas sensing is experimentally demonstrated. Progress towards monolithic integration of lasers and detectors are discussed.

We report on a theoretical and experimental study of the energy transfer between an optical evanescent wave, propagating in vacuum along the planar boundary of a dielectric material, and a beam of sub-relativistic electrons. The evanescent wave is excited via total internal reflection in the dielectric by an infrared (λ = 2 μm) femtosecond laser pulse. By matching the electron propagation velocity to the phase velocity of the evanescent wave, energy modulation of the electron beam is achieved. A maximum energy gain of 800 eV is observed, corresponding to the absorption of more than 1000 photons by one electron. The maximum observed acceleration gradient is 19 ± 2 MeV/m. The striking advantage of this scheme is that a structuring of the acceleration element's surface is not required, enabling the use of materials with high laser damage thresholds that are difficult to nano-structure, such as SiC, Al2O3 or CaF2.

Single molecule detection and fluorescence correlation spectroscopy on surfaces

Multi-component gas sensing based on slotted photonic crystal waveguide with liquid infiltration

Transmission properties of the periodic dielectric waveguide (PDWG) formed by aligning a sequence of dielectric cylinders in air are investigated theoretically. Unlike photonic crystal waveguides (PCWs), light confinement in a PDWG is due to total internal reflection. Besides, the dispersion relation of the guided modes is strongly influenced by the dielectric periodicity along the waveguide. The band structure for the guided modes is calculated using a finite-difference time-domain (FDTD) method. The first band is used for guiding light, which makes PDWG single mode. Transmission is calculated using the multiple scattering method for various S shaped PDWGs, each containing two opposite bends. When PDWG operates in appropriate frequency ranges, high transmission (above 90%) is observed, even if the radius of curvature of the bends is reduced to three wavelengths. This feature indicates that the guiding ability of PDWG can be made better than the conventional waveguide when used in an optical circuit. In addition, PDWG has the advantage that it can be bent to any arbitrary shape while still preserves the high transmission, avoiding the geometric restriction that PCWs are subject to.

Plasmonics-based waveguiding structures can deliver unprecedented degrees of wave-matter interaction, enhancing linear and nonlinear optical processes such as spectroscopy, sensing and signal processing, all within small form-factors and staggering device density. However, the main obstacle to a wider adoption has always been the excessive losses that scale poorly with optical confinement / localization [1-6]. Also in this work we demonstrate how can one utilize composite plasmonic waveguides with unparalleled alleviation of the loss-confinement tradeoff to achieve record Purcell factors within plasmonic waveguides [6]. Also I this talk we plan to discuss a novel class of nanoscale devices that address unmet performance demands for applications in data communications [1-6]. The performance of emerging generations of high-speed, integrated electronic circuits is increasingly dictated by interconnect density and latency as well as by power consumption. To alleviate these limitations, data communications using photons has been deployed, where photonic circuits and devices are integrated on platforms compatible with conventional electronic technologies. Within the dominant platform; namely Si, dielectric waveguides confine light via total internal reflection. This imposes bounds on minimizing device dimensions and density of integration. Those bounds arise due to the diffraction limit and the cross-coupling between neighbouring waveguides. Nanoscale Plasmonic waveguides provide the unique ability to confine light within a few nanometers and allow for near perfect transmission through sharp bends as well as efficient light distribution between orthogonally intersecting junctions. With these structures as a building block, new levels of optoelectronic integration and performance metrics for athermal transceivers with achievable bandwidths in excess of 500 Gbps as will be overviewed in this talk. In addition opportunities for the role that 2D materials may pay in propelling these record performance metrics even further will be projected [2]. 4.

To realize a high sensitivity gas sensor, a large wideband, temperature stabilized and large light confinement slow light structure in slotted photonic crystal waveguide (SPCW) is designed. The simulation results indicate that with a proper optimization of the SPCW structure, a very high group index of 105 can be achieved and the electric field enhancement factor would reach to 12. Combined with the correlation spectroscopy for signal processing, the detection limit of 10 ppm is achieved.

A comprehensive analysis of Raman mediated solitonic compression of pulses inside a silicon nanocrystal embedded slotted photonic crystal waveguide (SPCW) is presented. The ultrahigh Raman gain coefficient of the silicon nanocrystal material, being fortified further by the intense optical confinement inside the SPCW, essentially outweighs the contribution of the Kerr in the self-phase modulation phenomenon that is responsible for the solitonic propagation of a pulse. This work particularly emphasizes the contribution of Raman nonlinearity in the soliton dynamics, which however has been neglected conventionally, leading to an unprecedented miniaturization in both the operating power and footprint. Spatiotemporal evolution of pulses inside the SPCW has been calculated using the nonlinear Schrödinger equation considering the slow-light regime. A compression factor as high as 44.3 is obtained using a 30 µm long SPCW with a peak input power of 8 mW at 1550 nm.

It is pointed out that the evanescent light wave occurring at total reflection does not possess a transverse spin angular momentum as Bliokh, Bekshaev, and Nori claimed recently in (2014) Nature Communications, 5,3300. This is not only because of the nonlocality of the photon spin but also because the evanescent wave is such a state whose angular momentum cannot be separated into spin and orbital parts.

Recently we proposed a paradigm shift in light confinement strategy showing how relaxed total internal reflection and photonic skin-depth engineering can lead to sub-diffraction waveguides without metal (S. Jahani and Z. Jacob, "Transparent sub-diffraction optics: nanoscale light confinement without metal," Optica 1, 96-100 (2014)). Here, we show that such extreme-skin-depth (e-skid) waveguides can counter-intuitively confine light better than the best-case all-dielectric design of high index silicon waveguides surrounded by vacuum. We also analytically establish that figures of merit related to light confinement in dielectric waveguides are fundamentally tied to the skin depth of waves in the cladding, a quantity surprisingly overlooked in dielectric photonics. We contrast the propagation characteristics of the fundamental mode of e-skid waveguides and conventional waveguides to show that the decay constant in the cladding is dramatically larger in e-skid waveguides, which is the origin of sub-diffraction confinement. We also propose an approach to verify the reduced photonic skin depth in experiment using the decrease in the Goos-Hanschen phase shift. Finally, we provide a generalization of our work using concepts of transformation optics where the photonic-skin depth engineering can be interpreted as a transformation on the momentum of evanescent waves.

Trapping light within cavities or waveguides in photonic crystals is an effective technology in modern integrated optics. Traditionally, cavities rely on total internal reflection or a photonic bandgap to achieve field confinement. Recent investigations have examined new localized modes that occur at a Dirac frequency that is beyond any complete photonic bandgap. We design Al2O3 dielectric cylinders placed on a triangular lattice in air, and change the central rod size to form a photonic crystal microcavity. It is predicted that waves can be localized at the Dirac frequency in this device without photonic bandgaps or total internal reflections. We perform a theoretical analysis of this new wave localization and verify it experimentally. This work paves the way for exploring localized defect modes at the Dirac point in the visible and infrared bands, with potential applicability to new optical devices.

We report on the observation of spontaneous Raman scattering in silicon photonic crystal waveguides. Continuous-wave measurements of both forward scattered and backscattered Stokes emissions are reported in single line-defect waveguides in hexagonal lattice photonic crystal silicon membranes. By utilizing the Bragg gap edge dispersion of the TM-like mode for pump enhancement and the TE-like fundamental mode onset for Stokes enhancement, the spontaneous Raman scattering coefficient was observed to increase by up to six times in the region of slow group velocity. The results show explicit nonlinear enhancement in a silicon photonic crystal slow-light waveguide device.