Soham Saha

Low-loss plasmon-assisted electro-optic modulator

For nearly two decades, researchers in the field of plasmonics1—which studies the coupling of electromagnetic waves to the motion of free electrons near the surface of a metal2—have sought to realize subwavelength optical devices for information technology3–6, sensing7,8, nonlinear optics9,10, optical nanotweezers11 and biomedical applications12. However, the electron motion generates heat through ohmic losses. Although this heat is desirable for some applications such as photo-thermal therapy, it is a disadvantage in plasmonic devices for sensing and information technology13 and has led to a widespread view that plasmonics is too lossy to be practical. Here we demonstrate that the ohmic losses can be bypassed by using ‘resonant switching’. In the proposed approach, light is coupled to the lossy surface plasmon polaritons only in the device’s off state (in resonance) in which attenuation is desired, to ensure large extinction ratios between the on and off states and allow subpicosecond switching. In the on state (out of resonance), destructive interference prevents the light from coupling to the lossy plasmonic section of a device. To validate the approach, we fabricated a plasmonic electro-optic ring modulator. The experiments confirm that low on-chip optical losses, operation at over 100 gigahertz, good energy efficiency, low thermal drift and a compact footprint can be combined in a single device. Our result illustrates that plasmonics has the potential to enable fast, compact on-chip sensing and communications technologies.Ohmic losses in plasmonic devices can be reduced by exploiting ‘resonant switching’, in which light couples to surface plasmon polaritons only when in resonance and bypasses them otherwise.

Hybrid plasmonic waveguides formed by metal coating of dielectric ridges

Bound hybrid plasmon-polariton modes supported by waveguides, which are formed by gold coating of ridges etched into a silica substrate, are analyzed using numerical simulations and investigated experimentally using near-field microscopy at telecom wavelengths (1425-1625 nm). Drastic modifications of the fundamental mode profile along with changes in the mode confinement and propagation loss are found when varying the ridge height. The main mode characteristics (effective mode index, propagation length, and mode profile) are determined from the experimental amplitude- and phase-resolved near-field images and compared with the simulations. The possibility of strongly influencing the mode properties along with subwavelength confinement found simultaneously with relatively long propagation can further be exploited in mode shaping and sensing applications.

Ultrafast all-optical switching in a continuous layer gap plasmon metasurface (Conference Presentation)

All-optical nanophotonic switches, not bound by the inherent RC delays of electronic circuits, have the potential to push data-processing speeds beyond the limits of Moore’s Law. This has lead to the investigation of light-matter interactions in nanostructured materials in several all-optical data processing applications. To have a true impact on the field of ultrafast data-transfer, it is important to demonstrate switching in the telecom frequency range. We have designed a continuous layer gap plasmon metasurface, comprising a layer of gold nanodisk resonators on a 20 nm film of ZnO deposited on an optically thick gold layer. The performance of the metasurface has been investigated through numerical studies, using the optical properties of as-grown gold and zinc oxide, characterized by ellipsometry. An on-off ratio of 10.6 dB has been observed in simulations. Experimental studies are underway. The findings of this research work will pave the pathway to the design of ultra-compact and ultrafast optical switches employing ultrafast, dynamically tunable metasurfaces.

Titanium nitride based hybrid plasmonic-photonic waveguides for on-chip plasmonic interconnects

Over the past few decades, photonic technologies have emerged as a promising technology for data communications. They offer advantages such as high data bandwidths at comparable or even lower power consumption than electronics. However, photonic integrated circuits suffer from the diffraction limit of light which is a major obstacle in achieving small device footprints and densely packed on-chip interconnects. In recent years, plasmonics has emerged as a possible solution for densely packed on-chip nanophotonic circuitry. The field of plasmonics deals with oscillations of free electrons in a metal coupled to an electromagnetic field. The large wave-vector associated with these oscillations enables light to be localized in volumes much smaller than the diffraction limit. Consequently, there have been many demonstrations of plasmonic interconnects for on-chip communications, using well known metals such as gold and silver. However these materials are not CMOS compatible and hence their use is not technologically feasible. The growing need for plasmonic materials which are robust, cost-effective, and CMOS-compatible has led to the study of alternate plasmonic materials. For the visible and near infrared ranges, transition metal nitrides have been shown to be suitable metals for plasmonic applications These materials have optical properties comparable to that of gold and are CMOS-compatible, hence, they can be easily integrated into a silicon platform for on-chip applications. In this work, we demonstrate titanium nitride based plasmonic interconnects in an all-solid state geometry which can be easily integrated on a silicon platform.

On-chip and planar optics with alternative plasmonic materials (Conference Presentation)

Plasmonic interconnects as well as metasurfaces manipulating the phase and amplitude of reflected and/or transmitted light, have attracted significant attention in the fields of planar optics and on-chip nanophotonics. The application space of plasmonics has recently been expanded by new materials classes including transparent conducting oxides (TCOs) and refractory transition metal nitrides (TMNs). These materials offer superior thermal stability, robustness, tailorability and CMOS compatibility, thus outperforming the conventional plasmonic components (e.g. gold and silver) in application-specific requirements. Here, we discuss recent progress in the areas of plasmonic interconnects, modulators and metasurfaces realized using TMNs and TCOs. Ultrafast nonlinear responses near the epsilon near zero (ENZ) region have been recently demonstrated for Al doped zinc oxide (AZO) (TCO). This has spurred the development of ultrafast, on-chip modulators with TCOs as an active material. Building from on our previous work on LRSPP waveguides with ultrathin Titanium Nitride (TiN) (5.5mm propagation length) and Zirconium Nitride (ZrN), we will report solid-state hybrid mode waveguides using TiN as well as a modulator based on this waveguide which provides all-optical tunability, low optical losses at the telecommunication window, and CMOS-compatibility. Successful integration of these alternative plasmonic components into the phase gradient metasurfaces platform without loss of performance have also been demonstrated. A metasurface employing ZrN brick antennas shows the photonic spin Hall Effect (PSHE), by reflecting the two circular polarizations in different directions. In another device, a metasurface based on nanostructures of Ga doped ZnO (TCO), functioning as a quarter-wave plate, have been realized.