Rutgers University

Electrical & Computer Engineering

The State University of New Jersey

* Rigorous formulas for superprism sensitivities

* A general theory of wave transmission through a surface of a periodic structure—for any surface orientation (either periodic or quasi-periodic surfaces)

* Publications: PRB05, PRB08, JAP07

* Board-level optical interconnects based on polymer photonics

* On-chip optical interconnects based on silicon photonics or nanophotonics

* Low-cost, high-throughput fabrication

* For photonic crystal nanostructures or large scale molding

* Strong optical confinement (lower figure) and slow light enhancement

* MMI coupling scheme (upper figure)

* High-speed Si MOS modulator demonstrated

* Publications: APL07, OL09

* US Patent No. 7,421,179 (2008).

Fabrication: Nanoimprint/molding, Holography

Optical Interconnects

Superprism effect & photonic crystal interface physics

Slot photonic crystal waveguides

* Publications: JAP07, APL07, APL05

* News report: Nature Photonics

* Publications: JLT08, OL07, APL05, APL07

* Publications: APL07, SSE07, JSTQE08, ...

* News reports:

* EE Times: 2006, 2007

* Nature Photonics: Research Highlights,

* NASA Tech Brief, Laser Focus World, more...

Active Silicon Photonic Devices based on photonic crystal structures

* The first 1GHz photonic crystal waveguide modulator

* Scaling law for the electric current density of high-speed silicon modulators (leading to the prediction of power consumption)

Research

Photonic crystal and optical metamaterials research had exciting development in the last two decades. Built upon many advances in physics and materials research, nanophotonic devices such as lasers and modulators have been demonstrated recently with orders of magnitude better performance than their conventional counterparts. Looking ahead, challenges remain in reducing slow light loss, further improving cavity quality factors, further reducing device power consumption, seeking the limit of photonic crystal dispersion (longitudinal and angular). We contributed to some of these past efforts (e.g. modulator, slot PCW, superprism, and slow light loss), and keep exploring the physics to push the limits of slow light and superprism effects. Occasionally, these highly challenging problems in photonic crystal research also brought deeper understanding of some foundational problems in solid state physics, such as wave transmission through the surface of a periodic lattice, and the cross-sectional eigenmode orthogonality in 1D-periodic structures with finite cross-sections (e.g. PCW or nanowire).

Silicon photonics emerged in recent years with the potential for low-cost large-scale photonic-electronic integration. Silicon photonic crystal devices offer novel mechanisms to improve the performance of conventional silicon photonic devices as demonstrated in our prior work (e.g. modulator, slot PCW). Furthermore, these efforts also brought us fundamental understanding of current density scaling and power consumption of Si electro-optic devices, and led us to new directions such as quadrature amplitude modulation, mode symmetry transform. When the low-cost large-scale monolithic integration capability of silicon photonics meets the intriguing physics in photonic crystals, an area of abundant opportunities is unfolding. To explore these opportunities we need innovative ideas, practical rationale, physical insight, efficient simulation/modeling, and careful fabrication. You are welcome to browse through our work on this website, and uncover new paths in this promised land.

Slow light loss in a photonic crystal waveguide

* Analytic theory reveals general characteristics of slow light loss

* Loss dominated by random scattering of photons arising from sidewall roughness

* Loss coefficient a=a1ng+a2ng2 (ng group index)

* a1 and a2 are analytically shown comparable in magnitude

* Interplay between the spatial phase of PCW mode and the roughness-correlation is critical

* Agree well with experiment

* Surprising byproduct: cross-sectional eigenmode orthogonality

* Publications: PRB10, JNN10, OE2012, APL11.

Dual racetrack Si micro-resonators for quadrature amplitude modulation

* Strong coherent cross-coupling between two parallel racetrack micro-resonators

* In over-coupling scenario, there is a delicate balance between the direct sum and “interactions” of two resonances

* Large amplitude & phase modulation ranges

* Suitable for arbitrary quadrature amplitude modulation (including DPSK, QPSK, 16-QAM, etc.)

* Resilient against fabrication imperfections

* Publications: OE2011.

* US Patent No. 9,239,477 (2016).

Slow light Thermo-optic Switches

* Power consumption and spatial temperature profile are found as explicit functions of structural, thermal and optical parameters

* Agree with FEM simulations and experiments

* Air-bridge (membrane) configuration is shown to enhance the temperature rise compared to the SOI structure.

* Scaling of power consumption with key parameters (buried oxide layer thickness, heater location & width, group index, etc.)

* Practical analysis of slow light loss in photonic crystal waveguide switches

* Sub-milliwatt switching power is possible

* Substrate effect can be precisely modeled

* Publications: OE2012, OE2013

Parity & Time Reversal Symmetry in Multi-dimensional Photonic Crystals

* Symmetry plays important roles in designing photonic crystal structures and devices or, more generally, 2D/3D photonic synthetic structures.

* Even-Odd mode-symmetry/parity transform in PCWs shows the potential of breaking time-reversal symmetry, enabling one-way waveguides or optical isolators.

* Symmetry-induced singularities can lead to ultra-high sensitivities in superprism effects while maintaining low loss.

* Symmetrical structures can enable a class of novel wavelength-division-multiplexing (WDM) devices.

* Publications: OL12, OE13, PRB08, PRL03,

* US Patent No. 9,086,583 (2015).

High-density waveguide integration &

Space division multiplexing

* High-density integration is the trend for silicon photonics. Integration density of waveguides—the most ubiquitous components of Si photonics, is crucial.

* Challenge: strong coupling as waveguides gets close

* Introduce a waveguide superlattice to suppress waveguide coupling at submicron pitches.

* Achieved high-density waveguide array at half-wavelength pitches. Filling factor 50%,

* Applications: (1) Space-division multiplexing (SDM); (2) Improve wavelength resolution of spectrometer & WDM demux. (3) Optical phased array.

* Publications: Nature Comm. 2015,

* News: Phys.org

* US Provisional Patent Application, 2013, PCT 2014 .