Jingqing Huang

Nonreciprocal light propagation in a silicon photonic circuit.

An engineered metallic-silicon waveguide allows for direction-dependent light propagation. Optical communications and computing require on-chip nonreciprocal light propagation to isolate and stabilize different chip-scale optical components. We have designed and fabricated a metallic-silicon waveguide system in which the optical potential is modulated along the length of the waveguide such that nonreciprocal light propagation is obtained on a silicon photonic chip. Nonreciprocal light transport and one-way photonic mode conversion are demonstrated at the wavelength of 1.55 micrometers in both simulations and experiments. Our system is compatible with conventional complementary metal-oxide-semiconductor processing, providing a way to chip-scale optical isolators for optical communications and computing.

Nonlinear Polymer-Clad Silicon Slot Waveguide Modulator with a Half Wave Voltage of 0.25 V

We report on an electro-optic modulator fabricated from a silicon slot waveguide and clad in a nonlinear polymer. In this geometry, the electrodes form parts of the waveguide, and the modulator driving voltage drops across a 120nm slot. As a result, a half wave voltage of 0.25V is achieved near 1550nm. This is one of the lowest values for any modulator obtained to date. As the nonlinear polymers are extremely resistive, our device also has the advantage of drawing almost no current, suggesting this type of modulator could operate at exceedingly low power.

Towards a Millivolt Optical Modulator with Nano-Slot Waveguides

We describe a class of modulator design involving slot waveguides and electro-optic polymer claddings. Such geometries enable massive enhancement of index tuning when compared to more conventional geometries. We present a semi-analytic method of predicting the index tuning achievable for a given geometry and electro-optic material. Based on these studies, as well as previous experimental results, we show designs for slot waveguide modulators that, when realized in a Mach-Zehnder configuration, will allow for modulation voltages that are orders of magnitude lower than the state of the art. We also discuss experimental results for nano-slot waveguides.

Room Temperature, Continuous-Wave Coupled-Cavity InAsP/InP Photonic Crystal Laser with Enhanced Far-field Emission Directionality

We demonstrate room temperature, continuous-wave lasing with enhanced far field emission directionality in coupled-cavity photonic crystal lasers, made with InAsP/InP quantum well material. These surface-emitting lasers can have a very low effective threshold power of 14.6 μW, with a linewidth of 60 pm, and 40% of the surface emitted power concentrated within a small divergence angle of ±30°.

Photonic Crystal Nanocavity Laser in an Optically Very Thick Slab

A photonic crystal (PhC) nanocavity formed in an optically very thick slab can support reasonably high-Q modes for lasing. Experimentally, we demonstrate room-temperature pulsed lasing operation from the PhC dipole mode emitting at 1324 nm, which is fabricated in an InGaAsP slab with thickness (T) of 606 nm. Numerical simulation reveals that when T≥800 nm, over 90% of the laser output power couples to the PhC slab modes, suggesting a new route toward an efficient in-plane laser for photonic integrated circuits.

From Vertical-Cavities to Hybrid Metal/Photonic-Crystal Nanocavities: Towards High-Efficiency Nanolasers

We provide a numerical study showing that a bottom reflector is indispensable to achieve unidirectional emission from a photonic-crystal (PhC) nanolaser. First, we study a PhC slab nanocavity suspended over a flat mirror formed by a dielectric or metal substrate. We find that the laser’s vertical emission can be enhanced by more than a factor of 6 compared with the device in the absence of the mirror. Then, we study the situation where the PhC nanocavity is in contact with a flat metal surface. The underlying metal substrate may serve as both an electrical current pathway and a heat sink, which would help achieve continuous-wave lasing operation at room temperature. The design of the laser emitting at 1.3 μm reveals that a relatively high cavity Q of over 1000 is achievable assuming room-temperature gold as a substrate. Furthermore, linearly polarized unidirectional vertical emission with the radiation efficiency over 50% can be achieved. Finally, we discuss how this hybrid design relates to various plasmonic cavities and propose a useful quantitative measure of the degree of the “plasmonic” character in a general metallic nanocavity.

Higher-Order Defect-Mode Laser in an Optically Thick Photonic Crystal Slab

The use of an optically thick slab may provide versatile solutions for the realization of a current injection-type laser using photonic crystals. Here, we show that a transversely higher-order defect mode can be designed to be confined by a photonic bandgap in such a thick slab. Using simulations, we show that a high Q of >10(5) is possible from a finely tuned second-order hexapole mode (2h). Experimentally, we achieve optically pumped pulsed lasing at 1347 nm from the 2h with a peak threshold pump power of 88 μW.

Unidirectional vertical emission from photonic crystal nanolasers

Here, we emphasize the importance of using a bottom reflector to achieve unidirectional vertical emission from an ultra-small light emitter. Specifically, we have considered a photonic crystal slab nanocavity laser placed in the vicinity of a perfect mirror. By choosing near one-wavelength distance between the bottom reflector and the cavity, over 80% of photons generated inside the laser cavity can funnel into a small divergence angle of ±30°. It is also interesting to observe that the natural radiation rate (∼1/Q factor) of the nanocavity mode can be modified by varying the gap size, which is analogous to the famous cavity quantum electrodynamics effect for a point dipole source placed near a perfect mirror. A simple, comprehensive plane wave interference model is presented to explain the observed over six-fold vertical emission enhancement. Furthermore, we propose some of the very practical nanolaser designs based on a metal bonding technology, which may enable continuous current injection operation at room-temperature.

Nonreciprocal light propagation on an integrated silicon photonic chip

We have designed, fabricated, and tested a CMOS compatible silicon waveguide system that demonstrates nonreciprocal light propagation on-chip, by mimicking microscopic non-Hermitian optical potentials for guided light and thus spontaneously breaking parity-time symmetry. Experiments were performed at 1.55 µm wavelength for potential telecommunication applications.

Highly directional emission from ultra-small photonic crystal resonators

Here, we emphasize the importance of a bottom reflector for achieving unidirectional far-field emission. As a result, over 80% of photons generated inside the cavity can be collected within a divergence angle of ±30° from the top. We also discuss interesting analogy in which the nanocavity-bottom reflector coupled system is treated as a point-like emitter in front of a mirror. Based on such a view point, the observed directivity is explained by using a comprehensive interference model. Finally, we propose a very practical form of an efficient photonic crystal nanolaser bonded on a flat metal surface, which may enable current injection and room-temperature continuous-wave operation.