Ehsan Shah Hosseini

Large-scale nanophotonic phased array

Electromagnetic phased arrays at radio frequencies are well known and have enabled applications ranging from communications to radar, broadcasting and astronomy. The ability to generate arbitrary radiation patterns with large-scale phased arrays has long been pursued. Although it is extremely expensive and cumbersome to deploy large-scale radiofrequency phased arrays, optical phased arrays have a unique advantage in that the much shorter optical wavelength holds promise for large-scale integration. However, the short optical wavelength also imposes stringent requirements on fabrication. As a consequence, although optical phased arrays have been studied with various platforms and recently with chip-scale nanophotonics, all of the demonstrations so far are restricted to one-dimensional or small-scale two-dimensional arrays. Here we report the demonstration of a large-scale two-dimensional nanophotonic phased array (NPA), in which 64 × 64 (4,096) optical nanoantennas are densely integrated on a silicon chip within a footprint of 576 μm × 576 μm with all of the nanoantennas precisely balanced in power and aligned in phase to generate a designed, sophisticated radiation pattern in the far field. We also show that active phase tunability can be realized in the proposed NPA by demonstrating dynamic beam steering and shaping with an 8 × 8 array. This work demonstrates that a robust design, together with state-of-the-art complementary metal-oxide–semiconductor technology, allows large-scale NPAs to be implemented on compact and inexpensive nanophotonic chips. In turn, this enables arbitrary radiation pattern generation using NPAs and therefore extends the functionalities of phased arrays beyond conventional beam focusing and steering, opening up possibilities for large-scale deployment in applications such as communication, laser detection and ranging, three-dimensional holography and biomedical sciences, to name just a few.

High quality planar silicon nitride microdisk resonators for integrated photonics in the visible wavelength range

High quality factor (Q approximately 3.4 x 10(6)) microdisk resonators are demonstrated in a Si(3)N(4) on SiO(2) platform at 652-660 nm with integrated in-plane coupling waveguides. Critical coupling to several radial modes is demonstrated using a rib-like structure with a thin Si(3)N(4) layer at the air-substrate interface to improve the coupling.

Systematic design and fabrication of high-Q single-mode pulley-coupled planar silicon nitride microdisk resonators at visible wavelengths

High quality (Q approximately 6 x 10(5)) microdisk resonators are demonstrated in a Si(3)N(4) on SiO(2) platform at 652-660 nm with integrated in-plane wrap-around coupling waveguides to enable critical coupling to specific microdisk radial modes. Selective coupling to the first three radial modes with >20dB suppression of the other radial modes is achieved by controlling the wrap-around waveguide width. Advantages of such pulley-coupled microdisk resonators include single mode operation, ease of fabrication due to larger waveguide-resonator gaps, the possibility of resist reflow during the lithography phase to improve microdisk etched surface quality, and the ability to realize highly over-coupled microdisks suitable for low-loss delay lines and add-drop filters.

Silicon nanophotonic devices for integrated sensing

The potentials of a nanophotonic platform, including compactness, low power consumption, integrability with other functionalities, and high sensitivity make them a suitable candidate for sensing applications. Strong light-matter interaction in such a platform enables a variety of sensing mechanisms, including refractive index change, fluorescence emission, and Raman scattering. Recent advances in nanophotonic devices include the demonstration of silicon and silicon-nitride microdisk resonators with high intrinsic Q values (0.5-2×106) for strong field enhancement and the realization of compact photonic crystal spectrometers (high spectral resolution at 100-µm length scales) for on-chip spectral analysis. These two basic building blocks, when combined with integrated fluidic channels for sample delivery, provide an efficient platform to implement different sensing mechanisms and architectures.

Monolithic erbium- and ytterbium-doped microring lasers on silicon chips

We demonstrate monolithic 160-µm-diameter rare-earth-doped microring lasers using silicon-compatible methods. Pump light injection and laser output coupling are achieved via an integrated silicon nitride waveguide. We measure internal quality factors of up to 3.8 × 105 at 980 nm and 5.7 × 105 at 1550 nm in undoped microrings. In erbium- and ytterbium-doped microrings we observe single-mode 1.5-µm and 1.0-µm laser emission with slope efficiencies of 0.3 and 8.4%, respectively. Their small footprints, tens of microwatts output powers and sub-milliwatt thresholds introduce such rare-earth-doped microlasers as scalable light sources for silicon-based microphotonic devices and systems.

Planar photonic crystal microspectrometers in silicon-nitride for the visible range

We demonstrate the feasibility of forming a compact integrated photonic spectrometer for operation in the visible wavelength range using the dispersive properties of a planar photonic crystal structure fabricated in silicon nitride. High wavelength resolution and compact device sizes in these spectrometers are enabled by combining superprism effect, negative diffraction effect, and negative refraction effect in a 45 degree rotated square lattice photonic crystal. Our experimental demonstration shows 1.2 nm wavelength resolution in a 70 microm by 130 microm photonic crystal structure with better performance than alternative structures for on-chip spectroscopy, confirming the unique capability of the proposed approach to realize compact integrated spectrometers.

Athermal performance in high- Q polymer-clad silicon microdisk resonators

We present a method for eliminating the temperature dependence of the resonance wavelength in high-Q silicon-based microdisk resonators by using a polymer cladding with a negative thermo-optic coefficient. Design requirements for athermal performance are derived based on theory and simulation, and their validity is experimentally verified.

Four-port integrated polarizing beam splitter.

In this Letter, we report on the first integrated four-port polarizing beam splitter. The device operates on the principle of mode evolution and was implemented in a silicon-on-insulator silicon photonics platform and fabricated on a 300 mm CMOS line using 193 nm optical immersion lithography. The adiabatic transition forming of the structure enabled over a 150 nm bandwidth from λ~1350 to λ~1500  nm, achieving a cross-talk level below -10  dB over the entire band.

Strong angular dispersion using higher bands of planar silicon photonic crystals

We present experimental evidence for strong angular dispersion in a planar photonic crystal (PC) structure by properly engineering the modes in the second PC band. We show that by using the second photonic band of a square lattice PC, angular dispersion of 4 degrees /nm can be achieved. We also show that major challenges in designing practical PC devices using second band modes can be addressed by engineering the lattice and adding input/output buffer stages designed to eliminate unwanted effects.

Two-dimensional apodized silicon photonic phased arrays

In this Letter, we demonstrate an 8×8 apodized silicon photonic phased array where the emission from each of 64 nanoantennas was tailored to exhibit Gaussian-shaped intensity distributions in the near field so that the sidelobes of the generated far-field optical beam were suppressed compared to that of a uniform phased array. With the aid of the 72 thermo-optic phase tuners directly integrated within the phased array, we dynamically shaped the generated optical beam in the far field in a variety of ways.