Framework for tunable polarization state generation using Berry's phase in silicon waveguides.

We present a framework for an arbitrary polarization state generator exploiting Berry's phase through a cascade of in-plane and out-of-plane silicon strip waveguides. We establish two criteria required for a passive device to achieve 90° polarization rotation, and derive explicit equations to satisfy the criteria. The results define regions within the parameter space where active tuning of the polarization state is possible over the entire Poincaré sphere. We use numerical modeling to show ±30 dB tuning of the polarization extinction ratio between the quasi-transverse electric and magnetic modes for a range of devices with deflection angles ranging from 5° to 45°, and modal birefringence from 0 to 0.05. We envision control of optical polarization on the chip-scale in integrated waveguides for communications, sensing, and computing applications.

Rotating polarization using Berry's phase in asymmetric silicon strip waveguides.

Light propagating in an out-of-plane curvilinear waveguide acquires a Berry's phase, which rotates optical polarization. The effect is promising for realizing waveguide polarization controllers. In high index contrast platforms, such as silicon-on-insulator, however, small waveguide cross-sectional asymmetries reduce the amount of polarization rotation. To overcome this, we present a method based on the periodic spatial modulation of Berry's phase. Ninety degree polarization rotation is achieved, even in the presence of waveguide asymmetry. Using a numerical model based on Jones calculus, we demonstrate the approach with 303×300 nm2 asymmetric silicon waveguides. We convert polarization from transverse electric to transverse magnetic with a polarization extinction ratio (PER) greater than 20 dB PER over a 100 nm bandwidth in a 110×240 µm2footprint.

Enhanced photoluminescence from ring resonators in hydrogenated amorphous silicon thin films at telecommunications wavelengths.

We report enhanced photoluminescence in the telecommunications wavelength range in ring resonators patterned in hydrogenated amorphous silicon thin films deposited via low-temperature plasma enhanced chemical vapor deposition. The thin films exhibit broadband photoluminescence that is enhanced by up to 5 dB by the resonant modes of the ring resonators due to the Purcell effect. Ellipsometry measurements of the thin films show a refractive index comparable to crystalline silicon and an extinction coefficient on the order of 0.001 from 1300 nm to 1600 nm wavelengths. The results are promising for chip-scale integrated optical light sources.

7 nm/V DC tunability and millivolt scale switching in silicon carrier injection degenerate band edge resonators.

We demonstrate electro-optical tuning of degenerate band edge resonances in Si photonic waveguides for applications including tunable filters, low voltage switches, and modulators. Carrier injection modulation is enabled by introducing periodic Si slabs to electrically connect the resonator to P and N dopants. Measured devices yield a large DC tunability of 7.1 nm/V and a peak switching slope of 206 dB/V. Digital data transmission measurements at 100 Mb/s show 3 dB of switching with a swing voltage of 6.8 mV, 91.4 aJ/bit switching energy, and 1.08 pJ/bit holding energy.

Zero-coupling-gap degenerate band edge resonators in silicon photonics.

Resonances near regular photonic band edges are limited by quality factors that scale only to the third power of the number of periods. In contrast, resonances near degenerate photonic band edges can scale to the fifth power of the number periods, yielding a route to significant device miniaturization. For applications in silicon integrated photonics, we present the design and analysis of zero-coupling-gap degenerate band edge resonators. Complex band diagrams are computed for the unit cell with periodic boundary conditions that convey characteristics of propagating and evanescent modes. Dispersion features of the band diagram are used to describe changes in resonance scaling in finite length resonators. Resonators with non-zero and zero coupling gap are compared. Analysis of quality factor and resonance frequency indicates significant reduction in the number of periods required to observe fifth power scaling when degenerate band edge resonators are realized with zero-coupling-gap. High transmission is achieved by optimizing the waveguide feed to the resonator. Compact band edge cavities with large optical field distribution are envisioned for light emitters, switches, and sensors.

Degenerate band edge resonances in periodic silicon ridge waveguides.

We experimentally demonstrate degenerate band edge resonances in periodic Si ridge waveguides that are compatible with carrier injection modulation for active electro-optical devices. The resonant cavities are designed using a combination of the plane-wave expansion method and the finite difference time domain technique. Measured and simulated quality factors of the first band edge resonances scale to the fifth power of the number of periods. Quality factor scaling is determined to be limited by fabrication imperfections. Compared to resonators based on a regular transmission band edge, degenerate band edge devices can achieve significantly larger quality factors in the same number of periods. Applications include compact electro-optical switches, modulators, and sensors that benefit from high-quality factors and large distributed electric fields.

Highly linear ring modulator from hybrid silicon and lithium niobate.

We present a highly linear ring modulator from the bonding of ion-sliced x-cut lithium niobate onto a silicon ring resonator. The third order intermodulation distortion spurious free dynamic range is measured to be 98.1 dB Hz(2/3) and 87.6 dB Hz(2/3) at 1 GHz and 10 GHz, respectively. The linearity is comparable to a reference lithium niobate Mach-Zehnder interferometer modulator operating at quadrature and over an order of magnitude greater than silicon ring modulators based on plasma dispersion effect. Compact modulators for analog optical links that exploit the second order susceptibility of lithium niobate on the silicon platform are envisioned.

Compensating thermal drift of hybrid silicon and lithium niobate ring resonances.

We present low-power compensation of thermal drift of resonance wavelengths in hybrid silicon and lithium niobate ring resonators based on the linear electro-optic effect. Fabricated devices demonstrate a resonance wavelength tunability of 12.5 pm/V and a tuning range of 1 nm. A capacitive geometry and low thermal sensitivity result in the compensation of 17°C of temperature variation using tuning powers at sub-nanowatt levels. The method establishes a route for stabilizing high-quality factor resonators in chip-scale integrated photonics subject to temperature variations.

Electrically tunable optical polarization rotation on a silicon chip using Berry's phase.

The continued convergence of electronics and photonics on the chip scale can benefit from the voltage control of optical polarization for applications in communications, signal processing and sensing. It is challenging, however, to electrically manipulate the polarization state of light in planar optical waveguides. Here we introduce out-of-plane optical waveguides, allowing access to Berry's phase, a quantum-mechanical phenomenon of purely topological origin. As a result, electrically tunable optical polarization rotation on the chip scale is achieved. Devices fabricated in the silicon-on-insulator material platform are not limited to a single static polarization state. Rather, they can exhibit dynamic tuning of polarization from the fundamental transverse electric mode to the fundamental transverse magnetic mode. Electrical tuning of optical polarization over a 19 dB range of polarization extinction ratio is demonstrated with less than 1 dB of conversion loss at infrared wavelengths. Compact system architectures involving dynamic control of optical polarization in integrated circuits are envisioned.

12.5 pm/V hybrid silicon and lithium niobate optical microring resonator with integrated electrodes.

We present a silicon microring resonator with a lithium niobate top cladding and integrated tuning electrodes. Submicrometer thin films of z-cut lithium niobate are bonded to silicon microring resonators via benzocyclobutene. Integrated electrodes are incorporated to confine voltage controlled electric fields within the lithium niobate thin film. The electrode design utilizes thin film metal electrodes and an optically transparent electrode wherein the silicon waveguide core serves as both an optical waveguide medium and as a conductive electrode medium. The hybrid material system combines the electro-optic functionality of lithium niobate with the high index contrast of silicon waveguides, enabling compact low tuning voltage microring resonators. Optical characterization of fabricated devices results in a measured loaded quality factor of 11,500 and a free spectral range of 7.15 nm in the infrared. The demonstrated tunability is 12.5 pm/V, which is over an order of magnitude greater than electrode-free designs.

Degenerate band edge resonances in coupled periodic silicon optical waveguides.

Using full three-dimensional analysis we show that coupled periodic optical waveguides can exhibit a giant slow light resonance associated with a degenerate photonic band edge. We consider the silicon-on-insulator material system for implementation in silicon photonics at optical telecommunications wavelengths. The coupling of the resonance mode with the input light can be controlled continuously by varying the input power ratio and the phase difference between the two input arms. Near unity transmission efficiency through the degenerate band edge structure can be achieved, enabling exploitation of the advantages of the giant slow wave resonance.

Compact electric field sensors based on indirect bonding of lithium niobate to silicon microrings.

An electric field sensor based on the indirect bonding of submicrometer thin films of lithium niobate to silicon microring resonators is presented using benzocyclobutene as an intermediate bonding layer. The hybrid material system combines the electro-optic functionality of lithium niobate with the high-index contrast of silicon waveguides, enabling compact and metal-free electric field sensors. A sensor is designed and fabricated using ion-sliced z-cut lithium niobate as the top cladding of a 20 µm radius silicon microring resonator. The optical quasi transverse magnetic mode is used to access the largest electro-optic coefficient in the lithium niobate. Optical characterization of the hybrid device results in a measured loaded quality factor of 13,000 in the infrared. Operation of the device as an electric field sensor is demonstrated by detecting the fringing fields from a microstrip electrical circuit operating at 1.86 GHz. The demonstrated sensitivity to electric fields is 4.5 V m-1 Hz-1/2.

Compact cantilever couplers for low-loss fiber coupling to silicon photonic integrated circuits.

We demonstrate coupling from tapered optical fibers to 450 nm by 250 nm silicon strip waveguides using compact cantilever couplers. The couplers consist of silicon inverse width tapers embedded within silicon dioxide cantilevers. Finite difference time domain simulations are used to design the length of the silicon inverse width taper to as short as 6.5 µm for a cantilever width of 2 µm. Modeling of various strip waveguide taper profiles shows reduced coupling losses for a quadratic taper profile. Infrared measurements of fabricated devices demonstrate average coupling losses of 0.62 dB per connection for the quasi-TE mode and 0.50 dB per connection for the quasi-TM mode across the optical telecommunications C band. In the wavelength range from 1477 nm to 1580 nm, coupling losses for both polarizations are less than 1 dB per connection. The compact, broadband, and low-loss coupling scheme enables direct access to photonic integrated circuits on an entire chip surface without the need for dicing or cleaving the chip.

Vertical chip-to-chip coupling between silicon photonic integrated circuits using cantilever couplers.

We demonstrate vertical chip-to-chip light coupling using silicon strip waveguide cantilever couplers. The guided-wave couplers consist of silicon strip waveguides embedded within silicon dioxide cantilevers. The cantilevers deflect 90° out-of-plane via residual stress, allowing vertical light coupling between separate chips. A chip-to-chip coupling loss of 2.5 dB per connection is measured for TE polarization and 1.1 dB for TM polarization at 1550 nm wavelength. The coupling loss varies by less than±0.8 dB within the wavelength range from 1500 nm to 1565 nm for both polarizations. The couplers enable broadband and compact system architectures involving high speed vertical data transport between photonic integrated circuits.

Submilliwatt thermo-optic switches using free-standing silicon-on-insulator strip waveguides.

A low power Mach-Zehnder interferometer thermo-optic switch using free-standing silicon-on-insulator strip waveguides is demonstrated. The air gap provides thermal isolation between the waveguide interferometer arms and the underlying silicon substrate. The highly confined optical modes of the strip waveguides enable miniature heated cross-sections. The heating efficiency from on-chip resistive heaters is enhanced. Measurements of fabricated devices using 100 microm arm lengths at 1550 nm wavelength result in a switching power of 540 microW, a 10% - 90% switching rise time of 141 micros, and an extinction ratio of 25 dB.

Low-power optical bistability in a free-standing silicon ring resonator.

We demonstrate low-power thermo-optic-based optical bistability in a free-standing silicon ring resonator. A bistable optical response is achieved at reduced pump powers by thermally isolating the ring resonator from its supporting substrate with an air gap. The conversion efficiency from optical power to temperature change in the silicon core is enhanced. The optical transfer function of the resulting free-standing resonator exhibits a hysteresis loop for 80 microW input optical power. Similar nonthermally isolated resonators at the same detuning do not exhibit a bistable mode for input powers less than 2 mW.

Multimode waveguide-cavity sensor based on fringe visibility detection.

Fringe visibility detection of the interaction of two bus spatial eigenmodes with a resonant cavity is investigated for the purpose of achieving a sensor platform with high sensitivity. The power distribution between the bus waveguide eigenmodes is modulated by the interaction with the cavity and is detected via fringe visibility lineshapes produced by twin-fiber interferometry. A test device is fabricated in a polymer-silica material system by a photolithographic process and is characterized by measuring the fringe visibility change as a function of analyte refractive index. Fringe visibility modulation from a straight two-mode waveguide coupled to a single mode ring resonator exposed to an aqueous glucose solution demonstrates a visibility change of 1.57 per weight percent, compared to a transmission change of 0.19 per weight percent for a single mode waveguide critically coupled to a ring with similar intrinsic quality factor. The demonstrated change in fringe visibility is 8.2 times larger.

Cantilever couplers for intra-chip coupling to silicon photonic integrated circuits.

An intra-chip coupling scheme from optical fibers to silicon strip waveguides is demonstrated. The couplers consist of silicon inverse width tapers embedded within silicon dioxide cantilevers that are deflected out-of-plane by residual stress. Deflection angles from 5 to 30 degrees are obtained and controlled by thermal annealing. Butt-coupling from tapered fibers or collimation-coupling from lensed fibers may be employed. The coupling scheme enables direct access to devices on the entire chip surface without dicing or cleaving the chip. Coupling efficiencies of 1.6 dB per connection for TE polarization and 2 dB per connection for TM polarization are achieved. The coupling efficiency shows little wavelength-dependence, with less than 1.6 dB fluctuation over the wavelength range of 1500 nm to 1560 nm.

Vertical coupling between gap plasmon waveguides.

This work examines vertical coupling between gap plasmon waveguides for use in high confinement power transfer and power splitting applications at 1.55 microm free space wavelength. The supermode interference method is used to obtain key coupler performance parameters such as coupling length, extinction ratio, net coupled output power, radiated power, and reflected power as a function of waveguide center-to-center spacing, core refractive index, and gap width. The initial power distribution among the two coupler supermodes is obtained via the mode matching method for a single input waveguide feed. Excellent agreement with three-dimensional finite difference time domain simulations is observed for the case of square 50 nm gaps with core refractive indices of 2.50 and a center-to-center spacing of 112 nm. Local maxima in the net coupled output power are found to coincide with local minima in the coupling length. An increase in the core refractive index from 1.00 to 2.5 increases the local maximum net coupled output power from 6.4% to 49% but decreases the extinction ratio from 12.7 to 6.94. A sweep of the width of the core from 25 to 100 nm increases the net coupled output power from 43.7% to 52.0%, but increases the coupling length from 1.58 to 3.19 ???m and decreases the extinction ratio from 7.39 to 6.57.

Real-time shape evolution of nanoimprinted polymer structures during thermal annealing.

The real-time shape evolution of nanoimprinted polymer patterns is measured as a function of annealing time and temperature using critical dimension small-angle X-ray scattering (CD-SAXS). Periodicity, line width, line height, and sidewall angle are reported with nanometer resolution for parallel line/space patterns in poly(methyl methacrylate) (PMMA) both below and above the bulk glass transition temperature (T(G)). Heating these patterns below T(G) does not produce significant thermal expansion, at least to within the resolution of the measurement. However, above T(G) the fast rate of loss in pattern size at early times transitions to a reduced rate in longer time regimes. The time-dependent rate of polymer flow from the pattern into the underlying layer, termed pattern "melting", is consistent with a model of elastic recovery from stresses induced by the molding process.