Nanosecond-scale shift-and-dump Mach-Zehnder switch.

We present a Mach-Zehnder-based optical switch with novel electrooptic shift-and-dump phase shifters that have no residual amplitude modulation. As a result, the switch can have a very high extinction ratio with no overhead in power dissipation or insertion loss compared to standard electrooptic Mach-Zehnder switches. We fabricate the switch in an all-digital silicon photonic platform that monolithically integrates the actuation and control electronics together with the photonics. The switch demonstrates an extinction ratio >33 dB, insertion loss <1.2 dB, and nanosecond-scale transients.

Carotenoid Nuclear Reorganization and Interplay of Bright and Dark Excited States.

We report quantum chemical calculations using multireference perturbation theory (MRPT) with the density matrix renormalization group (DMRG) plus photothermal deflection spectroscopy measurements to investigate the manifold of carotenoid excited states and establish their energies relative to the bright state (S2) as a function of nuclear reorganization. We conclude that the primary photophysics and function of carotenoids are determined by interplay of only the bright (S2) and lowest-energy dark (S1) states. The lowest-lying dark state, far from being energetically distinguishable from the lowest-lying bright state along the entire excited-state nuclear reorganization pathway, is instead computed to be either the second or first excited state depending on what equilibrium geometry is considered. This result suggests that, rather than there being a dark intermediate excited state bridging a non-negligible energy gap from the lowest-lying dark state to the lowest-lying bright state, there is in fact no appreciable energy gap to bridge following photoexcitation. Instead, excited-state nuclear reorganization constitutes the bridge from S2 to S1, in the sense that these two states attain energetic degeneracy along this pathway.

Nanosecond photonic switch architectures demonstrated in an all-digital monolithic platform.

We present an all-digital, fully programmable, nanosecond-scale photonic switch platform, monolithically integrating electronics for actuation, tuning, and power-monitoring alongside switching elements, resulting in a scalable, packageable solution for high-radix photonic switch fabrics. In this platform, we achieve record loss and extinction performances utilizing a 2×2 Mach-Zehnder switch (MZS) with 0.8 dB of loss and 28 dB extinction and a 2×2 nested MZS with 1.3 dB of loss and 38 dB extinction.

Transparent Conductive Adhesives for Tandem Solar Cells Using Polymer-Particle Composites.

Transparent conductive adhesives (TCAs) can enable conductivity between two substrates, which is useful for a wide range of electronic devices. Here, we have developed a TCA composed of a polymer-particle blend with ethylene-vinyl acetate as the transparent adhesive and metal-coated flexible poly(methyl methacrylate) microspheres as the conductive particles that can provide conductivity and adhesion regardless of the surface texture. This TCA layer was designed to be nearly transparent, conductive in only the out-of-plane direction, and of practical adhesive strength to hold the substrates together. The series resistance was measured at 0.3 and 0.8 Ω cm2 for 8 and 0.2% particle coverage, respectively, while remaining over 92% was transparent in both cases. For applications in photovoltaic devices, such as mechanically stacked multijunction III-V/Si cells, a TCA with 1% particle coverage will have less than 0.5% power loss due to the resistance and less than 1% shading loss to the bottom cell.

Ultralow crosstalk nanosecond-scale nested 2 × 2 Mach-Zehnder silicon photonic switch.

We present the design and characterization of a novel electro-optic silicon photonic 2×2 nested Mach-Zehnder switch monolithically integrated with a CMOS driver and interface logic. The photonic device uses a variable optical attenuator in order to balance the power inside the Mach-Zehnder interferometer leading to ultralow crosstalk performance. We measured a crosstalk as low as -34.5 dB, while achieving ∼2 dB insertion loss and 4 ns transient response.

Quasi-Direct Optical Transitions in Silicon Nanocrystals with Intensity Exceeding the Bulk.

Comparison of the measured absolute absorption cross section on a per Si atom basis of plasma-synthesized Si nanocrystals (NCs) with the absorption of bulk crystalline Si shows that while near the band edge the NC absorption is weaker than the bulk, yet above ∼ 2.2 eV the NC absorbs up to 5 times more than the bulk. Using atomistic screened pseudopotential calculations we show that this enhancement arises from interface-induced scattering that enhances the quasi-direct, zero-phonon transitions by mixing direct Γ-like wave function character into the indirect X-like conduction band states, as well as from space confinement that broadens the distribution of wave functions in k-space. The absorption enhancement factor increases exponentially with decreasing NC size and is correlated with the exponentially increasing direct Γ-like wave function character mixed into the NC conduction states. This observation and its theoretical understanding could lead to engineering of Si and other indirect band gap NC materials for optical and optoelectronic applications.

Polarization-independent nonuniform grating couplers on silicon-on-insulator.

Grating couplers are proposed for polarization-independent coupling of light between a single-mode fiber and a 220-nm-thick channel waveguide on silicon-on-insulator. The grating couplers have nonuniform grating periods that are composed of the intersection or union of a set of two near-optimal TE- and TM-grating periods. The proposed grating couplers have a coupling efficiency greater than 20% and polarization dependent loss (PDL) lower than 0.5 dB within 3-dB bandwidth in design. For the evaluation of the design concept, a fabricated intersection grating coupler has the PDL of less than 0.8 dB within the wavelength range of 1540 to 1560 nm, and the coupling efficiency is ∼18%.

30-Gb/s 90-nm CMOS-driven equalized multimode optical link.

We report an 850-nm vertical cavity surface emitting laser (VCSEL)-based optical link that achieves a new record in speed. The laser driver and receiver ICs are fabricated in standard 90-nm bulk CMOS, and the optoelectronic devices are commercial components. Operation at 30 Gb/s with a bit-error rate < 10(-12) is achieved, representing to the authors' knowledge the highest speed reported to date for a CMOS-based full optical link. Transmitter feed-forward equalization is shown to improve maximum data rate from 25 to 30 Gb/s, timing margin by 17% at 23.5 Gb/s, and receiver sensitivity by 4 dB at 23.5 Gb/s.

High-speed receiver based on waveguide germanium photodetector wire-bonded to 90nm SOI CMOS amplifier.

The performance of a receiver based on a CMOS amplifier circuit designed with 90nm ground rules wire-bonded to a waveguide germanium photodetector is characterized at data rates up to 40Gbps. Both chips were fabricated through the IBM Silicon CMOS Integrated Nanophotonics process on specialty photonics-enabled SOI wafers. At the data rate of 28Gbps which is relevant to the new generation of optical interconnects, a sensitivity of -7.3dBm average optical power is demonstrated with 3.4pJ/bit power-efficiency and 0.6UI horizontal eye opening at a bit-error-rate of 10(-12). The receiver operates error-free (bit-error-rate < 10(-12)) up to 40Gbps with optimized power supply settings demonstrating an energy efficiency of 1.4pJ/bit and 4pJ/bit at data rates of 32Gbps and 40Gbps, respectively, with an average optical power of -0.8dBm.

Diffractive light trapping in crystal-silicon films: experiment and electromagnetic modeling.

Diffractive light trapping in 1.5 µm thick crystal silicon films is studied experimentally through hemispherical reflection measurements and theoretically through rigorous coupled-wave analysis modeling. The gratings were fabricated by nanoimprinting of dielectric precursor films. The model data, which match the experimental results well without the use of any fitting parameters, are used to extract the light trapping efficiency. Diffractive light trapping is studied as a function of incidence angle, and an enhancement of light absorption is found for incidence angles up to 50° for both TE and TM polarizations.

Non-blocking 4x4 electro-optic silicon switch for on-chip photonic networks.

We present a 4x4 spatially non-blocking Mach-Zehnder based silicon optical switch fabricated using processes fully compatible with standard CMOS. We successfully demonstrate operation in all 9 unique switch states and 12 possible I/O routing configurations, with worst-case cross-talk levels lower than -9 dB, and common spectral bandwidth of 7 nm. High-speed 40 Gbps data transmission experiments verify optical data integrity for all input-output channels.

Dispersion-compensated wavelength beam combining of quantum-cascade-laser arrays.

A multiwavelength array of distributed feedback (DFB) quantum cascade lasers (QCLs) that spans λ = 8.28 to 9.62 µm is wavelength beam combined (WBC) using both single-grating and dual-grating designs. WBC with a single grating results in a pointing error of 3-times the beam divergence for a single laser and arises from the nonlinear dispersion of the grating. By adding a second grating to compensate for the nonlinear dispersion, the pointing error is reduced to only 13% of the beam divergence for a single laser. A transceiver based on the dual-grating-WBC QCL was used to measure the transmittance of a polymer sheet placed between itself and a retroreflector over a round-trip distance of 70 meters.

Wavelength multicasting in silicon photonic nanowires.

We demonstrate a scalable, energy-efficient, and pragmatic method for high-bandwidth wavelength multicasting using FWM in silicon photonic nanowires. We experimentally validate up to a sixteen-way multicast of 40-Gb/s NRZ data using spectral and temporal responses, and evaluate the resulting data integrity degradation using BER measurements and power penalty performance metrics. We further examine the impact of this wavelength multicasting scalability on conversion efficiency. Finally, we experimentally evaluate up to a three-way multicast of 160-Gb/s pulsed-RZ data using spectral and temporal responses, representing the first on-chip wavelength multicasting of pulsed-RZ data.

Beam combining of quantum cascade laser arrays.

Wavelength beam combining was used to co-propagate beams from 28 elements in an array of distributed-feedback quantum cascade lasers (DFB-QCLs). The beam-quality product of the array, defined as the product of near-field spot size and far-field divergence for the entire array, was improved by a factor of 21 by using wavelength beam combining. To demonstrate the applicability of wavelength beam combined DFB-QCL arrays for remote sensing, we obtained the absorption spectrum of isopropanol at a distance of 6 m from the laser array.

Optical 4x4 hitless slicon router for optical networks-on-chip (NoC).

We demonstrate here a spatially non-blocking optical 4x4 router with a footprint of 0.07 mm(2) for use in future integrated photonic interconnection networks. The device is dynamically switched using thermo-optically tuned silicon microring resonators with a wavelength shift to power ratio of 0.25nm/mW. The design can route four optical inputs to four outputs with individual bandwidths of up to 38.5 GHz. All tested configurations successfully routed a single-wavelength laser and provided a maximum extinction ratio larger than 20 dB.

Design and fabrication of photonic crystal quantum cascade lasers for optofluidics.

We present novel designs and demonstrate a fabrication platform for electrically driven lasers based on high quality-factor photonic crystal cavities realized in mid-infrared quantum cascade laser material. The structures are based on deep-etched ridges with their sides perforated with photonic crystal lattice, using focused ion beam milling. In this way, a photonic gap is opened for the emitted TM polarized light. Detailed modeling and optimization of the optical properties of the lasers are presented, and their application in optofluidics is investigated. Porous photonic crystal quantum cascade lasers have potential for on-chip, intracavity chemical and biological sensing in fluids using mid infrared spectroscopy. These lasers can also be frequency tuned over a large spectral range by introducing transparent liquid in the photonic crystal holes.

Transmission of high-data-rate optical signals through a micrometer-scale silicon ring resonator.

The effects of a micrometer-scale silicon ring resonator with a FWHM of 0.078 nm (9.6 GHz) on a nonreturn to zero amplitude-modulated optical signal with a modulation rate of 10 Gbps are experimentally investigated. By transmitting the optical signal through the device, significant spectral distortion and side band attenuation is introduced, as characterized by amplitude Bode plots, and a power penalty of 0.8 dB is observed. Carrier wavelengths within the transmission resonance, but detuned from the center wavelength, are investigated as well. Numerical simulations further support the experimental results.

On cascades of resonators for high-bandwidth integrated optical interconnection networks.

The power penalty characteristics of high-speed optical signals transmitted through a variety of filters based on multiple microring resonator devices are analyzed by numerical simulation. The technique used here has been verified with single-ring experimental measurements. Butterworth and Chebyshev filters are investigated, as are serial cascades of resonant devices. Although the power penalty is generally not prohibitive, it is a parameter which cannot be ignored for the design of complex high-bandwidth photonic interconnect systems that utilize microring resonators as filters and switches.

Microfluidic tuning of distributed feedback quantum cascade lasers.

In this Letter, we report the tuning of the emission wavelength of a single mode distributed feedback quantum cascade laser by modifying the mode effective refractive index using fluids. A fabrication procedure to encapsulate the devices in polymers for microfluidic delivery is also presented. The integration of microfluidics with semiconductor laser (optofluidics) is promising for new compact and portable lab-on-a-chip applications.