Ultrasensitive silicon photonic-crystal nanobeam electro-optical modulator: design and simulation

Ultra-compact on-chip spectrometer based on thermally tuned topological miniaturized bound states in the continuum cavity

On-chip spectrometers are key components in many spectral sensing applications owing to their unique advantages in size and in situ detection. In this work, we propose and demonstrate a class of thermally tunable spectrometers by utilizing topological miniaturized bound states in the continuum (mini-BIC) cavities in a photonic crystal (PhC) slab combined with a metal micro-ring heater. We achieve a resolution of 0.19 nm in a spectral range of ∼6 nm, while the device's footprint is only 42×42μm2. The mini-BIC spectrometer works in nearly vertical incidence and is compatible with array operation. Our work sheds light on the new possibilities of high-performance on-chip spectrometers for applications ranging from bio-sensing to medicine.

A hybrid photonic-plasmonic resonator based on a partially encapsulated 1D photonic crystal waveguide and a plasmonic nanoparticle

In this paper, a hybrid photonic-plasmonic resonator is proposed. The device consists of a partially encapsulated 1D photonic crystal waveguide and a plasmonic nanoparticle to yield high radiation efficiency for integrated photonic platforms, owing to a high Q-factor and a small mode volume. The design of the resonator is accomplished in two consecutive steps: first of all, a partially encapsulated photonic crystal nanobeam with a robust mechanical stability and a high-Q factor is prepared; secondly, a plasmonic nanoparticle is placed on the surface of the nanobeam to interact the optical mode with the localized surface plasmons of the gold nanoparticle which is being present in the vicinity of the radiating dipole. Strongly enhanced electromagnetic field, regenerated through the optical mode field inside the hybrid resonator, enables to reduce the optical mode volume of the device and significantly enhance the Purcell factor.

Electro-optical logic using dual-nanobeam Mach-Zehnder interferometer switches

The maturity of integrated photonics enables many applications including high-performance computing. Digital photonic computing always considers resonator-based modulators as the key active components due to their compactness as compared to broad-spectrum Mach-Zehnder interferometers (MZIs). In this paper, we investigate the dual-nanobeam (NB) based MZI 2 × 2 switches with much smaller footprint for realizing electro-optical logic circuits. New logic gates and scalable circuits assisted by multiplexing techniques are proposed. Results show that the NB MZI is another promising candidate for electronic-photonic digital computing.

Compact resonant 2 × 2 crossbar switch using three coupled waveguides with a central nanobeam

This theoretical simulation paper presents designs and projected performance of ∼1550-nm silicon-on-insulator (SOI) and ∼2000-nm Ge-on-Si-on-nitride and Ge-on-nitride 2×2 optical crossbar switches based upon a three-waveguide coupler in which the central waveguide is a nanobeam actuated by the thermo-optical (TO) effect. A TO heater stripe is located atop the central nanobeam. To implement accurate and realistic designs, the 3D finite difference time domain approach was employed. The metrics of crossbar switching, insertion loss (IL) and crosstalk (CT) were evaluated for choices of 3-waveguide structure parameters and TO-induced index changes. The predicted ILs and CTs were excellent, enabling the designed devices to be considered as fundamental building blocks in wavelength-division-multiplexed cross-connect (WXC) applications. Proposed here are compact, nonblocking space-and-wavelength routing switches to be constructed in a monolithic, industry-standard SOI chip (and in Ge-on-SON and GON chips). Specifics are given for realizing 16 × 16 × Mλ WXCs as well as reconfigurable, multi-resonant, programmable hexagonal and diamond meshes.

Reflectionless dual standing-wave microcavity resonator units for photonic integrated circuits

We propose a novel photonic circuit element configuration that emulates the through-port response of a bus coupled traveling-wave resonator using two standing-wave resonant cavities. In this "reflectionless resonator unit", the two constituent cavities, here photonic crystal (PhC) nanobeams, exhibit opposite mode symmetries and may otherwise belong to a single design family. They are coupled evanescently to the bus waveguide without mutual coupling. We show theoretically, and verify using FDTD simulations, that reflection is eliminated when the two cavities are wavelength aligned. This occurs due to symmetry-induced destructive interference at the bus coupling region in the proposed photonic circuit topology. The transmission is equivalent to that of a bus-coupled traveling-wave (e.g. microring) resonator for all coupling conditions. We experimentally demonstrate an implementation fabricated in a new 45 nm silicon-on-insulator complementary metal-oxide semiconductor (SOI CMOS) electronic-photonic process. Both PhC nanobeam cavities have a full-width half-maximum (FWHM) mode length of 4.28 μm and measured intrinsic Q's in excess of 200,000. When the resonances are tuned to degeneracy and coalesce, transmission dips of the over-coupled PhC nanobeam cavities of -16 dB and -17 dB nearly disappear showing a remaining single dip of -4.2 dB, while reflection peaks are simultaneously reduced by 10 dB, demonstrating the quasi-traveling-wave behavior. This photonic circuit topology paves the way for realizing low-energy active devices such as modulators and detectors that can be cascaded to form wavelength-division multiplexed links with smaller power consumption and footprint than traveling wave, ring resonator based implementations.

Tunable all-optical microwave filter with high tuning efficiency

We propose and experimentally demonstrate a continuously tunable all-optical microwave filter based on a photonic crystal (PC) L3 cavity. Due to the small cavity mode volume and prominent optical properties, the required power to arouse the cavity nonlinear effects is low as microwatt level. Moreover, the cavity resonance could be continuously shifted by finely adjusting the input powers. Therefore, under optical single sideband modulation, the frequency interval between the optical carrier and cavity resonance could be controllable. In this case, the central frequency of the microwave photonic filter (MPF) could be continuously tuned with low power consumption. To the best of our knowledge, the experimental tuning efficiency of 101.45 GHz/mW is a record for on-chip tunable all-optical microwave filters. With dominant features of all-optical control, ultra-high tuning efficiency (101.45 GHz/mW), large rejection ratios (48 dB) and compact footprint (100 µm²), the proposed silicon nanocavity is competent to process microwave signals, which has many useful applications in on-chip energy-efficient microwave photonic systems.

Designing an electro-optical encoder based on photonic crystals using the graphene-Al2O3 stacks

In this paper, an electro-optical 4-to-2 encoder based on a photonic crystal is presented. The structure is composed of four silicon waveguides, four photonic crystal structures including the graphene-${{\rm Al}_2}{{\rm O}_3}$Al₂O₃ stacks, and two optical combiners. Two one-dimensional arrays of air holes in the silicon background are designed parallel to the waveguides. Also, a graphene-${{\rm Al}_2}{{\rm O}_3}$Al₂O₃ stack is placed at the center of each array, which provides the desired interferences. This feature is used for controlling the optical wave transmission through the waveguides. Using two optical combiners at the end of two waveguides, the received signals from the waveguides will be guided toward the output ports. The amount of the transmitted signal from input ports to the output of the encoder can be controlled by applying the proper chemical potential to the graphene-based stacks. The simulation results show that the encoding operation can be achieved by using 0.2 eV and 0.8 eV for chemical potentials. In addition, the normalized output power margins for logic 0 and 1 are calculated to be 8.2% and 46.7%, respectively. The footprint for the proposed structure is approximately equal to ${127}\;\unicode{x00B5} {{\rm m}^2}$127µm². Also, the required optical power intensity at input ports is ${100}\;{\rm mW/}\unicode{x00B5} {{\rm m}^2}$100mW/µm².

High-Q antisymmetric multimode nanobeam photonic crystal cavities in silicon waveguides

Antisymmetric multimode nanobeam photonic crystal cavities (AM-NPCs) are proposed and demonstrated in this paper. Due to transverse symmetry-breaking of the antisymmetric multimode periodic waveguide, anti-crossing of the fundamental mode and 1st-order mode is realized and confirmed by band structure calculation. Two-mode filtering and reflection-free cavity filters based on this characteristic are demonstrated. Experimental results on silicon-on-insulator platform shows that broadband (> 100 nm) reflection suppression (< -10 dB) and high-Q (7 × 10⁴) AM-NPCs can be achieved using existed design methodology and fabrication facility. We also explain resonance splitting of the measured transmission spectra and find resonance-enhanced mode-conversion phenomena in the AM-NPCs.

Silicon microdisk-based full adders for optical computing

Due to the projected saturation of Moore's law, as well as the drastically increasing trend of bandwidth with lower power consumption, silicon photonics has emerged as one of the most promising alternatives that has attracted a lasting interest due to the accessibility and maturity of ultra-compact passive and active integrated photonic components. In this Letter, we demonstrate a ripple-carry electro-optic 2-bit full adder using microdisks, which replaces the core part of an electrical full adder by optical counterparts and uses light to carry signals from one bit to the next with high bandwidth and low power consumption per bit. All control signals of the operands are applied simultaneously within each clock cycle. Thus, the severe latency issue that accumulates as the size of the full adder increases can be circumvented, allowing for an improvement in computing speed and a reduction in power consumption. This approach paves the way for future high-speed optical computing systems in the post-Moore's law era.

Improved 2 × 2 Mach-Zehnder switching using coupled-resonator photonic-crystal nanobeams

Design and simulation results are presented for an on-chip 2×2 Mach-Zehnder-based optical switch where each arm of the interferometer is composed of a coupled-resonator optical waveguide. The individual resonators are one-dimensional photonic crystal nanobeam cavities, and switching occurs through thermally induced changes in the refractive index of the silicon structure using integrated heating pads. The performance of the coupled-resonator device is directly compared to its single resonator counterpart, and significant improvement is found in the bar-state CT metric.

Tunable Design of Structural Colors Produced by Pseudo-1D Photonic Crystals of Graphene Oxide

It is broadly observed that graphene oxide (GO) films appear transparent with a thickness of about several nanometers, whereas they appear dark brown or almost black with thickness of more than 1 μm. The basic color mechanism of GO film on a sub-micrometer scale, however, is not well understood. This study reports on GO pseudo-1D photonic crystals (p1D-PhCs) exhibiting tunable structural colors in the visible wavelength range owing to its 1D Bragg nanostructures. Striking structural colors of GO p1D-PhCs could be tuned by simply changing either the volume or concentration of the aqueous GO dispersion during vacuum filtration. Moreover, the quantitative relationship between thickness and reflection wavelength of GO p1D-PhCs has been revealed, thereby providing a theoretical basis to rationally design structural colors of GO p1D-PhCs. The spectral response of GO p1D-PhCs to humidity is also obtained clearly showing the wavelength shift of GO p1D-PhCs at differently relative humidity values and thus encouraging the integration of structural color printing and the humidity-responsive property of GO p1D-PhCs to develop a visible and fast-responsive anti-counterfeiting label. The results pave the way for a variety of potential applications of GO in optics, structural color printing, sensing, and anti-counterfeiting.

Simulation of germanium nanobeam electro-optical 2 × 2 switches and 1 × 1 modulators for the 2 to 5 µm infrared region

This paper proposes and analyzes resonant Si-based electro-optical modulators and switches that use Ge-on-Si₃N₄ nanobeams (NBs) operating at 2 to 5 µm wavelengths. The wavelength of operation can be extended to 15 µm by mounting the Ge channel waveguides on a bulk Si chip. Electrons and holes are injected into the intrinsic Ge NB cavity center via thin P- and N- doped Ge wings on the NB (a lateral PIN diode at ~0.5 V forward bias). Simulations of the carrier-induced resonance-wavelength shift-and-damping in a 1 × 1 modulator show 6 dB of extinction at ~60 fJ/bit over the mid infrared. The NB's active length is λ-scale. The cavity uses tapered-diameter air holes. Intensity modulation at ~1 Gb/s appears feasible. High-performance 2 × 2 switching is predicted by embedding one NB in each arm of a Mach-Zehnder device. The resonance of each identical NB is shifted by the same Δλ via carrier injection. Calculations show very low insertion loss and crosstalk in both the cross and bar states; however, the cross-to-bar energy, around 8 pJ/bit, is much higher than that in the 2 × 2 version that employs PN-junction carrier depletion.

Proposed ultralow-energy dual photonic-crystal nanobeam devices for on-chip N x N switching, logic, and wavelength multiplexing

Silicon-on-insulator Mach-Zehnder interferometer structures that utilize a photonic crystal nanobeam waveguide in each of two connecting arms are proposed here as efficient 2 × 2 resonant, wavelength-selective electro-optical routing switches that are readily cascaded into on-chip N × N switching networks. A localized lateral PN junction of length ~2 μm within each of two identical nanobeams is proposed as a means of shifting the transmission resonance by 400 pm within the 1550 nm band. Using a bias swing ΔV = 2.7 V, the 474 attojoules-per-bit switching mechanism is free-carrier sweepout due to PN depletion layer widening. Simulations of the 2 × 2 outputs versus voltage are presented. Dual-nanobeam designs are given for N × N data-routing matrix switches, electrooptical logic unit cells, N × M wavelength selective switches, and vector matrix multipliers. Performance penalties are analyzed for possible fabrication induced errors such as non-ideal 3-dB couplers, differences in optical path lengths, and variations in photonic crystal cavity resonances.

Analysis of an electro-optic modulator based on a graphene-silicon hybrid 1D photonic crystal nanobeam cavity

We propose and numerically study an on-chip graphene-silicon hybrid electro-optic (EO) modulator operating at the telecommunication band, which is implemented by a compact 1D photonic crystal nanobeam (PCN) cavity coupled to a bus waveguide with a graphene sheet on top. Through electrically tuning the Fermi level of the graphene, both the quality factor and the resonance wavelength can be significantly changed, thus the in-plane lightwave can be efficiently modulated. Based on finite-difference time-domain (FDTD) simulation results, the proposed modulator can provide a large free spectral range (FSR) of 125.6 nm, a high modulation speed of 133 GHz, and a large modulation depth of ~12.5 dB in a small modal volume, promising a high performance EO modulator for wavelength-division multiplexed (WDM) optical communication systems.

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.

Unambiguous demonstration of soliton evolution in slow-light silicon photonic crystal waveguides with SFG-XFROG

We demonstrate the temporal and spectral evolution of picosecond soliton in the slow light silicon photonic crystal waveguides (PhCWs) by sum frequency generation cross-correlation frequency resolved optical grating (SFG-XFROG) and nonlinear Schrödinger equation (NLSE) modeling. The reference pulses for the SFG-XFROG measurements are unambiguously pre-characterized by the second harmonic generation frequency resolved optical gating (SHG-FROG) assisted with the combination of NLSE simulations and optical spectrum analyzer (OSA) measurements. Regardless of the inevitable nonlinear two photon absorption, high order soliton compressions have been observed remarkably owing to the slow light enhanced nonlinear effects in the silicon PhCWs. Both the measurements and the further numerical analyses of the pulse dynamics indicate that, the free carrier dispersion (FCD) enhanced by the slow light effects is mainly responsible for the compression, the acceleration, and the spectral blue shift of the soliton.