Ultrafast optical control using the Kerr nonlinearity in hydrogenated amorphous silicon microcylindrical resonators

Mid-infrared optical modulator based on silicon D-shaped photonic crystal fiber with VO2 material

Recently, photonic crystal fibers (PCFs) have become of significant interest due to their various applications, especially in the mid-infrared (mid-IR) regime. In this work, an optical mid-IR modulator based on silicon D-shaped PCF (Si-D-PCF) with vanadium dioxide (VO₂) as a phase changing material (PCM) is presented and analyzed. Thanks to the phase transition of the VO₂ material between insulating (ON) and conducting (OFF) states, the modulation process can be attained. The well-known full vectorial finite element method is utilized to numerically analyze the proposed design. Further, the propagation of light through the suggested structure is studied using the 3D finite difference time domain method. The optical losses of the fundamental TM mode supported by the Si-D-PCF structure in both ON and OFF states are investigated. The obtained results reveal that the extinction ratio (ER) of the reported modulator approaches 236 dB, while the insertion loss (IL) is less than 1.3 dB over the studied wavelength range 3-7 µm at a device length (L_D) of 0.5 mm. Additionally, the ER of the proposed modulator is higher than 56 dB through the whole studied wavelength range. Therefore, the proposed modulator could be utilized in photonic integrated circuits that require high ER, low IL, and large bandwidth. To the best of the authors' knowledge, this is the first time an infrared optical modulator based on Si-D-PCF with VO₂ material has been presented.

Controlled Transportation of Light by Light at the Microscale

We show how light can be controllably transported by light at microscale dimensions. We design a miniature device that consists of a short segment of an optical fiber coupled to transversally oriented input-output microfibers. A whispering gallery soliton is launched from the first microfiber into the fiber segment and slowly propagates along its mm-scale length. The soliton loads and unloads optical pulses at designated input-output microfibers. The speed of the soliton and its propagation direction is controlled by the dramatically small, yet feasible to introduce, permanently or all-optically, nanoscale variations of the effective fiber radius.

Analysis of photonic noise generated due to Kerr nonlinearity in silicon ring resonators

The Kerr effect in silicon ring resonators (RRs) is widely used for switching and regeneration of optical communications signals. In addition, it has been shown to considerably limit the performance of refractive index sensors based on high quality-factor RRs. While the Kerr effect's impact on output signals of silicon RRs is well known, its influence on the properties of the output noise is yet to be explored. In this work, we analytically and numerically analyze the noise properties of Kerr effect in silicon RRs. We show that the input power, RR's bandwidth, and input optical signal to noise ratio (OSNR) have significant influence on the power and distribution of the output noise. We use the developed noise model to evaluate the RR's noise figure and output noise distribution for optical communications and sensing applications. These noise properties can be used for the design and performance evaluation of optical communications systems and sensors using silicon photonic RRs.

Ultracompact CMOS-compatible optical logic using carrier depletion in microdisk resonators

We present a CMOS-compatible optoelectronic directed logic architecture that achieves high computational throughput (number of operations per second per unit area) by its ultracompact form factor. High speed-to-power performance is also achieved, by the low capacitance and high junction-to-mode overlap of low-radii SOI vertical pn junction microdisk switches. By using wavelength-division multiplexing and two electrical control signals per disk, each switch performs (N)OR, (N)AND, and X(N)OR operations simultaneously. Connecting multiple switches together, we demonstrate higher-order scalability in five fundamental N-bit logic circuits: AND/OR gates, adders, comparators, encoders, and decoders. To the best of our knowledge, these circuits achieve the lowest footprint of silicon-based multigigabit-per-second optical logic devices in literature.

Electrically tunable whispering gallery mode microresonator based on a grapefruit-microstructured optical fiber infiltrated with nematic liquid crystals

An electrically tunable whispering gallery mode (WGM) microresonator based on an HF-etched microstructured optical fiber (MOF) infiltrated with nematic liquid crystals (NLCs) is proposed and experimentally demonstrated. Experimental results indicate that as the peak-to-peak voltage of the applied AC electric field increases from 160 to 220 V, WGM resonance peaks gradually move toward a shorter wavelength region by 0.527 nm with a wavelength sensitivity up to 0.01  nm/V for a TM1691 mode, and the Q-factor for each WGM resonance peak rapidly decreases with the increment of applied electric voltage. The proposed electrically controlled WGM tuning scheme shows a linear resonance wavelength shift with good spectral reversibility, which makes it a promising candidate to serve as an integrated functional photonic device in practical use and in related fundamental scientific studies.

Experimental demonstration of a reconfigurable electro-optic directed logic circuit using cascaded carrier-injection micro-ring resonators

We experimentally demonstrate a reconfigurable electro-optic directed logic circuit which can perform any combinatorial logic operation using cascaded carrier-injection micro-ring resonators (MRRs), and the logic circuit is fabricated on the silicon-on-insulator (SOI) substrate with the standard commercial Complementary Metal-Oxide-Semiconductor (CMOS) fabrication process. PIN diodes embedded around MRRs are employed to achieve the carrier injection modulation. The operands are represented by electrical signals, which are applied to the corresponding MRRs to control their switching states. The operation result is directed to the output port in the form of light. For proof of principle, several logic operations of three-operand with the operation speed of 100 Mbps are demonstrated successfully.

Self-phase modulation in highly confined submicron Ta2O5 channel waveguides

Optical spectra broadening as a result self-phase modulation in a channel waveguide fabricated on a high quality tantalum pentoxide (Ta₂O₅) film by using RF sputtering is measured. The full-width at half maximum of the optical spectra for transverse electric (TE)/transverse magnetic (TM) polarizations of 42.5/31.7 nm is obtained using pulses of 10 nm at a wavelength of 800 nm with a peak-coupled power of 43.77 W. The nonlinear Kerr coefficients of 2.14 × 10^-14 cm²/W and 1.92 × 10^-14 cm²/W for TE and TM polarizations, respectively, are then extracted from the experiments using a theoretical model based on the method of moments. The obtained results on the nonlinearity further suggest that Ta₂O₅ is a promising material to develop nonlinear waveguide devices for integrated photonics.

Air-mode photonic crystal ring resonator on silicon-on-insulator

In this report, we propose and demonstrate an air-mode photonic crystal ring resonator (PhCRR) on silicon-on-insulator platform. Air mode is utilized to confine the optical field into photonic crystal (PhC) air holes, which is confirmed by the three-dimensional finite-difference time-domain simulation. PhCRR structure is employed to enhance the light-matter interaction through combining the whispering-gallery mode resonance of ring resonator with the slow-light effect in PhC waveguide. In the simulated and measured transmission spectra of air-mode PhCRR, nonuniform free spectral ranges are observed near the Brillouin zone edge of PhC, indicating the presence of the slow-light effect. A maximum group index of 27.3 and a highest quality factor of 14600 are experimentally obtained near the band edge. Benefiting from the strong optical confinement in the PhC holes and enhanced light-matter interaction in the resonator, the demonstrated air-mode PhCRR is expected to have potential applications in refractive index sensing, on-chip light emitting and nonlinear optics by integration with functional materials.

Microscopic optical buffering in a harmonic potential

In the early days of quantum mechanics, Schrödinger noticed that oscillations of a wave packet in a one-dimensional harmonic potential well are periodic and, in contrast to those in anharmonic potential wells, do not experience distortion over time. This original idea did not find applications up to now since an exact one-dimensional harmonic resonator does not exist in nature and has not been created artificially. However, an optical pulse propagating in a bottle microresonator (a dielectric cylinder with a nanoscale-high bump of the effective radius) can exactly imitate a quantum wave packet in the harmonic potential. Here, we propose a tuneable microresonator that can trap an optical pulse completely, hold it as long as the material losses permit, and release it without distortion. This result suggests the solution of the long standing problem of creating a microscopic optical buffer, the key element of the future optical signal processing devices.

Laser-tuned whispering gallery modes in a solid-core microstructured optical fibre integrated with magnetic fluids

A laser-assisted tuning method of whispering gallery modes (WGMs) in a cylindrical microresonator based on magnetic-fluids-infiltrated microstructured optical fibres (MFIMOFs, where MF and MOF respectively refer to magnetic fluid and microstructured optical fibre) is proposed, experimentally demonstrated and theoretically analysed in detail. The MFIMOF is prepared by infiltrating the air-hole array of the MOF using capillary action effect. A fibre-coupling system is set up for the proposed MFIMOF-based microresonator to acquire an extinction ratio up to 25 dB and a Q-factor as large as 4.0 × 10⁴. For the MF-infiltrated MOF, the light propagating in the fibre core region would rapidly spread out and would be absorbed by the MF-rod array cladding to induce significant thermal effect. This has been exploited to achieve a WGM resonance wavelength sensitivity of 0.034 nm/mW, which is ~20 times higher than it counterpart without MF infiltration. The wavelength response of the resonance dips exhibit linear power dependence, and owing to such desirable merits as ease of fabrication, high sensitivity and laser-assisted tunability, the proposed optical tuning approach of WGMs in the MFIMOF would find promising applications in the areas of optical filtering, sensing, and signal processing, as well as future all-optical networking systems.

Low-loss and high-Q Ta(2)O(5) based micro-ring resonator with inverse taper structure

A low-loss and high-Q Ta(2)O(5) based micro-ring resonator is presented. The micro-ring resonator and channel waveguide with core area of the 700 by 400 nm(2) were fabricated on amorphous Ta(2)O(5) thin films prepared by reactive sputtering at 300°C and post annealing at 650°C for 3 hours. The Ta(2)O(5) micro-ring resonator with a diameter of 200 μm was coupled to the channel waveguide with a coupled Q up to 38,000 at a 0.9 μm coupling gap. By fitting the transmission spectrum of the resonator, the extracted loss coefficient inside the ring cavity and transmission coefficient of TE mode were 8.1dB/cm and 0.9923, leading to the estimated unloaded Q of higher than 44,000. In addition, based on the cut-back method, the propagation loss and the coupling loss of Ta(2)O(5) channel waveguide with an inverse taper were 1.5dB/cm and 3.2 dB, respectively. The proposed Ta(2)O(5) technology offers an unique alternative for fabricating high performance guided wave devices, and may well lead to novel applications in photonic integrated circuits.

Tunable continuous wave emission via phase-matched second harmonic generation in a ZnSe microcylindrical resonator

Whispering gallery mode microresonators made from crystalline materials are of great interest for studies of low threshold nonlinear phenomena. Compared to amorphous materials, crystalline structures often exhibit desirable properties such as high indices of refraction, high nonlinearities, and large windows of transparency, making them ideal for use in frequency comb generation, microlasing and all-optical processing. In particular, crystalline materials can also possess a non-centrosymmetric structure which gives rise to the second order nonlinearity, necessary for three photon processes such as frequency doubling and parametric down-conversion. Here we report a novel route to fabricating crystalline zinc selenide microcylindrical resonators from our semiconductor fibre platform and demonstrate their use for tunable, low power continuous wave second harmonic generation. Visible red light is observed when pumped with a telecommunications band source by a process that is phase-matched between different higher order radial modes, possible due to the good spatial overlap between the pump and signal in the small volume resonator. By exploiting the geometrical flexibility offered by the fibre platform together with the ultra-wide 500-22000 nm transmission window of the ZnSe material, we expect these resonators to find use in applications ranging from spectroscopy to quantum information systems.

Whispering gallery mode sensors

We present a comprehensive overview of sensor technology exploiting optical whispering gallery mode (WGM) resonances. After a short introduction we begin by detailing the fundamental principles and theory of WGMs in optical microcavities and the transduction mechanisms frequently employed for sensing purposes. Key recent theoretical contributions to the modeling and analysis of WGM systems are highlighted. Subsequently we review the state of the art of WGM sensors by outlining efforts made to date to improve current detection limits. Proposals in this vein are numerous and range, for example, from plasmonic enhancements and active cavities to hybrid optomechanical sensors, which are already working in the shot noise limited regime. In parallel to furthering WGM sensitivity, efforts to improve the time resolution are beginning to emerge. We therefore summarize the techniques being pursued in this vein. Ultimately WGM sensors aim for real-world applications, such as measurements of force and temperature, or alternatively gas and biosensing. Each such application is thus reviewed in turn, and important achievements are discussed. Finally, we adopt a more forward-looking perspective and discuss the outlook of WGM sensors within both a physical and biological context and consider how they may yet push the detection envelope further.

Kerr nonlinear switching in a hybrid silica-silicon microspherical resonator

A hybrid silicon-core, silica-clad microspherical resonator has been fabricated from the semiconductor core fiber platform. Linear and nonlinear characterization of the resonator properties have shown it to exhibit advantageous properties associated with both materials, with the low loss cladding supporting high quality (Q) factor whispering gallery modes which can be tuned through the nonlinear response of the crystalline core. By exploiting the large wavelength shift associated with the Kerr nonlinearity, we have demonstrated all-optical modulation of a weak probe on the timescale of the femtosecond pump pulse. This novel geometry offers a route to ultra-low loss, high-Q silica-based resonators with enhanced functionality.

Si-rich SiNx based Kerr switch enables optical data conversion up to 12 Gbit/s

Silicon photonic interconnection on chip is the emerging issue for next-generation integrated circuits. With the Si-rich SiNx micro-ring based optical Kerr switch, we demonstrate for the first time the wavelength and format conversion of optical on-off-keying data with a bit-rate of 12 Gbit/s. The field-resonant nonlinear Kerr effect enhances the transient refractive index change when coupling the optical data-stream into the micro-ring through the bus waveguide. This effectively red-shifts the notched dip wavelength to cause the format preserved or inversed conversion of data carried by the on-resonant or off-resonant probe, respectively. The Si quantum dots doped Si-rich SiNx strengthens its nonlinear Kerr coefficient by two-orders of magnitude higher than that of bulk Si or Si3N4. The wavelength-converted and cross-amplitude-modulated probe data-stream at up to 12-Gbit/s through the Si-rich SiNx micro-ring with penalty of -7 dB on transmission has shown very promising applicability to all-optical communication networks.

Cavity ring-up spectroscopy for ultrafast sensing with optical microresonators

Spectroscopy of whispering-gallery mode microresonators has become a powerful scientific tool, enabling the detection of single viruses, nanoparticles and even single molecules. Yet the demonstrated timescale of these schemes has been limited so far to milliseconds or more. Here we introduce a scheme that is orders of magnitude faster, capable of capturing complete spectral snapshots at nanosecond timescales—cavity ring-up spectroscopy. Based on sharply rising detuned probe pulses, cavity ring-up spectroscopy combines the sensitivity of heterodyne measurements with the highest-possible, transform-limited acquisition rate. As a demonstration, we capture spectra of microtoroid resonators at time intervals as short as 16 ns, directly monitoring submicrosecond dynamics of their optomechanical vibrations, thermorefractive response and Kerr nonlinearity. Cavity ring-up spectroscopy holds promise for the study of fast biological processes such as enzyme kinetics, protein folding and light harvesting, with applications in other fields such as cavity quantum electrodynamics and pulsed optomechanics.

Whispering-gallery mode microresonators are powerful sensing tools, but spectrum acquisition has taken milliseconds or longer. Here, Rosenblum et al. introduce cavity ring-up spectroscopy, in which sharply rising detuned probe pulses capture spectra of microresonators on nanosecond timescales.

Generation of coherent supercontinuum in a-Si:H waveguides: experiment and modeling based on measured dispersion profile

Hydrogenated amorphous silicon (a:Si-H) has recently been recognized as a highly nonlinear CMOS compatible photonic platform. We experimentally demonstrate the generation of a supercontinuum (SC) spanning over 500 nm in a-Si:H photonic wire waveguide at telecommunication wavelengths using femtosecond input pulse with energy lower than 5 pJ. Numerical modeling of pulse propagation in the waveguide, based on the experimentally characterized dispersion profile, shows that the supercontinuum is the result of soliton fission and dispersive wave generation. It is demonstrated that the SC is highly coherent and that the waveguides do not suffer from material degradation under femtosecond pulse illumination. Finally, a direct comparison of SC generation in c-Si and a-Si:H waveguides confirms the higher performances of a-Si:H over c-Si for broadband low power SC generation at telecommunication wavelengths.

All-optical switching using Kerr effect in a silica toroid microcavity

We demonstrate experimentally an all-optical switching operation using the Kerr effect in a silica toroid microcavity. Thanks to the small mode volume and high quality factor of the silica toroid microcavity, we achieved on-chip optical Kerr switching with an input power of 2 mW. This value is the smallest among all previously reported on-chip optical Kerr switches. We also show that this value can be reduced to a few tens of μW by employing a mode with a Q factor of > 2 × 10⁷.

Hybrid polymer photonic crystal fiber with integrated chalcogenide glass nanofilms

The combination of chalcogenide glasses with polymer photonic crystal fibers (PCFs) is a difficult and challenging task due to their different thermo-mechanical material properties. Here we report the first experimental realization of a hybrid polymer-chalcogenide PCF with integrated As₂S₃ glass nanofilms at the inner surface of the air-channels of a poly-methyl-methacrylate (PMMA) PCF. The integrated high refractive index glass films introduce distinct antiresonant transmission bands in the 480–900 nm wavelength region. We demonstrate that the ultra-high Kerr nonlinearity of the chalcogenide glass makes the polymer PCF nonlinear and provides a possibility to shift the transmission band edges as much as 17 nm by changing the intensity. The proposed fabrication technique constitutes a new highway towards all-fiber nonlinear tunable devices based on polymer PCFs, which at the moment is not possible with any other fabrication method.

Picosecond all-optical switching in hydrogenated amorphous silicon microring resonators

We utilize cross-phase modulation to observe all-optical switching in microring resonators fabricated with hydrogenated amorphous silicon (a-Si:H). Using 2.7-ps pulses from a mode-locked fiber laser in the telecom C-band, we observe optical switching of a cw telecom-band probe with full-width at half-maximum switching times of 14.8 ps, using approximately 720 fJ of energy deposited in the microring. In comparison with telecom-band optical switching in undoped crystalline silicon microrings, a-Si:H exhibits substantially higher switching speeds due to reduced impact of free-carrier processes.