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The ultrafast concentration of electromagnetic energy in nanoscale volumes is one of the key features of optical nanoantennas illuminated at their surface plasmon resonances. Here, we drive the insulator to metal phase transition in vanadium dioxide (VO2) using a laser-induced pumping effect obtained by positioning a single gold nanoantenna in proximity to a VO2 thermochromic material. We explore how the geometry of the single nanoantenna affects the size and permittivity of the nanometer-scale VO2 regions featuring phase transition under different pumping conditions. The results reveal that a higher VO2 phase transition effect is obtained for pumping of the longitudinal or transversal localized surface plasmon depending on the antenna length. This characterization is of paramount importance since the single nanoantennas are the building blocks of many plasmonic nanosystems. Finally, we demonstrate the picosecond dynamics of the VO2 phase transition characterizing this system, useful for the realization of fast nano-switches. Our work shows that it is possible to miniaturize the hybrid plasmonic-VO2 system down to the single-antenna level, still maintaining a controllable behavior, fast picosecond dynamics, and the features characterizing its optical and thermal response.

The high speed conversion of signals between the optical and electrical domains is crucial for many key applications of silicon photonics. Electro-optic modulators integrated with electronic drive amplifiers are typically used to convert an electrical signal to the optical domain. Design of these individual elements is important to achieve high performance, however a true optimisation requires careful co-design of the photonic and electronic components considering the properties of each other. Here we present our recent results in this area together with a MOSCAP type modulator with the potential for high speed, high efficiency and highly linear modulation.

The ultrafast concentration of electromagnetic energy in nanoscale volumes is one of the key features of optical nanoantennas illuminated at their surface plasmon resonances. Here, we drive the insulator to metal phase transition in vanadium dioxide (VO2) using a laser-induced pumping effect obtained by positioning a single gold nanoantenna in proximity to a VO2 thermochromic material. We explore how the geometry of the single nanoantenna affects the size and permittivity of the nanometer-scale VO2 regions featuring phase transition under different pumping conditions. The results reveal that a higher VO2 phase transition effect is obtained for pumping of the longitudinal or transversal localized surface plasmon depending on the antenna length. This characterization is of paramount importance since the single nanoantennas are the building blocks of many plasmonic nanosystems. Finally, we demonstrate the picosecond dynamics of the VO2 phase transition characterizing this system, useful for the realization of fast nano-switches. Our work shows that it is possible to miniaturize the hybrid plasmonic-VO2 system down to the single-antenna level, still maintaining a controllable behavior, fast picosecond dynamics, and the features characterizing its optical and thermal response. [ABSTRACT FROM AUTHOR]

Featured Application: The CORNERSTONE silicon photonics rapid prototyping service can be used by academic and industry researchers alike. The platform targets those who require flexibility in their fabrication process flows for applications in datacoms, metrology, sensing and more. The field of silicon photonics has experienced widespread adoption in the datacoms industry over the past decade, with a plethora of other applications emerging more recently such as light detection and ranging (LIDAR), sensing, quantum photonics, programmable photonics and artificial intelligence. As a result of this, many commercial complementary metal oxide semiconductor (CMOS) foundries have developed open access silicon photonics process lines, enabling the mass production of silicon photonics systems. On the other side of the spectrum, several research labs, typically within universities, have opened up their facilities for small scale prototyping, commonly exploiting e-beam lithography for wafer patterning. Within this ecosystem, there remains a challenge for early stage researchers to progress their novel and innovate designs from the research lab to the commercial foundries because of the lack of compatibility of the processing technologies (e-beam lithography is not an industry tool). The CORNERSTONE rapid-prototyping capability bridges this gap between research and industry by providing a rapid prototyping fabrication line based on deep-UV lithography to enable seamless scaling up of production volumes, whilst also retaining the ability for device level innovation, crucial for researchers, by offering flexibility in its process flows. This review article presents a summary of the current CORNERSTONE capabilities and an outlook for the future. [ABSTRACT FROM AUTHOR]

Symmetries of waveguides determine fundamental properties of photons such as mode profiles, polarisation, and effective refractive indexes as well as practical properties affecting the propagation loss. Here, we review our recent progress on manipulating symmetries of silicon (Si) photonic waveguides. Starting from the strategic choice of Si-on-insulator (SOI) wafer specifications, we established the process technologies to fabricate Si wire and slot waveguides with atomically-flat interfaces, defined by Si (111) planes. These waveguides have relatively low propagation loss (∼1 dB cm−1), even though they were fabricated in a university line. By combining patterning and re-growth of deposited amorphous Si, we also fabricated an Si slot waveguide with a nano-meter-scale vertical oxide layer, which is useful for optical modulators and various sensing applications. We also fabricated a horizontal slot waveguide using our manually bonded double-SOI substrate. The self-limited alkali-wet-etching allowed us to pattern the bottom SOI layer on top of the top SOI layer, by properly designing the mask to align along the mirror asymmetric Si (110) surface, allowing to access to top and bottom SOI layers individually through connected multiple-fin arrays. The patterning technique can be readily applicable to the other platform such as Si/LiNbO3-hybrid wafers, and we discuss our design of electro-optic modulator towards zero power consumptions. We also investigate photonic crystal waveguides with broken mirror symmetries. By manipulating the mismatch between adjacent photonic crystals across the waveguide made of line defects, we could continuously control the band-gap of the photonic crystals. Moreover, the phase profiles of modes exhibited photonic graphene and poly-acetylene shapes, made of optical vortices with optical orbital angular momentum. This shows that the most energetically favourable configuration of a photonic material under the triangular lattice is topologically equivalent to an organic material. We discuss the potential for the photonic organic chemistry and possible applications in quantum technologies. [ABSTRACT FROM AUTHOR]

Grapheneis an optical material of unusual characteristics because of its linearlydispersive conduction and valence bands and the strong interband transitions.It allows broadband light-matter interactions with ultrafast responsesand can be readily pasted to surfaces of functional structures forphotonic and optoelectronic applications. Recently, graphene-basedoptical modulators have been demonstrated with electrical tuning ofthe Fermi level of graphene. Their operation bandwidth, however, waslimited to about 1 GHz by the response of the driving electrical circuit.Clearly, this can be improved by an all-optical approach. Here, weshow that a graphene-clad microfiber all-optical modulator can achievea modulation depth of 38% and a response time of ∼2.2 ps, limitedonly by the intrinsic carrier relaxation time of graphene. This modulatoris compatible with current high-speed fiber-optic communication networksand may open the door to meet future demand of ultrafast optical signalprocessing. [ABSTRACT FROM AUTHOR]

Germanium (Ge) ion implantation into silicon waveguides will induce lattice defects in the silicon, which can eventually change the crystal silicon into amorphous silicon and increase the refractive index from 3.48 to 3.96. A subsequent annealing process, either by using an external laser or integrated thermal heaters can partially or completely remove those lattice defects and gradually change the amorphous silicon back into the crystalline form and, therefore, reduce the material's refractive index. Utilising this change in optical properties, we successfully demonstrated various erasable photonic devices. Those devices can be used to implement a flexible and commercially viable wafer-scale testing method for a silicon photonics fabrication line, which is a key technology to reduce the cost and increase the yield in production. In addition, Ge ion implantation and annealing are also demonstrated to enable post-fabrication trimming of ring resonators and Mach–Zehnder interferometers and to implement nonvolatile programmable photonic circuits. [ABSTRACT FROM AUTHOR]

High-quality single-crystalline Li 2 Sn(IO 3 ) 6 microwires (MWs) have been successfully prepared by using a facile hydrothermal method. The as-synthesized Li 2 Sn(IO 3 ) 6 MWs exhibit regular hexagonal prism morphology, excellent surface smoothness, and remarkable diameter uniformity. The optical propagation loss has been determined to be as low as 0.026 dB μm -1 at 785 nm wavelength, implying the low-loss optical waveguiding capability of the Li 2 Sn(IO 3 ) 6 MWs. The effective frequency-doubling conversions of the fundamental frequency light source in the wavelength range from 916 to 1560 nm have been observed, and the second-harmonic generation (SHG) conversion efficiency has been measured to be 2.1% with a 1560 nm fundamental pump source (pulse duration of 10 ns, and average power of 9.06 nW) transmitted through a 1.32-μm-diameter and 300-μm-length Li 2 Sn(IO 3 ) 6 MW. These intriguing optical waveguiding and strong SHG conversion capabilities of the Li 2 Sn(IO 3 ) 6 MWs suggest its potential applications for photonic devices in micrometer scale.

In this work, we propose and experimentally demonstrate a broadband polarization splitter-rotator (PSR) on the lithium niobate on insulator (LNOI). With multiple sequentially connected adiabatic tapers for waveguide mode conversion and directional coupling, the PSR shows a 160-nm bandwidth covering the C and L bands, an insertion loss of less than 2 dB, and an extinction ratio of more than 11 dB. Benefiting from the conversion-enhanced adiabatic tapers, the broadband device has a short length of 405 µm. Further optimization is performed to reduce the device length to 271 µm and comparable performances are achieved, demonstrating the feasibility of higher device compactness. The proposed design and principle can contribute to high-performance polarization management for integrated lithium niobate photonics.

Modal inspection of optical fibers is important for multimode application but it is challenging to collect in-situ information of propagating modes for evaluation and manipulation. Here we demonstrate direct observation of multimode interference in Er 3+ /Yb 3+ co-doped micro/nanofibers. Luminescent interference patterns are visualized by visible up-conversion of Er 3+ ions and are used for establishing the existence of higher order modes co-propagating with fundamental modes. We use fast Fourier transform to analyze the patterns in detail and obtain excellent agreement between experiment and calculation on beat lengths of the interference. Effective index differences among higher order modes and a fundamental mode of a microfiber are also experimentally investigated with the assistance of interference patterns, revealing the characteristic of modal dispersions.

Fabrication errors pose significant challenges on silicon photonics, promoting post-fabrication trimming technologies to ensure device performance. Conventional approaches involve multiple trimming and characterization steps, impacting overall fabrication complexity. Here we demonstrate a highly accurate trimming method combining laser annealing of germanium implanted silicon waveguide and real-time monitoring of device performance. Direct feedback of the trimming process is facilitated by a differential spectroscopic technique based on photomodulation. The resonant wavelength trimming accuracy is better than 0.15 nm for ring resonators with 20-µm radius. We also realize operating point trimming of Mach-Zehnder interferometers with germanium implanted arms. A phase shift of 1.2π is achieved by annealing a 7-μm implanted segment.

Advanced photonic probing techniques are of great importance for the development of non-contact wafer-scale testing of photonic chips. Ultrafast photomodulation has been identified as a powerful new tool capable of remotely mapping photonic devices through a scanning perturbation. Here, we develop photomodulation maps into a quantitative technique through a general and rigorous method based on Lorentz reciprocity that allows the prediction of transmittance perturbation maps for arbitrary linear photonic systems with great accuracy and minimal computational cost. Excellent agreement is obtained between predicted and experimental maps of various optical multimode-interference devices, thereby allowing direct comparison of a device under test with a physical model of an ideal design structure. In addition to constituting a promising route for optical testing in photonics manufacturing, ultrafast perturbation mapping may be used for design optimization of photonic structures with reconfigurable functionalities.

Hybrid integration of nanoplasmonic devices with silicon photonic circuits holds promise for a range of applications in on-chip sensing, field-enhanced and nonlinear spectroscopy, and integrated nanophotonic switches. Here, we demonstrate a new regime of photon-plasmon coupling by combining a silicon photonic resonator with plasmonic nanoantennas. Using principles from coherent perfect absorption, we make use of standing-wave light fields to maximize the photon-plasmon interaction strength. Precise placement of the broadband antennas with respect to the narrowband photonic racetrack modes results in controlled hybridization of only a subset of these modes. By combining antennas into groups of radiating dipoles with opposite phase, far-field scattering is effectively suppressed. We achieve ultrafast tuning of photon-plasmon hybridization including reconfigurable routing of the standing-wave input between two output ports. Hybrid photonic-plasmonic resonators provide conceptually new approaches for on-chip integrated nanophotonic devices.

On the basis of the transverse second harmonic generation (TSHG) in a highly nonlinear subwavelength-diameter CdTe nanowire, we demonstrate a single-nanowire optical correlator for femto-second pulse measurement with pulse energy down to femtojoule (fJ) level. Pulses to be measured were equally split and coupled into two ends of a suspending nanowire via tapered optical fibers. The couterpropagating pulses meet each other around the central area of the nanowire, and emit TSHG signal perpendicular to the axis of the nanowire. By transferring the spatial intensity profile of the transverse second harmonic (TSH) image into the time-domain temporal profile of the input pulses, we operate the nanowire as a miniaturized optical correlator. Benefitted from the high nonlinearity and the very small effective mode area of the waveguiding CdTe nanowire, the input energy of the single-nanowire correlator can go down to fJ-level (e.g., 2 fJ/pulse for 1064 nm 200 fs pulses). The miniature fJ-pulse correlator may find applications from low power on-chip optical communication, biophotonics to ultracompact laser spectroscopy.

We demonstrate ultrafast optical modulation using a single 1-μm-diameter graphene-decorated microfiber, which is fabricated with a convenient and controllable evanescent-field-induced deposition method. Benefitting from the significantly enhanced light-graphene interaction of the subwavelength transvers dimension of the microfiber and accumulation of the saturable absorption of the piled graphene flakes, the microfiber shows nonlinear saturable absorption with a peak power threshold down to 1.75 W (60 MW/cm(2)), with a measured response time of about 3.5 ps.

We report the fabrication and characterization of freestanding graphene coated ZnO nanowires (GZNs) for optical waveguiding. The GZNs are fabricated using a tape-assist transfer under micromanipulation. Owing to the deep-subwavelength diameter and high index contrast of the ZnO nanowire waveguide, light-graphene interaction is significantly enhanced by the strong surface optical fields, resulting in a linear absorption as high as 0.11 dB/µm in a 606-nm-diameter GZN at 1550-nm wavelength. Launched by 1550-nm-wavelength femto-second pulses, a 475-nm-diameter GZN with a graphene coating length of merely 24 µm exhibits evident nonlinear saturable absorption with a peak power threshold down to 1.3 W. In addition, we also demonstrate a transmission modulation for 1550-nm-wavelength signal with a 590-nm-diameter GZN, showing the possibility of using GZN waveguides as nanoscale bulding blocks for nanophotonic devices.

We demonstrate a dynamic surface plasmonic modulation based on graphene-nanowire (grapheme-NW) hybrid structures in the visible light range. A static modulation depth of as high as 0.07 dB/μm has been achieved experimentally. Through careful simulation and systematical experimental investigation, we found that the dual-confinement effect of charge density and electromagnetic energy around the vicinity of the NW will dramatically enhance the light-matter interaction and increase the Fermi level shifting, which are the key roles for bringing the optical response of the device to the visible range. The carrier concentration near the vicinity of a Ag NW is estimated to reach 0.921×10(14) cm(-2) after applying more than 25 V voltages, which is enough to shift the Fermi level for visible light. Furthermore, the modulation behaviors near the Dirac point of monolayer graphene and the singularity of gap-induced bilayer graphene are investigated. Calculated optical conductivity as a function of Fermi level predicts a minimum value near the Dirac point, which is consistent with the experimental results.