The crystallinity of a material often plays a significant role in determining its material properties. Aluminum nitride (AlN), which has emerged as a promising material for photonics in the past few decades, can be polycrystalline or monocrystalline in nature. Previously, the electro-optic (EO) coefficient of polycrystalline and bulk monocrystalline AlN has been reported. However, to the best of our knowledge, the EO coefficient of thin-film monocrystalline AlN has not yet been reported. In this work, we report the EO coefficient of thin-film monocrystalline AlN and make a side-by-side comparison with the EO coefficient of polycrystalline AlN. We used the resonant shift of a microring resonator to measure the EO coefficients for both transverse electric (TE) and transverse magnetic (TM) modes in the telecom C-band. For monocrystalline AlN, we observe surface effects that cause bias drift, which can be eliminated through annealing. The EO coefficients we measured for monocrystalline and polycrystalline AlN are comparable, with the EO of annealed monocrystalline AlN being slightly higher.

We present Self-Aligned Fiber Entry Low-loss Optical Couplers (SAFELOCs), three-dimensionally tapered polymer waveguides that efficiently couple tapered optical fibers to on-chip waveguides across multiple material platforms with low insertion loss (≲ 1 dB) over the C and L -bands. SAFELOCs incorporate 3D fiber receptacles for passive, mechanical alignment, enabling broadband coupling without precision stages or active feedback. Experimental measurements confirm low insertion loss consistent with simulations, and the design can achieve minimal polarization dependence. The tapered fiber receptacle also allows permanent fiber attachment while expanding alignment tolerance, facilitating simultaneous coupling of fiber arrays for multi-port photonic circuits. While demonstrated in the C and L bands on Si₃N₄, Si, and SU-8, the two-photon lithography fabrication process can be applied to other materials, and the polymer's broad transparency window suggests compatibility down to 400 nm, highlighting the versatility and scalability of SAFELOCs.

We demonstrate efficient coupling of multiple fibers using 3D-printed coupling structures that interface with on-chip grating and adiabatic couplers, achieving low-loss, compact integration. The technique is readily scalable for high-density and alignment-tolerant photonic packaging applications.

We present SOI foundry-compatible, switchable, and nonvolatile micromechanical beams for post-fabrication tuning of microresonator-based photonic integrated circuits. Utilizing mechanical bistability, these devices provide an energy-efficient, passive, and digital approach for aligning optical resonances to target wavelengths.

We demonstrate an adiabatic polarization splitter-rotator on a 500 nm Si 3 N 4 -on-SiO 2 platform, with < 1 dB insertion loss, < −12 dB crosstalk, and > 50 nm bandwidth near 1550 nm, tolerant to ±50 nm width variations.

Using the resonant shift in a microring resonator, we measure the electro-optic coefficient for both TE and TM modes in monocrystalline and polycrystalline AlN in the C band.

Evanescent couplers are fabricated on multicore fibers using direct laser writing. The devices are coupled to microdisk, microring, and photonic crystal cavities in-situ, demonstrating direct fiber-to-device coupling and the potential for wafer-scale optical probing.

We present an ultrasensitive technique for probing transient optical changes in atomically thin molybdenum disulfide (MoS_2) layers integrated onto silicon nitride (Si_3N_4) ring resonators. The MoS_2 is illuminated by a femtosecond laser, while a tunable near-infrared (NIR) continuous-wave laser probes the microresonator resonance. The NIR light polarization can be adjusted to either transverse electric (TE, parallel to the 2D material) or transverse magnetic (TM, perpendicular), a configuration that is impossible to achieve with conventional normal-incidence pump-probe techniques. By capturing the transmitted signal on a fast oscilloscope, we detect transient optical shifts with unprecedented sensitivity, observing phenomena over time scales ranging from picoseconds to microseconds. Our results reveal both a rapid, carrier-induced nonlinear optical shift in the resonance, and a slower thermo-optic transient. The ability to simultaneously measure these fast and slow dynamics offers new insight into the complex optoelectronic behavior of 2D materials when integrated with microresonators. This method provides a significant advance over traditional pump-probe approaches, enabling the detection of exceedingly small transient signals and opening new avenues for exploring the optical properties of atomically thin materials. Our findings highlight the potential of this approach for investigating polarization-dependent nonlinear effects, with applications in photonics, sensing, and optoelectronics.

In a future quantum network, two spatially separated individuals sharing a polarization entanglement source may need to assess the entanglement quality of the source without the presence of classical auxiliary signals. When the two are separated by a single-mode fiber, automated methods are necessary for the validation and estimation of polarization entanglement. We experimentally examine real-time iterative methods to search for maximal Bell violations, specifically the Clauser–Horne–Shimony–Holt (CHSH) inequality, between two observers sharing polarization entangled photons to evaluate the entanglement quality of the source. Our source allows us to tune the degree of entanglement of their shared photon pair by changing the temporal overlap of the two photons at an entangling 50/50 beam splitter so that we can compare the CHSH parameter these methods find to the theoretical values our source produces. The iterative methods used within our experiment are the Nelder–Mead optimization method, stochastic gradient descent, and Bayesian optimization. This is the first feedback experiment to study automated Bell violations in fiber and the first to compare all three of these iterative methods to one another in a quantum polarization control experiment. In our experiment, all methods are able to find Bell violations, but the Nelder–Mead method performed the best in terms of the speed and accuracy in finding the maximal violation.

We measured the optical transmission through an SU-8 microring resonator inside a cryostat and analyzed the shift of the resonant wavelengths to determine the thermo-optic behavior around a wavelength of 1600 nm. As the temperature was decreased from room temperature (RT) to 3K, the refractive index of crosslinked SU-8 was measured to increase from 1.571 to 1.584, while the thermo-optic coefficient decreased by two orders of magnitude.

We demonstrate low-loss (< 1 dB), broadband (BW∼100 nm near λ ∼ 1550 nm) and polarization-independent fiber-to-chip couplers using 3D nano-printed polymer structures on Si3N4-on-SiO2 platform.

This paper demonstrates a passive, integrated electro-optic receiver for detection of free -space microwave radiation. Unlike a traditional microwave receiver, which relies on conductive antennas and electrical amplifiers, this receiver uses only passive, optically probed elements with no electrodes or electronic components. The receiver employs two co -resonant structures: a dielectric resonator antenna (DRA) to concentrate incoming microwave radiation and an integrated aluminum nitride (AlN) racetrack resonator to resonantly enhance the optical carrier. The microwave field of the DRA modulates the built-up optical carrier in the resonator via the electro-optic response of AlN. We successfully detected 15 GHz microwave radiation through co -resonant electro-optic up -conversion, without the need for any conducting electrodes, amplifiers, or electronic components. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

We study the generation of spin-orbit (SO) modes via four-wave mixing (FWM)-based parametric amplification. SO modes carry quantized total angular momentum (TAM), and we show that FWM processes that generate new signals conserve TAM. This is a generalization of prior research which operated in a regime where FWM processes conserved spin and orbital angular momenta independently. We calculate the growth rates of new modes for both degenerate and nondegenerate pump configurations. Our theory is validated against numerical simulations for the cases where the generated signals are in the same SO mode(s) as the pump(s). We also calculate the growth rates of signals in SO modes other than the pumps.

With woodland expansion providing a nature-based solution to help tackle anthropogenic climate change, the persistence and regeneration of fragmented Atlantic oak woodland is a critical feature of the wilding of uplands and conservation of Temperate Rainforests in Western Europe. Predicted shifts in precipitation with climate change are likely to be a major determinant of tree recruitment here, yet our understanding of the threshold effects of high soil moisture on oak establishment is limited, particularly at early-life-history stages when trees are most vulnerable. We address this gap in knowledge by investigating the effect of soil saturation states on; 1) the capacity for oak seedling development from acorns in peaty soils; and 2) the survival and performance of juvenile (one-year-old) saplings planted in upland pasture. In container experiments, we quantified Quercus robur establishment from acorns in four soil saturation states. There was complete recruitment failure in 'flooded' soils (water level 20 mm above acorn), and reduction (43% survival) at 'high' saturation (water level 81 mm below acorn) compared to 'medium' (77% survival - water level 155 mm below acorn) and 'low' (83% survival - water level 220 mm below acorn) treatments. Surviving seedlings exhibited reduced root:shoot ratio, leaf photosynthesis, and lower likelihood of late season shoot growth in soils of high saturation. In the field experiment, juvenile oak (Q. robur, Q. petraea) saplings were planted in UK upland freely draining, seasonally waterlogged, and waterlogged pastures soils and subject to livestock browsing treatments. Q. robur had greater shoot growth and leaf photosynthesis response to soil saturation than closely related Q. petraea. Results highlight the need for better understanding of soil-dependent influence on tree browsing. We conclude that climate-resilient restoration of Atlantic oak woodland must utilise both European oak species to support seedling and sapling recruitment events in the face of rapid but uncertain changes in precipitation and soil conditions. Such an approach might better allow future natural colonisation events across a gradient of projected soil scenarios and landscape trajectories. Further research should look to better characterise the role of soil moisture on the early-life-history stages of trees. Furthermore, understanding the soil factors that limit seedling and sapling establishment, especially when these factors are themselves perturbed by climate change, is central to both our wider understanding of plant community response to climate change and nature-based solutions to mitigate its effects.

Developing low-power, high-sensitivity photodetectors for the terahertz (THz) band that operate at room temperature is an important challenge in optoelectronics. In this study, we introduce a photo-thermal-electric (PTE) effect detector based on quasi-free standing bilayer graphene (BLG) on a silicon carbide (SiC) substrate, designed for the THz frequency range. Our detector's performance hinges on a quasi-optical coupling scheme, which integrates an aspherical silicon lens, to optimize impedance matching between the THz antenna and the graphene p-n junction. At room temperature, we achieved a noise equivalent power (NEP) of less than 300 pW/root Hz. Through an impedance matching analysis, we coupled a planar antenna with a graphene p-n junction, inserted in parallel to the nano-gap of the antenna, via two coupling capacitors. By adjusting the capacitors and the antenna arm length, we tailored the antenna's maximum infrared power absorption to specific frequencies. The sensitivity, spectral properties, and scalability of our material make it an ideal candidate for future development of far-infrared detectors operating at room temperature.

We designed and demonstrated 3D polymer coupler structures to adiabatically couple light into polymer waveguides. We measured a coupling loss of 2.1 dB per coupler facet around 1550 nm.

We demonstrate a record 14 bits-per-incident-photon Photon Information Efficiency (PIE) detection using a new transmitter design that produces an average extinction ratio of 89 dB and operates at four times higher rates than previous high-PIE demonstrations.

Graphene plasmonic elements enhance the light-matter interaction at resonance. Intense optical excitation results in a nonlinear response, based on two main causes: thermal effects and the Kerr effect. Here we present polarization dependent pump-probe experiments on graphene disks. Those revealed that both effects are similar in strength, though the Kerr nonlinearity is much faster, making it a candidate for efficient harmonic generation. Besides excitation with linear polarization, we investigated the impact of circularly polarized pump radiation. The circulating currents cause an effective quasi-static magnetic field perpendicular to the disks and thus Faraday rotation without the need of external magnetic field.

Strong circularly polarized excitation opens up the possibility to generate and control effective magnetic fields in solid state systems, e.g., via the optical inverse Faraday effect or the phonon inverse Faraday effect. While these effects rely on material properties that can be tailored only to a limited degree, plasmonic resonances can be fully controlled by choosing proper dimensions and carrier concentrations. Plasmon resonances provide new degrees of freedom that can be used to tune or enhance the light-induced magnetic field in engineered metamaterials. Here we employ graphene disks to demonstrate light-induced transient magnetic fields from a plasmonic circular current with extremely high efficiency. The effective magnetic field at the plasmon resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong ( ~ 1°) ultrafast Faraday rotation ( ~ 20 ps). In accordance with reference measurements and simulations, we estimated the strength of the induced magnetic field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ cm −2 .