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Silicon-based micro-devices are considered promising candidates for consolidating several terahertz technologies into a common and practical platform. The practicality stems from the relatively low loss, device compactness, ease of fabrication, and wide range of available passive and active functionalities. Nevertheless, typical device footprints are limited by diffraction to several hundreds of micrometers, which hinders emerging nanoscale applications of terahertz frequencies. While metallic gap modes provide nanoscale terahertz confinement, efficiently coupling to them is difficult. Here we present and experimentally demonstrate a strategy for efficiently interfacing sub-terahertz radiation (\lambda=1 mm) to a waveguide formed by a nanogap, etched in a gold film, that is 200 nm (\lambda/5000) wide and up to 4.5 mm long. The design principle relies on phase matching dielectric and nanogap waveguide modes, resulting in efficient directional coupling between them when placed side-by-side. Broadband far field terahertz transmission experiments through the dielectric waveguide reveal a transmission dip near the designed wavelength due to resonant coupling. Near field measurements on the surface of the gold layer confirm that such a dip is accompanied by a transfer of power to the nanogap, with an estimated coupling efficiency of ~10%. Our approach provides a pathway for efficiently interfacing millimeter-wave and near-infrared photonic circuits, providing controlled and tailored nanoscale terahertz confinement, with important implications for on-chip nanospectroscopy, telecommunications, and quantum technologies.

Chip-based terahertz (THz) devices are emerging as versatile tools for manipulating mm-wave frequencies in the context of integrated high-speed communication technologies for potential sixth-generation (6G) wireless applications. The characterization of THz devices is typically performed using far-field techniques that provide limited information about the underlying physical mechanisms producing them. As the library of chip-based functionalities expands, e.g., for tailoring the emission and directional propagation properties of THz antennas and waveguides, novel characterization techniques will likely be beneficial for observing subtle effects that are sensitive to a device's structural parameters. Here we present near-field measurements showing the emission properties of a broadband THz emitter placed in the vicinity of a photonic crystal (PHC) slab. These experiments reveal long-predicted emission properties, but which to our knowledge have yet to be experimentally observed at THz frequencies. We demonstrate three distinct effects between 0.3-0.5 THz: (i) field suppression at frequencies corresponding to quasi-TE bandgaps (ii) a frequency-dependent directed emission along two distinct pathways for two neighboring frequencies, resulting in a local field concentration; (iii) a re-direction of the emission, achieved by rotating the PHC with respect to the dipole orientation. Simulations reveal that the observed behavior can be predicted from the underlying band structure. Our results highlight the opportunities that PHCs can potentially provide for alignment-free, chip-based 6G technologies. Our experimental technique extends the applicability realms of THz spectroscopy and will find use for characterizing the THz modes supported by true samples, whose inherent imperfections cannot realistically be accounted for by simulations, particularly in highly dispersive frequency bands.

The excitation of microresonators using focused intensity modulated light, known as photothermal excitation, is gaining significant attention due to its capacity to accurately excite microresonators without distortions, even in liquid environments, which is driving key advancements in atomic force microscopy and related technologies. Despite progress in the development of coatings, the conversion of light into mechanical movement remains largely inefficient, limiting resonator movements to tens of nanometres even when milliwatts of optical power are used. Moreover, how photothermal efficiency depends on the relative position of a microresonator along the propagation axis of the photothermal beam remains poorly studied, hampering our understanding of the conversion of light into mechanical motion. Here, we perform photothermal measurements in air and water using cantilever microresonators and a custom-built picobalance, and determine how photothermal efficiency changes along the propagation beam axis. We identify that far out-of-band laser emission can lead to visual misidentification of the beam waist, resulting in a drop of photothermal efficiency of up to one order of magnitude. Our measurements also unveil that the beam waist is not always the position of highest photothermal efficiency, and can reduce the efficiency up to 20% for silicon cantilevers with trapezoidal cross section.

Paradoxically, imaging with resolution much below the wavelength $\lambda$ - now common place in the visible spectrum - remains challenging at lower frequencies, where arguably it is needed most due to the large wavelengths used. Techniques to break the diffraction limit in microscopy have led to many breakthroughs across sciences, but remain largely confined to the optical spectrum, where near-field coupled fluorophores operate. At lower frequencies, exponentially decaying evanescent waves must be measured directly, requiring a tip or antenna to be brought into very close vicinity to the object. This is often difficult, and can be problematic as the probe can perturb the near-field distribution itself. Here we show the information encoded in evanescent waves can be probed further than previously thought possible, and a truthful image of the near-field reconstructed through selective amplification of evanescent waves - akin to a virtual superlens reversing the evanescent decay. We quantify the trade-off between noise and measurement distance, and experimentally demonstrate reconstruction of complex images with subwavelength features, down to a resolution of $\lambda/7$ and amplitude signal-to-noise ratios below 25dB between 0.18-1.5THz. Our procedure can be implemented with any near field probe far from the reactive near field region, greatly relaxes experimental requirements for subwavelength imaging in particular at sub-optical frequencies, and opens the door to non-perturbing near-field scanning.

We report for the first time the direct growth of Molybdenum disulfide (MoS$_2$) monolayers on nanostructured silicon-on-insulator waveguides. Our results indicate the possibility of utilizing the Chemical Vapour Deposition (CVD) on nanostructured photonic devices in a scalable process. Direct growth of 2D material on nanostructures rectifies many drawbacks of the transfer-based approaches. We show that the van der Waals materials grow conformally across the curves, edges, and the silicon-SiO$_2$ interface of the waveguide structure. Here, the waveguide structure used as a growth substrate is complex not just in terms of its geometry but also due to the two materials (Si and SiO$_2$) involved. A transfer-free method like this yields a novel approach for functionalizing nanostructured, integrated optical architectures with an optically active direct semiconductor.

Terahertz (THz) technology is a growing and multi-disciplinary research field, particularly for sensing and telecommunications. A number of THz waveguides have emerged over the past years, which are set to complement the capabilities of existing and bulky free space setups. In most designs however, the guiding region is physically separated from the surroundings, making interactions between light and the environment inefficient. We present photonic THz light cages (THzLCs) operating at THz frequencies, consisting of free-standing dielectric strands, which guide light within a hollow core with immediate access to the environment. We show the versatility and design flexibility of this concept, by 3D-printing several cm-length-scale modules using a single design and four different polymer- and ceramic- materials, which are either rigid, flexible, or resistant to high temperatures. We characterize propagation- and bend-losses for straight- and curved- waveguides, which are of order ~1 dB/cm in the former, and ~2-8 dB/cm in the latter for bend radii below 10 cm, and largely independent of the material. Our transmission experiments are complemented by near-field measurements at the waveguide output, which reveal antiresonant guidance for straight THzLCs, and a deformed fundamental mode in the bent waveguides, in agreement with numerical conformal mapping simulations. We show that these THzLCs can be used either as: (i) flexible, reconfigurable, and bendable modular assemblies; (ii) in-core sensors of structures contained directly inside the hollow core; (iii) high-temperature sensors, with potential applications in industrial monitoring. These THzLCs are a novel and useful addition to the growing library of THz waveguides, marrying the waveguide-like advantages of reconfigurable, diffractionless propagation, with the free-space-like immediacy of direct exposure to the surrounding environment.

We evaluate the sensing properties of plasmonic waveguide sensors by calculating their resonant transmission spectra in different regions of the non-Hermitian eigenmode space. We elucidate the pitfalls of using modal dispersion calculations in isolation to predict plasmonic sensor performance, which we address by using a simple model accounting for eigenmode excitation and propagation. Our transmission calculations show that resonant wavelength and spectral width crucially depend on the length of the sensing region, so that no single criterion obtained from modal dispersion calculations alone can be used as a proxy for sensitivity. Furthermore, we find that the optimal detection limits occur where directional coupling is supported, where the narrowest spectra occur. Such narrow spectral features can only be measured by filtering out all higher-order modes at the output, e.g., via a single-mode waveguide. Our calculations also confirm a characteristic square root dependence of the eigenmode splitting with respect to the permittivity perturbation at the exceptional point, which we show can be identified through the sensor beat length at resonance. This work provides a convenient framework for designing and characterizing plasmonic waveguide sensors when comparing with experimental measurements.

Silica-based optical fibers are a workhorse of nonlinear optics. They have been used to demonstrate nonlinear phenomena such as solitons and self-phase modulation. Since the introduction of the photonic crystal fiber, they have found many exciting applications, such as supercontinuum white light sources and third-harmonic generation, among others. They stand out by their low loss, large interaction length, and the ability to engineer its dispersive properties, which compensate for the small chi(3) nonlinear coefficient. However, they have one fundamental limitation: due to the amorphous nature of silica, they do not exhibit second-order nonlinearity, except for minor contributions from surfaces. Here, we demonstrate significant second-harmonic generation in functionalized optical fibers with a monolayer of highly nonlinear MoS2 deposited on the fiber guiding core. The demonstration is carried out in a 3.5 mm short piece of exposed core fiber, which was functionalized in a scalable process CVD-based process, without a manual transfer step. This approach is scalable and can be generalized to other transition metal dichalcogenides and other waveguide systems. We achieve an enhancement of more than 1000x over a reference sample of equal length. Our simple proof-of-principle demonstration does not rely on either phase matching to fundamental modes, or ordered growth of monolayer crystals, suggesting that pathways for further improvement are within reach. Our results do not just demonstrate a new path towards efficient in-fiber SHG-sources, instead, they establish a platform with a new route to chi(2)-based nonlinear fiber optics, optoelectronics, and photonics platforms, integrated optical architectures, and active fiber networks.

We consider the motion of a light sail that is accelerated by a powerful laser beam. We derive the equations of motion for two proof-of-concept sail designs with damped internal degrees of freedom. Using linear stability analysis we show that perturbations of the sail movement in all lateral degrees of freedom can be damped passively. This analysis also shows complicated behaviour akin to that associated with exceptional points in PT-symmetric systems in optics and quantum mechanics. The excess heat that is produced by the damping mechanism is likely to be substantially smaller than the expected heating due to the partial absorption of the incident laser beam by the sail.

The quest for practical waveguides operating in the terahertz range faces two major hurdles: large losses and high rigidity. While recent years have been marked by remarkable progress in lowering the impact of material losses using hollow-core guidance, such waveguides are typically not flexible. Here we experimentally and numerically investigate antiresonant dielectric waveguides made of polyurethane, a commonly used dielectric with a low Young's modulus. The hollow-core nature of antiresonant fibers leads to low transmission losses using simple structures, whereas the low Young's modulus of polyurethane makes them extremely flexible. The structures presented enable millimeter-wave manipulation in centimeter-thick waveguides in the same spirit as conventional (visible- and near-IR-) optical fibers, i.e. conveniently and reconfigurably. We investigate two canonical antiresonant geometries formed by one- and six-tubes, experimentally comparing their transmission, bend losses and mode profiles. The waveguides under investigation have loss below 1 dB/cm in their sub-THz transmission bands, increasing by 1 dB/cm for a bend radius of about 10 cm, which is analogous to bending standard $125 \mu{\rm m}$ diameter fiber to a 1.2 mm radius.

Bulk materials possessing a relative electric permittivity $\varepsilon$ close to zero exhibit giant Kerr nonlinearities. However, harnessing this response in guided-wave geometries is not straightforward, due to the extreme and counter-intuitive properties of epsilon-near-zero materials. Here we investigate, through rigorous calculations of the Kerr nonlinear coefficient, how the remarkable nonlinear properties of such materials can be exploited in several different types of structures, including bulk films, plasmonic nanowires, and metal nanoapertures. We find the largest Kerr nonlinear response when both the modal area and the group velocity are simultaneously minimized, corresponding to omnidirectional field enhancement. The physical insights developed will be key for understanding and engineering nonlinear nanophotonic systems with extreme nonlinearities and point to new design paradigms.

Atomically thin transition metal dichalcogenides are highly promising for integrated optoelectronic and photonic systems due to their exciton-driven linear and nonlinear interaction with light. Integrating them into optical fibers yields novel opportunities in optical communication, remote sensing, and all-fiber optoelectronics. However, scalable and reproducible deposition of high quality monolayers on optical fibers is a challenge. Here, we report the chemical vapor deposition of monolayer MoS2 and WS2 crystals on the core of microstructured exposed core optical fibers and their interaction with the fibers' guided modes. We demonstrate two distinct application possibilities of 2D-functionalized waveguides to exemplify their potential. First, we simultaneously excite and collect excitonic 2D material photoluminescence with the fiber modes, opening a novel route to remote sensing. Then we show that third harmonic generation is modified by the highly localized nonlinear polarization of the monolayers, yielding a new avenue to tailor nonlinear optical processes in fibers. We anticipate that our results may lead to significant advances in optical fiber based technologies.

We introduce a modular approach for efficiently interfacing photonic integrated circuits with deep-sub-wavelength hybrid plasmonic functionality. We demonstrate that an off-the-shelf silicon-on-insulator waveguide can be post-processed into an integrated hybrid plasmonic circuit by evaporating a silica and gold nanolayer. The circuit consists of a plasmonic rotator and a nanofocusser module, which together result in nano-scale, nonlinear wavelength conversion. We experimentally characterize each module, and demonstrate an intensity enhancement of $>200$ in a calculated mode area of $50\,{\rm nm}^2$ at $\lambda = 1320\,{\rm nm}$ using second harmonic generation. This work opens the door to customized plasmonic functionalities on industry-standard waveguides, bridging conventional integrated photonic circuits with hybrid plasmonic devices. This approach promises convenient access to nanometre-scale quantum information processing, nonlinear plasmonics, and single-molecule sensing.

We experimentally observe an effective PT-phase transition through the exceptional point in a hybrid plasmonic-dielectric waveguide system. Transmission experiments reveal fundamental changes in the underlying Eigenmode interactions as the environmental refractive index is tuned, which can be unambiguously attributed to a crossing through the plasmonic exceptional point. These results extend the design opportunities for tuneable non-Hermitian physics to plasmonic systems.