Ultrafast all-optical second harmonic wavefront shaping

Optical communication can be revolutionized by encoding data into the orbital angular momentum of light beams. However, state-of-the-art approaches for dynamic control of complex optical wavefronts are mainly based on liquid crystal spatial light modulators or miniaturized mirrors, which suffer from intrinsically slow (µs-ms) response times. Here, we experimentally realize a hybrid meta-optical system that enables complex control of the wavefront of light with pulse-duration limited dynamics. Specifically, by combining ultrafast polarization switching in a WSe2 monolayer with a dielectric metasurface, we demonstrate second harmonic beam deflection and structuring of orbital angular momentum on the femtosecond timescale. Our results pave the way to robust encoding of information for free space optical links, while reaching response times compatible with real-world telecom applications.

Two-Color Spatially Resolved Tuning of Polymer-Coated Metasurfaces

For the realization of truly reconfigurable metasurface technologies, dynamic spatial tuning of the metasurface resonance is required. Here we report the use of organic photoswitches as a means for the light-induced spatial tuning of metasurface resonances. Coating of a dielectric metasurface, hosting high-quality-factor resonances, with a spiropyran (SPA)-containing polymer enabled dynamic resonance tuning up to 4 times the resonance full-width at half-maximum with arbitrary spatial precision. A major benefit of employing photoswitches is the broad toolbox of chromophores available and the unique optical properties of each. In particular, SPA and azobenzene (AZO) photoswitches can both be switched with UV light but exhibit opposite refractive index changes. When applied to the metasurface, SPA induced a red shift in the metasurface resonance with a figure of merit of 97 RIU-1, while AZO caused a blue shift in the resonance with an even greater sensitivity of 100 RIU-1. Critically, SPA and AZO can be individually recovered with red and blue light, respectively. To exploit this advantage, we coated a dielectric metasurface with spatially offset SPA- and AZO-containing polymers to demonstrate wavelength-dependent, spatially resolved control over the metasurface resonance tuning.

Structured illumination ptychography and at-wavelength characterization with an EUV diffuser at 13.5 nm wavelength

Structured illumination is essential for high-performance ptychography. Especially in the extreme ultraviolet (EUV) range, where reflective optics are prevalent, the generation of structured beams is challenging and, so far, mostly amplitude-only masks have been used. In this study, we generate a highly structured beam using a phase-shifting diffuser optimized for 13.5 nm wavelength and apply this beam to EUV ptychography. This tailored illumination significantly enhances the quality and resolution of the ptychography reconstructions. In particular, when utilizing the full dynamics range of the detector, the resolution has been improved from 125 nm, when using an unstructured beam, to 34 nm. Further, ptychography enables the quantitative measurement of both the amplitude and phase of the EUV diffuser at 13.5 nm wavelength. This capability allows us to evaluate the influence of imperfections and contaminations on its "at wavelength" performance, paving the way for advanced EUV metrology applications and highlighting its importance for future developments in nanolithography and related fields.

Generation of infrared photon pairs by spontaneous four-wave mixing in a CS2-filled microstructured optical fiber

We experimentally demonstrate frequency non-degenerate photon-pair generation via spontaneous four-wave mixing from a novel CS2-filled microstructured optical fiber. CS2 has high nonlinearity, narrow Raman lines, a broad transmission spectrum, and also has a large index contrast with the microstructured silica fiber. We can achieve phase matching over a large spectral range by tuning the pump wavelength, allowing the generation of idler photons in the infrared region, which is suitable for applications in quantum spectroscopy. Moreover, we demonstrate a coincidence-to-accidental ratio of larger than 90 and a pair generation efficiency of about [Formula: see text] per pump pulse, which shows the viability of this fiber-based platform as a photon-pair source for quantum technology applications.

Color Routing of the Emission from Magnetic and Electric Dipole Transitions of Eu3+ by Broken-Symmetry TiO2 Metasurfaces

Manipulation of magnetic dipole emission with resonant photonic nanostructures is of great interest for both fundamental research and applications. However, obtaining selective control over the emission properties of magnetic dipole transitions is challenging, as they usually occur within a manifold of spectrally close emission lines associated with different spin states of the involved electronic levels. Here we demonstrate spectrally selective directional tailoring of magnetic dipole emission using designed photonic nanostructures featuring a high quality factor. Specifically, we employ a hybrid nanoscale optical system consisting of a Eu3+ compound coupled to a designed broken-symmetry TiO2 metasurface to demonstrate directional color routing of the compound's emission through its distinct electric and magnetic-dominated electronic transition channels. Using low numerical aperture collection optics, we achieve a fluorescence signal enhancement of up to 33.13 for the magnetic-dominated dipole transition at 590 nm when it spectrally overlaps with a spectrally narrow resonance of the metasurface. This makes the, usually weak, magnetic dipole transition the most intense spectral line in our recorded fluorescence spectra. By studying the directional emission properties for the coupled system using Fourier imaging and time-resolved fluorescence measurements, we demonstrate that the high-quality-factor modes in the metasurface enable free-space light routing, where forward-directed emission is established for the magnetic-dominated dipole transition, whereas the light emitted via the electric dipole transition is mainly directed sideways. Our results underpin the importance of magnetic light-matter interactions as an additional degree of freedom in photonic and optoelectronic systems. Moreover, they facilitate the development of spectrometer-free and highly integrated nanophotonic imaging, sensing, and probing devices.

Photoluminescence Enhancement of Monolayer WS2 by n-Doping with an Optically Excited Gold Disk

In nanophotonics and quantum optics, we aim to control and manipulate light with tailored nanoscale structures. Hybrid systems of nanostructures and atomically thin materials are of interest here, as they offer rich physics and versatility due to the interaction between photons, plasmons, phonons, and excitons. In this study, we explore the optical and electronic properties of a hybrid system, a naturally n-doped monolayer WS2 covering a gold disk. We demonstrate that the nonresonant excitation of the gold disk in the high absorption regime efficiently generates hot carriers via localized surface plasmon excitation, which n-dope the monolayer WS2 and enhance the photoluminescence emission by regulating the multiexciton population and stabilizing the neutral exciton emission. The results are relevant to the further development of nanotransistors in photonic circuits and optoelectronic applications.

Precision Tailoring Quasi-BIC Resonance of a-Si:H Metasurfaces

The capability of tailoring the resonance wavelength of metasurfaces is important as it can alleviate the manufacturing precision required to produce the exact structure according to the design of the nanoresonators. Tuning of Fano resonances by applying heat has been theoretically predicted in the case of silicon metasurfaces. Here, we experimentally demonstrate the permanent tailoring of quasi-bound states in the continuum (quasi-BIC) resonance wavelength in an a-Si:H metasurface and quantitatively analyze the modification in the Q-factor with gradual heating. A gradual increment in temperature leads to a spectral shift in the resonance wavelength. With the support of ellipsometry measurements, the spectral shift resulting from the short-duration (ten minutes) heating is identified to be due to refractive index variations in the material rather than a geometric effect or amorphous/polycrystalline phase transition. In the case of quasi-BIC modes in the near-infrared, resonance wavelength could be adjusted from T = 350 °C to T = 550 °C without affecting the Q-factor considerably. Apart from the temperature-induced resonance trimming, large Q-factors can be attained at the highest analyzed temperature (T = 700 °C) in the near-infrared quasi-BIC modes. Resonance tailoring is just one of the possible applications of our results. We expect that our study is also insightful in the design of a-Si:H metasurfaces where large Q-factors are required at high temperatures.

Quantum ghost imaging based on a "looking back" 2D SPAD array

Quantum ghost imaging (QGI) is an intriguing imaging protocol that exploits photon-pair correlations stemming from spontaneous parametric down-conversion (SPDC). QGI retrieves images from two-path joint measurements, where single-path detection does not allow us to reconstruct the target image. Here we report on a QGI implementation exploiting a two-dimensional (2D) single-photon avalanche diode (SPAD) array detector for the spatially resolving path. Moreover, the employment of non-degenerate SPDC allows us to investigate samples at infrared wavelengths without the need for short-wave infrared (SWIR) cameras, while the spatial detection can be still performed in the visible region, where the more advanced silicon-based technology can be exploited. Our findings advance QGI schemes towards practical applications.

Lasing modes in ZnO nanowires coupled to planar metals

Semiconductor nanowire lasers can be subject to modifications of their lasing threshold resulting from a variation of their environment. A promising choice is to use metallic substrates to gain access to low-volume Surface-Plasmon-Polariton (SPP) modes. We introduce a simple, yet quantitatively precise model that can serve to describe mode competition in nanowire lasers on metallic substrates. We show that an aluminum substrate can decrease the lasing threshold for ZnO nanowire lasers while for a silver substrate, the threshold increases compared with a dielectric substrate. Generalizing from these findings, we make predictions describing the interaction between planar metals and semiconductor nanowires, which allow to guide future improvements of highly-integrated laser sources.

Photothermomechanical Nanopump: A Flow-Through Plasmonic Sensor at the Fiber Tip

Optical fibers equipped with plasmonic flow sensors at their tips are fabricated and investigated as photothermomechanical nanopumps for the active transport of target analytes to the sensor surface. The nanopumps are prepared using a bottom-up strategy: i.e., by sequentially stacking a monolayer of a thermoresponsive polymer and a plasmonic nanohole array on an optical fiber tip. The temperature-dependent collapse and swelling of the polymer is used to create a flow-through pumping mechanism. The heat required for pumping is generated by exploiting the photothermal effect in the plasmonic nanohole array upon irradiation with laser light (405 nm). Simultaneous detection of analytes by the plasmonic sensor is achieved by monitoring changes in its optical response at longer wavelengths (∼500-800 nm). Active mass transport by pumping through the holes of the plasmonic nanohole array is visualized by particle imaging velocimetry. Finally, the performance of the photothermomechanical nanopumps is investigated for two types of analytes, namely nanoscale objects (gold nanoparticles) and molecules (11-mercaptoundecanoic acid). In the presence of the pumping mechanism, a 4-fold increase in sensitivity was observed compared to the purely photothermal effect, demonstrating the potential of the presented photothermomechanical nanopumps for sensing applications.

Acid-base responsive photoluminescence switching of CdSe/ZnS quantum dots coupled to plasmonic gold film using nanometer-thick poly[(2-diethylamino)ethyl methacrylate] layer

The control of plasmon-nanoemitter interactions at the nanoscale enables the tailored modulation of optical properties, namely, the photoluminescence (PL) intensity of the nanoemitters. In this contribution, using a nanometer-thick poly[(2-diethylamino) ethyl methacrylate] (39 to 74 nm) as pH responsive spacer layer (pK_a ∼ 6 to 6.5) between a plasmonic gold film and CdSe/ZnS Quantum Dots (QDs) nanoemitters, we could achieve reversible pH-responsive PL switching in QDs. In fact, the swelling (at pH 5) and shrinking (at pH 11) function of the pH-responsive spacer layer modulates the distance between the QDs and the gold surface, which dictates the plasmonic film-QDs nanoemitter interaction. Notably, we observed a high QDs' PL enhancement of up to a factor of 3.1 ± 0.4 through changing the pH value from 5 to 11. Furthermore, based on a systematic analysis of several samples with different spacer layer thicknesses and multiple pH cycles, our developed system revealed substantial stability, reversibility and PL enhancement reproducibility. Thus, the established acid-base responsive switchable systems may represent an appealing platform for applications such as sensors, biochemical assays, optoelectronics and logic gates and can be easily evolved to other multifunctional switchable systems using alternative stimuli-responsive polymers.

Spectral tailoring of photon pairs from microstructured suspended-core optical fibers with liquid-filled nanochannels

We theoretically study the generation of photon pairs via spontaneous four-wave mixing (SFWM) in a liquid-filled microstructured suspended-core optical fiber. We show that it is possible to control the wavelength, group velocity, and bandwidths of the two-photon states. Our proposed fiber structure shows a large number of degrees of freedom to engineer the two-photon state. Here, we focus on the factorable state, which shows no spectral correlation in the two-photon components of the state, and allows the heralding of a single-photon pure state without the need for spectral post-filtering.

Subdiffraction Quantum Imaging with Undetected Photons

We propose a nonlinear imaging scheme with undetected photons that overcomes the diffraction limit by transferring near-field information at one wavelength to far-field information of a correlated photon with a different wavelength generated through spontaneous photon-pair generation. At the same time, this scheme allows for retrieval of high-contrast images with zero background, making it a highly sensitive scheme for imaging of small objects at challenging spectral ranges with subdiffraction resolutions.

Material-specific high-resolution table-top extreme ultraviolet microscopy

Microscopy with extreme ultraviolet (EUV) radiation holds promise for high-resolution imaging with excellent material contrast, due to the short wavelength and numerous element-specific absorption edges available in this spectral range. At the same time, EUV radiation has significantly larger penetration depths than electrons. It thus enables a nano-scale view into complex three-dimensional structures that are important for material science, semiconductor metrology, and next-generation nano-devices. Here, we present high-resolution and material-specific microscopy at 13.5 nm wavelength. We combine a highly stable, high photon-flux, table-top EUV source with an interferometrically stabilized ptychography setup. By utilizing structured EUV illumination, we overcome the limitations of conventional EUV focusing optics and demonstrate high-resolution microscopy at a half-pitch lateral resolution of 16 nm. Moreover, we propose mixed-state orthogonal probe relaxation ptychography, enabling robust phase-contrast imaging over wide fields of view and long acquisition times. In this way, the complex transmission of an integrated circuit is precisely reconstructed, allowing for the classification of the material composition of mesoscopic semiconductor systems.

Toward Perfect Optical Diffusers: Dielectric Huygens' Metasurfaces with Critical Positional Disorder

Conventional optical diffusers, such as thick volume scatterers (Rayleigh scattering) or microstructured surface scatterers (geometric scattering), lack the potential for on-chip integration and are thus incompatible with next-generation photonic devices. Dielectric Huygens' metasurfaces, on the other hand, consist of 2D arrangements of resonant dielectric nanoparticles and therefore constitute a promising material platform for ultrathin and highly efficient photonic devices. When the nanoparticles are arranged in a random but statistically specific fashion, diffusers with exceptional properties are expected to come within reach. This work explores how dielectric Huygens' metasurfaces can implement wavelength-selective diffusers with negligible absorption losses and nearly Lambertian scattering profiles that are largely independent of the angle and polarization of incident waves. The combination of tailored positional disorder with a carefully balanced electric and magnetic response of the nanoparticles is shown to be an integral requirement for the operation as a diffuser. The proposed metasurfaces' directional scattering performance is characterized both experimentally and numerically, and their usability in wavefront-shaping applications is highlighted. Since the metasurfaces operate on the principles of Mie scattering and are embedded in a glassy environment, they may easily be incorporated in integrated photonic devices, fiber optics, or mechanically robust augmented reality displays.

Spatio-temporal propagation dynamics of Airy plasmon pulses

We investigate numerically the evolution of a particular type of non-diffracting pulsed plasmonic beam called Airy plasmon pulses. A suitable diffraction grating is obtained by optimizing a grating (e.g., [Phys. Rev. Lett.107, 116802 (2011)10.1103/PhysRevLett.107.116802]) for maximum generation bandwidth and efficiency to excite ultrashort Airy plasmon pulses. The optimization process is based on Airy and non-Airy plasmons contributions from the diffraction grating. The time-averaged Airy plasmon pulse generated from the grating shows a bent trajectory and quasi non-diffracting properties similar to CW excited Airy plasmons. A design-parameter-dependent geometrical model is developed to explain the spatio-temporal dynamics of the Airy plasmon pulses, which predicts the pulse broadening in Airy plasmon pulses due to non-Airy plasmons emerging from the grating. This model provides a parametric design control for the potential engineering of temporally focused 2D non-diffracting pulsed plasmonic beams.

Tailoring Magnetic Dipole Emission by Broken-Symmetry TiO2 Metasurfaces

Strong magnetic dipole emission is offered by rare earth ions such as trivalent lanthanides, due to selection rule forbidden electric dipole (ED) transitions. This stimulates the study of optical nanostructures, which efficiently tailor magnetic dipole emission. High refractive index all dielectric nanostructures are promising candidates in this regard due to their strong magnetic response and negligible absorption loss in the visible spectral range. Here, we design and experimentally realize a broken-symmetry titanium dioxide (TiO2) metasurface supporting an out-of-plane magnetic dipole (MD) resonance at 590 nm wavelength, corresponding to the MD transition of trivalent Europium ions (Eu3+). A strong photoluminescence (PL) enhancement of the MD transition up to a factor of 15.5 is observed.

Experimental observation of the short-range surface plasmon polariton mode and its longitudinal adiabatic compression in a metallic wedge

In this study, we explore analytically and experimentally long- and short-range surface plasmon polariton (LR-SPP and SR-SPP, respectively) modes in gold wedges. Especially, we aim to observe the 2-dimensional confinement of the electromagnetic field in gold wedges as it could enhance the light-matter interaction by offering a local density of states which depends on the propagation constant, consequently on the wedge height. The LR-SPP mode can propagate over a long distance, but the real part of the propagation constant remains relatively insensitive to the decreasing wedge height. This mode also experiences cut-off at a wedge height of about 50 nm in our experimental condition. Meanwhile, the SR-SPP mode has a large propagation constant that increases further with decreasing wedge height. As a result, the effective wavelength of the mode shrinks confining the electromagnetic wave longitudinally along the propagation direction in addition to enhancing the transverse confinement of SR-SPP. In the experiment, we use gold wedges with different edge heights to excite each SPP mode individually and image the electromagnetic near field by using a pseudo-heterodyne scattering scanning near-field optical microscope. By imaging the LR-SPP mode field, we demonstrate that the theoretical and measured values of the effective wavelength agree quite well. By using short wedges, we measure the SR-SPP mode field and demonstrate that the effective wavelength decreases to 47% in about half a micrometer of propagation distance. This corresponds to a 3.5 times decrease of the vacuum wavelength or an effective index of 3.5. It is important to note that this value is, by no means, the limit of the electromagnetic field's longitudinal confinement in a gold wedge. Rather, we were only able to measure the electromagnetic field up to this point due to our measurement limitations. The electromagnetic field will be propagating further, and the longitudinal confinement will increase as well. In conclusion, we measured the SR-SPP in a gold wedge and demonstrate the electromagnetic field confinement in the visible spectrum in gold wedges.

Ultra-compact, broadband adiabatic passage optical couplers in thin-film lithium niobate on insulator waveguides

We report the first demonstration of broadband adiabatic directional couplers in thin-film lithium niobate on insulator (LNOI) waveguides. A three LN-waveguide configuration with each waveguide having a ridge cross section of less than 1 square micron, built atop a layer of SiO₂ based on a 500-µm-thick Si substrate, has been designed and constructed to optically emulate a three-state stimulated Raman adiabatic passage system, with which a unique counterintuitive adiabatic light transfer phenomenon in a high coupling efficiency of >97% (corresponding to a >15 dB splitting ratio) spanning telecom S, C, and L bands for both TE and TM polarization modes has been observed for a 2-mm long coupler length. An even broader operating bandwidth of >800 nm of the device can be found from the simulation fitting of the experimental data. The footprint of the realized LNOI adiabatic coupler has been reduced by >99% compared to its bulk counterparts. Such an ultra-compact, broadband LNOI adiabatic coupler can be further used to implement or integrate with various photonic elements, a potential building block for realizing large-scale integrated photonic (quantum) circuits in LN.

Photon Pairs from Resonant Metasurfaces

All-dielectric optical metasurfaces are a workhorse in nano-optics, because of both their ability to manipulate light in different degrees of freedom and their excellent performance at light frequency conversion. Here, we demonstrate first-time generation of photon pairs via spontaneous parametric-down conversion in lithium niobate quantum optical metasurfaces with electric and magnetic Mie-like resonances at various wavelengths. By engineering the quantum optical metasurface, we tailor the photon-pair spectrum in a controlled way. Within a narrow bandwidth around the resonance, the rate of pair production is enhanced up to 2 orders of magnitude, compared to an unpatterned film of the same thickness and material. These results enable flat-optics sources of entangled photons-a new promising platform for quantum optics experiments.

Modeling of surface-induced second-harmonic generation from multilayer structures by the transfer matrix method

We analytically and numerically investigate surface second-harmonic generation (SHG) from a stack of dielectric layers. We develop a theoretical formalism based on the transfer matrix method for the calculation of the surface-driven second-harmonic radiation from multilayer structures and elaborate it for the case of ultrathin dielectric layers using a power series expansion to derive the effective surface nonlinear tensor for the whole stack. We show that for deeply subwavelength thicknesses of the layers the surface responses from all interfaces can efficiently sum up, leading to largely enhanced efficiency of SHG. As a result, such surface-driven nonlinearity can become comparable to the bulk nonlinearity in noncentrosymmetric semiconductors and can yield high performance for nonlinear nanophotonic applications.

Manipulation of quantum dot emission with semiconductor metasurfaces exhibiting magnetic quadrupole resonances

Optical metasurfaces were suggested as a route for engineering advanced light sources with tailored emission properties. In particular, they provide a control over the emission directionality, which is essential for single-photon sources and LED applications. Here, we experimentally study light emission from a metasurface composed of III-V semiconductor Mie-resonant nanocylinders with integrated quantum dots (QDs). Specifically, we focus on the manipulation of the directionality of spontaneous emission from the QDs due to excitation of different magnetic quadrupole resonances in the nanocylinders. To this end, we perform both back focal plane imaging and momentum-resolved spectroscopy measurements of the emission. This allows for a comprehensive analysis of the effect of the different resonant nanocylinder modes on the emission characteristics of the metasurface. Our results show that the emission directionality can be manipulated by an interplay of the excited quadrupolar nanocylinder modes with the metasurface lattice modes and provide important insights for the design of novel smart light sources and new display concepts.

Second-Harmonic Generation in Resonant Nonlinear Metasurfaces Based on Lithium Niobate

Lithium niobate is an excellent and widely used material for nonlinear frequency conversion due to its strong optical nonlinearity and broad transparency region. Here, we report the fabrication and experimental investigation of resonant nonlinear metasurfaces for second-harmonic generation based on thin-film lithium niobate. In the fabricated metasurfaces, we observe pronounced Mie-type resonances leading to enhanced second-harmonic generation in the direction normal to the metasurface. We find the largest second-harmonic generation efficiency for the resonance dominated by the electric contributions because its specific field distribution enables the most efficient usage of the largest element of the lithium niobate nonlinear susceptibility tensor. This is confirmed by polarization-resolved second-harmonic measurements, where we study contributions from different elements of the nonlinear susceptibility tensor to the total second-harmonic signal. Our work facilitates establishing lithium niobate as a material for resonant nanophotonics.

Chiral Bilayer All-Dielectric Metasurfaces

Three-dimensional chiral plasmonic metasurfaces were demonstrated to offer enormous potential for ultrathin circular polarizers and applications in chiral sensing. However, the large absorption losses in the metallic systems generally limit their applicability for high-efficiency devices. In this work, we experimentally and numerically demonstrate three-dimensional chiral dielectric metasurfaces exhibiting multipolar resonances and examine their chiro-optical properties. In particular, we demonstrate that record high circular dichroism of 0.7 and optical activity of 2.67 × 10⁵ degree/mm can be achieved based on the excitation of electric and magnetic dipolar resonances inside the chiral structures. These large values are facilitated by a small amount of dissipative loss present in the dielectric nanoresonator material and the formation of a chiral supermode in a 4-fold symmetric metasurface unit cell. Our results highlight the mechanisms for maximizing the chiral response of photonic nanostructures and offer important opportunities for high-efficiency, ultrathin polarizing elements, which can be used in miniaturized devices, for example, integrated circuits.

Scalable Functionalization of Optical Fibers Using Atomically Thin Semiconductors

Atomically thin transition metal dichalcogenides are highly promising for integrated optoelectronic and photonic systems due to their exciton-driven linear and nonlinear interactions with light. Integrating them into optical fibers yields novel opportunities in optical communication, remote sensing, and all-fiber optoelectronics. However, the scalable and reproducible deposition of high-quality monolayers on optical fibers is a challenge. Here, 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 are reported. Two distinct application possibilities of 2D-functionalized waveguides to exemplify their potential are demonstrated. First, the excitonic 2D material photoluminescence is simultaneously excited and collected with the fiber modes, opening a novel route to remote sensing. Then it is shown 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. It is anticipated that the results may lead to significant advances in optical-fiber-based technologies.

Electro-optically spectrum switchable, multiwavelength optical parametric oscillators based on aperiodically poled lithium niobate

We report the first fast switchable multiwavelength optical parametric oscillator based on aperiodic optical superlattice technology. The constructed aperiodically poled lithium niobate (APPLN) integrates the functionalities of two quasi-phase-matching devices on a chip to work simultaneously as an electro-optic (EO) switchable notch-like filter and a multiline optical parametric downconverter. When such an APPLN is built in a 1064-nm-pumped optical resonator system, we achieve the oscillation of dual signals at 1540 and 1550 nm, for a single signal at 1540 nm, and a single signal at 1550 nm in the system when the 3-cm-long APPLN is driven by 0 V, 354 V, and 805 V, respectively. The switching among the three signal spectra is operationally simple and electro-optically fast. The electro-optically switched signals also feature enhanced power spectral density due to the unique EO gain-spectrum filtering mechanism employed in this work.

Generating path entangled states in waveguide systems with second-order nonlinearity

Spontaneous parametric down-conversion in coupled nonlinear waveguides is a flexible approach for generating tunable path entangled states. We describe a formalism based on the Cayley-Hamilton theorem to compute the quantum states generated by waveguide arrays for arbitrary system parameters. We find that all four Bell states can be generated in directional couplers with non-degenerate photons. Our method enables one to efficiently explore the phase space of waveguide systems and readily assess the robustness of any given state to variations in the system's parameters. We believe it represents a valuable tool for quantum state engineering in coupled waveguide systems.

Discrete dispersion scan setup for measuring few-cycle laser pulses in the mid-infrared

In this work, we demonstrate a discrete dispersion scan scheme using a low number of flat windows to vary the dispersion of laser pulses in discrete steps. Monte Carlo simulations indicate that the pulse duration can be retrieved accurately with less than 10 dispersion steps, which we verify experimentally by measuring few-cycle pulses and material dispersion curves at 3 and 10 µm wavelength. This minimal measuring scheme using only five optical components without the need for linear positioners and interferometric alignment can be readily implemented in many wavelength ranges and situations.

Single-shot characterization of strongly focused coherent XUV and soft X-ray beams

In this Letter, we present a novel, to the best of our knowledge, single-shot method for characterizing focused coherent beams. We utilize a dedicated amplitude-only mask, in combination with an iterative phase retrieval algorithm, to reconstruct the amplitude and phase of a focused beam from a single measured far-field diffraction pattern alone. In a proof-of-principle experiment at a wavelength of 13.5 nm, we demonstrate our new method and obtain an RMS phase error of better than λ/70. This method will find applications in the alignment of complex optical systems, real-time feedback to adaptive optics, and single-shot beam characterization, e.g., at free-electron lasers or high-order harmonic beamlines.

Nanostructure-modulated planar high spectral resolution spectro-polarimeter

We present a planar spectro-polarimeter based on Fabry-Pérot cavities with embedded polarization-sensitive high-index nanostructures. A 7 µm-thick spectro-polarimetric system for 3 spectral bands and 2 linear polarization states is experimentally demonstrated. Furthermore, an optimal design is theoretically proposed, estimating that a system with a bandwidth of 127 nm and a spectral resolution of 1 nm is able to reconstruct the first three Stokes parameters with a signal-to-noise ratio of -13.14 dB with respect to the the shot noise limited SNR. The pixelated spectro-polarimetric system can be directly integrated on a sensor, thus enabling applicability in a variety of miniaturized optical devices, including but not limited to satellites for Earth observation.

Far-field polarization signatures of surface optical nonlinearity in noncentrosymmetric semiconductors

We analyse possibilities to quantitatively evaluate the surface second-order optical nonlinearity in noncentrosymmetric materials based on polarization-resolved analysis of far-field radiation patterns of second-harmonic generation. We analytically demonstrate that for plane-wave illumination the contribution to the second-harmonic signal from the surface of a nonlinear medium exhibits different polarization properties and angular dependencies compared to the contribution from the bulk. In view of this, we optimize the illumination geometry in order to enable the most efficient separation and comparison of both nonlinearities. Furthermore, we consider the illumination of an AlGaAs slab by a tightly-focused linearly-polarized Gaussian beam as an alternative measurement geometry. It is found that the reliable separation of the surface nonlinearity contribution as well as a wide range of detectable values can be achieved with this geometry as well.

Direct and High-Throughput Fabrication of Mie-Resonant Metasurfaces via Single-Pulse Laser Interference

High-index dielectric metasurfaces featuring Mie-type electric and magnetic resonances have been of great interest in a variety of applications such as imaging, sensing, photovoltaics, and others, which led to the necessity of an efficient large-scale fabrication technique. To address this, here we demonstrate the use of single-pulse laser interference for direct patterning of an amorphous silicon film into an array of Mie resonators a few hundred nanometers in diameter. The proposed technique is based on laser-interference-induced dewetting. A precise control of the laser pulse energy enables the fabrication of ordered dielectric metasurfaces in areas spanning tens of micrometers and consisting of thousands of hemispherical nanoparticles with a single laser shot. The fabricated nanoparticles exhibit a wavelength-dependent optical response with a strong electric dipole signature. Variation of the predeposited silicon film thickness allows tailoring of the resonances in the targeted visible and infrared spectral ranges. Such direct and high-throughput fabrication is a step toward a simple realization of spatially invariant metasurface-based devices.

General design formalism for highly efficient flat optics for broadband applications

The use of flat diffractive optical elements (DOEs) for broadband applications, e.g. conventional optical systems, requires DOEs that maintain high efficiencies across the required range of wavelengths, angles of incidence, and grating periods. Here we introduce a general framework for how dispersion engineering can be used to design DOEs that fulfill these requirements and use our approach to determine design rules for broadband DOEs. Our analysis shows that the key to making échelette-type gratings (EGs) suitable for broadband optical systems is the development of new optical materials with specific uncommon dispersion properties. Subsequently, we use our framework to design a representative range of prototype EGs, which allows us to link the specifications of an optical system to the requirements on the EGs' materials. Finally, we show that our design rules apply to all DOEs based on propagation delays including GRIN DOEs and metagratings. Our design rules therefore guide the way towards unlocking the full potential of DOEs for different kinds of broadband applications.

Laser-induced spatially-selective tailoring of high-index dielectric metasurfaces

Optically resonant high-index dielectric metasurfaces featuring Mie-type electric and magnetic resonances are usually fabricated by means of planar technologies, which limit the degrees of freedom in tunability and scalability of the fabricated systems. Therefore, we propose a complimentary post-processing technique based on ultrashort (≤ 10 ps) laser pulses. The process involves thermal effects: crystallization and reshaping, while the heat is localized by a high-precision positioning of the focused laser beam. Moreover, for the first time, the resonant behavior of dielectric metasurface elements is exploited to engineer a specific absorption profile, which leads to a spatially-selective heating and a customized modification. Such technique has the potential to reduce the complexity in the fabrication of non-uniform metasurface-based optical elements. Two distinct cases, a spatial pixelation of a large-scale metasurface and a height modification of metasurface elements, are explicitly demonstrated.

Controlling second-harmonic diffraction by nano-patterning MoS2 monolayers

Monolayers of transition metal dichalcogenides have a strong second-order nonlinear response enabling second-harmonic generation. Here, we control the spatial radiation properties of the generated second harmonic by patterning MoS₂ monolayers using focused ion beam milling. We observe diffraction of the second harmonic into the zero and first diffraction orders via an inscribed one-dimensional grating. Additionally, we included a fork-like singularity into the grating to create a vortex beam in the first diffraction order.

A fully automated dual-tip scanning near-field optical microscope for localized optical excitation and detection in the visible and near-infrared

Near-field optical microscopes with two independent tips for simultaneous excitation and detection can be essential tools for studying localized optical phenomena on the subwavelength scale. Here, we report on the implementation of a fully automated and robust dual-tip scanning near-field optical microscope (SNOM), in which the excitation tip is stationary, while the detection tip automatically scans the surrounding area. To monitor and control the distance between the two probes, mechanical interactions due to shear forces are used. We experimentally investigate suitable scan parameters and find that the automated dual-tip SNOM can operate stably for a wide range of parameters. To demonstrate the potential of the automated dual-tip SNOM, we characterize the propagation of surface plasmon polaritons on a gold film for visible and near-infrared wavelengths. The good agreement of the measurements with numerical simulations verifies the capability of the dual-tip SNOM for the near-field characterization of localized optical phenomena.

Manipulation of Magnetic Dipole Emission from Eu3+ with Mie-Resonant Dielectric Metasurfaces

Mie-resonant high-index dielectric nanoparticles and metasurfaces have been suggested as a viable platform for enhancing both electric and magnetic dipole transitions of fluorescent emitters. While the enhancement of the electric dipole transitions by such dielectric nanoparticles has been demonstrated experimentally, the case of magnetic-dipole transitions remains largely unexplored. Here, we study the enhancement of spontaneous emission of Eu³⁺ ions, featuring both electric and magnetic-dominated dipole transitions, by dielectric metasurfaces composed of Mie-resonant silicon nanocylinders. By coating the metasurfaces with a layer of an Eu³⁺ doped polymer, we observe an enhancement of the Eu³⁺ emission associated with the electric (at 610 nm) and magnetic-dominated (at 590 nm) dipole transitions. The enhancement factor depends systematically on the spectral proximity of the atomic transitions to the Mie resonances as well as their multipolar order, both controlled by the nanocylinder size. Importantly, the branching ratio of emission via the electric or magnetic transition channel can be modified by carefully designing the metasurface, where the magnetic dipole transition is enhanced more than the electric transition for cylinders with radii of about 130 nm. We confirm our observations by numerical simulations based on the reciprocity principle. Our results open new opportunities for bright nanoscale light sources based on magnetic transitions.

Analyzing the polarization response of a chiral metasurface stack by semi-analytic modeling

We investigate a class of stacked metasurfaces where the interaction between layers is dominated by their respective far-field response. Using a semi-analytic scattering matrix approach, we exploit the Fabry-Perot-type response for different layer distances to show the spectral tunability of the resonant effect. This method presents a faster and more intuitive route to modeling Fabry-Perot-type effects than rigorous numerical simulations. The results are illustrated for a chiral metasurface stack that exhibits asymmetric transmission. Here, the effect of asymmetric transmission is highly sensitive to the layer distance, which is used as a free parameter in our model. To prove our theoretical findings we fabricate two variants of the stack with different layer distances and show that far-field interaction between layers is sufficient to generate the effect while being accessible by semi-analytic modeling. The analyticity of the approach is promising for designing sophisticated layered media containing stacks of arbitrary metasurfaces.

Disorder-Induced Phase Transitions in the Transmission of Dielectric Metasurfaces

Light interaction with disordered materials is both complex and fascinating at the same time. Here, we reveal disorder-induced phase transitions in a dielectric Huygens' metasurface made from silicon nanocylinders that simultaneously support an electric and magnetic dipole resonance. Depending on the degree of positional disorder and the spectral detuning of the two resonances, the phase angle of the transmission coefficient exhibits a clear phase transition from normal to anomalous dispersion. Combined with the considerations of whether the resonances of spectrally detuned particles appear as separated or overlapping, we distinguish four different phase states. We study this phenomenon analytically by employing dipole particles and disclose the entire phase diagram, support our insights with full-wave simulations of actual structures, and corroborate the findings with experimental results. Unveiling this phenomenon is a milestone simultaneously in the growing fields of metamaterial-inspired silicon nanophotonics, photonics in disordered media, and the fundamental physics of phase transitions.

Photonic crystal waveguides as sources of counterpropagating factorizable biphoton states

We demonstrate numerically that photonic crystal slab waveguides can generate spectrally unentangled biphoton states, highly desired for heralding of single photons. We achieve this by modally phase matching a counterpropagating spontaneous parametric down-conversion process, in a fully integrated scheme and without the need for periodic poling. Such a configuration is an ideal integrated source of heralded single photons, as it spatially separates the photons of a pair at the source without any extra components, while allowing for generation of spectrally narrow photons on a very short length scale.

Design of a 2 diopter holographic progressive lens

In this contribution, we investigate the use of holographic optical elements (HOEs) as progressive addition lenses (PALs). We design HOEs with high diffraction efficiency (DE) using the Fourier Modal Method (FMM) and optimize an optical system comprising two of these HOEs to fulfill the optical function of a 2 diopter (dpt) PAL. The resulting design is a holographic PAL (hPAL) exhibiting high DE and limited angular color error (CE) with a distribution of spherical power and astigmatism equivalent to its refractive counterpart. To our knowledge, our contribution is the first complete design of an hPAL. While our HOE design method is shown for PALs here, it has the potential to improve other applications of HOEs as well.

Light-Emitting Metasurfaces: Simultaneous Control of Spontaneous Emission and Far-Field Radiation

Light-emitting sources and devices permeate every aspect of our lives and are used in lighting, communications, transportation, computing, and medicine. Advances in multifunctional and "smart lighting" would require revolutionary concepts in the control of emission spectra and directionality. Such control might be possible with new schemes and regimes of light-matter interaction paired with developments in light-emitting materials. Here we show that all-dielectric metasurfaces made from III-V semiconductors with embedded emitters have the potential to provide revolutionary lighting concepts and devices, with new functionality that goes far beyond what is available in existing technologies. Specifically, we use Mie-resonant metasurfaces made from semiconductor heterostructures containing epitaxial quantum dots. By controlling the symmetry of the resonant modes, their overlap with the emission spectra, and other structural parameters, we can enhance the brightness by 2 orders of magnitude, as well as reduce its far-field divergence significantly.

Active Tuning of Spontaneous Emission by Mie-Resonant Dielectric Metasurfaces

Mie-resonant dielectric metasurfaces offer comprehensive opportunities for the manipulation of light fields with high efficiency. Additionally, various strategies for the dynamic tuning of the optical response of such metasurfaces were demonstrated, making them important candidates for reconfigurable optical devices. However, dynamic control of the light-emission properties of active Mie-resonant dielectric metasurfaces by an external control parameter has not been demonstrated so far. Here, we experimentally demonstrate the dynamic tuning of spontaneous emission from a Mie-resonant dielectric metasurface that is situated on a fluorescent substrate and embedded into a liquid crystal cell. By switching the liquid crystal from the nematic state to the isotropic state via control of the cell temperature, we induce a shift of the spectral position of the metasurface resonances. This results in a change of the local photonic density of states, which, in turn, governs the enhancement of spontaneous emission from the substrate. Specifically, we observe spectral tuning of both the electric and magnetic dipole resonances, resulting in a 2-fold increase of the emission intensity at λ ≈ 900 nm. Our results demonstrate a viable strategy to realize flat tunable light sources based on dielectric metasurfaces.

Atom-mediated spontaneous parametric down-conversion in periodic waveguides

We propose the concept of atom-mediated spontaneous parametric down-conversion, in which photon-pair generation can take place only in the presence of a single two-level emitter, relying on the bandgap evanescent modes of a nonlinear periodic waveguide. Using a guided signal mode, an evanescent idler mode, and an atom-like emitter with the idler's transition frequency embedded in the structure, we find a heralded excitation mechanism, in which the detection of a signal photon outside the structure heralds the excitation of the embedded emitter. We use a rigorous Green's function quantization method to model this heralding mechanism in a 1D periodic waveguide and determine its robustness against losses.

Adaptive pre-amplification pulse shaping in a high-power, coherently combined fiber laser system

We report on the successful implementation of an adaptive pre-amplification pulse shaping technique in a high-power, coherently combined fiber laser system to achieve sub-300-fs pulse durations at 320 W average power and 3.2 mJ pulse energy. The pulse shaper is utilized to impose a gain flattening mask to increase the spectral width of the amplified pulse by 60%. Simultaneously, it pre-compensates the spectral phase acquired in the multi-stage amplification and subsequent compression including the eight-channel, coherently combined main amplification stage. This result does significantly enhance the performance of the fiber laser system and the subsequent nonlinear compression stages.

Towards 3D-photonic, multi-telescope beam combiners for mid-infrared astrointerferometry

In the past two decades high precision optical astronomical interferometry has benefited from the use of photonic technologies. Today, near-infrared interferometric instruments deliver high-resolution, hyperspectral images of astronomical objects and combine up to 4 independent telescopes at a time thanks to integrated optics (IO). Following the success of IO interferometry, several initiatives aim at developing components which could combine simultaneously more telescopes and extend their operation beyond the near-infrared bands. Here we report on the development of multi-telescope IO beam combiners for mid-infrared interferometry exploiting the three-dimensional (3D) structuring capabilities of ultrafast laser inscription. We characterise the capability of a 2-telescope and a 4-telescope beam combiner to retrieve the visibility amplitude and phase of monochromatic light fields at a wavelength of 3.39 µm. The combiner prototypes exploit different 3D architectures and are written with a femtosecond laser on substrates of Gallium Lanthanum Sulfide. Supporting numerical simulations of the performance of the beam combiners show that there is still room for improvement and indicate a roadmap for the development of future prototypes.

High-bit rate ultra-compact light routing with mode-selective on-chip nanoantennas

Optical nanoantennas provide a promising pathway toward advanced manipulation of light waves, such as directional scattering, polarization conversion, and fluorescence enhancement. Although these functionalities were mainly studied for nanoantennas in free space or on homogeneous substrates, their integration with optical waveguides offers an important "wired" connection to other functional optical components. Taking advantage of the nanoantenna's versatility and unrivaled compactness, their imprinting onto optical waveguides would enable a marked enhancement of design freedom and integration density for optical on-chip devices. Several examples of this concept have been demonstrated recently. However, the important question of whether nanoantennas can fulfill functionalities for high-bit rate signal transmission without degradation, which is the core purpose of many integrated optical applications, has not yet been experimentally investigated. We introduce and investigate directional, polarization-selective, and mode-selective on-chip nanoantennas integrated with a silicon rib waveguide. We demonstrate that these nanoantennas can separate optical signals with different polarizations by coupling the different polarizations of light vertically to different waveguide modes propagating into opposite directions. As the central result of this work, we show the suitability of this concept for the control of optical signals with ASK (amplitude-shift keying) NRZ (nonreturn to zero) modulation [10 Gigabit/s (Gb/s)] without significant bit error rate impairments. Our results demonstrate that waveguide-integrated nanoantennas have the potential to be used as ultra-compact polarization-demultiplexing on-chip devices for high-bit rate telecommunication applications.

Dual-SNOM investigations of multimode interference in plasmonic strip waveguides

The ability of squeezing and guiding light in nanoscale plasmonic waveguides makes them especially interesting for photonic circuits. In spite of reported realizations of plasmonic waveguides, experimental studies on the content of plasmonic modes and mode-selective excitation methods are rare. We apply here a Dual-SNOM technique, incorporating two aperture scanning near-field optical microscopes, for simultaneous near-field excitation and detection of plasmonic modes in gold strip waveguides. Depending on the waveguide width, either a single waveguide mode or a beating pattern of several modes is observed. The relative excitation strengths of the individual modes in multi-mode waveguides are shown to be controllable by the lateral position of the excitation tip. The excitation coefficients are described by an analytical model and the results are fully corroborated by analytical calculations and full-wave numerical simulations. The Dual-SNOM technique provides a "non-invasive" method of local excitation and detection of photonic modes thus making it a valuable tool for in situ characterization of complex photonic micro- and nanostructures.