Crystal superlattices for versatile and sensitive quantum spectroscopy.

Nonlinear interferometers with quantum correlated photons have been demonstrated to improve optical characterization and metrology. These interferometers can be used in gas spectroscopy, which is of particular interest for monitoring greenhouse gas emissions, breath analysis and industrial applications. Here, we show that gas spectroscopy can be further enhanced via the deployment of crystal superlattices. This is a cascaded arrangement of nonlinear crystals forming interferometers, allowing the sensitivity to scale with the number of nonlinear elements. In particular, the enhanced sensitivity is observed via the maximum intensity of interference fringes that scales with low concentration of infrared absorbers, while for high concentration the sensitivity is better in interferometric visibility measurements. Thus, a superlattice acts as a versatile gas sensor since it can operate by measuring different observables, which are relevant to practical applications. We believe that our approach offers a compelling path towards further enhancements for quantum metrology and imaging using nonlinear interferometers with correlated photons.

Photon pairs bi-directionally emitted from a resonant metasurface.

Metasurfaces are artificially structured surfaces able to control the properties of light at subwavelength scales. While, initially, they have been proposed as means to control classical optical fields, they are now emerging as nanoscale sources of quantum light, in particular of entangled photons with versatile properties. Geometric resonances in metasurfaces have been recently used to engineer the frequency spectrum of entangled photons, but the emission directivity was so far less studied. Here, we generate photon pairs via spontaneous parametric down conversion from a metasurface supporting a quasi-bound state in the continuum (BIC) leading to remarkable emission directivities. The pair generation rate is enhanced 67 times compared to the case of an unpatterned film of the same thickness and material. At the wavelength of the quasi-BIC resonance, photons are mostly emitted backwards, while their partners, spectrally detuned by only 8 nm, are emitted forwards. This behavior demonstrates fine spectral splitting of entangled photons and their bi-directional emission, never before observed in nanoscale sources. We expect this work to be a starting point for the efficient demultiplexing of photons in nanoscale quantum optics.

Tuning of silicon nitride micro-cavities by controlled nanolayer deposition.

Integration of single-photon emitters (SPEs) with resonant photonic structures is a promising approach for realizing compact and efficient single-photon sources for quantum communications, computing, and sensing. Efficient interaction between the SPE and the photonic cavity requires that the cavity's resonance matches the SPE's emission line. Here we demonstrate a new method for tuning silicon nitride (Si3N4) microring cavities via controlled deposition of the cladding layers. Guided by numerical simulations, we deposit silicon dioxide (SiO2) nanolayers onto Si3N4 ridge structures in steps of 50 nm. We show tuning of the cavity resonance exceeding a free spectral range (FSR) of 3.5 nm without degradation of the quality-factor (Q-factor) of the cavity. We then complement this method with localized laser heating for fine-tuning of the cavity. Finally, we verify that the cladding deposition does not alter the position and spectral properties of nanoparticles placed on the cavity, which suggests that our method can be useful for integrating SPEs with photonic structures.

Sum-Frequency Generation in High-Q GaP Metasurfaces Driven by Leaky-Wave Guided Modes.

Resonant metasurfaces provide a unique platform for enhancing multiwave nonlinear interactions. However, the difficulties over mode matching and material transparency place significant challenges in the enhancement of these multiwave processes. Here we demonstrate efficient nonlinear sum-frequency generation (SFG) in multiresonant GaP metasurfaces based on guided-wave bound-state in the continuum resonances. The excitation of the metasurface by two near-infrared input beams generates strong SFG in the visible spectrum with a conversion efficiency of 2.5 × 10-4 W-1, 2 orders of magnitude higher than the one reported in Mie-type resonant metasurfaces. In addition, we demonstrate the nontrivial polarization dependence on the SFG process. In contrast to harmonic generation, the SFG process is enhanced when using nonparallel polarized input-beams. Importantly, by varying the input pump beam polarization it is possible to direct the SFG emission to different diffraction orders, thereby opening up new opportunities for nonlinear light sources and infrared to visible light conversion.

Generation of even and odd high harmonics in resonant metasurfaces using single and multiple ultra-intense laser pulses.

High harmonic generation (HHG) opens a window on the fundamental science of strong-field light-mater interaction and serves as a key building block for attosecond optics and metrology. Resonantly enhanced HHG from hot spots in nanostructures is an attractive route to overcoming the well-known limitations of gases and bulk solids. Here, we demonstrate a nanoscale platform for highly efficient HHG driven by intense mid-infrared laser pulses: an ultra-thin resonant gallium phosphide (GaP) metasurface. The wide bandgap and the lack of inversion symmetry of the GaP crystal enable the generation of even and odd harmonics covering a wide range of photon energies between 1.3 and 3 eV with minimal reabsorption. The resonantly enhanced conversion efficiency facilitates single-shot measurements that avoid material damage and pave the way to study the controllable transition between perturbative and non-perturbative regimes of light-matter interactions at the nanoscale.

Second harmonic generation in gallium phosphide nano-waveguides.

We designed, fabricated and tested gallium phosphide (GaP) nano-waveguides for second harmonic generation (SHG). We demonstrate SHG in the visible range around 655 nm using modal phase matching. We observe phase matched SHG for different combinations of interacting modes by varying the widths of the waveguides and tuning the wavelength of the pump. We achieved a normalized internal SHG conversion efficiency of 0.4% W-1cm-2 for a continuous-wave pump at wavelength of 1283.5 nm, the highest reported in the literature for a GaP waveguide. We also demonstrated temperature tuning of the SHG wavelength with a slope of 0.17 nm/°C. The presented results contribute to the development of integrated photonic platforms with efficient nonlinear wave-mixing processes for classical and quantum applications.

Integrated avalanche photodetectors for visible light.

Integrated photodetectors are essential components of scalable photonics platforms for quantum and classical applications. However, most efforts in the development of such devices to date have been focused on infrared telecommunications wavelengths. Here, we report the first monolithically integrated avalanche photodetector (APD) for visible light. Our devices are based on a doped silicon rib waveguide with a novel end-fire input coupling to a silicon nitride waveguide. We demonstrate a high gain-bandwidth product of 234 ± 25 GHz at 20 V reverse bias measured for 685 nm input light, with a low dark current of 0.12 µA. We also observe open eye diagrams at up to 56 Gbps. This performance is very competitive when benchmarked against other integrated APDs operating in the infrared range. With CMOS-compatible fabrication and integrability with silicon photonic platforms, our devices are attractive for sensing, imaging, communications, and quantum applications at visible wavelengths.

Entangled photons from subwavelength nonlinear films.

Miniaturized entangled photon sources, in particular based on subwavelength metasurfaces, are highly demanded for the development of integrated quantum photonics. Here, as a first step towards the development of quantum optical metasurfaces (QOMs), we demonstrate generation of entangled photons via spontaneous parametric down-conversion (SPDC) from subwavelength films. We achieve photon pair generation with a high coincidence-to-accidental ratio in lithium niobate and gallium phosphide nanofilms. By implementing the fiber spectroscopy of SPDC in nanofilms, we measure a spectrum with a bandwidth of 500 nm, limited only by the overall detection efficiency. The spectrum reveals vacuum field enhancement due to a Fabry-Perot resonance inside the nonlinear films. It also suggests a strategy for observing SPDC from QOM. Our experiments lay the groundwork for future development of flat SPDC sources, including QOM.

Frequency conversion in nano-waveguides using bound-state-in-continuum.

Optical frequency conversion in semiconductor nanophotonic devices usually imposes stringent requirements on fabrication accuracy and etch surface roughness. Here, we adopt the concept of bound-state-in-continuum (BIC) for waveguide frequency converter design, which obviates the limitations in nonlinear material nano-fabrication and requires to pattern only a low-refractive-index strip on the nonlinear slab. Taking gallium phosphide (GaP) as an example, we study second-harmonic generation using horizontally polarized pump light at 1.55 µm phase matching to vertically polarized BIC modes. A theoretical normalized frequency conversion efficiency of 1.1×104 % W -1 c m -2 is obtained using the fundamental BIC mode, which is comparable to that of conventional GaP waveguides.

Continuous Wave Second Harmonic Generation Enabled by Quasi-Bound-States in the Continuum on Gallium Phosphide Metasurfaces.

Resonant metasurfaces are an attractive platform for enhancing the nonlinear optical processes, such as second harmonic generation (SHG), since they can generate large local electromagnetic fields while relaxing the phase-matching requirements. Here, we demonstrate visible range, continuous wave (CW) SHG by combining the attractive material properties of gallium phosphide with high quality-factor photonic modes enabled by bound states in the continuum. We obtain efficiencies around 5e-5% W-1 when the system is pumped at 1200 nm wavelength with CW intensities of 1 kW/cm2. Moreover, we measure external efficiencies of 0.1% W-1 with pump intensities of only 10 MW/cm2 for pulsed irradiation. This efficiency is higher than the values previously reported for dielectric metasurfaces, but achieved here with pump intensities that are two orders of magnitude lower. These results take metasurface-based SHG a step closer to practical applications.

Hyperspectral infrared microscopy with visible light.

Hyperspectral microscopy is an imaging technique that provides spectroscopic information with high spatial resolution. When applied in the relevant wavelength region, such as in the infrared (IR), it can reveal a rich spectral fingerprint across different regions of a sample. Challenges associated with low efficiency and high cost of IR light sources and detector arrays have limited its broad adoption. Here, we introduce a new approach to IR hyperspectral microscopy, where the IR spectral map is obtained with off-the-shelf components built for visible light. The method is based on the nonlinear interference of correlated photons generated via parametric down-conversion. In this proof-of-concept we demonstrate the chemical mapping of a patterned sample, where different areas have distinctive IR spectroscopic fingerprints. The method provides a wide field of view, fast readout, and negligible heat delivered to the sample, which opens prospects for its further development for applications in material and biological studies.

Efficient ultrafast all-optical modulation in a nonlinear crystalline gallium phosphide nanodisk at the anapole excitation.

High-refractive index nanostructured dielectrics have the ability to locally enhance electromagnetic fields with low losses while presenting high third-order nonlinearities. In this work, we exploit these characteristics to achieve efficient ultrafast all-optical modulation in a crystalline gallium phosphide (GaP) nanoantenna through the optical Kerr effect (OKE) and two-photon absorption (TPA) in the visible/near-infrared range. We show that an individual GaP nanodisk can yield differential reflectivity modulations of up to ~40%, with characteristic modulation times between 14 and 66 fs, when probed at the anapole excitation (AE). Numerical simulations reveal that the AE represents a unique condition where both the OKE and TPA contribute with the same modulation sign, maximizing the response. These findings highly outperform previous reports on sub-100-fs all-optical switching from resonant nanoscale dielectrics, which have demonstrated modulation depths no larger than 0.5%, placing GaP nanoantennas as a promising choice for ultrafast all-optical modulation at the nanometer scale.

Nonlinear interference in crystal superlattices.

Nonlinear interferometers with correlated photons hold promise to advance optical characterization and metrology techniques by improving their performance and affordability. These interferometers offer subshot noise phase sensitivity and enable measurements in detection-challenging regions using inexpensive and efficient components. The sensitivity of nonlinear interferometers, defined by the ability to measure small shifts of interference fringes, can be significantly enhanced by using multiple nonlinear elements, or crystal superlattices. However, to date, experiments with more than two nonlinear elements have not been realized, thus hindering the potential of nonlinear interferometers. Here, we build a nonlinear interferometer with up to five nonlinear elements, referred to as superlattices, in a highly stable and versatile configuration. We study the modification of the interference pattern for different configurations of the superlattices and perform a proof-of-concept gas sensing experiment with enhanced sensitivity. Our approach offers a viable path towards broader adoption of nonlinear interferometers with correlated photons for imaging, interferometry, and spectroscopy.

Polarization effects in nonlinear interference of down-converted photons.

We study polarization effects in the nonlinear interference of photons generated via frequency nondegenerate spontaneous parametric-down conversion. Signal and idler photons, which are generated in the visible and infrared (IR) range, respectively, are split into different arms of a nonlinear Michelson interferometer, and the interference pattern for signal photons is detected. Due to the effect of induced coherence, the interference pattern for the signal photons depends on the polarization rotation of idler photons, which are introduced by a birefringent sample. Based on this concept, we realize two new methods of measuring sample retardation in the IR range by using well-developed and inexpensive components for visible light. The methods' accuracy reaches specifications that are reported for industrial-grade optical elements. The developed IR polarimetry technique is relevant to material research, optical inspection, and quality control.

2D Monte Carlo simulation of a silicon waveguide-based single-photon avalanche diode for visible wavelengths.

Integrated photonics platforms are crucial to the development and implementation of scalable quantum information and networking schemes, but many such devices still rely on external bulk photodetectors. We report the design and simulation of a waveguide-based single-photon avalanche diode (SPAD) for visible wavelengths. The SPAD consists of a p-n junction implemented in a doped silicon waveguide, which is end-fire coupled to an input silicon nitride waveguide. We developed a 2D Monte Carlo model to simulate the avalanche multiplication process of charge carriers following the absorption of an input photon, and calculated the photon detection efficiency (PDE) and timing jitter of the SPAD. We investigated the SPAD performance at a wavelength of 640 nm and temperature of 243K for different device dimensions and device doping configurations. For our simulated parameters, we obtained a maximum PDE of 0.45 at a reverse bias voltage of ~20 V, and full-width-half-max (FWHM) timing jitter values <8 ps.

Varying temperature and silicon content in nanodiamond growth: effects on silicon-vacancy centres.

Nanodidamonds containing colour centres open up many applications in quantum information processing, metrology, and quantum sensing. However, controlling the synthesis of nanodiamonds containing silicon vacancy (SiV) centres is still not well understood. Here we study nanodiamonds produced by a high-pressure high-temperature method without catalyst metals, focusing on two samples with clear SiV signatures. Different growth temperatures and relative content of silicon in the initial compound between the samples altered their nanodiamond size distributions and abundance of SiV centres. Our results show that nanodiamond growth can be controlled and optimised for different applications.

A Metalens with a Near-Unity Numerical Aperture.

The numerical aperture (NA) of a lens determines its ability to focus light and its resolving capability. Having a large NA is a very desirable quality for applications requiring small light-matter interaction volumes or large angular collections. Traditionally, a large NA lens based on light refraction requires precision bulk optics that ends up being expensive and is thus also a specialty item. In contrast, metasurfaces allow the lens designer to circumvent those issues producing high-NA lenses in an ultraflat fashion. However, so far, these have been limited to numerical apertures on the same order of magnitude as traditional optical components, with experimentally reported NA values of <0.9. Here we demonstrate, both numerically and experimentally, a new approach that results in a diffraction-limited flat lens with a near-unity numerical aperture (NA > 0.99) and subwavelength thickness (∼λ/3), operating with unpolarized light at 715 nm. To demonstrate its imaging capability, the designed lens is applied in a confocal configuration to map color centers in subdiffractive diamond nanocrystals. This work, based on diffractive elements that can efficiently bend light at angles as large as 82°, represents a step beyond traditional optical elements and existing flat optics, circumventing the efficiency drop associated with the standard, phase mapping approach.

Diamond in a Nanopocket: A New Route to a Strong Purcell Effect.

Light emission from the color centers in diamonds can be significantly enhanced by their interaction with optical microcavities. In the conventional chip-based hybrid approach, nanodiamonds are placed directly on the surface of microcavity chips created using fabrication-matured material platforms. However, the achievable enhancement due to the Purcell effect is limited because of the evanescent interaction between the electrical field of the cavity and the nanodiamond. Here, we propose and statistically analyze a diamond in a nanopocket structure as a new route to achieve a high enhancement of light emission from the color center in the nanodiamond, placed in an optical microcavity. We demonstrate that by creating a nanopocket within the photonic crystal L3 cavity and placing the nanodiamond in, a significant and a robust control over the local density of states can be obtained. The antinodes of the electric field relocate to the nanosized air gaps within the nanopocket, between the nanodiamond and the microcavity. This creates an elevated and uniform electric field across the nanodiamond that is less sensitive to perturbations in the shape and orientation of the nanodiamond. Using a silicon nitride photonic crystal L3 cavity and aiming at silicon-vacancy and nitrogen-vacancy color centers in diamond, we performed a statistical analysis of light emission, assuming random positions of color centers and dipole moment orientations. We showed that in cavities with experimentally feasible quality factors, the diamond in the nanopocket structure produces Purcell factor distributions with mean and median that are tenfold larger compared to what can be achieved when the diamond is on the surface of the microcavity.

Quantum interference in the presence of a resonant medium.

Interaction of light with media often occurs with a femtosecond response time. Its measurement by conventional techniques requires the use of femtosecond lasers and sophisticated time-gated optical detection. Here we demonstrate that by exploiting quantum interference of entangled photons it is possible to measure the dephasing time of a resonant media on the femtosecond time scale (down to 100 fs) using accessible continuous wave laser and single-photon counting. We insert a sample in the Hong-Ou-Mandel interferometer and observe the modification of the two-photon interference pattern, which is driven by the coherent response of the medium, determined by the dephasing time. The dephasing time is then inferred from the observed pattern. This effect is distinctively different from the basic effect of spectral filtering, which was studied in earlier works. In addition to its ease of use, our technique does not require compensation of group velocity dispersion and does not induce photo-damage of the samples. Our technique will be useful for characterization of ultrafast phase relaxation processes in material science, chemistry, and biology.

Nonlinear infrared spectroscopy free from spectral selection.

Infrared (IR) spectroscopy is an indispensable tool for many practical applications including material analysis and sensing. Existing IR spectroscopy techniques face challenges related to the inferior performance and the high cost of IR-grade components. Here, we develop a new method, which allows studying properties of materials in the IR range using only visible light optics and detectors. It is based on the nonlinear interference of entangled photons, generated via Spontaneous Parametric Down Conversion (SPDC). In our interferometer, the phase of the signal photon in the visible range depends on the phase of an entangled IR photon. When the IR photon is traveling through the media, its properties can be found from observations of the visible photon. We directly acquire the SPDC signal with a visible range CCD camera and use a numerical algorithm to infer the absorption coefficient and the refraction index of the sample in the IR range. Our method does not require the use of a spectrometer and a slit, thus it allows achieving higher signal-to-noise ratio than the earlier developed method.