With recent advances in nanophotonics, metasurfaces based on nano-resonators have facilitated novel types of optical devices. In particular, the interplay between different degrees of freedom, involving polarization and spatial modes, boosted classical polarization measurements and imaging applications. However, the use of metasurfaces for measuring the quantum states of light remains largely unexplored. Conventionally, the task of quantum state tomography is realized with several bulk optical elements, which need to be reconfigured multiple times. Such setups can suffer from decoherence, and there is a fundamental and practical interest in developing integrated solutions for measurement of multi-photon quantum states. We present a new concept and the first experimental realization of all-dielectric metasurfaces with no tuneable elements for imaging-based reconstruction of the full quantum state of entangled photons. Most prominently, we implement multi-photon interferometric measurements on a sub-wavelength thin optical element, which delivers ultimate miniaturization and extremely high robustness. Specifically, we realize a highly transparent all-dielectric metasurface, which spatially splits different components of quantum-polarization states. Then, a simple one-shot measurement of correlations with polarization-insensitive on-off click detectors enables complete reconstruction of multi-photon density matrices with high precision. In our experiment, we prepare sets of polarization states and reconstruct their density matrices with a high fidelity of over 99% for single photon states and above 95% for two-photon states. Our work provides a fundamental advance in the imaging of quantum states, where multi-photon quantum interference takes place at sub-wavelength scale.
The processing of information encoded in frequency combs or spectral lattices has multiple applications for both classical and quantum states of light ranging from communications to spectroscopy. There is a strong interest in all-optical approaches for ultra-fast processing on integrated platforms. Here, we develop a concept and demonstrate experimentally all-optical flexible spectral comb reshaping in a nonlinear waveguide for two novel applications. First, we reveal that the evolution of an optical spectral comb can emulate wave dynamics in multi-dimensional lattices, which is a nontrivial generalization of previous theoretical proposals. In our experiment, a discrete signal spectrum is modulated by stronger pumps co-propagating in a nonlinear fiber with Kerr-type nonlinearity. Four-wave mixing Bragg scattering then induces coupling between many spectral lines, including nonlocal couplings between spectral lines which are further apart. We find that such a configuration can be exactly mapped to wave dynamics in complex multi-dimensional lattices, and as a representative example we realize a tube of triangular lattice. Importantly, the nontrivial phase of complex-valued couplings can give rise to synthetic gauge fields, and we directly measure corresponding asymmetric spectral reshaping. Our approach is scalable to higher-dimensional synthetic lattices. Second, we show that such a lattice with nonlocal couplings can enable the full reconstruction of the input spectra, including information on the phase and coherence, with a single-shot spectral intensity measurement. We demonstrate the reconstruction of input states composed of four frequency channels. Remarkably, the coherent nature of nonlinearly induced couplings is applicable for quantum states with spectral encoding.
Recently discovered photon sources based on 2D materials are much more practical compared to their earlier counterparts due to high emission rate, robust performance in a range of environmental conditions and ease of photonic integration. It is expected that this platform will make a substantial contribution to a range of quantum optical applications, including quantum communication, computing and sensing.
Dielectric nanoantennas and metasurfaces have proven to be able to manipulate the wavefront of incoming waves with high transmission efficiency. The important next question is: Can they enable enhanced interaction with the light to transform its colour or to be able to control one light beam with another? Here we show how a dielectric nano-resonator of subwavelength size can enable enhanced light matter interaction for efficient nonlinear frequency conversion. In particular, we show how AlGaAs or silicon nanoantennas can enhance second and third harmonic generation, respectively. Importantly, by controlling the size of the antennas, we can achieve control of directionality and polarisation state of the emission of harmonics. Our results open novel applications in ultra-thin light sources, light switches and modulators, ultra-fast displays, and other nonlinear optical metadevices based on low loss subwavelength dielectric resonant nanoparticles.
Optical nanoantennas possess great potential for controlling the spatial distribution of light in the linear regime as well as for frequency conversion of the incoming light in the nonlinear regime. However, the usually used plasmonic nanostructures are highly restricted by Ohmic losses and heat resistance. Dielectric nanoparticles like silicon and germanium can overcome these constrains [1,2], however second harmonic signal cannot be generated in these materials due to their centrosymmetric nature. GaAs-based III-V semiconductors, with non-centrosymmetric crystallinity, can produce second harmonic generation (SHG) . Unfortunately, generating and studying SHG by AlGaAs nanocrystals in both backward and forward directions is very challenging due to difficulties to fabricate III-V semiconductors on low-refractive index substrate, like glass. Here, for the first time to our knowledge, we designed and fabricated AlGaAs nanoantennas on a glass substrate. This novel design allows the excitation, control and detection of backwards and forwards SHG nonlinear signals. Different complex spatial distribution in the SHG signal, including radial and azimuthal polarization originated from the excitation of electric and magnetic multipoles were observed. We have demonstrated an unprecedented SHG conversion efficiency of 10-4; a breakthrough that can open new opportunities for enhancing the performance of light emission and sensing .
 A. S. Shorokhov et al. Nano Letters 16, 4857 (2016).
 G. Grinblat et al. Nano Letters 16, 4635 (2016).
 S. Liu et al. Nano Letters 16, 7191 (2016).
 R. Camacho et al. Nano Lett. 16, 7191 (2016).
Metallic nanoantenna possess versatile scattering properties enabling to engineer the emission directionality at the nanoscale. However, due to their Ohmic losses and low heat resistance they cannot be practically applied in nonlinear optical processes for optical frequency conversion. Dielectric nanoparticles, e.g. silicon and germanium, are good candidates to overcome these limitations [1, 2]. Nevertheless, the centrosymmetric nature of these materials have voided the second-harmonic generation (SHG). Alternatively, the use of GaAs-based III-V semiconductors, with non-centrosymmetric structures, can overcome this difficulty [3,4]. However, fabrication of III-V semiconductor nanoantennas on low refractive index substrates remains very challenging, blocking the possibility to explore the SHG directionality in both forward and backward direction. Here, for the first time to our knowledge, we design and fabricate high-quality AlGaAs nanostructures on a glass substrate. Through this novel platform, we manage to excite, control and detect backward and forward nonlinear signals by SHG in AlGaAs nanodisks [5,6]. In particular, we observe that for certain size of nanoantenna, the SHG emission has a complex spatial distribution polarization state corresponding to radial polarization in the forward direction and a polarization state of a more general nature in the backward direction. Furthermore, we demonstrate an unprecedented SHG conversion efficiency of 10-4. Our breakthrough can open new avenues for enhancing the performance of photodetection, light emission and sensing.v
Quantum walks have attracted significant attention due to the possibility to propagate and reshape large-scale photon entanglement based on the superposition of possible photon paths. Entangling photons brings the promise of secure communication and ultra-fast quantum computing. Another phenomenon called optical nonlinearity allows interaction between electro-magnetic waves through matter. Bringing the concepts of quantum walks and optical nonlinearity together, and integrating them on a chip, opens a way to efficiently generate entangled photons and tune the entanglement. In this talk we will show the first experiments and theoretical studies featuring such tunable integrated sources.
We describe the process of parametric amplification in a directional coupler of quadratically nonlinear and lossy waveguides, which belong to a class of optical systems with spatial parity-time (PT) symmetry in the linear regime. We identify a distinct spectral parity-time anti-symmetry associated with optical parametric interactions, and show that pump-controlled symmetry breaking can facilitate spectrally selective mode amplification in analogy with PT lasers. We also establish a connection between breaking of spectral and spatial mode symmetries, revealing the potential to implement unconventional regimes of spatial light switching through ultrafast control of PT breaking by pump pulses.