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 .
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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.
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.