Here we review our journey from metallic nanostructures to dielectric and semiconductor metasurfaces. We show how to employ metal non-linearity to stimulate a strongly anisotropic nonlinear response by symmetry breaking, despite their high Ohmic losses. Furthermore, we show how an ultra-thin surface of subwavelength dielectric nanostructures, e.g. silicon with negligible losses and multipolar characteristics, can enable enhanced light matter interaction for efficient third harmonic generation and ultra-fast light modulation. However, the centrosymmetric structure of silicon and the lack of quadratic nonlinearity, guided us towards exploiting semiconductor nanostructures, particularly III-V semiconductors. Subsequently, we demonstrate dielectric realization of AlGaAs nanoantennas for an efficient second harmonic generation, allowing the control of both directionality and polarization of nonlinear emission. This is enabled through the fabricated high-quality AlGaAs nanostructures embedded in an optically transparent low-index material. Our results open novel applications in ultra-thin light sources, light switches and modulators, ultra-fast displays, night-vision and other nonlinear optical metadevices based on 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).
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
Bridging the gap in scale between the THz wavelength and the biomolecule sample sizes to be sensed is a challenging
task. We tackle this mismatch by developing sensing platforms based on the concepts of designer surface plasmon
polaritons and localized plasmons. We show that corrugated metallic surfaces, complementary split ring resonators and
arrays of micro-dipoles provides enhanced THz-matter interaction times and strong interrogating evanescent fields. We
will also demonstrate how transformation optics can be used to design broadband plasmonic semiconductor and metallic
gap micro-antennas for terahertz-to-visible applications.