NANOPHOTONICS
Optical Metasurfaces and Metamaterials
Metasurfaces, i.e., periodic arrangements of either resonant or non-resonant meta-atoms, enable light management at the subwavelength scale. Flat lenses, ultra-compact beam steerers, polarization converters, angular filters, and frequency-selective filters can be realized by properly engineering the meta-atom. These structures can be designed with dielectrics, metals, or hybrid material systems, at any desired frequency, from radio-frequencies up to optical frequencies.
Our research mainly deals with plasmonic and dielectric metasurfaces for applications in the optical regime, with emphasis on nonlinear optical functionalities and tunable nanophotonic devices.
In collaboration with Sandia National Lab., we have recently demonstrated a hybrid approach for realizing strong coupling and efficient second-harmonic generation in an ultrathin dielectric-semiconductor metasurface. The device shows low losses, high damage threshold, large bandwidth, wavelength scalability, dual mode operation in transmission and reflection, monolithic integrability, and ease of fabrication. Our design uses high quality factor leaky mode resonances that are coupled to intersubband transitions of semiconductor quantum wells. Our results open a new direction for designing low loss, broadband, and efficient ultrathin nonlinear optical devices.
One of the most challenging tasks in nanophotonics is to achieve tunable functionalities. We are currently investigating a new path to endow the optical response of resonant gratings with tunability. In collaboration with University of Brescia, Politecnico di Bari and Istituto Italiano di Tecnologia, we have demonstrated modulation of reflectance in response to a thermal stimulus in an array of pillars grown on top of a grounded, high-index slab. This photonic structure may host a variety of resonances that can be tuned across infrared and visible ranges, namely guided-mode resonances, Mie resonances, plasmonic resonances and Fabry-Pérot resonances.
When the structure is spin-coated with a nanofilm of vanadium-dioxide – a phase change material that switches from an insulator to a metal phase at 68°C – we have experimentally observed reflectance changes on the order of 10% near the resonances in the visible, and find good agreement with predictions based on numerical simulations. We are now focusing our efforts to find design strategies that improves the tunability in the visible range and to extend it in other portions of the spectrum.
Slow light
Among all nonlinear optical phenomena, stimulated Brillouin scattering (SBS) is often overlooked, essentially because of its small natural bandwidth. Nonetheless, the relative low power needed to induce this effect makes SBS a very attractive effect for photonics. Recently the realization of dynamic Brillouin gratings (DBGs) in optical fibers [1,2] demonstrated to be an extremely versatile technique to achieve, with a single experimental setup, several all-optical signal processing functions: delay lines [3], all-optical time differentiation, time integration and true time reversal [4]. This research activity explore the additional potential applications of DBGs in all-optical processing.
1. Z. Zhu, D.J. Gauthier and R.W. Boyd, Science 318, 17481750 (2007);
2. K. Y. Song, W. Zou, Z. He, and K. Hotate, Optics Letters 33, 926-928 (2008).
3. L. Thevenaz, S. Chin, Laser & Photonics Reviews 6, 728-734 (2012).
4. M. Santagiustina, S. Chin, N. Primerov, L. Ursini, L. Thevenaz, Scientific Reports 3, 1594 (2013).
Plasmonics
The interaction of light with electrons in motion produces plasmon polaritons, i.e., light-driven collective oscillations of conduction electrons. Usual electromagnetic signatures of plasmon polaritons include subwavelength optical confinement, absorption peaks in angular and frequency spectra, sensitivity to p-polarized light, and large electric-field enhancement. Owing to these properties, structures that support plasmonic modes improve sensors, photodetectors, absorbers, and thermal management devices, and hold the promise to merge electronics and photonics at the nanoscale. Indeed, current nanotechnologies allow fabrication of plasmonic structures with unprecedented quality and precision, with sub-nanometer and, in some cases, atomic-size features. This ever-increasing miniaturization trend has stimulated the growth of new research areas, such as nanoplasmonics, nonlinear plasmonics and quantum plasmonics. A fundamental understanding of microscopic events occurring in the bulk and on the surface of these systems, and the ability to account for these phenomena in simple, yet accurate theoretical models, are necessary steps to advance these fields of studies. Important examples of microscopic interactions include quantum, nonlocal (e.g., spatially-dispersive), and nonlinear effects induced by optically-excited charge carriers in nanoplasmonic structures.
We have recently demonstrated elasticity and viscosity effects associated with the oscillations of free electrons near the plasma frequency of doped metal oxides – the theory has been developed at UniPD, the experiments have been carried out at Sandia National Labs.
In this field, in collaboration with Dr Michael Scalora (US Army) and under the guidance of Prof. Joe Haus at University of Dayton, we have developed a methodology based on quantum mechanics for assigning quantum conductivity when an ac field is applied across a variable gap between two plasmonic nanoparticles with an insulator sandwiched between them. The quantum tunneling effect is portrayed by a set of quantum conductivity coefficients describing the linear ac conductivity responding at the frequency of the applied field, and nonlinear coefficients that modulate the field amplitude at the fundamental frequency and its harmonics. The quantum conductivity, determined with no fit parameters, has both frequency and gap dependence that can be applied to determine the nonlinear quantum effects of strong applied electromagnetic fields, even when the system is composed of dissimilar metal nanostructures.
Our methodology compares well to results on quantum tunneling effects reported in the literature, and is simple to extend to a number of systems with different metals and different insulators between them.
Nanoantennas
The manipulation of light with optical nanoantennas may unlock a plethora of new opportunities for microscopy and spectroscopy devices, enhanced photovoltaic cells, and for a large variety of optical and electro-optical applications that are typically limited by diffraction.
Efficient transformation of propagating light waves into highly confined fields, and vice versa, is mediated by the excitation of plasmon resonances localized at the surface of metallic nanostructures, in the case of plasmonic nanoantennas, or by the excitation of Mie scattering resonances in the volume of high-index dielectrics in the case of dielectric nanoantennas.
Research on nanoantennas at PEG is mainly focused on the enhancement of light-matter interactions with emphasis on nonlinear optical effects.
Nanoantennas may enhance nonlinear processes initiated in the surrounding media, in the bulk of the antenna, and at the interfaces. For example, the most relevant boosting mechanism for nonlinear interactions in metallic nanostructures is the field enhancement induced by surface plasmon resonances and by the lightning-rod effect.
We have investigated the role of antenna resonances and field enhancement in second-harmonic generation from plasmonic nanoantennas, the origin of third harmonic generation from hybrid metal-semiconductor nanoantennas and we have proposed a design in which the nanoantenna provides super-field enhancement mediated by nested plasmonic resonances. Recently we have also started investigating all-dielectric systems. In particular we have designed doubly resonant all-dielectric nanoantennas made of GaAs for highly-efficient generation of second-harmonic light and multi-quantum well nanoantennas for light-matter interactions in the strong coupling regime.
