Achieving fast and active temporal control of optical metasurfaces remains an open challenge in nanophotonics. Such control would dramatically enhance the scopes of metasurfaces, as their functionalities could be tuned on demand, beyond the limits of their static properties. Here we report on our approach to design and demonstrate the control of all-optically tunable metasurfaces in the ultrafast regime in different scenarios. First, we theoretically predict and experimentally prove by ultrafast transient absorption spectroscopy that spatio-temporal dynamics of hot electrons can promote and control a sub-picosecond photoinduced anisotropy in plasmonic metasurfaces, enabling active reconfiguration of the nanostructure nonlinear response. Then, we demonstrate a giant all-optical modulation of dichroism in an anisotropic all-dielectric metasurface. Finally, we propose a new paradigm exploiting the unique properties of active ultrafast metasurfaces for controlling physico-chemical processes activated by light to improve their efficiency.
We study the spatio-temporal evolution of hot electrons generated in plasmonic nanostructures under resonant excitation with fs-laser pulses. A spatially inhomogeneous version of the Three-Temperature Model for hot-electrons dynamics, coupled to semiclassical calculations of third-order optical nonlinearity in gold, enabled us to engineer a transient symmetry breaking of the optical properties at the nanoscale. This effect is exploited to achieve all-optical control of light with unprecedented speed. For instance, a photoinduced broadband dichroism, fully reversible and transiently vanishing in less than 1 picoseconds (overcoming the speed bottleneck caused by slower, electron-phonon and phonon-phonon relaxation processes), has been experimentally demonstrated in plasmonic metasurfaces with nanocross metaatoms. Also, we designed a nonlinear plasmonic metagrating (based on cross-polarized gold nanostrip dimer metaatoms), where the nanoscale symmetry breaking enables ultrafast reconfiguration of diffraction orders via control laser pulses. The photoinduced power imbalance between symmetrical diffraction orders is calculated to exceed 20% under moderate (∼2 mJ/cm2) laser fluence, and returns to the balanced diffraction in about 2 ps. Our design has been developed for gold nanomaterials, but the concept of ultrafast all-optical symmetry breaking can be exploited beyond plasmonics (e.g. in semiconductor nanostructures), with potential impact on a broad range of applications in nanophotonics.
Nanoscale optical integration is nowadays a strategic technological challenge and the ability of generating and manipulating nonlinear optical processes in sub-wavelength volumes is pivotal to realize efficient sensing probes and photonic sources for the next-generation communication technologies. Yet, confining nonlinear processes below the diffraction limit remains a challenging task because phase-matching is not a viable approach at the nanoscale. The localized fields associated to the resonant modes of plasmonic and dielectric nanoantennas offer a route to enhance and control nonlinear processes in highly confined volumes. In my talk I will discuss two nonlinear platforms based on plasmonic and dielectric nanostructures. The first relies on a broken symmetry antenna design, which brings about an efficient second harmonic generation (SHG). We recently applied this concept to an extended array of non-centrosymmetric nanoantennas for sensing applications. I will also show the evidence of a cascaded second-order process in Third Harmonic Generation (THG) in these nanoantennas.
Recently, dielectric nanoantennas emerged as an alternative to plasmonic nanostructures for nanophotonics applications, thanks to their sharp magnetic and electric Mie resonances along with the low ohmic losses in the visible/near-infrared region of the spectrum. I will present our most recent studies on the nonlinear properties of AlGaAs dielectric nanopillars. The strong localized modes along with the broken symmetry in the crystal structure of AlGaAs allow obtaining more than two orders of magnitude higher SHG efficiency with respect to plasmonic nanoantennas with similar spatial footprint and using the same pump power. I will also discuss a few key strategies we recently adopted to optically switch the SHG in these antennas even on the ultrafast time scale. Finally, I will show how to effectively engineer the sum frequency generation via the Mie resonances in these nanoantennas. These results draw a viable blueprint towards room-temperature all optical logic operation at the nanoscale.
Here the engineering of anisotropic plasmonic metasurfaces in the form of nanostripes or nanostripe dimers is demonstrated by a novel self-organization technique. Subwavelength quasi-1D glass templates are fabricated over large (cm2) area by ion beam induced wrinkling, enabling the maskless confinement of out-of-plane tilted gold nanostripe arrays supporting localized plasmon resonances easily tunable from the Visible to the Near-Infrared spectrum. A multi-step variant of the method allows to achieve plasmon hybridization in Au-silica-Au nanostrip dimer arrays with excitation of plasmonic electric dipole and magnetic dipole mode featuring strong subradiant near-field enhancement. The selforganized method enables to tune the hybridized plasmonic mode in the Visible and Near-Infrared spectrum opening the possibility to exploit these templates in highly sensitive biosensing and/or nonlinear optical spectroscopies.
Here we discuss the light transmission modulation by periodic and disordered one dimensional (1D) photonic structures. In particular, we will present some theoretical and experimental findings highlighting the peculiar optical properties of: (i) 1D periodic and disordered photonic structures made with two or more materials1,2; (ii) 1D photonic structures in which the homogeneity3 or the aggregation4 of the high refractive index layers is controlled. We will focus also on the fabrication aspects of these structures.
We present the possibility to tailor the optical properties of 1D photonic structures by using more than two materials and by clustering the high refractive index (hRI) layer in the structures. In particular, we show that: i) with a photonic crystal made of i different materials, the photonic band gap splits in i-1 bands; ii) with a proper choice of the layer thickness, disordered photonic structures made with a high number of layers show periodic transmission peaks; iii) when the size of the hRI layer clusters, randomly distributed within the low refractive index layers, follows a power law distribution, the total light transmission follows a sigmoidal function. Furthermore, we discuss the fabrication aspects to realize the above mentioned photonic structures.
Single-frequency waveguide lasers are very promising candidate for applications were insensitivity to technical noise
and possible integration with other devices are key issue. However for applications such as airbone radar and LIDAR
output power of the order of 100 mW at a eye-safe wavelength is required. In this paper we propose a suitable way to
build high-power robust single-frequency waveguide lasers at 1.5 micron and we demonstrate up to 80 mW with 40%
efficiency in an almost single-frequency operation.
We report results on a single-end pumped waveguide laser for sensing applications Output power in excess of 20 mW
with 17% slope efficiency in robust single-frequency operation at 1533.5 nm is demonstrated. The overall laser cavity
laser was 60-mm long but the active medium, an Er:Yb-doped phosphate glass, was only 9-mm long. The waveguide
was fabricated by two-step Ag-Na ion-exchange technique. The overall cavity length including butt-coupled fiber-
Bragg-grating mirrors was <60 mm. We also reports on recent work to reach 100-mW single-frequency output power.
To extend the operation wavelength to 2-micron wavelength region we also developed new tellurite glasses. Preliminary
results on glass investigation are also reported.
Scattering resonances of metal nano-strip resonators are described as a consequence of formation of standing waves due
to counter-propagating short-range (and slow) surface plasmon polaritons and gap plasmon polaritons, which are
electromagnetic waves bound to and propagating along a nanometer-thin metal film, and a nanometer-sized gap between
metal surfaces, respectively. Scattering spectra and resonant fields are presented for single-metal-nano-strip resonators
and gap plasmon resonators with two closely spaced metal nano-strips. It is shown how strip resonators can be designed
for any resonance wavelength in the range from 600nm to 1600nm.
Recently, great effort has been devoted to waveguide lasers, because of their inherent simplicity with respect to
fiber lasers. Actually, due to their compactness, such lasers are expected to achieve a higher temporal coherence,
making them attracting for fiber optical reflectometry, distribute sensing, and range finding applications. Furthermore,
the availablity of fast saturable absorbers based on carbon nanotubes allows for a cheap and reliable
implementation of the passive mode-locking technique with the potential for generating high repetition rate pulse
trains. Such lasers will provide low-noise and inexpensive pulsed sources for applications in optical communications,
optically sampled analog-to-digital converters, and spectral line-by-line pulse shaping. We report here on
advanced waveguide lasers, operating both in continuous wave and pulsed regimes, based on active waveguides
fabricated by femtosecond laser writing in a phosphate glass substrate. A single longitudinal mode waveguide
laser providing more than 50 mW with 21% slope efficiency was demonstrated. Furthermore, by combining a high
gain waveguide and an innovated fiber-pigtailed saturable absorber based on carbon nanotubes, a mode-locked
ring laser providing transform limited 1.6-ps pulses was also demonstrated.
A theoretical and experimental analysis of group velocity reduction in periodic super-structured Bragg gratings
is presented. Experimental demonstration of group velocity reduction of sub-nanosecond pulses at the 1.5 μm wavelength of optical communications is reported using either a 20-cm-long Moire and a periodically-spaced πphase shift fiber gratings. Time delays up to approximately 690 ps for 250-ps-duration optical pulses have been achieved leading to the realization of an optical buffer.
The fabrication of telecom active devices, such as waveguide amplifiers and lasers, with femtosecond laser pulses is of great industrial interest due to the simplicity, low cost and 3D capabilities of this technology with respect to the standard ones. In this work we will present the various improvements that brought us to demonstrate net gain and the first waveguide laser fabricated with femtosecond laser pulses on an erbium-ytterbium-doped phosphate glass. The first results have been obtained with an amplified, low repetition rate (1 kHz), Ti:Sapphire system. The target of matching the mode field of the fabricated waveguides to that of standard telecom fibers pushed us to develop a novel astigmatic focusing of the writing beam to overcome the asymmetry of the waveguide transverse profile intrinsic in the transversal writing geometry. Despite the circularization of the transverse profile, the high coupling losses allowed only for internal gain in an all-fiber coupling configuration. The best results have been obtained with a very compact, unamplified, diode-pumped Yb:glass laser, with a higher repetition rate (166/505 kHz) and lower energy. In this case, the waveguides exhibited almost perfect mode matching with a telecom fiber allowing coupling losses as low as 0.18 dB and propagation losses of 0.5 dB/cm. Such figures enabled net gain when pumping with 980-nm laser diodes and laser action by terminating the waveguide with two fiber Bragg gratings. These results pave the way to a transfer of femtosecond waveguide writing into the industrial arena for the realization of practical telecom components.
A review on the results achieved by our group in the development of novel solid-state lasers for Lidar applications at 2 μm is presented. These lasers, based on fluoride crystals (YLF4, BaY2F8, and KYF4) doped with Tm and Ho ions, are characterized by high-efficiency and wide wavelength tunability around 2 μm. Single crystals of LiYF4, BaY2F8, and KYF4 codoped with the same Tm3+ and Ho3+ concentrations were successfully grown by the Czochralski method. The full spectroscopic characterization of the different laser crystals and the comparison between the laser performance are presented. Continuous wave operation was efficiently demonstrated by means of a CW diode-pumping. These oscillators find interesting applications in the field of remote sensing (Lidar and Dial systems) as well as in high-resolution molecular spectroscopy, frequency metrology, and biomedical applications.
The fabrication of telecom active devices, such as waveguide amplifiers and lasers, with femtosecond laser pulses is of great industrial interest due to the simplicity, low cost and 3D capabilities of this technology with respect to the standard ones. In this work we will present the various improvements that brought us to demonstrate net gain and the first waveguide laser fabricated with femtosecond laser pulses on an erbium-ytterbium-doped phosphate glass. The first results have been obtained with an amplified, low repetition rate (1 kHz), Ti:Sapphire system. The target of matching the mode field of the fabricated waveguides to that of standard telecom fibers pushed us to develop a novel astigmatic focusing of the writing beam to overcome the asymmetry of the waveguide transverse profile intrinsic in the transversal writing geometry. Despite the circularization of the transverse profile, the high coupling losses allowed only for internal gain in an all-fiber coupling configuration. The best results have been obtained with a very compact, unamplified, diode-pumped Yb:glass laser, with a higher repetition rate (166/505 kHz) and lower energy. In this case, the waveguides exhibited almost perfect mode matching with a telecom fiber allowing coupling losses as low as 0.18 dB and propagation losses of 0.5 dB/cm. Such figures enabled net gain when pumping with 980-nm laser diodes and laser action by terminating the waveguide with two fiber Bragg gratings. These results pave the way to a transfer of femtosecond waveguide writing into the industrial arena for the realization of practical telecom components.
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