arXiv subjects

Valeria Giliberti

Publications and source records attributed to Valeria Giliberti.

10 recordsLinked to original sources

arXiv2026

Plasmonic-cavity Modulator for the Mid-IR with a Semi-transparent and Nonlinear Heavily-doped Semiconductor Mirror

We present a free-space plasmonic modulator based on a single heavily-doped semiconductor layer. We investigate its ability to modulate both the linear and nonlinear response at mid-infrared frequencies slightly below the plasma frequency of the semiconductor. We demonstrate electric control of the linear transmittance and reflectance, and of the efficiency of third-harmonic generation with a field-effect gate structure. We discuss further performance optimization of the device in terms of modulation speed and depth towards a fast modulator with very simple active material requirements. Our results establish a viable route toward practical plasmonic modulators and mixers operating in the mid-infrared atmospheric window available for free-space communications at wavelengths between 8 and 12 um.

physics.optics↗

arXiv2024

Origin of optical nonlinearity in plasmonic semiconductor nanostructures

The development of nanoscale nonlinear elements in photonic integrated circuits is hindered by the physical limits to the nonlinear optical response of dielectrics, which requires that the interacting waves propagate in transparent volumes for distances much longer than their wavelength. Here we present experimental evidence that optical nonlinearities in doped semiconductors are due to free-electron and their efficiency could exceed by several orders of magnitude that of conventional dielectric nonlinearities. Our experimental findings are supported by comprehensive computational results based on the hydrodynamic modeling, which naturally includes nonlocal effects, of the free-electron dynamics in heavily doped semiconductors. By studying third-harmonic generation from plasmonic nanoantenna arrays made out of heavily n-doped InGaAs with increasing levels of free-carrier density, we discriminate between hydrodynamic and dielectric nonlinearities. As a result, the value of maximum nonlinear efficiency as well as its spectral location can now be controlled by tuning the doping level. Having employed the common material platform InGaAs/InP that supports integrated waveguides, our findings pave the way for future exploitation of plasmonic nonlinearities in all-semiconductor photonic integrated circuits.

physics.optics↗

arXiv2022

Detection of strong light-matter interaction in a single nano-cavity with a thermal transducer

Recently, the concept of strong light-matter coupling has been demonstrated in semiconductor structures, and it is poised to revolutionize the design and implementation of components, including solid state lasers and detectors. We demonstrate an original nanospectroscopy technique that permits to study the light-matter interaction in single subwavelength-sized nano-cavities where far-field spectroscopy is not possible using conventional techniques. We inserted a thin ($\approx$ 150 nm) polymer layer with negligible absorption in the mid-IR (5 $μ$m < $λ$ < 12 $μ$m) inside a metal-insulator-metal resonant cavity, where a photonic mode and the intersubband transition of a semiconductor quantum well are strongly coupled. The intersubband transition peaks at $λ$ = 8.3 $μ$m, and the nano-cavity is overall 270 nm thick. Acting as a non-perturbative transducer, the polymer layer introduces only a limited alteration of the optical response while allowing to reveal the optical power absorbed inside the concealed cavity. Spectroscopy of the cavity losses is enabled by the polymer thermal expansion due to heat dissipation in the active part of the cavity, and performed using an atomic force microscope (AFM). This innovative approach allows the typical anticrossing characteristic of the polaritonic dispersion to be identified in the cavity loss spectra at the single nano-resonator level. Results also suggest that near-field coupling of the external drive field to the top metal patch mediated by a metal-coated AFM probe tip is possible, and it enables the near-field mapping of the cavity mode symmetry including in the presence of strong light-matter interaction.

physics.optics↗

arXiv2021

Modelling photothermal induced resonance microscopy: the role of interface thermal resistances

Infrared (IR) nanospectroscopy by photothermal induced resonance (PTIR) is a novel experimental technique that combines the nanoscale resolution granted by atomic force microscopy (AFM) and the chemical labelling made possible by IR absorption spectroscopy. While the technique has developed enormously over the last decade from an experimental point of view, the theoretical modelling of the signal still varies significantly throughout the literature and misses a solid benchmark. Here, we report an analysis focused on the electromagnetic and thermal simulations of a PTIR experiment. Thanks to a control experiment where the signal is acquired as a function of the thickness of a polymer film and for different tip geometries, we find clear evidence that the interface thermal resistances play a key role in the determination of the measured signal and should therefore always be accounted for by any quantitative modelling.

cond-mat.mtrl-sci↗

arXiv2019

Tip-Enhanced Infrared Difference-Nanospectroscopy of the Proton Pump Activity of Bacteriorhodopsin in Single Purple Membrane Patches

Photosensitive proteins embedded in the cell membrane (about 5 nm thickness) act as photoactivated proton pumps, ion gates, enzymes, or more generally, as initiators of stimuli for the cell activity. They are composed of a protein backbone and a covalently bound cofactor (e.g. the retinal chromophore in bacteriorhodopsin (BR), channel rhodopsin, and other opsins). The light-induced conformational changes of both the cofactor and the protein are at the basis of the physiological functions of photosensitive proteins. Despite the dramatic development of microscopy techniques, investigating conformational changes of proteins at the membrane monolayer level is still a big challenge. Techniques based on atomic force microscopy (AFM) can detect electric currents through protein monolayers and even molecular binding forces in single-protein molecules but not the conformational changes. For the latter, Fourier-transform infrared spectroscopy (FTIR) using difference-spectroscopy mode is typically employed, but it is performed on macroscopic liquid suspensions or thick films containing large amounts of purified photosensitive proteins. In this work, we develop AFM-assisted, tip-enhanced infrared difference-nanospectroscopy to investigate light-induced conformational changes of the bacteriorhodopsin mutant D96N in single submicrometric native purple membrane patches. We obtain a significant improvement compared with the signal-to-noise ratio of standard IR nanospectroscopy techniques by exploiting the field enhancement in the plasmonic nanogap that forms between a gold-coated AFM probe tip and an ultraflat gold surface, as further supported by electromagnetic and thermal simulations. IR difference-spectra in the 1450-1800 cm^{-1} range are recorded from individual patches as thin as 10 nm, with a diameter of less than 500 nm, well beyond the diffraction limit for FTIR microspectroscopy...

physics.bio-ph↗

arXiv2018

Surface Thermal Gradients Activated by Enhanced Molecular Absorption in Mid-infrared Vertical Antenna Arrays

We investigate local heat generation by molecules at the apex of polymer-embedded vertical antennas excited at resonant mid-infrared wavelengths, exploiting the surface enhanced infrared absorption (SEIRA) effect. The embedding of vertical nanoantennas in a non-absorbing polymer creates thermal isolation between the apical hotspot, the locus of heat generation, and the heat sink represented by the substrate. Vibrational mid-infrared absorption by strongly absorbing molecules located at the antenna apex then generates nanoscale temperature gradients at the surface. We imaged the thermal gradients by using a nano-photothermal expansion microscope, and we found values up to 10 K/microm in conditions where the radiation wavelength resonates with both the molecule vibrations and the plasmonic mode of the antennas. Values up to 1000 K/microm can be foreseen at maximum quantum cascade laser power. The presented system provides a promising thermoplasmonic platform for antenna-assisted thermophoresis and resonant mid-infrared photocatalysis.

physics.app-ph↗

arXiv2018

Out-of-equilibrium collective oscillation as phonon condensation in a model protein

In the first part of the present paper (theoretical), the activation of out-of-equilibrium collective oscillations of a macromolecule is described as a classical phonon condensation phenomenon. If a macromolecule is modeled as an open system, that is, it is subjected to an external energy supply and is in contact with a thermal bath to dissipate the excess energy, the internal nonlinear couplings among the normal modes make the system undergo a non-equilibrium phase transition when the energy input rate exceeds a threshold value. This transition takes place between a state where the energy is incoherently distributed among the normal modes, to a state where the input energy is channeled into the lowest frequency mode entailing a coherent oscillation of the entire molecule. The model put forward in the present work is derived as the classical counterpart of a quantum model proposed long time ago by H. Fröhlich in the attempt to explain the huge speed of enzymatic reactions. In the second part of the present paper (experimental), we show that such a phenomenon is actually possible. Two different and complementary THz near-field spectroscopic techniques, a plasmonic rectenna, and a micro-wire near-field probe, have been used in two different labs to get rid of artefacts. By considering a aqueous solution of a model protein, the BSA (Bovine Serum Albumin), we found that this protein displays a remarkable absorption feature around 0.314 THz, when driven in a stationary out-of-thermal equilibrium state by means of optical pumping. The experimental outcomes are in very good qualitative agreement with the theory developed in the first part, and in excellent quantitative agreement with a theoretical result allowing to identify the observed spectral feature with a collective oscillation of the entire molecule.

cond-mat.soft↗

arXiv2018

Thermoplasmonic Effect of Surface Enhanced Infrared Absorption in Vertical Nanoantenna Arrays

The temperature increase and temperature gradients induced by mid-infrared laser illumination of vertical gold nanoantenna arrays embedded into polymer layers was measured directly with a photothermal expansion nanoscope. Nanoscale thermal hotspot images and local temperature increase spectra were both obtained, the latter by broadly tuning the emission wavelength of a quantum cascade laser. The spectral analysis indicates that plasmon-enhanced mid-infrared vibrations of molecules located in the antenna hotspots are responsible for some of the thermoplasmonic resonances, while Joule heating in gold is responsible for the remaining resonances. In particular, plasmonic dark modes with low scattering cross-section mostly produce surface-enhanced infrared absorption (SEIRA), while bright modes with strong radiation coupling produce Joule heating. The dark modes do not modify the molecular absorption lineshape and the related temperature increase is chemically triggered by the presence of molecules with vibrational fingerprints resonant with the plasmonic dark modes. The bright modes, instead, are prone to Fano interference, display an asymmetric molecular absorption lineshape and generate heat also at frequencies far from molecular vibrations, insofar lacking chemical specificity. For focused mid-infrared laser power of 50 mW, the measured nanoscale temperature increases are in the range of 10 K and temperature gradients reach 5 K/$μ$m in the case of dark modes resonating with strong infrared vibrations such as the C=O bond of poly-methylmethacrylate at 1730 cm$^{-1}$.

physics.app-ph↗

arXiv2016

Tunability and Losses of Mid-infrared Plasmonics in Heavily Doped Germanium Thin Films

Heavily-doped semiconductor films are very promising for application in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in this wavelength range. In this work we investigate heavily n-type doped germanium epilayers grown on different substrates, in-situ doped in the $10^{17}$ to $10^{19}$ cm$^{-3}$ range, by infrared spectroscopy, first principle calculations, pump-probe spectroscopy and dc transport measurements to determine the relation between plasma edge and carrier density and to quantify mid-infrared plasmon losses. We demonstrate that the unscreened plasma frequency can be tuned in the 400 - 4800 cm$^{-1}$ range and that the average electron scattering rate, dominated by scattering with optical phonons and charged impurities, increases almost linearly with frequency. We also found weak dependence of losses and tunability on the crystal defect density, on the inactivated dopant density and on the temperature down to 10 K. In films where the plasma was optically activated by pumping in the near-infrared, we found weak but significant dependence of relaxation times on the static doping level of the film. Our results suggest that plasmon decay times in the several-picosecond range can be obtained in n-type germanium thin films grown on silicon substrates hence allowing for underdamped mid-infrared plasma oscillations at room temperature.

cond-mat.mtrl-sci↗

arXiv2015

Plasmon-phonon interactions in topological insulator rings

The great potential of Dirac electrons for plasmonics and photonics has been readily recognized after their discovery in graphene, followed by applications to smart optical devices. Dirac carriers are also found in topological insulators (TI) --quantum systems having an insulating gap in the bulk and intrinsic Dirac metallic states at the surface--. Here, we investigate the plasmonic response of ring structures patterned in Bi$_2$Se$_3$ TI films, which we investigate through terahertz (THz) spectroscopy. The rings are observed to exhibit a bonding and an antibonding plasmon modes, which we tune in frequency by varying their diameter. We develop an analytical theory based on the THz conductivity of unpatterned films, which accurately describes the strong plasmon-phonon hybridization and Fano interference experimentally observed as the bonding plasmon is swiped across the promineng 2\,THz phonon exhibited by this material. This work opens the road for the investigation of plasmons in topological insulators and for their application in tunable THz devices.

cond-mat.mtrl-sci↗