Giovanni Pellegrini

Tunability of the dielectric function of heavily doped germanium thin films for mid-infrared plasmonics

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 to 10 cm 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 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 ntype germanium thin films grown on silicon substrates hence allowing for underdamped mid-infrared plasma oscillations at room temperature.

Polarization properties of second-harmonic generation in AlGaAs optical nanoantennas

Manipulating light at the nanoscale by means of dielectric nanoantennas recently received renewed attention thanks to the development of key enabling fabrication tools in semiconductor technology, combined with the extremely low losses exhibited by dielectrics in the optical regime. Nanostructures based on III-V type semiconductors, characterized by an intrinsic broken symmetry down to a single elementary cell, has already demonstrated remarkable nonlinear conversion efficiencies at scales well below the operating wavelength. In this Letter, we thoroughly investigate the emission properties of second-harmonic generation (SHG) in AlGaAs monolithic nanoantennas. Our findings point toward the pivotal role of volume susceptibility in SHG, further unraveling the physics behind the nonlinear processes in these systems. The extremely high SHG efficiency attained, together with the control over the polarized emission in these nanoantennas, constitute key ingredients for the development of tunable nonlinear metasurfaces.

Chiral surface waves for enhanced circular dichroism

We present a novel chiral sensing platform that combines a one-dimensional photonic crystal design with a birefringent surface defect. The platform sustains simultaneous transverse electric and transverse magnetic surface modes, which are exploited to generate chiral surface waves. The present design provides homogeneous and superchiral fields of both handednesses over arbitrarily large areas in a wide spectral range, resulting in the enhancement of the circular dichroism signal by two orders of magnitude, thus paving the road toward the successful combination of surface-enhanced spectroscopies and electromagnetic superchirality.

n-Ge on Si for mid-infrared plasmonic sensors

The detection and amplification of molecular absorption lines from a mustard gas simulant is demonstrated using plasmonic antennas fabricated from n-Ge epitaxially grown on Si. Approaches to integrated sensors will be presented along with a review of n-Ge compared to other mid-infrared plasmonic materials.

Competing second and third order nonlinear effects in plasmonic nanoantennas (Conference Presentation)

The optimization of nonlinear optical processes at the nanoscale is a key challenge in nanoscience. In this framework, plasmon-enhanced nonlinear effects together with the development of innovative nanoantenna designs and hybrid nanostructures are receiving a lot of attention [1-2]. We recently devised a plasmonic nanoantenna working in the near-infrared region of the electromagnetic spectrum, which allows boosting the SHG efficiency. This is achieved by optimizing the nanoantenna geometry to feature (i) a double resonant response at both the excitation and emission wavelengths, (ii) a spatial overlap between the modes involved in the process and (iii) a broken symmetry, to enable dipole-allowed SHG. We found that this nanoantenna behaves like a strongly coherent nanoscale light source, featuring a marked THG along with an intense SHG [3-4].nWe find evidence of a SHG-mediated cascaded effect in THG [5]. We have identified a THG polarization behavior that strongly deviates from that of a bulk (3)-mediated effect and unveils a significant contribution coming from the cascaded coherent sum of a SHG photon and a pump photon.nnnnReferencesn[1] Kauranen, M.; Zayats, A. V., Nature Photon. 2012, 6, 737-748.n[2] Butet, J.; Brevet, P. F.; Martin, O. J. F., ACS Nano 2015, 9, 10545–10562.n[3] Celebrano, M.; et al., Nat. Nanotechnol. 2015, 10, 412−417.n[4] Sartorello, G.; et al., ACS Photon. 2016, 3, 1517−1522.n[5] Mu, X.; et al., Y., Opt. Lett. 2000, 25, 117-119.

Surface-enhanced chiroptical spectroscopy with superchiral surface waves

We study the chiroptical properties of one-dimensional photonic crystals supporting superchiral surface waves by introducing a simple formalism based on the Fresnel reflection matrix. We show that the proposed framework provides useful insights on the behavior of all the relevant chiroptical quantities, allowing for a deeper understanding of surface-enhanced chiral sensing platforms based on one-dimensional photonic crystals. Finally, we analyze and discuss the limitations of such platforms as the surface concentration of the target chiral analytes is gradually increased.

Metal–dielectric hybrid nanoantennas for efficient frequency conversion at the anapole mode

Background: Dielectric nanoantennas have recently emerged as an alternative solution to plasmonics for nonlinear light manipulation at the nanoscale, thanks to the magnetic and electric resonances, the strong nonlinearities, and the low ohmic losses characterizing high refractive-index materials in the visible/near-infrared (NIR) region of the spectrum. In this frame, AlGaAs nanoantennas demonstrated to be extremely efficient sources of second harmonic radiation. In particular, the nonlinear polarization of an optical system pumped at the anapole mode can be potentially boosted, due to both the strong dip in the scattering spectrum and the near-field enhancement, which are characteristic of this mode. Plasmonic nanostructures, on the other hand, remain the most promising solution to achieve strong local field confinement, especially in the NIR, where metals such as gold display relatively low losses. Results: We present a nonlinear hybrid antenna based on an AlGaAs nanopillar surrounded by a gold ring, which merges in a single platform the strong field confinement typically produced by plasmonic antennas with the high nonlinearity and low loss characteristics of dielectric nanoantennas. This platform allows enhancing the coupling of light to the nanopillar at coincidence with the anapole mode, hence boosting both second- and third-harmonic generation conversion efficiencies. More than one order of magnitude enhancement factors are measured for both processes with respect to the isolated structure. Conclusion: The present results reveal the possibility to achieve tuneable metamixers and higher resolution in nonlinear sensing and spectroscopy, by means of improved both pump coupling and emission efficiency due to the excitation of the anapole mode enhanced by the plasmonic nanoantenna.

Mid-infrared n-Ge on Si plasmonic based microbolometer sensors

The detection and amplification of molecular absorption lines from a chemical weapons simulant is demonstrated using plasmonic antennas fabricated from n-Ge epitaxially grown on Si. A free-standing Si0.25Ge0.75 microbolometer detector with n-Ge plasmonic antenna is demonstrated as an integrated mid-infrared plasmonic sensor.

Heavily-doped germanium on silicon with activated doping exceeding 10 20 cm −3 as an alternative to gold for mid-infrared plasmonics

Ge-on-Si has been demonstrated as a platform for Si foundry compatible plasmonics. We use laser thermal annealing to demonstrate activated doping levels >10²⁰ cm⁻³ which allows most of the 3 to 20 μm mid-infrared sensing window to be covered with enhancements comparable to gold plasmonics.

Germanium nanoantennas for plasmon-enhanced third harmonic generation in the mid infrared

Recent advances in semiconductor film deposition allow for the growth of heavily-doped germanium with effective plasma frequencies above 60 THz, corresponding to wavelengths below 5 μm. This technology paves the way for mid-infrared nanoplasmonics with application in integrated telecommunication systems and enhanced molecular sensing in the so-called vibrational fingerprint spectral region [1].