Marco P. Fischer

Nonlinear photoluminescence spectrum of single gold nanostructures.

We investigate the multiphoton photoluminescence characteristics of gold nanoantennas fabricated from single crystals and polycrystalline films. By exciting these nanostructures with ultrashort pulses tunable in the near-infrared range, we observe distinct features in the broadband photoluminescence spectrum. By comparing antennas of different crystallinity and shape, we demonstrate that the nanoscopic geometry of plasmonic devices determines the shape of the emission spectra. Our findings rule out the contribution of the gold band structure in shaping the photoluminescence.

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.

Optical Activation of Germanium Plasmonic Antennas in the Mid-Infrared

Impulsive interband excitation with femtosecond near-infrared pulses establishes a plasma response in intrinsic germanium structures fabricated on a silicon substrate. This direct approach activates the plasmonic resonance of the Ge structures and enables their use as optical antennas up to the mid-infrared spectral range. The optical switching lasts for hundreds of picoseconds until charge recombination redshifts the plasma frequency. The full behavior of the structures is modeled by the electrodynamic response established by an electron-hole plasma in a regular array of antennas.

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.

Optical switching of mid-infrared plasmonic nanoantennas based on germanium

Germanium nanoantennas are activated by triggering a mid-infrared plasma response via ultrafast interband excitation. Femtosecond control of the intrinsic semiconductor allows complete activation of the plasmonic resonance for hundreds of picoseconds.

Coherent field transients below 15 THz by difference frequency generation in 4H-SiC

The frequency range between 5 THz and 15 THz is a spectral region of particular interest since it includes various fundamental excitations in condensed-matter systems such as phonons in solids, molecular vibrations and low-energy collective modes of correlated materials. However, the generation and coherent detection of highly stable THz transients for ultrafast experiments in this spectral interval is hindered by the fact that most second-order nonlinear materials display a forbidden region due to the presence of the Reststrahlen band and also a lack of birefringence for proper phase matching. In this work, we present 4H silicon carbide (4H-SiC) as a new nonlinear crystal for difference frequency generation (DFG) in this elusive spectral regime. SiC features a large variety of advantageous properties including a Reststrahlen band at relatively high frequencies between 20 THz and 50 THz [1] and thus complete transparency from 17 THz to below 1 THz as well as excellent transmission in the visible range without two-photon absorption of near-infrared pump light. Furthermore, high-quality crystals of outstanding thermal and mechanical robustness are available. Large second-order nonlinearity [2, 3] plus strong uniaxial birefringence allow for phase matching and high conversion efficiency in thick crystals.

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].

Dispersion of the nonlinear susceptibility in gold nanoantennas

Optical nanoantennas are excellent tools for accessing the nonlinear response of noble metals owing to the strong enhancement of light-matter interaction occurring in the near-field. For example, excitation of gold nanostructures with intense radiation triggers both coherent and incoherent phenomena such as third-harmonic generation (THG) [1] and multi-photon photoluminescence (MPPL), respectively [2, 3]. In this work, we study the interplay between these frequency conversion mechanisms in order to unveil the effective 2(3) nonlinear response of metals.

Mid-infrared third-harmonic emission from heavily-doped Germanium plasmonic nanoantennas

We investigate the nonlinear optical properties of single resonant plasmonic antennas fabricated from heavily-doped Germanium films. Excitation with intense and ultrashort mid-infrared pulses at 10.8 μm wavelength produces emission at 3.7 μm via third-harmonic generation.

Multi-Photon Photoluminescence Spectral Behavior of Single Gold Nanorods

The spectral shape and nonlinear order of optical emission from single gold nanorods is investigated. The results highlight the complex absorption cascade in the out-of-equilibrium electronic distribution after few-cycle excitation by near-infrared pulses.