Alexander Grupp

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.

Tunneling breakdown of a strongly correlated insulating state in V O 2 induced by intense multiterahertz excitation

We directly trace the near- and mid-infrared transmission change of a VO

Ultrafast pseudospin dynamics in graphene

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Ultrafast Insulator-Metal Transition in VO2 Driven by Intense Multi-THz Pulses

thin film during an ultrafast insulator-to-metal transition triggered by high-field multi-terahertz transients. Non-thermal switching into a metastable metallic state is governed solely by the amplitude of the applied terahertz field. In contrast to resonant excitation below the threshold fluence, no signatures of excitonic self-trapping are observed. Our findings are consistent with the generation of spatially separated charge pairs and a cooperative transition into a delocalized metallic state by THz field-induced tunneling. The tunneling process is a condensed-matter analogue of the Schwinger effect in nonlinear quantum electrodynamics. We find good agreement with the pair production formula by replacing the Compton wavelength with an electronic correlation length of 2.1

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

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

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Incoherent Pathways of Charge Separation in Organic and Hybrid Solar Cells

We investigate pseudospin-selective ultrafast optical excitation in monolayer graphene. We track the evolution of anisotropy in momentum space as a function of excited carrier density. The results are well described by an analytical model.

Depletion of the superconducting condensate in Bi 2 Sr 2 CaCu 2 O 8+δ induced by intense multi-THz pulses

We demonstrate a non-thermal metallization of VO2 induced by a non-resonant excitation at frequencies around 25 THz. An ultrafast switching time of 80 fs comprises only two cycles of the driving multi-THz field.

Below-gap excitation of semiconducting single-wall carbon nanotubes

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].

Broadly tunable ultrafast pump-probe system operating at multi-kHz repetition rate

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.