Grupp, Alexander
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Control of excitonic absorption by thickness variation in few-layer GaSe
We control the thickness of GaSe on the level of individual layers and study the corresponding optical absorption via highly sensitive differential transmission measurements. Suppression of excitonic transitions is observed when the number of layers is smaller than a critical value of 8. Through ab initio modelling we are able to link this behavior to a fundamental change in the band structure that leads to the formation of a valence band shaped as an inverted Mexican hat in thin GaSe. The thickness-controlled modulation of the optical properties provides attractive resources for the development of functional optoelectronic devices based on a single material.
Incoherent Pathways of Charge Separation in Organic and Hybrid Solar Cells
In this work, we investigate the exciton dissociation dynamics occurring at the donor:acceptor interface in organic and hybrid blends employed in the realization of photovoltaic cells. Fundamental differences in the charge separation process are studied with the organic semiconductor polymer poly(3-hexylthiophene) (P3HT) and either [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or titanium dioxide (TiO2) acting as the acceptor. By using ultrafast broad-band transient absorption spectroscopy with few-fs temporal resolution, we observe that in both cases the incoherent formation of free charges dominates the charge generation process. From the optical response of the polymer and by tracking the excited-state absorption, we extract pivotal similarities in the incoherent energy pathways that follow the impulsive excitation. On time scales shorter than 200 fs, we observe that the two acceptors display similar dynamics in the exciton delocalization. Significant differences arise only on longer time scales with only an impact on the overall photocarrier generation efficiency.
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]. In this work, we demonstrate that Ge antenna structures are also suitable for nonlinear optical processes such as third harmonic generation (THG) in the mid infrared [2], owing to the strong resonant enhancement. Subwavelength-confined light emitters are of high interest for experiments targeting single molecules or other isolated quantum systems [3].
Ultrafast Insulator-Metal Transition in VO2 Driven by Intense Multi-THz Pulses
We demonstrate sub-100 fs metallization of VO2 induced by few-cycle electric transients at frequencies around 25 THz. Interband tunneling is identified as an instantaneous excitation mechanism.
Suppression of Excitonic Absorption by Thickness Variation in Few-layer GaSe
We study the optical absorption of GaSe via sensitive differential transmission measurements while controlling the sample thickness with individual layer precision. Below a critical value of eight layers, suppression of the excitonic transitions is observed.
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.
Germanium plasmonic nanoantennas for third-harmonic generation in the mid infrared
Recent advances in semiconductor film deposition allow the growth of heavily doped germanium with effective plasma frequencies of up to 30 THz. This technology paves the way for mid-infrared (MIR) nanoplasmonics with application to integrated telecommunications systems and to precise sensing in the spectral region defined as the vibrational fingerprint of molecules. Characteristics like CMOS compatibility, low electron effective mass and tunable dielectric function give advantage to Ge over metal plasmonics or other semiconductors.
Broadly tunable ultrafast pump-probe system operating at multi-kHz repetition rate
Femtosecond systems based on ytterbium as active medium are ideal for driving ultrafast optical parametric amplifiers in a broad frequency range. The excellent stability of the source and the repetition rate tunable to up to hundreds of kHz allow for the implementation of an advanced two-color pump probe setup with the capability to achieve excellent signal-to-noise performances with sub-10 fs temporal resolution.
Tunability of the dielectric function of heavily doped germanium thin films for mid-infrared plasmonics
Heavily doped semiconductor thin 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 the 5 to 50μm wavelength range at least. In this work, we investigate the electrodynamics of heavily n-type-doped germanium epilayers at infrared frequencies beyond the assumptions of the Drude model. The films are grown on silicon and germanium substrates, are in situ doped with phosphorous in the 1017 to 1019 cm−3 range, then screened plasma frequencies in the 100 to 1200cm−1 range were observed. We employ infrared spectroscopy, pump-probe spectroscopy, and dc transport measurements to determine the tunability of the plasma frequency. Although no plasmonic structures have been realized in this work, we derive estimates of the decay time of mid-infrared plasmons and of their figures of merit for field confinement and for surface plasmon propagation. The average electron scattering rate increases almost linearly with excitation frequency, in agreement with quantum calculations based on a model of the ellipsoidal Fermi surface at the conduction band minimum of germanium accounting for electron scattering with optical phonons and charged impurities. Instead, we found weak dependence of plasmon losses on neutral impurity density. In films where a transient plasma was generated by optical pumping, we found significant dependence of the energy relaxation times in the few-picosecond range on the static doping level of the film, confirming the key but indirect role played by charged impurities in energy relaxation. Our results indicate that underdamped mid-infrared plasma oscillations are attained in n-type-doped germanium at room temperature.