Claudia Ruppert

Optical Properties and Band Gap of Single- and Few-Layer MoTe2 Crystals

Single- and few-layer crystals of exfoliated MoTe2 have been characterized spectroscopically by photoluminescence, Raman scattering, and optical absorption measurements. We find that MoTe2 in the monolayer limit displays strong photoluminescence. On the basis of complementary optical absorption results, we conclude that monolayer MoTe2 is a direct-gap semiconductor with an optical band gap of 1.10 eV. This new monolayer material extends the spectral range of atomically thin direct-gap materials from the visible to the near-infrared.

The Role of Electronic and Phononic Excitation in the Optical Response of Monolayer WS2 after Ultrafast Excitation

Transient changes of the optical response of WS2 monolayers are studied by femtosecond broadband pump-probe spectroscopy. Time-dependent absorption spectra are analyzed by tracking the line width broadening, bleaching, and energy shift of the main exciton resonance as a function of time delay after the excitation. Two main sources for the pump-induced changes of the optical response are identified. Specifically, we find an interplay between modifications induced by many-body interactions from photoexcited carriers and by the subsequent transfer of the excitation to the phonon system followed by cooling of the material through the heat transfer to the substrate.

Thermochromic modulation of surface plasmon polaritons in vanadium dioxide nanocomposites

We propose and implement a new concept for thermochromic plasmonic elements. It is based on vanadium dioxide (VO2) nanocrystals located in the near field of surface plasmon polaritons supported by an otherwise unstructured gold thin film. When the VO2 undergoes the metal-insulator phase transition, the coupling conditions for conversion of light into propagating surface plasmon polaritons change markedly. In particular, we realize thermochromic plasmonic grating couplers with substantial switching contrast as well as tunable plasmonic couplers in a Kretschmann configuration. The use of VO2 nanocrystals permits highly repetitive switching and room temperature operation. Simulations based on the actual dielectric function of our VO2 nanocrystals agree well with the experiment.

Ultrafast field-resolved semiconductor spectroscopy utilizing quantum interference control of currents.

We implement a versatile concept to time-resolve optical nonlinearities of semiconductors in amplitude and phase. A probe pulse transmitted through the optically pumped sample is superimposed with first subharmonic spectral components derived from the same laser source. This effective ω/2ω pulse pair induces a coherently controlled current in a time-integrating semiconductor detector. Current interferograms obtained by scanning the ω/2ω time delay then reveal the electric field of the 2ω part as well as its pump-induced modifications. As a paradigm we analyze the excitonic optical nonlinearity of a CdTe thin film at frequencies around 385 THz. We then move on to resolve the pump-induced amplitude- and phase-distortions of a probe pulse related to two-photon absorption and cross-phase modulation in ZnSe.

Phase-retrieval of femtosecond pulses utilizing ω/2ω quantum interference control of electrical currents.

We propose and implement a versatile scheme to analyze the phase structure of femtosecond pulses. It relies on second harmonic generation in combination with phase-sensitive χ(3)-current injection driven by two time-delayed portions of the emerging ω/2ω pulse pair. Most strikingly, the group velocity dispersions of both the ω and 2ω components can be unambiguously determined from a simple Fourier transformation of the resulting current interferogram. We test the concept for 45 fs pulses at 1.45 μm and directly compare it to second harmonic frequency resolved optical gating. By choosing appropriate frequency doublers and semiconductor detectors, the scheme is applicable to many wavelength regimes and to pulses as short as a few optical cycles.