Nikolai G. Kalugin

Measurement of Filling-Factor-Dependent Magnetophonon Resonances in Graphene Using Raman Spectroscopy

We perform polarization-resolved Raman spectroscopy on graphene in magnetic fields up to 45 T. This reveals a filling-factor-dependent, multicomponent anticrossing structure of the Raman G peak, resulting from magnetophonon resonances between magnetoexcitons and E(2g) phonons. This is explained with a model of Raman scattering taking into account the effects of spatially inhomogeneous carrier densities and strain. Random fluctuations of strain-induced pseudomagnetic fields lead to increased scattering intensity inside the anticrossing gap, consistent with the experiments.

Magnetophonon resonance in graphite: High-field Raman measurements and electron-phonon coupling contributions

We perform Raman scattering experiments on natural graphite in magnetic fields up to 45 T, observing a series of peaks due to interband electronic excitations over a much broader magnetic field range than previously reported. We also explore electron-phonon coupling in graphite via magnetophonon resonances. The Raman G peak shifts and splits as a function of magnetic field, due to the magnetically tuned coupling of the E2g optical phonons with the K -a ndH-point inter-Landau-level excitations. The analysis of the observed anticrossing behavior allows us to determine the electron-phonon coupling for both K -a ndH-point carriers. In the highest field range (>35 T) the G peak narrows due to suppression of electron-phonon interaction.

Different components of far-infrared photoresponse of quantum Hall detectors

We have performed time-resolved measurements of the far-infrared photoresponse of two-dimensional electron systems in the quantum Hall regime. The photoresponse consists of two equally important components: the longitudinal component, caused by the photoinduced change of the longitudinal resistance Rxx, and the transversal component, caused by the photoinduced Hall currents and by the photoinduced change of Rxy. Both these components are connected with two mechanisms of the photoresponse: a nonresonant bolometric, and a cyclotron-resonant contribution.

Oxidation of ultrathin GaSe

Oxidation of exfoliated gallium selenide (GaSe) is investigated through Raman, photoluminescence, Auger, and X-ray photoelectron spectroscopies. Photoluminescence and Raman intensity reductions associated with spectral features of GaSe are shown to coincide with the emergence of signatures emanating from the by-products of the oxidation reaction, namely, Ga2Se3 and amorphous Se. Photoinduced oxidation is initiated over a portion of a flake highlighting the potential for laser based patterning of two-dimensional heterostructures via selective oxidation.

Graphene-based quantum Hall effect infrared photodetector operating at liquid Nitrogen temperatures

We demonstrate a quantum Hall effect (QHE) graphene-based infrared photodetector that functions at 70K—a temperature achievable using liquid Nitrogen in vapor evacuation regime. This result opens up the possibility of wider use of QHE photodetectors if graphene is utilized instead of “classical” materials with two-dimensional electron gas. The potential advantages to this approach are increased operating temperature, reduced magnetic field, and wider range of operating frequency compared to previous implementations of QHE detectors.

Concentration dependence of femtosecond coherent anti-Stokes Raman scattering in the presence of strong absorption

We study near-resonant femtosecond coherent anti-Stokes Raman scattering (CARS) of dipicolinic acid (DPA) by exciting the molecular system with a pair of visible pump and Stokes pulses and probing the resultant molecular coherence with a time-delayed UV probe pulse. We record the generated Stokes and anti-Stokes pulse energies as functions of DPA concentration. We observe that the CARS signal has a maximum and the power-law dependence is steeper than the well-known quadratic one. We present a model that describes the propagation of the generated signal through the medium. From this model, we derive an analytical expression that closely agrees with our experimental data. Since DPA serves as a marker molecule for bacterial spores, our results help to establish the detectability limits for a lethal spore dosage when the present technique is applied.

Efficient generation of short terahertz pulses via stimulated Raman adiabatic passage

We have analyzed the efficiency of coherent scattering of infrared radiation in molecular gases for the production of intense, short terahertz (THz) pulses by using stimulated Raman adiabatic passage for the preparation of coherence. We show that coherently driven molecular media potentially yield strong, controllable, short pulses of THz radiation. The pulses have energies ranging from several nanojoules to microjoules and time durations from several femtoseconds to nanoseconds at room temperature.

Fast terahertz detectors with spectral tunability based on quantum Hall Corbino devices

We present THz photoconductivity measurements on Corbino-shaped GaAs∕AlGaAs heterostructures. The THz source is a pulsed p-Ge laser, which provides photon frequencies of 1.7THzto2.5THz (corresponding to wavelengths of 180–120μm). We investigate the relaxation process from the dissipative state to the quantum Hall state time-resolved and find that the relaxation time depends on the applied voltage and on the mobility of the sample. Relaxation times of approximately 10ns to over 200ns are observed. A simple picture is suggested to explain the results. In addition, spectrally resolved measurements are discussed. The short response time and the useful spectral selectivity together with the high sensitivity make QH devices promising for high-performance THz detectors.We present THz photoconductivity measurements on Corbino-shaped GaAs∕AlGaAs heterostructures. The THz source is a pulsed p-Ge laser, which provides photon frequencies of 1.7THzto2.5THz (corresponding to wavelengths of 180–120μm). We investigate the relaxation process from the dissipative state to the quantum Hall state time-resolved and find that the relaxation time depends on the applied voltage and on the mobility of the sample. Relaxation times of approximately 10ns to over 200ns are observed. A simple picture is suggested to explain the results. In addition, spectrally resolved measurements are discussed. The short response time and the useful spectral selectivity together with the high sensitivity make QH devices promising for high-performance THz detectors.

Gate-tunable THz detector based on a quantum Hall device

Recent investigations showed that the THz photoresponse of quantum Hall systems (QHS) is a complex combination of different mechanisms. In this work, we separate these parts by varying the electron concentration and the source–drain current. For our photoconductivity measurements, we used a pulsed p-Ge cyclotron laser as a monochromatic THz radiation source and changed the magnetic field, the source–drain current and the electron concentration (via a backgate) of the QH sample. The spectral resolution of the QHS was found to improve with increasing values of the source–drain current up to the breakdown of the QH effect.

Effects of electron-electron interactions on the electronic Raman scattering of graphite in high magnetic fields

We report the observation of strongly temperature (T)-dependent spectral lines in electronic Raman-scattering spectra of graphite in a high magnetic field up to 45 T applied along the c axis. The magnetic field quantizes the in-plane motion, while the out-of-plane motion remains free, effectively reducing the system dimension from 3 to 1. Optically created electron-hole pairs interact with, or shake up, the one-dimensional Fermi sea in the lowest Landau subbands. Based on the Tomonaga-Luttinger liquid theory, we show that interaction effects modify the spectral line shape from (ω-Δ)-1/2 to (ω-Δ)2α-1/2 at T = 0. At finite T, we predict a thermal broadening factor that increases linearly with T. Our model reproduces the observed T-dependent line shape, determining the electron-electron interaction parameter α to be ∼0.05 at 40 T.