Georgy Fedorov

Role of elastic scattering mechanisms in GaInAs/AlInAs quantum cascade lasers

Electron scattering spectroscopy has been performed on a GaInAs∕AlInAs midinfrared quantum cascade laser by applying a strong magnetic field along its growth axis. The interpretation of the experimental data, supported by our model of the electron lifetime in the presence of a strong magnetic field, very clearly demonstrates that the elastic contribution to the total scattering rate has a weight comparable to that of the optical phonons. The authors believe that this contribution has its origin in the alloy disorder, an efficient mechanism in this material system, which limits the lifetime of the excited subband of the laser transition.

Intervalley carrier transfer in short-wavelength InP-based quantum-cascade laser

The scattering of electrons out of the upper laser state into indirect valleys in quantum-cascade lasers is demonstrated by investigating the operation of the laser under the influence of magnetic fields up to 45 T. A quantum-cascade laser based on strain-compensated AlAs barriers and In0.73Ga0.27As/InAs wells, emitting with wavelength 3.1 μm, is investigated as a function of magnetic field normal to the surface. Minima in emission power are observed when Landau levels of the upper laser state are brought into resonance with states derived from the indirect valleys, leading to the partial depopulation of the upper laser level. The energy for the indirect valley states is determined to be about 640 meV above the bottom of the In0.73Ga0.27As Γ valley, about 70 meV above the upper laser level.

Electronic transport and possible superconductivity at van Hove singularities in carbon nanotubes

Van Hove singularities (VHSs) are a hallmark of reduced dimensionality, leading to a divergent density of states in one and two dimensions and predictions of new electronic properties when the Fermi energy is close to these divergences. In carbon nanotubes, VHSs mark the onset of new subbands. They are elusive in standard electronic transport characterization measurements because they do not typically appear as notable features and therefore their effect on the nanotube conductance is largely unexplored. Here we report conductance measurements of carbon nanotubes where VHSs are clearly revealed by interference patterns of the electronic wave functions, showing both a sharp increase of quantum capacitance, and a sharp reduction of energy level spacing, consistent with an upsurge of density of states. At VHSs, we also measure an anomalous increase of conductance below a temperature of about 30 K. We argue that this transport feature is consistent with the formation of Cooper pairs in the nanotube.

Probing the population inversion in intersubband laser by magnetic field spectroscopy

The magnetic field dependence of the midinfrared quantum-cascade laser emission spectra is used to identify the particular Wannier-Stark states responsible for the laser action in two different laser designs. The active regions in both quantum-cascade lasers are based on a modified bound-to-continuum design, but have differing degrees of coupling between the injector miniband and the bound state. The effects of the magnetic field and the injection-barrier width on the emission wavelength indicate that the laser emission in the quantum-cascade laser with less coupling between the injector and the bound state originates from a transition between the injector and extractor minibands. The transition from injector miniband to extractor miniband has both a lower energy and a lower oscillator strength than the transition originating from the bound state, but dominates because of the low population of the upper bound state. This result has important implications for further miniband engineering of quantum-cascade...

Response of asymmetric carbon nanotube network devices to sub-terahertz and terahertz radiation

Demand for efficient terahertz radiation detectors resulted in intensive study of the asymmetric carbon nanostructures as a possible solution for that problem. It was maintained that photothermoelectric effect under certain conditions results in strong response of such devices to terahertz radiation even at room temperature. In this work, we investigate different mechanisms underlying the response of asymmetric carbon nanotube (CNT) based devices to sub-terahertz and terahertz radiation. Our structures are formed with CNT networks instead of individual CNTs so that effects probed are more generic and not caused by peculiarities of an individual nanoscale object. We conclude that the DC voltage response observed in our structures is not only thermal in origin. So called diode-type response caused by asymmetry of the device IV characteristic turns out to be dominant at room temperature. Quantitative analysis provides further routes for the optimization of the device configuration, which may result in appear...

Tuning the band gap of semiconducting carbon nanotube by an axial magnetic field

We have investigated the magnetic field dependence of transfer characteristics of a device fabricated in a configuration of a field-effect transistor with a conduction channel formed by a semiconducting multiwalled carbon nanotube. Our results unambiguously indicate that an axial magnetic field suppresses the band gap of the nanotube. Quantitative analysis of the data indicates linear dependence of the band gap on magnetic field as well as a linear splitting between the K and K′ subbands of the band structure of the nanotube.

Two-dimensional plasmons in lateral carbon nanotube network structures and their effect on the terahertz radiation detection

We consider the carrier transport and plasmonic phenomena in the lateral carbon nanotube (CNT) networks forming the device channel with asymmetric electrodes. One electrode is the Ohmic contact to the CNT network and the other contact is the Schottky contact. These structures can serve as detectors of the terahertz (THz) radiation. We develop the device model for collective response of the lateral CNT networks which comprise a mixture of randomly oriented semiconductor CNTs (s-CNTs) and quasi-metal CNTs (m-CNTs). The proposed model includes the concept of the collective two-dimensional (2D) plasmons in relatively dense networks of randomly oriented CNTs (CNT “felt”) and predicts the detector responsivity spectral characteristics exhibiting sharp resonant peaks at the signal frequencies corresponding to the 2D plasmonic resonances. The detection mechanism is the rectification of the ac current due the nonlinearity of the Schottky contact current-voltage characteristics under the conditions of a strong enhancement of the potential drop at this contact associated with the plasmon excitation. The detector responsivity depends on the fractions of the s- and m-CNTs. The burning of the near-contact regions of the m-CNTs or destruction of these CNTs leads to a marked increase in the responsivity in agreement with our experimental data. The resonant THz detectors with sufficiently dense lateral CNT networks can compete and surpass other THz detectors using plasmonic effects at room temperatures.

Radiative quantum efficiency in an InAs/AlSb intersubband transition

The quantum efficiency of an electroluminescent intersubband emitter based on

Dual origin of room temperature sub-terahertz photoresponse in graphene field effect transistors

\mathrm{In}\mathrm{As}∕\mathrm{Al}\mathrm{Sb}

Graphene-based lateral Schottky diodes for detecting terahertz radiation

has been measured as a function of the magnetic field up to