Maxim Ryzhii

Negative dynamic conductivity of graphene with optical pumping

We study the dynamic ac conductivity of a nonequilibrium two-dimensional electron-hole system in optically pumped graphene. Considering the contribution of both interband and intraband transitions, we demonstrate that at sufficiently strong pumping the population inversion in graphene can lead to the negative net ac conductivity in the terahertz range of frequencies. This effect might be used in graphene-based coherent sources of terahertz radiation.

Feasibility of terahertz lasing in optically pumped epitaxial multiple graphene layer structures

A multiple graphene layer (MGL) structure with a stack of GLs and a highly conducting bottom GL on SiC substrate pumped by optical radiation is considered as an active region of terahertz and far infrared lasers with external metal mirrors. The dynamic conductivity of the MGL structure is calculated as a function of the signal frequency, the number of GLs, and the optical pumping intensity. The utilization of optically pumped MGL structures might provide the achievement of lasing with the frequencies of about 1 THz at room temperature due to a high efficiency of pumping.

Toward the creation of terahertz graphene injection laser

We study the effect of population inversion associated with the electron and hole injection in graphene p-i-n structures at the room and slightly lower temperatures. It is assumed that the recombination and energy relaxation of electrons and holes are associated primarily with the interband and intraband processes assisted by optical phonons. The dependences of the electron-hole and optical phonon effective temperatures on the applied voltage, the current-voltage characteristics, and the frequency-dependent dynamic conductivity are calculated. In particular, we demonstrate that at low and moderate voltages, the injection can lead to a pronounced cooling of the electron-hole plasma in the device i-section to the temperatures below the lattice temperature. However at higher voltages, the voltage dependences can be ambiguous exhibiting the S-shape. It is shown that the frequency-dependent dynamic conductivity can be negative in the terahertz (THz) range of frequencies at certain values of the applied voltage...

Injection and population inversion in electrically induced p-n junction in graphene with split gates

We study electron and hole injection processes in a forward biased p–n junction electrically induced in a graphene heterostructure with split gates and calculate the ac conductivity associated with the interband and intraband transitions under the conditions of population inversion. It is shown that the net conductivity can be negative in the terahertz range of frequencies, so that the electrically induced p–n junctions in graphene heterostuctures might be used in sources of coherent terahertz radiation.

Comparison of dark current, responsivity and detectivity in different intersubband infrared photodetectors

This paper deals with the comparison of quantum well, quantum wire and quantum dot infrared photodetectors (QWIPs, QRIPs and QDIPs, respectively) based on physical analysis of the factors determining their operation. The operation of the devices under consideration is associated with the intersubband (intraband) electron transitions from the bound states in QWs, QRs and QDs into the continuum states owing to the absorption of infrared radiation. The redistribution of the electric potential across the device active region caused by the photoionization of QWs, QRs and QDs affects the electron injection from the emitting contact. The injection current provides the effect of current gain. Since the electron thermoemission and capture substantially determine the electric potential distribution and, therefore, the injection current, these processes are also crucial for the device performance. To compare the dark current, responsivity and detectivity of QWIPs, QRIPs and QDIPs we use simplified but rather general semi-phenomenological formulae which relate these device characteristics to the rates of the thermoemission and photoemission of electrons from and their capture to the QWs and the QR and QD arrays. These rates are expressed via the photoemission cross-section, capture probability and so on, and the structural parameters. Calculating the ratios of the QWIP, QRIP and QDIP characteristics using our semi-phenomenological model, we show that: the responsivity of QRIPs and QDIPs can be much higher than the responsivity of QWIPs, however, higher responsivity is inevitably accompanied by higher dark current; the detectivity of QRIPs and QDIPs with low-density arrays of relatively large QRs and QDs is lower than that of QWIPs; the detectivity of QRIPs and QDIPs based on dense arrays can significantly exceed the detectivity of QWIPs.

Double graphene-layer plasma resonances terahertz detector

We propose a detector of terahertz radiation based on a double graphene-layer heterostructure utilizing the tunnelling between graphene layers and the resonant excitation of plasma oscillations (standing plasma waves). Using the developed device model, we substantiate the detector operation and calculate the spectral characteristics. It is shown that the detector responsivity exhibits the resonant peaks when the frequency of incoming terahertz radiation approaches the resonant plasma frequencies. These frequencies are tuned by the bias voltage. The height of the responsivity resonant peaks in sufficiently perfect double graphene-layer heterostructures can markedly exceed those in the resonant plasma–wave detectors based on the standard heterostructures and utilizing the plasma hydrodynamic nonlinearity.

Graphene bilayer field-effect phototransistor for terahertz and infrared detection

A graphene bilayer phototransistor (GBL-PT) is proposed and analyzed. The GBL-PT under consideration has the structure of a field-effect transistor with a GBL as the channel sandwiched between the back and the top gates. The positive bias of the back gate creates the conducting source and drain sections in the channel, while the negatively biased top gate provides the potential barrier which is controlled by the charge of the photogenerated holes. The features of the GBL-PT operation are associated with the variation in both the potential distribution and the energy gap in different sections of the channel when the gate voltages change. Using the developed GBL-PT device model, the spectral characteristics, dark current, responsivity, photoelectric gain, and detectivity are calculated as functions of the applied voltages, energy of incident photons, intensity of electron and hole scattering, and geometrical parameters. It is shown that the GBL-PT spectral characteristics are voltage tuned. The GBL-PT performance as a photodetector in the terahertz and infrared regions of the spectrum can markedly exceed the performance of other photodetectors.

Device Model for Graphene Nanoribbon Phototransistor

An analytical device model for a graphene nanoribbon phototransistor (GNR-PT) is presented. GNR-PT is based on an array of graphene nanoribbons with the side source and drain contacts, which is sandwiched between the highly conducting substrate and the top gate. Using the developed model, we derive the explicit analytical relationships for the source–drain current as a function of the intensity and frequency of the incident radiation and find the detector responsivity. It is shown that GNR-PTs can be rather effective photodetectors in infrared and terahertz ranges of spectrum.

Characteristics of a terahertz photomixer based on a high-electron mobility transistor structure with optical input through the ungated regions

We develop a device model for a terahertz photomixer that utilizes the excitation of plasma oscillations in the channel of a device similar to a high-electron mobility transistor (HEMT). The device design assumes vertical optical input through the ungated source–gate and gate–drain regions. Using this model, we calculate the characteristics of the HEMT photomixer: the responsivity as a function of the signal frequency for devices with different geometrical and physical parameters, and the dependence of resonant frequency on the length of the gated and ungated portions of the channel and the gate voltage. We compare also the performance of the HEMT photomixer with that of a similar device but one in which the optical input is through the substrate.

Terahertz and infrared photodetection using p-i-n multiple-graphene-layer structures

We propose to utilize multiple-graphene-layer structures with lateral p-i-n junctions for terahertz and infrared (IR) photodetection and substantiate the operation of photodetectors based on these structures. Using the developed device model, we calculate the detector dc responsivity and detectivity as functions of the number of graphene layers and geometrical parameters and show that the dc responsivity and detectivity can be fairly large, particularly, at the lower end of the terahertz range at room temperatures. Due to relatively high quantum efficiency and low thermogeneration rate, the photodetectors under consideration can substantially surpass other terahertz and IR detectors. Calculations of the detector responsivity as a function of modulation frequency of THz and IR radiation demonstrate that the proposed photodetectors are very fast and can operate at the modulation frequency of several tens of gigahertz.