Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userQuantum ratchet effects induced by terahertz radiation in GaN-based two-dimensional structures
392
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Photogalvanic effects are observed and investigated in wurtzite (0001)-oriented GaN/AlGaN low-dimensional structures excited by terahertz radiation. The structures are shown to represent linear quantum ratchets. Experimental and theoretical analysis exhibits that the observed photocurrents are related to the lack of an inversion center in the GaN-based heterojunctions.
rate research
Read More
We report on a strong nonlinear behavior of the photogalvanics and photoconductivity under excitation of HgTe quantum wells (QWs) by intense terahertz (THz) radiation. The increasing radiation intensity causes an inversion of the sign of the photocurrent and transition to its superlinear dependence on the intensity. The photoconductivity also shows a superlinear raise with the intensity. We show that the observed photoresponse nonlinearities are caused by the band-to-band emph{light} impact ionization under conditions of a photon energy less than the forbidden gap. The signature of this kind of impact ionization is that the angular radiation frequency $omega=2pi f$ is much higher than the reciprocal momentum relaxation time. Thus, the impact ionization takes place solely because of collisions in the presence of a high-frequency electric field. The effect has been measured on narrow HgTe/CdTe QWs of 5.7,nm width; the nonlinearity is detected for linearly and circularly polarized THz radiation with different frequencies ranging from $f=0.6$ to 1.07,THz and intensities up to hundreds of kW/cm$^2$. We demonstrate that the probability of the impact ionization is proportional to the exponential function, $exp(-E_0^2/E^2)$, of the radiation electric field amplitude $E$ and the characteristic field parameter $E_0$. The effect is observable in a wide temperature range from 4.2 to 90,K, with the characteristic field increasing with rising temperature.
Experimental and theoretical studies on ratchet effects in graphene with a lateral superlattice excited by alternating electric fields of terahertz frequency range are presented. A lateral superlatice deposited on top of monolayer graphene is formed either by periodically repeated metal stripes having different widths and spacings or by inter-digitated comb-like dual-grating-gate (DGG) structures. We show that the ratchet photocurrent excited by terahertz radiation and sensitive to the radiation polarization state can be efficiently controlled by the back gate driving the system through the Dirac point as well as by the lateral asymmetry varied by applying unequal voltages to the DGG subgratings. The ratchet photocurrent includes the Seebeck thermoratchet effect as well as the effects of linear and circular ratchets, sensitive to the corresponding polarization of the driving electromagnetic force. The experimental data are analyzed for the electronic and plasmonic ratchets taking into account the calculated potential profile and the near field acting on carriers in graphene. We show that the photocurrent generation is based on a combined action of a spatially periodic in-plane potential and the spatially modulated light due to the near field effects of the light diffraction.
We report on the observation of terahertz (THz) radiation induced band-to-band impact ionization in HgTe quantum well (QW) structures of critical thickness, which are characterized by a nearly linear energy dispersion. The THz electric field drives the carriers initializing electron-hole pair generation. The carrier multiplication is observed for photon energies less than the energy gap under the condition that the product of the radiation angular frequency $omega$ and momentum relaxation time $tau_{text l}$ larger than unity. In this case, the charge carriers acquire high energies solely because of collisions in the presence of a high-frequency electric field. The developed microscopic theory shows that the probability of the light impact ionization is proportional to $exp(-E_0^2/E^2)$, with the radiation electric field amplitude $E$ and the characteristic field parameter $E_0$. As observed in experiment, it exhibits a strong frequency dependence for $omega tau gg 1$ characterized by the characteristic field $E_0$ linearly increasing with the radiation frequency $omega$.
We report on the observation of terahertz radiation induced photoconductivity and of terahertz analog of the microwave-induced resistance oscillations (MIRO) in HgTe-based quantum well (QW) structures of different width. The MIRO-like effect has been detected in QWs of 20 nm thickness with inverted band structure and a rather low mobility of about 3 $times$ 10$^5$ cm$^2$/V s. In a number of other structures with QW widths ranging from 5 to 20 nm and lower mobility we observed an unconventional non-oscillatory photoconductivity signal which changes its sign upon magnetic field increase. This effect was observed in structures characterized by both normal and inverted band ordering, as well as in QWs with critical thickness and linear dispersion. In samples having Hall bar and Corbino geometries an increase of the magnetic field resulted in a single and double change of the sign of the photoresponse, respectively. We show that within the bolometric mechanism of the photoresponse these unusual features imply a non-monotonic behavior of the transport scattering rate, which should decrease (increase) with temperature for magnetic fields below (above) the certain value. This behavior is found to be consistent with the results of dark transport measurements of magnetoresistivity at different sample temperatures. Our experiments demonstrate that photoconductivity is a very sensitive probe of the temperature variations of the transport characteristics, even those that are hardly detectable using standard transport measurements.
Based on a structure consisting of a single graphene layer situated on a periodic dielectric grating, we show theoretically that intense terahertz (THz) radiations can be generated by an electron bunch moving atop the graphene layer. The underlying physics lies in the fact that a moving electron bunch with rather low electron energy ($sim$1 keV) can efficiently excite graphene plasmons (GPs) of THz frequencies with a strong confinement of near-fields. GPs can be further scattered into free space by the grating for those satisfying the phase matching condition. The radiation patterns can be controlled by varying the velocity of the moving electrons. Importantly, the radiation frequencies can be tuned by varying the Fermi level of the graphene layer, offering tunable THz radiations that can cover a wide frequency range. Our results could pave the way toward developing tunable and miniature THz radiation sources based on graphene.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
