Content uploaded by Michael S. Shur
Author content
All content in this area was uploaded by Michael S. Shur on Aug 25, 2015
Content may be subject to copyright.
A preview of the PDF is not available
Negative terahertz conductivity in remotely doped graphene bilayer heterostructures
Abstract and Figures
Injection or optical generation of electrons and holes in graphene bilayers
(GBLs) can result in the interband population inversion enabling the terahertz
(THz) radiation lasing. The intraband radiative processes compete with the
interband transitions. We demonstrate that remote doping enhances the indirect
interband generation of photons in the proposed GBL heterostructures. Therefore
such remote doping helps surpassing the intraband (Drude) absorption and
results in large absolute values of the negative dynamic THz conductivity in a
wide range of frequencies at elevated (including room) temperatures. The
remotely doped GBL heterostructure THz lasers are expected to achieve higher
THz gain compared to previously proposed GBL-based THz lasers.
Figures - uploaded by Michael S. Shur
Author content
All figure content in this area was uploaded by Michael S. Shur
Content may be subject to copyright.
... They have suggested that the remotely doped BLG heterostructure THz lasers can achieve higher THz gain and are of considerable interest for applications as the active medium in the THz lasers. 51 Still, the complete numerical calculations of phonon emission and phonon intensity in BLG remain untouched in which the chiral nature of carriers in BLG is accounted for. This warrants an investigation into the phonon emission process in the BLG system as the requirements for the phonon emission holds good. ...
We present a theoretical investigation on the generation of Cerenkov emission of terahertz acoustic phonons in bilayer graphene (BLG) in the presence of a driving dc electric field. We have numerically and analytically studied the Cerenkov phonon emission spectrum, [Formula: see text], and phonon intensity, [Formula: see text], dependence on the phonon frequency [Formula: see text], drift velocity [Formula: see text], electron temperature [Formula: see text], concentration n, and phonon emission angle [Formula: see text] in BLG with and without considering the chirality of the charge carriers. We find that the magnitude of [Formula: see text] increases at larger drift velocities and applied electric fields with the peak of the spectrum shifting toward the higher frequency side. The spectrum magnitude in BLG is found to be much enhanced as compared to conventional 2D semiconductors and transition metal dichalcogenides, which makes it viable for SASER and other practical device applications. The chiral nature of carriers strongly influences the [Formula: see text] behavior and sharpens the spectrum peak but with a decrease in the magnitude. The chirality favors the negative emission spectrum caused by the absorption of acoustic phonons. [Formula: see text] and [Formula: see text] are found to be strongly dependent on temperature but independent of carrier concentration in the equipartition regime. The study is significant from the point of application of BLG as an acousto/optoelectronic device and high-frequency phonon spectrometers.
... [32][33][34][35] Experimental and theoretical studies with individual graphene layers and graphene meta-surfaces suggest that graphene interacts efficiently with EM radiation in the THz range. [36][37][38][39][40] Available data suggest that graphene particularly absorbs radiation well in the high GHz frequency range rather than reflecting it back to the surroundings. 33 Further work is needed to understand EM characteristics of such composites at different loading of graphene and assess their application potential. ...
The interaction of terahertz electromagnetic radiation with dilute graphene-epoxy composites was studied experimentally at frequencies from 0.25 to 4 THz. Composites with low graphene loading (≤1.2 wt. %) below the electrical percolation threshold revealed the total shielding effectiveness above ∼70 dB (1 mm thickness) at 1.6 THz frequency. The unexpected high shielding effectiveness of dilute graphene composites in blocking terahertz radiation was mostly achieved by absorption rather than reflection. The shielding effectiveness increases with increasing frequency. Our results suggest that even the thin-film or spray coatings of the lightweight, electrically insulating graphene composites with thicknesses in the few-hundred-micrometer range can be sufficient for blocking terahertz radiation in many practical applications.
... [32][33][34][35] Experimental and theoretical studies with individual graphene layers and graphene meta-surfaces suggest that graphene interacts efficiently with EM radiation in the THz range. [36][37][38][39][40] Available data suggest that graphene particularly absorbs radiation well in the high GHz frequency range rather than reflecting it back to the surroundings. 33 Further work is needed to understand EM characteristics of such composites at different loading of graphene and assess their application potential. ...
We demonstrate that polymer composites with a low loading of graphene, below 1.2 wt. %, are efficient as electromagnetic absorbers in the THz frequency range. The epoxy-based graphene composites were tested at frequencies from 0.25 THz to 4 THz, revealing total shielding effectiveness of 85 dB (1 mm thickness) with graphene loading of 1.2 wt. % at the frequency f=1.6 THz. The THz radiation is mostly blocked by absorption rather than reflection. The efficiency of the THz radiation shielding by the lightweight, electrically insulating composites, increases with increasing frequency. Our results suggest that even the thin-film or spray coatings of graphene composites with thickness in the few-hundred-micrometer range can be sufficient for blocking THz radiation in many practical applications.
... It has been shown that the transmittance and the reflectance of the FLG can be adjusted by changing the number of the layers and the intensity of the OP. Ryzhii et al. [17] theoretically studied a bilayer graphene (BLG), in the frequency range from 1 to 10 THz. It has been shown that the remote doping enhances the indirect interband generation of photons in the BLG. ...
Terahertz time-domain spectroscopic polarimetry (THz-TDSP) method was used to study polarization properties of a few-layer graphene (FLG) on a silicon (Si) substrate in terahertz (THz) frequency range under an external optical pumping and an external static magnetic field. Frequency dependencies of azimuth and ellipticity angles of a polarization ellipse and the polarization ellipse at various frequencies of the electromagnetic waves transmitted through the Si substrate and the FLG on the Si substrate were obtained experimentally and theoretically. The results confirm the fact that, based on the FLG, it is possible to devise efficient tunable THz polarization modulators for use in the latest security and telecommunication systems.
... Comparing these reports, the explanations for the mechanism involved in negative photoconductivity have yet to reach consensus despite similar experimental phenomena being observed. Therefore, further experiments are required in order to achieve deeper insights into the [39,40]. These approaches could increase the indirect interband radiative transitions and scattering of carriers, resulting in population inversion and negative contribution to the conductivity. ...
The remarkable specialties of 2D layered materials like the wide spectral coverage, high strength and great flexibility endow ultrathin 2D layered materials the potential to meet the criteria of next generation optoelectronic devices. Photoconductivity is one of the critical parameters of materials applied to optoelectronics. Different to the traditional semiconductors, specific ultrathin 2D layers present anomalous negative photoconductivity. This opens a new avenue for designing novel optoelectronic devices. Deep understanding of the fundamentals in this anomalous response is important for design and optimization of devices. In this review, we provide an overview into the observation of negative photoconductivity in 2D layered materials including graphene, topological insulators and transitional mental dichalcogenides. We also summarize the recent reports about the investigations of the fundamental mechanism using ultrafast terahertz spectroscopies. Finally, we conclude the review by discussing the existing challenges and proposing the possible prospects of this research direction.
The gapless energy band spectra make the structures based on graphene and
graphene bilayers with the population inversion created by optical or injection
pumping to be promising media for the interband terahertz (THz) lasing.
However, a strong intraband absorption at THz frequencies still poses a
challenge for efficient THz lasing. In this paper, we show that in the pumped
graphene bilayer structures, the indirect interband radiative transitions
accompanied by scattering of carriers caused by disorder can provide a
substantial negative contribution to the THz conductivity (together with the
direct interband transitions).
In the graphene bilayer structures on high-$\kappa$ substrates with point
charged defects, these transitions almost fully compensate the losses due to
the intraband (Drude) absorption. We also demonstrate that the indirect
interband contribution to the THz conductivity in a graphene bilayer with the
extended defects (such as the charged impurity clusters, surface corrugation,
and nanoholes) can surpass by several times the fundamental limit associated
with the direct interband transitions and the Drude conductivity. These
predictions can affect the strategy of the graphene-based THz laser
implementation.
The recent demonstration of saturable absorption and negative optical
conductivity in the Terahertz range in graphene has opened up new opportunities
for optoelectronic applications based on this and other low dimensional
materials. Recently, population inversion across the Dirac point has been
observed directly by time- and angle-resolved photoemission spectroscopy
(tr-ARPES), revealing a relaxation time of only ~ 130 femtoseconds. This
severely limits the applicability of single layer graphene to, for example,
Terahertz light amplification. Here we use tr-ARPES to demonstrate long-lived
population inversion in bilayer graphene. The effect is attributed to the small
band gap found in this compound. We propose a microscopic model for these
observations and speculate that an enhancement of both the pump photon energy
and the pump fluence may further increase this lifetime.
We theoretically reveal a new mechanism of light amplification in graphene
under the conditions of interband population inversion. It is enabled by the
indirect interband transitions, with the photon emission preceded or followed
by the scattering on disorder. The emerging contribution to the optical
conductivity, which we call the interband Drude conductivity, appears to be
negative for the photon energies below the double quasi-Fermi energy of pumped
electrons and holes. We find that for the Gaussian correlated distribution of
scattering centers, the real part of the net Drude conductivity (interband plus
intraband) can be negative in the terahertz and near-infrared frequency ranges,
while the radiation amplification by a single graphene sheet can exceed 2.3%.
Optical pump-terahertz probe differential transmission measurements of
as-prepared single layer graphene (AG)(unintentionally hole doped with Fermi
energy $E_F$ at $\sim$180 meV), nitrogen doping compensated graphene (NDG) with
$E_F$ $\sim$10 meV and thermally annealed doped graphene (TAG) are examined
quantitatively to understand the opposite signs of photo-induced dynamic
terahertz conductivity $\Delta\sigma$. It is negative for AG and TAG but
positive for NDG. We show that the recently proposed mechanism of multiple
generations of secondary hot carriers due to Coulomb interaction of
photoexcited carriers with the existing carriers together with the intraband
scattering can explain the change of photoinduced conductivity sign and its
magnitude. We give a quantitative estimate of $\Delta\sigma$ in terms of
controlling parameters - the Fermi energy $E_F$ and momentum relaxation time
$\tau$. Further, the cooling of photoexcited carriers is analyzed using
super-collision model which involves defect mediated collision of the hot
carriers with the acoustic phonons, thus giving an estimate of the deformation
potential.
We theoretically study absorption by an undoped graphene layer decorated with arrays of small particles. We discuss periodic and random arrays within a common formalism, which predicts a maximum absorption of 50% for suspended graphene in both cases. The limits of weak and strong scatterers are investigated, and an unusual dependence on particle-graphene separation is found and explained in terms of the effective number of contributing evanescent diffraction orders of the array. Our results can be important to boost absorption by single-layer graphene due to its simple setup with potential applications to light harvesting and photodetection based on energy (Förster) rather than charge transfer.
The diverse applications of terahertz radiation and its importance to
fundamental science makes finding ways to generate, manipulate, and detect
terahertz radiation one of the key areas of modern applied physics. One
approach is to utilize carbon nanomaterials, in particular, single-wall carbon
nanotubes and graphene. Their novel optical and electronic properties offer
much promise to the field of terahertz science and technology. This article
describes the past, current, and future of the terahertz science and technology
of carbon nanotubes and graphene. We will review fundamental studies such as
terahertz dynamic conductivity, terahertz nonlinearities, and ultrafast carrier
dynamics as well as terahertz applications such as terahertz sources,
detectors, modulators, antennas, and polarizers.
Nonlinear carrier relaxation/recombination dynamics and the resultant stimulated terahertz (THz) photon emission with excitation of surface plasmon polaritons (SPPs) in photoexcited monolayer graphene has been experimentally studied using an optical pump/THz probe and an optical probe measurement. We observed the spatial distribution of the THz probe pulse intensities under linear polarization of optical pump and THz probe pulses. It was clearly observed that an intense THz probe pulse was detected only at the area where the incoming THz probe pulse takes a transverse magnetic (TM) mode capable of exciting the SPPs. The observed gain factor is in fair agreement with the theoretical calculations. Experimental results support the occurrence of the gain enhancement by the excitation of SPPs on THz stimulated emission in optically pumped monolayer graphene.
We propose a novel concept of terahertz lasing based on stimulated generation of plasmons in a planar array of graphene resonant micro/nanocavities strongly coupled to terahertz radiation. Due to the strong plasmon confinement and superradiant nature of terahertz emission by the array of plasmonic nanocavities, the amplification of terahertz waves is enhanced by many orders of magnitude at the plasmon resonance frequencies. We show that the lasing regime is ensured by the balance between the plasmon gain and plasmon radiative damping.
Graphene is establishing itself as a new photonic material with huge potential in a variety of applications ranging from transparent electrodes in displays and photovoltaic modules to saturable absorber in mode-locked lasers. Its peculiar bandstructure and electron transport characteristics naturally suggest graphene could also form the basis for a new generation of high-performance devices operating in the terahertz (THz) range of the electromagnetic spectrum. The region between 300 GHz and 10 THz is in fact still characterized by a lack of efficient, compact, solid state photonic components capable of operating well at 300 K. Recent works have already shown very promising results in the development of high-speed modulators as well as of bolometer and plasma-wave detectors. Furthermore, several concepts have been proposed aiming at the realization of lasers and oscillators. This paper will review the latest achievements in graphene-based THz photonics and discuss future perspectives of this rapidly developing research field.
This paper reviews recent advances in spectroscopic study on ultrafast
carrier dynamics and terahertz (THz) stimulated emission in optically
pumped graphene. The gapless and linear energy spectra of electrons and
holes in graphene can lead to nontrivial features such as negative
dynamic conductivity in the THz spectral range, which may lead to the
development of new types of THz lasers. First, the non-equilibrium
carrier relaxation/recombination dynamics is formulated to show how
photoexcited carriers equilibrate their energy and temperature via
carrier-carrier and carrier-phonon scatterings and in what photon
energies and in what time duration the dynamic conductivity can take
negative values as functions of temperature, pumping photon
energy/intensity, and carrier relaxation rates. Second, we conduct
time-domain spectroscopic studies using an optical pump and a terahertz
probe with an optical probe technique at room temperature and show that
graphene sheets amplify an incoming terahertz field. Two different types
of samples are prepared for the measurement; one is an exfoliated
monolayer graphene on SiO2/Si substrate and the other is a
heteroepitaxially grown non-Bernal stacked multilayer graphene on a
3C-SiC/Si epi-wafer.