Interband Absorption Research Articles

Ultrafast two-dimensional THz spectroscopy of graphene

With two-dimensional THz spectroscopy the dynamics of low-energy carriers in graphene is determined. Both intra- and interband absorption contribute to the observed ultrafast pump-probe signals.

Optical properties of Bi2Te2Se at ambient and high pressures

The temperature dependence of the complex optical properties of the three-dimensional topological insulator Bi2Te2Se is reported for light polarized in the a-b planes at ambient pressure, as well as the effects of pressure at room temperature. This material displays a semiconducting character with a bulk optical gap of 300 meV at 295 K. In addition to the two expected infrared-active vibrations observed in the planes, there is additional fine structure that is attributed to either the removal of degeneracy or the activation of Raman modes due to disorder. A strong impurity band located at 200 cm^{-1} is also observed. At and just above the optical gap, several interband absorptions are found to show a strong temperature and pressure dependence. As the temperature is lowered these features increase in strength and harden. The application of pressure leads to a very abrupt closing of the gap above 8 GPa, and strongly modifies the interband absorptions in the mid-infrared spectral range. While ab initio calculations fail to predict the collapse of the gap, they do successfully describe the size of the band gap at ambient pressure, and the magnitude and shape of the optical conductivity.

Interplay of intra- and interband absorption in a disordered graphene

The absorption of heavily doped graphene in the terahertz (THz) and mid-infrared (MIR) spectral regions is considered taking into account both the elastic scattering due to finite-range disorder and the variations of concentration due to long-range disorder. Interplay between intra- and interband transitions is analyzed for the high-frequency regime of response, near the Pauli blocking threshold. The gate voltage and temperature dependencies of the absorption efficiency are calculated. It is demonstrated that for typical parameters, the smearing of the interband absorption edge is determined by a unscreened part of long-range disorder while the intraband absorption is determined by finite-range scattering. The latter yields the spectral dependencies which deviate from those following from the Drude formula. The obtained dependencies are in good agreement with recent experimental results. The comparison of the results of our calculations with the experimental data provides a possibility to extract the disorder characteristics.

Band gap and type of optical transitions at the interband absorption edge in solid solutions based on bismuth telluride

The spectra of optical absorption in multicomponent n- and p-type solid solutions based on bismuth and antimony chalcogenides with substitutions in both sublattices of Bi2Te3 have been investigated. It has been found that, in all the solid solutions studied, just as in the parent compound Bi2Te3, direct allowed transitions occur at the interband absorption edge at T = 300 K. The band gap Eg in the n-Bi2 − xSbxTe3 − y − zSeySz solid solutions weakly increases with increasing number of substituted atoms in the Bi and Te sublattices. These atomic substitutions do not leads to an increase in Eg as compared to that of the n-Bi2Te2.7Se0.3 composition. An analysis of the optical absorption spectra suggests that the solid solutions under consideration are weakly degenerate, a conclusion supported by the earlier studies of the thermoelectric and galvanomagnetic properties. It has been established that, in the conduction band of the Bi1.8Sb0.2Te2.7Se0.15S0.15 solid solution, there is an additional extremum lying above the main extremum at a distance no more than 0.1 eV.

ChemInform Abstract: Optoelectronic Properties of Single‐Wall Carbon Nanotubes

AbstractReview: 288 refs.

Photoinduced currents in graphene and carbon nanotubes

AbstractThe paper overviews experimental and theoretical studies of photogalvanic effects (PGEs) induced in graphene and carbon nanotubes (CNs) by polarized electromagnetic radiation. The following items are discussed: the helicity‐dependent photon‐drag and edge electric currents in graphene sheets, the circular PGEs in asymmetric graphene superlattices and chiral CNs, and pure valley currents generated in graphene by the interband absorption of linearly polarized light.

Evaluation of electronic transport properties and conduction band offsets of asymmetric InAs/InxGa1−x As/GaAs dot-in-well structures

The charge carrier distribution profile, carrier dynamics, barrier height and conduction band offset for asymmetric InAs/InxGa1−x As/GaAs dot-in-well (DWELL) structures are evaluated from temperature-dependent electronic transport and spectroscopic measurements. The charge carrier leakages via defects lead to less charge carrier accumulation in the DWELL architecture in comparison with the associated QWs. Furthermore, the differential capacitance–voltage measurements show the resonance of charge carrier escape rate with the ground and excited states of the DWELL structure. The peak of the charge carrier escape rate related to the excited state is almost independent of the temperature while that of the ground state changes significantly with heating. It is explained on the basis of changes in the accumulated carrier density due to carrier localization and nonradiative recombination processes, via defects/dislocations/Auger recombination, which are dominant at moderate and high temperatures, respectively. Furthermore, the obtained values of barrier height and conduction band offsets for asymmetric InAs/InxGa1−x As/GaAs (x = 0.18) DWELL structures are 194 ± 10 meV and 287 ± 10 meV, respectively. Downward transitions from both ground and excited states of quantum dots (QDs) are observed in photoluminescence while upward transitions only to the excited state energy levels of QDs are observed in photoreflectance spectroscopy. This is due to the filling of the ground state energy levels of QDs, which is confirmed by the electron transport measurements. Hence, it is concluded that interband absorption occurs to the excited states, whereas intersubband absorption predominantly takes place from the ground states to the continuum of the QD ensemble.

Optoelectronic Properties of Single‐Wall Carbon Nanotubes

Single-wall carbon nanotubes (SWCNTs), with their uniquely simple crystal structures and chirality-dependent electronic and vibrational states, provide an ideal laboratory for the exploration of novel 1D physics, as well as quantum engineered architectures for applications in optoelectronics. This article provides an overview of recent progress in optical studies of SWCNTs. In particular, recent progress in post-growth separation methods allows different species of SWCNTs to be sorted out in bulk quantities according to their diameters, chiralities, and electronic types, enabling studies of (n,m)-dependent properties using standard macroscopic characterization measurements. Here, a review is presented of recent optical studies of samples enriched in 'armchair' (n = m) species, which are truly metallic nanotubes but show excitonic interband absorption. Furthermore, it is shown that intense ultrashort optical pulses can induce ultrafast bandgap oscillations in SWCNTs, via the generation of coherent phonons, which in turn modulate the transmission of a delayed probe pulse. Combined with pulse-shaping techniques, coherent phonon spectroscopy provides a powerful method for studying exciton-phonon coupling in SWCNTs in a chirality-selective manner. Finally, some of the basic properties of highly aligned SWCNT films are highlighted, which are particularly well-suited for optoelectronic applications including terahertz polarizers with nearly perfect extinction ratios and broadband photodetectors.

Dynamical conductivity of AA-stacked bilayer graphene

We calculate the dynamical conductivity of AA-stacked bilayer graphene as a function of frequency and in the presence of a finite chemical potential due to charging. Unlike the monolayer, we find a Drude absorption at charge neutrality in addition to an interband absorption with onset of twice the interlayer hopping energy. At finite doping, the interband absorption exhibits two edges which depend on both chemical potential and interlayer hopping energy. We study the behaviour as a function of varying chemical potential relative to the interlayer hopping energy scale and compute the partial optical sum. The results are contrasted with the previously published case of AB-stacking. While we focus on in-plane conductivity, we also provide the perpendicular conductivity for both AB and AA stacking. We also examine conductivity for other variations with AA-stacking, such as AAA-stacked trilayer. Based on proposed models for topological insulators discussed in the literature, we also consider the effect of spin orbit coupling on the optical properties of an AA-stacked bilayer which illustrates the effect of an energy gap opening at points in the band structure.

A quantum pseudodot system with two-dimensional pseudoharmonic oscillator in external magnetic and Aharonov-Bohm fields

Using the Nikiforov-Uvarov (NU) method, the energy levels and the wave functions of an electron confined in a two-dimensional (2D) pseudoharmonic quantum dot are calculated under the influence of temperature and an external magnetic field inside dot and Aharonov-Bohm (AB) field inside a pseudodot. magnetic field and geometrical size of quantum pseudodot. The temperature dependence energy levels for GaAs semiconductor are also calculated. The exact solutions for energy eigenvalues and wave functions are computed as functions of the chemical potential parameters, applied magnetic field strength, AB flux field, magnetic quantum number and temperature. Analytical expression for the light interband absorption coefficient and absorption threshold frequency are found as functions of applied

Ultrafast and spatially resolved studies of charge carriers in atomically thin molybdenum disulfide

Atomically-thin molybdenum disulfide is emerging as a new nanomaterial with potential applications in the fields of electronic and photonics. Charge carrier dynamics plays an essential role in determining its electronic and optical properties. We report spatially and temporally resolved pump-probe studies of charge carriers in atomically-thin molybdenum disulfide samples fabricated by mechanical exfoliation. Carriers are injected by interband absorption of a 390-nm pump pulse and detected by measuring differential reflection of a time-delayed and spatially-scanned probe pulse that is tuned to an exciton transition. Several parameters on charge carrier dynamics are deduced, including carrier lifetime, diffusion coefficient, diffusion length, and mobility.

Terahertz and Infrared Spectroscopy of Gated Large-Area Graphene

We have fabricated a centimeter-size single-layer graphene device with a gate electrode, which can modulate the transmission of terahertz and infrared waves. Using time-domain terahertz spectroscopy and Fourier-transform infrared spectroscopy in a wide frequency range (10-10 000 cm(-1)), we measured the dynamic conductivity change induced by electrical gating and thermal annealing. Both methods were able to effectively tune the Fermi energy, E(F), which in turn modified the Drude-like intraband absorption in the terahertz as well as the "2E(F) onset" for interband absorption in the mid-infrared. These results not only provide fundamental insight into the electromagnetic response of Dirac fermions in graphene but also demonstrate the key functionalities of large-area graphene devices that are desired for components in terahertz and infrared optoelectronics.

A simple analysis of interband absorption in quantum well structure of III-V ternary and quaternary semiconductors

A simple generalized theory is presented for the determination of interband optical absorption coefficient (IOAC) around band edges, in quantum well (QW) structure of non-parabolic semiconducting materials whose band structures obey the three band model of Kane. The dependence of absorption coefficient on wave-vector (k→) has also been investigated. IOAC has been calculated for a wide range of III–V compound semiconductors, such as InAs, InSb, Hg1−xCdxTe, and In1−xGaxAsyP1−y lattice matched to InP. It has been found that IOAC for QWs increases in steps with increasing photon energy and the positions of jumps of the coefficient are more closely spaced in the three band model of Kane than those with parabolic energy band approximations in all the cases. IOAC for QWs is largely dependent on the polarization direction of the incident light and on the well dimension as well. The expressions of IOAC presented in this article can easily be extended to incorporate the effects of different external conditions like strain, electric, and magnetic fields on IOAC. It has also been shown that under certain limiting conditions the expressions for IOAC reduces to the previously known results for wide band gap materials and this is the indication for it to be a generalized theory for IOAC.

Interband optical absorption in a circular graphene quantum dot

We investigate the energy levels and optical properties of a circular graphene quantum dot in the presence of an external magnetic field perpendicular to the dot. Based on the Dirac–Weyl equation and assuming zero outward current at the edge of the dot we present the results for two different types of boundary conditions, i.e. infinite-mass (IMBC) and zigzag boundary conditions. We found that the dot with zigzag edges displays a zero-energy state in the energy spectra while this is not the case for the IMBCs. For both boundary conditions, the confinement becomes dominated by the magnetic field, where the energy levels converge to the Landau levels as the magnetic field increases. The effect of boundary conditions on the electron- and hole-energy states is found to affect the interband absorption spectra, where we found larger absorption in the case of IMBCs. The selection rules for interband optical transitions are determined and discussed for both boundary conditions.

Optical spectroscopy of graphene: From the far infrared to the ultraviolet

The unique electronic structure of graphene leads to several distinctive optical properties. In this brief review, we outline the current understanding of two general aspects of optical response of graphene: optical absorption and light emission. We show that optical absorption in graphene is dominated by intraband transitions at low photon energies (in the far-infrared spectral range) and by interband transitions at higher energies (from mid-infrared to ultraviolet). We discuss how the intraband and interband transitions in graphene can be modified through electrostatic gating. We describe plasmonic resonances arising from the free-carrier (intraband) response and excitonic effects that are manifested in the interband absorption. Light emission, the reverse process of absorption, is weak in graphene due to the absence of a band gap. We show that photoluminescence from hot electrons can, however, become observable either through femtosecond laser excitation or strong electrostatic gating.

Role of interband and photoinduced absorption in the nonlinear refraction and absorption of resonantly excited PbS quantum dots around 1550 nm

The nonlinear refractive index ${n}_{2}$ and absorption coefficient $\ensuremath{\beta}$ of PbS quantum dots (Qdots) is determined around 1550 nm with the $Z$-scan technique, using a picosecond pulsed laser. We find that ${n}_{2}$ is wavelength tunable and follows the PbS absorbance spectrum. At fixed wavelength, ${n}_{2}$ is constant in the optical intensity range used (1--25 MW/cm${}^{2}$), indicative of a third order nonlinear effect. The figure of merit is larger than 1 in the quoted intensity range, demonstrating that PbS Qdots are efficient nonlinear materials at telecom wavelengths. It is argued that the creation of excitons and the resulting photoinduced absorption in the PbS Qdots lie at the origin of the observed ${n}_{2}$ and $\ensuremath{\beta}$. Analyzing the measured intensity dependence of the absorption at the first energy transition using a rate-equation model with a full bleaching at 4 excitons per Qdot, we find that photoinduced absorption inhibits the full bleaching of the Qdot first energy transition. Femtosecond four-wave mixing (FWM) experiments at low intensities show that the dynamics of the nonlinear optical response shows a nanosecond decay, which is attributed to exciton thermalization, while at higher excitation intensities an additional $\ensuremath{\sim}$100 ps component appears, which reflects carrier-carrier assisted processes. The analysis of the phase of the FWM signal shows that at high excitation the signal is dominated by photoinduced absorption once charge separation occurs. Considering the facile (wet) processing of colloidal Qdots, these results demonstrate the potential of PbS Qdots for low-cost photonic devices.

Effects of hydrostatic pressure and temperature on interband optical transitions in InAs/GaAs vertically coupled double quantum dots

We consider the effect of hydrostatic pressure, temperature and the variations of structure's sizes on interband transition energy and absorption coefficient in InAs/GaAs vertically coupled double quantum dots. The threshold energy of interband optical transitions is examined as a function of hydrostatic pressure and temperature for the different geometries of the structure. We also investigated the dependencies of the interband light absorption coefficient on the incident photon energy.

Direct interband absorption of light in a strongly oblate truncated ellipsoidal quantum dot's ensemble

Within the framework of adiabatic approximation the energy levels and direct interband absorption of light in a strongly oblate truncated ellipsoidal quantum dot's ensemble are studied. Analytical expressions for the particle energy spectrum and absorption threshold frequencies in the regime of strong size quantization are obtained. Selection rules for quantum transitions are revealed. To facilitate the comparison of obtained results with the probable experimental data, size dispersion distribution of growing quantum dots by the minor semiaxe by two experimentally realizing distribution functions have been taken into account. Distribution functions of Lifshits-Slezov and Gaussian have been considered.

LÉVY FLIGHT OF HOLES IN InP SEMICONDUCTOR SCINTILLATOR

High radiative efficiency in moderately doped n- InP results in the transport of holes dominated by photon-assisted hopping, when radiative hole recombination at one spot produces a photon, whose interband absorption generates another hole, possibly far away. Due to "heavy tails" in the hop probability, this is a random walk with divergent diffusivity (process known as the Lévy flight). Our key evidence is derived from the ratio of transmitted and reflected luminescence spectra, measured in samples of different thicknesses. These experiments prove the non-exponential decay of the hole concentration from the initial photo-excitation spot. The power-law decay, characteristic of Lévy flights, is steep enough at short distances (steeper than an exponent) to fit the data for thin samples and slow enough at large distances to account for thick samples. The high radiative efficiency makes possible a semiconductor scintillator with efficient photon collection. It is rather unusual that the material is "opaque" at wavelengths of its own scintillation. Nevertheless, after repeated recycling most photons find their way to one of two photodiodes integrated on both sides of the semiconductor slab. We present an analytical model of photon collection in two-sided slab, which shows that the heavy tails of Lévy-flight transport lead to a high charge collection efficiency and hence high energy resolution. Finally, we discuss a possibility to increase the slab thickness while still quantifying the deposited energy and the interaction position within the slab. The idea is to use a layered semiconductor with photon-assisted collection of holes in narrow-bandgap layers spaced by distances far exceeding diffusion length. Holes collected in these radiative layers emit longwave radiation, to which the entire structure is transparent. Nearly-ideal calculated characteristics of a mm-thick layered scintillator can be scaled up to several centimeters.

TiO2–Au plasmonic nanocomposite for enhanced dye-sensitized solar cell (DSSC) performance

Anatase TiO2 nanoparticles dressed with gold nanoparticles were synthesized by hydrothermal process by using mixed precursor and controlled conditions. Diffused Reflectance Spectra (DRS) reveal that in addition to the expected TiO2 interband absorption below 360nm gold surface plasmon feature occurs near 564nm. It is shown that the dye sensitized solar cells made using TiO2–Au plasmonic nanocomposite yield superior performance with conversion efficiency (CE) of ∼6% (no light harvesting), current density (JSC) of ∼13.2mA/cm2, open circuit voltage (Voc) of ∼0.74V and fill factor (FF) 0.61; considerably better than that with only TiO2 nanoparticles (CE∼5%, JSC∼12.6mA/cm2, Voc∼0.70V, FF∼0.56).