Interband Optical Absorption Research Articles

Phonon-Assisted Optical Absorption in Silicon from First Principles

The phonon-assisted interband optical absorption spectrum of silicon is calculated at the quasiparticle level entirely from first principles. We make use of the Wannier interpolation formalism to determine the quasiparticle energies, as well as the optical transition and electron-phonon coupling matrix elements, on fine grids in the Brillouin zone. The calculated spectrum near the onset of indirect absorption is in very good agreement with experimental measurements for a range of temperatures. Moreover, our method can accurately determine the optical absorption spectrum of silicon in the visible range, an important process for optoelectronic and photovoltaic applications that cannot be addressed with simple models. The computational formalism is quite general and can be used to understand the phonon-assisted absorption processes in general.

Pseudospin in Optical and Transport Properties of Graphene

We show that the pseudospin, being an additional degree of freedom for carriers in graphene, can be efficiently controlled by means of the electron-electron interactions which, in turn, can be manipulated by changing the substrate. In particular, an out-of-plane pseudospin component can occur leading to a zero-field Hall current as well as to polarization-sensitive interband optical absorption.

Electronic structure and optical absorption ofGaAs/AlxGa1−xAsandAlxGa1−xAs/GaAscore-shell nanowires

The electronic structure of $\text{GaAs}/{\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}$ and ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}/\text{GaAs}$ core-shell nanowires grown in the [001] direction is studied. The $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ method with the $6\ifmmode\times\else\texttimes\fi{}6$ Kohn-L\"uttinger Hamiltonian, taking into account the split-off band is used. The variation in the energy level dispersion, the spinor contribution to the ground state and the optical interband absorption are studied. For some range of parameters the top of the valence band exhibits a camelback structure which results in an extra peak in the optical absorption.

Optoelectronic properties of graphene in the presence of optical phonon scattering

We study in detail the optoelectronic properties of graphene. Considering the electron interactions with photons and phonons, we employ the mass- and energy-balance equations to self-consistently evaluate the photoinduced carrier densities, the optical conductance, and the transmission coefficient in the presence of a linearly polarized radiation field. We demonstrate that the photoinduced carrier densities increase around the electron-photon-phonon resonant transition. They depend strongly on the radiation intensity and frequency, temperature, and dark carrier density. For short-wavelength radiation $(\mathcal{L}l3\text{ }\ensuremath{\mu}\text{m})$, we obtain the universal optical conductance ${\ensuremath{\sigma}}_{0}={e}^{2}/(4\ensuremath{\hbar})$. Importantly, there exists an optical-absorption window in the radiation wavelength range $4--100\text{ }\ensuremath{\mu}\text{m}$, which is induced by different transition energies required for interband and intraband optical absorption. The position and width of this window depend sensitively on the temperature and the carrier density of the system. These theoretical results are in line with recent experimental findings and indicate that graphene exhibits important features not only in the visible regime but also in the midinfrared bandwidth.

Template-free Solvothermaly Synthesized ZnSe and ZnSe: Eu3+ Nanoparticles, Structural, Optical and Raman Studies

Lanthanide ions are visible to near-infrared ultra-narrowband emitters with long lifetime thus have potential applications in lasing, up-conversion and in bioimaging. Present work report a general, single step and easy solvothermal method to synthesize well crystalline, pure and Eu3+ doped ZnSe nanocrystals in a unified system. The products were well characterized by powder X-ray diffraction (XRD), UV-visible (UV-vis) spectroscopy, photoluminescence spectroscopy (PL), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy dispersive X-ray diffraction (EDAX) and by Raman spectroscopy. Semiconducting NPs exhibiting cubic phases were obtained at low temperature under mild conditions. The products were found to show significant finite size effect as characterized by broadened XRD peaks, blue shift of the interband optical absorption edge and by Raman scatterings. The structure and properties were correlated and detailed growth mechanism has also been discussed. Keywords: X-ray diffraction, Photoluminescence, Raman scattering, ZnSe, Semiconducting materials

Structural, optical and Raman studies of template-free solvothermally synthesized ZnSe/ZnSe:Ce 3+ nanoparticles

The present work reports a general, single step and easy solvothermal method to synthesize well crystallized and pure phase Ce 3+ doped ZnSe nanocrystals for the first time in a unified system. The products were well characterized by powder X-ray diffraction (XRD), UV–vis spectroscopy (UV–vis), photoluminescence spectroscopy (PL), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy dispersive X-ray diffraction (EDAX) and Raman spectroscopy. The products were found to show significant finite size effect as characterized by broadened XRD peaks, blue-shifts of the interband optical absorption edge and the asymmetric broadening of Raman spectra. The emission intensity of Ce 3+ ion doped ZnSe was found to be considerably increased with respect to the pure one. The observed lineshape of LO modes indicates broadening that is due to the nanosized effect. The structure and properties were correlated and detailed growth mechanism is also discussed.

Atomic‐Layer Graphene as a Saturable Absorber for Ultrafast Pulsed Lasers

AbstractThe optical conductance of monolayer graphene is defined solely by the fine structure constant, α = $e^2 /\hbar c$ (where e is the electron charge, $\hbar $ is Dirac's constant and c is the speed of light). The absorbance has been predicted to be independent of frequency. In principle, the interband optical absorption in zero‐gap graphene could be saturated readily under strong excitation due to Pauli blocking. Here, use of atomic layer graphene as saturable absorber in a mode‐locked fiber laser for the generation of ultrashort soliton pulses (756 fs) at the telecommunication band is demonstrated. The modulation depth can be tuned in a wide range from 66.5% to 6.2% by varying the graphene thickness. These results suggest that ultrathin graphene films are potentially useful as optical elements in fiber lasers. Graphene as a laser mode locker can have many merits such as lower saturation intensity, ultrafast recovery time, tunable modulation depth, and wideband tunability.

Doping of Conjugated Polythiophenes with Alkyl Silanes

AbstractA strong modification of the electronic properties of solution‐processable conjugated polythiophenes by self‐assembled silane molecules is reported. Upon bulk doping with hydrolized fluoroalkyl trichlorosilane, the electrical conductivity of ultrathin polythiophene films increases by up to six orders of magnitude, reaching record values for polythiophenes: (1.1 ± 0.1) × 103 S cm−1 for poly(2,5‐bis(3‐tetradecylthiophen ‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT) and 50 ± 20 S cm−1 for poly(3‐hexyl)thiophene (P3HT). Interband optical absorption of the polymers in the doped state is drastically reduced, making these highly conductive films transparent in the visible range. The dopants within the porous polymer matrix are partially crosslinked via a silane self‐polymerization mechanism that makes the samples very stable in vacuum and nonpolar environments. The mechanism of SAM‐induced conductivity is believed to be based on protonic doping by the free silanol groups available within the partially crosslinked SAM network incorporated in the polythiophene structure. The SAM‐doped polythiophenes exhibit an intrinsic sensing effect: a drastic and reversible change in conductivity in response to ambient polar molecules, which is believed to be due to the interaction of the silanol groups with polar analytes. The reported electronic effects point to a new attractive route for doping conjugated polymers with potential applications in transparent conductors and molecular sensors.

Franz–Keldysh effect in the interband optical absorption of semiconducting nanostructures

We present a theoretical calculation of the effect of an electric field applied either parallel or perpendicular to the direction of confinement on the interband optical absorption of semiconducting nanostructures such as quantum wells, quantum wires, and quantum boxes. We find that the application of the electric field decreases the optical absorption coefficient for both the parallel and perpendicular to the direction of carrier confinement. The absorption is greater when the electric field is along the direction of carrier confinement than when it is perpendicular to the direction of carrier confinement. Our work differs from that of others in that we present detailed theoretical calculations of the effect of the electric field on the optical absorption coefficient and obtain analytical expressions for the absorption coefficient. We find that there are additional oscillations in the absorption above the effective band gap in addition to the structure in the absence of the electric field, which are due to the Franz–Keldysh effect for the electric field parallel to the axis of the wire and the plane of the well. Numerical results also show that there is a smaller threshold for the optical absorption of cylindrical wires than for rectangular wires with the same cross sectional area.

Preparation and characterization of ZnSe nanocrystals in silica gel-glasses

Transparent ZnSe/SiO2 nanocomposites were prepared using a sol–gel technique which was followed by a reductive thermal treatment. The several approaches for avoiding the aggregation of ZnSe nanocrystals (NCs) doped in silica gel-glass were presented and adopted compositely during the preparation process. Transmission electron microscopy (TEM), X-ray diffraction (XRD), UV–Vis optical absorption, and Raman scattering were used to study the microstructural properties of the nanocomposites. All nanocomposites were found to show significant finite size effects as characterized by the broadened XRD peaks, blueshifts of the interband optical absorption edge, and the asymmetric broadening of the Raman scatterings. The structure and properties were correlated and discussed.

Light-matter interaction in doped microcavities

We discuss theoretically the light-matter coupling in a microcavity containing a quantum well with a two-dimensional electron gas. The high density limit where the bound exciton states are absent is considered. The matrix element of an interband optical absorption demonstrates the Mahan singularity [Phys. Rev. B153, 882 (1967); 163, 612 (1967)] due to strong Coulomb effect between the electrons and a photocreated hole. We extend the nonlocal dielectric response theory to calculate the quantum well reflection and transmission coefficients as well as the microcavity transmission spectra. The new eigenmodes of the system are discussed. Their implications for the steady state and time-resolved spectroscopy experiments are analyzed.

Dynamical Conductivity in Disordered Quantum Wire Array

The dynamical conductivity of quantum wire arrays with disorder in a corrugation period, direction, and height are calculated in a self-consistent Born approximation. Anisotropic scattering associated with diffuse Bragg peaks causes the formation of pseudo-band-structure and gives rise to inter-band optical absorption in the dynamical conductivity in the direction perpendicular to quantum wires.

Absorption in the Fractional Quantum Hall Regime: Trion Dichroism and Spin Polarization

We present measurements of optical interband absorption in the fractional quantum Hall regime in a GaAs quantum well in the range 0<nu<or=1. We investigate the mechanism of singlet trion absorption, and show that its circular dichroism can be used as a probe of the spin polarization of the ground state of the two-dimensional electron system (2DES). We find that at nu<or=1/3 the 2DES is fully spin polarized. Increasing the filling factor results in a gradual depolarization, with a sharp minimum in the dichroism near nu=2/3. We find that in the range 0.5<or=nu<0.85 the 2DES remains partially polarized for the broad range of magnetic fields from 2.75 to 11 T. This is consistent with the presence of a mixture of polarized and depolarized regions.

One-electron states and interband optical absorption in single-wall carbon nanotubes

Explicit expressions for the wavefunctions and dispersion equation for the bandπ-electrons in single-wall carbon nanotubes are obtained within the method of zero-rangepotentials. They are then used to investigate the absorption spectrum of polarizedlight caused by direct interband transitions in isolated nanotubes. It is shownthat, at least, under the above approximations, circular dichroism is absent inchiral nanotubes for a light wave propagating along the tube axis. The resultsobtained are compared with those calculated in a similar way for a graphite plane.

Effect of Hydrostatic Pressure on Interband Transitions in CoupledQuantum Wires

We calculate the exciton binding energy and interband opticalabsorption in a rectangular coupled quantum wire under the hydrostaticpressure in the effective-mass approximation, using the variationalapproach. It is found that the interband optical absorption stronglydepend on the hydrostatic pressure and the coupling parameter, and thatthe magnitude of the absorption coefficient for the HH1–E1 transitionin the coupled quantum wire is larger than that of the single quantumwire.

Optical Evidence for the Existence of a Stripe Structure in the Normal and Superconducting Phases of YBa2cu3O6+x

We study optical interband absorption spectra of insulator and metallic YBa2Cu3O6+ x films in a wide temperature region which includes the pseudogap and superconducting states. We show that in the pseudogap state a stripe structure appears with polaron-like charge carriers having an antiferromagnetic core.

Electronic states and interband light absorption in semi-spherical quantum dot under the influence of strong magnetic field

In the framework of effective mass approximation the electronic states in semi-spherical quantum lens under the influence of strong magnetic field are investigated. We have used the adiabatic approximation for the case of strong magnetic field. The eigenfunctions and eigenvalues of this problem are determined. In strong confinement regime interband optical absorption of light is investigated in quantum lens from InAs. The threshold frequencies of absorption are determined. The comparison with the case of film under the influence of strong magnetic field with infinitely high confinement potential is performed.

Franz-Keldysh effect and dynamical Franz-Keldysh effect of cylindrical quantum wires

We investigate the interband optical absorption spectra near the band edge of a cylindrical semiconductor quantum wire in the presence of a static electric field and a terahertz electric field polarized along the axis. Optical absorption spectra are nonperturbatively calculated by solving the low-density semiconductor Bloch equations in real space and real time. The influence of the Franz-Keldysh (FK) effect and dynamical FK effect on the absorption spectrum is investigated. To highlight the physics behind the FK effect and dynamical FK effect, the spatiotemporal dynamics of the polarization wave packet are also presented. Under a reasonable static electric field, substantial and tunable absorption oscillations appear above the band gap. A terahertz field, however, will cause the Autler-Townes splitting of the main exciton peak and the emergence of multiphoton replicas. The presented results suggest that semiconductor quantum wires have potential applications in electro-optical devices.

Optical properties of ellipsoidal CdSe quantum dots

The energy spectrum and optical absorption of confined carriers in ellipsoidal CdSe quantum dots are calculated. Our model relies on the effective mass approximation and Fermi's golden rule for the electronic structure and optical properties, respectively. We demonstrate that the electronic structure and the interband optical absorption are highly dependent on the ellipsoid aspect ratio.

Simple theoretical analysis of the interband optical absorption coefficient in wide-gap semiconductors in the presence of an external electric field and its dependence on a longitudinal magnetic field

An attempt is made to present a simplified theoretical analysis of the interband optical absorption coefficient (OAC) due to a constant uniform electric field on the basis of the electron wave vector dependence of the optical matrix element (OME) for the incident photon energy, ( ℏ ω ), below and above the band-gap ( E g). It has been found, taking n-GaAs as an example for numerical computation, that the expression of the OAC exhibits an exponential fall-off with the electric field and the photon energy without the consideration of the Wannier–Stark levels, which generally exists in a band due to the external electric field. The effect of a longitudinal magnetic field on the OAC is also studied on the basis of the fact that the transverse wave vector ( k ⊥) is quantized due to parallel magnetic field and is conserved in the interband optical transition of electrons. Similar results, such as singularity for the case ℏ ω < E g in the OAC and the oscillations in the OAC for the case ℏ ω ⩾ E g in presence of electric field are also obtained with modifications in the expressions for the OAC in presence of electric and parallel magnetic fields. Present study explains the modification of the band-gap of the semiconductor in the presence of electric and parallel magnetic fields, respectively. The numerical analyses are performed and discussed in details taking n-GaAs as an example.