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.     

Photocurrent resonances in short-period AlAs/GaAs superlattices in an electric field

The photocurrent was measured as a function of the external electric field in short-period AlAs/GaAs superlattices for various photon energies. Transport resonances, whose positions do not depend on the photon energy, were observed in these dependences together with optical resonances due to interband transitions in Wannier-Stark levels. It is shown that the transport resonances are due to tunneling of photoelectrons from the p-GaAs contact region into the first level in GaAs wells located 2–5 lattice periods from the contact layer. Fiz. Tverd. Tela (St. Petersburg) 41, 159–164 (January 1999)     

Wannier-Stark states of a quantum particle in 2D lattices

A simple method of calculating the Wannier-Stark resonances in 2D lattices is suggested. Using this method we calculate the complex Wannier-Stark spectrum for a nonseparable 2D potential realized in optical lattices and analyze its general structure. The dependence of the lifetime of Wannier-Stark states on the direction of the static field (relative to the crystallographic axis of the lattice) is briefly discussed.     

Structure of excited-state transitions of individual semiconductor nanocrystals probed by photoluminescence excitation spectroscopy

We perform for the first time photoluminescence excitation (PLE) studies of individual nanocrystals (NCs) that reveal the structure of excited-state transitions not obscured by ensemble averaging. Single-NC PLE spectra strongly deviate from a traditional idealized picture of sharp, quasiatomic resonances. We detect only a few relatively narrow transitions (3-4 meV) at the band edge, while at higher spectral energies, we observe a broad structureless feature separated from the band-edge peaks by a >50 meV "minigap." These observations can be rationalized by analyzing hole intraband relaxation behavior.     

Efficient frequency conversion in slab waveguide by cascaded nonreciprocal interband photonic transitions

A slab-waveguide-based configuration for efficient frequency conversion is proposed. In the proposed structure, frequency conversion is induced through cascaded multistep nonreciprocal interband photonic transitions activated by external refractive-index modulations. The advantages of interband photonic transition over intraband photonic transition for frequency conversion are analyzed with the coupled-mode theory.     

Calculation of Wannier-Bloch and Wannier-Stark states

The paper discusses the metastable states of a quantum particle in a periodic potential under a constant force (the model of a crystal electron in a homogeneous electric field), which are known as the Wannier-Stark ladder of resonances. An efficient procedure to find the positions and widths of resonances is suggested and illustrated by numerical calculations for a cosine potential, which are in excellent agreement with complex scaling resonance energies. Received: 27 April 1998 / Revised: 21 July 1998 / Accepted: 3 August 1998     

Monopolar optical orientation of electron spins in bulk semiconductors and heterostructures

It is shown that intraband absorption of circularly polarized light leads to spin polarization of the electron gas. A theory of this monopolar optical spin orientation is developed for indirect intraband transitions in bulk semiconductors and for indirect intrasubband and direct intersubband transitions in quantum wells.     

Effect of the electron population on intraband absorption in InAs/GaAs self-assembled quantum dots

We present a comprehensive study of the intraband transitions in n-type InAs/GaAs quantum dots (QDs) with a filling varying from 0.5 to 4 electrons per dot, using both polarization-dependent absorption and photocurrent spectroscopy. Applying these complementary mid- and far-infrared spectroscopies over a wide energy range allows us to obtain a detailed picture of the intraband transitions and energy levels in self-assembled QDs.     

Discrete breathers and delocalization in nonlinear disordered systems

We find exact localized time-periodic solutions with frequencies inside the linearized spectrum [intraband discrete breathers (IDBs)] in random nonlinear models using a new self-consistent method. The IDB frequencies belong to intervals between forbidden gaps generated by resonances with the linear modes, becoming fat Cantor sets in infinite systems. When localized IDBs are continued versus frequency, they delocalize and become multisite IDBs (not predicted by existing theorems), which can propagate energy. Some implications for energy relaxation in glasses are discussed.     

Influence of direct transitions between spin branches on free-carrier absorption of ferromagnetic semiconductors

The direct spin-flip intraband absorption coefficient due to transitions of conduction electrons between the spin branches in degenerate ferromagnetic semiconductors is calculated. It is shown that the absorption coefficient consists essentially of a sharp peak at the band splitting frequency which shifts towards low frequencies as the temperature increases from T = 0 K.     

Wannier-Stark resonances under strong localization conditions in natural silicon-carbide superlattices

A detailed experimental investigation is made of the electronic transport under conditions of Wannier-Stark localization of carriers in a natural superlattice of hexagonal polytypes of silicon carbide. The 4H and 6H polytypes, which possess different superlattice and miniband spectrum parameters, are employed. Direct measurements of the electronic current versus the average electric field in the active region of the sample revealed a series of regions of negative differential conductivity in fields ranging from 500 to 2100 kV/cm. Analysis of the results shows that the observed current resonances are associated with the development of the Wannier-Stark quantization process and are due to conduction mechanisms such as hopping conduction, induced between the levels of a Wannier-Stark ladder by a resonant electron-phonon interaction, and the resonant interminiband tunneling from the first into the second miniband. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 2, 105–109 (25 July 1996)     

Bloch oscillations and Wannier-Stark ladder in the coupled LC circuits

We present a new scheme for realizing Bloch oscillations and Wannier-Stark ladder based on a lattice of coupled LC circuits. By converting the second order dynamical ODEs of the system into a first order Schrödinger-like equation, we propose an equivalent tight binding Hamiltonian to describe the circuit. We show that a synthesized electric field is produced by introducing a frequency mismatch into the resonant frequency of the adjacent LC resonators. The Wannier-Stark modes are the normal modes of the circuit and the Bloch oscillations can be observed in a coupled LC lattice. By addition of coupling capacitors between nodes of the circuit, we study the Bloch oscillation in the presence of long-range couplings. We also show that the circuit converts to a transmission line simulating synthetic electric fields in the continuum limit. The coupled LC circuit is, in some sense, amongst the simplest physical systems exhibiting Bloch oscillation and Wannier-Stark Ladder.     

Infrared absorption in parabolic multiquantum well structures

The absorption constant for intraband transitions in parabolic multiquantum well structures is calculated and compared with intraband absorption in a square well superlattice. The parabolic multiple quantum well structure may be used as an Infrared detector with the possibility of lower leakage current compared to one made of square wells.     

Intraband optical transitions in semiconductor quantum dots: Radiative electronic-excitation lifetime

Intraband optical transitions in semiconductor quantum dots (QDs) in the forms of a parallelepiped, sphere, and cylinder have been considered. It is shown that the size dependence of the matrix elements of electron-photon interaction, which includes the intraband transitions, differs in the E · r and A · p representations of electron-photon coupling, which are widely used to describe various optical processes. The radiative intraband relaxation rates of QD electron excitations have been calculated, depending on the QD size and shape and the parameters of the QD material. It is shown that the radiative intraband transition rate may reach 10⁹ s⁻¹.     

Electrodynamic response of a quantum nanotube in a parallel magnetic field

This paper reports on a theoretical study of intraband resonances arising in the absorption of electromagnetic radiation by a quantum nanotube both with and without involvement of optical phonons. Explicit relations are derived for the absorption coefficients. The relative intensities of the resonance peaks are analyzed.     

Infrared probe of the electronic structure and charge dynamics of Na0.7CoO2

We present measurements of the optical spectra on Na(0.7)CoO(2) single crystals. The optical conductivity shows two broad interband transition peaks at 1.6 eV and 3.1 eV, and a weak midinfrared peak at 0.4 eV. The intraband response of conducting carriers is different from that of a simple Drude metal. A peak at low but finite frequency is observed, which shifts to higher frequencies with increasing temperature, even though the dc resistivity is metallic. The origin of the interband transitions and the low-frequency charge dynamics have been discussed and compared with other experiments.     

Doppler-free spectroscopy of neon atoms dressed by optical photons in nearly-degenerate four-wave mixing

We have investigated nearly degenerate four-wave mixing in neon on the 607 nm line. We have considered the case where one pump beam saturates the medium while the two other incident beams have a much weaker intensity. The frequency of the most intense pump beam is equal to the frequency of the probe beam. When the frequency of the second pump beam is scanned, narrow resonances are observed. These resonances correspond to transitions between the energy levels of the atom dressed by the photons of the intense pump beam.     

Intraband Rabi Oscillations Involving Photons and Acoustic Phonons

Optics and Spectroscopy - Probabilities Wexc of intraband electron transitions that involve the participation of photons and longitudinal acoustic phonons in a covalent crystal are determined. It...     

Vertical Conductance in Superlattice in Strong Magnetic Field during Inter- and Intraband Transitions

Russian Physics Journal - The paper considers the influence of interband and intraband transitions on vertical conductance oscillations in superlattices in a strong longitudinal magnetic field. It...     

垂直电场下扭转双层石墨烯光学吸收性质的理论研究

层间扭转角度是对石墨烯物理性质宽波段可调谐的一个新参量.本文采用2°<θ<15°扭转角度下的连续近似模型,获得了不同扭转角度双层石墨烯分别在有、无电场下的能带结构,通过电子-光子相互作用跃迁速率,计算模拟了范霍夫奇点附近电子带内跃迁和带间跃迁所引起的光学吸收谱.结果表明,在无外加电场时,带间跃迁吸收峰的位置随着扭转角度的增大而发生从红外到可见光波段的蓝移,且吸收系数增大,带内跃迁的光学吸收系数相对于带间跃迁高出2个数量级;而存在外加电场时,两个范霍夫奇点在波矢空间的位置发生偏移,带间跃迁吸收峰发生分裂,且两个分裂的吸收峰位置随着电场强度的不断增大而反向行进.上述研究结果对石墨烯材料在光电器件方面的应用有一定指导作用.