Over the past three decades a new spectroscopic technique with unique possibilities has emerged. Based on coherent and time-resolved detection of the electric field of ultrashort radiation bursts in the far-infrared, this technique has become known as terahertz time-domain spectroscopy (THz-TDS). In this review article the authors describe the technique in its various implementations for static and time-resolved spectroscopy, and illustrate the performance of the technique with recent examples from solid-state physics and physical chemistry as well as aqueous chemistry. Examples from other fields of research, where THz spectroscopic techniques have proven to be useful research tools, and the potential for industrial applications of THz spectroscopic and imaging techniques are discussed.

Terahertz spectroscopy and imaging – Modern techniques and applications

We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

A review on TiO2-based nanotubes synthesized via hydrothermal method: Formation mechanism, structure modification, and photocatalytic applications

We review pioneering and recent studies of the conductivity of solid state systems at terahertz frequencies. A variety of theoretical formalisms that describe the terahertz conductivity of bulk, mesoscopic and nanoscale materials are outlined, and their validity and limitations are given. Experimental highlights are discussed from studies of inorganic semiconductors, organic materials (such as graphene, carbon nanotubes and polymers), metallic films and strongly correlated electron systems including superconductors.

A Review of the Terahertz Conductivity of Bulk and Nano-Materials

Diffusion, coalescence, and reconstruction of vacancy defects in graphene layers are investigated by tight-binding molecular dynamics (TBMD) simulations and by first principles total energy calculations. It is observed in the TBMD simulations that two single vacancies coalesce into a 5-8-5 double vacancy at the temperature of 3000 K, and it is further reconstructed into a new defect structure, the 555-777 defect, by the Stone-Wales type transformation at higher temperatures. First principles calculations confirm that the 555-777 defect is energetically much more stable than two separated single vacancies, and the energy of the 555-777 defect is also slightly lower than that of the 5-8-5 double vacancy. In TBMD simulation, it is also found that the four single vacancies reconstruct into two collective 555-777 defects which is the unit for the hexagonal haeckelite structure proposed by Terrones et al. [Phys. Rev. Lett. 84, 1716 (2000)].

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Diffusion, Coalescence, and Reconstruction of Vacancy Defects in Graphene Layers

Self-assembled nanodrill technology based on droplet epitaxy growth was developed to obtain nanoholes on a GaAs(100) surface. In this technology, the gallium droplets act like “electrochemical drills” etching away the GaAs substrate beneath to give rise to nanoholes more than 10nm deep. The driving force of the nanodrill is attributed to the arsenic desorption underneath the gallium droplet at high growth temperatures and Ga-rich condition. This nanodrill technology provides an easy and flexible method to fabricate nanohole templates on GaAs(100) surface and has great potential for developing quantum dots and quantum dot molecules for quantum computation applications.

Nanoholes fabricated by self-assembled gallium nanodrill on GaAs(100)

Several distinctive self-assembled InGaAs quantum dot molecules (QDMs) are studied. The QDMs self-assemble around nanoscale-sized GaAs moundlike templates fabricated by droplet homoepitaxy. Depending on the specific InAs monolayer coverage, the number of QDs per GaAs mound ranges from two to six (bi-QDMs to hexa-QDMs). The Ga contribution from the mounds is analyzed in determining the morphologies of the QDMs, with respect to the InAs coverages ranging between 0.8 and 2.4 ML. Optical characterization shows that the resulting nanostructures are high-quality nanocrystals.

InGaAs quantum dot molecules around self-assembled GaAs nanomound templates

Anisotropic surface diffusion and strain are used to explain the formation of three-dimensional (In,Ga)As quantum dot lattices. The diffusion characteristics of the surface, coupled with the elastic anisotropy of the matrix, provides an excellent opportunity to influence the dot positions. In particular, quantum dots that are laterally organized into long chains or chessboard two-dimensional arrays vertically organized with strict vertical ordering or vertical ordering that is inclined to the sample surface normal are accurately predicted and observed.

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Controlling Planar and Vertical Ordering in Three-Dimensional (In,Ga)As Quantum Dot Lattices by GaAs Surface Orientation

A growth technique combining droplet epitaxy and molecular beam epitaxy (MBE) is developed to obtain a low density of InAs quantum dots (QDs) on GaAs nanoholes. This growth technique is simple, flexible, and does not require additional substrate processing. It makes possible separate control of the QD density via droplet epitaxy and the QD quality via MBE growth. In this letter the authors report the use of this technique to produce InAs QDs with a low density of 2.7×108cm−2 as well as good photoluminescence properties. The resulting samples are suitable for single QD device fabrication and applications.

Low density InAs quantum dots grown on GaAs nanoholes

Transient photoluminescence from a series of asymmetric InAs quantum-dot bilayers with a GaAs barrier layer thickness varying from 30 to 60 monolayers between the quantum-dot planes is investigated. The interdot carrier transfer process is analyzed. In the framework of a three-level system, interdot carrier transfer times between 200 and 2500ps are derived and compared with similar data from the literature. Within the semiclassical Wentzel–Kramers–Brillouin approximation, the observed “transfer time-barrier thickness-relation” supports nonresonant tunneling as the microscopic carrier transfer mechanism.

Zh. M. Wang

Papers

Nanoholes fabricated by self-assembled gallium nanodrill on GaAs(100)

InGaAs quantum dot molecules around self-assembled GaAs nanomound templates

Controlling Planar and Vertical Ordering in Three-Dimensional (In,Ga)As Quantum Dot Lattices by GaAs Surface Orientation

Low density InAs quantum dots grown on GaAs nanoholes

Interdot carrier transfer in asymmetric bilayer InAs∕GaAs quantum dot structures