Research into terahertz technology is now receiving increasing attention around the world, and devices exploiting this waveband are set to become increasingly important in a very diverse range of applications. Here, an overview of the status of the technology, its uses and its future prospects are presented.

Cutting-edge terahertz technology

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

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 widely used term, “photocatalysis”, generally addresses photocatalytic (energetically downhill) and photosynthetic (energetically uphill) reactions and refers to the use of photonic energy as a driving force for chemical transformations, i.e., electron reorganization to form/break chemical bonds. Although there are many such important reactions, this contribution focuses on the fundamental aspects of photocatalytic water splitting into hydrogen and oxygen by using light from the solar spectrum, which is one of the most investigated photosynthetic reactions. Photocatalytic water splitting using solar energy is considered to be artificial photosynthesis that produces a solar fuel because the reaction mimics nature’s photosynthesis not only in its redox reaction type but also in its thermodynamics (water splitting: 1.23 eV vs glucose formation: 1.24 eV). To achieve efficient photocatalytic water splitting, all of the parameters, though involved at different time scales and spatial resolutions, should be op...

http://pubs.acs.org/doi/pdf/10.1021/acscatal.7b02662

Photocatalytic Water Splitting: Quantitative Approaches toward Photocatalyst by Design

The photoresponse of a uni-traveling-carrier photodiode (UTC-PD), which is configured with a neutral narrow-gap light absorption layer and a depleted wide-gap carrier collecting layer, is investigated by small-signal analysis. Drift-diffusion model was used for analyzing carrier dynamics in the absorption layer. For accurately predicting the frequency response, a boundary condition at the edge of the absorption layer was carefully treated by taking into account the electron thermionic emission velocity. High electron mobility in the absorption layer and high drift velocity in the carrier collecting layer associated with the velocity overshoot effect are both essential for short response times. Calculations performed on InP/InGaAsP UTC-PDs with the same absorption and carrier collecting layer thicknesses show that the response can be dominated by the electron transport in the absorption layer provided that the significant velocity overshoot occurs in the carrier collecting layer. Furthermore, a UTC-PD with a quasi-field in the absorption layer can generate a several times broader bandwidth than conventional pin PDs, while maintaining a similar internal quantum efficiency.

https://iopscience.iop.org/article/10.1143/JJAP.36.6263/pdf

High-Speed Response of Uni-Traveling-Carrier Photodiodes

Photonic generation of continuous millimetre- and sub-millimetre waves up to the THz range using antenna-integrated uni-travelling-carrier photodiodes is described. A device integrating a wideband log-periodic antenna exhibits a maximum output power of 2.6 ?W at 1.04 THz with good linearity. A module with a quasi-optical output port fabricated for practical use generates almost the same output power as the chip at around 1 THz and operates at frequencies of up to 1.5 THz. The output power level and the operation frequency are records for wideband photodiodes operating at 1.55 ?m. Devices integrating resonant narrowband dipole antennae have also been fabricated and the output power increases at resonant peak frequencies confirmed. The device having a peak at 1.04 THz exhibits a maximum (detected) output power of 10.9??W at 1.04 THz with good linearity. This output power is the highest value ever directly generated from a photodiode in the THz range, and several times higher than the maximum value reported by the low-temperature-grown GaAs photoconductive switch at around 1 THz.

https://iopscience.iop.org/article/10.1088/0268-1242/20/7/008/pdf

Continuous THz-wave generation using antenna-integrated uni-travelling-carrier photodiodes

We have observed strong photoluminescence (PL) from a well-defined two-dimensional (2D) Si structure formed on a SIMOX (separation by implanted oxygen) wafer, where the thin ( <5 nm) single crystalline silicon film is sandwiched between SiO2 layers. The PL intensity has a sharp maximum at a Si thickness of about 2 nm, whereas the peak photon energy of the PL spectra (1.65 eV) is almost independent of the Si thickness. These results can be interpreted with a three-region model in which electron-hole pairs are excited in the Si well and luminescence occurs at the upper and lower Si/SiO2 interfaces. Furthermore, temperature dependence of PL intensity in the present 2D system is found to be different from previously reported dependence in 0D or 1D Si structures.

Photoluminescence from a silicon quantum well formed on separation by implanted oxygen substrate

We have recently found that quantum‐box‐like structures are formed during spontaneous reorganization of a sequence of AlGaAs and strained InGaAs epitaxial films grown on GaAs (311)B substrates by metalorganic vapor‐phase epitaxy into InGaAs islands (disks) buried beneath AlGaAs. The size of the disks is directly controlled by the In content in the range 200–30 nm. Strong photoluminescence (PL) efficiency at room temperature is observed in these strained quantum disks. Even for the 30 nm disk the radiative efficiency is not reduced compared to the reference (100) quantum well. The PL spectra are characterized by narrow linewidth and well resolved exciton resonances in excitation spectroscopy.

Strong photoluminescence emission at room temperature of strained InGaAs quantum disks (200–30 nm diameter) self‐organized on GaAs (311)B substrates

We have studied the ultrafast response of uni-traveling-carrier photodiodes with photo-absorption layer doping levels from 2.5 ×1017 to 2.5 ×1018 cm-3. It is found that 3-dB band width increases with the output voltage in the low output region. This enhancement is more prominent for a lower doping level in the photo-absorption layer. From the analysis of the carrier transport in the photo-absorption layer, we attribute the observed enhanced bandwidth as a result of the self-induced electric field associated with carrier injection.

https://iopscience.iop.org/article/10.1143/JJAP.37.1424/pdf

Tomofumi Furuta

Papers

High-Speed Response of Uni-Traveling-Carrier Photodiodes

Continuous THz-wave generation using antenna-integrated uni-travelling-carrier photodiodes

Photoluminescence from a silicon quantum well formed on separation by implanted oxygen substrate

Strong photoluminescence emission at room temperature of strained InGaAs quantum disks (200–30 nm diameter) self‐organized on GaAs (311)B substrates

Improved Response of Uni-Traveling-Carrier Photodiodes by Carrier Injection