In this abstract, a brief summary of Terahertz (THz) generated by two color mixing which induced nonlinearity in semiconductor is presented. Indium Phosphide have shown saturable absorption like behavior with MV/cm THz field strengths. The free carriers, which are generated by leading edge of THz pulse, achieve high kinetic energies. These energetic carriers in lowest intervalley of the conduction band get scattered in to next highest intervalley of the conduction band. The effective mass of the electrons in these valleys reduce leading to decrease in the conductivity of the sample increasing the high transmission of the THz at focus.

Metal organic vapor phase epitaxy is used to grow gallium arsenide (GaAs) nanocrystals (NCs) on germanium (Ge) templates on nanoscopic silicon (Si) threads prepared by reactive ion etching. Scanning transmission electron microscopy with energy dispersive X-ray measurements shows an epitaxial growth of the GaAs on the Ge template that is supported by the Si thread, and that Ge doping is induced to the GaAs by the template. On Ge templates of about 60 nm diameter, as-grown GaAs NCs show a very regular rhombic-dodecahedral outer shape that can be explained by a preferential growth along the <110> plane. Photoluminescence measurements of the Ge/GaAs structures reveal radiative emission peaks on top of the GaAs band-to-band emission and at sub-band gap energies. While high energy peaks are originating from Ge acceptor levels in GaAs, sub-band gap peaks can be explained by radiation from Ge donor and acceptor bands that are amplified by photonic modes hosted in the rhombic-dodecahedral GaAs NCs. This study shows that a template-assisted crystal growth at the nanoscale opens up routes for a versatile integration of strongly emitting nanomaterials for a use in on-chip solid state lighting and photonics.

We have fiber-coupled an array of n-i-pn-i-p superlattice photomixers using a fiber array of same pitch of 145 mu m. We experimentally investigate the effect of the finite size of the implemented silicon lens on the interference between the array elements in the far field. We compare the results from a geometry optimized for a collimated terahertz (THz) beam to theory and simulations. Further, beam steering is demonstrated by controlling the optical phase of the individual photomixers. Due to broadband antennas attached to each array element, the array is frequency tunable. It is exemplarily characterized at 165 and 310 GHz. Such arrays can overcome power limitations of individual photomixers. In contrast to bulky individually packaged free space solutions, this array can be packaged to a compact terahertz source, limited in size only by the size of the silicon lens. The investigated 2 x 2 array features a spot diameter (full-width at half-maximum) of 12.1 mm at a distance of 19 cm at 310 GHz with a silicon lens of only 20-mm diameter.

The theoretical background of terahertz generation by photomixing will be discussed in detail. Basic design rules are specified for obtaining highly efficient optical to THz power conversion for both photoconductive and high frequency p-i-n diodes, considering pulsed as well as continuous-wave operation. State-of-the-art realizations of photomixers at 800 and 1550 nm laser wavelength are shown. Limiting electrical and thermal constraints to the achievable THz power are also addressed. Finally, this chapter gives an overview of electronic means for THz generation, such as Schottky diodes, negative differential resistor oscillators, and plasmonic effects that are used in THz generation. The chapter starts with a quick overview of the most relevant THz generation schemes based on nonlinear media, accelerating electrons, and actual THz lasers. This serves to place in context the two schemes discussed in detail thereafter: photomixing and electronic generation. The chapter covers the theoretical frameworks, principles of operation, limitations, and reported implementations of both schemes for pulsed and continuous-wave operation when applicable. It also covers to a lesser extent the recently explored use of plasmonics to improve the efficiency of THz generation in photomixing, nonlinear media, and laser schemes.

The theoretical background of antenna theory, especially in relation to terahertz applications, is presented in this chapter. A general discussion of the issues of THz antennas, especially when matching the photomixer is presented. Array theory is presented, together with an exhaustive and precise analysis of one of the most promising new solutions for generating THz emission with high power levels, that is, the large area emitter concept.

Arrays of coherently driven photomixers with antenna (antenna emitter arrays, AEAs) have been evaluated as a possibility to overcome the power limitations of individual conventional photomixers with antenna (“antenna emitters”, AEs) for the generation of CW THz radiation. In this paper “large area emitters” (LAEs) are proposed as an alternative approach. In this antenna-free new scheme of photomixing, the THz radiation originates directly from the acceleration of photo-induced charge carriers generated within a large semiconductor area. The quasi-continuous distribution of emitting elements corresponds to a high-density array and results in favorable radiation profiles without side lobes. Moreover, the achievable THz power is expected to outnumber even large AEAs. Last not least, the technological challenge of fabricating LAEs appears to be significantly less demanding. The radiation pattern of a vertical LAE is shown from 0.1 THz up to 1 THz.

Arrays of coherently driven photomixers with antenna (antenna emitter arrays, AEAs) have been evaluated as a possibility to overcome the power limitations of individual conventional photomixers with antenna (“antenna emitters”, AEs) for the generation of continuous-wave (CW) THz radiation. In this paper, “large area emitters” (LAEs) are proposed as an alternative approach, and compared with AEAs. In this antenna-free new scheme of photomixing, the THz radiation originates directly from the acceleration of photo-induced charge carriers generated within a large semiconductor area. The quasi-continuous distribution of emitting elements corresponds to a high-density array and results in favorable radiation profiles without side lobes. Moreover, the achievable THz power is expected to outnumber even large AEAs. Last not least, the technological challenge of fabricating LAEs appears to be significantly less demanding.

A new geometry for the design of antennas in the Terahertz band is presented. The structure is based on a horn antenna etched in the substrate and fed with a planar printed antenna used for generation of terahertz radiation, designed for the 200 GHz to 3 THz range. For the proposed antenna, the energy distribution through the substrate is reduced towards an increase in the gain of the system, at least, 8 dB in a 1:10 bandwidth. The structure has been measured showing the expected behavior in the low band.

We report on a theoretical and experimental investigation of magneto-absorption in quantum wells (QWs) in the presence of a uniform in-plane electric field. From a simple theoretical model a linear real-space shift of the electron Landau level wave functions relative to the hole counterparts is expected under increasing in-plane electric field. As a result, quadratic Stark shifts, fading of the zero-field allowed transitions, and increasing oscillator strength for the originally forbidden transitions are predicted. These predictions are confirmed by experiments on GaAs/AlGaAs-QWs and found to be in reasonably good agreement with simulations based on a simple model. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.