During the past decade, enormous progress has been made in growing ultrathin organic films and multilayer structures with a wide range of exciting optoelectronic properties. This progress has been made possible by several important advances in our understanding of organic films and their modes of growth. Perhaps the single most important advance has been the use of ultrahigh vacuum (UHV) as a means to achieve, for the first time, monolayer control over the growth of organic thin films with extremely high chemical purity and structural precision.1-3 Such monolayer control has been possible for many years using well-known techniques such as Langmuir-Blodgett film deposition,4 and more recently, self-assembled monolayers from solution have also been achieved.5 However, ultrahighvacuum growth, sometimes referred to as organic molecular beam deposition (OMBD) or organic molecular beam epitaxy (OMBE), has the advantage of providing both layer thickness control and an atomically clean environment and substrate. When combined with the ability to perform in situ highresolution structural diagnostics of the films as they are being deposited, techniques such as OMBD have provided an entirely new prospect for understanding many of the fundamental structural and optoelectronic properties of ultrathin organic film systems. Since such systems are both of intrinsic as well as practical interest, substantial effort worldwide has been invested in attempting to grow and investigate the properties of such thin-film systems. This paper is a review of recent progress made in organic thin films grown in ultrahigh vacuum or using other vapor-phase deposition methods. We will describe the most important work which has been published in this field since the emergence of OMBD in the mid-1980s. Both the nature of thin-film growth and structural ordering will be discussed, as well as some of the more interesting consequences to the physical properties of such organic thin-film systems will be considered both from a theoretical as well as an experimental viewpoint. Indeed, it will 1793 Chem. Rev. 1997, 97, 1793−1896

Ultrathin Organic Films Grown by Organic Molecular Beam Deposition and Related Techniques.

Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz–30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.

https://iopscience.iop.org/article/10.1088/1361-6463/50/4/043001/pdf

The 2017 terahertz science and technology roadmap

Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequilibrium time evolution of carriers and low-energy excitations with sub-ps time resolution. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the conductivity of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.

/pdf/carrier-dynamics-in-semiconductors-studied-with-time-1u551csnya.pdf

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

A highly strained ultrathin membrane of MoS2 could lead to the creation of a solar funnel, a new form of solar cell which absorbs a much broader range of the solar spectrum that a usual single junction device.

/pdf/strain-engineered-artificial-atom-as-a-broad-spectrum-solar-zr8r1jqfeg.pdf

Strain-engineered artificial atom as a broad-spectrum solar energy funnel

We experimentally demonstrate the implementation of three-dimensional optical metamaterials. We investigate the interaction between adjacent stacked layers using the method of plasmon hybridization and analyze the optical properties of stacked metamaterials with increasing layer numbers.

/pdf/three-dimensional-metamaterials-at-optical-frequencies-9jvt5b6330.pdf

Three-dimensional metamaterials at optical frequencies

We propose a generally applicable velocity matching method for THz-pulse generation by optical rectification in the range below the phonon frequency of the nonlinear material. Velocity matching is based on pulse front tilting of the ultrashort excitation pulse and is able to produce a large-area THz beam. Tuning of the THz radiation by changing the tilt angle is experimentally demonstrated for a narrow line in the range between 0.8-0.97 times the phonon frequency. According to model calculations broadband THz radiation can be generated at lower frequencies. Advantages of the new velocity matching technique in comparison to the electro-optic Cherenkov effect and non-collinear beam mixing are discussed.

Velocity matching by pulse front tilting for large area THz-pulse generation.

We numerically study the spectral response of ‘U’-shaped split-ring-resonators at normal incidence with respect to the resonator plane. Based on the evaluation of the near-field patterns of the resonances and their geometry-dependent spectral positions, we obtain a comprehensive and consistent picture of their origin. We conclude that all resonances can be understood as plasmonic resonances of increasing order of the entire structure. In particular, for an electrical field polarized parallel to the gap the so-called LC-resonance corresponds to the fundamental plasmonic mode and, contrary to earlier interpretations, the electrical resonance is a second order plasmon mode of the entire structure. The presence of further higher order modes is discussed.

On the reinterpretation of resonances in split-ring-resonators at normal incidence

The ultrafast relaxation of excitons in GaAs, coherently excited by a short optical pulse and subjected to collisions with free carriers and noncoherent excitons which are independently created by a second synchronized light pulse, is studied directly in the time domain by a probing of the excitonic phase coherence. The relaxation rates reveal strong exciton-exciton scattering with a collision efficiency of 1.6 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\ensuremath{-}1}$ and even 10 times more efficient exciton-free-carrier scattering.

Ultrafast phase relaxation of excitons via exciton-exciton and exciton-electron collisions.

The phase relaxation of two-dimensional (2D) heavy-hole excitons in a 12-nm GaAs single quantum well subjected to collisions with either free carriers or incoherent heavy-hole excitons is investigated by time-resolved degenerate four-wave mixing. The homogeneous linewidth corresponding to the phase coherence time reveals a collisional broadening due to exciton-exciton scattering and an 8 times stronger exciton-free carrier scattering. Both scattering processes are enhanced for 2D excitons as compared to 3D excitons.

Collision broadening of two-dimensional excitons in a GaAs single quantum well

Picosecond luminescence experiments on $\mathrm{GaAs}/{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum well structures reveal a significant influence of localization on the transition probabilities of photoexcited carriers. The spontaneous lifetime of electrons and holes within the quantum well decreases with well thickness ${L}_{z}$ from 1 ns for ${L}_{z}=14$ nm to 350 ps for ${L}_{z}=5$ nm because of the enhanced recombination due to localization of the carriers.

Jürgen Kuhl

Papers

Velocity matching by pulse front tilting for large area THz-pulse generation.

On the reinterpretation of resonances in split-ring-resonators at normal incidence

Ultrafast phase relaxation of excitons via exciton-exciton and exciton-electron collisions.

Collision broadening of two-dimensional excitons in a GaAs single quantum well

Recombination Enhancement due to Carrier Localization in Quantum Well Structures