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 interaction of subpicosecond laser pulses with magnetically ordered materials has developed into a fascinating research topic in modern magnetism. From the discovery of subpicosecond demagnetization over a decade ago to the recent demonstration of magnetization reversal by a single 40 fs laser pulse, the manipulation of magnetic order by ultrashort laser pulses has become a fundamentally challenging topic with a potentially high impact for future spintronics, data storage and manipulation, and quantum computation. Understanding the underlying mechanisms implies understanding the interaction of photons with charges, spins, and lattice, and the angular momentum transfer between them. This paper will review the progress in this field of laser manipulation of magnetic order in a systematic way. Starting with a historical introduction, the interaction of light with magnetically ordered matter is discussed. By investigating metals, semiconductors, and dielectrics, the roles of nearly free electrons, charge redistributions, and spin-orbit and spin-lattice interactions can partly be separated, and effects due to heating can be distinguished from those that are not. It will be shown that there is a fundamental distinction between processes that involve the actual absorption of photons and those that do not. It turns out that for the latter, the polarization of light plays an essential role in the manipulation of the magnetic moments at the femtosecond time scale. Thus, circularly and linearly polarized pulses are shown to act as strong transient magnetic field pulses originating from the nonabsorptive inverse Faraday and inverse Cotton-Mouton effects, respectively. The recent progress in the understanding of magneto-optical effects on the femtosecond time scale together with the mentioned inverse, optomagnetic effects promises a bright future for this field of ultrafast optical manipulation of magnetic order or femtomagnetism.

/pdf/ultrafast-optical-manipulation-of-magnetic-order-2ms2t5k1zo.pdf

Ultrafast optical manipulation of magnetic order

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

Ultrafast charge and spin excitations in the elusive terahertz regime1,2 of the electromagnetic spectrum play a pivotal role in condensed matter3,4,5,6,7,8,9,10,11,12,13. The electric field of free-space terahertz pulses has provided a direct gateway to manipulating the motion of charges on the femtosecond timescale6,7,8,9. Here, we complement this process by showing that the magnetic component of intense terahertz transients enables ultrafast control of the spin degree of freedom. Single-cycle terahertz pulses switch on and off coherent spin waves in antiferromagnetic NiO at frequencies as high as 1 THz. An optical probe pulse with a duration of 8 fs follows the terahertz-induced magnetic dynamics directly in the time domain and verifies that the terahertz field addresses spins selectively by means of the Zeeman interaction. This concept provides a universal ultrafast means to control previously inaccessible magnetic excitations in the electronic ground state. Researchers report the direct observation of ultrafast magnetic dynamics using the magnetic component of highly intense terahertz wave pulses with a time resolution of 8 fs. This concept provides a universal ultrafast method of visualizing magnetic excitations in the electronic ground state.

/pdf/coherent-terahertz-control-of-antiferromagnetic-spin-waves-46l6412ep4.pdf

Coherent terahertz control of antiferromagnetic spin waves

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

https://link.aps.org/accepted/10.1103/RevModPhys.89.025006

Interface-induced phenomena in magnetism

It is shown that the laser induced ultrafast demagnetization of ferromagnetic films results in the emission of a terahertz electromagnetic pulse. This emission has been detected from Ni films using free-space electro-optic sampling. The radiated electric field E(t) is explained by Maxwell equations (radiation from a time dependent magnetic dipole), and is expected to be proportional to the second time derivative of the magnetization d2M/dt2, as measured in the far field. This technique opens appealing perspectives in the context of measuring and understanding the ultrafast spin dynamics as well as the interaction of electrons (both charge and spin) with electromagnetic fields.

Coherent terahertz emission from ferromagnetic films excited by femtosecond laser pulses

ZnTe(110) is widely used as a source of terahertz radiation generated by optical rectification. However, when ZnTe(110) is excited with 800 nm light, optical rectification is not the only process which can occur. Specifically, second harmonic generation and two-photon absorption are also possibilities. In addition, free carriers generated by two-photon absorption can absorb terahertz radiation, further reducing the efficiency of optical rectification. We have used terahertz emission spectroscopy to study these effects by analyzing the dependence of the terahertz waveform on excitation fluence. At high excitation fluences, the overall efficiency is reduced and the trailing edge of the waveform is attenuated. A simple model reproduces the measured behavior.

http://ftp.aip.org/epaps/journ_appl_phys/E-JAPIAU-107-069003/SupplementaryInformation.pdf

Influence of free-carrier absorption on terahertz generation from ZnTe(110)

The influence of elliptically and circularly polarized excitation on terahertz emission from unbiased bulk GaAs at normal incidence and room temperature is reported. Illumination of GaAs above the bandgap produces both spin-polarized electrons and shift currents. The induced currents are monitored via terahertz emission spectroscopy. The terahertz emission amplitude is compared to theoretical calculations as a function of excitation beam ellipticity. Exciting slightly above the bandgap (800 nm at room temperature) with elliptical polarization generates shift currents that deviate substantially from theoretical predictions. On the other hand, exciting either below the bandgap (835 nm at 77 K) to produce optical rectification or far above the bandgap (400 nm at room temperature) to produce shift currents generates emission in agreement with theoretical calculations. Spin-polarized electrons created by elliptically polarized excitation are the source of the observed discrepancy.

Effect of spin-polarized electrons on terahertz emission from photoexcited GaAs

Much can be learned about charge and magnetization dynamics at surfaces and in nanometer thickness films through terahertz emission spectroscopy (TES) In some respects, TES is the difference-frequency analog of second harmonic generation (SHG) As such, interface-specific properties contribute to the generation of a THz pulse upon ultrafast optical excitation In addition, there can be bulk contributions as well, as is also true in SHG The dependence of THz pulse emission on surface orientation has been used to study carrier dynamics, both real and virtual, in GaAs(111) We find that the dependence on the angle of linearly polarized excitation is well described by known theory Magnetization dynamics in polycrystalline nickel films ranging in thickness from 5 nm to 60 nm have also been characterized with TES Distinct bulk and surface contributions each play a role, and their origins are discussed

Studies of interface polarization and magnetization dynamics using terahertz emission spectroscopy

The advent of rapid, cost-effective, high-powered lasers are driving forward advancements in the fields of for soft X-ray and EUV imaging, ptychography, spectroscopy and tomography. This coupled with increasing investment in industrial EUV lithography and semiconductor metrology processes is creating demand for fast, high resolution and sensitive detectors in the 20 eV to 10 keV range. Here we present the latest detector developments in the field of sCMOS for direct EUV and soft X-ray detection. We demonstrate how sCMOS technology is able to overcome the limitations of slow scan CCD technology to provide high resolution direct EUV and soft X-ray detection with a greater than 16-fold speed increase whilst maintaining low noise compared to classical CCD detectors.

Next generation fast sCMOS detector development for EUV and soft X-ray high-harmonic generation, semiconductor metrology, X-ray absorption spectroscopy, soft X-ray microscopy, and tomography

Shayne M. Harrel

Papers

Coherent terahertz emission from ferromagnetic films excited by femtosecond laser pulses

Influence of free-carrier absorption on terahertz generation from ZnTe(110)

Effect of spin-polarized electrons on terahertz emission from photoexcited GaAs

Studies of interface polarization and magnetization dynamics using terahertz emission spectroscopy

Next generation fast sCMOS detector development for EUV and soft X-ray high-harmonic generation, semiconductor metrology, X-ray absorption spectroscopy, soft X-ray microscopy, and tomography