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

One of the most intriguing features of some high-temperature cuprate superconductors is the interplay between one-dimensional "striped" spin order and charge order, and superconductivity. We used mid-infrared femtosecond pulses to transform one such stripe-ordered compound, nonsuperconducting La(1.675)Eu(0.2)Sr(0.125)CuO(4), into a transient three-dimensional superconductor. The emergence of coherent interlayer transport was evidenced by the prompt appearance of a Josephson plasma resonance in the c-axis optical properties. An upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which is significantly faster than expected. This places stringent new constraints on our understanding of stripe order and its relation to superconductivity.

https://science.sciencemag.org/content/sci/331/6014/189.full.pdf

Light-Induced Superconductivity in a Stripe-Ordered Cuprate

We used 600-femtosecond electron pulses to study the structural evolution of aluminum as it underwent an ultrafast laser–induced solid-liquid phase transition. Real-time observations showed the loss of long-range order that was present in the crystalline phase and the emergence of the liquid structure where only short-range atomic correlations were present; this transition occurred in 3.5picoseconds for thin-film aluminum with an excitation fluence of 70 millijoules per square centimeter. The sensitivity and time resolution were sufficient to capture the time-dependent pair correlation function as the system evolved from the solid to the liquid state. These observations provide an atomic-level description of the melting process, in which the dynamics are best understood as a thermal phase transition under strongly driven conditions.

/pdf/an-atomic-level-view-of-melting-using-femtosecond-electron-3dgv8l1htf.pdf

An Atomic-Level View of Melting Using Femtosecond Electron Diffraction

We review studies of the electromagnetic response of various classes of correlated electron materials including transition metal oxides, organic and molecular conductors, intermetallic compounds with $d$- and $f$-electrons as well as magnetic semiconductors. Optical inquiry into correlations in all these diverse systems is enabled by experimental access to the fundamental characteristics of an ensemble of electrons including their self-energy and kinetic energy. Steady-state spectroscopy carried out over a broad range of frequencies from microwaves to UV light and fast optics time-resolved techniques provide complimentary prospectives on correlations. Because the theoretical understanding of strong correlations is still evolving, the review is focused on the analysis of the universal trends that are emerging out of a large body of experimental data augmented where possible with insights from numerical studies.

/pdf/electrodynamics-of-correlated-electron-materials-2qg1x9aaaz.pdf

Electrodynamics of correlated electron materials

Surface plasmon polaritons, propagating bound oscillations of electrons and light at a metal surface, have great potential as information carriers for next-generation, highly integrated nanophotonic devices [1,2]. Since the term 'active plasmonics' was coined in 2004 [3], a number of techniques for controlling the propagation of guided surface plasmon polariton signals have been demonstrated [4-7]. However, with sub-microsecond or nanosecond response times at best, these techniques are likely to be too slow for future applications in such fields as data transport and processing. Here we report that femtosecond optical frequency plasmon pulses can propagate along a metal-dielectric waveguide and that they can be modulated on the femtosecond timescale by direct ultrafast optical excitation of the metal, thereby offering unprecedented terahertz modulation bandwidth - a speed at least five orders of magnitude faster than existing technologies.

/pdf/ultrafast-active-plasmonics-29wmd0qszw.pdf

Ultrafast active plasmonics

Laser induced dynamic spectral weight transfer in La0.7Ca0.3MnO3

The operation of a 1030 nm, single, thin disk laser which produced 6.5 kW of laser output power with 57 percent slope efficiency is reported. The Yb:YAG ceramic gain element is 200 μm thick, bonded to a 1 mm, undoped, ceramic YAG cap. The gain element is pumped by diodes at 940 nm. The maximum incident pump intensity was 5 kW/cm2, which yielded an output intensity of 2.6 kW/cm2 of multimode laser radiation. Rigrod analysis suggests that the laser operates with inhomogeneous gain saturation. This is attributed to the enhanced, spatial‐hole‐burning effect when the gain element is adjacent to a mirror. The pump threshold and output intensities were independent of pump spot size, which validates area scaling. Observed thermal lensing contributions include thermal‐expansion‐induced disk flexure, pump‐edge‐induced temperature profile and a strong thermal imprint of the cooling nozzle due to the direct jet impingement on the high reflection (HR) coated side. Weak absorption of the 1030 nm intracavity intensity i...

6.5 kW, Yb:YAG Ceramic Thin Disk Laser

Coherent phonon generation mechanism in solids

We investigated the ultrafast carrier dynamics in InSb using a nonlinear pump-probe technique. The measurement involves two pump pulses chopped at different frequencies and the detection of the probe modulation at their sum frequency. We show that the nonlinear signature and dynamics of the transmission-bleaching signal enables the differentiation of carrier thermalization from cooling dynamics. We follow the evolution of the injected carrier distribution from a nonthermal to a thermal one and calculate its absolute temperature and energy decay rates as a function of time from a single transmission pump-probe measurement. The carrier scattering rate in the thermalization stage increases by (3 ps)−1 with doubling the injected carrier density. For an initial injected excess energy of 650 K, carrier thermalization is complete after 1.7 ps with a thermal distribution of 370 K. We also demonstrate the ability to follow carrier diffusion through the decay of the carrier grating decay in a transmission measurement.

Carrier thermalization, cooling and diffusion in InSb through a nonlinear pump-probe technique

A femtosecond, tunable color center laser was used to conduct degenerate pump-probe transmission spectroscopy of thin film low temperature grown molecular beam epitaxy In0.53Ga0.47As samples. Low temperature molecular beam epitaxy In0.53Ga0.47As exhibits a growth-temperature dependent femtosecond optical response when probed near the conduction band edge. Below Tg=250°C, the optical response time of the material is subpicosecond in duration, and we observed induced absorption, which we suggest is due to the formation of a quasi-“three-level system”.

Femtosecond optical response of low temperature grown In 0.53 Ga 0.47 As

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