Light with a self-torque: extreme-ultraviolet beams with time-varying orbital angular momentum

Twisted light fields carrying orbital angular momentum (OAM) provide powerful capabilities for applications in optical communications, microscopy, quantum optics and microparticle rotation. Here we introduce and experimentally validate a new class of light beams, whose unique property is associated with a temporal OAM variation along a pulse: the self-torque of light. Self-torque is a phenomenon that can arise from matter-field interactions in electrodynamics and general relativity, but to date, there has been no optical analog. In particular, the self-torque of light is an inherent property, which is distinguished from the mechanical torque exerted by OAM beams when interacting with physical systems. We demonstrate that self-torqued beams in the extreme-ultraviolet (EUV) naturally arise as a necessary consequence of angular momentum conservation in non-perturbative high-order harmonic generation when driven by time-delayed pulses with different OAM. In addition, the time-dependent OAM naturally induces an azimuthal frequency chirp, which provides a signature for monitoring the self-torque of high-harmonic EUV beams. Such self-torqued EUV beams can serve as unique tools for imaging magnetic and topological excitations, for launching selective excitation of quantum matter, and for manipulating molecules and nanostructures on unprecedented time and length scales.Comment: 24 pages, 4 figure

FAST CARS: Engineering a Laser Spectroscopic Technique for Rapid Identification of Bacterial Spores

Airborne contaminants, e.g., bacterial spores, are usually analyzed by time consuming microscopic, chemical and biological assays. Current research into real time laser spectroscopic detectors of such contaminants is based on e.g. resonant Raman spectroscopy. The present approach derives from recent experiments in which atoms and molecules are prepared by one (or more) coherent laser(s) and probed by another set of lasers. The connection with previous studies based on "Coherent Anti-Stokes Raman Spectroscopy" (CARS) is to be noted. However generating and utilizing maximally coherent oscillation in macromolecules having an enormous number of degrees of freedom is much more challenging. This extension of the CARS technique is called FAST CARS (Femtosecond Adaptive Spectroscopic Techniques for Coherent Anti-Stokes Raman Spectroscopy), and the present paper proposes and analyses ways in which it could be used to rapidly identify pre-selected molecules in real time.Comment: 43 pages, 21 figures; replacement with references added. Submitted to the Proceedings of National Academy of Science

Tuning the electronic band structure of metal surfaces for enhancing high-order harmonic generation

High harmonic generation (HHG) from condensed matter phase holds promise to promote future cutting-edge research in the emerging field of attosecond nanoscopy. The key for the progress of the field relies on the capability of the existing schemes to enhance the harmonic yield and to push the photon energy cutoff to the extreme ultraviolet (XUV, 10-100 eV) regime and beyond towards the spectral "water window" region (282-533 eV). Here, we demonstrate a coherent control scheme of HHG, which we show to give rise to quantum modulations in the XUV region. The control scheme is based on exploring surface states in transition-metal surfaces, and specifically by tuning the electronic structure of the metal surface itself together with the use of optimal chirped pulses. Moreover, we show that the use of such pulses having moderate intensities permits to push the harmonic cutoff further to the spectral water window region, and that the extension is found to be robust against the change of the intrinsic properties of the material. The scenario is numerically implemented using a minimal model by solving the time-dependent Schrodinger equation for the metal surface Cu(111) initially prepared in the surface state. Our findings elucidate the importance of metal surfaces for generating coherent isolated attosecond XUV and soft-x-ray pulses and for designing compact solid-state HHG devices.Comment: 9 pages, 4 figure

Mapping Atomic Motions with Electrons: Toward the Quantum Limit to Imaging Chemistry

Recent advances in ultrafast electron and X-ray diffraction have pushed imaging of structural dynamics into the femtosecond time domain, that is, the fundamental time scale of atomic motion. New physics can be reached beyond the scope of traditional diffraction or reciprocal space imaging. By exploiting the high time resolution, it has been possible to directly observe the collapse of nearly innumerable possible nuclear motions to a few key reaction modes that direct chemistry. It is this reduction in dimensionality in the transition state region that makes chemistry a transferable concept, with the same class of reactions being applicable to synthetic strategies to nearly arbitrary levels of complexity. The ability to image the underlying key reaction modes has been achieved with resolution to relative changes in atomic positions to better than 0.01 Å, that is, comparable to thermal motions. We have effectively reached the fundamental space-time limit with respect to the reaction energetics and imaging the acting forces. In the process of ensemble measured structural changes, we have missed the quantum aspects of chemistry. This perspective reviews the current state of the art in imaging chemistry in action and poses the challenge to access quantum information on the dynamics. There is the possibility with the present ultrabright electron and X-ray sources, at least in principle, to do tomographic reconstruction of quantum states in the form of a Wigner function and density matrix for the vibrational, rotational, and electronic degrees of freedom. Accessing this quantum information constitutes the ultimate demand on the spatial and temporal resolution of reciprocal space imaging of chemistry. Given the much shorter wavelength and corresponding intrinsically higher spatial resolution of current electron sources over X-rays, this Perspective will focus on electrons to provide an overview of the challenge on both the theory and the experimental fronts to extract the quantum aspects of molecular dynamics

Fiber Laser Based Nonlinear Spectroscopy

To date, nonlinear spectroscopy has been considered an expensive technique and confined mostly to experimental laboratory settings. Over recent years, optical-fiber lasers that are highly reliable, simple to operate and relatively inexpensive have become commercially available, removing one of the major obstacles to widespread utilization of nonlinear optical measurement in biochemistry. However, fiber lasers generally offer relatively low output power compared to lasers traditionally used for nonlinear spectroscopy, and much more careful design is necessary to meet the excitation power thresholds for nonlinear signal generation. On the other hand, reducing the excitation intensity provides a much more suitable level of user-safety, minimizes damage to biological samples and reduces interference with intrinsic chemical processes. Compared to traditional spectroscopy systems, the complexity of nonlinear spectroscopy and imaging instruments must be drastically reduced for them to become practical. A nonlinear spectroscopy tool based on a single fiber laser, with electrically controlled wavelength-tuning and spectral resolution enhanced by a pulse shaping technique, will efficiently produce optical excitation that allows quantitative measurement of important nonlinear optical properties of materials. The work represented here encompasses the theory and design of a nonlinear spectroscopy and imaging system of the simplest architecture possible, while solving the difficult underlying design challenges. With this goal, the following report introduces the theories of nonlinear optical propagation relevant to the design of a wavelength tunable system for nonlinear spectroscopy applications, specifically Coherent Anti-Stokes Spectroscopy (CARS) and Förster Resonance Energy Transfer (FRET). It includes a detailed study of nonlinear propagation of optical solitons using various analysis techniques. A solution of the generalized nonlinear Schrödinger equation using the split-step Fourier method is demonstrated and investigation of optical soliton propagation in fibers is carried out. Other numerical methods, such as the finite difference time domain approach and spectral-split step Fourier methods are also described and compared. Numerical results are contrasted with various measurements of wavelength shifted solitons. Both CARS and FRET test-bed designs and experiments are presented, representing two valuable biochemical measurement applications. Two-photon excitation experiments with a simplified calibration process for quantitative FRET measurement were conducted on calmodulin proteins modified with fluorescent dyes, as well as modified enhanced green fluorescent protein. The resulting new FRET efficiency measurements showed agreement with those of alternative techniques which are slower and can involve destruction of the sample. In the second major application of the nonlinear spectroscopy system, CARS measurement with enhanced spectral resolution was conducted on cyclohexane as well as on samples of mouse brain tissue containing lipids with Raman resonances. The measurements of cyclohexane verified the ability of the system to precisely determine its Raman resonances, thus providing a benchmark within a similar spectral range for biological materials which have weaker Raman signal responses. The improvement of spectral resolution (resonance frequency selectivity), was also demonstrated by measuring the closely-spaced resonances of cyclohexane. Finally, CARS measurements were also made on samples of mouse brain tissue which has a lipids-based Raman signature. The CARS spectrum of the lipid resonances matched well with other cited studies. The imaging of mouse brain tissue with Raman resonance contrast was also partially achieved, but it was hindered by low signal to noise ratio and limitations of the control hardware that led to some dropout of the CARS signal due to power coupling fluctuations. Nevertheless, these difficulties can be straightforwardly addressed by refinement of the wavelength tuning electronics. In conclusion, it is hoped that these efforts will lead to greater accessibility and use of CARS, FRET and other nonlinear spectral measurement instruments, in line with the promising advances in optics and laser technology