Summary form only given. Fundamental processes in atoms, molecules, as well as condensed matter are triggered or mediated by the motion of electrons inside or between atoms. Electronic dynamics on atomic length scales tends to unfold within tens to thousands of attoseconds (1 attosecond [as] = 10-18 s). Recent breakthroughs in laser science are now opening the door to watching and controlling these hitherto inaccessible microscopic dynamics. The key to accessing the attosecond time domain is the control of the electric field of (visible) light, which varies its strength and direction within less than a femtosecond (1 femtosecond = 1000 attoseconds). Atoms exposed to a few oscillations cycles of intense laser light are able to emit a single extreme ultraviolet (XUV) burst lasting less than one femtosecond. Full control of the evolution of the electromagnetic field in laser pulses comprising a few wave cycles have recently allowed the reproducible generation and measurement of isolated sub-femtosecond XUV pulses, demonstrating the control of microscopic processes (electron motion and photon emission) on an attosecond time scale. These tools have enabled us to visualize the oscillating electric field of visible light with an attosecond "oscilloscope", to control single-electron and probe multi-electron dynamics in atoms, molecules and solids. Recent experiments hold promise for the development of an attosecond X-ray source, which may pave the way towards 4D electron imaging with sub-atomic resolution in space and time.

Attosecond Physics

This review discusses how low-energy, valence excitations created by swift electrons can render information on the optical response of structured materials with unmatched spatial resolution. Electron microscopes are capable of focusing electron beams on sub-nanometer spots and probing the target response either by analyzing electron energy losses or by detecting emitted radiation. Theoretical frameworks suited to calculate the probability of energy loss and light emission (cathodoluminescence) are revisited and compared with experimental results. More precisely, a quantum-mechanical description of the interaction between the electrons and the sample is discussed, followed by a powerful classical dielectric approach that can be in practice applied to more complex systems. We assess the conditions under which classical and quantum-mechanical formulations are equivalent. The excitation of collective modes such as plasmons is studied in bulk materials, planar surfaces, and nanoparticles. Light emission induced by the electrons is shown to constitute an excellent probe of plasmons, combining sub-nanometer resolution in the position of the electron beam with nanometer resolution in the emitted wavelength. Both electron energy-loss and cathodoluminescence spectroscopies performed in a scanning mode of operation yield snap shots of plasmon modes in nanostructures with fine spatial detail as compared to other existing imaging techniques, thus providing an ideal tool for nanophotonics studies.

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Optical excitations in electron microscopy

http://nathan.instras.com/ResearchProposalDB/doc-108.pdf

Radiative decay engineering 5: metal-enhanced fluorescence and plasmon emission

It is shown that the ordinary semiclassical theory of the absorption of light by exciton states is not completely satisfactory (in contrast to the case of absorption due to interband transitions). A more complete theory is developed. It is shown that excitons are approximate bosons, and, in interaction with the electromagnetic field, the exciton field plays the role of the classical polarization field. The eigenstates of the system of crystal and radiation field are mixtures of photons and excitons. The ordinary one-quantum optical lifetime of an excitation is infinite. Absorption occurs only when "three-body" processes are introduced. The theory includes "local field" effects, leading to the Lorentz local field correction when it is applicable. A Smakula equation for the oscillator strength in terms of the integrated absorption constant is derived.

a Quantum-Mechanical Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals.

A fiber-optic chemical sensor is presented which utilizes surface plasmon resonance excitation The sensing element of the fiber has been fabricated by removing a section of the fiber cladding and symmetrically depositing a thin layer of highly reflecting metal onto the fiber core A white-light source is used to introduce a range of wavelengths into the fiber optic Changes in the sensed parameters (eg, bulk refractive index, film thinkness and film refractive index) are determined by measuring the transmitted spectral-intensity distribution Experimental results of the sensitivity and the dynamic range in the measurement of the refractive indices of aqueous solutions are in agreement with the theoretical model of the sensor

A fiber-optic chemical sensor based on surface plasmon resonance

Fundamentals of quantum electronics

A theoretical analysis of the pump-induced temperature change and associated thermal phase shift occurring in a pumped doped fiber is presented. Although the primary devices targeted are all-optical switches based on doped fibers, where such effects can be detrimental, this analysis is also applicable to lasers, amplifiers, and other doped fiber devices. The effects of a single pump pulse, multiple pulses and continuous wave (CW) pumping are investigated, both in the dynamic and steady-state regimes. Simple expressions are derived for the thermal relaxation time constant of a fiber, and for its steady-state temperature rise and thermal phase shift under CW pumping. This study predicts that in all-optical fiber switches utilizing a reasonably good dopant the thermal effect due to a single short pulse is negligible in all interferometers, while the steady-state effect can be sizable in a standard fiber Mach-Zehnder but is negligible in a twin-core fiber, a two-mode fiber, and a specially designed Mach-Zehnder interferometer.

Thermal effects in doped fibers

A method for spanning the 100−1000−μ portion of the spectrum with continuously tunable coherent radiation is described. The approach is based upon laser light scattering from the long−wavelength side of the A1−symmetry soft mode in LiNbO3. In contrast with other techniques, this method uses a single fixed−frequency pump source, requires no magnetic field, provides continuous rather than discrete tuning, can cover most of the 100−1000−μ range, operates at room temperature, and is simple to tune. The experimental data show that tuning was obtained from approximately 150 to 700 μ.

Continuously tunable submillimeter wave source

A laser method for accelerating electrons is described, based on the inverse Cherenkov effect in a gas. The laser fields are in the form of a cylindrical cone of plane waves on whose axis travel the electrons, with the cone angle and the gas refraction index such that each electron sees constant fields in time. Expressions are obtained relating the overall energy transfer to total laser power and wavelength, and to gas index and interaction length. With laser powers now available, energy increments of tens of GeV are possible. For comparative purposes, a related alternative scheme involving electrons in vacuum and evanescent laser fields is also analyzed. It is found that the method applies particularly well to adding energy to the electron bunches produced by large microwave accelerators, as collision effects are less troublesome at high injection energies.

A high‐energy, laser accelerator for electrons using the inverse Cherenkov effect

Radiation from 56-MeV and 28-MeV electrons channeled along major planes or along a major axis of an 18-..mu..m thick silicon crystal has been observed. Unpredicted spectral peaks in the range from 10 to 130 keV were observed and resolved for planar-channeling electrons, whereas only a large low-energy enhancement was seen for axial-channeling electrons.

Richard H. Pantell

Papers

Fundamentals of quantum electronics

Thermal effects in doped fibers

Continuously tunable submillimeter wave source

A high‐energy, laser accelerator for electrons using the inverse Cherenkov effect

Observation of Channeling Radiation from Relativistic Electrons