There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

The field of nanoparticle research has drawn much attention in the past decade as a result of the search for new materials. Size confinement results in new electronic and optical properties, possibly suitable for many electronic and optoelectronic applications. A characteristic feature of noble metal nanoparticles is the strong color of their colloidal solutions, which is caused by the surface plasmon absorption. This article describes our studies of the properties of the surface plasmon absorption in metal nanoparticles that range in size between 10 and 100 nm. The effects of size, shape, and composition on the plasmon absorption maximum and its bandwidth are discussed. Furthermore, the optical response of the surface plasmon absorption due to excitation with femtosecond laser pulses allowed us to follow the electron dynamics (electron−electron and electron−phonon scattering) in these metal nanoparticles. It is found that the electron−phonon relaxation processes in nanoparticles, which are smaller than t...

Spectral properties and relaxation dynamics of surface plasmon electronic oscillations in gold and silver nanodots and nanorods

The properties of a material depend on the type of motion its electrons can execute, which depends on the space available for them (i.e., on the degree of their spatial confinement). Thus, the properties of each material are characterized by a specific length scale, usually on the nanometer dimension. If the physical size of the material is reduced below this length scale, its properties change and become sensitive to its size and shape. In this Account we describe some of the observed new chemical, optical, and thermal properties of metallic nanocrystals when their size is confined to the nanometer length scale and their dynamical processes are observed on the femto- to picosecond time scale.

Some interesting properties of metals confined in time and nanometer space of different shapes.

1 Introductory Concepts.- 1.1 Spontaneous and Stimulated Emission, Absorption.- 1.1.1 Spontaneous Emission.- 1.1.2 Stimulated Emission.- 1.1.3 Absorption.- 1.2 The Laser Idea.- 1.3 Pumping Schemes.- 1.4 Properties of Laser Beams.- 1.4.1 Monochromaticity.- 1.4.2 Coherence.- 1.4.3 Directionality.- 1.4.4 Brightness.- Problems.- 2 Interaction of Radiation with Matter.- 2.1 Summary of Blackbody Radiation Theory.- 2.2 Absorption and Stimulated Emission.- 2.2.1 Rates of Absorption and Stimulated Emission.- 2.2.2 Allowed and Forbidden Transitions.- 2.2.3 Transition Cross Section, Absorption and Gain Coefficient.- 2.3 Spontaneous Emission.- 2.3.1 Semiclassical Approach.- 2.3.2 Quantum Electrodynamic Approach.- 2.3.3 Einstein Thermodynamic Treatment.- 2.3.4 Radiation Trapping, Superradiance, Superfluorescence, and Amplified Spontaneous Emission.- 2.4 Nonradiative Decay.- 2.5 Line Broadening Mechanisms.- 2.5.1 Homogeneous Broadening.- 2.5.2 Inhomogeneous Broadening.- 2.5.3 Combined Effect of Line Broadening Mechanisms.- 2.6 Saturation.- 2.6.1 Saturation of Absorption: Homogeneous Line.- 2.6.2 Gain Saturation: Homogeneous Line.- 2.6.3 Inhomogeneously Broadened Line.- 2.7 Degenerate Levels.- 2.8 Relation between Cross Section and Spontaneous Radiative Lifetime.- 2.9 Molecular Systems.- 2.9.1 Energy Levels of a Molecule.- 2.9.2 Level Occupation at Thermal Equilibrium.- 2.9.3 Radiative and Nonradiative Transitions.- Problems.- References.- 3 Pumping Processes.- 3.1 Introduction.- 3.2 Optical Pumping.- 3.2.1 Pumping Efficiency.- 3.2.2 Pump Light Distribution.- 3.2.3 Pumping Rate.- 3.3 Electrical Pumping.- 3.3.1 Electron Impact Excitation.- 3.3.2 Spatial Distribution of the Pump Rate.- 3.3.3 Pumping Efficiency.- 3.3.4 Excitation by (Near) Resonant Energy Transfer.- Problems.- References.- 4 Passive Optical Resonators.- 4.1 Introduction.- 4.2 Plane-Parallel Resonator.- 4.2.1 Approximate Treatment of Schawlow and Townes.- 4.2.2 Fox and Li Treatment.- 4.3 Confocal Resonator.- 4.4 Generalized Spherical Resonator.- 4.4.1 Mode Amplitudes, Diffraction Losses, and Resonance Frequencies.- 4.4.2 Stability Condition.- 4.5 Unstable Resonators.- Problems.- References.- 5 Continuous Wave and Transient Laser Behavior.- 5.1 Introduction.- 5.2 Rate Equations.- 5.2.1 Four-Level Laser.- 5.2.2 Three-Level Laser.- 5.3 CW Laser Behavior.- 5.3.1 Four-Level Laser.- 5.3.2 Three-Level Laser.- 5.3.3 Optimum Output Coupling.- 5.3.4 Reasons for Multimode Oscillation.- 5.3.5 Single-Line and Single-Mode Oscillation.- 5.3.6 Two Numerical Examples.- 5.3.7 Frequency Pulling and Limit to Monochromaticity.- 5.3.8 Lamb Dip and Active Stabilization of Laser Frequency.- 5.4 Transient Laser Behavior.- 5.4.1 Spiking Behavior of Single-Mode and Multimode Lasers.- 5.4.2 Q-Switching.- 5.4.2.1 Methods of Q-Switching.- 5.4.2.2 Operating Regimes.- 5.4.2.3 Theory of Q-Switching.- 5.4.2.4 A Numerical Example.- 5.4.3 Mode Locking.- 5.4.3.1 Methods of Mode Locking.- 5.4.3.2 Operating Regimes.- 5 5 Limits to the Rate Equations.- Problems.- References.- 6 Types of Lasers.- 6.1 Introduction.- 6.2 Solid-State Lasers.- 6.2.1 The Ruby Laser.- 6.2.2 Neodymium Lasers.- 6.3 Gas Lasers.- 6.3.1 Neutral Atom Lasers.- 6.3.2 Ion Lasers.- 6.3.2.1 Ion Gas Lasers.- 6.3.2.2 Metal Vapor Lasers.- 6.3.3 Molecular Gas Lasers.- 6.3.3.1 Vibrational-Rotational Lasers.- 6.3.3.2 Vibronic Lasers.- 6.3.3.3 Excimer Lasers.- 6.4 Liquid Lasers (Dye Lasers).- 6.4.1 Photophysical Properties of Organic Dyes.- 6.4.2 Characteristics of Dye Lasers.- 6.5 Chemical Lasers.- 6.6 Semiconductor Lasers.- 6.6.1 Photophysical Properties of Semiconductor Lasers.- 6.6.2 Characteristics of Semiconductor Lasers.- 6.7 Color-Center Lasers.- 6.8 The Free-Electron Laser.- 6.9 Summary of Performance Data.- Problems.- References.- 7 Properties of Laser Beams.- 7.1 Introduction.- 7.2 Monochromaticity.- 7.3 First-Order Coherence.- 7.3.1 Complex Representation of Polychromatic Fields.- 7.3.2 Degree of Spatial and Temporal Coherence.- 7.3.3 Measurement of Spatial and Temporal Coherence.- 7.3.4 Relation between Temporal Coherence and Monochromaticity.- 7.3.5 Some Numerical Examples.- 7.4 Directionality.- 7.5 Laser Speckle.- 7.6 Brightness.- 7.7 Higher-Order Coherence.- Problems.- References.- 8 Laser Beam Transformation.- 8.1 Introduction.- 8.2 Transformation in Space. Gaussian Beam Propagation.- 8.3 Transformation in Amplitude: Laser Amplification.- 8.4 Transformation in Frequency: Second-Harmonic Generation and Parametric Oscillation.- 8.4.1 Physical Picture.- 8.4.1.1 Second-Harmonic Generation.- 8.4.1.2 Parametric Oscillation.- 8.4.2 Analytical Treatment.- 8.4.2.1 Parametric Oscillation.- 8.4.2.2 Second-Harmonic Generation.- Problems.- References.- 9 Applications of Lasers.- 9.1 Introduction.- 9.2 Applications in Physics and Chemistry.- 9.3 Applications in Biology and Medicine.- 9.4 Material Working.- 9.5 Optical Communications.- 9.6 Measurement and Inspection.- 9.7 Thermonuclear Fusion.- 9.8 Information Processing and Recording.- 9.9 Military Applications.- 9.10 Holography.- 9.11 Concluding Remarks.- References.- Appendixes.- A Space-Dependent Rate Equations.- B Physical Constants.- Answers to Selected Problems.

Principles of Lasers

Summary form only given. In this work, we present a novel laser pulse compression technique for Ti:sapphire chirped pulse laser amplifiers, based on spectral broadening in a hollow cylindrical quartz fiber filled with noble gases at high pressure, which is suitable for high-energy pulses. Pulse recompression is then achieved using two conventional quartz prism pairs.

Generation of high-energy 10-fs pulses by a new pulse compression technique

High-energy 20-fs pulses generated by a Ti:sapphire laser system were spectrally broadened to more than 250 nm by self-phase modulation in a hollow fiber filled with noble gases and subsequently compressed in a broadband high-throughput dispersive system. Pulses as short as 4.5 fs with energy up to 20-microJ were obtained with krypton, while pulses as short as 5 fs with energy up to 70 microJ were obtained with argon. These pulses are, to our knowledge, the shortest generated to date at multigigawatt peak powers.

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Compression of high-energy laser pulses below 5 fs

We demonstrate generation of 3.8-fs pulses with energies of up to 15 μJ from a supercontinuum produced in two cascaded hollow fibers. Ultrabroadband dispersion compensation was achieved through a closed-loop combination of a spatial light modulator for adaptive pulse compression and spectral-phase interferometry for direct electric-field reconstruction (SPIDER) measurements as feedback signal.

/pdf/generation-of-3-8-fs-pulses-from-adaptive-compression-of-a-4wdc78l6wg.pdf

Generation of 3.8-fs pulses from adaptive compression of a cascaded hollow fiber supercontinuum.

Powerful techniques for spectral broadening and ultrabroadband dispersion control, which allow the compression of high-energy femtosecond pulses to a duration of a few optical cycles, are presented. Spectral broadening by propagation along hollow-core fused silica fiber filled with atomic and molecular gases is studied under two excitation regimes with high-energy input pulses of 140 fs and 20 fs duration respectively. Conditions for optimum pulse compression are outlined considering the role of self-phase modulation and gas dispersion in the two regimes. With 20 fs input pulses and under optimum compression conditions we demonstrate a pulse shortening down to 4.5 fs with output energy up to 70 μJ using a high-throughput prism-chirped-mirror delay line. These pulses are the shortest generated to date at multigigawatt peak power. PACS: 42.65.Re; 42.65.Vh Ultrashort-pulse lasers are the most important experimental tools for investigating fast-evolving atomic and molecular dynamics in physics, chemistry, and biology. In the last few years, great technological advances have been made in the field of ultrafast pulse generation. New mode-locking techniques such as additive-pulse mode-locking and Kerr-lens mode-locking have been successfully used for femtosecond pulse generation from a wide range of solid-state laser oscillators [1]. Using chirped mirrors [2] for intracavity dispersion control, pulses down to 7.5 fs have been directly generated by a Kerr-lens mode-locked Ti:sapphire oscillator [3] and, more recently, 6.5-fs pulses have been obtained using broadband semiconductor saturable absorbers for self-starting [4]. Ti:sapphire amplifiers seeded by femtosecond laser oscillators can now generate pulses of 20–30 fs with gigawatt [5, 6] or terawatt [7–9] peak power at repetition rates in the kHz and 10 Hz regimes, respectively. Ultrashort pulses can also be generated by extracavity compression techniques, in which the pulses are spectrally broadened upon propagation in a suitable nonlinear waveguide and subsequently compressed in a carefully designed optical dispersive delay line. Spectral broadening of laser pulses by self-phase modulation (SPM) in a single-mode optical fiber is a well-established technique: pulses down to 6 fs were obtained in 1987 from 50-fs pulses from a mode-locked dye laser [10]. More recently 13-fs pulses from a cavity-dumped Ti:sapphire laser were compressed to 5 fs with the same technique [11]. However, the use of single-mode fibers limits the pulse energy to a few nanojoules. A powerful pulse compression technique based on spectral broadening in an hollow fiber filled with noble gases has demonstrated the capability of handling highenergy pulses (sub-mJ range) [12]. This technique presents the advantages of a guiding element with a large diameter mode and of a fast nonlinear medium with high threshold for multiphoton ionization. New concepts in the construction of dispersive delay lines have been applied in the development of specially designed chirped mirrors for fine control of cubic and quartic phase dispersion terms over a large spectral bandwidth [3]. The implementation of the hollow-fiber technique using 20-fs seed pulses from a Ti:sapphire system [5] and a high-throughput broadband dispersive delay line consisting of prisms and chirped mirrors has recently permitted the generation of multigigawatt sub-5 fs pulses [13]. In this paper we present a comprehensive analysis of compression experiments with high-energy femtosecond pulses performed using gas-filled hollow fibers. Spectral broadenings obtained in different gases are compared for 140-fs and 20-fs input pulses generated by Ti:sapphire laser systems, and the optimum conditions for pulse compression are outlined considering the role of SPM and gas dispersion. A new ultrabroadband prism-chirped-mirror dispersive delay line, characterized by a high throughput and dispersion control up to the fourth order, is described in detail. The paper is organized as follows. In Sect. 1 we provide a description of hollow fiber modes and discuss the major advantages of this device compared to optical fibers. Sect. 2 reports on typical spectral broadenings achieved under different excitation conditions. In Sect. 3 we report on the characteristics of the prism-chirped-mirror compressor and discuss the experimental results obtained with 20-fs input pulses. Under optimum compression conditions we show a pulse shortening down to 4.5 fs with output energy up to 70 μJ. These pulses are the

Orazio Svelto

Papers

Principles of Lasers

Generation of high-energy 10-fs pulses by a new pulse compression technique

Compression of high-energy laser pulses below 5 fs

Generation of 3.8-fs pulses from adaptive compression of a cascaded hollow fiber supercontinuum.

A novel-high energy pulse compression system: generation of multigigawatt sub-5-fs pulses