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This chapter describes lasers and other sources of coherent light that operate in a wide wavelength range. First, the general principles for the generation of coherent continuous-wave and pulsed radiation are treated including the interaction of radiation with matter, the properties of optical resonators and their modes as well as such processes as Q-switching and mode-locking. The general introduction is followed by sections on numerous types of lasers, the emphasis being on todayʼs most important sources of coherent light, in particular on solid-state lasers and several types of gas lasers. An important part of the chapter is devoted to the generation of coherent radiation by nonlinear processes with optical parametric oscillators, difference- and sum-frequency generation, and high-order harmonics. Radiation in the extended ultraviolet (EUV) and X-ray ranges can be generated by free electron lasers (FEL) and advanced X-ray sources. Ultrahigh light intensities up to 10 W/cm open the door to studies of relativistic laser–matter interaction and laser particle acceleration. The chapter closes with a section on laser stabilization.

In this contribution some basic properties of femtosecond laser pulses are summarized. In Sect.  we start with the linear properties of ultrashort light pulses. Nonlinear optical effects that would alter the frequency spectrum of an ultrashort pulse are not considered. However, due to the large bandwidth, the linear dispersion is responsible for dramatic effects. For example, a 10 fs laser pulse at a center wavelength of 800 nm propagating through 4 mm of BK7 glass will be temporally broadened to 50 fs. In order to describe and manage such dispersion effects a mathematical description of an ultrashort laser pulse is given first before we continue with methods how to change the temporal shape via the frequency domain. The chapter ends with a paragraph on the powerful technique of pulse shaping, which can be used to create complex-shaped ultrashort laser pulses with respect to phase, amplitude and polarization state.

In Sect.  the generation of femtosecond laser pulses via mode locking is described in simple physical terms. As femtosecond laser pulses can be generated directly from a wide variety of lasers with wavelengths ranging from the ultraviolet to the infrared no attempt is made to cover the different technical approaches.

In Sect.  we deal with the measurement of ultrashort pulses. Traditionally a short event has been characterized with the aid of an even shorter event. This is not an option for ultrashort light pulses. The characterization of ultrashort pulses with respect to amplitude and phase is therefore based on optical correlation techniques that make use of the short pulse itself. Methods operating in the time–frequency domain are especially useful.

Spectroscopy is the most important method for gaining detailed information on the structure and dynamics of atoms and molecules. The essential criteria of any spectroscopic technique are the attainable spectral resolution and the sensitivity.

Since lasers have been introduced as coherent narrow-band, intense radiation sources, spectroscopy has seen impressive progress. With single-mode lasers the spectral resolution can be greatly increased and finer details, generally hidden within the Doppler width of spectral lines, can be resolved. The available high intensity of lasers allows nonlinear spectroscopic techniques and the possibility of generating ultra-short light pulses has opened access to studies of very fast dynamical processes, such as the breaking of chemical bonds or the time-resolved redistribution of energy pumped into molecules by the absorption of photons.

In this first part we will concentrate on stationary methods of spectroscopy where the spectral resolution, the maximum achievable sensitivity and the development of optimum detectors are the main subjects.

Quantum optics is the study of the quantum theory of light at low energies and interactions with bound electronic systems. We discuss physically achievable states of the electromagnetic field, including squeezed states and single photons states, as well as schemes by which they may be generated and measured. Measured systems are necessarily open systems and we discuss how dissipation, noise and decoherence is treated in quantum optics in terms of Markov master equations, quantum trajectories and quantum stochastic differential equations. Quantum optics has recently proved a valuable test-bed to implement new communication protocols such as teleportation and quantum information processing and we discus some of these new schemes including ion traps and linear optics quantum computing.

Nanooptics deals with optical near fields, the electromagnetic fields that mediate the interaction between nanometric particles located in close proximity to each other. The projection-operator method is a theoretical description of how a virtual exciton–polariton is exchanged between these particles, corresponding to the nonresonant interaction. The optical near field mediates this interaction, and is represented by a Yukawa function, which means that the optical near-field energy is localized around the nanometric particles like an electron cloud around an atomic nucleus. Its decay length is proportional to the particle size. This chapter is primarily a review of nanophotonics, a leading branch of nanooptics, which is the technology utilizing the optical near field. The true nature of nanophotonics is to realize qualitative innovation in photonic devices, fabrication, and systems by utilizing novel functions and phenomena caused by optical near-field interactions, which are impossible as long as conventional propagating light is used. As evidence of such qualitative innovation, this chapter describes novel nanophotonic devices, nanophotonic fabrication, nanophotonic systems, and extensions related to science.

In this chapter we show that stimulated emission depletion (STED) microscopy and its derivative concepts are able to radically overcome the diffraction barrier in far-field fluorescence imaging, thus disclosing fluorescent details on the macromolecular scale even with diffracted beams of light.

The optical microscope is an invaluable tool in the life sciences because it is able to noninvasively image structures within cells and tissues. Unfortunately, due to the fact that light propagates as a wave, the smallest possible size of a focal spot in a far-field light microscope is limited by diffraction, putting a lower limit on the size of the structures which can be observed. Concretely, this means that for a lens of semiaperture angle , the full-width-half-maximum (FWHM) Δ of the main diffraction maximum of the point-spread function (PSF) in the focal plane of the lens is Δ = /(2 sin ), with and denoting the wavelength of light and the refractive index, respectively []. If the distance between two objects is smaller than this FWHM, the objects cannot be readily resolved from one another. The diffraction resolution limit is particularly disadvantageous in the life sciences where about 80% of all microscopy applications are carried out with far-field fluorescence systems.

Ultrafast THz photonics the topic of this chapter describes the union of optical and ultrafast laser capability with electronics to achieve frequency performance and bandwidths extending well into the THz frequency range. With the demonstrated capability to create and to measure subpicosecond electrical signals that are much faster than those produced and measured by any other method, THz photonics will help determine the direction for the development of new materials and ultra high performance technologies of the future.

In the Guided-Wave THz Photonics Section the generation, measurement and applications of these very short electrical pulses are described. They can be used as probes for short electrical pulse studies and the consequent characterization of transmission lines is described, including the measurement of Cherenkov radiation. The performance of transmission lines is compared with metal THz waveguides, whose characterization is also presented. Results for superconducting transmission lines and dielectric THz waveguides are given.

In the Freely Propagating Wave THz Photonics Section recent work demonstrating the generation of freely propagating THz radiation (1 THz =33.3  cm =4.1  meV) via material and electronic excitation by ultrashort laser pulses is presented. The generation of short pulses of THz radiation, by the passage of a short optical pulse through a nonlinear optical material, is also described. A cw photomixer capable of producing tunable radiation by beating together two laser beams is presented. The most developed THz application is THz time-domain spectroscopy (THz-TDS) which is described in detail. The combination of THz-TDS with THz beams will be shown to have some powerful advantages compared to traditional c.w. spectroscopy. The efficacy of THz-TDS is demonstrated by the presented characterizations of water vapor, flames, sapphire, high-resistivity silicon, n and p-type semiconductors, normal and high Tc superconductors, molecular vapors and liquids.

Due to the weak interaction of hard X rays with matter it is generally difficult to manipulate X rays by optical components. As a result, there have been many complementary approaches to making X-ray optics, exploiting refraction, reflection, and diffraction of X-rays by matter. In this chapter, we describe the physics that underly X-ray optics and explain the work principles and performances of a variety of X-ray optics, including refractive X-ray lenses, reflective optics, such as mirrors and waveguides, and diffractive optics, such as multilayer and crystal optics and Fresnel zone plates.

This chapter describes the fundamentals of radiation transport in general and in the Earthʼs atmosphere. The role of atmospheric aerosol and clouds are discussed and the connections between radiation and climate are described. Finally, natural optical phenomena of the atmosphere are discussed.

The term holography is composed of the Greek words (= whole) and (= to record, to write), and thus summarizes the key aspects of its underlying principle: recording the complete wavefront of an object, i.e., its intensity as well as its phase. Interference and diffraction phenomena are employed to record and retrieve the full information, a technique pioneered by Dennis Gabor in 1948. He was honored with the Nobel prize in Physics in 1971, reflecting the general impact of holography on modern physics.

Holography plays an essential role in todayʼs science and industry. Relevant applications making use of its principle have been developed, including three-dimensional (3-D) displays and holographic cameras, interferometers for nondestructive material analysis, archival data storage systems, diffractive optical systems, and embossed display holograms for security features. The success of holography was made possible in particular by the availability of coherent laser-light sources. In the meantime holography has even been performed using microwaves, neutrons, electrons, X-rays, and acoustic waves.

The first part of this chapter is devoted to holography itself. It provides an introduction to the historical development and reviews the principle of wavefront reconstruction. This section also includes an overview of hologram classification, recording/read-out geometries, holographic techniques and recording materials. Special emphasis is given to explaining the principles of some of the most important holographic applications, finishing with a brief insight into a few of the latest discoveries making use of Gaborʼs principle, such as holographic scattering and neutron diffractive optics.

The second part of this chapter addresses trends in optical storage, focussing on holographic data storage. It highlights different approaches to achieving increased optical storage density. This section also discusses the historical development of optical storage, the need for increased storage densities (and hence storage capacities) and the role of optical storage systems in todayʼs life.

Various approaches to increasing the areal density of optical storage systems are introduced. Next, the advantages of and approaches to volume optical recording that are currently under consideration for future generations of optical storage systems are presented. The state of the art as well as physical and technical attempts to realize holographic data storage are discussed in detail.