The impact of the invention of masers and lasers on science and technology has been profound. Here, the histories of the invention of the ruby laser and He-Ne laser are described in detail. The applications of lasers to basic science and engineering are also reviewed,and the effects of laser technology on society are outlined.

The progress in semiconductor lasers over the past half-century is reviewed. Semiconductor lasers, which are key devices in our information-based society, have the following salient features : Among the various kinds of lasers, semiconductor lasers are produced in by far the greatest quantity. They are used in the widest range of applications. Japan has been playing a leading role in the research, development and production of semiconductor lasers. The science and engineering of semiconductor lasers has made continuous progress over the past half-century.

A free electron laser (FEL) is a prospective X-ray coherent light source. A self-amplified spontaneous emission (SASE)-type FEL, which produces intense X-ray pulses by single-pass amplification in a high-gain medium, has recently been realized, about 50 years after the invention of the laser. In this article, after a brief summary of the principle of FEL emission, the present status of SASE-FEL developments in Europe, USA, and Japan is reported together with future plans for achieving an X-ray FEL with high temporal coherence. Future X-ray experiments using SASE-FEL are also discussed.

This paper reviews 50 years of advances in laser-based information communication technology(ICT), and mainly focuses on optical fiber communication. The advances are classified into three.

The technique of chirped-pulse amplification (CPA) has opened new avenues for the production of high-energy ultrafast laser pulses without optical damage to laser amplifiers. The combination of CPA and ultrabroad-band solid-state laser materials has made it possible to produce 10-fs-range pulses with petawatt peak power. Such lasers create superstrong fields equivalent to 170 times the field binding the ground-state electron in the hydrogen atom. At such intensities the electron velocity in the laser field also becomes relativistic. Thus, the free electrons move at close to the speed of light and their mass changes dramatically compared with their rest mass. In this article, recent advances in ultrafast, high-peak-power lasers using CPA and optical parametric chirped-pulse amplification (OPCPA) are reviewed. Further developments of emerging CPA/OPCPA lasers towards higher intensity, achieving a higher repetition rate and shorter pulse duration are also described.

The laser was first realized in 1960 by the development of maser in the frequency of optical range based on stimulated emission of radiation. The frequency of the coherent radiation of a laser jumped to several hundred THz in a single step from GHz, which was the highest frequency of coherent electromagnetic waves when lasers were first realized. For a long time the intermediate frequency was left as an unexplored area called the “terahertz gap”. However, in recent years the development of THz-wave sources by various techniques has been accomplished, and a variety of applications have been investigated. The progress in the development of THz-wave sources and their future prospects are described.

Photonic crystals, in which the refractive index changes periodically, are a valuable tool for the manipulation of photons, in which substantial progress has been made in recent years. In this article, the auther reviws two topics on photonic crystal lasers : (i) ultimate nanolasers based on the photonic band-gap effect and high-Q nanocavities, and (ii) broad-area photonic crystal lasers based on the photonic band-edge effect.

More than 30 years have passed since lasers were introduced in industry. Laser beam can be focused precisely and can process materials with negligible reaction force. Laser-processing machines have been developed utilizing these advantages in combination with modern numerical control technologies. Laser-processing machines are thus recognized as one of the essential tools in manufacturing industries.

Excimer laser lithography was developed in the 2nd half of 1980's by combinating the concepts of monochromatic projection optics and line-narrowed excimer lasers. Excimer laser has now been lithography used in industry for 15 years. During this period, KrF, ArF and F₂ excimer lasers, and the tequniques of immersion, double patterning are developed very rapidly. The power of the first KrF excimer laser power was only 1W, but the power of the latest ArF laser GT62A (developed in 2009) is 90W. On the other hand EUV lithography is mainly used below the 32nm node. We have already demonstrated the power of 69W at an intermediate focus using with an LPP-EUV source. A 400W EUV light source appears seems feasible. The history, current status, and future development in light sources for microlithography are described.

One of the subjects of the 2005 Nobel Prize in Physics was the development of laser-based precision spectroscopy, including the optical frequency comb technique. As a technology integrating ultrafast laser physics and ultraprecision spectroscopy, the optical frequency comb has become an essential tool for various studies on precision measurement and standards. In this report, we review the history of the optical frequency comb and outline the principles of comb measurement. The development and applications of the optical frequency comb are also introduced.

The definition and physical discription of temperature are explained in terms of energy and entropy. Temperature corresponds to the average energy shared equally among each degree of freedom in thermal equilibrium. As an example, the translational kinetic energy in ideal gases is calculated. An important viewpoint is that temperature does not mean the total energy, but the average energy of a system. In this article, temperature is originally defined as the coefficient of entropy change with respect to energy transfer between two systems. Another important viewpoint is that temperature is a crucial index for determining the thermal equilibrium condition based on the maximal entropy.