1. Introduction

1.1. Motivation

Nuclear magnetic resonance spectroscopy is a powerful tool for the investigation of material properties, which has found applications in many areas of physics, chemistry, biology and medicine. Its physical foundations, the excitation of transitions between nuclear spin states, was explored in the years after the second world war, and is well characterised by now. Today's interest in the field is based largely on the immense potential for applications: spins can serve as probes for their environment because they are weakly coupled to other degrees of freedom. In most magnetic resonance experiments, these couplings are used to monitor the environment of the nuclei, like spatial structure or molecular dynamics.
While the direct excitation of nuclear spin transitions requires radio frequency irradiation, it is often possible to use light for polarizing the spin system or for observing its dynamics. This possibility arises from the coupling of spins with the electronic degrees of freedom: optical photons excite transitions between states that differ both in electronic excitation energy as well as in their angular momentum states. In general, the light can serve three different purposes: it can polarize the spin system, thereby creating the 'raw material' for spectroscopic experiments; it can measure the spin polarization, serving as a detector; and it can influence the dynamics of the system. Some motivations for using light in magnetic resonance experiments include

1.2. Laser Spectroscopy

Apart from magnetic resonance, the interaction of matter with laser light is the second most important ingredient for the experiments that will be discussed below. In laser spectroscopy, light induces transitions between different electronic states. The differences between laser spectroscopy and magnetic resonance are related to the different frequency scales: optical frequencies are more than 6 orders of magnitude higher than rf frequencies. An important consequence is that laser spectroscopy is much more sensitive as a technique: optical experiments on individual atoms [Hurst et. al. 1979; Dehmelt 1990], and molecules [Nguyen et. al. 1987; Moerner et. al. 1989] have become routine in the last few years. While the first experiments of this type were performed in ultrahigh vacuum, they can now also be performed in condensed matter, like solutions [Nguyen et al. 1987] or solid matrices [Moerner et al. 1989].
Another characteristic property of laser spectroscopy is the high time resolution which is of course also intimately linked to the high frequencies of optical radiation. Technical limits to the time resolution are now of the order of a few femtoseconds, corresponding to only a few optical cycles. These characteristics are of course used extensively in various applications. In chemistry, the high time resolution has been used for 'real time' monitoring of chemical reactions, e.g. by measuring the intermolecular potential as a function of time during the reaction. Also in solid state physics, laser spectroscopy is used extensively to monitor fast processes, like excitation and recombination of electrons and holes in semiconductors.
In the field of high-resolution spectroscopy, it is also possible to achieve very high frequency resolution: some systems can now reach resolutions of better than 1 Hz, corresponding to a relative frequency stability of some 1014. This high frequency resolution is especially attractive for metrology, where many groups try to build new, more precise frequency standards that are based on optical transitions. The properties of laser spectroscopy, high frequency resolution and high temporal resolution can potentially also be used in optically enhanced magnetic resonance experiments - obviously an attractive perspective for many applications.
The article is structured as follows: the next section discusses the most important physical phenomena that permit the use of laser radiation in magnetic resonance experiments. Examples of applications to specific systems are discussed in the third section.