摘要： Multiferroics, that is, materials with a coexistence of long-range magnetic and electric order have been attracting tremendous interest because of pronounced coupling effects between magnetic and electric properties that may be the basis for novel devices in which a magnetization is controlled by an electric voltage rather than by energy-intensive electric-current-driven magnetic fields. For monitoring as well as controlling the magnetoelectric coupling, it is essential to have simultaneous access to the magnetic and electric phase of a multiferroic. Only then, the spatial relation between the magnetic and electric domain structures and their response to external perturbations like applied magnetic or electric fields can be studied. Nonlinear optics is particularly well suited for this purpose. The simplest nonlinear optical process is second harmonic generation (SHG) (cid:650) doubling of the frequency of a light wave in a material. SHG is a very symmetry-sensitive process. Any reduction of the symmetry of the frequency-doubling medium can lead to new SHG contributions. Since all types of ferroic order reduce the point group symmetry, SHG can be used as background-free probe of the occurring order. In general, the magnetic and the electric order of a multiferroic change symmetry in different ways so that they are represented by different SHG contributions. These are readily separated by polarization filters so that SHG is an ideal tool to observe the coexistence and correlation between the ferroic phases and their domains in a single experiment. Polarization-dependent SHG spectroscopy and imaging experiments are therefore ideal for analysing the magnetoelectric correlation in a broad variety of multiferroic materials. Some of the most important achievements of nonlinearoptics on multiferroics are: (cid:120) Detection of a coupling between antiferromagnetic and ferroelectric order in hexagonal manganites that occurs in the domain walls rather than in the domains. The result anticipated the current interest in domain walls as functional oxide interfaces [1]. (cid:120) A coupling between magnetic and electric domains in TbMnO3 sustained even across a first-order (cid:120) phase transition [2]. Inversion of a multiferroic domain pattern: The direction of the magnetic or electric order parameter of a multiferroic is reversed across the entire sample, but the domain patterns such is retained [3]. (cid:120) Reversal of a multiferroic domain state without application of external DC fields, just by light [4]. (cid:120) In-situ tracking of the emergence of ferroic order in multiferroic oxide thin films by monitoring the formation of the ordered state by SHG during growth [5]. Optical access in all these experiments involves lasers emitting light pulses of either 100 fs or 5 ns. The laser light can be used to monitor the multiferroic order by nonlinear optics but also, prior to that, to manipulate the ordered state by with an intense pump light beam and analyse the ensuing dynamics. In view of the versatility of the optical access to multiferroics and the generality of the symmetry-based approach to the detection of ferroic phases, it is to be expected that the field of nonlinear optics on multiferroics will continue to develop at the highest speed.