10.1117/2.1201301.004631

Opto-magnonics: light pulses
manipulating spin waves
Takuya SatohSpatially shaped ultrashort light pulses achieve non-contact, ultrafast,
microfabrication-free directional-control of spin wave emission.
In the electronics that support our modern information society,information is borne by two fundamental aspects—degrees offreedom—of electrons: electron charge and its flow (current).
With traditional electronics, Joule heating caused by the flow ofcharge is unavoidable, and the achievable density of practicalelectronic devices is reaching its limits. The relatively new technology of spintronics, which exploits electron spin, overcomes
these problems and, as a result, has been studied extensively.1 In
a magnetic field, the spins of individual electrons precess aroundthe direction of the magnetic field. In a magnetically ordered material, the interaction of spins leads to a propagating wave of
spin precessional motion. This ‘spin wave’ shows promise as a
new kind of information carrier even in an insulator.2–4 This phenomenon has attracted much attention in the field of ‘magnonics,’ that is, using the spin wave that is generated, controlled, anddetected as an information carrier. One critical challenge, however, is to establish a suitable control technology for the propagation of spin waves.
Until now, spin waves have been generated by microwaves
from an antenna or by spin-polarized current.5–7 However, once
the antenna and electrode have been microfabricated and a magnetic field applied, the propagation characteristics of the spinwave cannot be tuned directly. As a first step toward arbitrary
tuning of the spin wave propagation without the use of microfabrication, my coworkers and I have proposed a model concerning the wavenumber distribution of a spin wave generated
by focusing an ultrashort light pulse on a magnetic substance.8
When a circularly polarized light pulse of high intensity(pump pulse) is focused on a magnetic material, a spin precessional motion will start within the light spot via the inverse Faraday effect, whereby an effective magnetic field isgenerated along the beam direction.9, 10 The precessional motion then propagates outside the pump light spot in 2D in theform of a spin wave. In our proposed model, the wavenumber

Figure 1. Experimental configuration for directional control of spinwave emission.
distribution of an initial state of the spin wave induced by thepump pulse is determined by the Fourier transform of the spatial intensity distribution of the pump pulse.
In experiments, we used a garnet ferrite single crystal as asample. Initially, the magnetization lay in the plane parallel tothe sample surface when in a 1kOe magnetic field also directed
parallel to the sample surface. When a circularly polarized pumppulse with a pulse width of 100fs was focused in a circularspot (50m in diameter) on the sample surface, spin precessional motion occurred within the spot via the inverse Faradayeffect. The magnetization deviated from the plane and theout-of-plane component became nonzero. This out-of-planemagnetization component was detected by the Faraday rotation(polarization rotation) of a low-intensity linearly polarized pulse(probe pulse). A time-resolved pump-probe experiment was carried out by measuring the Faraday rotation angle of the probe
pulse, which is time-delayed with respect to the pump pulse.
The precessional motion propagated in 2D out of the pumplight spot in the form of a spin wave (see Figure 1). By scanning the relative position of the probe light spot on the sample with respect to the pump light spot, we observed time- andspace-resolved spin wave propagation: see Figure 2(a).

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are now working on spatial light modulation using computergenerated holograms toward arbitrary tuning of the spin wave
propagation.
Author Information

Figure 2. 2D spin waveforms obtained by (a) experiment and (b) simulation. The time delay was 1.5ns. Red and blue represent positive andnegative Faraday rotation (i.e., the z-component of the magnetizationis positive or negative), respectively. H: The magnetic field.

Takuya SatohThe University of TokyoTokyo, Japan
Takuya Satoh is an assistant professor at the University of Tokyo,
Japan and is supported by the Japan Science and Technology
Agency Precursory Research for Embryonic Science and Technology (JST-PRESTO) program.
References

Figure 3. Amplitude maps of spin waves obtained by simulations. The
longer side of the aperture was (a) parallel and (b) perpendicular to themagnetic field. The time delay was 1.5ns.
We found that the wavelength of the spin wave was 200–
300m, and the group velocity was about 100km/s. A simulation based on the model was able to reproduce the experimental
result almost exactly: see Figure 2(b).
According to our model, it is essential to optimize the shape of
the pump light spot at the sample surface to control the directionof spin wave emission. Thus, we inserted a rectangular aperture
into the front focal plane of the pump beam lens to make an elliptical spot shape on the sample surface (see Figure 1). When themajor axis of the ellipse was aligned parallel or perpendicular
to the applied magnetic field, the experiment and the simulation
both showed a spin wave propagating perpendicular or parallelto the magnetic field, respectively (see Figure 3). In this way, weachieved directional control of the spin wave emission.
Recently, a spatio-temporal pulse shaper has been used to control lattice vibrational waves but not spin waves.11 In our technique, neither microwaves nor spin-polarized current are used
at all; the spin wave is generated and directed only by the spatially shaped light pulse. We expect that our technique will allow
much higher design flexibility in magnonics, and that this technique will pave the way for a new field of ‘opto-magnonics’ formanipulating spin waves with light spots of various shapes. We

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