Magnon Accumulation by Clocked Laser Excitation as Source of Long-Range Spin Waves in Transparent Magnetic Films

Optical tools are promising for spin-wave generation because of the possibilities of ultrafast manipulation and local excitation. However, a single laser pulse can inject spin waves (SWs) only with a broad frequency spectrum, resulting in short propagation distances and low wave amplitudes. Here, we excite a magnetic garnet film by a train of fs-laser pulses with a 1-GHz repetition rate so that the pulse separation is shorter than the decay time of magnetic modes, which allows us to achieve a collective impact on the magnetization and establish a quasistationary source of spin waves, namely, a coherent accumulation of magnons (“magnon cloud”). This approach has several appealing features: (i) The magnon source is tunable, (ii) the SW amplitude can be significantly enhanced, (iii) the SW spectrum is quite narrow, providing long-distance propagation, (iv) the periodic pumping results in an almost constant-in-time SW amplitude for the distances larger than $20\mu \mathrm{m}$ away from the source, and (v) the SW emission shows pronounced directionality. These results expand the capabilities of ultrafast coherent optical control of magnetization and pave the way for applications in data processing, including the quantum regime. The quasistationary magnon accumulation might also be of interest for applications in magnon Bose-Einstein condensates.

Popular Summary

In traditional computing and data processing, electrical charges are used to move and manipulate information. But some researchers are investigating a new paradigm that relies on “magnetization waves,” also known as spin waves or magnons. Spin waves are essentially ripples in the orientation of electrons in a magnetic substance. The advantages of spin waves are that they are free from energy losses that plague electrical currents, and they can operate at higher frequencies. Microwave radiation is conventionally used to generate spin waves, but laser pulses are an important alternative as they offer more precise location control and tunability. However, lasers usually generate spin waves with a wide range of frequencies, preventing further progress. We tackle this problem with a series of very short laser pulses that builds up a long-lasting source of spin waves, or a “magnon cloud.”

We target a film of magnetic garnet with a sequence of femtosecond laser pulses at a repetition rate of 1 GHz. The interval between pulses is much shorter than the time it takes for the magnetic oscillations to decay, thus creating a quasistationary magnon cloud. Our approach has several appealing features: The source is tunable, the amplitude of the spin waves can be significantly enhanced, the spectrum of the spin wave is quite narrow (which provides a longer propagation distance), and the spin waves show pronounced directionality.

These results expand the capabilities of ultrafast optical control of magnetization waves and pave the way for novel applications in data processing, including in the quantum regime.

Figure 1

Excitation of magnetization precession in an iron-garnet film by a train of fs optical pulses. (a) Experimental scheme. The periodic laser pulses are incident on the magnetic film. The probe beam is detected from the substrate side of the sample. (b) Сomparison of magnetization oscillations excited by ${\sigma }^{+}$ and ${\sigma }^{-}$ circularly polarized pumps (red circles and blue squares, respectively). Solid curves are fits with a decaying sine function. The fit parameters are given in Table SI (see Ref. [30]), $H=2038\mathrm{Oe}$. (c) Magnetization oscillations at different magnetic fields (open symbols) corresponding to maximal and minimal amplitudes. Fits to the data by decaying sines are given by the thin blue curves. The incident light is ${\sigma }^{+}$ polarized. Fit parameters are given in Table SII (see Ref. [30]). (d) Measured precession amplitude vs magnetic field strength (circles). The solid curve is calculated after Eq. (3) using the parameters $h=6.2\mathrm{Oe}$, $\alpha =0.02$ ($\tau =0.77\mathrm{ns}$ at $H=1500\mathrm{Oe}$) to fit the experimental data. Experimental results are presented for sample 1.

Figure 2

Excitation of magnetization oscillation showing two precession modes in an iron-garnet film by a laser pulse train. (a) Magnetization oscillations at different magnetic fields. (b) Consecutive resonances of the slowly decaying precession mode B for different magnetic fields. The green thin lines are fits with decaying sine functions. The fit parameters are given in Table SIII (see Ref. [30]). The experimental data are presented for sample 2.

Figure 3

Excitation of magnetization oscillation mode of high quality factor (mode B). (a) Measured precession amplitude in sample 2 vs magnetic field (open circles). The solid curve is calculated by solving Eq. (S11) (see Ref. [30]) with $h=0.8\mathrm{Oe}$, and $\alpha =0.005$ ($\tau =8.8\mathrm{ns}$ at $H=1500\mathrm{Oe}$) to fit the experimental data. (b) Calculated precession amplitude vs $\omega$ for modes of different quality factors [and therefore different values of $\tau$: 0.6 ns (black line), 1.5 ns (blue line), 4 ns (green line), and 10 ns (red line) at $H=1500\mathrm{Oe}$]. Here, $h=0.8\mathrm{Oe}$.

Figure 4

Generation of spin waves by a cloud of periodically pumped, coherently oscillating spins. (a) Magnetization oscillations observed in sample 1 in the probe beam shifted by $5–35\mu \mathrm{m}$ relative to the pump beam along the external magnetic field. Solid curves are fits with a decaying sine function. The fit parameters are given in Table SII (see Ref. [30]). Here, $H=1060\mathrm{Oe}$. (b) Oscillation amplitude (red and orange) and decay rate (blue) at different distances between the pump and probe for the probe shifted along the $x$ axis (open symbols and solid lines) and the $y$ axis (filled symbols and dashed lines). The experimental data are shown by the symbols, while the calculation results are given by the lines. (с) Calculated dispersion of the SWs propagating along the $x$ and $y$ directions. The blue region indicates the range of $k$ generated by a single laser pulse with spot radius $r$. The dispersion was calculated using the theoretical model in Ref. [35]. (d) Calculated distribution of $m_z$ in SWs launched at $H=1060\mathrm{Oe}$ ($\omega T/2\pi = 3$) and $H=1320\mathrm{Oe}$ ($\omega T/2\pi = 3.75$). In the former case, SWs are generated in a narrow region along H (phenomenon of SW directionality), while in the latter one, SWs are generated in the perpendicular direction as well.