Supplemental Material: All-optical switching of magnetoresistive
devices using telecom-band femtosecond laser
Li He, Jun-Yang Chen, Jian-Ping Wang, and Mo Li
Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455, USA
Issue of unsynchronized optical pulse picker
Fig. S1 shows the schematics of our acousto-optic pulse picking system used in the experiments.
Optical pulse train out from the fiber laser has a base repetition rate of 35 MHz and is sent into the
acoustic-optic modulator (AOM) to reduce the repetition rate before exposing the sample.
Electrical pulses provided by the signal generator serve as the gating signal to control the optical
transmission of the AOM, where high and low gate voltage corresponds to high and low optical
transmission, respectively. In all experiments electrical pulse width is set to be 25 ns with a rise/fall
time of 5 ns to ensure only one optical pulse can transmit through AOM for each electrical pulse
in most cases. Components like RF driver and AOM would further increase the rise/fall time in
the effective gate window.
Due to the unsynchronization between laser pulse train and electrical pulses, the temporal location
of optical pulse with respect to the effective gate window varies from pulse to pulse, which
eventually gives rise to fluctuation of the transmitted laser pulse energy.
FIG. S1 Schematics illustrating the optical pulse picking system.
To further show optical pulse energy variation caused by the pulse picker, we characterize the
optical transmission of the AOM under various gating condition. Fig. S2a shows the optical pulse
train measured right after the seed laser (yellow) and the AOM (magenta) to which a constant high
gate voltage is applied. As one may expect, the latter has the same repetition rate as seed laser and
the peak voltages are constant from pulse to pulse. In comparison, when an electrical pulse train
with 1 MHz repetition rate is applied to control the AOM, as shown in Fig. S2b, the transmitted
optical energy varies from pulse to pulse, although the repetition rate matches electrical pulse
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frequency. In addition, the periodic change in terms of peak voltage is due to the beating between
the base optical frequency (35 MHz) and electrical pulse frequency.
FIG. S2 (a) Seed laser pulse train (yellow) and optical pulses measured after AOM (magenta)
with a photodetector. A constant high voltage is applied to AOM. (b) Optical pulses measured
after AOM when the electrical gate signal is 1 MHz.
To evaluate the laser pulse energy variation in the single shot experiment, we set the electrical
pulse frequency at 1 Hz and measure the optical pulses after the AOM. Fig. S3 summarizes the
statistics of the peak voltage measured for 1000 counts. It confirms that optical pulse energy
varies significantly from pulse to pulse due to the unsynchronized pulse picker.
FIG. S3 Statistical summary of the peak voltage measured after the AOM for 1000 counts. The
frequency of the electrical gate signal applied to the AOM is 1 Hz.
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However, there is no simple statistical correlation between the peak voltage distribution and the
optically induced VH change presented in the manuscript (Fig. 4(b)) due to the nature of threshold
behavior associated with AOS and that VH is also sensitivity to the shape and location of the
switched domain relative to the electrodes. In fact, even the peak voltage measured by the
photodetector, which has a response time of 100 ns, may not reflect optical pulse energy linearly.
Therefore, it is currently not possible to find out any enlightening correlation between optical pulse
energy and Hall voltage signal. We are currently improving our measuring system by replacing
the acousto-optic pulse picker with an electro-optic pulse picker system with a phase-locked loop
that can synchronize with the laser pulses.
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