Magneto-optical spectroscopy based on pump-probe strobe light

Abstract

We demonstrate a pump-probe strobe light spectroscopy for sensitive detection of magneto-optical dynamics in the context of hybrid magnonics. The technique uses a combinatorial microwave-optical pump-probe scheme, leveraging both the high-energy resolution of microwaves and the high-efficiency detection using optical photons. In contrast to conventional stroboscopy using a continuous-wave light, we apply microwave and optical pulses with varying pulse widths, and demonstrate magnetooptical detection of magnetization dynamics in Y3Fe5O12 films. The detected magneto-optical signals strongly depend on the characteristics of both the microwave and the optical pulses as well as their relative time delays. We show that good magneto-optical sensitivity and coherent stroboscopic character are maintained even at a microwave pump pulse of 1.5 ns and an optical probe pulse of 80 ps, under a 7 megahertz clock rate, corresponding to a pump-probe footprint of ~1% in one detection cycle. Our results show that time-dependent strobe light measurement of magnetization dynamics can be achieved in the gigahertz frequency range under a pump-probe detection scheme.

Figures (10)

• Figure 1: Schematic illustration of the optoelectronic setup. (Left) Fiber-optic panel: the delay generator synchronizes the laser pulse and the microwave (MW) pulse controlled by distinct but locked channels, CH1 and CH2. The optical pulse train, after passing through the optical amplifier, the fiber delayline, and the polarizer, is intensity-modulated at the Mach-Zehnder Modulator (MZM), at a frequency of the MW signal (upper branch after the rf splitter). The MW pulse train is generated from the same MW signal by a rf mixer (lower branch after the splitter). The MW pulse width is controlled by the delay generator. This signal is then in-phase and quadrature (I-Q) mixed to enable a heterodyne detection mechanism. The I and Q signals (100 kHz in the present work) are sourced by an arbitrary function generator with appropriate voltage level and phase lag. (Right) Free-space panel: after the fiber collimator, the light pulses are polarized at 45$^\circ$, passing through the beam splitter, and reaching the sample device-under-test (DUT). The magnetization dynamics of the DUT induces a polarization modulation that can be sensitively captured -- with the birefringence effect -- using a polarization beam splitter (PBS), a balanced photodetector, and the lock-in amplifier. The excitation MW pulses are amplified, passing through a 6-dB coupler, and reaching the sample DUT. To monitor the relative time delay between light and MW pulses arriving at the DUT, two sampling branches are installed: optically, from the beam splitter (BS) to an ultrafast detector (optical sampling path, $L_\mathrm{Os}$), and electrically, at the coupler end (electrical sampling path, $L_\mathrm{Es}$). Additionally, the laser alignment and sample viewing is achieved via a visible light path, consisting of a red laser, a 1550/635 wavelength division multiplexing (WDM), a dichroic mirror, and a CMOS camera. Between the two panels, fiber-optic and microwave patch cables with appropriate lengths are inserted to compensate for any outstanding path differences. IF: intermediate frequency (mixer), LO: local-oscillator (mixer), RF: radio-frequency (mixer), arb.Func.generator: arbitrary functional generator, Amp: Amplifier, M: mirror, dich.M: dichroic mirror.
• Figure 2: Sample and measurement scheme: the sample DUT consists of a YIG(350-$\mu$m)/Py(80-nm) bilayer sample centered atop a CPW under the flip-chip configuration (Py facing CPW). An external bias magnetic field is applied along the $y$ direction ($H_y^\textrm{bias}$), and the oscillation rf field is along $x$. The optical pulse train is intensity-modulated at $f_\textrm{pump}$, and the MW pulse train, at IQ(100 kHz)$+f_\textrm{pump}$, is delivered via the CPW signal-line, so that the coherent strobe feature is maintained in the measurement. The optical pulse train probes at a sample location that is a distance ($D_x$) away from the CPW's signal-line.
• Figure 3: Pulse strobe light detection of magnon dispersion. (a-c) Magnon dispersion [$f,H$]-contour plots from the lock-in amplifier's (a) $X$, (b) $Y$, and (c) $R$ channels, measured from 2 -- 5 GHz. The MW pulse is 25 ns and the optical pulse is 20 ns. Apart from the central FMR mode, selective PSSW and BWVSW modes are also excited.
• Figure 4: Optical pulse width dependence: (a-f) (Left panel) magnon spectrum measured at a fixed MW pump pulse (20 ns, 4-GHz) but with various optical probe pulse widths (unit in ns): (a) 20, (b) 14, (c) 8, (d) 4, (e) 2, and (f) 0.08. (Right panel) The relative time positions of the MW pump and the optical pulses are monitored in real-time using an oscilloscope, taking advantage of the MW and optical sampling paths of our setup.
• Figure 5: Optical power dependence: (a) insertion of a fiber-optic attenuator. (b) Magnon spectrum measured at different optical power attenuation levels and displayed in [$H,V$]-contour plot (0V: max optical power, 5V: min optical power), using a 25-ns MW pulse at 4 GHz, and a 12-ns optical probe pulse. (c) The total signal amplitude plotted against different attenuation levels (symbols) and fitted to the response function (curve) of the attenuator.