Frequency comb enhanced Brillouin microscopy

Abstract

Brillouin light scattering (BLS) microscopy is a well established and powerful technique to study acoustic and magnetic excitations in the frequency domain with sub-micron spatial resolution. Many other spectroscopic techniques have benefited from the introduction of femtosecond laser sources to optically pump and stimulate the sample under investigation. In BLS microscopy, the use of femtosecond lasers as the excitation source introduces several challenges, primarily since the measured frequency shift is small and the signal levels are weak due to the low duty cycle of typical femtosecond lasers. Here we present a method to evade these challenges. A strong enhancement of the weak scattering amplitude on selected modes is observed by pumping the sample with a high repetition rate frequency comb laser source. The laser beam can be focused to the diffraction limit, providing a micron pumping area. We can thus preserve the innate high frequency and spatial resolution of BLS microscopy. Furthermore, we are able to induce a point-like source of mode-selected elementary excitations which propagate away from the pumping spot. We conclude that we have demonstrated frequency comb pumped BLS microscopy as an attractive tool for studies of ultrafast induced laser dynamics directly in the frequency domain.

Femtosecond Laser Pulse Driven Caustic Spin Wave Beams

“…tained SWs in a focused region close to the diffraction limit of the light, which is required to excite SWs with high wavevectors, where caustics can be formed. Here we overcome this limitation by utilizing a 1 GHz repetitionrate femtosecond laser comb capable of sustaining continuous SWs [24,26,27]. The emitted SWs contain a range of k-vectors and by slight detuning of the applied magnetic field off the ferromagnetic resonance (FMR), we observe X-shaped caustic SW patterns at around 70 • angles as predicted by theory.…”

“…A more in-depth description can be found in Ref. 27. Briefly, the pump-probe experiment uses a mode-locked femtosecond laser operating at 816 nm and a repetition rate of 1 GHz with 120 fs long pulses to induce rapid demagnetization.…”

Acoustic, Phononic, Brillouin Light Scattering and Faraday Wave-Based Frequency Combs: Physical Foundations and Applications

“…Recently, the BLS spectroscopy has been employed to produce FC-like signals originating from spin wave modes excited in a ferromagnetic thin film structure [ 18 , 97 ]. In those works, a thin Permalloy ( alloy) film—a standard building block of many magnonic and spintronic devices [ 82 ]—was deposited onto a sapphire substrate using a DC magnetron sputtering technique.…”

“…Note that studies reported in [ 18 , 97 ] did not aim to generate FC signals for use in applications where other kinds FCs have been typically used. Instead, a method of FC-enhanced BLS microscopy was introduced to coherently excite vibrational and spin wave modes in the sample.…”

Acoustic, Phononic, Brillouin Light Scattering and Faraday Wave Based Frequency Combs: Physical Foundations and Applications

“…Recently, the BLS spectroscopy has been employed to produce FC-like signals originating from spin wave modes excited in a ferromagnetic thin film structure [89,90]. In those works, a thin Permalloy (Ni 80 Fe 20 alloy) film-a standard building block of many magnonic and spintronic devices [73]-was deposited onto a sapphire substrate using a dc magnetron sputtering technique.…”