Nanoscale polarization transient gratings

Abstract

Light manipulation at the nanoscale is essential both for fundamental science and modern technology. The quest to shorter lengthscales, however, requires the use of light wavelengths beyond the visible. In particular, in the extreme ultraviolet regime these manipulation capabilities are hampered by the lack of efficient optics, especially for polarization control. Here, we present a method to create periodic, polarization modulations at the nanoscale using a tailored configuration of the FERMI free electron laser and demonstrate its capabilities by comparing the dynamics induced by this polarization transient grating with those driven by a conventional intensity grating on a thin ferrimagnetic alloy. While the intensity grating signal is dominated by the thermoelastic response, the polarization grating excitation minimizes it, uncovering helicity-dependent responses previously undetected. We anticipate nanoscale polarization transient gratings to become useful for the study of physical, chemical and biological systems possessing chiral symmetry.

Fig. 1. Intensity vs polarization grating excitation.

a PP beams generate an intensity grating with period Λ_TG and constant polarization. b OP beams generate a modulated polarization with the same periodicity and constant intensity. c Diffraction from an intensity TG resulting from surface modulations u_z and refractive index variations. The first typically dominates the backward-diffracted signal and the latter the forward diffraction. d Diffraction from a polarization TG, where the thermal modulation depends on the dichroic absorption Δβ, reducing ${\mathbf{u}}_z^{\star }$. The signal can still originate from refractive index variations associated with the chiral dispersion Δδ.

Fig. 2. Experimental setup.

a Scheme of the TIMER instrument in the OP beam configuration. The two beams generated by the FEL source are split vertically by a planar mirror PM1 to generate the probe (see panel c). Of the remaining two half beams, the variably polarized one (LH/LV) is steered by the second planar mirror PM2 whereas the LV branch propagates unaffected. Both half beams constitute the pump and are crossed and focussed at the sample at an angle 2θ by two toroidal mirrors FM1 and FM2. Of the two half beams reflected by PM1, the variably polarized one is geometrically blocked while the LV is steered to the delay line and then focussed at the sample at an angle θ_pr. The signal beam is drawn exemplary to propagate in forward diffraction at an angle θ_sig. b Scheme of the OP beam generation in the FERMI undulator chain (see text for discussion).

Fig. 3. Dynamics induced by intensity vs polarization grating excitation.

a Intensity grating. Blue circles: backward-diffracted TG signal from PP beams. b Polarization grating. Red triangles: backward-diffracted signal from OP beams. Dashed line: residual sinusoidal component (see text for discussion). Error bars in a and b are the variance calculated over three scans. Center value is the mean.

Fig. 4. Modulation of sample parameters induced by intensity vs polarization grating excitation.

a Temperature gradient ΔT (blue curve) induced by intensity TG compared with the residual temperature modulation ΔT^⋆ (red curve) around T^⋆ (dotted line) associated with dichroic absorption in polarization TG. b Changes in magnetization ΔM and density Δρ driven by the temperature modulation ΔT and the corresponding ΔM^⋆ and Δρ^⋆ due to ΔT^⋆. Please note that here ΔM^⋆, Δρ^⋆ and ΔT^⋆ are multiplied by a factor 5 to be visible on the scale.

Fig. 5. Dynamics induced by stochastic vs deterministic magnetic modulations.

a Experimental data (red line with markers) vs. calculations for deterministic (yellow) and stochastic (blue) initial conditions. See text for discussion. Center value and error bars are, respectively, mean and variance of the data. b and c Two-dimensional plots of the initial conditions in the deterministic (polarization TG) and stochastic (intensity TG) simulations, respectively. The color scale is the same for both plots.