The probe of antiferromagnetic (AFM) spin dynamics in thin films has traditionally relied on the short x-ray pulses based on the magnetic dichroism or diffraction effect. Here, we demonstrate the direct optical probe of ultrafast laser-induced AFM spin dynamics of thin AFM CoO films using time-resolved magnetic linear dichroism effect in reflection geometry. The ultrafast laser excitation of the CoO film leads to the quench of AFM order, manifested as large polarization rotations of the reflected probe light. Far below the Néel temperature (TN), the quench of the AFM order occurs within 300 fs, which is faster than the lattice thermalization (∼1ps) via electron-lattice scattering. This AFM quench process, however, slows down near TN, where an additional slower quench process with the time constant longer than 20 ps is emergent. Such an AFM spin dynamics is dramatically different than the transient reflectivity dynamics which is nearly invariant in a wide temperature range across TN. We attribute the quench of AFM order in CoO to the charge-transfer excitation and thermal effect, but the latter mechanism only plays a significant role near TN.

Magnetization (M) precessions driven by ultrafast laser-induced nonthermal effects are observed in undoped yttrium iron garnet (YIG) films of (100) and (111) orientations using pump-probe time-resolved magneto-optical Kerr effect. The M precessions show a strong dependence on the polarization direction of linearly polarized pump pulses of 400 nm. In contrast, we can barely observe any M precession using circularly polarized pump pulses, which indicates that the inverse Faraday effect is negligible. For the case of linear pump polarization, a phenomenological model is introduced, based on the modulation of M via a modulation of fourth-rank susceptibility tensors by a laser pulse. This allows one to distinguish the contributions of the inverse Cotton-Mouton effect (ICME) from those of the photoinduced magnetic anisotropy (PMA). Using the formula derived from the phenomenological model, we perform the fitting of the polarization-direction-dependent precession phase and amplitude in (100)- and (111)-oriented YIG films. The fitting results reveal that the M-precession excitation originates from a combination effect of ICME and PMA, but the ICME plays the dominant role.

Improved charge generation via fast and effective hole transfer in all‐polymer solar cells (all‐PSCs) with large highest occupied molecular orbital (HOMO) energy offset (ΔEH) is revealed utilizing ultrafast transient absorption (TA) spectroscopy. Blending the same nonfullerene acceptor poly{[N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene) (N2200) with three different donor polymers produces all‐polymer blends with different ΔEH. The selective excitation of N2200 component in blends enables to uncover the hole transfer process from hole polaron‐induced bleaching and absorption signals probed at different wavelength. As the ΔEH is enhanced from 0.14 to 0.37 eV, the hole transfer rate rises more than one order and the hole transfer efficiency increases from 12.9% to 86.8%, in agreement with the trend of internal quantum efficiency in the infrared region where only N2200 has absorption. Additionally, Grazing‐incidence wide‐angle X‐ray scattering measurements indicate that face‐on crystal orientation in both polymer donor and acceptor also plays an important role in facilitating the charge generation via hole transfer in all‐PSCs. Hence, large ΔEH and proper crystal orientation should be considered in material design for efficient hole transfer in N2200‐based heterostructures. These results can provide valuable guidance for fabrication of all‐PSCs to further improve power conversion efficiency.

Ultrafast laser induced magnetization reversal in L10 FePt films with high perpendicular magnetic anisotropy was investigated using single- and double-pulse excitations. Single-pulse excitation beyond 10 mJ cm−2 caused magnetization (M) reversal at the applied fields much smaller than the static coercivity of the films. For double-pulse excitation, both coercivity reduction and reversal percentage showed a rapid and large decrease with the increasing time interval (Δt) of the two pulses in the range of 0–2 ps. In this Δt range, the maximum demagnetization (ΔMp) was also strongly attenuated, whereas the integrated demagnetization signals over more than 10 ps, corresponding to the average lattice heat effect, showed little change. These results indicate that laser induced M reversal in FePt films critically relies on ΔMp. Because ΔMp is determined by spin temperature, which is higher than lattice temperature, utilizing an ultrafast laser instead of a continuous-wave laser in laser-assisted M reversal may reduce the overall deposited energy and increase the speed of recording. The effective control of M reversal by slightly tuning the time delay of two laser pulses may also be useful for ultrafast spin manipulation. The work at the Department of Optical Science and Engineering, Fudan University, was supported by the National Key Research and Development Program of China (Grant No. 2016YFA0300703), the National Key Basic Research Program of China (Grant No. 2015CB921403), and the National Natural Science Foundation of China (Nos. 11774064 and 51371052). The work at the Department of Physics, Fudan University, was supported by the National Key Basic Research Program (No. 2015CB921401) and the National Natural Science Foundation (Nos. 11434003 and 11474066) of China.

Controlling the wave fronts of surface waves (including surface-plamon polaritons and their equivalent counterparts) at will is highly important in photonics research, but the available mechanisms suffer from the issues of low efficiency, bulky size, and/or limited functionalities. Inspired by recent studies of metasurfaces that can freely control the wave fronts of propagating waves, we propose to use metawalls placed on a plasmonic surface to efficiently reshape the wave fronts of incident surface waves (SWs). Here, the metawall is constructed by specifically designed meta-atoms that can reflect SWs with desired phases and nearly unit amplitudes. As a proof of concept, we design and fabricate a metawall in the microwave regime (around 12 GHz) that can anomalously reflect the SWs following the generalized Snell’s law with high efficiency (approximately 70%). Our results, in excellent agreement with full-wave simulations, provide an alternative yet efficient way to control the wave fronts of SWs in different frequency domains. We finally employ full-wave simulations to demonstrate a surface-plasmon-polariton focusing effect at telecom wavelength based on our scheme.

In the current research field of non-fullerene solar cells, the device performance of all polymer solar cells (all-PSCs) lags significantly behind those based on polymer donor and molecule acceptor. To further improve the device performance of all-PSCs, the small molecule electron acceptor ITIC was introduced for the first time to the all-PSCs based on PTP8/P(NDI2HD-T). The combinative polymer/molecule acceptor P(NDI2HD-T)/ITIC with only a small amount of ITIC, 15 wt% in acceptors and 6 wt% in total, can significantly improve the device PCE from 6% to over 7%, which is among the highest-efficiencies for reported all-PSCs. The improved device performance can be attributed to the broadened absorption and optimized blend morphology, which can further enhance charge carrier generation and balance charge carrier mobility in the ternary blend. More importantly, we discovered that this strategy can be successfully applied to all-PSCs based on other polymer donors like widely spread P3HT or PTB7. Therefore, our work not only reveals the detailed effect of polymer-small molecule acceptor alloy in all-PSCs, but also suggest that the ternary cell strategy of polymer donor/polymer:molecule acceptor may become a general and facile approach to further boost the performance of current all-PSCs.

Optical control of magnetic anisotropy in ferromagnetic (FM) metals via non-thermal effects offers an intriguing route for the ultrafast magnetization control. Here, we report on strong modification of exchange-coupling induced uniaxial magnetic anisotropy (UMA) in Fe/CoO below the Néel temperature of CoO owing to the charge transfer excited by ultrafast laser pulses. This UMA modification by nonthermal effects is manifested as much smaller frequencies of the Fe spin precession in the initial 100-ps time range under the 400-nm pump with charge transfer excitation, compared to the 800-nm pump with pure thermal effects. From the time-dependent frequency shift under a moderate pump fluence of 0.5 mJ/cm2, we determined the magnitude of the UMA attenuation with a highest value of more than 1000 Oe in a duration of 100 ps. The strong UMA attenuation is attributed to the large suppression of the interface exchange coupling as a result of the diminishment of antiferromagnetic (AFM) spin order in CoO. Our results give insights into the ultrafast spin modulation in AFM CoO dominated by the electronic process, which can be utilized for efficient driving of the coherent spin motion in the proximate FM metal exchange coupled to CoO. This work was supported by the National Key Basic Research Program of China (Grant Nos. 2015CB921403 and 2015CB921401), the National Key Research and Development Program of China (Grant No. 2016YFA0300703), and the National Natural Science Foundation of China (Grant Nos. 51371052, 11474066, and 11434003).

Ultrafast transient absorption (TA) spectroscopy was employed to investigate the thermal annealing effect on the charge transfer (CT) in bulk heterojunction (BHJ) all-polymer solar cells (all-PSCs) utilizing an n-type polymer P(NDI2OD-T2) (Polyera, N2200) as acceptor and a low bandgap polymer PBPT as donor. The CT generates hole polarons residing in the PBPT and electron polarons belonging to N2200, manifested in the TA spectra of the BHJ films as the long-lived absorption peak centered at ∼850 nm. The CT is most efficient in the film annealed at 160 °C and its efficiency declines monotonically when enhancing or reducing the annealing temperature, displaying a positive correlation with the power conversion efficiency (PCE) of the corresponding solar cell devices. This correlation is analyzed in terms of the crystallinity and phase separation, which are the key factors determining the performance of all-PSCs. Our results can provide valuable guidance for the fabrication of BHJ all-PSCs to improve their PCE.

Current all polymer solar cells still suffer from low fill factors (FF) and short-circuit current density (Jsc) compared with the conventional polymer/fullerene system. Herein in this work, devices using PTP8 as the electron donor and [70]PCBM as well as widely used polymer N2200 as the electron acceptor were systematically studied and compared. The major loss mechanisms in the all polymer solar cells were investigated to understand their relatively lower performance than the PTP8/fullerene system. By performing in-depth analysis on ultrafast transient transmission spectroscopy results, we estimated that in PTP8/N2200 device nearly half of the charges recombine geminately, which is confirmed as the major factor hindering the device performance of all polymer solar cells compared with polymer/fullerene system. Through thorough morphology analysis, the low charge generation efficiency is attributed to the reduced crystallinity of N2200 in the blend film and the unfavorable face-to-edge orientation at the donor/acceptor heterojunction. Coupling these results with knowledge from efficient polymer/fullerene systems, the future design of new polymers can devote to increase the attraction between the π face of donor and acceptor, leading to enhanced face-to-face orientation at the heterojunction, while maintaining a high π-π stacking order for each polymer.

The conductivity of Fe3O4 drops two orders of magnitude below the Verwey temperature Tv, known as the Verwey transition, due to the formation of charge ordering (CO). Here, we report the discovery of a large birefringence effect correlated with the CO in Fe3O4 controlled by ultrafast-laser-assisted magnetic field cooling. The polarization rotation (PR) of the light reflected from a single crystalline Fe3O4 film below Tv shows a twofold symmetry as the cooling field (CF) rotates through 360∘ within the film plane. The maximum PR occurs for the CF parallel to the cubic ⟨110⟩ axes, and its amplitude depends on the sample orientation. These results are well interpreted by taking into account two CO patterns with orthogonal CO orientations, with their fractional areas determined by the ratio of the field components along the [110] and [1¯10] axes. Our results indicate that application of the CF along ⟨110⟩ axes may result in the single orientation CO state, which is highly desirable for unraveling the subtle CO structure to better understand the driving mechanism of the Verwey transition. In addition, ultrafast pump-probe measurements reveal a diminishment of the twofold PR at 0.8 ps due to fast melting of the CO state by the ultrafast laser pulse.