In particular, their optical anisotropy has enabled novel light-polarization-driven applications, such as highly polarizationsensitive light detectors and emitters [3,[10][11][12][13][14][15][16][17] and optical information encryption. [18] Recently, the ultrafast control of the anisotropic optical properties of 2D materials has become a topic of interest, since it not only enables high-speed, polarizationcontrolled optical switches but also offers novel physical insights into anisotropic light-matter interactions and quantum coherences. [19][20][21][22][23][24][25][26][27][28][29] Moreover, their ultrafast spatiotemporal dynamics exhibit unique quasi-1D behavior of photocarriers with direction-dependent mobilities and diffusivities, offering essential information for improving the anisotropic devices. [30,31] However, despite the continuous emergence of novel anisotropic 2D materials, [6][7][8][9] ultrafast optical anisotropy has so far been explored for only a very limited class of materials.ZrTe 5 is a transition metal pentatelluride that has been extensively studied since the 1980s owing to its unique physical properties, such as resistivity anomaly (Figure S1, Supporting information), [32,33] large thermoelectric power, [34,35] and pressure-induced superconductivity. [36] In particular, ZrTe 5 exhibits different topological phases (weak and strong topological insulators and Dirac semimetal) in diverse experiments, its topological nature has been a topic of active debate in the last decade. [37][38][39][40][41][42][43] Further, recent studies demonstrated that transitions between these topological Layered nanomaterials with in-plane anisotropy exhibit unique orientationdependent responses to external stimuli, enabling the development of novel devices with additional degrees of freedom. In particular, their anisotropic optical properties enable ultrafast nanophotonic modulators to be controlled by light polarization. However, achieving high controllability is still challenging due to incomplete optical anisotropy in most materials. Here, this work presents a completely anisotropic, ultrafast optical modulation in zirconium pentatelluride (ZrTe 5 ), a layered nanomaterial that has recently attracted renewed attention. The transient absorption (TA) microscopy reveals anisotropic ultrafast picosecond optical modulation in a broad range of 1.2-2.2 eV. In particular, at a certain photon-energy of 1.62 eV, complete on/off switching with a near-unity degree of anisotropy is achieved solely by changing the light polarization, suggesting that ZrTe 5 is a promising material for polarization-selective high-speed optical modulators. The theoretical analysis of the transition dipole moments attributes this sharp anisotropy to strongly polarization-dependent excited-state absorption. Furthermore, this work directly observes direction-dependent photocarrier transport using scanning TA microscopy. It yields the anisotropic diffusivity, mobility, and diffusion lengths of the photocarriers, which are essential parameters for de...

Layered quasi‐1D nanomaterials exhibit strong linear dichroism and effective light‐matter interactions, promising for novel information devices driven by light‐polarization. In particular, their optical anisotropy and ultrafast photoresponse allow for polarization‐controlled nanophotonic switches and modulators. However, the technology still requires substantial further studies due to an insufficient understanding of modulation mechanisms and limited investigations of a few materials. Here, transient absorption (TA) microscopy is employed to investigate the ultrafast polarization‐based optical switching in TiS3 nanoribbons, a rising quasi‐1D material of the transition metal trichalcogenide family. Highly anisotropic sub‐picosecond near‐infrared modulation is observed, which is most pronounced when the polarization of the probe pulse is aligned with the nanoribbon axis but disappears as the polarization is rotated to the perpendicular orientation. The authors attribute this significant dependence on the probe polarization to the hot carrier‐induced broadening of an anisotropic interband resonance, as supported by analyses of transient TA line shapes and first principles calculated optical transitions. Furthermore, a strong pump polarization‐dependent modulation is obtained by tuning the pumping energy, enabling highly anisotropic all‐optical switching controlled solely by polarization. These results offer comprehensive information on both probe‐ and pump‐polarization‐based modulations and a deeper physical understanding of polarization‐dependent photoresponse through ultrafast hot carrier dynamics.