Altermagnets host unconventional spin-polarized bands despite zero net magnetization, but controlling their spin structure remains challenging. We theoretically propose a multifield approach to engineer spin polarization in $d$-wave altermagnets using gating, optical driving, and in-plane electric fields, which enable tunable and switchable polarizations along multiple directions. Optical driving induces out-of-plane ($z$) polarization, while gating and in-plane fields generate $x$ and $y$ polarizations via the Edelstein effect, all of which are experimentally detectable. We further find that spin- and band-selective doping induces chiral optical activity, a feature unique to altermagnets. Our approach offers a versatile means to control the direction of spin polarization in altermagnets.

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• Altermagnetism
• Magnetic susceptibility
• Magneto-optical Kerr effect
• Spin polarization
• Ultrafast magnetic effects
• Altermagnets
• Floquet systems
• Bloch-Floquet theorem
• Hall bar
• Linear response theory