Broadband and ultra-broadband modular half-wave plates

• Experimental demonstration of broadband and ultra-broadband half-wave plates.
• Both modular devices comprise an array of rotated single half-wave plates.
• The elements in the arrays are rotated at specific angles.

We experimentally demonstrate broadband and ultra-broadband spectral bandwidth modular half-wave plates. Both modular devices comprise an array of rotated single half-wave plates (HWPs), whereby for broadband and ultra-broadband performance we use standard and commercial achromatic HWPs, respectively. The bandwidth of the modular HWPs depends on the number N   of individual HWPs used and in this paper we experimentally investigate this for N={3,5,7,9}N={3,5,7,9}. The elements in the arrays are rotated at specific angles with respect to their fast-polarization axes, independent of the nature of the birefringent material. We find the rotation angles using an analogy to the technique of composite pulses, which is widely used for control in nuclear magnetic resonance.