Polarization Shaping

Recently we built a polarization shaper (Fig. 1) which can modulate the phase, amplitude and polarization of mid-IR light independently. Two identical beams are sent through the shaper along different paths such that their frequency-dependent amplitudes and phases can be programmed independently. The polarization is then rotated by 90° for one of the two beams, which are then recombined on a wire-grid polarizer so that they propagate collinearly.

Polarization shaper layout

Figure 1. Optical layout of mid-IR polarization shaper. WGP: wire-grid polarizer, WP: MgF2 zero-order λ/2 wave plate, ZW: ZnSe wedge pair, AOM: Ge acousto-optical modulator, BS: 50/50 ZnSe beamsplitter, H and V are horizontal and vertical polarizations.

This polarization shaper is capable of producing pulses with a wide variety of polarization profiles, shown in Fig. 2.  We expect that polarization shaping will find use in discriminating between degenerate modes that have different transition dipole directions, such as the amide I normal modes of peptides and protein secondary structures, or in eliminating diagonal peaks in 2D IR spectra to make it easier to extract cross-peaks.

Figure 2. Cross-correlations of polarization-shaped pulses with a reference pulse: (a) a 45° pulse, (b) two perpendicular pulses with a delay, (c) a pulse with time-dependent linear polarization rotating from 0° to 90°, (d) a right-circularly polarized pulse. Traces are colored based on instantaneous polarization angle.