Mid-IR Pulse Shaping

Femtosecond pulse shaping has led to dramatic advances in the control of physical phenomena. Based on work by Warren and coworkers who pioneered TeO2 AOM pulse shaping in the visible, we have built a pulse shaper from a germanium acousto-optic modulator (Ge AOM) that works directly in the mid-IR (Fig. 1). The Ge AOM has a resolution and transmission efficiency comparable to commercially available visible shapers, but is capable of programmable phase and amplitude modulation between 2 and 18 μm. Our Ge AOM can generate shaped pulses with 50% throughput efficiency and with a resolution equivalent to 500 pixels with high phase stability.

Mid-IR Pulse Shaper

Figure 1. (a) Optical layout of mid-IR pulse shaper. (b) Top down photograph of shaper. WP: MgF2 zero-order λ/2 waveplate, CM: cylindrical mirror, GR: diffraction grating, AOM: Ge acousto-optic modulator.

With our shaper the pulse sequence time-delays can be updated with every laser shot, leading to very fast data acquisition (0.2 s for a full 2D scan). This makes possible the acquisition of 2D IR spectra of rapidly evolving systems. The high phase stability of the shaper also affords us to implement a wide variety of different types of pulse sequences to take 2D IR spectra (Fig. 2) and phase cycling schemes to eliminate scatter from the spectra (Fig. 3).

Pump pulse shapes

Figure 2. Various pulse shapes used to collect 2D IR spectra. Pulse sequences in time domain: (a) a traditional etalon; (d) time-reversed etalon; (g) a Gaussian; (j) a pulse pair. 2D IR spectra of W(CO)6 in hexane at T = 2.0 and 0.2 ps using the pulse shapes: (b–c) traditional etalon; (e–f) time-reversed etalon; (h–i) Gaussian; (k–l) a pulse pair. Each row shows spectra taken from the same pump shape, while each column aligns spectra with the same T.

Figure 3. Removing scatter by phase cycling of the pump pulses. 2D IR spectra of hIAPP fiber in D2O were collected with (a) no phase cycling and (b) with phase cycling. The scatter which appears along the diagonal in (a) is now absent in (b).