Pulse shaping in the mid-infrared by a deformable mirror

Abstract

Summary form only given. Electromagnetic radiation in the mid-infrared (MIR) wavelength region offers unique possibilities to steer the phase state of condensed matter. Selective excitation of low-energy modes, such as lattice vibrations [1], by intense few-cycle pulses in this spectral range has been shown to control insulator-metal transitions [2] or magnetism in manganites and to induce superconductivity in cuprates [3]. The availability of precisely tailored light pulses may open up new path ways for the control of such phase transitions at ultrahigh speeds.Changing the temporal shape of short laser pulses with a given bandwidth requires control of the spectral phase of their electric field. Widely used devices in the near-infrared or visible range, such as acousto-optic or liquid crystals modulators, are not suitable for operation at wavelengths longer than 12 μm: as they require propagation in bulk materials, absorption in this spectral range prevents their applicability. For this reason, reflective schemes based on deformable mirrors are promising candidates for pulse shaping at these wavelengths. Their spectral coverage is indeed only limited by the reflectivity of their metallic coating. However their development requires the realization of membranes capable of deformations large enough to apply the necessary phase shifts for pulse manipulation at the long MIR wavelengths. In this work we present a pulse shaper based on a new type of deformable mirror, consisting of a series of 32 piezoelectric actuators. The mirror employs a continuous gold coated substrate, has high reflectivity in the MIR range, and can be applied to wavelengths even longer than 20 μm. The combined action of all actuators can produce deformations as large as 110 μm in both directions, thus giving either concave or convex shape. The mirror is placed in the Fourier plane of a 4-f shaper (see Fig. 1(a)). The light dispersed by a diffraction grating with 40 lines/mm is collected by a spherical mirror with f = 50 cm and sent onto the deformable mirror; this scheme is optimized in order to reduce aberrations of the shaped beam, as suggested by ray-tracing simulations. In the spectral range between 15 and 30 THz, the throughput of the shaper is close to 50%.The MIR pulses are generated by difference frequency generation (DFG) from two near-infrared pulses generated by tunable optical parametric amplifiers (OPAs). This system can deliver carrier-envelope phasestable MIR pulses tunable from 15 and 30 THz (i.e. 10-20 μm) [4]. The pulses were measured by electro-optical sampling, which allows direct access to the electric-field profile of the processed pulse and to its spectral phase via Fourier and Wigner transformations. Figure 1(b) provides the distribution map of a MIR pulse at 19 THz, after application of a linear chirp with GDD of 200x103 fs2. This map shows the very good agreement with the desired group delay, indicated by the solid line. The shaper is also able to split the input pulse into two pulses with chosen delay and arbitrarily split spectral content: Fig. 1(c) shows a pulse tuned at 22 THz and shaped into two replicas with a relative delay of 2 ps.