Liquid crystals for ultrafast optics

The scientific ground covered since Maiman's first laser demonstration in 1960 and the first non-linear optics experiment the following year is immense. In particular, activities linked to femtosecond pulsed laser sources have been expanding steadily since the 1990s. At that time, the demonstration of CPA (Chirped-Pulse Amplification, Nobel Prize in Physics 2018), the emergence of titanium-doped sapphire as a near-perfect material for broad-spectrum lasing, and the discovery of Magic Mode Locking created a craze for ultra-short light sources that has never since waned. Today, femtosecond lasers are well established in many fields of fundamental research, including physics, chemistry (winner of the 1999 Nobel Prize for Chemistry), biology, medicine, micromachining and even archaeology. It is perhaps in the field of high intensities, and its applications to particle acceleration and the generation of secondary sources in various areas of the electromagnetic spectrum, that progress has been most spectacular. At the dawn of the 2000s, mastery of the spatio-temporal quality of energy pulses enabled intensities as high as 10 ²¹ W.cm ^–2 to be achieved on target. A few years later, the ability to generate intense pulses lasting no more than a few oscillations of the electric field (optical cycle regime), combined with phase stabilization between the field carrier and its envelope, turned time references upside down and opened the door to attosecond physics (Nobel Prize in Physics 2023).

In optics, research topics related to ultra-short lasers are extremely varied and constantly developing: laser physics, ultra-fast nonlinear optics, all-optical measurement methods and spatial and/or temporal shaping. These topics also open up a wide range of investigations into materials physics (crystals, electro-optic and acousto-optic media, optical fibers and even soft matter). Finally, new source architectures are being studied, such as third-generation femtosecond systems based on parametric amplification.

The vast majority of ultrafast laser applications require control of the spectral content and temporal shape of femtosecond pulses, either to benefit from a maximum electric field, or to follow the dynamics of the system under study. Phase control, ideally programmable, is therefore a key issue in ultrafast optics. To achieve this, numerous combinations of optical systems are available, and are the subject of ongoing technological development.

Liquid crystals provide an elegant and relevant technological solution to complement current phase control devices for femtosecond sources. Indeed, these mesomorphic materials possess intrinsically very interesting optical properties:...