Apart from the fundamental interest of discovering the physical effects that are behind the light-matter interaction processes that make it possible, slowing and storing light currently attracts a lot of attention because the ability to control the propagation speed of optical pulses, to stop and to release them on demand, is a key tool in the manipulation of optical signals. Two main applications directly benefit from the ability to slow and store light: the first is the buffering/multiplexing of optical pulses, concerning telecommunications and optical delay lines, and the second is phase sensing, as employed for detecting small phase variations, realizing compact, highly-sensitive interferometers or implementing phase compensation in optical array systems.
With respect to other exiting methods to obtain slow light, the novelty of the method proposed here is the use of a photochemical process of dye molecules hosted in a chiral liquid crystal structure and the associated photo-isomerization induced transparency effect1. The idea of using a molecular medium is, in itself, new, but also the type of molecular assembly, the chiral structure, is important. Indeed, the helical structure is sufficiently rigid to keep the dyes in their places, and to avoid optically induced orientation of the liquid crystals, while soft enough to allow local and small changes of the molecular order around the dyes with transformed shape (cis state). When a pulse is sent onto the medium together with a reference beam, the change of shape of the dyes in the illuminated regions produces a dynamic hologram that keeps the memory of the pulse and allows slowing it down via the induced variations of the medium dispersion properties.
Group delays of the order of tens of milliseconds were achieved. To understand the advantage brought in by slow light in phase sensing applications, one has to introduce the concept of group index, which is the equivalent of the refractive index felt by a light pulse while it traverses a medium within which it is being delayed. In other words, the group index is the ratio of the speed of light in vacuum to the speed of the light pulse (group velocity) in the medium. Now, the sensitivity of an interferometer is proportional to the optical path difference between the arms of the interferometer. If a slow light medium is inserted in one of the arms, then the equivalent optical path difference for light pulses becomes amplified because the optical thickness of the slow light medium becomes dilated by a factor that is nothing else but the group index. In media where large group delays are achieved, the group index can become very large and, therefore, the sensitivity of slow-light based interferometers can be considerably increased with respect to conventional ones. A recent demonstration of this concept has been realized by performing extremely sensitive Doppler shift measurements in a liquid crystal slow light medium2.
While slow light is realized in several systems, only a few media have shown the ability of storing light, among them, atomic vapors and a particular class of solid crystals when operated at very low temperature. In these systems, the physical effect is the so-called electromagnetically induced transparency (EIT) and the coherent states, allowing the information of the pulse to be stored in the medium, are atomic levels, either those of atoms in vapors or those of color centers locked in a crystal lattice.
In our system, the states into which the optical information is stored are those of the dye molecules hosted in the chiral structure formed by the liquid crystal matrix. These dyes are able to change their conformation under light irradiations. More precisely, our medium is made of an ordered arrangement of organic molecules, the liquid crystals, disposed in a helical structure, in which has been inserted a small amount (doping) of dyes. Such dyes possess an azo group that makes them sensitive to light irradiation.
When light irradiates the medium, the dyes undergo a photo-excitation process – an electron is excited by photon absorption, then, when decaying to the ground state the molecule changes its shape, bending around the azo group, from an elongated (trans) to a v-like (cis) form, thus keeping memory of the optical process undergone. The concentration of molecules in the cis-state is related to the propagation of the pulse in the sample. The two molecular forms are called “isomers” and the photo-excitation process is called photo-isomerization. It is this process that we exploit to keep the memory of the pulse. Indeed, because the absorption cross-sections of the trans and cis states are different, it happens that the absorption properties of the sample become different in the bright (illuminated) and in the dark regions.
When two beams are sent to interact in the medium, a cis population grating is correspondingly created and a transparency window appears around the resonance condition for which the two beams have the same frequency. The width of the transparency window is determined by the decay rate of the cis towards the trans state. The effect is similar to the EIT but, in our case, it is the photo-isomerization process acting on the dyes that induces the transparency window and the involved energy levels are those of molecules instead of atoms.
Soft matter systems are easy to implement, there is a large flexibility in the choice of the materials (liquid crystals, dyes, chiral dopants) so that, for instance, various wavelength ranges could be addressed, and large transverse size samples could be realized by using the standard technology of liquid crystal displays.
In future investigations, steps could be taken in order to extend the range of applicability of this method to other molecular arrangements, different compositions, different concentrations of chiral dopants, and different types of dyes. These new functionalized soft matter materials should be able to increase the operational wavelength range, introduce tunability, and enlarge frequency bandwidth for more applicability and performances in phase sensing applications.
- D. Wei, U. Bortolozzo, J.P. Huignard and S. Residori, Slow and stored light by photoisomerization induced transparency in dye doped chiral nematics, Optics Express 21, 1954, 2013.
- U. Bortolozzo, S. Residori and J.C. Howell,Precision Doppler measurements with steep dispersion, Optics Letters 38, 3107, 2013.