Researchers have found that using helical beams in disordered solids shows that the solids exhibit dichroism. [Image: Ravi Bhardwaj]
Amorphous solids are generally thought to respond identically to beams of light with different polarizations. An amorphous solid has no long-range order because its atoms are randomly arranged. This means that it is isotropic, so its response does not vary according to light's polarization―whether left- or right-circularly polarized, incoming light is absorbed to the same degree.
But new research reportedly shows that what is known as helical light can induce different levels of absorption in disordered solids depending on its direction of twist―and it has a similar effect on crystalline solids (Nature Commun., doi: 10.1038/s41467-024-45735-9).
Generating helical light
Whereas standard polarization regards the orientation of a light wave's electric field oscillations and is associated with spin angular momentum, helical light possesses finite orbital angular momentum. The wavefront of such a beam is not fixed at right angles to the direction of propagation but instead rotates around the propagation axis, creating a “phase singularity” at its center. The greater the angular momentum, the more cycles the wavefront completes in the space of a wavelength.
Ravi Bhardwaj and colleagues at the University of Ottawa, Canada, generated helical light by shining femtosecond pulses from an infrared laser through a q-plate, a birefringent material that molds the phase profile of a beam's wavefront. This light then passed through a sample of amorphous solid, and its transmitted fraction was measured using a photodetector. The researchers mounted the q-plate on a moveable platform to vary the position of the singularity within the beam profile, which creates a void in the intensity profile like the hole in a donut.
When generating light with one or three twists per wavelength in both helical directions and directing that light at fused silica and borosilicate glass, the team measured identical levels of absorption for a fully centered singularity. But when shifting the singularity and therefore the void slightly, Bhardwaj and colleagues found that the beams with opposing orbital angular momenta were absorbed to different extents―an effect known as helical dichroism―with the magnitude of the difference depending on the size of the void offset.
Arguing that the result “challenges the conventional knowledge that dichroism does not exist in amorphous solids,” the researchers explain that, although such materials do not possess long-range order, they do exhibit regularities on a scale of about 2 µm―roughly the thickness of a strand in a spider's web.
Looking at crystals
The researchers also carried out the same procedure using crystalline solids and obtained very similar results. They found that for a beam with a noncentered void, a sample of magnesium oxide also displayed significant helical dichroism.
What's more, the team also looked at chiral crystals. These materials have lattice structures that lack mirror symmetry, so in principle they absorb left- and right-circularly polarized light to different extents. But because the signal from this dichroism is often masked by the effects of other crystal properties, such as linear birefringence, its detection requires the use of sophisticated imaging techniques and large, very pure crystals.
Bhardwaj and colleagues found that they could amplify this signal by using helical light in their bench-top experiment―light that had linear polarization with either positive or negative orbital angular momentum. They say that the signal is an order of magnitude stronger than that from solid-state techniques that use circularly polarized light and comparable with that obtained using hard X-rays at the roughly 700-m-long SwissFEL free electron laser in Switzerland.
Applications and hurdles
According to the researchers, this capability could have a number of applications, including for certain types of catalysis and molecular synthesis and also in chiral sensors.
According to the researchers, this capability could have a number of applications, including for certain types of catalysis and molecular synthesis and also in chiral sensors. As for measuring helical dichroism in amorphous solids, they say that their setup could help shed light on short-range order in polymers used in printed and flexible organic electronics.
One technical hurdle that still needs to be overcome, however, stems from the fact that environmental fluctuations can lead to errors when measuring very small dichroism signals. The current setup is susceptible to this problem because the signal is obtained in two stages―with absorbance of left-helical light being measured before that of right-helical light. This problem, explains Bhardwaj, can be reduced by rapidly modulating the helical light. “When such techniques are developed, it opens up new opportunities and applications,” he says.
