The well defined regularity of so-called photonic crystals enables the control of properties of light propagating through the crystals. Recently, there is growing interest in production of photonic crystals whose photonic properties can be tuned by external stimuli. For example, the transmission of light has been controlled by stretching photonic crystals to shift the photonic stop-bands—the spectral region where light does not propagate through the crystal.

Now, researchers from the Chinese State Key Laboratory of Bioelectronics in Nanjing1 have fabricated photonic crystals that show different transmission properties for light polarized perpendicular and parallel to the orientation of liquid crystals embedded within the photonic crystal structure.

Fig. 1: Birefringent photonic crystals. a, The microscope image shows the initial structure of the photonic crystal. b, Image of a stretched photonic crystal. Infiltrated with liquid crystals, birefringence can be achieved.

In their approach, Zhuo-Ying Xie and colleagues infiltrated flexible photonic crystals with liquid crystals, which show birefringence—the transmission properties being dependent on the polarization of light (Fig.1).

“A photonic crystal with polarization-dependent stop-bands is a promising candidate for devices such as polarizing displays, beam splitters or optical waveguides,” says Xie.

Stretching photonic crystals infiltrated with liquid crystals causes the liquid crystals to align themselves along the stretch axis, leading to birefringent photonic crystals. As a result, light polarized parallel to the stretching axis is reflected, while light polarized perpendicular to this axis is transmitted.

Importantly, the birefringent behavior can be tuned by optical irradiation, where UV radiation causes a loss of birefringent properties and the crystal reflects light equally for both polarizations. This process can be reversed by irradiation with visible light.

Notably, the opposite effect occurs by applying an electrical field, leads to a rotation of the liquid crystal molecules within the photonic crystal, so that their preferred axis is in the direction perpendicular to the stretching plane of the photonic crystal. Accordingly, this destroys the stop band so that polarized light both parallel and perpendicular to the stretching axis is transmitted.

Therefore, three different transmission states can be achieved through external control either through external light or an applied electric field, allowing for the promising use of birefringent photonic crystals as optical switches in photonic circuits.