A high birefringence liquid crystal for lenses with large aperture

Abstract

This work presents the application of an experimental nematic liquid crystal (LC) mixture (1929) in a large aperture lens. The LC material is composed of terphenyl and biphenyl derivatives compounds with an isothiocyanate terminal group and fluorinated lateral substituents. The substitution with a strongly polar isothiocyanate group and an aromatic rigid core provides [Formula: see text]-electron coupling, providing high birefringence ([Formula: see text] at 636 nm and 23 °C) and low viscosity ([Formula: see text] = 17.03 mPa s). In addition, it also shows high values of birefringence at near infrared (0.318 at 1550 nm). The synthesis process is simple when comparing materials with high melting temperatures. The excellent properties of this LC mixture are demonstrated in a large aperture LC-tunable lens based on a transmission electrode structure. Thanks to the particular characteristics of this mixture, the optical power is high. The high birefringence makes this LC of specific interest for lenses and optical phase modulators and devices, both in the visible and infrared regions.

Figure 1

(a) General formula of mixture 1929. (b) The chemical formula of prepared isothiocyanatoterphenyls.

Figure 2

(a) Schematic depiction of the LC-tunable large-aperture lens and its various constituent parts. The top substrate shows the electrode configuration for the distribution of the applied voltage profile. (b) Detail of the top substrate electrode. The figure was generated using Inkscape software with version no. 1 and link https://inkscape.org/es/.

Figure 3

Transmission spectra for: (a) UV–VIS and (b) NIR.

Figure 4

Wavelength dispersion of: (a) the extraordinary ($n_{\text{e}}$) and ordinary $n_{\text{o}}$ LC indices (symbols are experimental data and solid line the Cauchy fit) and (b) the LC birefringence ($\mathrm{\Delta }n=n_{\text{e}}-n_{\text{o}}$).

Figure 5

Frequency dependence of complex relative permittivity. (a) Real (${\epsilon }^{\prime }$) and (b) imaginary ${\epsilon }^{\prime \prime }$. indices.

Figure 6

Temperature dependence of complex relative permittivity. (a) Real (${\epsilon }^{\prime }$) and (b) imaginary ${\epsilon }^{\prime \prime }$. indices.

Figure 7

DCS thermographs upon heating/cooling cycles of mixture 1929.

Figure 8

(a) Schematic of the optical system for measuring (a) the fringe patterns by placing the LC lens between crossed polarizers, (b) the focal length of TELCL and (c) the MTF function. The figure was generated using Inkscape software with version no. 1 and link https://inkscape.org/es/.

Figure 9

Interference patterns measured by placing the LC lens between crossed polarizers. Positive lens (V${}_{\text{RMS}}$ values): (a) $V_1=1.75$, $V_2=0.5$, (b) $V_1=1.5$, $V_2=0.5$, (c) $V_1=1.35$, $V_2=0.5$, (d) $V_1=1.25$, $V_2=0.5$. Negative lens (V${}_{\text{RMS}}$ values): (e) $V_1=1.25$, $V_2=3.5$, (f) $V_1=1.4$, $V_2=3.5$, (g) $V_1=1.55$, $V_2=3.5$, (h) $V_1=1.65$, $V_2=3.5$. The figure was generated using Spinview software with version no. 3.1 and link https://www.flir.es/products/spinnaker-sdk/?vertical=machine+vision &segment=iis.

Figure 10

(a) Experimental phase shift profiles extracted from the fringe patterns of Fig. 5. (b) Optical power for different applied voltages.

Figure 11

Negative lens focal spot for (a) $\text{f}=-200$ cm, (b) $\text{f}=-147$ cm, (c) $\text{f}=-120$ cm.

Figure 12

Different PSFs for (a) $V_1=1.75$  V${}_{\text{RMS}}$ and $V_2=4.5$ V${}_{\text{RMS}}$. (b) $V_1=1.80$ V${}_{\text{RMS}}$ and $V_2=0.6$ V${}_{\text{RMS}}$ (c) $V_1=1.85$ V${}_{\text{RMS}}$ and $V_2=0.6$ V${}_{\text{RMS}}$.

Figure 13

MTFs for (a) horizontal and (b) diagonal cross-sections for different applied voltages and the diffraction limit (Diff.).

Figure 14

Different images of the lens performance (a) $V_1$= 1.4 V${}_{\text{RMS}}$ and $V_2=4.5$ V${}_{\text{RMS}}$ (focused). (b) Lens switched off. (c) $V_1=1.85$  V${}_{\text{RMS}}$ and $V_2$ = 0.6 V${}_{\text{RMS}}$ (focused).