Improvement of the dynamic response of an ultrasonically driven liquid crystal tunable neutral density filter

Conventional neutral density (ND) filters are designed with a constant attenuation level, necessitating manual replacement to adapt to various environments or applications. To address this limitation, tunable ND filters utilizing ultrasonic vibrations and liquid crystal (LC) molecules have been developed. The filter consists of a twisted nematic LC layer sandwiched between a glass disc and an annular-shaped piezoelectric ultrasonic transducer, enabling a compact and simple design. This paper examined the relationship between the LC layer thickness and the optical performance of the LC ND filter. The resonant flexural vibration mode of the filter altered the attenuation level of the transmitted light through a change in the LC molecular orientation induced by acoustic radiation force; the attenuation level decreased as the vibrational amplitude increased. The response time of the filter was found to be dependent significantly on the LC layer thickness, with a thinner LC layer resulting in a shorter response time. The time constant of the dynamic response for the filter with an LC layer of 10 μm was 0.27 ms.