Liquid-crystal neutral density filter that can be tuned using ultrasound

Conventional fixed neutral density (ND) filters are designed with a specific, predetermined attenuation level. Thus, users need to manually swap filters depending on the environment or application. Existing variable ND filters often require complex fabrication processes and control mechanisms. In this study, we proposed a variable ND filter with no mechanical moving parts in which the light exposure can be adjusted by using ultrasonic vibration to control the twisted-nematic (TN) liquid-crystal (LC) molecular orientation. This ultrasonic variable ND filter consists of glass discs, a TN LC layer, and an ultrasonic transducer. The application of an electrical signal induced a resonant flexural vibration mode, generating an acoustic radiation force. This force effectively disrupted the twisted orientation of the LC layer, enabling the transmitted light intensity to be controlled by the input voltage. This filter, with a thin and simple structure, could facilitate the development of thinner and lower-cost optical devices.