Under-display ultrasonic fingerprint sensors for foldable displays

An apparatus, comprising:

• ultrasonic sensor means;
• a display stack, comprising:
• a high-impedance layer;
• and
• one or more display stack resonators configured to enhance ultrasonic waves transmitted by the ultrasonic sensor means in a first ultrasonic frequency range;
• and
• a transmission enhancement layer, the high-impedance layer, the transmission enhancement layer and at least a portion of the ultrasonic sensor means forming a transmission enhancement resonator configured to enhance the ultrasonic waves transmitted by the ultrasonic sensor means in the first ultrasonic frequency range.

A method, comprising:

• controlling, via a control system, an ultrasonic transceiver layer of an ultrasonic sensor system to transmit ultrasonic waves through at least a first resonator configured for causing a first local maximum of ultrasonic wave transmission in a first ultrasonic frequency range and a second resonator configured for causing a second local maximum of ultrasonic wave transmission in the first ultrasonic frequency range, the first resonator including a high-impedance layer and a transmission enhancement layer, the high-impedance layer having an acoustic impedance of 10 MRayls or more, the second resonator including one or more display stack layers;
• receiving, by the control system and from the ultrasonic sensor system, ultrasonic sensor signals corresponding to reflections of transmitted ultrasonic waves from a portion of a target object positioned on an outer surface of an apparatus that includes the ultrasonic sensor system;
• and
• performing, by the control system, an authentication process based, at least in part, on the ultrasonic sensor signals.