Figure 1. Principle of iTRUE.

(a) Schematic experimental setup consisting of a DOPC system and an ultrasound modulation module. The probe beam reflected off the SLM is scattered by the sample and modulated by the ultrasound. The phase map of the ultrasound modulated light is measured by the camera and the conjugated phase map is displayed on the SLM, which modulates the probe beam again. (b)–(e) Schematic demonstration of signal enhancement with iTRUE. (b) Without ultrasound modulation, the probe beam diffuses inside the scattering sample when the SLM is initially uncontrolled. (c) With ultrasound modulation, a small fraction of the probe beam is shifted in frequency and backscattered to the DOPC system. (d) The phase-conjugated copy of the measured ultrasound-modulated light is sent back to the sample. Although SNR of the measured wavefront is low, owing to the small amount of light being modulated in c, the playback light is still able to create a focus at the focus of the ultrasound. This playback light focus is modulated simultaneously by the ultrasound, and a more accurate wavefront can be measured for the next step. (e) A stronger light focus can thus be obtained. By repeating the playback and recording process, the light intensity focused at the focus of the ultrasound can be progressively increased.