Fig. 1.

Principle of magnetic-particle-guided optical focusing. (a) A magnetic particle is embedded in a piece of scattering tissue. A portion of the impinging laser beam interacts with the particle and the resulting tagged light is detected interferometrically using the camera of a DOPC system. (b) After capturing the field of the tagged light, the conjugate wavefront is displayed on the spatial light modulator (SLM) of the DOPC system. The reconstructed conjugate light field then retraces the scattering paths and forms a focus at the location of the magnetic particle. Panels (c) and (d) show two methods to separate the tagged light field from the background unmodulated light. The field subtraction method in (c) captures two optical fields before and after a magnetic field displaces the magnetic particle. The differential field nullifies the contribution from the background, which is not scattered by the particle. The frequency modulation method shown in (d) uses an AC magnetic field to make the magnetic particle oscillate, which shifts the frequency of the light, which interacts with the particle. By matching the frequency of a planar reference beam with that of the tagged light, the DOPC system detects the tagged light field via phase-shifting holography. (e) After imprinting the conjugate wavefront of the tagged light on a planar reference beam using the SLM, the conjugate wave forms a bright focus on top of a dim background at the location of the magnetic particle inside the scattering medium.