To exploit photonics technologies for in vivo studies in life science and biomedicine, it is necessary to efficiently deliver light energy to the target objects embedded deep within complex biological tissues. However, light waves diffuse randomly inside complex media due to multiple scattering, and only a small fraction reaches the target object. Here we present a method to counteract the random diffusion and to focus ‘snake-like’ multiple-scattered waves to the embedded target. To realize this, we experimentally identified time-gated reflection eigenchannels that have extraordinarily large reflectance at a specific flight time where most of the multiple-scattered waves have interacted with the target object. By injecting light to these eigenchannels, we achieved more than 10-fold enhancement in light energy delivery compared to ordinary wave diffusion cases. This method works up to depths of approximately 2 times the transport mean free path at which target objects are completely invisible by ballistic optical imaging. This work will lay a foundation for enhancing the working depth of imaging, sensing, and light stimulation.
When waves travel through disordered media such as ground glass and skin tissues, they are scattered multiple times. Most of the incoming energy bounces back at the superficial layers and only a small fraction can penetrate deep inside. This has been a limiting factor for the working depth of various optical techniques. We present a systematic method to enhance wave penetration to the scattering media. Specifically, we measured the reflection matrix of a disordered medium with wide angular coverage for each orthogonal polarization states. From the reflection matrix, we identified reflection eigenchannels of the medium, and shaped the incident wave into the reflection eigenchannel with smallest eigenvalue, which we call anti-reflection mode. This makes reflectance reduced and wave penetration increased as a result of the energy conservation. We demonstrated transmission enhancement by more than a factor of 3 by the coupling of the incident waves to the anti-reflection modes. Based on the uneven distribution of eigenvalues of reflection eigenchannels, we further developed an iterative feedback control method for finding and coupling light to anti-reflection modes. Since this adaptive control method can keep up with sample perturbation, it promotes the applicability of exploiting reflection eigenchannels. Our approach of delivering light deep into the scattering media will contribute to enhancing the sensitivity of detecting objects hidden under scattering layers, which is universal problem ranging from geology to life science.