Letter to the Editor: More on the transmission matrix in optics
In regard to the Viewpoint by van Putten and Mosk , which discusses results on measurement of the transmission matrix in optics by S. M. Popoff et al., I would like to point out several facts. Aspects of propagation of waves through random media are at the heart of mesoscopic physics, and have therefore been studied extensively in the last two decades. The fact that the transmission matrix (TM) of disordered media depends in a complicated, seemingly random manner on the properties of the input field has been known for a long time. The use of random media as traditional optical components or systems was suggested twenty years ago by Isaac Freund  and a broader discussion of these proposals can also be found in a review by van Rossum and Nieuwenhuizen . Since then, there have been several developments that exploit the existence of the transmission matrix without explicitly knowing it, as well as a number of practical applications that rely on measuring the transmission matrices of random media. However, the present essay does not provide an inclusive perspective of the entire field. The Viewpoint by van Putten and Mosk fails to acknowledge the original proposal by Freund in 1990 and other experimental demonstrations of the use of multiple scattering media as high-precision optical instruments. The control and use of light propagating through random media is indeed a topic of both significant fundamental importance and high practical relevance and, therefore, deserves to be placed in the appropriate factual context.