  In conventional molecular communication (MC) systems, the signaling molecules
used for information transmission are stored, released, and then replenished by
a transmitter (TX). However, the replenishment of signaling molecules at the TX
is challenging in practice. Furthermore, in most envisioned MC applications,
e.g., in the medical field, it is not desirable to insert the TX into the MC
system, as this might impair natural biological processes. In this paper, we
propose the concept of media modulation based MC where the TX is placed outside
the channel and utilizes signaling molecules already present inside the system.
The signaling molecules can assume different states which can be switched by
external stimuli. Hence, in media modulation based MC, the TX modulates
information into the state of the signaling molecules. In particular, we
exploit the group of photochromic molecules, which undergo light-induced
reversible state transitions, for media modulation. We study the usage of these
molecules for information transmission in a three-dimensional duct system,
which contains an eraser, a TX, and a receiver for erasing, writing, and
reading of information via external light, respectively. We develop a
statistical model for the received signal which accounts for the distribution
of the signaling molecules in the system, the initial states of the signaling
molecules, the reliability of the state control mechanism, the randomness of
irrepressible, spontaneous state switching, and the randomness of molecule
propagation. We adopt a maximum likelihood detector and a threshold based
detector. Furthermore, we derive analytical expressions for the optimal
threshold value and the resulting bit error rate (BER), respectively. Our
results reveal that media modulation enables reliable information transmission,
validating it as a promising alternative to MC based on molecule emitting TXs.
