The idea is to activate two different
modes lying at a given distance in a
titanium dioxide structure. The modes
are found to emit at the same frequency
when they are both excited by independent control pump-beams. In addition,
one mode may act as a gate to transport
the spectral content of the other, like a
standard transistor, and, in a way, that
is inherently parallel because it involves
several wavelengths. 4
By exploiting adaptive optics, we can
select and control spectral features of
RLs. This ability turns RLs into fertile
ground where novel devices may be
envisaged, implemented and tested.
The ability to control the flow of energy in a random medium has a wide range
of potential applications. However, the
mechanisms underlying signal processing
and transport in the presence of disorder
are mostly unknown. Random lasers (RL)
are light sources based on disordered
active media, such as intergalactic powders
or laboratory synthesized micron-sized
titanium dioxide resonators randomly
distributed in a dye-doped solution. 1, 2
An RL sustains a large number of modes,
and, generally speaking, it is impossible to
predict the ones that will lase.
We reached a key milestone in this field
by demonstrating selective activation of RL
modes by exploiting adaptive optics to control the shape of the pump beam. In systems
with fixed scatterers, it is possible to excite
specific individual lasing modes, resulting
in a tunable subnanometer spectral emission.
In this approach, we obtain pump-beam
tailoring by directional amplified stimulated
emission (ASE). 3
Large spatial regions of population inversion give rise to a directional emission, which
may be exploited to pump microresonators.
We obtained ASE by pumping rod-shaped
volumes of red liquid rhodamine layers. 3
By shifting the position and orientation of
the green pump beam, different scattering
particles can be illuminated by the ASE and
drive the RL. We demonstrated that spatial
pump-beam shaping allows subnanometer
single-mode emission tunable in a 20-nm
In RLs, the disordered dielectric cavity
is open; hence, energy easily flows from one
mode to another, as modes are inherently
coupled by radiative losses. This feature may
be exploited to achieve a controlled interaction and logic gating in disordered systems.
ASE demonstration: (a) Scattering particles (spheres) are illuminated by ASE
(red) when we shift the green pump beam (b), driving the random laser.
(Bottom) Optical images of the intensity emitted by the titanium dioxide cluster
in various pumping configurations: (c) source-only pumping, source mode on;
(d) gate-only pumping, drain mode on; and (e) source and gate simultaneously.
Marco Leonetti (marco-
and Claudio Conti
University of Rome
La Sapienza, Rome, Italy
Institute of Materials
Sciences, Madrid, Spain
1. S. Lethokov et al. IEEE
J. Quantum Electron. 2,
2. D. S. Wiersma. Nature
Phys. 4, 359, (2008).
3. M. Leonetti et al. Appl.
Phys. Lett. 102, 071105,
4. M. Leonetti et al. Nat.
Commun. 4, 1740, (2013).
Random Lasers: Active Mode
Control and Gating