42 research outputs found
Control of mesoscopic transport by modifying transmission channels in opaque media
While controlling particle diffusion in a confined geometry is a popular
approach taken by both natural and artificial systems, it has not been widely
adopted for controlling light transport in random media, where wave
interference effects play a critical role. The transmission eigenchannels
determine not only light propagation through the disordered system but also the
energy concentrated inside. Here we propose and demonstrate an effective
approach to modify these channels, whose structures are considered to be
universal in conventional diffusive waveguides. By adjusting the waveguide
geometry, we are able to alter the spatial profiles of the transmission
eigenchannels significantly and deterministically from the universal ones. In
addition, propagating channels may be converted to evanescent channels or vice
versa by tapering the waveguide cross-section. Our approach allows to control
not only the transmitted and reflected light, but also the depth profile of
energy density inside the scattering system. In particular geometries perfect
reflection channels are created, and their large penetration depth into the
turbid medium as well as the complete return of probe light to the input end
would greatly benefit sensing and imaging applications. Absorption along with
geometry can be further employed for tuning the decay length of energy flux
inside the random system, which cannot be achieved in a common waveguide with
uniform cross-section. Our approach relies solely on confined geometry and does
not require any modification of intrinsic disorder, thus it is applicable to a
variety of systems and also to other types of waves
Transmission channels for light in absorbing random media: from diffusive to ballistic-like transport
While the absorption of light is ubiquitous in nature and in applications,
the question remains how absorption modifies the transmission channels in
random media. We present a numerical study on the effects of optical absorption
on the maximal transmission and minimal reflection channels in a
two-dimensional disordered waveguide. In the weak absorption regime, where the
system length is less than the diffusive absorption length, the maximal
transmission channel is dominated by diffusive transport and it is equivalent
to the minimal reflection channel. Its frequency bandwidth is determined by the
underlying quasimode width. However, when the absorption is strong, light
transport in the maximal transmission channel undergoes a sharp transition and
becomes ballistic-like transport. Its frequency bandwidth increases with
absorption, and the exact scaling varies with the sample's realization. The
minimal reflection channel becomes different from the maximal transmission
channel and becomes dominated by absorption. Counterintuitively, we observe in
some samples that the minimum reflection eigenvalue increases with absorption.
Our results show that strong absorption turns open channels in random media
from diffusive to ballistic-like.Comment: 11 pages, 7 figure
Topological defect lasers
We demonstrate topological defect lasers in a GaAs membrane with embedded
InAs quantum dots. By introducing a disclination to a square-lattice of
elliptical air holes, we obtain spatially confined optical resonances with high
quality factor. Such resonances support powerflow vortices, and lase upon
optical excitation of quantum dots, embedded in the structure. The spatially
inhomogeneous variation of the unit cell orientation adds another dimension to
the control of a lasing mode, enabling the manipulation of its field pattern
and energy flow landscape