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MotivationThin- and ultra-thin film materials for efficient light absorption.
Large variety of possible applications: sensing and detection, energy production, integrated multimedia applications, optical neuro-stimulators, etc..
Pros:Reduced weight .Efficient carrier extraction.Low-cost (in terms of material employed).Flexible (sometimes). Etc…
Cons:The thinner you go, the lower is absorption
Guided modes for trapping light
Light coupling to these forbidden modes would increase the effective optical thickness of the thin-film
Trapped light
Reffracted light
Quasi-trapped modes for light coupling
Quasi-trapping Angle-independent trapping
Periodic and quasi-periodic patterning can provide discrete number of Fourier resonances in the shaded area.
A WHITE response in frequency- AND reciprocal-space would be ideal!
Then, chop the light randomly short!
Breaking the wave with strong multiple scattering
x (m)
y (
m)
0 2 4 6 8
0
1
2
3
4
5
6
7
8
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
Coupling wavevectors
λ= 500 nm
• Silicon slab• Holes diameter 400 nm • thickness 100 nm
Disorder traps - Light is absorbed
Remarkable absorption enhancement on a broad range of wavelengths and angle of incidence
K. Vynck et al, Nature Materials (2012)
Random
Bare slab
630
1600
940
1370
1115
810
710
Wavelength (nm)
Wavenumber (1/um)62503825120
Absorption length fixed for all wavelengths
Engineering disorder
Correlated disorder for tunable spectral response
Bare slab
Correlated
Periodic630
1600
940
1370
1115
810
710
Wavelength (nm)
Wavenumber (1/um)503825120 62
K. Vynck et al, Nature Materials (2012)
Random
Coherent light trapping Vs absorption<a>/li=0.14
<a>/li=1.4
<a>/li=8.4
For strong absorption only the single hole scattering matters
Realistic solar cells
F. Pratesi et al, Sumbitted to Opt. Express
a-Si c-Si
Correlated-disorder and order provide the same integrated absorption.
ConclusionDisordered Modes can be used for an efficient
absorption enhancement of solar radiation;
Disorder can be tuned by introducing correlation in the system and, thus control the optical response of the system;
Disordered structures have trapping performances comperable with deterministic ones
AcknowledgementsPeople involved in this works
Kevin VynckFilippo PratesiFrancesco RiboliDiederik S. Wiersma
Collaborators
Massimo Tormen(TASC-Trieste)
