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Resonant gratings for narrow band pass filtering applications. Olga Boyko, Fabien Lemarchand , Anne Talneau, Anne-Laure Fehrembach and Anne Sentenac Laboratoire de Photonique et Nanostructures, CNRS UPR20, Route de Nozay, 91460 Marcoussis, France - PowerPoint PPT Presentation
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Resonant gratings for narrow band pass filtering
applications Olga Boyko, Fabien Lemarchand, Anne Talneau, Anne-Laure
Fehrembach and Anne Sentenac
Laboratoire de Photonique et Nanostructures, CNRS UPR20, Route de Nozay, 91460 Marcoussis, France
Institut Fresnel, CNRS UMR6133, Aix-Marseille universités, D.U. de Saint Jérome, 13397 Marseille, France
Ultra narrowband inverse (notch) filters FWHM < 1 nm
with polarization independence and good angular tolerance
kxoy
Kx
x
yz
O
Incident light
kg guided mode
Subwavelength grating
Dielectric AR structure
Incident plane wave for R = 1
Resonance /coupling condition: |kxoy + m Kx| = kg (m integer)
proj. of the incident wavevector
evanescent wave
Reflected light
R
at the resonance R = 1 (and T=0)
NOTCH (INVERSE) FILTERS
A single guided mode kg is excited with a single evanescent wave under oblique incidence
kxoy
K
kg
Different configurations for exciting a guided mode
1.
Resonance very sensitive to the incident angle and the incident polarization
Typically = 5nm for a = 0.2degBad performances with standard collimated beam
A single guided mode kg is excited with the two +/-1 evanescent waves under normal incidence
kxoy =0 (normal incidence)
+K
kg
Different configurations for exciting a guided mode
2.
Resonance very sensitive to the incident polarizationThe angular tolerance may be good with specific grating profiles
-K
A single guided mode kg is twice excited with the two +/-1 evanescent waves under oblique incidence
Different configurations for exciting a guided mode
3.
Resonance very sensitive to the incident polarizationThe angular tolerance may be good with specific grating profiles
kx0y
Kx-Kx
kg
Two guided modes kg are excited with the two +/-1 evanescent waves under oblique incidence
Different configurations for exciting a guided mode
4.
Resonance with a possible good angular tolerance BUT design sensitive to fabrication errors
K
kg1
-K
kxOy
kg2
Lamellar grating profiles leading to a good angular tolerance
(x)
x
(f)
f
Kx 2Kx 3Kx 4Kx
1
2
Single mode excitation with single evanescent wave: 1
dd1 d2 d1 and d2 d/2
Guide Mode excitation with two evanescent waves: 1 and 2
Combining angular tolerance and polarization independence
kg
+Kx
+Ky
-Kx
-Ky
normal incidence
kg1
kxOykg2
oblique incidence
Polarization independence:excitation with two orthogonal grating vectors Kx and Ky (2D gratings)
Angular tolerance: excitation with several evanescent waves and |2| >>|1|
Design and fabrication
• 4 DIBS layers
SiO2PMMA
272.5nm
365nm
180nm
d/4
272.5nm
d=940nm
d/4
• electronic lithography (Leica EBPG 5000+)
tunable laser 1520-1570nm
Pigtailed collimator
2w0 = 0.58 mm
T photodiode
RGF
reference fluxNon polarising
polarizing beamsplitter
R photodiode
/2 waveplate
Experimental characterization set-up
Transmittance of the normal incidence notch filter
T T
5.4 5.6 5.8 6 6.21.535
1.54
1.545
1.55
(°)
(
m)
ps
theory
Oblique incidence filter: location of the minima of transmittivity versus and for s and p polarizations
• experimental and theoretical curves are similar
(same gap width ~ 5nm, opening around 5.8°)
• spectral shift: due uncertainty on layer thickness or layer index
A
5.4 5.6 5.8 6.0 6.21.545
1.550
1.555
1.560 p s
(m
)
experience
B’A’
B
Points A and A’: polarization independence
Gaussian beam: diameter at waist 580µm, full angle divergence 0.2°
theoretically =0.2nm (Plane wave: =0.1nm )
experimentally =0.4nm
Points B and B’: s and p resonances split and filter performances deteriorated
Theory (gaussian beam)Experience
Conclusion
• Few number of layers and subwavelength grating
• Specific 2D grating design => polarization
independence and good angular tolerance
• Experimental demonstration of ultra narrowband inverse
filters =0.4nm
• Improvement of the maximum R value: larger grating
surface (4mm2) and designs with even higher angular
tolerance