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