Metallic diffractive lens for terahertz · PDF fileA. Siemion, A. Siemion, M. Makowski and M....
Transcript of Metallic diffractive lens for terahertz · PDF fileA. Siemion, A. Siemion, M. Makowski and M....
Metallic diffractive lens for terahertz beams
E. Hérault, F. Garet, and J.-L. CoutazIMEP-LAHC, Université de Savoie
A. Siemion, A. Siemion, M. Makowski and M. SypekFaculty of Physics, Warsaw University of Technology
Wokshop of the GDRI-CNRS, Tignes, March 29 – April 1 2011
Outline
• Motivations
• Diffractive structure
• Spatial and spectral filter
• 2D prism
• Conclusion and prospective
Motivations
• Teraeye : European project
– To design a THz camera
– THz filter for thermal load
– Focusing element
• Idea : to realize both functions with one element
– Off axis detector
– Diffractive component
�To deflect the THz beam
�To focus the beam onto the detector
360 mm
50 mm
50 mm
object
plane
D
Diffractive structure• Composition of two lenses :
• Coding of the phase distribution into amplitude
binary form
Tstructure=T1xT2
Ideal phase distribution
Amplitude binary form
Binary phase distribution
( )− +
=
2 2
121( , )
ikx y
FT x y e ( )− ⋅ + + +=
22 22
2( , )ik x y d F
T x y e
Diffractive structure
• Laser cut curved strips
• Varying width of both metal and air
strips
• Average periodicity : 0.81 µm
• Two operating regimes :
• f > 0.4 THz : “normal” regime
���� Application : Spatial and spectral filter
• f < 0.4 THz : sub-wavelength regime
���� Application : 2D prismManufactured by Ad-Venta (Loriol)
Spatial and spectral filter
• Experimental setup : THz time-domain spectroscopy
emitter
moving
diffracted and
focused THz beam
non-deviatedGaussian
THz beam
device
receiver
∆θ∆θ∆θ∆θ ∼∼∼∼ 5.75.75.75.7°°°°
Spatial and spectral filter
• Is the structure deflecting
the THz beam ?
� Best contrast at 0.5 THz
� Deflection angle = 46 °°°°
� Diffractive efficiency = 10 %
0
2
4
6
8
0 10 20 30 40 50 60 70
TH
z si
gn
al
am
pli
tud
e
Angle (degree)
0.41 THz
0.44 THz
0.46 THz
0.60 THz
0.50 THz
0.80 THz
1.00 THz
0.90 THz
0.70 THz
Spatial and spectral filter
• Is the structure focusing the beam at 0.5 THz ?
z
� Waist at z = 86.7 mm
� w0 = 2.05 mm
� Focusing property limited by diffraction
Blade cutting experiment
1.5
2
2.5
3
3.5
4
4.5
5
60 65 70 75 80 85 90 95 100
Radiu
s (m
m)
Distance z (mm)
Spatial and spectral filter
• Comparison modeling/experiment
� Good agreement
Theoretical modeling Extrapolation from measurements
30 35 40 45 50 55 60 65 70 75 80 85 90
30
35
40
45
50
55
60
65
70
75
80
0.8-1
0.6-0.8
0.4-0.6
0.2-0.4
0-0.2
70 mm
80 mm
60 mm
30 35 40 45 50 55 60 65 70 75 80 85 9030
35
40
45
50
55
60
65
70
75
80
0.8-1
0.6-0.8
0.4-0.6
0.2-0.4
0-0.2
f = 0.5 THz
2D prism
• Deflection for the non diffracted
beam
• Sub-wavelength regime
• Is it an effect of the variation of
periodicity ?
0
2
4
6
8
10
12
0 8 16 24 32 40 48 56
TH
z fi
eld
mag
nit
ud
e (A
.U.)
Angle (degree)
0.28 THz
0.30 THz
0.32 THz
0.37 THz
0.39 THz
0.44 THz
0.50 THz
0.60 THz
0.70 THz
0.80 THz
0.23 THz
2D prism
• Comparison between two structures
-5
0
5
10
15
20
25
30
35
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
θ (
degre
e)
Frequency (THz)
Varying periodicity
Constant periodicity
� No deflection for the constant periodicity structure
� Deviation angle related to the gradient of index
( , ) 2 ( , )tan
φ πθ
∂ ∂= =
∂ ∂
x f f n x fh
x c x
Conclusion and prospective
• Design and realization of a metallic diffractive structure
• Spatial and spectral filter
– Deflection and focusing of the beam at 0.5 THz
– Good agreement between experiment and modeling
• 2D prism (sub-wavelength regime)
– Periodic structure seen as gradient of index
Conclusion and prospective
• Design and realization of a metallic diffractive structure
• Spatial and spectral filter
– Deflection and focusing of the beam at 0.5 THz
– Good agreement between experiment and modeling
�To improve the diffractive efficiency by using dielectric material
• 2D prism (sub-wavelength regime)
– Periodic structure seen as gradient of index
�To improve the efficiency by optimizing the grid geometry
Thank you for your attention