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