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Status of the THz Imaging

Franz Roeder and Amrutha Gopal

PEB meeting 19.11.2019

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A.Gopal

Objectives

1. Design the optimum THz source

2. Find the required laser parameters

3. Choose the right THz wavelength

4. Find the suitable experimental geometry

5. Influence of Plasma cell parameters

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Visualization of wakefields

ω p =nee

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(ε0me )Plasma frequency

Refractive index n(ω ) = 1−ω p2

ω 2Plasma parameters:Rubidium vapor1014 – 1015 cm−3 λP = 1.1 – 3.3 mm

ωP~0.1THz

Expected plasma wavelength is of the order of mm

Wavelength (cm)

THz

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THz source

Optical rectification

Sufficient power

Compact - no ionised radiation, in air

Simple set up in air and require only 10‘s mj laser energy

∇2E (ω ) − µ0ε(ω )∂2E (ω )

∂t 2= µ0

∂2P(ω )NL

∂t 2

Optical rectification

Difference Frequency mixing

Sum Frequencymixing

SHG

PNL- nonlinear polarization

Pi = ε0χij(1)Ej + ε0χijk

(2)EjEk + ε0χijl(3)EjEkEl + .....

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THz crystal

High nonlinearityOptimum phase matching at the available laser wavelengthAgility of the setupLow absorption of both Pump and THz radiationSpectrum of the generated THz radiation

Material ng @800nm nph @1THz deff (pm/V) Setup Abs. Coeff (cm-1)

THz spectrum

ZnTe 3.13 3.17 68.5 Collinear 1.3 up to 4 THz

LiNbO3 2.25 4.96 168 Tilted pulse front

16 0.1-2.5THz

BNA 1.9 2.1-2.25 234 Collinear 100-150 2.5 THz

High nonlinearityLaser Wavelength : Ti-Saph (800 nm)

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THz generation and detection

EO detection

Pyrometer

Ti-Saph laser

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Our Laser system

Typical spectrum Modified spectrumdue to SPM

THz generation

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Fluence dependence

Irradiated area: 0.77cm2

Dazzler setting

THz generation -Optimal laser parameters

T = xctan(θ )

J. Shan et al., Opt. Lett. 25, 426 (2000)

THz detection- Noncollinear EO scheme

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THz Detection

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Window Characterization

Sapphire

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THz Detection

1. Coherent detection

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EO measurements with sample

8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13t in ps

-50

0

50

100

150

200

|E THz| in a.u.

no samplepolycarbonatesapphire

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EO measurements with samples

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EO measurements with samples

Dispersion 500 µm ZnTe EO-crystal allows minimal time resolution of about 0.3 ps

polycarbonate: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.306.3833&rep=rep1&type=pdf

sapphire: https://www.researchgate.net/publication/263835051_Optical_properties_of_silicon_sapphire_silica_and_glass_in_the_Terahertz_range

Temporal shift (ps)

Polycarbonate (n=1.6)

5mm thick

Sapphire (n=3.3)

1.6 mm thick No sample

Measured 10.83 ps 10.83 ps

Calculated 11ps 12.32 ps

THz pulse duration (ps)

1.06 1.25 0,568

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Transmission measurements with samples

Reference: transmission of THz through 7 mm aperture to match size of Sapphire windows (2.02(±0.51) µJ)

2sapphire+2polycarbonate

2sapphire+1polycarbonate

2sapphire 1sapphire

TransmissiononPyrodetector

57% 66% 78% 89%

onlyfilter polycarbonate sapphire poly+sapphire

BPF@1THz 5.6% 79% 78% 64%

BPF@1.5THz 5.5% 84% 86% 68%

BPF@3THz 5.2% 84% 78% 65%

Bandpass filter measurementsEnergy without BP-filter: 12.9(±2.2) µJ

Transmission ratios are calculated in respect to transmission of filter as reference

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Summary of the current work

•The best source for generating THz crystal is tested and crystal size and efficiency estimated.

•BNA crystals are optimum for generating THz with 800 nm Ti-Saph laser.

•0.2% is the conversion efficiency.

•For single-shot plasma imaging (1cm plasma) requires about 150 muJ of THz radiation (1.5 THz central freq)

•We propose coherent and incoherent THz imaging.

•Suppliers for the crystals and camera identified.

•Tested the transmission properties of the window materials of the plasma column.

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THz Shadowgraphy

Current work at FSU/HIJ

Temporal resolution of the THz pulse is enough to observe the plasma wave

λp = 1-3 mm THz pulse < 3 ps

Other effects which need to taken into consideration

1. Diffraction 2. Scattering

Coherent Imaging

Detec.

onschem

eTi-Saph

Visualization of the plasma waveVisualization of the plasma wave +spectrally resolved density information

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THz Shadowgraphy

Price ~ 95, 000 Euros

For single-shot plasma imaging (1cm plasma) requires about 20-25 muJ of THz radiation (1.5 THz central frequency)

320x240 pixels (50 µm) high sensitivity: 20 pw at 2,5 THzrange of coverage : 0.3 THz to 5 THz

BNA CRYSTAL LENSPLASMA

Ti-Saph

THz

i2S THz microbolometer camera

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CoherentImaging

Large aperture EO crystal andBNA crystal

Estimated cost : 20,000 EurosFor single-shot plasma imaging (1cm plasma) requires about 150 muJ of THz radiation (1.5 THz)

Required laser energy : 150 mJ

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BNA CRYSTAL

CCD

LENS

ANALYZER

EOCRYSTALLENSPLASMA

THz

Ti-Saph

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Current work at FSU/HIJ

Next Beam time : January 2020 - 5 weeks

Delivery of large aperture BNA crystal : expected at the end of Dec 2019

Develop the Coherent imaging technique and image the plasma channel in a vacuum chamber equipped with Sapphire and poly carbonate windows - Dec’19

Design the plasma cell : December 2019

Explore funding for the i2s Camera

Development of algorithm for data extraction from the EO image : Dec’19 - Feb’20

Investigate the effects of diffraction and scattering in plasma for THz radiation : Dec’19 - Feb’20

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Diffrac.oneffects

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The spatial resolution of a far-field imaging system is limited by diffraction of the carrier wave

Δ = 1.22λ lD

When depth of field is much smaller than the object distance, it can be described as

L = δDlδ ′D ± D

δD required spatial resolution on the target

δ ′D required spatial resolution on the imaging plane

l distance from the target to the imaging lens

D aperture diameter of the lens

δ ′D = δD( ′l / l) ′l image distance (focal length of the lens for far field imaging)

In our case, to have a spatial resolution of 1 mm lD~ 2.73 to have a spatial resolution of 0.6 mm

lD~ 1.63

Size of the EO crystal DS ≈ DTfl

DT Dimension of the target

Thickness of the EO crystal ′L = δ ′D fD

≈ δ ′D2NA

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Current work at FSU/HIJ100 GW, Ti-Saph laser, 25fs, 3 mJ @ 10Hz