N. Guignot PSICHE Beamline, Synchrotron SOLEIL

The LH-DAC in synchrotrons: general principles and an overview of some important techniques

N. Guignot

PSICHE beamline, Synchrotron SOLEIL Why the LH-DAC?

Give access to the highest P-T conditions (static) Diamonds are transparent in the visible and IR: used for spectroscopy (ex: Raman, IR), but also very useful to focus an IR laser on the sample through the diamonds and to measure the thermal radiation during heating

Emitted light

Laser X-ray beam Pressure transmitting Ruby medium

Gasket

Sample

XRD source : CalTech Why the LH-DAC?

Emitted light

Laser X-ray beam Pressure transmitting Ruby medium IR laser

Gasket

Sample

XRD source : CalTech Why the LH-DAC?

Emitted light

Laser X-ray beam Pressure transmitting Ruby medium

Spectral range used for the Gasket thermal emission analysis

Sample

XRD source : CalTech Why the LH-DAC?

Laser X-ray beam Pressure transmitting Ruby medium

Diamond is also a low Z material (low density). Gasket Makes it ideal for in-situ synchrotron studies coupled with LH (XRD, EXAFS)

Sample

XRD A- Laser focusing wavelength lens focal length λ YAG = 1,064 µm λ CO2 = 10,6 µm

D θ

ΝΑ = θ) F n.sin( laser beam waist

laser beam diameter

The laser beam waist increases Ex : a 15 µm YAG laser with the wavelength and by hotspot can be produced decreasing the NA with a 5 mm laser beam and intensity a focal length of 55 mm

The hot spot diameter can be adjusted either by (de)focussing or with a beam expander A- Laser focusing wavelength lens focal length λ YAG = 1,064 µm λ CO2 = 10,6 µm

D θ

ΝΑ = θ) F n.sin( laser beam waist

laser beam diameter

Ex : a 15 µm YAG laser hotspot can be produced with a 5 mm laser beam and a focal length of 55 mm (depth of field: 328 µm )

The hot spot diameter can be adjusted either by (de)focussing or with a beam expander A- Laserfocusing intensity 15 µm A- Laserfocusing intensity 15 µm (~10 %variation)isverysmall power distributionishomogeneous Problem :theregionwhere A- Laserfocusing intensity 15 µm 3.5 µm most beamlines X-ray beamdimensionsavailableon 3.5 µm. This isusuallyinlinewiththe In ourexampleofa15µmhotspot: (~10 %variation)isverysmall power distributionishomogeneous Problem :theregionwhere sensitivity tomisalignment the samplesurface+increased but strongGaussian T gradients on A- Laserfocusing intensity 15 µm 3.5 µm tion isbeamshaping But amoreelegantandefficient solu the NA A solutionistodefocusordecrease “Flat-top”, “Top-hat” profiles (Prakapenka etal.2008) - A- Laserfocusing intensity 12 µm 15 µm 3.5 µm tion isbeamshaping. But amoreelegantandefficient solu the NA. A solutionistodefocusordecrease “Flat-top”, “Top-hat” profiles - A- Laser focusing

source: A. Laskin, AdlOptica Beam shaping is done by distorsion of the laser beam wavefront inside the optical system (refractive beam shapers) A- Laser focusing

beam shape (focus)

source: A. Laskin, AdlOptica Measurement

Model

“Donut” “Flat-top” “Peak” A- Laser focusing Another type of beam shaper: Diffractive optical element (DOE)

Uses thin micro-structure patterns to alter the phase of the light that is propagated through it Input: Input: Gaussian profile Focusing system Gaussian profile Focusing system

source: Eksmaoptics F F BS Without beam shaper: Gaussian profile at focal plane With beam shaper: Top-Hat profile at focal plane

Available shapes: square and round B- Laser heating Emitted light

Laser Laser beam absorption depends on: Pressure transmitting X-ray beam medium

Gasket

Sample

XRD B- Laser heating λ Emitted light Laser Laser beam absorption depends on: Pressure transmitting X-ray beam medium Laser: wavelength

Gasket

Sample

XRD B- Laser heating λ Emitted light Laser Laser beam absorption depends on: Pressure transmitting X-ray beam medium α Laser: wavelength, angle of inci- dence

Gasket

Sample

XRD B- Laser heating λ Emitted light Laser Laser beam absorption depends on: Pressure transmitting X-ray beam medium α Laser: wavelength, angle of inci- dence

Gasket

“On axis geometry”: laser and visualisation Sample share the same optics path, α = 0 “Off axis”: different paths, α > 0

XRD B- Laser heating λ Emitted light Laser Laser beam absorption depends on: Pressure transmitting X-ray beam medium α Laser: wavelength, angle of inci- dence, plane of polarisation

Gasket

Sample

XRD B- Laser heating λ Emitted light Laser Laser beam absorption depends on: Pressure transmitting X-ray beam medium α Laser: wavelength, angle of inci- dence, plane of polarisation

Gasket Sample: composition, surface roughness, impurities, temperature, isolation from the diamonds (pressure medium), thickness Sample Diamond anvil = heat sink XRD Transparent medium Absorbing medium B- Laser heating Reflected Laser beam absorption inside the Incident intensity sample follows the Beer-Lambert law: intensity RI −αz I I e-αz I(z) = I0e 0

I0/e The intensity of the light decays expo- I0 = (1-R)I nentially with depth at a rate determined from the Handbook of the z = 0 z = 1/α by the material’s absorption coefficient α. Eurolaser Academy (1998) Transparent medium Absorbing medium B- Laser heating Reflected Laser beam absorption inside the Incident intensity sample follows the Beer-Lambert law: intensity RI −αz I I e-αz I(z) = I0e 0

I0/e The intensity of the light decays expo- I0 = (1-R)I nentially with depth at a rate determined from the Handbook of the z = 0 z = 1/α by the material’s absorption coefficient α. Eurolaser Academy (1998) 105 Brown and Arnold, 2010 Aluminum Chromium The optical penetration or absorption 104 Copper α Germanium depth is defined as z = 1/ 103 Gold Iron Silicon (Single Crystal) That means that for metals most of the 2 10 energy of the incoming beam is ab-

101 sorbed in the first 20 nm @1064 nm Optical Absorption Depth (nm) 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Wavelength (nm) Optical absorption depths for several materials over a range of wavelengths B- Laser heating But the typical thickness of a sample is in the 3-10 µm range. What about the axial gradient, that will affect X-ray diffraction/absorption measurements? The LH-DAC temperature distributions are dominated by conductive heat transfer in the sample, the pressure medium, the diamonds and the gasket. Rainey et al. (2013)

2 sides 1 side 2 sides +asym

Dynamic 3D models show that the axial gradient is actually low, as long as laser absorption by the sample is strong enough, sample thickness low enough and insulation from the diamonds and gasket good enough. B- Laser heating But the typical thickness of a sample is in the 3-10 µm range. What about the axial gradient, that will affect X-ray diffraction/absorption measurements? The LH-DAC temperature distributions are dominated by conductive heat transfer in the sample, the pressure medium, the diamonds and the gasket. Rainey et al. (2013)

2 sides 1 side 2 sides +asym

+ thermal equilibrium is achieved very fast (a few µs at most) B- Laser heating Heating during short period of times can be desirable to reduce chemical reaction, migration of liquid, etc...

If steady state and low axial T gradient is re- CW quired, pulse durations in the range of a few µs are needed. At the nano scale the amount of material that is being heated is < 1 µm

In conjunction with continuous (CW) laser heating, nano-pulse laser heating is very useful to measure the sample thermal con- ductivity (e.g. Konopkova et al. 2016) nano-pulse

Goncharov et al. (2009) B- Laser heating

The absorption coefficient depends on T (polynomial law) and a strong jump can be observed during melting

Here are represented models of the evo- lution of the absorption coefficients for different metals @10.6 µm and ambient P (from Prokhorov et al., 1990)

This can be the source of heating unstabilities, especially during melting B- Laser heating The sample surface roughness has also a strong effect on laser absorption I a b Bergstrom (2008)

R 1 I R R 2 T

Brown and Arnold (2010)

(a) Light specularly reflecting from a flat surface (b) Multiple reflections from protruding structures enhance coupling into the material

Feature size Influence on reflectivity Light trapping due to multiplereflections enhances coupling into the material. Light refracted at oblique angles increases the eective optical path length Small features can successively scatter light, increasing the eective optical path length and enhancing absorption Subwavelength structures (SWS) can reduce reflections through the moth-eye eect Xie and Kar (1999) B- Laser heating

In practice the heating heterogeneities that can be observed are due to a combination of subtle variations of all these parameters: surface quality, sample thickness, pressure medium thickness, melting, etc... B- Laser heating Soret effect (also called thermal diffusion) is chemical diffusion driven by a temperature gradient (Mg,Fe)(Al,Si)O perovskite, 1900 K, 20 min 3 Walker and DeLong, 1982

Sinmyo and Hirose, 2010

Migration of elements along the T gradient: from hot to cold regions, ex : Fe / or from cold to hot regions, ex: Si. Often very difficult to predict, making multi-phases/components samples studies very challenging + source of further laser heating unstabilities Another good reason to minimize T gradients, regardless of X-ray beam size! C- T measurement Temperature is measured via spectroradiometry, using the “gray body” approximation Black body: body in thermodynamic equilibrium with its environment, emitting light that only depends on its T Spectral radiance is given by the Planck law:

source: hyperphysics.phy-astr.gsu.edu C- T measurement Temperature is measured via spectroradiometry, using the “gray body” approximation Black body: body in thermodynamic equilibrium with its environment, emitting light that only depends on its T Spectral radiance is given by the Planck law: ε emissivity, 0< ε <1 In practice materials are not ideal black bodies: introduction of emissivity source: hyperphysics.phy-astr.gsu.edu Emissivity is considered constant (gray body approximation). The thermal emission spectrum can thus be fitted with only 2 parameters : T and ε C- T measurement laser filter BS Spectrometer

entrance pinhole Emitted light grating Laser X-ray beam objective

camera

vizualisation

Sample General overview of a basic temperature measurement setup (one side) XRD C- T measurement g b 3000 r y Chromatic aberrations can be problematic in the case 2900 (K ) 2800 of a heterogeneous heating spot (typically: gaussian) ature 2700

2600 Tempe r 2500

14 14 12 12 10 10 8 8 distance ( µm) 6 6 distance ( µm) 4 4 2 2

ideal blackbody curve (3000 K) 2.8 2970 K r

y 2.6 y 2940 K en si t 2910 K

In t 2.4

chromatically dispersed g blackbody curve 2.2

2 The focal length of a lens or refractive objective is 650 700 750 800 850 wavelength (nm) wavelength dependant (different wavelength focused Walter and Koga (2004) at different positions along the optical axis) C- T measurement g b 3000 r y Chromatic aberrations can be problematic in the case 2900 (K ) 2800 of a heterogeneous heating spot (typically: gaussian) ature 2700

2600 Tempe r 2500

14 14 12 12 10 10 8 8 distance ( µm) 6 6 distance ( µm) 4 4 2 2

ideal blackbody curve (3000 K) 2.8 2970 K r

y 2.6 y 2940 K en si t 2910 K

In t 2.4

chromatically dispersed g blackbody curve 2.2

2 That means that the wavelength focused at a given 650 700 750 800 850 wavelength (nm) position (e.g. entrance of the spectrometer) originate Walter and Koga (2004) from different areas of the hot spot C- T measurement g b 3000 r y Chromatic aberrations can be problematic in the case 2900 (K ) 2800 of a heterogeneous heating spot (typically: gaussian) ature 2700

2600 Tempe r 2500

14 14 12 12 10 10 8 8 distance ( µm) 6 6 distance ( µm) 4 4 2 2

ideal blackbody curve (3000 K) 2.8 2970 K r

y 2.6 y 2940 K en si t 2910 K

In t 2.4

chromatically dispersed g blackbody curve 2.2

2 As a consequence the thermal radiation measure- 650 700 750 800 850 wavelength (nm) ment is distorted Walter and Koga (2004) C- T measurement g b 3000 r y Chromatic aberrations can be problematic in the case 2900 (K ) 2800 of a heterogeneous heating spot (typically: gaussian) ature 2700

2600 Tempe r 2500

14 14 12 12 10 10 8 8 distance ( µm) 6 6 distance ( µm) 4 4 2 2

ideal blackbody curve (3000 K) 2.8 2970 K r

y 2.6 y 2940 K en si t 2910 K

In t 2.4

chromatically dispersed g blackbody curve 2.2

2 Reducing the NA helps by increasing the DOF, but at 650 700 750 800 850 wavelength (nm) the expense of image resolution Walter and Koga (2004) two-colour plots 1 2400 0.9 Reﬂective optics, T=1725 2 K 2200 C- T measurement 0.8 0.7 2000 0.6 1800 0.5 0.4 1600

Comparisons between measurements Intensity [a.u.] 0.3 Temperature [K] 1400 0.2 done with reflective objectives, free of 0.1 1200

550 600 650 700 750 800 850 900 550 600 650 700 750 800 850 chromatic aberrations (Schwarzchild Wavelength [nm] Wavelength [nm] 1 2400 objectives) and refractive objectives 0.9 Reduced aperture lenses, T=1786 3 K 0.8 2200 0.7 show that a good agreement is ob- 2000 0.6 0.5 1800 served at relatively low T (up to 2600 0.4 1600 Intensity [a.u.]

0.3 Temperature [K] K) by lowering the NA. 0.2 1400 0.1 1200

550 600 650 700 750 800 850 900 550 600 650 700 750 800 850 Wavelength [nm] Wavelength [nm] At higher T the difference becomes 1 2400 0.9 Lenses, T=1710 7 K 2200 significative. 0.8 0.7 2000 0.6 1800 0.5 0.4 1600 Intensity [a.u.]

0.3 Temperature [K] 1400 0.2 1200 Minimizing chromatic aberrations is 0.1

550 600 650 700 750 800 850 900 550 600 650 700 750 800 850 desirable Wavelength [nm] Wavelength [nm]

Giampaoli et al. 2018 C- T measurement The 4-color pyrometry method

Measurement of temperature distributions across laser heated samples by multispectral imaging radiometry Andrew J. Campbell Department of Geology, University of Maryland, College Park, Maryland 20742, USA

Two-dimensional temperature mapping of laser heated diamond anvil cell samples is performed by processing a set of four simultaneous images of the sample, each ob- tained at a narrow spectral range in the visible to near infrared.

The images are correlated spatially, and each set of four points is fitted to the Planck radiation function to determine the temperature and the emissivity of the sample, using the gray body approximation. C- T measurement The 4-color pyrometry method

60 500 BS2 F1 580 640 C3 M2 50 400 F2 40 300 BS2 BS2

el 30

Pi x F3 200 20 BS2 BS2

PD2 Intensity (x 1000) 100 F4 10 766 905 M2 0 0 BS2 F7 400 600 800 1000 0 200 400 600 4-color box Pixel Wavelength (nm)

M2 L3 L3 BS3 Laser BS3 Spectrometer

PXB

PCB F6 C2 F5 BS1 C1 M1 DAC L1 M2 L2 F7 PH (removable) White Light PD1 Source Du et al.2013 C- T measurement The 4-color pyrometry method

After the cross correlation of the 4 sub-images, 4 intensities at different wavelengths are attrib- uted to each pixel

Lord and Wang 2018 (MIRRORS) C- T measurement The 4-color pyrometry method Du et al. 2013 Surface T mapping

Good correspondance of flash heating T maps and melting structures observed by microscopy C- T measurement The 4-color pyrometry method Du et al. 2013 Surface T mapping

Also a good agreement between measured melting T for different metals and literature values C- T measurement The 4-color pyrometry method

(Lord and Wang 2018) In general the agreement between the T measured by spectroradiometry (single point) and 4-color spectrometry looks very good

ex: Heating ramp on a Fe–S–Si ternary alloy at ~50 GPa. Blue dots : peak T measured by the 4-color pyrometry system, red dots: spectroradiometry C- T measurement The 4-color pyrometry method Advantages of the method: (Lord and Wang 2018) - truly achromatic (each wavelength focused independantly) - the peak temperature can always be determined - insensitive to misalignment (no entrance pinhole) - observation of the dynamic changes in sample temperature in 2D, especially during melting

Disadvantage:

- larger uncertainties C- T measurement The PSICHE Laser heating setup C- T measurement

The Schwarzchild objective is an in-house development

WD: 180 mm, NA 0.24, diffraction limited (1.5 µm resolution)

6th element of the group #7 resolved (line width 2.19 µm), USAF 1951 C- T measurement The Schwarzchild objective is an in-house development

test image of 2 facing 50 µm bevel Diamond fluorescence using the diamonds + dust X-ray beam