Pump-probe Dark-eld X-ray Microscopy Ishwor Poudyal12 Zhi Qiao2 Arndt Last3 Michael R. Armstrong4 and Zahir Islam2 1Materials Science Division Argonne National Laboratory Lemont IL 60439 USA

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Pump-probe Dark-ﬁeld X-ray Microscopy

Ishwor Poudyal1,2,*, Zhi Qiao2, Arndt Last3, Michael R. Armstrong4, and Zahir Islam2,*

1Materials Science Division, Argonne National Laboratory Lemont, IL 60439 (USA)

2X-ray Science Division, Argonne National Laboratory, Lemont, IL 60439 (USA)

3Karlsruhe Institute of Technology, Institute of Microstructure Technology,

Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen D-76344 (Germany)

4Physical and Life Sciences, Lawrence Livermore National Laboratory, Livermore, CA 94550,

(USA)

*Corresponding author: ipoudyal@anl.gov, zahir@anl.gov

Abstract

A pump-probe dark-ﬁeld X-ray microscopy (DFXM) experiment was carried out at the Advanced

Photon Source (APS) with nanosecond drive laser pulses and hybrid mode X-ray probe pulses. We

observe a thermal decay due to laser-induced heat diﬀusion in a Germanium single crystal which matches

the theoretical prediction. The single pulse DFXM imaging in combination with the laser-pump X-ray

probe method could reveal thermal strain formation and propagation of the acoustic wave “on-the-ﬂy”

generated by a laser-induced lattice deformation. This will open a new avenue of materials research on

laser-induced dynamic phenomena in solids at sub-nanosecond time scales.

1 Introduction

Materials undergo several non-equilibrium minima across the energy landscape as a function of time against

external perturbations like temperature, pressure, and stress [1]. These non-equilibrium minima do not

exist under steady-state conditions. Understanding these non-equilibrium processes helps to determine the

structure, dynamics, and thermodynamics of a given system and also provides the conceptual and com-

putational framework for condensed matter science [1]. However, it remains challenging to systematically

drive materials into these states for detailed study. The studies from the ground-state targets to excited

states are enabled by the laser-induced pump-probe experiments. Other perturbations such as electric

ﬁeld [2] and mechanical loading [3] have also been used to induce disorder in the materials. Observing and

quantifying the response of materials like phase transitions and lattice distortion to external stimuli would

shed new light on their behavior.

Pump-probe measurements have been used to explore the dynamic properties of the interaction of

materials, such as interfaces, defects, and surfaces on material properties [4]. This technique has been used

extensively for data collection in both solid-state materials [4] and biological materials [5, 6]. A pump

pulse (laser pulse) initiates dynamics and after a controllable time delay, a probe pulse (X-rays) is used

to characterize the resultant change. The temporal resolution is deﬁned by the duration of the pump and

probe pulses. For a nondestructive experiment and weak diﬀraction signal from a single probe-pulse, pump-

probe cycles can be repeated many times for a given ﬁxed time delay to increase the signal-to-noise ratio.

Yet, this repetitive measurement only allows the characterization of reversible processes. For destructive

single-shot pump-probe experiments of non-reversible phenomena (e.g. diﬀractive imaging of proteins at

X-ray Free-Electron Lasers), the sample must be replenished for each probe pulse [7, 8].

The study of dynamic strain events with the pump-probe experimental techniques may help to un-

derstand fundamental dissipation and transformation mechanisms in materials undergoing inelastic defor-

mation. The inelastic transformation processes, at low strain rates, have been studied using transmission

electron microscopy (TEM) but are limited to sample thickness to 100 nm scales. Nano-scale material

phenomena are diﬃcult to study with TEM owing to short time scale phenomena at the length scale of

arXiv:2210.06243v3 [cond-mat.mtrl-sci] 14 Nov 2022

material defects [9]. Even the highest time resolution dynamic TEM cannot resolve acoustic wave mod-

ulated nm-scale material transformations due to the blurring from the strain wave motion [10]. Under-

standing fundamental phenomena in materials undergoing transformations (such as dislocation migration

in plasticity) will require nm-scale spatial resolution. Dark-ﬁeld X-ray microscopy (DFXM) [11, 12, 13], a

material characterization tool that emerged in the past decade, can potentially help to obtain the neces-

sary spatial resolution [14] for understanding heterogeneous phenomena in materials. DFXM is a full-ﬁeld

X-ray imaging technique and has been established as a tool for mapping orientation, and strain in deeply

embedded structures [11]. This technique has also been employed for studying in situ dynamic processes

such as dislocation movement as a function of temperature [15] and structural transformations during

phase transitions in ferroelectric materias [16]. However, the pump-probe laser scheme incorporating the

DFXM technique at synchrotron sources has not yet been reported. Here, we demonstrate the feasibility

of combining DFXM with pump-probe experiments at Sector 6 at APS (Advanced Photon Source).

The experiments reported here examine the laser-induced strains near the surface of a Germanium single

crystal in low X-ray energy and non-destructive mode. Germanium is a popular material for high-speed

metal-oxide-semiconductor transistors and silicon-based optoelectronics [17]. It has good optical properties

and provides carrier mobility much higher than that of silicon [17, 18]. The study of time-resolved full-

ﬁeld X-ray imaging, which can resolve features within the beam size, of strain propagation in Germanium

semiconductor could be important for the high-speed device application purposes. A similar experimental

approach could be used for direct imaging of propagation of short pulse strain waves, due to laser heating,

through the bulk single crystal. These types of studies will inform models of laser-induced electron-phonon

interactions which will ultimately provide comprehensive information and underlying mechanisms about

laser-induced melting processes in semiconductors.

2 Experimental Setup

A schematic of the DFXM geometry for this experiment is shown in Fig.(1). Our experiments were carried

out at Beamline 6-IDC at the Advanced Photon Source [19]. An X-ray beam with an energy of 13.0 keV

selected by a Si (111) double crystal monochromator, with a bandwidth of ∆E/E = 10−3. The beam

was condensed using a Be Compound Refractive Lens (CRL) comprised of eight 2D Be lenslets, with a

radius of curvature R= 50 µm that produces an eﬀective focal length of 1.5m, to generate a focused

beam of size 6 µm ×6µm on the sample. A 5 mm ×5mm ×0.5mm single crystal Germanium sample is

mounted on the high-precision translation and rotation stages such that the (111) Bragg peak is measured

in the reﬂection geometry in the horizontal scattering plane. Because of the reﬂection geometry, there

is a lower axial resolution (more blurring along the beam-direction due to the footprint on the sample

(see Fig.(3))) in our measured images. The transverse spatial resolution is given by the focused-beam size

produced by the condenser lens and is roughly equal to one half of the focused-beam size. The single-

crystalline nature of the sample was veriﬁed through static X-ray diﬀraction measurements carried out at

the same beamline during the same experiment, which shows the capability of switching the setup for both

diﬀraction and imaging experiments. The 10 ns (FWHM) pulsed laser (1064 nm wavelength, Quantel-

ICE450) with maximum repetition rate of 10 Hz was used to induce surface heating of the Ge crystal,

as shown in Fig.(1).The X-ray attenuation length of Germanium is small at 13 keV energy and only the

eﬀect on the surface of the Germanium is visible during measurements as shown in Fig(3). The excited

sample is probed after a ﬁxed time delay with the X-ray pulse. The direction of the diﬀracted beam in the

horizontal plane is characterized by the scattering angle for a nominal reﬂection (111). The objective lens,

polymeric CRL (pCRL)1, is aligned so that the optical axis lies along the diﬀracted beam to produce a

magniﬁed image on the 2D Pi-MAX IV detector. The objective lens has a design focal length of ∼95 mm

at 13 keV with an eﬀective aperture of ∼60 µm. The detector housing is coupled with a scintillator,

which converts X-ray photons to visible light, and the optical setup with Mo= 5×magniﬁcation [19]. The

objective lens projected a magniﬁed image of the diﬀracting sample onto the far-ﬁeld detector, with an

X-ray magniﬁcation of MX−ray = 19.0×. This yields a total magniﬁcation of Mt= 95×.

1Layout 1921 00 A0 #18, lot 2018-04 400-6.

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Pump-probeDark-eldX-rayMicroscopyIshworPoudyal1,2,*,ZhiQiao2,ArndtLast3,MichaelR.Armstrong4,andZahirIslam2,*1MaterialsScienceDivision,ArgonneNationalLaboratoryLemont,IL60439(USA)2X-rayScienceDivision,ArgonneNationalLaboratory,Lemont,IL60439(USA)3KarlsruheInstituteofTechnology,InstituteofMicrostruct...

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Pump-probe Dark-eld X-ray Microscopy Ishwor Poudyal12 Zhi Qiao2 Arndt Last3 Michael R. Armstrong4 and Zahir Islam2 1Materials Science Division Argonne National Laboratory Lemont IL 60439 USA

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