Periodic Reporting for period 1 - SR-XTRS-2DLayMat (Self-Referenced XUV Transient Reflectivity Spectroscopy: Attosecond Electron Dynamics of 2D Layered Materials)
Période du rapport: 2022-07-01 au 2024-06-30
The aim of the project is to advance the method of attosecond extreme ultra-violet (XUV) transient reflectivity spectroscopy (XTRS) beyond its current state of the art with the aim to better understand ultrafast electron dynamics in solid state materials, especially novel layered and 2D materials.
Since solid state materials are truly many-body systems, reaching high densities, electrons can quickly exchange their energy, spin and momentum with other electrons or the crystal lattice. These fast relaxation processes require sophisticated time-resolved spectroscopy methods with resolutions at the attosecond range to be understood in their fundamental behavior.
Attosecond XTRS uses extremely short laser pulses of attosecond duration in the XUV spectral range, where core-level transitions are available. These core-level transitions, in which an electron is excited from a highly localized core-orbital at element-characteristic wavelengths into unoccupied valence states can give us a localized picture into the complex electronic structure of solids, aiming in the understanding and interpretation of experimental results.
The method is closely related to attosecond XUV transient absorption spectroscopy (XTAS): While samples are investigated and spectroscopic data is recorded in a transmission geometry in XTAS, a reflection spectroscopy is used in XTRS. This has several advantages over the transmission geometry including the availability of higher quality samples, larger signal contrasts and surface sensitivity, to name a few.
However, the method comes with a significant drawback, which has so far hindered its widespread usage. This is due to the increased complexity in the data analysis due to the mixing of dispersive and absorptive parts (real and imaginary) of the optical response (dipole response function).
The proposed solution to this problem within this action is to implement a self-referenced, interferometric approach, which will allow to directly reconstruct the spectral amplitude and phase of the optical response in an XTRS experiment and thus recover the seperation of real and imaginary part of the dipole response function.
Additionally, the proposed optical setup will allow to use a fast modification of the XUV spectrum to apply lock-in approaches to XTRS to increase the signal-to-noise ratio achievable in state-of-the-art XUV beamlines.
Both parts will be crucial to further our understanding of ultrafast carrier dynamics in solid materials. We set out to apply these methods to few- and monolayer materials, such as heterojunctions of transition metal dichalcogenides and study their (coherent) carrier transport properties, interactions with the lattice and spin dynamics with attosecond resolution. To set out to this goal we plan to address these phenomena in prototypical samples and apply our knowledge gained there to novel and more unfamiliar materials. The advancement of XTRS will help to further establish this method as a powerful tool to understand carrier dynamics in solid state materials.
Since solid state materials are truly many-body systems, reaching high densities, electrons can quickly exchange their energy, spin and momentum with other electrons or the crystal lattice. These fast relaxation processes require sophisticated time-resolved spectroscopy methods with resolutions at the attosecond range to be understood in their fundamental behavior.
Attosecond XTRS uses extremely short laser pulses of attosecond duration in the XUV spectral range, where core-level transitions are available. These core-level transitions, in which an electron is excited from a highly localized core-orbital at element-characteristic wavelengths into unoccupied valence states can give us a localized picture into the complex electronic structure of solids, aiming in the understanding and interpretation of experimental results.
The method is closely related to attosecond XUV transient absorption spectroscopy (XTAS): While samples are investigated and spectroscopic data is recorded in a transmission geometry in XTAS, a reflection spectroscopy is used in XTRS. This has several advantages over the transmission geometry including the availability of higher quality samples, larger signal contrasts and surface sensitivity, to name a few.
However, the method comes with a significant drawback, which has so far hindered its widespread usage. This is due to the increased complexity in the data analysis due to the mixing of dispersive and absorptive parts (real and imaginary) of the optical response (dipole response function).
The proposed solution to this problem within this action is to implement a self-referenced, interferometric approach, which will allow to directly reconstruct the spectral amplitude and phase of the optical response in an XTRS experiment and thus recover the seperation of real and imaginary part of the dipole response function.
Additionally, the proposed optical setup will allow to use a fast modification of the XUV spectrum to apply lock-in approaches to XTRS to increase the signal-to-noise ratio achievable in state-of-the-art XUV beamlines.
Both parts will be crucial to further our understanding of ultrafast carrier dynamics in solid materials. We set out to apply these methods to few- and monolayer materials, such as heterojunctions of transition metal dichalcogenides and study their (coherent) carrier transport properties, interactions with the lattice and spin dynamics with attosecond resolution. To set out to this goal we plan to address these phenomena in prototypical samples and apply our knowledge gained there to novel and more unfamiliar materials. The advancement of XTRS will help to further establish this method as a powerful tool to understand carrier dynamics in solid state materials.
To achieve the scientific goals, we addressed the underlying fundamental scientific questions in prototypical materials.
For this, we studied the carrier thermalization, that is the timescale that optically excited carriers take to reach the thermodynamical equilibrium within the carriers (to reach a thermal distribution) in a series of different metals. Our study shows that the timescale of this process vary between metals over orders of magintude, reaching from hundreds of femtoseconds to only a few femtoseconds. Using relatively simple theoretical models, we could show that this depends on several key electronic structure properties, such as Fermi energy, plasma frequency, density of states and the carrier localization, which affects the rate of energy exchange between carriers. The latter also affects an observable spectroscopic signature of core-level transitions, as expressed through the local field effect (LFE). The results are currently undergoing peer-review.
We also studied the effect of carrier relaxation on coherent phonon motion that follows the change in energy that follows to photoexcitation. We were able to show that the relaxation of carriers and formation of a thermal carrier distribution affects the phase of the phonon oscillation. This is due to the difference in force that the carriers enact onto to lattice during the first femtoseconds following optical excitation while they occupy higher energetic states than after they form their thermal distribution. The results are currently undergoing peer-review.
To study the spin-polarization in 2D materials, we studied the possibilities to follow electron spin dynamics in semiconductors using circular polarized XUV laser pulses of attosecond duration. For this we first examined the angular-momentum selection rules that arise when two circular polarized pulses, pump and probe are used. Our results examining circular-dichroic attosecond transient absorption spectroscopy (cDATAS) in helium demonstrate that the different optical response between a co- and counter-rotating pump pulse (with respect to the probe pulse) are dependent on the magnetic spin quantum number and therefore spin-selectivity can be achieved.
We then applied the principle to a simple semiconductor, where our results indicate that the spin-polarization of holes, following optical excitation, decays much faster than shown in previous experimental results using optical methods in the visible and infra-red spectral regions. The results are currently being compared to theoretical modelling and prepared for publication.
For this, we studied the carrier thermalization, that is the timescale that optically excited carriers take to reach the thermodynamical equilibrium within the carriers (to reach a thermal distribution) in a series of different metals. Our study shows that the timescale of this process vary between metals over orders of magintude, reaching from hundreds of femtoseconds to only a few femtoseconds. Using relatively simple theoretical models, we could show that this depends on several key electronic structure properties, such as Fermi energy, plasma frequency, density of states and the carrier localization, which affects the rate of energy exchange between carriers. The latter also affects an observable spectroscopic signature of core-level transitions, as expressed through the local field effect (LFE). The results are currently undergoing peer-review.
We also studied the effect of carrier relaxation on coherent phonon motion that follows the change in energy that follows to photoexcitation. We were able to show that the relaxation of carriers and formation of a thermal carrier distribution affects the phase of the phonon oscillation. This is due to the difference in force that the carriers enact onto to lattice during the first femtoseconds following optical excitation while they occupy higher energetic states than after they form their thermal distribution. The results are currently undergoing peer-review.
To study the spin-polarization in 2D materials, we studied the possibilities to follow electron spin dynamics in semiconductors using circular polarized XUV laser pulses of attosecond duration. For this we first examined the angular-momentum selection rules that arise when two circular polarized pulses, pump and probe are used. Our results examining circular-dichroic attosecond transient absorption spectroscopy (cDATAS) in helium demonstrate that the different optical response between a co- and counter-rotating pump pulse (with respect to the probe pulse) are dependent on the magnetic spin quantum number and therefore spin-selectivity can be achieved.
We then applied the principle to a simple semiconductor, where our results indicate that the spin-polarization of holes, following optical excitation, decays much faster than shown in previous experimental results using optical methods in the visible and infra-red spectral regions. The results are currently being compared to theoretical modelling and prepared for publication.
The increase in experimental sensitivity that underlie our results enabled us to efficiently examine the target materials with higher detail in shorter time. Overall these results highlight the great versatility of attosecond transient specrtroscopy for materials research and will aid in wider spread of adaption of the technique.
Our results on coherent phonon motion imply optimization potential to shape the long-lasting phonon oscillation by tailoring the optical excitation for potential optomechanic applications.
Compared to previous studies using circular polarized XUV pulses that targeted magnetic properties, we could show that our method allows a band-resolved picture of purely electronic spin polarization, which we anticipate will be met with large interest by the material science community. The results furthermore could have large implications for technical applications that rely on spin-polarization, such as spintronic devices.
Our results on coherent phonon motion imply optimization potential to shape the long-lasting phonon oscillation by tailoring the optical excitation for potential optomechanic applications.
Compared to previous studies using circular polarized XUV pulses that targeted magnetic properties, we could show that our method allows a band-resolved picture of purely electronic spin polarization, which we anticipate will be met with large interest by the material science community. The results furthermore could have large implications for technical applications that rely on spin-polarization, such as spintronic devices.