Periodic Reporting for period 2 - DANCE (Dynamical Band Structure Engineering)
Okres sprawozdawczy: 2022-05-01 do 2023-10-31
As early as the Middle Ages, alchemists tried to turn lead into gold - admittedly with very dubious success. Over the past century, modern materials science has developed from these rudimentary beginnings thanks to new theoretical concepts and experimental methods. Today, it is possible, for example, to modify the electronic properties of materials by varying their chemical composition or by external parameters such as temperature, or pressure. All these methods are now well established, but they have one major drawback: they are slow.
We are exploring a new alternative method that is capable of changing the electronic properties of a material within a few femtoseconds (one millionth of a billionth of a second). The underlying idea can be explained quite simply using a children's toy: a spherical spinning top with a cylindrical hole surrounding the central rod. When the top is at rest, the rod points upward (see Fig. 1a). If one sets the top in rotation, it will turn upside down (see Fig. 1b). The properties of the rotating top are different from those of the top at rest.
In our research we replace the top with a solid and the child that sets the top in rotation with the extremely strong light fields of a femtosecond laser amplifier, which coherently accelerate electrons and atoms inside the solid. To this end, we are developing tailored laser pulses that selectively excite different degrees of freedom of the materials we study. Changes of the electronic properties then occur on the timescale of a single oscillation cycle of the driving laser field. There are no limits to the imagination: using periodic driving one can turn an insulator into a metal, a metal into a superconductor, a trivial semiconductor into a semiconductor with topologically protected edge states, and much more. There are many exciting theoretical predictions on this topic as well as first experimental successes.
To find out if periodic driving of the solid with strong laser pulses has the desired effect, one needs a method to study the electronic properties of driven solids. We are using time- and angle-resolved photoelectron spectroscopy (trARPES) which provides direct access to the transient band structure E(k) of a material.
If successful, our research will establish driven solids as a completely new class of materials with tunable functionalities. The ability to control band structure and electron spin with light will certainly find applications in novel ultrafast optoelectronic and optospintronic devices.
We are exploring a new alternative method that is capable of changing the electronic properties of a material within a few femtoseconds (one millionth of a billionth of a second). The underlying idea can be explained quite simply using a children's toy: a spherical spinning top with a cylindrical hole surrounding the central rod. When the top is at rest, the rod points upward (see Fig. 1a). If one sets the top in rotation, it will turn upside down (see Fig. 1b). The properties of the rotating top are different from those of the top at rest.
In our research we replace the top with a solid and the child that sets the top in rotation with the extremely strong light fields of a femtosecond laser amplifier, which coherently accelerate electrons and atoms inside the solid. To this end, we are developing tailored laser pulses that selectively excite different degrees of freedom of the materials we study. Changes of the electronic properties then occur on the timescale of a single oscillation cycle of the driving laser field. There are no limits to the imagination: using periodic driving one can turn an insulator into a metal, a metal into a superconductor, a trivial semiconductor into a semiconductor with topologically protected edge states, and much more. There are many exciting theoretical predictions on this topic as well as first experimental successes.
To find out if periodic driving of the solid with strong laser pulses has the desired effect, one needs a method to study the electronic properties of driven solids. We are using time- and angle-resolved photoelectron spectroscopy (trARPES) which provides direct access to the transient band structure E(k) of a material.
If successful, our research will establish driven solids as a completely new class of materials with tunable functionalities. The ability to control band structure and electron spin with light will certainly find applications in novel ultrafast optoelectronic and optospintronic devices.
The ERC project DANCE officially started in November 2020. The first seven months of the project were used to optimize the growth of KxC60 films on different substrates for the envisioned experiments on putative light-induced superconductivity in this compound.
Upon completion of our new laser laboratory in June 2021, we started to reassemble our 1kHz trARPES setup. First trARPES experiments using 1.55eV pump and 21.7eV probe pulses were successfully performed in November 2021.
The trARPES experiments for DANCE require wavelength tunable mid-infrared (MIR) and terahertz (THz) pump pulses. The generation of these pulses requires two different homebuilt setups seeded by a commercial optical parametric amplifier (OPA): (1) difference frequency generation (DFG) in GaSe and (2) chirped pulse difference frequency generation (CPDFG) in DAST or DSTMS, respectively. The implementation of these sources includes several steps: (1) successful implementation of the OPA in the trARPES setup, (2) generation, characterization, and focusing of the MIR and THz pulses, respectively, and (3) proof-of-principle trARPES experiments with MIR and THz pump pulses. The first step was achieved in February 2022, where we obtained the first trARPES data using the second harmonic of the OPA as a pump pulse. Steps (2) and (3) were achieved shortly afterwards for the GaSe-based MIR source with a successful proof-of-principle experiment in May 2022.
The CPDFG THz source requires a grating stretcher to chirp the signal pulses that come out of the OPA before DFG. This stretcher was assembled, characterized, and optimized by December 2022. In May 2023 we obtained first THz emission from the CPDFG source. We are currently designing a box that will allow us to purge the CPDFG source with nitrogen to avoid absorption of the THz pulses in air and the focusing optics that will be mounted close to the sample inside the trARPES chamber.
Since February 2022, we performed a series of trARPES experiments with visible pump pulses on different two-dimensional materials (monolayer WS2 as well as Sn-intercalated graphene). In both cases we observed a strong light-induced momentum-dependent band structure renormalization that we now try to explain with support from theory.
To improve the signal-to noise ratio of the trARPES data we started with the implementation of a second laser amplifier with a repetition rate of 100kHz in December 2022.
Upon completion of our new laser laboratory in June 2021, we started to reassemble our 1kHz trARPES setup. First trARPES experiments using 1.55eV pump and 21.7eV probe pulses were successfully performed in November 2021.
The trARPES experiments for DANCE require wavelength tunable mid-infrared (MIR) and terahertz (THz) pump pulses. The generation of these pulses requires two different homebuilt setups seeded by a commercial optical parametric amplifier (OPA): (1) difference frequency generation (DFG) in GaSe and (2) chirped pulse difference frequency generation (CPDFG) in DAST or DSTMS, respectively. The implementation of these sources includes several steps: (1) successful implementation of the OPA in the trARPES setup, (2) generation, characterization, and focusing of the MIR and THz pulses, respectively, and (3) proof-of-principle trARPES experiments with MIR and THz pump pulses. The first step was achieved in February 2022, where we obtained the first trARPES data using the second harmonic of the OPA as a pump pulse. Steps (2) and (3) were achieved shortly afterwards for the GaSe-based MIR source with a successful proof-of-principle experiment in May 2022.
The CPDFG THz source requires a grating stretcher to chirp the signal pulses that come out of the OPA before DFG. This stretcher was assembled, characterized, and optimized by December 2022. In May 2023 we obtained first THz emission from the CPDFG source. We are currently designing a box that will allow us to purge the CPDFG source with nitrogen to avoid absorption of the THz pulses in air and the focusing optics that will be mounted close to the sample inside the trARPES chamber.
Since February 2022, we performed a series of trARPES experiments with visible pump pulses on different two-dimensional materials (monolayer WS2 as well as Sn-intercalated graphene). In both cases we observed a strong light-induced momentum-dependent band structure renormalization that we now try to explain with support from theory.
To improve the signal-to noise ratio of the trARPES data we started with the implementation of a second laser amplifier with a repetition rate of 100kHz in December 2022.
To date, we mainly worked on the (re-)assembly of the 1kHz trARPES setup with variable wavelength strong-field MIR and THz pump and XUV probe pulses. When finished, this will be a unique setup that – to the best of our knowledge – exists only once worldwide. With its present performance the setup is now ready to start experiments on light-induced topological phase transitions, dynamical localization, and light-induced superconductivity. The envisioned experiments aiming at the control of dispersion, spin, and various order parameters require the successful implementation of the CPDFG THz source that we hope to demonstrate by the end of 2023.
DANCE also relies on a second setup operating at high repetition rate to improve the signal-to-noise ratio of the trARPES data and thereby reduce the risk that the desired light-induced band structure changes will be too small to be resolved. In January 2023 we submitted a research grant for major equipment to the German Research Foundation that, when approved, will allow us to purchase a second trμARPES experiment with additional financial support from DANCE. This experiment will provide MIR pump and ultraviolet probe pulses at high repetition rate. ARPES spectra will be taken with a novel state-of-the-art time-of-flight photoelectron emission microscope (TOF-PEEM) that allows for band structure measurements from a pre-selected micron-sized area on the sample surface. We expect the funding to be approved in summer 2023. Delivery and installation of the components are envisioned for summer 2024, such that first trµARPES experiments at high repetition rate might be performed in the second half of 2024. To the best of our knowledge, also this setup with its ability of MIR pumping at high repetition rate in combination with 6eV µARPES, will be unique.
These two setups will enable various experiments revealing the transient electronic properties of driven solids. If successful, these experiments will establish dynamical band structure engineering as a new method for electronic structure control and pave the way for novel optoelectronic and optospintronic devices.
Preliminary experiments using the 1kHz trARPES setup revealed a strong k-dependent band structure renormalization in different two-dimensional materials (monolayer WS2 and Sn-intercalated graphene) induced by excitation with visible light. These results now pave the way for future experiments with more complex driving schemes.
DANCE also relies on a second setup operating at high repetition rate to improve the signal-to-noise ratio of the trARPES data and thereby reduce the risk that the desired light-induced band structure changes will be too small to be resolved. In January 2023 we submitted a research grant for major equipment to the German Research Foundation that, when approved, will allow us to purchase a second trμARPES experiment with additional financial support from DANCE. This experiment will provide MIR pump and ultraviolet probe pulses at high repetition rate. ARPES spectra will be taken with a novel state-of-the-art time-of-flight photoelectron emission microscope (TOF-PEEM) that allows for band structure measurements from a pre-selected micron-sized area on the sample surface. We expect the funding to be approved in summer 2023. Delivery and installation of the components are envisioned for summer 2024, such that first trµARPES experiments at high repetition rate might be performed in the second half of 2024. To the best of our knowledge, also this setup with its ability of MIR pumping at high repetition rate in combination with 6eV µARPES, will be unique.
These two setups will enable various experiments revealing the transient electronic properties of driven solids. If successful, these experiments will establish dynamical band structure engineering as a new method for electronic structure control and pave the way for novel optoelectronic and optospintronic devices.
Preliminary experiments using the 1kHz trARPES setup revealed a strong k-dependent band structure renormalization in different two-dimensional materials (monolayer WS2 and Sn-intercalated graphene) induced by excitation with visible light. These results now pave the way for future experiments with more complex driving schemes.