| 1. |
- Alber, Lukas, et al.
(författare)
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NTMpy : An open source package for solving coupled parabolic differential equations in the framework of the three-temperature model
- 2021
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Ingår i: Computer Physics Communications. - : Elsevier BV. - 0010-4655 .- 1879-2944. ; 265
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Tidskriftsartikel (refereegranskat)abstract
- The NTMpy code package allows for simulating the one-dimensional thermal response of multilayer samples after optical excitation, as in a typical pump-probe experiment. Several Python routines are combined and optimized to solve coupled heat diffusion equations in one dimension, on arbitrary piecewise homogeneous material stacks, in the framework of the so-called three-temperature model. The energy source deposited in the material is modelled as a light pulse of arbitrary cross-section and temporal profile. A transfer matrix method enables the calculation of realistic light absorption in presence of scattering interfaces as in multilayer samples. The open source code is fully object-oriented to enable a user-friendly and intuitive interface for adjusting the physically relevant input parameters. Here, we describe the mathematical background of the code, we lay out the workflow, and we validate the functionality of our package by comparing it to commercial software, as well as to experimental transient reflectivity data recorded in a pump-probe experiment with femtosecond light pulses.
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| 2. |
- Basini, Martina, et al.
(författare)
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Terahertz electric-field-driven dynamical multiferroicity in SrTiO3
- 2024
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Ingår i: Nature. - 0028-0836 .- 1476-4687.
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Tidskriftsartikel (refereegranskat)abstract
- The emergence of collective order in matter is among the most fundamental and intriguing phenomena in physics. In recent years, the dynamical control and creation of novel ordered states of matter not accessible in thermodynamic equilibrium is receiving much attention1,2,3,4,5,6. The theoretical concept of dynamical multiferroicity has been introduced to describe the emergence of magnetization due to time-dependent electric polarization in non-ferromagnetic materials7,8. In simple terms, the coherent rotating motion of the ions in a crystal induces a magnetic moment along the axis of rotation. Here we provide experimental evidence of room-temperature magnetization in the archetypal paraelectric perovskite SrTiO3 due to this mechanism. We resonantly drive the infrared-active soft phonon mode with an intense circularly polarized terahertz electric field and detect the time-resolved magneto-optical Kerr effect. A simple model, which includes two coupled nonlinear oscillators whose forces and couplings are derived with ab initio calculations using self-consistent phonon theory at a finite temperature9, reproduces qualitatively our experimental observations. A quantitatively correct magnitude was obtained for the effect by also considering the phonon analogue of the reciprocal of the Einstein–de Haas effect, which is also called the Barnett effect, in which the total angular momentum from the phonon order is transferred to the electronic one. Our findings show a new path for the control of magnetism, for example, for ultrafast magnetic switches, by coherently controlling the lattice vibrations with light.
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| 3. |
- Basini, Martina, 1985-, et al.
(författare)
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Terahertz ionic Kerr effect : Two-phonon contribution to the nonlinear optical response in insulators
- 2024
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 109:2
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Tidskriftsartikel (refereegranskat)abstract
- The THz Kerr effect measures the birefringence induced in an otherwise isotropic material by a strong THz pulse driving the Raman-active excitations of the systems. Here we provide experimental evidence of a sizable Kerr response in insulating SrTiO3 due to infrared-active lattice vibrations. Such a signal, named the ionic Kerr effect, is associated with the simultaneous excitation of multiple phonons. Thanks to a theoretical modeling of the time, polarization, and temperature dependence of the birefrengence, we can disentangle the ionic Kerr effect from the off-resonant electronic excitations, providing an alternative tunable mechanism to modulate the refractive index on ultrashort timescales via infrared active phonons.
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| 4. |
- Ksenzov, Dmitriy, et al.
(författare)
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Nanoscale Transient Magnetization Gratings Created and Probed by Femtosecond Extreme Ultraviolet Pulses
- 2021
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 21:7, s. 2905-2911
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Tidskriftsartikel (refereegranskat)abstract
- We utilize coherent femtosecond extreme ultraviolet (EUV) pulses from a free electron laser (FEL) to generate transient periodic magnetization patterns with periods as short as 44 nm. Combining spatially periodic excitation with resonant probing at the M-edge of cobalt allows us to create and probe transient gratings of electronic and magnetic excitations in a CoGd alloy. In a demagnetized sample, we observe an electronic excitation with a rise time close to the FEL pulse duration and similar to 0.5 ps decay time indicative of electron-phonon relaxation. When the sample is magnetized to saturation in an external field, we observe a magnetization grating, which appears on a subpicosecond time scale as the sample is demagnetized at the maxima of the EUV intensity and then decays on the time scale of tens of picoseconds via thermal diffusion. The described approach opens multiple avenues for studying dynamics of ultrafast magnetic phenomena on nanometer length scales.
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| 5. |
- Maznev, A. A., et al.
(författare)
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Generation and detection of 50 GHz surface acoustic waves by extreme ultraviolet pulses
- 2021
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 119:4
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Tidskriftsartikel (refereegranskat)abstract
- We use femtosecond extreme ultraviolet pulses derived from a free electron laser to excite and probe surface acoustic waves (SAWs) on the (001) surface of single crystal SrTiO3. SAWs are generated by a pair of 39.9 nm pulses crossed at the sample with the crossing angle defining the SAW wavelength at 84 nm. Detection of SAWs is performed via diffraction of a time-delayed 13.3 nm probe pulse by SAW-induced surface ripples. Despite the low reflectivity of the sample in the extreme ultraviolet range, the reflection mode detection is found to be efficient because of an increase in the diffraction efficiency for shorter wavelengths. We describe a methodology for extracting the SAW attenuation in the presence of a thermal grating, which is based on measuring the decay of oscillations at twice the SAW frequency. The proposed approach can be used to study ultrahigh frequency SAWs in a broad range of materials and will bridge the wave vector gap in surface phonon spectroscopy between Brillouin scattering and He atom scattering.
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| 6. |
- Unikandanunni, Vivek, et al.
(författare)
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Anisotropic ultrafast spin dynamics in epitaxial cobalt
- 2021
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 118:23
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the ultrafast spin dynamics in an epitaxial hcp(11⎯⎯00) cobalt thin film. By performing pump-probe magneto-optical measurements with the magnetization along either the easy or hard magnetic axis, we determine the demagnetization and recovery time for the two axes. We observe an average of 33% slower dynamics along the easy magnetization axis, which we attribute to magneto-crystalline anisotropy of the electron-phonon coupling, supported by our ab initio calculations. This points toward an unambiguous and previously undisclosed role of anisotropic electron–lattice coupling in ultrafast magnetism.
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| 7. |
- Unikandanunni, Vivek, et al.
(författare)
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Inertial spin dynamics : the role of magneto-crystalline anisotropy
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We investigate the spin dynamics driven by terahertz magnetic fields in epitaxial thin films of cobalt in its three crystalline phases with distinct magneto-crystalline anisotropy. The terahertz magnetic field generates a torque on the magnetization which causes it to precess for about 1 ps, with a sub-picosecond temporal lag from the driving force. Then, the magnetization undergoes natural damped oscillations in the THz range at a characteristic frequency of the crystalline phase, containing higher-order harmonics of the fundamental. We describe the experimental observations solving the inertial Landau-Lifshitz-Gilbert equation. Using the results from the relativistic theory of magnetic inertia, we find that the angular momentum relaxation time η is the only material parameter needed to describe all the experimental evidence. Our experiments suggest a correlation between η and the strength of the magneto-crystalline anisotropy
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| 8. |
- Unikandanunni, Vivek, 1993-, et al.
(författare)
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Inertial Spin Dynamics in Epitaxial Cobalt Films
- 2022
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 129:23
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the spin dynamics driven by terahertz magnetic fields in epitaxial thin films of cobalt in its three crystalline phases. The terahertz magnetic field generates a torque on the magnetization which causes it to precess for about 1 ps, with a subpicosecond temporal lag from the driving force. Then, the magnetization undergoes natural damped THz oscillations at a frequency characteristic of the crystalline phase. We describe the experimental observations solving the inertial Landau-Lifshitz-Gilbert equation. Using the results from the relativistic theory of magnetic inertia, we find that the angular momentum relaxation time η is the only material parameter needed to describe all the experimental evidence. Our experiments suggest a proportionality between η and the strength of the magnetocrystalline anisotropy.
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| 9. |
- Unikandanunni, Vivek, et al.
(författare)
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Ultrafast electron dynamics in platinum and gold thin films driven by optical and terahertz fields
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We investigate the ultrafast electron dynamics triggered by terahertz and optical pulses in thin platinum and gold films by probing their transient optical reflectivity. The response of the platinum film to an intense terahertz pulse is similar to the optically-induced dynamics of both films and can be described by a two-temperature model. Surprisingly, gold exhibits a much smaller terahertz pulse-induced reflectivity change and with opposite sign. For platinum, we estimate a 20% larger electron-phonon coupling for the terahertz-driven dynamics compared to the optically-induced one, which we ascribe to an additional nonthermal electron-phonon coupling contribution. We explain the remarkable response of gold to the terahertz field as the field emission of electrons due to Fowler-Nordheim tunneling. Our results provide fundamental insight into the ultrafast processes relevant to modern electro- and magneto-optical applications.
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| 10. |
- Unikandanunni, Vivek, 1993-, et al.
(författare)
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Ultrafast electron dynamics in platinum and gold thin films driven by optical and terahertz fields
- 2022
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 120:2
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the ultrafast electron dynamics triggered by terahertz and optical pulses in thin platinum and gold films by probing their transient optical reflectivity. The response of the platinum film to an intense terahertz pulse is similar to the optically induced one and can be described by a two-temperature model with a 20% larger electron–phonon coupling for the terahertz-driven dynamics compared to the optically induced one, ascribed to an additional nonthermal electron–phonon coupling contribution. Surprisingly, gold films exhibit a much smaller terahertz pulse-induced reflectivity change and with a sign opposite to the optical case. We explain this remarkable observation with field emission of electrons due to Fowler–Nordheim tunneling, enabled in samples with thicknesses below the structural percolation threshold, where nanostructuring promotes near-field enhancement. Our results provide a fundamental insight into the ultrafast processes relevant to modern electro- and magneto-optical applications.
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