Terahertz Magnonics

The potential of terahertz time domain spectroscopy has until recently been neglected in the field of the ultrafast magnetism. At the same time this technique can serve as a useful complementary tool with respect with conventional methods to investigate ultrafast magnetization dynamics. This thesis aims to implement time domain terahertz spectroscopy to observe high frequency spin waves excited optically in different magnetic systems. This work covers several distinct phenomena related to the study of spin waves (magnonics) at terahertz frequencies. The generation of transient broadband nonlinear magnetization via inverse Faraday effect in terbium gallium garnet is described in chapter 4. We demonstrate a remarkable discrepancy of at least two orders of magnitude between the strengths of the direct and inverse Faraday effects, thereby challenging the commonly accepted understanding of their relationship. Additionally, a striking nonlocality of the optical response is found. In chapter 5 the results of THz absorption spectroscopy of the terbium gallium garnet are reported. The garnet exhibits an intricate paramagnetic state with several magnetic sub-lattices at cryogenic temperatures under the application of strong magnetic fields. Some precessional modes of these sub-lattices were measured. The components of the g-tensor of terbium ions were extracted from the data. In chapter 6 the ultrafast magnetization dynamics of thulium orthoferrite, studied my means of terahertz spectroscopy, is described. It is demonstrated that terahertz response of the orthoferrite provides crucial additional information with respect to the optical pump-probe signal. A novel exchange driven mechanism of optical manipulation of the magnetic state is demonstrated. Finally, chapter 7 is a theoretical discussion of so called planar magnonic metamaterials. It is shown that the arrays of ferromagnetic films may exhibit negative refraction index at sub-terahertz frequencies, provided the mechanism of spin wave quantization is introduced. The thesis ends with a brief conclusions chapter where a short summary of the results is given. Some possible future extensions of the conducted research are drawn as well

Colossal magneto-optical modulation at terahertz frequencies by counterpropagating femtosecond laser pulses in Tb3Ga5O12

Single-frequency terahertz modulation of the magneto-optical Faraday effect with a record amplitude of the polarization rotation of ∼0.5° is achieved using a slab of the etalon Faraday rotator crystal Tb3Ga5O12. The modulation is the result of the interaction of two counterpropagating laser pulses via the optical Kerr effect. The frequency of the modulation is determined by the applied magnetic field and is continuously tunable in a terahertz frequency range between 0 and 0.7 THz

Resonant Pumping of d−d Crystal Field Electronic Transitions as a Mechanism of Ultrafast Optical Control of the Exchange Interactions in Iron Oxides

The microscopic origin of ultrafast modification of the ratio between the symmetric (J) and antisymmetric (D) exchange interaction in antiferromagnetic iron oxides is revealed, using femtosecond laser excitation as a pump and terahertz emission spectroscopy as a probe. By tuning the photon energy of the laser pump pulse we show that the effect of light on the D/J ratio in two archetypical iron oxides FeBO3 and ErFeO3 is maximized when the photon energy is in resonance with a spin and parity forbidden d−d transition between the crystal-field split states of Fe3+ ions. The experimental findings are supported by a multielectron model, which accounts for the resonant absorption of photons by Fe3+ ions. Our results reveal the importance of the parity and spin-change forbidden, and therefore often underestimated, d−d transitions in ultrafast optical control of magnetism

Negative permeability due to exchange spin-wave resonances in thin magnetic films with surface pinning

Copyright © 2010 The American Physical SocietyWe report a theory of the effective permeability of multilayered metamaterials containing thin ferromagnetic layers with magnetization pinned on either one or both surfaces. Because of the pinning and small film thickness, the lowest frequency magnetic resonances are due to nonuniform exchange spin waves with frequencies far above those expected for uniform ferromagnetic resonance in known magnetic materials. Yet, the coupling of the nonuniform spin-wave modes to the electromagnetic field is shown to be strong enough to lead, for magnetic parameters characteristic for conventional transition metal alloys, to negative values of the effective permeability at frequencies of several hundred gigahertzs. The permittivity of metals is already negative in this frequency range. Hence, this system represents a negative refractive index metamaterial at subterahertz frequencies. The ways by which to maximize the frequency and the strength of the negative magnetic response are analyzed

Terahertz emission spectroscopy of laser-induced spin dynamics in TmFeO3 and ErFeO3 orthoferrites

Copyright © 2014 American Physical SocietyUsing the examples of laser-induced spin-reorientation phase transitions in TmFeO3 and ErFeO3 orthoferrites, we demonstrate that terahertz emission spectroscopy can obtain novel information about ultrafast laser-induced spin dynamics, which is not accessible by more common all-optical methods. The power of the method is evidenced by the fact that, in addition to the expected quasi-ferromagnetic and quasi-antiferromagnetic modes of the iron sublattices, terahertz emission spectroscopy enables detection of a resonance optically excited at an unexpected frequency of ∼0.3–0.35 THz. By recording how the amplitude and phase of the excited oscillations depend on temperature and applied magnetic field, we show that the unexpected mode has all the features of a spin resonance of the Fe3+ ions. We suggest that it can be assigned to transitions between the multiplet sublevels of the 6A1 ground state of the Fe+3 ions occupying rare-earth positions.European Commission's 7th Framework Program (FP7/2007–2013)Engineering and Physical Sciences Research Council (EPSRC)Netherlands Organization for Scientific Research (NWO)Foundation for Fundamental Research on Matter (FOM)ERCRussian GovernmentRFB

Terahertz magnonics in antiferromagnetic iron oxides

Antiferromagnets, magnetic materials with antiparallel spin ordering and therefore lacking magnetic moment have for a long time been studied for academic interest only. However, the realization that antiferromagnets possess intrinsic magnetic resonance frequencies in the Terahertz range, which is orders of magnitude higher than the Gigahertz ferromagnetic resonance frequencies, has recently renewed the interest for antiferromagnets for applications in energy efficient data storage and processing. Moreover waves of spin precession, or magnons, have been proposed as new methods for wave-based computing. The miniaturization of such potential technological devices requires the spin waves to have nanometer scale wavelengths, which has proven to be challenging to achieve in anitferromagnets. In this thesis, we will study the ultrafast spin dynamics and magnons in a specific class of antiferromagnetic iron oxides, the orthoferrites, RFeO3, where R is a rare-earth element. These antiferromagnets possess a weak ferromagnetic moment due to the canting of the antiparallel spins. After introducing the field of ultrafast magnetism and magnonics and associated concepts in Chapter 1, in the first part of this thesis, we will describe how the challenge of generating nanoscale spin waves can be overcome by exciting a confined region of spins near the sample face. We will show how strongly absorbed laser pulses will generate a propagating broad-band wavepacket of spin waves. In Chapter 2, we will introduce the basic concepts behind the experiments performed in this work. We will introduce the principle of ultrafast pump-probe spectroscopy experiments that can be used to measure such spin waves, and describe the design of the setup that allows us to drive the spin dynamics with intense Terahertz pulses. In Chapter 3, a thorough theoretical description of the technique to launch propagating broadband wavepackets of magnons will be given. Additionally, we will model the detection of these generated packets of spin waves acts in Magneto-Optical Kerr Effect experiments. We find that through the emergence of the Brillouin condition, by the appropriate choice of the wavelength of the probe pulse, we can select the detected wavenumber component of the wave packet, resulting in a probe wavelength dependent frequency observed in the experiment. In Chapter 4, we proceed to the experiment and search for the spin wave packets in HoFeO3. We will show that by exciting the spin dynamics with high energy photons above the bandgap energy, we can launch such propagating packets of spin waves. We find the theoretically predicted dependence of the detected magnon frequency on the wavelength of the probe light, and find that we excite a broad range of components of the spin wave packet. In Chapter 5, we build upon the experiment in Chapter 4, and study how the propagating spin waves can be controlled. We find that we can achieve a nonlinear control of the spin waves by introducing a second pump pulse. From theoretical calculations, we show that the coupling between the propagating magnon and photon acts as an additional nonlinear torque on the spins. We will see that this nonlinear torque allows for the conversion of the low frequency uniform precession mode of the spins into the higher frequency and higher wavenumber modes of the propagating spin wave packet. In Chapter 6, we will study the spin dynamics in ErFeO3 and TmFeO3 induced by intense THz pulses. Despite the magnetic similarities of these materials, the spin dynamics shows a very different trend at the Spin Reorientation Transition temperatures. In ErFeO3, we observe an unexpected giant enhancement of the amplitude, whereas in TmFeO3, this amplitude is suppressed. We will show that this difference in the dynamics can be attributed to the effect of the coupling between the iron spins and rare-earth ions. Finally, in Chapter 7, we will conclude our findings, and provide a concise outlook that shows that the intense THz setup is not only suitable for the study of antiferromagnetic oxides, but can also be used to study metallic thin films. We will demonstrate this with a short summary of experimental data measured in the FeRh, which is an antiferromagnet at room temperature and exhibits a phase transition to the ferromagnetic phase at high temperatures

Selective Excitation of Terahertz Magnetic and Electric Dipoles in Er3+ Ions by Femtosecond Laser Pulses in ErFeO3

We show that femtosecond laser pulse excitation of the orthoferrite ErFeO3 triggers pico- and subpicosecond dynamics of magnetic and electric dipoles associated with the low energy electronic states of the Er3+ ions. These dynamics are readily revealed by using polarization sensitive terahertz emission spectroscopy. It is shown that by changing the polarization of the femtosecond laser pulse one can excite either electric dipole-active or magnetic dipole-active transitions between the Kramers doublets of the 4I15/2 ground state of the Er3+(4f11) ions. These observations serve as a proof of principle of polarization-selective control of both electric and magnetic degrees of freedom at terahertz frequencies, opening up new vistas for optical manipulation of magnetoelectric materials

Ultrafast optical modification of exchange interactions in iron oxides

Ultrafast non-thermal manipulation of magnetization by light relies on either indirect coupling of the electric field component of the light with spins via spin-orbit interaction or direct coupling between the magnetic field component and spins. Here we propose a scenario for coupling between the electric field of light and spins via optical modification of the exchange interaction, one of the strongest quantum effects with strength of 10(3) Tesla. We demonstrate that this isotropic opto-magnetic effect, which can be called inverse magneto-refraction, is allowed in a material of any symmetry. Its existence is corroborated by the experimental observation of terahertz emission by spin resonances optically excited in a broad class of iron oxides with a canted spin configuration. From its strength we estimate that a sub-picosecond modification of the exchange interaction by laser pulses with fluence of about 1 mJ cm(-2) acts as a pulsed effective magnetic field of 0.01 Tesla.European Commission’s 7th Framework Program (FP7/2007–2013)EPSRCNetherlands Organization for Scientific Research (NWO)Foundation for Fundamental Research on Matter (FOM)European Research CouncilRussian Ministry of Education and ScienceRFBR-NSFC projectBureau of International CooperationNSFC projectNSFC-NWOEU Seventh Framework ProgramNWO by a Rubicon grantEuropean Commission (FP7-ICT-2013-613024–GRASP

Sub-wavelength focusing of mid-IR light using metal/diamond/metal campanile probe for ultra-broadband SPM

Developing methods for efficient nanoscale probing of light-matter interaction, especially in the Mid-IR and THz spectral range, is essential for studying fundamental physical phenomena as well as chemical properties at micrometer to nanometer length scales. A highly efficient nanoscale focusing of visible and near-IR radiation light was reported recently using Au-SiO2-Au tapered gap campanile plasmon waveguide with an 830 nm wavelength that couples free space light into the nanoscale domain, enabling probing of materials in the visible and near-IR spectral range [1]. We expand this capability to the highly important mid-IR and THz range providing valuable information on local nanoscale chemistry and physical processes of materials and devices using a campanile shaped diamond tetragonal pyramid [2]. Our finite difference time domain (FDTD) simulation reveals that nanoscale focusing of mid-IR light is possible within the range of geometries and metal coatings including Au, Al and Cu. Here we report linked modeling and experimental results showing the confining efficiency of diamond pyramid in the mid-IR range (8-10 µm). Furthermore, we will demonstrate the integration of Au/diamond/Au light concentrator into a scanning probe microscope for performing sub-wavelength spectroscopy of various materials in both reflection and transmission geometries

An effective magnetic field from optically driven phonons

Light fields at terahertz and mid-infrared frequencies allow for the direct excitation of collective modes in condensed matter, which can be driven to large amplitudes. For example, excitation of the crystal lattice has been shown to stimulate insulator–metal transitions, melt magnetic order or enhance superconductivity. Here, we generalize these ideas and explore the simultaneous excitation of more than one lattice mode, which are driven with controlled relative phases. This nonlinear mode mixing drives rotations as well as displacements of the crystal-field atoms, mimicking the application of a magnetic field and resulting in the excitation of spin precession in the rare-earth orthoferrite ErFeO3. Coherent control of lattice rotations may become applicable to other interesting problems in materials research—for example, as a way to affect the topology of electronic phases