Image via WikipediaMY NOTE: This is more technical than much of what I try to post here, but it demonstrates, yet one more entire field, (quantum computation),where THz is likely to have a significant role.
Spintronics: Coherent terahertz control
NATURE PHOTONICS | NEWS AND VIEWS
Junichiro Kono
Nature Photonics 5, 5–6 (2011)
Spin and charge terahertz excitations in solids are promising for implementing future technologies such as spintronics and quantum computation, but coherently controlling them has been a significant challenge. Researchers have now manipulated coherent spin waves in an antiferromagnet using the intense magnetic field of ultrashort terahertz pulses.
Controlling quantum coherent states in solids is a key technical requirement for implementing future information technologies such as spintronics (using the spin of elementary particles to store data and perform processing tasks) and quantum computation. The realization of such technologies requires an appropriate excitation or transition in a solid with a long coherence time that can be driven and manipulated by external means.
Electromagnetic pulses in the terahertz (THz, or 1012 Hz) frequency range can excite a wide variety of spin and charge excitations in solids, including plasmons, phonons, magnons, intersubband transitions, cyclotron and spin resonances, and superconducting gap excitations. However, coherently exciting and controlling such excitations has proved a challenge, primarily because there are few sources that produce ultrashort and intense pulses of electromagnetic fields in the THz range. This frequency range is also known as the ‘technology gap’ between electronics and photonics, and is considered to be the last frontier of the electromagnetic spectrum remaining to be exploited by solid-state technology.
Now, writing in Nature Photonics 1, Tobias Kampfrath and co-workers demonstrate that the magnetic component of intense, ultrashort THz pulses can be used to coherently control collective spin excitations, known as magnons, in solids. The researchers used single-cycle THz pulses to turn on and off coherent magnons in antiferromagnetically ordered nickel oxide at room temperature with a resonance frequency of ~1 THz. An ultrashort (~8 fs) optical probe pulse was used to monitor the THz-induced magnetic dynamics directly in the time domain through time-resolved Faraday rotation (a magnetic-field-induced change in the polarization of the probe light). Through pump-power-dependent measurements and detailed simulations, Kampfrath et al. concluded that the magnetic field component of the THz field was addressing the spins selectively through a magnetic dipole interaction (Fig. 1).
Figure 1: The magnetic field component (B) of an ultrashort terahertz pulse — perpendicular to both the electric field component (E) and the propagation direction (the Poynting vector P) — excites coherent magnons through the magnetic dipole interaction.
The torque exerted by the magnetic field on the spin (S; shown inset) induces spin precession. The magnon, shown by the line connecting the spin tips, has a period of ~1 ps.
This work should be considered in context with the ongoing efforts of many groups in the area of spintronics to manipulate electronic spins optically 2. In particular, over the past decade, ultrafast magneto-optical studies of magnetically ordered systems have produced an assortment of exciting and sometimes conflicting results, whose microscopic understanding has been elusive 3, 4, 5, 6. It has been experimentally established that an optical pulse can induce changes in magnetism on ultrashort timescales, but exactly how light can modify magnetism in such a quasi-instantaneous manner is a matter of controversy. An important point to note is that in these previous studies of ultrafast magneto-optics, the light field itself never directly coupled with the magnetic order — its primary role has been to excite a transient, non-equilibrium distribution of carriers, which subsequently relaxed in energy and momentum while interacting with the lattice and ordered spins. As a result, magnetism was only indirectly affected and modified by light. This is distinctly different from the work of Kampfrath and co-workers 1, in which the magnetic field component of the intense THz field directly interacts with and modifies the magnetic state of the system via the Zeeman torque ? S × B, where S is the electron spin angular momentum and B is the transient magnetic field, thereby inducing and manipulating coherent magnons (Fig. 1).
The use of pulsed a.c. magnetic fields with megahertz and gigahertz frequencies for coherent manipulation of spins has a long history in the field of nuclear magnetic resonance (NMR), dating back to the pivotal spin-echo experiments of Erwin Hahn in 1950 7. More recently, pulsed NMR methods have been used to perform quantum computation using molecules8. However, decoherence times in solids are so short that ultrafast (femtosecond or picosecond) pulses of high-intensity magnetic fields are necessary for performing coherent control experiments on magnetic excitations in solids. Large pulsed magnetic fields (in excess of 100 T) can be produced using various destructive methods9, but the microsecond-scale pulse widths remain too large for studying coherent magnetic phenomena in solids. The existence of a transient high magnetic field within ultrashort optical pulses has long been recognized by laser scientists as a potentially useful tool, but an effective demonstration of their utility did not exist until now 1.
Kampfrath et al. used a transient magnetic field with a peak field-strength of 0.13 T in their experiments, which gave a correspondingly high-intensity electric field component of ~0.4 MV cm?1. The effect of the huge electric field was surprisingly small, however, with no sign of heat deposited by the field. This was primarily because nickel oxide is an insulator, in which the free-carrier absorption of THz electric fields is negligibly small. The Zeeman torque exerted on the spins by the transient magnetic fields initiated magnon oscillations, which were then either enhanced or reduced, depending on the arrival time of the second magnetic field pulse. Simulations based on a Landau–Lifshitz–Gilbert equation for magnetic coupling successfully reproduced these experimental observations.
The findings of Kampfrath et al. open up an entirely new field of research into the interactions of intense and ultrashort pulses of magnetic fields with matter. A range of exciting new experiments using such coherent magnetic field pulses in the THz range are now on the horizon, including the coherent manipulation of spins in solids — possibly even single electron spins — for the purposes of quantum information processing. Transient megagauss (>100 T) fields might one day become available through further improvements in generation techniques, and such intense and transient magnetic fields may allow the development of magnetic-dipole-excited nonlinear optics in non-perturbative regimes. There are, however, several technical challenges that must be overcome before further progress can be made. The main problem is related to the heating and damage problems associated with the intense electric field component of the THz field, which is orders of magnitude larger than the magnetic field component. Such problems were avoided in the work of Kampfrath et al. by using an insulating antiferromagnetic specimen, but coherent excitations and manipulations in magnetic and/or superconducting systems containing itinerant electrons are expected to be even more exotic and interesting to study with intense magnetic fields. New ideas are now needed to enable such innovative experiments.
References
1. Kampfrath, T. et al. Nature Photon. 5, 31–34 (2011).
2. Awschalom, D. D. & Samarth, N. in Semiconductor Spintronics and Quantum Computation (eds Awschalom, D. D. et al.) 147–194 (Springer, 2002).
3. Zhang, G., Hübner, W., Beaurepaire, E. & Bigot, J.-Y. in Spin Dynamics in Confined Magnetic Structures I (eds Hillebrands, B. & Ounadjela, K.) 245–288 (Springer, 2002).
4. Wang, J. et al. J. Phys. Cond. Matt. 18, R501–R530 (2006).
5. Koopmans, B. et al. Nature Mater. 9, 259–265 (2010).
6. Kirilyuk, A., Kimel, A. V. & Rasing, Th. Rev. Mod. Phys. 82, 2731–2784 (2010).
7. Hahn, E. L. Phys. Rev. 80, 580–594 (1950).
8. Gershenfeld, N. A. & Chuang, I. L. Science 275, 350–356 (1997).
9. Herlach, F. & Miura, N. High Magnetic Fields: Science and Technology Vol. 1 (World Scientific, 2003).
Junichiro Kono
Nature Photonics 5, 5–6 (2011)
Spin and charge terahertz excitations in solids are promising for implementing future technologies such as spintronics and quantum computation, but coherently controlling them has been a significant challenge. Researchers have now manipulated coherent spin waves in an antiferromagnet using the intense magnetic field of ultrashort terahertz pulses.
Controlling quantum coherent states in solids is a key technical requirement for implementing future information technologies such as spintronics (using the spin of elementary particles to store data and perform processing tasks) and quantum computation. The realization of such technologies requires an appropriate excitation or transition in a solid with a long coherence time that can be driven and manipulated by external means.
Electromagnetic pulses in the terahertz (THz, or 1012 Hz) frequency range can excite a wide variety of spin and charge excitations in solids, including plasmons, phonons, magnons, intersubband transitions, cyclotron and spin resonances, and superconducting gap excitations. However, coherently exciting and controlling such excitations has proved a challenge, primarily because there are few sources that produce ultrashort and intense pulses of electromagnetic fields in the THz range. This frequency range is also known as the ‘technology gap’ between electronics and photonics, and is considered to be the last frontier of the electromagnetic spectrum remaining to be exploited by solid-state technology.
Now, writing in Nature Photonics 1, Tobias Kampfrath and co-workers demonstrate that the magnetic component of intense, ultrashort THz pulses can be used to coherently control collective spin excitations, known as magnons, in solids. The researchers used single-cycle THz pulses to turn on and off coherent magnons in antiferromagnetically ordered nickel oxide at room temperature with a resonance frequency of ~1 THz. An ultrashort (~8 fs) optical probe pulse was used to monitor the THz-induced magnetic dynamics directly in the time domain through time-resolved Faraday rotation (a magnetic-field-induced change in the polarization of the probe light). Through pump-power-dependent measurements and detailed simulations, Kampfrath et al. concluded that the magnetic field component of the THz field was addressing the spins selectively through a magnetic dipole interaction (Fig. 1).
Figure 1: The magnetic field component (B) of an ultrashort terahertz pulse — perpendicular to both the electric field component (E) and the propagation direction (the Poynting vector P) — excites coherent magnons through the magnetic dipole interaction.
The torque exerted by the magnetic field on the spin (S; shown inset) induces spin precession. The magnon, shown by the line connecting the spin tips, has a period of ~1 ps.
This work should be considered in context with the ongoing efforts of many groups in the area of spintronics to manipulate electronic spins optically 2. In particular, over the past decade, ultrafast magneto-optical studies of magnetically ordered systems have produced an assortment of exciting and sometimes conflicting results, whose microscopic understanding has been elusive 3, 4, 5, 6. It has been experimentally established that an optical pulse can induce changes in magnetism on ultrashort timescales, but exactly how light can modify magnetism in such a quasi-instantaneous manner is a matter of controversy. An important point to note is that in these previous studies of ultrafast magneto-optics, the light field itself never directly coupled with the magnetic order — its primary role has been to excite a transient, non-equilibrium distribution of carriers, which subsequently relaxed in energy and momentum while interacting with the lattice and ordered spins. As a result, magnetism was only indirectly affected and modified by light. This is distinctly different from the work of Kampfrath and co-workers 1, in which the magnetic field component of the intense THz field directly interacts with and modifies the magnetic state of the system via the Zeeman torque ? S × B, where S is the electron spin angular momentum and B is the transient magnetic field, thereby inducing and manipulating coherent magnons (Fig. 1).
The use of pulsed a.c. magnetic fields with megahertz and gigahertz frequencies for coherent manipulation of spins has a long history in the field of nuclear magnetic resonance (NMR), dating back to the pivotal spin-echo experiments of Erwin Hahn in 1950 7. More recently, pulsed NMR methods have been used to perform quantum computation using molecules8. However, decoherence times in solids are so short that ultrafast (femtosecond or picosecond) pulses of high-intensity magnetic fields are necessary for performing coherent control experiments on magnetic excitations in solids. Large pulsed magnetic fields (in excess of 100 T) can be produced using various destructive methods9, but the microsecond-scale pulse widths remain too large for studying coherent magnetic phenomena in solids. The existence of a transient high magnetic field within ultrashort optical pulses has long been recognized by laser scientists as a potentially useful tool, but an effective demonstration of their utility did not exist until now 1.
Kampfrath et al. used a transient magnetic field with a peak field-strength of 0.13 T in their experiments, which gave a correspondingly high-intensity electric field component of ~0.4 MV cm?1. The effect of the huge electric field was surprisingly small, however, with no sign of heat deposited by the field. This was primarily because nickel oxide is an insulator, in which the free-carrier absorption of THz electric fields is negligibly small. The Zeeman torque exerted on the spins by the transient magnetic fields initiated magnon oscillations, which were then either enhanced or reduced, depending on the arrival time of the second magnetic field pulse. Simulations based on a Landau–Lifshitz–Gilbert equation for magnetic coupling successfully reproduced these experimental observations.
The findings of Kampfrath et al. open up an entirely new field of research into the interactions of intense and ultrashort pulses of magnetic fields with matter. A range of exciting new experiments using such coherent magnetic field pulses in the THz range are now on the horizon, including the coherent manipulation of spins in solids — possibly even single electron spins — for the purposes of quantum information processing. Transient megagauss (>100 T) fields might one day become available through further improvements in generation techniques, and such intense and transient magnetic fields may allow the development of magnetic-dipole-excited nonlinear optics in non-perturbative regimes. There are, however, several technical challenges that must be overcome before further progress can be made. The main problem is related to the heating and damage problems associated with the intense electric field component of the THz field, which is orders of magnitude larger than the magnetic field component. Such problems were avoided in the work of Kampfrath et al. by using an insulating antiferromagnetic specimen, but coherent excitations and manipulations in magnetic and/or superconducting systems containing itinerant electrons are expected to be even more exotic and interesting to study with intense magnetic fields. New ideas are now needed to enable such innovative experiments.
References
1. Kampfrath, T. et al. Nature Photon. 5, 31–34 (2011).
2. Awschalom, D. D. & Samarth, N. in Semiconductor Spintronics and Quantum Computation (eds Awschalom, D. D. et al.) 147–194 (Springer, 2002).
3. Zhang, G., Hübner, W., Beaurepaire, E. & Bigot, J.-Y. in Spin Dynamics in Confined Magnetic Structures I (eds Hillebrands, B. & Ounadjela, K.) 245–288 (Springer, 2002).
4. Wang, J. et al. J. Phys. Cond. Matt. 18, R501–R530 (2006).
5. Koopmans, B. et al. Nature Mater. 9, 259–265 (2010).
6. Kirilyuk, A., Kimel, A. V. & Rasing, Th. Rev. Mod. Phys. 82, 2731–2784 (2010).
7. Hahn, E. L. Phys. Rev. 80, 580–594 (1950).
8. Gershenfeld, N. A. & Chuang, I. L. Science 275, 350–356 (1997).
9. Herlach, F. & Miura, N. High Magnetic Fields: Science and Technology Vol. 1 (World Scientific, 2003).
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