170
7 Opto-spintronics
From this, it becomes clear that right- and left-handed circularly polarized waves
should act as magnetic fields of opposite sign. Hence, from Eq. (7.6), we can say
that, in addition to magneto-optical Faraday effect where the polarization of light is
affected by the magnetization M, the same susceptibility α also determines the inverse
opto-magnetic effect where circularly polarized light modifies the magnetization via
inverse Faraday effect (IFE). In recent years, IFE has acquired renewed interest in
solid-state physics, especially in magnetic material. It is recognized that IFE could
offer an alternative and new path to ultrafast, all-optical magnetization reversal.
The situation may be different and interesting when the product E(ω) × E(ω)* is
changed much faster than the fundamental time scales in a magnetically ordered
material, given by the spin precession period and the spin–lattice relaxation time.
Let us consider the excitation of spins by a laser pulse with duration t = 100 fs
(spectrally broad ω ∼5 THz). Initially the electron is in the ground state E 1 and
its spin is up. If photon is acted on this electron, there will be an increase in orbital
momentum. This effectively increases SOI and hence the probability of a spin-flip
process. If the photon energy is less than the band gap energy (difference between
the ground state E 1 and the nearest excited state E 2 ) then the electron will not jump
to the excited state. Instead, the electron will have spin flip in its ground state.
This process is associated with the coherent reemission of a photon of energy èω 2
= è(ω 1 − m ). Here, è m corresponds to the energy of a magnon in magnetically
ordered materials. The presence of frequencies ω 1 and ω 2 in the laser pulse can stimulate coherent spin-flip process (see Fig. 7.6). Materials having large magneto-optical
susceptibility can show ultrafast spin-flip process, which is around 20 femtosecond.
This mechanism does not require annihilation of a photon and hence much more
effective than magnetic dipole transitions.
7.6 Experimental Techniques
The study of ultrafast spin dynamics needs methods to detect the changes that
occurred in given magnetic medium with high temporal resolution. Several methods
are used in this regard like.
• Pump and probe
• Optical probe
• Far infrared probe
• X-ray probe.
A brief description is given below about the above-mentioned probing techniques.
7 Opto-spintronics
From this, it becomes clear that right- and left-handed circularly polarized waves
should act as magnetic fields of opposite sign. Hence, from Eq. (7.6), we can say
that, in addition to magneto-optical Faraday effect where the polarization of light is
affected by the magnetization M, the same susceptibility α also determines the inverse
opto-magnetic effect where circularly polarized light modifies the magnetization via
inverse Faraday effect (IFE). In recent years, IFE has acquired renewed interest in
solid-state physics, especially in magnetic material. It is recognized that IFE could
offer an alternative and new path to ultrafast, all-optical magnetization reversal.
The situation may be different and interesting when the product E(ω) × E(ω)* is
changed much faster than the fundamental time scales in a magnetically ordered
material, given by the spin precession period and the spin–lattice relaxation time.
Let us consider the excitation of spins by a laser pulse with duration t = 100 fs
(spectrally broad ω ∼5 THz). Initially the electron is in the ground state E 1 and
its spin is up. If photon is acted on this electron, there will be an increase in orbital
momentum. This effectively increases SOI and hence the probability of a spin-flip
process. If the photon energy is less than the band gap energy (difference between
the ground state E 1 and the nearest excited state E 2 ) then the electron will not jump
to the excited state. Instead, the electron will have spin flip in its ground state.
This process is associated with the coherent reemission of a photon of energy èω 2
= è(ω 1 − m ). Here, è m corresponds to the energy of a magnon in magnetically
ordered materials. The presence of frequencies ω 1 and ω 2 in the laser pulse can stimulate coherent spin-flip process (see Fig. 7.6). Materials having large magneto-optical
susceptibility can show ultrafast spin-flip process, which is around 20 femtosecond.
This mechanism does not require annihilation of a photon and hence much more
effective than magnetic dipole transitions.
7.6 Experimental Techniques
The study of ultrafast spin dynamics needs methods to detect the changes that
occurred in given magnetic medium with high temporal resolution. Several methods
are used in this regard like.
• Pump and probe
• Optical probe
• Far infrared probe
• X-ray probe.
A brief description is given below about the above-mentioned probing techniques.