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▲ [14p-P10-2] Terahertz electric-field induced Franz-Keldysh effect and magnetization modulation in the ferromagnetic semiconductor GaMnAs
Keywords:THz, Pump probe, ferromagnetic semiconductor
Generally, it takes more than a few hundred ps to reverse the magnetization direction. Meanwhile, using a terahertz light pulse, we can induce strong fluctuations of electromagnetic fields in ferromagnetic films within an extremely short time less than ~1 ps. Thus, we can expect ultrafast magnetization reversal within a few ps because the energy relaxation of spin is too slow to follow it. Previous studies of terahertz pump probe measurements for ferromagnetic metals have detected ultra-fast magnetization modulation (Δθ) of the polarization rotation of the probe pulse. However, the origin of the terahertz response has not been clarified yet.
Ferromagnetic semiconductor GaMnAs is quite sensitive to an external optical stimulus. In this study, to clarify the effect of the electro-optic effect in GaMnAs, we compare the terahertz response of GaMnAs with that of GaAs:Be. The terahertz-pump probe measurements were performed using a pulsed-light source with a repetition rate of 1 kHz. Both pump and probe pulses were linearly polarized. The strong terahertz-pump pulse with the centered frequency of 1 THz, whose electric field ETHz was along the [110] axis, was generated by the optical rectification using a LiNbO3 crystal.
We have found that Δθ is induced by ETHz even in the non-magnetic GaAs:Be sample. This result indicates that ETHz induces the birefringence in the GaAs:Be film. This difference can cause Δθ when the electric field vector Eprobe of the probe pulse and ETHz are not parallel. In fact, when the angle between Eprobe and ETHz is 30 deg, the maximum value of |Δθ| observed in the time-evolution of Δθ tends to saturate as the field intensity (ETHz2) increases, which is a typical behavior observed in the transient reflectivity induced by the Franz-Keldysh effect. For the GaMnAs film, we observed similar saturating behavior. These results indicate that Δθ is mainly attributed to the birefringence due to the Franz-Keldysh effect. When Eprobe//ETHz, Δθ induced by the birefringence should become negligible. Actually, the observed Δθ was almost symmetric around Δθ=0 when the direction of the external magnetic field applied perpendicular to the sample surface was reversed. This indicates that Δθ mainly originates from the Polar Kerr effect, which is thought to be induced by the Franz-Keldysh effect, when Eprobe//ETHz. Our results show that the Franz-Keldysh effect plays an important role while the terahertz pulse is irradiated to the GaMnAs film.
Ferromagnetic semiconductor GaMnAs is quite sensitive to an external optical stimulus. In this study, to clarify the effect of the electro-optic effect in GaMnAs, we compare the terahertz response of GaMnAs with that of GaAs:Be. The terahertz-pump probe measurements were performed using a pulsed-light source with a repetition rate of 1 kHz. Both pump and probe pulses were linearly polarized. The strong terahertz-pump pulse with the centered frequency of 1 THz, whose electric field ETHz was along the [110] axis, was generated by the optical rectification using a LiNbO3 crystal.
We have found that Δθ is induced by ETHz even in the non-magnetic GaAs:Be sample. This result indicates that ETHz induces the birefringence in the GaAs:Be film. This difference can cause Δθ when the electric field vector Eprobe of the probe pulse and ETHz are not parallel. In fact, when the angle between Eprobe and ETHz is 30 deg, the maximum value of |Δθ| observed in the time-evolution of Δθ tends to saturate as the field intensity (ETHz2) increases, which is a typical behavior observed in the transient reflectivity induced by the Franz-Keldysh effect. For the GaMnAs film, we observed similar saturating behavior. These results indicate that Δθ is mainly attributed to the birefringence due to the Franz-Keldysh effect. When Eprobe//ETHz, Δθ induced by the birefringence should become negligible. Actually, the observed Δθ was almost symmetric around Δθ=0 when the direction of the external magnetic field applied perpendicular to the sample surface was reversed. This indicates that Δθ mainly originates from the Polar Kerr effect, which is thought to be induced by the Franz-Keldysh effect, when Eprobe//ETHz. Our results show that the Franz-Keldysh effect plays an important role while the terahertz pulse is irradiated to the GaMnAs film.