Ultrafast magneto-optics in ferromagnetic III–V semiconductors

Abstract

We investigate various ultrafast optical processes in ferromagnetic (III,Mn)V semiconductors induced by femtosecond laser pulses. Two-colour timeresolved magneto-optical spectroscopy has been developed, which allows us to observe a rich array of dynamical phenomena. We isolate several distinct temporal regimes in spin dynamics, interpreting the fast (<1 ps) dynamics as spin heating through sp-d exchange interaction between photo-carriers and Mn ions while the ∼100 ps component is interpreted as a manifestation of spinlattice relaxation. Charge carrier and phonon dynamics were also carefully studied, showing an ultrashort charge lifetime of photo-injected electrons (∼2 ps) and propagating coherent acoustic phonon wavepackets with a strongly probe energy dependent oscillation period, amplitude and damping.

Probing and Manipulation of Coherent States in Ferromagnetic Narrow Gap Semiconductors with an Eye Towards Developing Concepts for New Device Functionalities

“…The initial sharp decrease in the differential reflectivity can result from free carrier Drude absorption whereas; the alteration of the dielectric function of the film through changes in the electron and hole distribution functions, can be responsible for the observed sign change of the differential reflectivity. A similar fast carrier dynamics and the sign change in the transient reflectivity have been reported in MBE grown InMnAs and InAs films [1,6] . The possibility of increasing the relaxation time of the photo-excited carriers by injection carrier above the gap into the satellite valley can be important in developing devices on the basis of these ferromagnetic materials.…”

“…Most of the understanding of the carrier relaxation in the MBE grown narrow gap (III,Mn)V ferromagnetic structures has been on the basis of two color differential reflectivity spectroscopy with pump and probe pulses ranging from 1.2-2μm and 650-850 nm, respectively [1] .The information extracted from the differential reflectivity can be affected by multi-reflections in the multi-layer structures. In this work, our TRDT measurements demonstrate complex and tunable dynamics of the photo-excited carriers in these ferromagnetic films which have not been reported before.…”

Manipulation of ferromagnets via the spin-selective optical Stark effect

“…22 Although their Curie temperature at present is below room temperature, studying these materials can improve our understanding of novel physical phenomena that are also present in other magnets. [23][24][25] Photoinjected carriers induced by linearly polarized light with frequency slightly above the or L band edges have been shown to induce magnetization dynamics in GaMnAs. 23,26 In contrast, we focus here on excitation with frequencies below the fundamental band gap, which is dissipationless, since no free carriers are excited.…”

“…[23][24][25] Photoinjected carriers induced by linearly polarized light with frequency slightly above the or L band edges have been shown to induce magnetization dynamics in GaMnAs. 23,26 In contrast, we focus here on excitation with frequencies below the fundamental band gap, which is dissipationless, since no free carriers are excited. Our approach is quite general and can be applied to arbitrary electronic structures and computed from first principles.…”

Relaxation of photoinduced spins and carriers in ferromagnetic InMnSb films

“…In this regime, the MOKE signal only lasts for less than 2 ps, similar to the relaxation observed in InMnAs and InGaMnAs with a similar photoinduced carrier density. 17 Figure 2͑a͒ ͑the two lowest traces͒ demonstrates the carrier/spin dynamics of the InMnSb ͑sample B͒ using the degenerate configuration at 10 K with pump fluence of ϳ20 J/cm 2 ; in addition, for comparison, the MOKE signal of sample C ͑the top trace͒ is shown at 11 K. Figure 2͑b͒ shows the MOKE signal and the differential reflectivity of InMnSb ͑sample C͒ with pump fluence of ϳ50 J/cm 2 using two different circular polarizations for the pump. In this pumping regime the observed spin relaxations were more than a factor of 3 longer compared to those observed with the two-color pumping regime, where the photoinduced carrier density is expected to be in the 10 17 -10 18 cm −3 range.…”