V. Zayets

Origin of the anomalous magnetic circular dichroism spectral shape in ferromagnetic Ga(1-x)MnxAs: impurity bands inside the band gap.

The electronic structure of a prototype dilute magnetic semiconductor (DMS), Ga(1-x)MnxAs, is studied by magnetic circular dichroism (MCD) spectroscopy. We prove that the optical transitions originated from impurity bands cause the strong positive MCD background. The MCD signal due to the E0 transition from the valence band to the conduction band is negative indicating that the p-d exchange interactions between the p carriers and d spin is antiferromagnetic. The negative E0 MCD signal also indicates that the hole doping of the valence band is not so large as previously assumed. The impurity bands seem to play important roles for the ferromagnetism of Ga(1-x)MnxAs.

Optical properties and functions of dilute magnetic semiconductors

The magneto-optical effect in dilute magnetic semiconductors (DMSs) is directly related to the interaction between the d electrons of the transition metal ions and the s, p electrons of the host semiconductor. We show the advantages of the magneto-optical effect of DMSs as regards fabricating magneto-optical waveguide devices that can be integrated with other semiconductor optical devices. We also discuss the advantages of magneto-optical spectroscopy for characterizing DMSs. Intrinsic ferromagnetism of In1−xMnxAs, Ga1−xMnxAs and Zn1−xCrxTe is confirmed by using magnetic circular dichroism (MCD) spectroscopy. The MCD analyses also show that Zn1−xTMxO (TM = Mn, Fe, Co, Ni or Cu), Ga1−xMnxN and Ga1−xCrxAs are paramagnetic DMSs with the s, p–d exchange interaction. Ferromagnetic behaviours observed in some transition metal doped ZnO, GaN and GaAs samples are attributed to unidentified precipitations not detectable by means of x-ray diffraction analysis.

Complete magneto-optical waveguide mode conversion in Cd1−xMnxTe waveguide on GaAs substrate

Complete magneto-optical mode conversion was attained in a waveguide of diluted magnetic semiconductor Cd1−xMnxTe grown on GaAs substrate. Mode conversion ratio 98%±2% under a magnetic field of 5 kG was achieved in the waveguide with graded-refractive-index clad layer. The Cd1−xMnxTe waveguide showed an optical loss below 1 dB/cm, and a high magneto-optical figure-of-merit, 200 deg/dB/kG at λ=730 nm. High efficiency magneto-optical mode conversion in a waveguide grown on a semiconductor substrate shows the feasibility of monolithical integration of an optical isolator with semiconductor optoelectronic devices.

Magnetization-dependent loss in an (Al,Ga)As optical waveguide with an embedded Fe micromagnet

The dependence of waveguiding loss on the magnetization of a Fe micromagnet embedded into the (Al,Ga)As optical waveguide was examined as a possible readout method for the spin-photon memory. The optical detection of the magnetization direction of a Fe micromagnet was demonstrated for the micromagnet sizes of 3 microm x 4 microm and 3 microm x 8 microm with signal-to-noise ratios of 4.8 and 6 dB, respectively. In the case of smaller sizes, the use of spin injection from the micromagnet into a semiconductor optical amplifier was proposed for the optical detection of the magnetization.

Room-temperature ferromagnetism in highly Cr-doped II–VI diluted magnetic semiconductor Zn1−xCrxTe

Highly Cr-doped II–VI diluted magnetic semiconductor (DMS) Zn1−xCrxTe films with a ferromagnetic long-range order have been grown. A phase diagram of Zn1−xCrxTe in relation to the growth temperature and Cr concentration was determined. Magnetic circular dichroism measurements revealed that a magnetically single phase of DMS Zn1−xCrxTe is obtained in the films with Cr concentration up to x=0.20. Spontaneous magnetization of the film with x=0.20 disappears around 300 K, indicating that the Zn1−xCrxTe is a DMS with room-temperature ferromagnetism.

Isolation effect in ferromagnetic-metal/semiconductor hybrid optical waveguide

The isolation effect in a ferromagnetic-metal/semiconductor hybrid optical waveguide was experimentally studied. Optical transmission in a Ga1−xAlxAs waveguide covered by Co was found to depend on the magnetization of the Co. The isolation direction was different for a waveguide with a SiO2 buffer layer and for a waveguide with a Ga1−xAlxAs buffer layer used between the waveguide core layer and Co layer. The physical origin of the isolation in this isolator structure was clarified.The isolation effect in a ferromagnetic-metal/semiconductor hybrid optical waveguide was experimentally studied. Optical transmission in a Ga1−xAlxAs waveguide covered by Co was found to depend on the magnetization of the Co. The isolation direction was different for a waveguide with a SiO2 buffer layer and for a waveguide with a Ga1−xAlxAs buffer layer used between the waveguide core layer and Co layer. The physical origin of the isolation in this isolator structure was clarified.

Optical Isolator Utilizing Surface Plasmons

Feasibility of usage of surface plasmons in a new design of an integrated optical isolator has been studied. In the case of surface plasmons propagating at a boundary between a transition metal and a double-layer dielectric, there is a significant difference of optical loss for surface plasmons propagating in opposite directions. Utilizing this structure, it is feasible to fabricate a competitive plasmonic isolator, which benefits from a broad wavelength operational bandwidth and a good technological compatibility for integration into the Photonic Integrated Circuits (PIC). The linear dispersion relation was derived for plasmons propagating in a multilayer magneto-optical slab.

Enhancement of the transverse non-reciprocal magneto-optical effect

The origin and properties of the transverse non-reciprocal magneto-optical (nMO) effect were studied. The transverse nMO effect occurs in the case when light propagates perpendicularly to the magnetic field. It was demonstrated that light can experience the transverse nMO effect only when it propagates in the vicinity of a boundary between two materials and the optical field at least in one material is evanescent. The transverse nMO effect is pronounced in the cases of surface plasmons and waveguiding modes. The magnitude of the transverse nMO effect is comparable to or greater than the magnitude of the longitudinal nMO effect. In the case of surface plasmons propagating at a boundary between the transition metal and the dielectric it is possible to magnify the transverse nMO effect and the magneto-optical figure-of-merit may increase from a few percents to above 100%. The scalar dispersion relation, which describes the transverse MO effect in cases of waveguide modes and surface plasmons propagating in a multilayer MO slab, was derived.The origin and properties of the transverse non-reciprocal magneto-optical (nMO) effect were studied. The transverse nMO effect occurs in the case when light propagates perpendicularly to the magnetic field. It was demonstrated that light can experience the transverse nMO effect only when it propagates in the vicinity of a boundary between two materials and the optical field at least in one material is evanescent. The transverse nMO effect is pronounced in the cases of surface plasmons and waveguiding modes. The magnitude of the transverse nMO effect is comparable to or greater than the magnitude of the longitudinal nMO effect. In the case of surface plasmons propagating at a boundary between the transition metal and the dielectric it is possible to magnify the transverse nMO effect and the magneto-optical figure-of-merit may increase from a few percents to above 100%. The scalar dispersion relation, which describes the transverse MO effect in cases of waveguide modes and surface plasmons propagating in a multilayer MO slab, was derived.

Optical isolation in Cd_1−xMn_xTe magneto-optical waveguide grown on GaAs substrate

The isolation effect was studied in a Cd_1−xMn_xTe magneto-optical waveguide grown on a GaAs substrate. By use of prism coupling, an isolation ratio of 20 dB was achieved at lambda = 740 nm under a magnetic field of 5 kG. The high isolation ratio obtained in the magneto-optical waveguide grown on the semiconductor substrate shows the feasibility of monolithical integration of an optical isolator with semiconductor optoelectronic devices.

Efficient spin injection into semiconductor from an Fe/GaOx tunnel injector

We examined the electrical injection of spin-polarized electrons into a GaAs-based light-emitting diode structure from a Fe/GaOx tunnel injector whose electron-charge injection efficiency was comparable to that of a conventional Fe/n+-AlGaAs ohmic injector. A high circular polarization of electroluminescence up to 20% was observed at 2 K. The combination of effective spin-and charge-injection efficiencies makes GaOx a promising tunnel barrier for GaAs-based spintronic devices.