M. C. Debnath

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.

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.

Enhancement of magneto-optical transverse electric–transverse magnetic mode conversion efficiency of (Cd,Mn)Te waveguide by graded-index clad layers

Graded-index clad layers with different thickness were used in (Cd,Mn)Te waveguide, and magneto-optical mode conversion was investigated as a function of the thickness of the graded-index layers. Magneto-optical mode conversion efficiency increased with the graded-index layer thickness, and high efficiency of 98%±2% was obtained for the waveguide with 4000- to 5000-A-thick graded layers. The waveguide also showed low optical loss of 0.5 dB/cm and high magneto-optical figure-of-merit above 500 deg/dB/kG.

(Cd,Mn)Te/(Cd,Zn)Te quantum well waveguide optical isolator with wide wavelength operational bandwidth

The authors developed a magneto-optical (Cd,Mn)Te∕(Cd,Zn)Te quantum well waveguide and demonstrated that the Faraday rotation in the waveguide was slightly dependent on the wavelength. For a waveguide with a quantum well width of 20A, constant Faraday rotation and a small amount of wavelength dispersion were simultaneously achieved in a wide 25nm wavelength range at room temperature. Thus the broadband waveguide optical isolator is operated over a wide bandwidth of 25nm. This result shows that it is feasible to monolithically integrate an optical isolator made of diluted magnetic semiconductor with semiconductor optoelectronic devices.

Thermal annealing of magneto-optical (Cd,Mn)Te waveguides for optical isolators with wider operational wavelength range

We obtained significant improvement of magneto-optical performance by thermal annealing of the graded-index Cd1−xMnxTe waveguide. For a waveguide annealed at 425°C, complete mode conversion was achieved in a wavelength range between 710 and 735nm, which is an expansion of more than eight times that of a waveguide without annealing. The annealed waveguide also showed very low optical loss of 0.2dB∕cm and a high magneto-optical figure-of-merit of more than 1000deg∕dB∕kG. In addition, an isolation ratio of more than 20dB was obtained at λ=715–735nm in magnetic fields, H=1.6–5.1kG. This result is an important step toward achieving a practical integrated optical isolator.

Cd1-xMnxTe optical isolator fabricated on GaAs substrate with wide wavelength operation range

Complete magneto-optical mode conversion, low optical loss, 0.2 dB/cm, high magneto-optical figure-of-merit, 800 deg/dB/kG, high optical isolation, 20 dB and wide wavelength operation range, 20 nm were demonstrated with annealed Cd1-xMnxTe waveguide grown on GaAs substrate.