Faraday Rotation Spectroscopy Research Articles

Synthesis and characterization of semimagnetic semiconductor nanocrystals for spin electronics

The technique of pulsed laser deposition and colloidal chemistry methods have been applied for synthesis of semimagnetic semiconductor Cd 1− x Mn x Te nanocrystals embedded in dielectric matrix. The performed analysis suggests a narrower size distribution for the colloidal nanoparticles. Optical absorption, Faraday rotation and photoluminescence spectroscopy have been used to characterize the grown quantum dot structures. The magneto-optical spectra of the Cd 1− x Mn x Te nanocrystals exhibit peculiarities typical for bulk semimagnetic semiconductors due to the strong spin-exchange interaction between band carriers and magnetic ions and simultaneously manifest some features because of confinement effects in low dimensional structures.

Coherent spin transfer between molecularly bridged quantum dots.

Femtosecond time-resolved Faraday rotation spectroscopy reveals the instantaneous transfer of spin coherence through conjugated molecular bridges spanning quantum dots of different size over a broad range of temperature. The room-temperature spin-transfer efficiency is approximately 20%, showing that conjugated molecules can be used not only as interconnections for the hierarchical assembly of functional networks but also as efficient spin channels. The results suggest that this class of structures may be useful as two-spin quantum devices operating at ambient temperatures and may offer promising opportunities for future versatile molecule-based spintronic technologies.

Ultrafast geometric manipulation of electron spin and detection of the geometric phase via Faraday rotation spectroscopy

Time-resolved Faraday rotation spectroscopy is currently exploited as a powerful technique to probe spin dynamics in semiconductors. We propose here an all-optical approach to geometrically manipulate electron spin and to detect the geometric phase by this type of extremely sensitive experiment. The global nature of the geometric phase can make the quantum manipulation more stable, which may find interesting application in quantum devices.

Spin and size-dependent effects in semimagnetic semiconductor nanostructures

The technique of pulsed laser deposition has been applied for growth of Cd1−xMnxTe thin films and CdTe/Cd1−xMnxTe quantum well structures. Different materials (silicon, glass, sapphire) have been used as substrates which where heated to 300 °C during deposition process. X-ray diffraction, scanning electron microscopy, optical absorption and Faraday rotation spectroscopy have been used to characterize the obtained thin films and quantum structures. The observation of exciton peaks in the absorption spectra at low temperatures demonstrates the high quality of the deposited layers. The Faraday rotation spectra of the thin films exhibit peculiarities typical for bulk semimagnetic semiconductors because of strong spin-exchange interaction between band carriers and magnetic ions. For the superlattice structures of CdTe/Cd1−xMnxTe, the peak value of the Faraday rotation increases and its position moves toward higher energies with decreasing barrier thickness because of exciton confinement effect.

Platelet-shaped nanoparticles of PbI 2 and PbMnI 2 embedded in polymer matrix

Nanocrystals of PbI 2 and Pb 1− x Mn x I 2 (0< x<0.15) embedded in transparent polymer matrix have been studied by optical absorption and Faraday rotation spectroscopy. Average size of semiconductor nanoparticles was controlled by choosing polymer material and growth conditions. The obtained nanoparticles are platelet-like, with thickness up to 0.7 nm. The observed exciton peaks in the absorption spectra for nanocrystals into gelatin matrix correspond to the discrete thickness of the nanoparticles, which include finite number of atomic layers. For the Faraday rotation of Pb 1− x Mn x I 2 nanocrystals, reversal negative sign and additional enhancement has been revealed as compared with bulk crystals.

Probing semiconductor carrier kinetics with ultrafast nonlinear Faraday rotation

We demonstrate that in semiconductors with strong Coulomb exchange correlations, transient nonlinear Faraday rotation spectroscopy (FRS) more accurately probes the ultrafast carrier relaxation than the widely used differential transmission spectroscopy (DTS). We calculate nonlinear FRS and DTS by solving the six-band semiconductor Bloch equations for magnetic ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ and CdTe quantum wells. We show that for highly excited carriers, nonlinear FRS rotation spectra closely monitor the ultrafast thermalization of the carrier distribution, unlike DTS signals. We further show that the magnetic band tuning capabilities (giant Zeeman splitting) of dilute magnetic semiconductors allows one to investigate the electron kinetics in selective bands.

Optical studies of spin precession in magnetic two-dimensional electron gases

Magnetic two-dimensional electron gases are studied using time-resolved Kerr and Faraday rotation spectroscopy in the Voigt geometry. The data directly reveal both electron and Mn spin precession in modest transverse fields. Scattering by Mn ions dominates the electron spin relaxation processes in these materials, and prevents the electron gas from acquiring a long-lived spin polarization as observed in non-magnetic structures. Nonetheless, a persistent Mn spin polarization occurs which creates a oscillating magnetic field within the electron gas for hundreds of picoseconds.

Spin coherence in semiconductor heterostructures

The addition of the spin degree of freedom to the nonlinear optical processes creates interesting possibilities for spin optoelectronics. In this theory paper, we attempt to explain in simple terms the concepts of optical and spin coherence, how the simultaneous occurrence of both types of coherence can be brought about by suitable arrangement of the magnetic field and polarization of exciting coherent light, and how the time development is governed by the nonlinear response of the system. We explain under what circumstances the single-particle effects dominate and under what circumstances the interaction effects between two excitons come to the fore. We relate to experiments by a theoretical formulation of how these effects are seen in a particular nonlinear optical experiment, namely the time-resolved Faraday rotation spectroscopy.

An optical alternative to the hall test

Effective mass ratios, m*, of electrons in n-type InSb, GaAs, and near intrinsic and n-type Hg1−xCdxTe for 0.20 < × < 0.30 over the temperature range 77K < T < 296K were measured using Faraday rotation spectroscopy. m* ranged from 0.0186 to 0.0357 for InSb with carrier concentrations, N, in the range 1.76 < N < 110 × 1016 cm−3 at 296K, in good agreement with available values in the literature. Effective masses of HgCdTe were found to be about twice as large at room temperature as band edge effective mass, m*be calculations. These calculations can be corrected for thermal excitation by adding a factor, m**, to the band edge calculation: m* = m** m*be, where m** was found empirically to be m** = 4.52 × 10−3T + 0.78. The electron’s mobility is proportional to the ratio of the electron’s Faraday rotation to its absorption; that is, the absorption due to the intraband transitions of the electron itself, not the sample’s total absorption, which may include holes, interband transitions, and the like. The constant of proportionality, or the “mobility constant”, was measured in n-type GaAs and InSb doped above 18 × 1016 cm−3 using absorption directly. Both HgCdTe and InSb have large intrinsic carrier concentrations, on the order of 1016 cm−3. Hole absorption is the majority component of the sample’s absorption at lower n-type dopant concentrations. In these cases, the mobility constant was determined using an absorption cross section.

Time-resolved Faraday rotation spectroscopy of spin dynamics in digital magnetic heterostructures

A time-resolved resonant Faraday rotation spectroscopy is employed to study the dynamical interplay between local magnetic moments and photoexcited carrier spins in quantum-confined semiconductor geometries. This highly sensitive technique functions as an energy selective, noninvasive, all-optical probe of spin dynamics ranging from femtosecond to microsecond timescales and is particularly suited to low-dimensional systems having small numbers of magnetic spins. Carrier spin-scattering rates, lifetimes, and the orientation and relaxation of perturbed magnetic ions are directly observed in the time domain. The utility of this technique is demonstrated through the study of a newly developed class of magnetic heterostructure, in which fractional monolayer planes of magnetic Mn/sup 2+/ ions are incorporated digitally into nonmagnetic II-VI ZnSe-ZnCdSe quantum wells. These digital magnetic heterostructures (DMH) possess large g-factors and exhibit enormous low-field resonant Faraday rotations in excess of 1.7/spl times/10/sup 7/ deg/T/spl middot/cm at low temperatures. Time-resolved Faraday rotation measurements identify a wealth of unexpected electronic and magnetic spin dynamics that are different from those generated in traditional semiconductors or alloyed diluted magnetic semiconductor structures.

The influence on the Paschen-Back effect on magneto-optical rotation spectra

The authors report new measurements, by Faraday rotation spectroscopy, of relative f-values and multiplet splittings for high Rydberg states in the potassium spectrum. They show that, in general, it is necessary to take full account of the gradual transition from Zeeman to Paschen-Back coupling, via intermediate coupling conditions, in order to represent magneto-optical spectra correctly. When this is done, very good agreement between theory and experiment can be achieved over the full extent of the MOR profiles, and much useful information can be extracted, even for very high Rydberg states. The results are compared with recent theoretical predictions.

Magneto-optical properties of the Ca:(yttrium iron garnet) epitaxial-layer–substrate interface on gadolinium gallium garnet

Calcium oxide is a growth rate inhibitor of the liquid-phase epitaxy (LPE) of garnet layers. Calcium oxide additions to a melt for the growth of yttrium iron garnet (YIG) will reduce the epitaxial growth rate from its typical value of 1.0 to 0.1 μm/min [W. H. De Roode and J. M. Robertson, J. Cryst. Growth 63, 105 (1983)], allowing precise thickness control for the fabrication of optical waveguides and magnetic heterostructures. LPE growth of YIG from calcium-free melts produces a 0.1-0.5 μm interface layer between the YIG layer and the gadolinium gallium garnet (GGG) substrate. This interface is caused by a transient crystal growth that occurs before equilibrium epitaxy, and it consists of a nonstoichiometric layer composition that includes flux and impurity components. Calcium oxide addition would be especially useful if it reduced the thickness of the interface layer in proportion to its inhibition of growth rate. This, however, is not the case. A series of Ca:YIG layers was grown by LPE on GGG substrates in the thickness range 0.05–10 μm. Faraday rotation spectroscopy in the wavelength range 475–575 nm distinguished the layer, interface, and substrate contributions to the optical rotation and revealed the presence of a 0.25-μm-thick interface layer with properties characteristic of Pb-substituted YIG. Double-crystal x-ray diffraction confirmed the existence of the interface by the angular broadening of the layer diffraction and a shifting to lattice constants intermediate between YIG and GGG as the layer thickness decreased. The importance of this interface as a graded refractive index layer in optical waveguides should not be overlooked.

SEARCH FOR Fe4+ IN YIG : Ca GARNET FILMS

Spectral and temperature dependences of the Faraday rotation and X-ray photoemission spectroscopy of YIG: Ca garnet films have shown features characteristic for the presence of tetravalent iron ions in tetrahedral positions. The charge compensation of ca2+ by ~ e * + ions becomes saturated at 0.16 Ca per formula unit suggesting the existence of 0ions at higher calcium conte~ts.

Study of the collisional broadening of the bismuth line 648 nn hyperfine components by faraday rotation spectroscopy

The spootral profiles of the Faraday effect for the hyperfine multiplet4S3/2–2D5/2 λ648 nm in atomic bismuth have been measured accurately using the tunable dye-laser and computer controlled experimental apparatus. The collisional line broadening and shift rate constants are determined for perturbation by helium, nitrogen, argon and xenon. The more accurate value is obtained for the ratio of the electric quadrupole and the magnetic dipole amplitudes for this transition.

A differential detection scheme for Faraday rotation spectroscopy with a color center laser

A differential detection scheme to reduce source noise and to improve sensitivity based upon the Faraday effect has been developed. This method is particularly useful in connection with low-power, high-noise laser sources and multipass arrangements where a degradation of polarization quality is substantial. The theoretical background of the differential method is developed. It is tested with magnetic rotation signals from transitions of stable species with low modulation sensitivity and signals from free radicals.

Analytical and spectral features of atomic magneto-optical rotation spectroscopy (the atomic Faraday effect) of Sb, Bi, Ag and Cu with a hollow cathode lamp operated in a pulse mode

In order to increase source intensity, hollow cathode lamps were operated in a pulse mode and combined with instrumentation for Faraday or atomic magneto-optical rotation spectroscopy. The analytical and spectral features of this method were studied for the trace determination of elements Sb, Bi, Ag and Cu. Novel line crossings between the σ ±-components in the analytical line Bi I 306.772 nm were found from the dependence of the transmitted intensity on the magnetic field strength. This is related to the theoretically calculated Zeeman splitting pattern. The enhancement in the source radiance by the pulse mode gave an increase in the detection power by a factor of ten.