Magneto-optical Absorption Research Articles

Nonlinear optical absorption via two-photon process in asymmetrical semi-parabolic quantum wells

The nonlinear optical absorption properties via two-photon process of an asymmetrical semi-parabolic quantum well (ASPQW) has been theoretically investigated. The analytical expression for the magneto-optical absorption coefficient (MOAC) is obtained by relating it to the transition probability for the absorption of photons. Meanwhile, the half width at half maximum (HWHM) of the resonant peaks is gained by the profile method. The numerical results are calculated for typical GaAs and GaAs/GaAlAs quantum wells. The obtained results show that MOAC and HWHM are strongly dependent on confinement frequency, magnetic field and temperature in both one- and two-photon processes. The increasing behavior of HWHM on temperature is generally in good accord with the previous reported ones, both theoretically and experimentally.

SA-phonon-assisted cyclotron resonance via two-photon process in graphene on GaAs substrate

The phonon-assisted cyclotron resonance (PACR) effect in a single-layer graphene on a GaAs substrate are theoretically studied via one and two-photon absorption processes. The result is limited by the combined interaction of the extrinsic potential of piezoelectric surface acoustic phonons of the substrate (PA) and of the intrinsic deformation potential of acoustic phonons of graphene (DA). The two-photon absorption process gives a significant contribution to magneto-optical absorption coefficient compared to one-photon process. It is shown that in the regime of low magnetic fields and temperatures the PA phonons dominate the magneto-optical absorption coefficient and half-width at half-maximum (HWHM). This finding would be useful to examine in subsequent research about HWHM in graphene. The present results have also demonstrated that the optical properties of graphene on a GaAs substrate can be controlled via extrinsic piezoelectric coupling.

Surface optical phonon-assisted cyclotron resonance in graphene on polar substrates

We theoretically study the influence of surface optical (SO) phonons on the phonon-assisted cyclotron resonance (PACR) effect in a single-layer graphene on different polar substrates via both one- and two-photon absorption processes. The two-photon absorption process gives a significant contribution to magneto-optical absorption coefficient (MOAC) compared to one-photon process. The shifts of the absorption peaks are larger for polar substrates than those in graphene on nonpolar substrates, where only the intrinsic optical phonons of graphene with higher energy contribute. Effects of temperature, graphene-substrate separation, and magnetic field on the magneto-optical absorption coefficient and the half-width are discussed. Our results provide a significantly quantitative picture for SO phonons interaction induced magneto-optical absorption in graphene on polar substrates.

Nonlinear phonon-assisted cyclotron resonance via two-photon process in asymmetrical Gaussian potential quantum wells

We theoretically study the combined effect of aluminum concentration and hydrostatic pressure on the nonlinear phonon-assisted cyclotron resonance (PACR) effect in asymmetrical Gaussian potential quantum wells (AGPQWs). The effect of confined and interface phonon modes on PACR is also studied. The analytical expression of the magneto-optical absorption coefficient (MOAC) is obtained by relating it to the transition probability for the absorption of photons. The numerical results are calculated for both bulk GaAs and GaAs/Ga1−xAlxAs materials. The obtained results show that the MOAC decreases in the magnitude and gives a red-shift as both concentration and pressure increase, while the half-width at half-maximum (HWHM) increases with concentration, but decreases with hydrostatic pressure in both one- and two-photon absorption processes. The aluminum concentration and hydrostatic pressure can be used to control the optical properties of the AGPQWs.

Interband magneto-spectroscopy in InSb square and parabolic quantum wells

We measure the magneto-optical absorption due to intersubband optical transitions between conduction and valence subband Landau levels in InSb square and parabolic quantum wells. InSb has the narrowest band gap (0.24 eV at low temperature) of the III–V semiconductors leading to a small effective mass (0.014 m0) and a large g–factor (−51). As a result, the Landau level spacing is large at relatively small magnetic fields (<8 T), and one can observe spin-splitting of the Landau levels. We examine two structures: (i) a multiple-square-well structure and (ii) a structure containing multiple parabolic wells. The energies and intensities of the strongest features are well explained by a modified Pidgeon-Brown model based on an 8-band k•p model that explicitly incorporates pseudomorphic strain. The strain is essential for obtaining agreement between theory and experiment. While modeling the square well is relatively straight-forward, the parabolic well consists of 43 different layers of various thickness to approximate a parabolic potential. Agreement between theory and experiment for the parabolic well validates the applicability of the model to complicated structures, which demonstrates the robustness of our model and confirms its relevance for developing electronic and spintronic devices that seek to exploit the properties of the InSb band structure.

Nonlinear phonon-assisted cyclotron resonance via two-photon process in parabolic quantum well

In this work, the combined effect of aluminum concentration, hydrostatic pressure, temperature, and confinement frequency of a parabolic quantum well (PQW) on the phonon-assisted cyclotron resonance (PACR) via two-photon absorption process under applied electric field is investigated theoretically. The analytical expression of the magneto-optical absorption coefficient (MOAC) is obtained by relating it to the transition probability for the absorption of photons. The numerical results are calculated for typical GaAs/GaAlAs quantum well. The obtained results show that the hydrostatic pressure, aluminum concentration, temperature, and confinement frequency have a significant effect on the MOAC as well as on the half-width. Moreover, it has been found that the optical properties of the GaAs/GaAlAs PQW can be modified by changing these parameters. This gives a new capacity for optical device applications.

Magneto-optical properties of ABC-stacked trilayer graphene.

The generalized tight-binding model is developed to investigate the magneto-optical absorption spectra of ABC-stacked trilayer graphene. The absorption peaks can be classified into nine categories of inter-Landau-level optical excitations, including three intra-group and six inter-group ones. Most of them belong to the twin-peak structures because of the asymmetric Landau level spectrum. The threshold absorption peak alone comes from a certain excitation channel, and its frequency is associated with a specific interlayer atomic interaction. The Landau-level anticrossings cause extra absorption peaks. Moreover, a simple relationship between the absorption frequency and the field strength is absent. The magneto-optical properties of ABC-stacked trilayer graphene are totally different from those of AAA- and ABA-stacked ones, such as the number, intensity and frequency of absorption peaks.

Emergence of novel phase of solid oxygen in ultrahigh magnetic Held

Faraday discovered the magnetism of molecular oxygen in 1850, and since then, oxygen has attracted significant interests as a ubiquitous but exotic molecular magnet. In solid oxygen, the magnetic energy through the exchange interaction is comparable to the van der Waals cohesive energy, resulting in a strong spin-lattice correlation. We reported the discovery of a magnetic-field-induced phase transition of solid oxygen using ultrahigh magnetic fields up to 186 T. This is the first experiments demonstrating the phase control through the spin by applying a magnetic field. Magneto-optical absorption measurements of the α phase at low temperatures revealed that visible light absorption suddenly ceases and the background transparency dramatically improves above the critical field. The large hysteresis in these data indicates a first-order transition with a relaxation time on the order of microseconds. The results strongly suggest the reconstruction of the O2 molecular geometry by the magnetic field, which causes that the antiferromagnetic phase collapses and the field-induced phase should have an isotropic crystal structure. Since all of the known phases of O2 are induced in high pressure, the discovery of a field-induced phase is noteworthy in the long history of solid O2 studies.

Magneto-optical absorption in semiconducting spherical quantum dots: Influence of the dot-size, confining potential, and magnetic field

Semiconducting quantum dots – more fancifully dubbed artificial atoms – are quasi-zero dimensional, tiny, man-made systems with charge carriers completely confined in all three dimensions. The scientific quest behind the synthesis of quantum dots is to create and control future electronic and optical nanostructures engineered through tailoring size, shape, and composition. The complete confinement – or the lack of any degree of freedom for the electrons (and/or holes) – in quantum dots limits the exploration of spatially localized elementary excitations such as plasmons to direct rather than reciprocal space. Here we embark on a thorough investigation of the magneto-optical absorption in semiconducting spherical quantum dots characterized by a confining harmonic potential and an applied magnetic field in the symmetric gauge. This is done within the framework of Bohm-Pines’ random-phase approximation that enables us to derive and discuss the full Dyson equation that takes proper account of the Coulomb interactions. As an application of our theoretical strategy, we compute various single-particle and many-particle phenomena such as the Fock-Darwin spectrum; Fermi energy; magneto-optical transitions; probability distribution; and the magneto-optical absorption in the quantum dots. It is observed that the role of an applied magnetic field on the absorption spectrum is comparable to that of a confining potential. Increasing (decreasing) the strength of the magnetic field or the confining potential is found to be analogous to shrinking (expanding) the size of the quantum dots: resulting into a blue (red) shift in the absorption spectrum. The Fermi energy diminishes with both increasing magnetic-field and dot-size; and exhibits saw-tooth-like oscillations at large values of field or dot-size. Unlike laterally confined quantum dots, both (upper and lower) magneto-optical transitions survive even in the extreme instances. However, the intra-Landau level transitions are seen to be forbidden. The spherical quantum dots have an edge over the strictly two-dimensional quantum dots in that the additional (magnetic) quantum number makes the physics richer (but complex). A deeper grasp of the Coulomb blockade, quantum coherence, and entanglement can lead to a better insight into promising applications involving lasers, detectors, storage devices, and quantum computing.

Impurity magneto-optical absorption with the participation of resonance states of D 2 − centers in quantum wells

The dependence of the average binding energy of the resonance g-state of a D2− center on the induction of an external magnetic field in a quantum well with a parabolic confining potential is studied using the zero-range potential method. It has been shown that with an increasing exchange interaction, the character of the dependence of the average binding energy of the resonance g-state of the D2− center on the induction of the external magnetic field changes. It has been assumed that in GaAs/AlGaAs quantum wells alloyed with small Si donors, resonance D2− states can exist under conditions of exchange interaction. It has been found that in spectra of impurity magneto-optical absorption in multiwall quantum structures, exchange interaction manifests itself as oscillations of interference origin.

Size effects on magneto‐optics in spherical quantum dots

The magneto-optical absorption in `spherical' quantum dots `completely' confined by a harmonic potential and exposed to an applied magnetic field in the symmetric gauge is investigated. This is done within the framework of Bohm-Pines' random phase approximation (RPA) that enables us to derive and discuss the full Dyson equation that takes proper account of the Coulomb interactions. Intensifying the confinement or magnetic field and reducing the dot-size yields a blue-shift in the absorption peaks. However, the size effects are seen to be predominant in this role. The magnetic field tends to maximise the localisation of the particle, but leaves the peak position of the radial distribution intact. The intra-Landau level transitions are forbidden.

Magnetic superfluid state in the frustrated spinel oxide CdCr2O4revealed by ultrahigh magnetic fields

We investigated the magnetic-field-induced phases of a typical three-dimensional frustrated magnet, CdCr${}_{2}$O${}_{4}$, in magnetic fields of up to 120 T using both the electromagnetic induction method and magneto-optical spectroscopy of the $d$-$d$ transitions and the exciton-magnon-phonon transitions. Anomalies were observed in the magneto-optical absorption intensity as well as the differential magnetization prior to a fully polarized magnetic phase, revealing a magnetic phase associated with changes in both the crystal and magnetic structures accompanied by the first-order phase transition. A magnetic superfluid state, such as an umbrella-like magnetic structure or a spin-nematic state, is proposed as a candidate for this phase, which is found universally in the series of chromium spinel oxides, $A$Cr${}_{2}$O${}_{4}$ ($A$ $=$ Zn, Cd, Hg).

Influences of Mechanical Deformation on Bilayer Graphene's Magneto-Optical Absorption Properties

The low-frequency magneto-optical absorption properties of bilayer Bernal graphene under uniaxial stress are investigated by using the gradient approximation. The magnetic field effectively condenses the electron states into discrete Landau levels, and the optical absorption spectra exhibit prominent peaks structures. There exists an optical selection rule for undeformed bilayer Bernal graphene due to the spatial symmetry of the electron wave functions. The stress distorts the wave functions, and destroys their spatial symmetry. The disruption of the spatial symmetry depends not only on the magnitude but also the direction of the strain. Therefore, the optical selection rule in bilayer Bernal graphene can be controlled by mechanical deformation. Many optical transitions violating the selection rule are allowed at sufficiently large strain. The variations of the absorption peak energies with the strain and the magnetic field strength are also explored. These theoretical predictions can be validated by opti...

Electric modulation effect on magneto-optical spectrum of monolayer graphene

The generalized tight-binding model is developed to largely reduce the numerical computation time in calculating optical properties. Modulated electric potentials can control the low-frequency magneto-optical absorption spectra of monolayer graphene. They induce the oscillatory energy dispersions of Landau subbands and the spatial symmetry breaking of the wave function; therefore, the original peaks and extra peaks of different selection rules come into existence simultaneously. Such peaks mainly arise from the band-edge states and/or the middle states. Their number, intensities, frequencies and structures are dominated by the modulation strength and period. More absorption peaks appear with an increase in potential. The extra peaks can relatively easily be observed for higher frequencies and stronger potentials. However, the absorption spectra remain unchanged for a fixed ratio of strength over period.

Generation of entangled photons in graphene in a strong magnetic field.

Entangled photon states attract tremendous interest as the most vivid manifestation of nonlocality of quantum mechanics and also for emerging applications in quantum information. Here we propose a mechanism of generation of polarization-entangled photons, which is based on the nonlinear optical interaction (four-wave mixing) in graphene placed in a magnetic field. Unique properties of quantized electron states in a magnetized graphene and optical selection rules near the Dirac point give rise to a giant optical nonlinearity and a high rate of photon production in the mid- or far-infrared range. A similar mechanism of photon entanglement may exist in topological insulators where the surface states have a Dirac-cone dispersion and demonstrate similar properties of magneto-optical absorption.

Theory of Triple Magnetopolarons in Quantum-Dimensional Nanostructures

A theory of triple magnetopolarons in an isolated quantum well in a strong magnetic field was developed. We study the behavior of the magnetooptical absorption peaks corresponding to the transitions of an electron at the Landau level with quantum numbers n ≥ 2. For n = 2 at the point of equality of cyclotron frequency and the frequency of optical phonon (LO), there is a cross of three terms of the electron-phonon system (the electron at the Landau level n = 2, the electron at n = 1, and the optical phonon and electron at n = 0 and two phonons), considered as a function of the cyclotron frequency. Interaction with phonons takes off the degeneracy of the terms and leads to three disjoint branches of the electron-phonon spectrum. The theory predicts that in the resonant magnetic field, the peak of magnetooptical absorption splits into three peaks, the intensity and position of which are dependent in a complex way on the magnitude of the magnetic field and the constant of the electron-phonon coupling.

Plasmonic enhancement of the magneto-optical response of MnP nanoclusters embedded in GaP epilayers

We report on the magneto-optical activity of MnP nanoclusters embedded in GaP epilayers and MnP thin film as a function of temperature, magnetic field, and wavelength in the near infrared and visible. The measured Faraday rotation originates from the ferromagnetic magnetization of the metallic MnP phase and exhibits a hysteretic behavior as a function of an externally applied magnetic field closely matching that of the magnetization. The Faraday rotation spectrum of MnP shows a magnetoplasmonic resonance whose energy depends on the MnP filling factor and surrounding matrix permittivity. At resonance, the measured rotary power for the epilayer systems increases by a factor of 2 compared to that of the MnP film in terms of degrees of rotation per MnP thickness for an applied magnetic field of 410 mT. We propose an effective medium model, which qualitatively reproduces the Faraday rotation and the magnetocircular dichroism spectra, quantitatively determines the spectral shift induced by variations in the MnP volume fraction, and demonstrates the influence of the shape and orientation distributions of ellipsoidal MnP nanoclusters on the magneto-optical activity and absorption spectra.

Magnetic Phases of ZnCr2O4Revealed by Magneto-Optical Studies under Ultra-High Magnetic Fields of up to 600 T

Faraday rotation and magneto-optical absorption spectral measurements were conducted on a spinel oxide, ZnCr 2 O 4 , a prototype of three-dimensional geometrically frustrated magnet. The measurements were carried out at temperatures down to 4.6 K under ultra-high magnetic fields of up to 600 T. The ultra-high magnetic fields were generated by the electro-magnetic flux compression method. We obtained a precise magnetization curve up to a fully polarized phase, where the phase transition takes place above 400 T. The experimental magnetization curves were compared with those obtained by Monte Carlo calculations with an effective spin model including spin–lattice coupling up to fully saturated magnetization. The absorption spectral peaks of the intra- d -band transitions in Cr 3+ ions as well as the exciton–magnon–phonon transition were used for monitoring the crystal and magnetic structures subjected to a strong external magnetic field. A novel magnetic phase was found prior to the fully polarized phase, whi...

Magnetic Phases of a Highly Frustrated Magnet,ZnCr2O4, up to an Ultrahigh Magnetic Field of 600 T

The Faraday rotation and magneto-optical absorption spectral measurements were conducted to reveal the full-magnetization process and map out a magnetic phase diagram of a typical geometrical frustrated magnet, ZnCr(2)O(4), by using the electro-magnetic flux compression method in ultrahigh magnetic fields up to 600 T. A fully polarized ferromagnetic phase is observed in which the absorption spectra associated with an exciton-magnon-phonon transition disappears. Furthermore, prior to the fully polarized ferromagnetic phase above 410 T, we found a novel magnetic phase above 350 T at 4.6 K followed by a canted 3∶1 phase.

Multicomponent magneto-optical conductivity of multilayer graphene on SiC

Far-infrared diagonal and Hall conductivities of multilayer epitaxial graphene on the C face of SiC were measured using magneto-optical absorption and Faraday rotation in magnetic fields up to 7 T and temperatures between 5 and 300 K. Multiple components were identified in the spectra, including (i) a quasiclassical cyclotron resonance, originating from the highly doped graphene layer closest to SiC, (ii) transitions between low-index Landau levels (LLs), which stem from weakly doped layers, and (iii) a broad optical absorption background. Electron- and hole-type LL transitions are optically distinguished and shown to coexist. An electron-hole asymmetry of the Fermi velocity of about 2% was found within one graphene layer, while the Fermi velocity varies by about 10% across the layers. The optical intensity of the LL transitions is several times smaller than is theoretically expected for isolated graphene monolayers without electron-electron and electron-phonon interactions.