We report the detection of zero-bias photocurrents induced by interband excitation using oblique near-infrared light (lambda = 0.780 mu m) along the directions parallel and perpendicular to the helical axis in a millimeter-sized Te crystal. The photocurrent parallel to the helical axis exhibits a circular-polarization-dependent component, namely circular photogalvanic effect. We create a framework for estimating the gyrotropic photoconductivity tensor beta, the ratio between the circular-polarization-dependent photocurrent density and intensity of light, through which extrinsic beta values ranging from a few to a few tens of nA W-1 are obtained. Searching the beta values through existing literatures reveals that these values vary significantly with sample conditions, excitation wavelengths, and forms of crystallite samples. A particular interesting point found in our study is that the beta values for the interband excitation may be greater than those for intraband excitation as far as bulk Te is concerned.

We present the magneto-optical Kerr effect (MOKE) of (111)-oriented antiferromagnetic L12-Mn3Ir films epitaxially grown on MgO (111) substrates. We observed that the amplitude and sign of the polar MOKE change depending on the growth temperature. The Mn3Ir films grown at 800 and 600 °C have rotation angles of 41.6 and −4.6 mdeg and ellipticity angles of −15.3 and 9.1 mdeg, respectively. Residual strains owing to heteroepitaxial growth on the order of a few tenths of a percent can play a critical role in determining the amplitude and sign of the MOKE of a noncollinear antiferromagnet, unlike ferromagnets.

We present femtosecond-pulse-induced precession of magnetization at low laser fluence (<5μJ/cm2) regime as a function of magnetic field and laser fluence in three Co/Pd multilayer (ML) systems. These systems belong to three different regimes of magnetic anisotropy that vary with Co thickness (tCo): in-plane (sample 1, tCo = 0.74 nm), weakly out-of-plane (sample 2, tCo = 0.6 nm), and out-of-plane (sample 3, tCo = 0.40 nm). Interestingly, we observed that the precession amplitudes increase significantly with decreasing the Co layer thickness. In this study, the influence of various spin dynamics and static magneto-optical parameters on precession amplitude is examined critically and compared with a previously proposed analytical expression that connects those quantities. It is found that the enhancement of structural-dependent energy transfer efficiency between charge and spin subsystems is indeed responsible for the observed variations in precession amplitudes. On the basis of this fact, we discuss that the spin–orbit interaction that yields perpendicularly spin-polarized electrons in the MLs through the Co/Pd interface is responsible for the observed increase in precession amplitudes of locally excited magnetization. Our approach of employing low-fluence laser excitation of magnetization precession could be practical for developing a non-thermal, all-optical magnetic switching toward photonic memory applications.

Spin-photonic devices, represented by spin-polarized light emitting diodes and spin-polarized photodiodes, have great potential for practical use in circularly polarized light (CPL) applications. Focusing on the lateral-type spin-photonic devices that can exchange CPL through their side facets, this review describes their functions in practical CPL applications in terms of: (1) Compactness and integrability, (2) stand-alone (monolithic) nature, (3) room temperature operation, (4) emission with high circular polarization, (5) polarization controllability, and (6) CPL detection. Furthermore, it introduces proposed CPL applications in a wide variety of fields and describes the application of these devices in biological diagnosis using CPL scattering. Finally, it discusses the current state of spin-photonic devices and their applications and future prospects.

Co/Pd thin film multilayers show large Perpendicular Magnetic Anisotropy (PMA) which is useful in MRAM devices for perpendicular magnetic recording. Co/Pd systems have been studied extensively through the use of ultrafast optical pump-probe methods in order to measure the Time Resolved Photo-excited Precession of Magnetization (TRPEPM). Most studies have been conducted at high laser fluence (> 1 mJ/cm2), where heating near the curie temperature occurs. In this study, we present low fluence measurements between 0.42 to 3.14 μJ/cm2 in Co/Pd systems with differing Co thickness between 0.4 to 0.74 nm to probe the role of interface anisotropy in low-power excitation.

Form of an optical memory unit is illustrated schematically in Figs. 1(a) and (b); (a) Mach-Zehnder interferometer (WG-MZI) and (b) WG splitter/selector.

A waveguide-type Mach–Zehnder interferometer (WG-MZI) incorporating a magnetic layer on each of two WG branches is proposed for use in an all-optical memory cell. The key concept is switching the relative magnetization configuration between parallel and anti-parallel with photonic or electronic excitation, which alters the interference condition of the WG-MZI and thus varies the intensity of its output light. In the present work, photo-excited precession of magnetization is assumed as a trigger for magnetization switching. The magnitude of changes in the effective refractive index (ΔnM) in the WG region optically coupled with magnetization and the output light intensity (ΔI) are studied theoretically by model calculation based on magneto-optical (MO) phase delay, through which target and practical values of ΔnM and ΔI are examined. Experimental works on the preparation and characterization of ultra-thin Co/Pd multi-layers are described as a prototype magnetic system for the proposed optical memory due to their large photo-induced precession angle.

Subpicosecond pumping of ferromagnetic semiconductor InMnAs in the ranges of intra- and interband electronic transitions can result in an efficient demagnetization of the medium up to 60% of the initial magnetic moment. Here, we report about the efficiency of ultrafast demagnetization by a duo of terahertz and infrared pulses that trigger intra- and interband electronic transitions, respectively. Varying the intensities of the pulses and the delay between them, we study the degree of demagnetization caused by the pulse duo. It is shown that the result of the excitation does not depend on the pulse sequence. Our findings indicate that both intra- and interband electronic transitions result in ultrafast demagnetization of the semiconductor via the very same mechanism, which evolves at a ps timescale. Independent of the origin of the electronic transition, ultrafast demagnetization is a result of a temperature increase in the free charge carriers (holes).

Ultrafast laser excitation of the ferromagnetic semiconductor InMnAs is shown to trigger spin precession with the largest amplitude reported for magnetic semiconductors so far. To reveal the electronic transitions mediating the coupling between light and spins, we compared the spin dynamics triggered by short terahertz (photon energy 5 meV) and midinfrared (photon energy 500 meV) pulses. The experiments reveal that terahertz pump pulses excite qualitatively similar spin dynamics, but are 100 times more energy efficient than the mid-IR pulses. This finding shows that in a semiconductor with hole-mediated ferromagnetism intraband electronic transitions mediate ultrafast and the most efficient coupling between light and spins.

We report lateral-type, GaAs/AlGaAs double hetero-structure (DH) spin-LEDs with stripe Fe spin injectors that exhibit nearly pure circular polarization (CP) electroluminescence (EL) with current densities of J ~ 10 A/cm2 or less. Experiments with such spin-LEDs have made it possible to obtain experimental data that were not accessible in the first report [1,2]: namely, clear experimental proof for the presence of a threshold J value at which the annihilation of minority-helicity CPEL component takes place.

A model for lateral-type refracting-facet spin-photodiodes based on ferromagnetic metal-insulator-semiconductor (FM-I-S) junctions is described. The model utilizes spin and charge drift-diffusion equations and spin-dependent tunneling equations which are simultaneously solved numerically in order to obtain a self-consistent solution. The model is used to analyze and optimize the refracting-facet spin-photodiode structure. The relation between the active layer thickness and the effective lifetime of photo-generated electrons is investigated. Results show that the optimum active layer thickness depends on both effective lifetimes of photo-generated electrons and spins. The influence of empty density-of-state of ferromangetic metals is also explored.

The circular polarization (CP) of light scattered by biological tissues provides valuable information about the structural changes in tissues. We investigate the spatial discrimination of cancer using CP light scattering within the in-plane and along the depth direction. In-plane spatial resolution was investigated using experiments on sliced biological tissues, which show a noticeable difference in polarization values between healthy and cancerous parts in a wide angular range. The resolution in the depth direction is examined with the Monte Carlo calculation method on pseudo-tissues having thin cancerous layers on healthy tissues. The calculation results suggest that the thickness of cancer can be estimated by detecting the degree of circular polarization values with different detection angles. The in-plane and depth resolutions are approximately 0.3 mm and 0.6 mm, respectively.

Using a two-color time-resolved pump-probe spectroscopy scheme, we studied the generation and detection of longitudinal coherent acoustic phonons, generated by ultrafast laser pulses in a semiconductor heterostructure. Our structure was a $p$-doped, 100-nm-thick ferromagnetic GaMnAs layer grown on a GaAs substrate. By probing the transient reflectivity in the time domain, we observed a strong dependence of the coherent phonon's amplitude on the external magnetic field. Our theoretical model relates this dependence to the increase in the detectability of coherent phonons in the presence of external magnetic fields. This enhancement comes from the formation of Landau levels in the absorption spectrum and leads to large changes in the real and imaginary parts of the dielectric function with strain. When the probe laser energy is close to an allowed Landau-level transition, the detectability of the coherent phonons can be significantly enhanced. Our results suggest that not only can one increase the detection of coherent phonons with magnetic fields, one can also enhance the generation by tuning the wavelength of the pump laser pulse to coincide with a Landau-level resonance.

The circular polarization of light scattered by biological tissues provides valuable information and has been considered as a powerful tool for the diagnosis of tumor tissue. We propose a non-staining, non-invasive and in vivo cancer diagnosis technique using an endoscope equipped with circularly polarized LEDs (spin-LEDs). We studied the scattering process of the circularly polarized light against cell nuclei in pseudo-healthy and cancerous tissues using the existing Monte Carlo method. The calculation results indicate that the resultant circular polarizations of light scattered in pseudo tissues shows a clear difference in a wide range of detection angles, and the sampling depth depends on those detection angles. The structure of the endoscope probe comprising spin-LEDs is designed based on the calculation results, providing structural and depth information regarding biological tissues simultaneously.

Electroluminescence with nearly pure circular polarization (CP) at room temperature (RT) together with electrical helicity control [1, 2] has been demonstrated by a lateral-type spin-polarized light emitting diodes (LT-spin-LEDs) consisting of AlGaAs/GaAs double-heterostructures and the crystalline AlOx (x-AlOx) tunneling barrier [3]. In this LT-spin-LED, relatively high current density of J > 100 A/cm2 was required to achieve the circular polarization of P ~ 0.95. Operation with J > 100 A/cm2, however, often resulted in irreversible breakdown and short-lived spin-LED. In order to suppress this breakdown, we have studied fabrication of LT-spin-LED devices incorporating x-AlOx/AlAs hybrid tunneling barriers. With AlAs layers that are inserted between the x-AlOx layer and a top n-GaAs layer of LT-spin-LED, we aim at reinforcing electrical robustness of x-AlOx layers that are formed by oxidation of Al epilayers at RT. Nearly pure CP emissions (~ 0.92) are obtained from LT-spin-LED devices with hybrid tunneling barriers at J ~ 10 A/cm2. To our surprise, current density for pure CP emission decreases to about one-tenth and the yield of device fabrication is significantly improved (~ 5 % -> ~ 67 %). It is supposed that growth of AlAs layers prior to the formation of x-AlOx layers gives rise to improvement of crystalline quality of x-AlOx layers in terms of suppression of defects in the oxide layer and/or those across x-AlOx/AlAs/n-GaAs interfaces. [1] N. Nishizawa et al., PNAS 114, 1783 (2017). [2] N. Nishizawa et al., APEX 11, 053003 (2018). [3] N. Nishizawa et al., JAP 114, 033507 (2013).