Jeongsu Lee

Spin Modulation in Semiconductor Lasers

We provide an analytic study of the dynamics of semiconductor lasers with injection (pump) of spin-polarized electrons, previously considered in the steady-state regime. Using complementary approaches of quasistatic and small signal analyses, we elucidate how the spin modulation in semiconductor lasers can improve performance, as compared to the conventional (spin-unpolarized) counterparts. We reveal that the spin-polarized injection can lead to an enhanced bandwidth and desirable switching properties of spin-lasers.

Mapping between quantum dot and quantum well lasers: From conventional to spin lasers

We explore similarities between the quantum wells and quantum dots used as optical gain media in semiconductor lasers. We formulate a mapping procedure which allows a simpler, often analytical, description of quantum well lasers to study more complex lasers based on quantum dots. The key observation in relating the two classes of laser is that the influence of a finite capture time on the operation of quantum dot lasers can be approximated well by a suitable choice of the gain compression factor in quantum well lasers. Our findings are applied to the rate equations for both conventional (spin-unpolarized) and spin lasers in which spin-polarized carriers are injected optically or electrically. We distinguish two types of mapping that pertain to the steady-state and dynamical operation respectively and elucidate their limitations.

Tailoring chirp in spin-lasers

The usefulness of semiconductor lasers is often limited by the undesired frequency modulation, or chirp, a direct consequence of the intensity modulation, and carrier dependence of the refractive index in the gain medium. In spin-lasers, realized by injecting, optically or electrically, spin-polarized carriers, we elucidate paths to tailoring chirp. We provide a generalized expression for chirp in spin-lasers and introduce modulation schemes that could simultaneously eliminate chirp and enhance the bandwidth, as compared to the conventional (spin-unpolarized) lasers.

Pressure dependence of Bi-Sr-Ca-Cu-O phases with laser deposition technique

Superconducting Bi2Sr2Can−1CunOx (n=1,2,3) films, with the c‐axis perpendicular to the films, were fabricated by laser deposition. Oxygen pressure was found to be important to control the phases, and a unique pressure dependence of Bi‐Sr‐Ca‐Cu‐O phases was shown by these films. Tc of 76 K and Jc of 1.5×105 A/cm2 at 40 K were achieved for the in situ Bi2Sr2CaCu2Ox film. The critical‐current densities in the external magnetic field up to 5 T with the fields parallel and perpendicular to the c‐axis of the film were measured.

Effect of oxygen pressure on properties of BiSrCaCuO thin films grown by laser deposition

Abstract We present the results of a study of the effect of oxygen pressure on the properties of BiSrCaCuO (BSCCO) thin films grown by laser ablation. Films of the same phase, which were deposited at different oxygen pressures, show quite different properties. Here, we define the phases of BSCCO films from their c -axis lattice constants calculated from the 2θ X -ray diffraction angle of the 002 peak. In-situ films deposited at high oxygen pressure (≥ 100 mTorr) show higher critical temperatures and critical current densities. However, by post annealing, transport properties of films deposited at low oxygen pressure (several 10 mTorr) are improved significantly. With respect to this difference, the results of X-ray diffraction and energy dispersive X-ray spectroscopy studies, and Hall effect measurements are discussed.

Spin Twists in a Transistor

By using electron spin in a transistor, researchers have developed a new approach for transferring and processing information. Transistors are the centerpiece of conventional electronics, with two key features of switching and amplification. However, transistors relying on electron charge are oblivious to another property of electrons: their spin. In a simple picture, these spins are compass needles, aligned by a magnetic field. With different orientations, “up” and “down,” spin lends itself to encoding binary information as ones and zeroes, as explored in the field of spintronics (1, 2). Whereas harnessing spin for robust information storage in computer hard drives and magnetic random access memories has been very successful, delivering a spin transistor has been challenging (1). On page 324 of this issue, Betthausen et al. (3) describe a newly discovered spin-transistor action. Considering that the pioneering work on another spin transistor (4) waited two decades for experimental realization (5), it is a remarkable feat that the work of Betthausen et al. presents the experiment and theory for their spin transistor.

Nodal \ground states" and orbital textures in semiconductor quantum dots

Conventional understanding implies that the ground state of a nonmagnetic quantum mechanical system should be nodeless. While this notion also provides a valuable guidance in understanding the ordering of energy levels in semiconductor nanostructures, there are reports that {\em nodal} ground states for holes are possible. However, the existence of such nodal states has been debated and even viewed merely as an artifact of a

Superconducting behavior of YBa2Cu3O6.8/Bi2Sr2CanCun+1 multilayers

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Semiconductor Spin-Lasers

model. Using complementary approaches of both

Spintronics stretches its arms to lasers

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