Doyeol Ahn

Entanglement induced by a single-mode heat environment

A thermal field, which frequently appears in problems of decoherence, provides us with minimal information about the field. We study the interaction of the thermal field and a quantum system composed of two qubits and find that such a chaotic field with minimal information can nevertheless entangle the qubits which are prepared initially in a separable state. This simple model of a quantum register interacting with a noisy environment allows us to understand how memory of the environment affects the state of a quantum register.

Optical gain in a strained-layer quantum-well laser

The optical gain and the refractive index change of a uniaxially stressed GaAs-Al/sub 2/Ga/sub 1-x/As quantum-well laser is studied theoretically using the multiband effective mass theory (k-p method) and density matrix formalism with intraband relaxations. It is found that uniaxial strain of the quantum well substantially alters the subband structures and the optical gain of the quantum-well laser. In particular, the gain of the TM mode increases while the gain of the TE mode decreases with increasing stress. Thus, the threshold current either decreases or increases with the stress, depending on whether the laser is operating in a TM or TE mode. >

Optical gain and gain suppression of quantum-well lasers with valence band mixing

The effects of valence band mixing on the nonlinear gains of quantum-well lasers are studied theoretically for the first time. The analysis is based on the multiband effective-mass theory and the density matrix formalism with intraband relaxation taken into account. The gain and the gain-suppression coefficient of a quantum-well laser are calculated from the complex optical susceptibility obtained by the density matrix formulation with the theoretical dipole moments obtained from the multiband effective-mass theory. The calculated gain spectrum shows that there are differences (both in peak amplitude and spectral shape) between this model with valence band mixing and the conventional parabolic band model. The shape of the gain spectrum calculated by the new model becomes more symmetric due to intraband relaxation together with nonparabolic energy dispersions. Optical intensity in the GaAs active region is estimated by solving rate equations for the stationary states with nonlinear gain suppression. >

Electric field control of optical second‐harmonic generation in a quantum well

We present theoretical predictions of the second‐harmonic susceptibility due to an intersubband transition within the conduction band of a quantum well in an external applied electric field. The asymmetry of the quantum well due to the electric field accounts for the nonvanishing of the second‐order susceptibilities. It is shown that for moderate values of an applied electric field of 10–70 kV/cm, the second‐harmonic susceptibility is generally 10–100 times larger than that of bulk GaAs. Furthermore, this procedure of second‐harmonic generation can be controlled by an external modulating voltage.

Intraband relaxation time effects on non-Markovian gain with many-body effects and comparison with experiment

Momentum-dependent and independent intraband relaxation time effects on a non-Markovian (Gaussian line shape) many-body optical gain spectrum are presented. Our theoretical results are compared with experimental data as well as those obtained from a many-body gain model with a conventional Lorentzian line shape function. We show that a Gaussian line shape gain model with a constant intraband relaxation time gives good agreement with experimental gain spectra and the inclusion of k -dependent intraband relaxation time yields slightly improved agreement. In the case of a Lorentzian line shape function, it is found that the inclusion of the k -dependent carrier-carrier scattering in the intraband relaxation time is important to obtain good agreement with the experiment. This is because the Gaussian line shape function is steeper than the Lorentzian for a constant intraband relaxation time. The Gaussian line shape function with a constant intraband relaxation time requires less computational time than that with a k -dependent intraband relaxation time; therefore, it is an efficient model for comparison with experimental data.

Nonlinear intersubband optical absorption in a semiconductor quantum well

The third‐order nonlinear intersubband absorption in a semiconductor quantum well is studied theoretically using the density matrix formalism including intrasubband relaxation. It is shown that the peak absorption is reduced by half for an optical intensity 1 MW/cm2 for the well size L=126.5 A with 3.0×1016/cm3 electrons.

Spontaneous and piezoelectric polarization effects in wurtzite ZnO∕MgZnO quantum well lasers

Spontaneous and piezoelectric polarization effects on electronic and optical properties of ZnO∕MgZnO quantum well (QW) structures are investigated by using the non-Markovian gain model with many-body effects. The spontaneous polarization constant for MgO determined from a comparison with the experiment is about −0.070C∕m2, which is larger than the value (−0.050C∕m2) for ZnO. The negligible internal field effect observed in the case of ZnO∕MgZnO QW structures with relatively low Mg composition (x<0.2) and thin well width (Lw<46A) can be explained by the cancelation of the sum of piezoelectric and spontaneous polarizations between the well and the barrier. The ZnO∕MgZnO QW laser has much larger optical gain than the GaN∕AlGaN QW laser. This is attributed to the fact that the ZnO∕MgZnO QW structure has a larger optical matrix element due to the relatively small internal field, compared to the GaN∕AlGaN QW structure.

Optical transitions in a parabolic quantum well with an applied electric field—analytical solutions

Both interband and intersubband optical transitions in a parabolic quantum well with an applied electric field are investigated. The wave functions for the electrons and holes are those of the displaced harmonic oscillators, which are shifted in opposite directions. The dipole moment matrices, the absorption coefficients, and the changes in the refractive indices are evaluated with the intrasubband relaxations taken into account. Interesting electroabsorption and electro‐optic effects are presented analytically and numerically.

Macromodeling of single-electron transistors for efficient circuit simulation

In this study, the possibility of compact modeling in single-electron circuit simulation has been investigated. It is found that each Coulomb island in single-electron circuits can be treated independently when the interconnections between single-electron transistors are large enough and a quantitative criterion for this condition is given. It is also demonstrated that, in those situations, SPICE macromodeling of single-electron transistors can be used for efficient circuit simulation. The developed macromodel produces simulation results with reasonable accuracy and with orders of magnitude less CPU time than usual Monte Carlo simulations.

Valence‐band mixing effects on the gain and the refractive index change of quantum‐well lasers

The effects of valence‐band mixing on the gain and on the refractive index change of the quantum‐well laser and the effect of an applied electric field perpendicular to the quantum wells for gain switching are studied theoretically. Our calculations are based on the multiband effective‐mass theory (k⋅p method) and the density‐matrix formalism with the intraband relaxation taken into account. First, we calculate the nonparabolic valence‐band structure by the finite difference method after making a unitary transformation of the Luttinger‐Kohn Hamiltonian. The calculated gain for our model shows remarkable differences in both spectral shape and peak amplitude as compared with those for the conventional model of the parabolic valence band. The peak gain is reduced considerably and the gain spectrum is more symmetric in our model compared with that for the conventional model. The refractive index change shows a negative increment in the active region for both the TE and TM polarizations resulting in the antigu...