Coupled Quantum Wells Structure Research Articles

Giant cross-Kerr nonlinearity in a four-level Y-type atomic system

We found the analytical expression for cross-Kerr nonlinear coefficient in a four-level Y-type atomic system. The analytical model is applied to 85Rb atoms and shown that under electromagnetically induced transparency, cross-Kerr nonlinear coefficient is enhanced by several orders of magnitude. At the same time, the amplitude and the sign of cross-Kerr nonlinear coefficient are controlled with respect to the intensity and the frequency of the coupling laser field. The analytical model can be useful to explain the experimental results and to study related effects in nonlinear optics. Full Text: PDF ReferencesC. Ottaviani, D. Vitali, M. Artoni, F. Cataliotti, P. Tombesi, "Polarization Qubit Phase Gate in Driven Atomic Media", Phys. Rev. Lett. 90, 197902 (2003). CrossRef C. Zhu, G. Huang, "Giant Kerr Nonlinearity, Controlled Entangled Photons and Polarization Phase Gates in Coupled Quantum-Well Structures", Opt. Express 19, 23364 (2011). CrossRef C. Hang, G. Huang, "Giant Kerr nonlinearity and weak-light superluminal optical solitons in a four-state atomic system with gain doublet", Opt. Express 18(3), 2952 (2010). CrossRef M. Fleischhauer, I. Mamoglu, and J. P. Marangos, "Electromagnetically induced transparency: Optics in coherent media", Rev. Mod. Phys. 77, 633 (2005). CrossRef H. Schmidt, And A. Imamogdlu, "Giant Kerr nonlinearities obtained by electromagnetically induced transparency", Opt. Lett., 21(23), 1936 (1996). CrossRef H. Kang And Y. Zhu, Phys. "Observation of Large Kerr Nonlinearity at Low Light Intensities", Rev. Lett., 91, 093601 (2003). CrossRef J. Kou, R. G. Wan, Z. H. Kang, H. H. Wang, L. Jiang, X. J. Zhang, Y. Jiang, and J. Y. Gao, "EIT-assisted large cross-Kerr nonlinearity in a four-level inverted-Y atomic system", J. Opt. Soc. Am. B. 27(10), 2035 (2010). CrossRef X. Yang, S. Li, C. Zhang, and H. Wang, "Enhanced cross-Kerr nonlinearity via electromagnetically induced transparency in a four-level tripod atomic system", J. Opt. Soc. Am. B. 26(7), 1423 (2009). CrossRef C. Ottaviani, D. Vitali, M. Artoni, F. Cataliotti and P. Tombesi, "Polarization Qubit Phase Gate in Driven Atomic Media", Phys. Rev. Lett. 90, 197902 (2003). CrossRef H. Sun, Y. Niu, S. Jin and S. Gong, "Phase control of cross-phase modulation with electromagnetically induced transparency", J. Phys. B: At. Mol. Opt. Phys. 40, 3037 (2007). CrossRef L.V. Doai, P.V. Trong, D.X. Khoa, and N.H. Bang, "Electromagnetically induced transparency in five-level cascade scheme of 85Rb atoms: An analytical approach", Optik, 125, 3666 (2014). CrossRef D. X. Khoa, P. V. Trong, L. V. Doai and N. H. Bang, "Electromagnetically induced transparency in a five-level cascade system under Doppler broadening: an analytical approach", Phys, Scr. 91, 035401 (2016). CrossRef D.X. Khoa, L.C. Trung, P.V. Thuan, L.V. Doai and N.H. Bang, "Measurement of dispersive profile of a multiwindow electromagnetically induced transparency spectrum in a Doppler-broadened atomic medium", J. Opt. Soc. Am. B 34 (6), 1255 (2017). CrossRef D. X. Khoa, L. V. Doai, D. H. Son, and N. H. Bang, "Enhancement of self-Kerr nonlinearity via electromagnetically induced transparency in a five-level cascade system: an analytical approach", J. Opt. Soc. Am. B., 31, 1330 (2014). CrossRef L.V. Doai, N.L.T. An, D.X. Khoa, V.N. Sau and N.H. Bang, "Manipulating giant cross-Kerr nonlinearity at multiple frequencies in an atomic gaseous medium", J. Opt. Soc. Am. B 36, 2856 (2019). CrossRef D. X. Khoa, L. V. Doai, L. N. M. Anh, L. C. Trung, P. V. Thuan, N. T. Dung, and N. H. Bang, "Optical bistability in a five-level cascade EIT medium: an analytical approach", J. Opt. Soc. Am. B, Vol. 33, 735 (2016). CrossRef N. T. Anh, L. V. Doai, and N. H. Bang, "Manipulating multi-frequency light in a five-level cascade-type atomic medium associated with giant self-Kerr nonlinearity", J. Opt. Soc. Am. B 35, 1233 (2018). CrossRef N. T. Anh, L. V. Doai, D. H. Son, and N. H. Bang, "Manipulating multi-frequency light in a five-level cascade EIT medium under Doppler broadening", Optik 171, 721 (2018). CrossRef D.A. Steck, Rb85 D Line Data: http://Steck.Us/Alkalidata/rubidium85numbers.pdf CrossRef

Large Rabi splitting in InGaN quantum wells microcavity at room temperature

Room-temperature luminescence of exciton polaritons was observed from InGaN/GaN quantum wells (QWs) sandwiched in Fabry–Perot (FP) microcavity (MC). Angle resolved photoluminescence measurements revealed unambiguous anti-crossing behavior, indicating strong coupling between excitons and photons. The Rabi splitting value varied from 40 meV–130 meV, depending on the excitation power, which was larger than previous publications. This large Rabi splitting value was ascribed to our high MC quality factor (Q > 3000) and large oscillator strength induced by coupled QWs structure. The sensitive dependence of the Rabi splitting in InGaN QWs on excitation power indicates that exciton-exciton scattering mechanism plays an important role in exciton- photon coupling.

Exciton states in asymmetric GaInNAs/GaAs coupled quantum wells in an applied electric field

The electronic and optical properties of exciton states in GaInNAs/GaAs coupled quantum well (CQW) structure have been theoretically investigated by solving the Schrödinger equation in real space. The effect of well width on the exciton states has been also studied by varying the well width from 5 nm to 10 nm in asymmetric structures. The electron, hole and exciton states are calculated in the presence of an applied electric field. It is found that there are two direct (bright) exciton states with the largest oscillator strengths. Their energies weakly depend on the electric field due to the compensation between the blue shift and red shift of the electron–hole pair states. In addition, these two states are overlap in the case of symmetric CQWs and one of them is then shifted to higher energy in asymmetric CQWs. The ground state exciton has the binding energy of approximately 7.3 meV and decrease to around 3.0 meV showing the direct to indirect transition of the ground state. The direct–indirect crossover is observed at different electric field for different structure. It happens at the electric field when the e1–e2 electron anticrossing or h1–h2 hole anticrossings is observed, so that the crossover can be controlled by the well width of CQWs structure.

Study of intersubband transitions in GaN-ZnGeN2 coupled quantum wells

In this work, we design and analyze a closely lattice-matched wide bandgap GaN-ZnGeN2 coupled quantum well (QW) structure targeting for near-infrared (IR) (λ ≤ 3 um) intersubband transition for quantum cascade laser applications. The coupled quantum well structure comprised two GaN wells separated by a thin ZnGeN2 barrier layer. The QW active region is surrounded by thick ZnGeN2 layers as barriers. The computations of the electron-phonon and electron-photon scattering rates are carried out by employing the Fermi's golden rule for transitions. The calculation takes into consideration the conservation of energy and momentum in scattering processes. The coupled QW structure is optimized through tuning the confined subband energy levels in the conduction band to achieve (1) electron-LO phonon resonant scattering when the energy separation between the first and second conduction subband levels matches the phonon energy of GaN (92 meV); and (2) dominant electron-photon transition in near-IR between the third and second conduction subband levels.

Direct and indirect excitons in semiconductor coupled quantum wells in an applied electric field

An accurate calculation of the exciton ground and excited states in AlGaAs and InGaAs coupled quantum wells (CQWs) in an external electric field is presented. An efficient and straightforward algorithm of solving the Schrodinger equation in real space has been developed and exciton binding energies, oscillator strengths, lifetimes, and absorption spectra are calculated for applied electric fields up to 100 kV/cm. It is found that in symmetric 8-4-8 nm GaAs/Al(0.33)Ga(0.67)As CQW structure, the ground state of the system switches from direct to indirect exciton at approximately 5 kV/cm with dramatic changes of its binding energy and oscillator strength while the bright excited direct-exciton state remains almost unaffected. It is shown that the excitonic lifetime is dominated either by the radiative recombination or by tunneling processes at small/large values of the electric field, respectively. The calculated lifetime of the exciton ground state as a function of the bias voltage is in a quantitative agreement with low-temperature photoluminescence measurements. We have also made freely available a numerical code for calculation of the optical properties of direct and indirect excitons in CQWs in an electric field.

Giant Kerr Nonlinearity, Controlled Entangled Photons and Polarization Phase Gates in Coupled Quantum-Well Structures

We study linear and nonlinear propagations of probe and signal pulses in a multiple quantum-well structure with a four-level, double Λ-type configuration. We show that slow, mutually matched group velocities and giant Kerr nonlinearity of the probe and the signal pulses may be achieved with nearly vanishing optical absorption. Based on these properties we demonstrate that two-qubit quantum polarization phase gates can be constructed and highly entangled photon pairs may be produced. In addition, we show that coupled slow-light soliton pairs with very low generation power can be realized in the system.

Effect of hydrostatic pressure on the donor impurity states in GaN/AlGaN asymmetric coupled quantum wells

Based on the effective-mass approximation, the ground-state donor binding energy in zinc-blende(ZB) GaN/AlGaN asymmetric coupled quantum wells(QWs) is investigated variationally, considering the hydrostatic pressure effect. Numerical results show that the donor binding energy is highly dependent on the impurity positions, the asymmetric coupled QWs structure parameters and the hydrostatic pressure. It is found that the donor binding energy increases with increase in the hydrostatic pressure for any impurity position. The hydrostatic pressure has an obvious influence on the donor binding energy of impurity localized inside the wide well of the asymmetric coupled QWs. For any hydrostatic pressure, our results show that the donor binding energy is distributed asymmetrically with respect to the center of the asymmetric coupled QWs. In particular, for the impurity located inside the wide well, the donor binding energy is insensitive to the middle barrier width in ZB GaN/AlGaN asymmetric coupled QWs if the middle barrier width is large.

Considerations on equilibration of two-dimensional excitons in coupled quantum wellstructures

In this paper we discuss the issues of the equilibration dynamics of spatially indirecttwo-dimensional excitons in a coupled quantum well structure, in particular those trappedby an in-plane harmonic potential trap. We discuss the relative timescales for severalprocesses. In the experiments with an in-plane trap, we can use the size of the excitoncloud as a measure of the temperature of particles. At low lattice temperatures the excitontemperature is higher than that of the lattice, even long after equilibrium hasobviously been reached. We discuss the effects that can be responsible for this.

Improved Push–Pull Polarization Modulation Using Coupled Quantum-Well Structures

Recently, we proposed a push-pull polarization-modulation scheme based on multiple repetitions of two InGaAlAs-InAlAs coupled quantum-well (CQW) structures. Additional numerical simulations showed one of the two CQW structures to be quite sensitive to layer thickness and compositional variations. Consequently, it would be challenging to reliably grow polarization modulators using this structure. Here, using the effective-mass envelope-function approximation, we design a more robust CQW structure to replace the sensitive one, and we also make improvements to the less sensitive of the two original structures

Design of polarization independent InGaAs/InGaAlAs coupled quantum well structure in 1.55 μm wavelength region

A novel polarization independent InGaAs/InGaAlAs quantum well (QW) structure in the 1.55 μm wavelength region is proposed. A coupled QW structure with tensile strain in the QW and/or barrier region is considered for the reduction of the optical gain difference between TE and TM modes in the wide spectral range. A triple-coupled QW structure with alternative strain (tensile/compressive/tensile) is found to be the most effective in reducing the polarization gain difference. This is because the transition strength difference of each polarization is reduced by energy states coupling. The optimized triple-coupled QW structure shows polarization independence for wide carrier density and wavelength range, which is suitable for polarization independent operation of QW based semiconductor devices, such as semiconductor optical amplifiers.

Electroabsorption in narrow coupled double quantum wells: Coulombic coupling effects

The effects of Coulombic coupling between different subband pairs on the electroabsorption spectra of narrow coupled double quantum wells (QWs) have been studied. It is shown via detailed comparison between electroabsorption spectra calculated using a full excitonic Green's function method, a decoupled excitonic Green's function method, a variational method, and experimental data that inclusion of the Coulombic coupling between different subband pairs is required for correct prediction of electroabsorption in the narrow well system. It is also shown that, due to Coulombic coupling, it is necessary to include unbound QW states, above the QW edge, in the simulation of electroabsorption and electrorefraction. These results are of particular significance for the accurate calculation of electrorefraction, by Kramers-Kronig transformation of QW electroabsorption spectra, in coupled QW structures containing narrow QWs.

SiGeC: Band gaps, band offsets, optical properties, and potential applications

Studying the structural and photoluminescence properties of pseudomorphic Si1−yCy and Si1−x−yGexCy multiple quantum well (QW) structures on (001) Si substrates offer a quantitative characterization of the band gap and band offset shifts caused by C alloying for y<3%. The main features of Si1−yCy alloys, which are a reduced lattice constant and a strong lowering of the conduction band energy, promise that C may serve as a counterpart to Ge in Si heteroepitaxy. The photoluminescent properties of Si1−yCy and SiGeC QWs are comparable to SiGe. Novel pseudomorphic Si1−yCy/SiGe coupled QW structures and Si1−yCy/Ge quantum dot structures result in a strong enhancement of the photoluminescent efficiency. The ternary SiGeC material system offers a higher degree of freedom in strain and band edge engineering of structures. We focus on our recent results on Si1−yCy and SiGeC QW layers embedded in Si concerning the growth by solid-source molecular beam epitaxy, structural properties, thermal stability, optical properties, and band offsets. The prospects of SiGeC alloys for realization of optoelectronic structures are discussed. First characteristics from 0.75 μm p-channel modulation-doped field-effect transistor devices containing an active SiGeC layer demonstrate good electrical properties.

Broadening of the absorption spectrum in a p-type InGaAs - InAlAs coupled quantum well

The intervalence subband absorption of normally incident infrared radiation in a p-type InGaAs - InAlAs coupled quantum well (CQW) is theoretically investigated by the multiband effective mass formalism. We calculate valence subband structures, intervalence subband transition matrix elements and the absorption coefficient spectrum in the CQW which consists of a wider well, a thinner well and a barrier between. We demonstrate that the absorption coefficient profile can be tailored to suit one's need by careful control of the flexible design parameters given to the valence band CQW structure. For the application to an infrared detector with a flat absorptivity over a wider wavelength range than the conventional detectors, the CQW is designed to have broadened absorption spectrum.

Very large blue and red Stark shifts of the excitonic transition in InGaAs-InP-InAsP antisymmetric coupled quantum wells

A new strained InGaAs-InP-InAsP antisymmetric coupled quantum well (CQW) structure with both very large blue and red quantum-confined Stark shifts for the first heavy-hole-to-electron excitonic transition, Ehh1fEe1, is studied theoretically in this paper. In the antisymmetric coupled quantum well, an antisymmetric-like pair of potential profiles between the shallow-deep conduction band profile and the deepshallow valence band profile are formed. The sub-band eigen-energies, E, and the associated envelope wave functions in the CQW structures with or without an applied electric field are calculated by the transfer-matrix method. The effect of strain on the pseudomorphic layers has been taken into account. Results indicate that the strained InGaAs-InP-InAsP antisymmetric CQW structure exhibits significant enhancement of the blue and red Stark effects in the Ehh1fEe1 transition. The influences of various antisymmetric CQW structural parameters, such as the total well width, the individual well width, the central barrier thickness and the composition of the strained layer on the quantum-confined Stark shift, as well as the envelope wave function overlap, are studied systematically. These strong Stark effects in the antisymmetric CQW structure may have potential applications in sophisticated new electronic devices, such as optical switching devices.

Strong blueshift of the excitonic transition in the InGaAs/InP/InAsP antisymmetric coupled quantum wells

A new strained InGaAs/InP/InAsP antisymmetric coupled-quantum-well (CQW) structure with significant enhancement of the blue and red Stark effects in the first heavy-hole-to-electron excitonic transition is proposed in this letter. The calculated amount of blueshift is about 48 meV as the applied electric field varied from 0 to 90 kV/cm and the red Stark shift of about 56 meV can be achieved with an applied electric field in the 0 to −90 kV/cm range. The results of the strong Stark effect in the antisymmetric CQW structure may have potential applications in sophisticated new electronic devices, such as optical switching devices and tunable lasers.

Electric-field dependence of optical absorption properties in coupled quantum wells and their application to 1.3 μm optical modulator

A 1.3 μm modulator using light-hole–to-electron interband Stark shift in the lattice-matched AlInAs/GaInAs coupled quantum wells (CQWs) is investigated theoretically. The operation of this device is based on the lowest-energy absorption resonance corresponding to the first light-hole–to–electron transition (ELh1→Ee1). The resonant nature of this process results in a sharp absorption peak when the incident photon energy is equal to the energy-level separation. This device utilizes the significant enhancement of the Stark effect on the electronic states and the strong field-dependence transition dipole moments. Under an applied electric field, the energy spacing between ELh1 and Ee1 changes due to the Stark shift. The contrast ratio can be improved from 8:1 for the symmetric CQW to as high as 20:1 for the proposed asymmetric CQW structure. These contrast ratios are achieved by varying the applied electric field in the 0–70 kV/cm range. This large variation of optical absorption at 1.3 μm is obtained both by the enhanced Stark shift and by varying the overlap between the hole and electron envelope wave functions with an applied electric field and Stark effect for the proposed AlInAs/GaInAs CQW system.

Enhancement of the Stark effect in AlInAs/GaInAs coupled quantum wells and their application to tunable midinfrared photodetectors

A tunable midinfrared photodetector using 1→4 intersubband Stark shift in the four-level AlInAs/GaInAs coupled quantum wells (CQWs) is proposed. The operation of this device is based on the infrared absorption by the electrons in the ground state transited from the ground-state subband E1 of the CQWs to the third-excited-state subband E4. A large variation of eigenenergy spacing ΔE41 between E1 and E4 under an applied electric field can be achieved for the proposed AlInAs/GaInAs CQW system. Since the infrared radiation is absorbed via the intersubband resonance absorption (ℏω=E4−E1), the detected infrared wavelength can be tuned by the spacing ΔE41 which can be adjusted by an applied electric field. The tunability of these AlInAs/GaInAs CQWs have been studied theoretically. Based on theoretical calculations, tuning ranges from 3 to 3.9 and 2.9 to 4.2 μm are predicted for the proposed asymmetric CQW structure and compositionally asymmetric CQW structure, respectively. This tuning capability is achieved by varying the applied electric field in the 90 to −90 kV/cm range.

Exciton Binding Energy as a Function of Barrier Width in Coupled Quantum Well Structures

Excitons in a coupled quantum well structure (CQWS), consisting of two wells separated by a thick barrier, and in the two boundaries, the potential being infinite, are studied. The binding energy is calculated as a function of the barrier width. It is shown that there is a maximum in the binding energy as a function of the barrier width for an asymmetric CQWS, whereas the binding energy is a monotonically decreasing function of the barrier width for a symmetric CQWS. Moreover, the binding energy in an asymmetric CQWS is larger than that in a symmetric CQWS.