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1
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Group Delay in Semiconductor Structures with Energy Dependent Effective Mass

100%
Kočinac S., Milanović V.
Acta Physica Polonica A
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2007
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vol. 112
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issue 5
1037-1042
EN
We investigate tunneling times of a particle with energy dependent effective mass for a one-dimensional real potential. General relations between phase, group and dwell times are obtained for a desired potential shape. For the textbook case of a real rectangular potential barrier the explicit relations for relevant times are derived, which reveal that the nonparabolicity, depending on the energy of incident particle, may substantially increase the group time in realistic structures. Further, we extend this theory to the case of absorptive media described by complex potentials, via introduction of a new absorptive tunneling time τ_a. Depending on whether the short wavelength or long wavelength limit is considered, maximization of τ_a results in a very different shape of a complex rectangular potential.
2
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Engineering and Advanced Digitalization of Photonic Structures with Bound Field in the Continuum

81%
Prodanović N., Milanović V., Radovanović J.
Acta Physica Polonica A
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2009
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vol. 116
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issue 4
607-610
EN
We describe a method for generation of complex optical potentials which support a bound state of the electric field in continuous part of the spectrum. It is based on deep analogy between quantum mechanical and electromagnetic phenomena and relies on the application of supersymmetric quantum mechanics to generate a smoothly varying complex optical potential, together with the corresponding electric field function for the (single) localized state. However, the obtained potential profile is generally a strongly oscillating function which requires additional processing to make it suitable for practical realization. With this goal in mind, i.e. the construction of a realizable photonic crystal with complex permittivity which supports one bound state in continuum, we have developed an original scheme of digital grading. It approximates the values of the complex relative permittivity in such manner that the final structure may be realized by assembling layers of homogeneous materials.
3
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Influence of the Goos-Hänchen Shift on Tunneling Times in Dispersive Nonlinear Media

71%
Ilić I., Beličev P., Milanović V., Radovanović J., Hadžievski L.
Acta Physica Polonica A
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2009
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vol. 116
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issue 4
638-641
EN
The Goos-Hänchen effect is an optical phenomenon defined as a displacement of the reflected beam of linearly polarized light after the total internal reflection, which occurs perpendicularly to the direction of propagation. Due to this effect, when an optical wave propagates through a barrier, tunneling times may change accordingly. In this paper, we consider the impact of the Goos-Hänchen effect on group delay and dwell time for electromagnetic wave propagating through a nonlinear dispersive slab placed inside linear dispersive surroundings. Numerical calculations are performed for the special case, namely a double negative index metamaterial embedded into a material with a saturable nonlinearity, when a background medium is vacuum. The numerical results for tunneling times are calculated when the Goos-Hänchen effect is observed. It is shown that this approach gives more accurate expressions for tunneling times when the angle of incidence has a non-zero value.
4
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Inter-Landau Level Scattering Processes in Magnetic Field Assisted THz Quantum Cascade Laser

71%
Radovanović J., Daničić A., Milanović V., Indjin D., Ikonic Z.
Acta Physica Polonica A
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2011
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vol. 120
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issue 2
227-230
EN
We present a detailed analysis of GaAs/AlGaAs terahertz quantum cascade laser in the presence of an intense external magnetic field. One of the objectives in further development of THz quantum cascade laser is the realization of structures operating at higher temperatures. This is difficult to obtain as the operating photon emission energy is smaller than the longitudinal-optical phonon energy in the semiconductor material. With increased temperature, electrons in the upper radiative state gain sufficient in-plane energy to emit an longitudinal-optical phonon, which represents a non-radiative scattering and reduces the optical gain. By applying strong magnetic field, two-dimensional continuous energy subbands become split into series of discrete Landau levels, and at particular values of B it is possible to quench these non-radiative channels. Numerical simulations are performed on two-well design quantum cascade laser operating at 4.6 THz, implemented in GaAs/Al_{0.15}Ga_{0.85}As, and the magnetic field is perpendicular to the epitaxial layers. Strong oscillations of carrier lifetimes for the upper state of the laser transition, as a function of magnetic field are observed, which can be attributed to interface roughness scattering and longitudinal-optical phonon scattering between Landau levels.
5
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Charge Carrier Transport in Quantum Cascade Lasers in Strong Magnetic Field

71%
Radovanović J., Milanović V., Indjin D., Ikonić Z., Harrison P.
Acta Physica Polonica A
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2011
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vol. 119
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issue 2
99-102
EN
We have developed a comprehensive rate equations based model for calculating the optical gain in the active region of a quantum cascade laser in magnetic field perpendicular to the structure layers, which takes into account all the relevant scattering channels. The model is applied to gain-optimized quantum cascade laser active region, obtained by a systematic optimization procedure based on the use of genetic algorithm, which we have previously set up for designing novel structures and improving performance of existing ones. It has proven to be very efficient in generating optimal structures which emit radiation at specified wavelengths corresponding to absorption fingerprints of particular harmful pollutants found in the atmosphere. We also illustrate another interesting prospective application of quantum cascade laser-type structures: the design of metamaterials with tunable complex permittivity, based on amplification via intersubband transitions. In this case, the role of the magnetic field is to assist the attainment of sufficient optical gain (population inversion), necessary to effectively manipulate the permittivity and fulfill the conditions for negative refraction (left-handedness).
6
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Quantum Cascade Laser Design for Tunable Output at Characteristic Wavelengths in the Mid-Infrared Spectral Range

71%
Daničić A., Radovanović J., Milanović V., Indjin D., Ikonić Z.
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issue 5
772-776
EN
We present a method for systematic optimization of quantum cascade laser active region, based on the use of the genetic algorithm. The method aims at obtaining a gain-maximized structure, designed to emit radiation at specified wavelengths suitable for direct absorption by pollutant gasses present in the ambient air. After the initial optimization stage, we introduce a strong external magnetic field to tune the laser output properties and to slightly modify the emission wavelength to match the absorption lines of additional compounds. The magnetic field is applied perpendicularly to the epitaxial layers, thus causing two-dimensional continuous energy subbands to split into series of discrete Landau levels. This affects all the relevant relaxation processes in the structure and consequently the lifetime of carriers in the upper laser level. Furthermore, strong effects of band nonparabolicity result in subtle changes of the lasing wavelength at magnetic fields which maximize the gain, thus providing a path for fine-tuning of the output radiation properties. Numerical results are presented for GaAs/Al_{x}Ga_{1-x}As based quantum cascade laser structures designed to emit at particular wavelengths in the mid-infrared part of the spectrum.
7
Content available remote

Time Delay in Thin Slabs with Kerr-Type Nonlinearity

71%
Radovanović J., Milanović V., Isić G., Ikonić Z., Indjin D.
Acta Physica Polonica A
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2007
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vol. 112
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issue 5
987-992
EN
In this paper we analyzed the following model: a thin slab with Kerr nonlinearity placed between two semi-infinite samples of linear and nonmagnetic materials. A general relation between the bidirectional group delay and the dwell time is derived for the thin slab. It is shown that the group delay is equal to the dwell time plus a self-interference delay. Particular attention is given to solving the Helmholtz equation for this case. Detailed and rigorous treatment revealed that the solutions of the Helmholtz equation are given via elliptic functions of the first kind. The boundary conditions at the interfaces are determined precisely. Finally, we provide an overall procedure for numerical calculation of the dwell times.
8
Content available remote

Time Delay in Thin Dielectric Slabs with Saturable Nonlinearity

71%
Beličev P., Ilić I., Radovanović J., Milanović V., Hadžievski L.
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EN
Time delays for an intense transverse electric wave propagating through a slab with saturable nonlinearity are investigated. The nonlinearity is assumed in a form of the Vinetskii-Kukhtarev model, which is relevant for the slabs made of nonlinear photorefractive crystals, such as GaAs and LiNbO_3, which feature a saturable nonlinearity. The expressions for the group delay and the dwell time are derived and the relation between them is studied. It is shown that the difference between them has three different contributions. The first one corresponds to the self-interference associated with the dispersion of the medium surrounding the slab. The other two appear due to the nonlinearity of the slab and oblique incidence of the transverse electric wave. All the results are compared with the case of dielectric slabs with cubic (Kerr) nonlinearity.
9
Content available remote

Spin Precession of Quasi-Bound States in Heterostructures with Spin-Orbit Interaction

62%
Isić G., Indjin D., Ikonić Z., Milanović V., Radovanović J., Harrison P.
Acta Physica Polonica A
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2009
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vol. 116
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issue 4
513-515
EN
We use a finite-difference model that is capable of describing the single state spin dynamics in a double-barrier AlGaAs heterostructure. The use of Green's functions enables a description of the double-barrier structure by a finite matrix while the interaction with contacts is described by appropriate self-energies. To account for interface roughness scattering, a self-energy Σp_{IR}(E, k) is derived within the random phase approximation. The dominant part is due to in-plane momentum relaxation while a smaller part describing spin-flip scattering is neglected. The former only decreases the state lifetime while the latter can also affect the spin precession frequency.
10
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Content available

Carrier Dynamics in Quantum Cascade Lasers

52%
Harrison P., Indjin D., Jovanović V., Mirčetić A., Ikonić Z., Kelsall R., McTavish J., Savić I., Vukmirović N., Milanović V.
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EN
A fully quantum-mechanical model for carrier scattering transport in semiconductor intersubband devices was applied to modelling of carrier dynamics in quantum cascade lasers. The standard model uses the envelope function and effective mass approximations to solve electron band structure under an applied bias. The k·p model has been employed in p-type systems where the more complex band structure requires it. The resulting wave functions are then used to evaluate all relevant carrier-phonon, carrier-carrier and alloy scattering rates from each quantised state to all others within the same and the neighbouring period. This piece of information is then used to construct a rate equation for the equilibrium carrier density in each subband and this set of coupled rate equations are solved self-consistently to obtain the carrier density in each eigenstate. The latter is a fundamental description of the device and can be used to calculate the current density and gain as a function of the applied bias and temperature, which in turn yields the threshold current and expected temperature dependence of the device characteristics. A recent extension which includes a further iteration of an energy balance equation also yields the electron (or hole) temperature over the subbands. This paper will review the method and describe its application to mid-infrared and terahertz, GaAs, GaN, and SiGe cascade laser designs.
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