Intersubband Optical Transitions Research Articles

Theoretical analysis of gate voltage-controlled subband states in an Al xGa 1−xN/GaN heterostructure

The subband structure and inter-subband transition as a function of gate voltage are determined by solving the Schrödinger and Poisson equations self-consistently in an Al x Ga 1− x N/GaN heterostructure. Different aluminum mole fraction and thickness of Al x Ga 1− x N barrier are considered. Calculation results show that energy difference between the first and second subband covers a wide range (from several tens to hundreds milli-electron volt) by applying different gate voltage, which corresponds to the midinfrared and long-wave infrared wavelength scope. Furthermore, such a modulation on the subband transition energy is much more pronounced for the structure with thin barrier. When the applied positive gate voltage is increased, the triangle well formed at the interface turns to be deeper and narrower, which enhances the confinement for electrons. As a result, the overlap between electron wave function at two subbands increases, and thus the optical intersubband transition also enhances its intensity. This tendency is in good agreement with the available data in the literature.

Near-infrared intersubband transitions in InGaAs–AlAs–InAlAs double quantum wells

Intersubband optical transitions at short wavelengths in strain-compensated In0.70Ga0.30As—AlAs double quantum wells are investigated by means of mid-infrared absorption. Trade-offs between achieving a high transition energy and a large oscillator strength of the two highest-energy intersubband transitions using our strain-compensation approach are analyzed as a function of the widths of the two wells. Two design strategies leading to relatively strong intersubband optical transitions at 800 meV, 1.55μm, are described and the corresponding structures grown using gas-source molecular-beam epitaxy on (001)InP are investigated. The strongest intersubband transitions obtained experimentally are generally between 300 and 600 meV, 2–4μm. Significant oscillator strength, however, also extends out to 800 meV, 1.55μm.

Optical intersubband transitions in quantum wires with an applied magnetic field

Optical intersubband transitions in quantum wires with an applied magnetic field

INTERSUBBAND OPTICAL ABSORPTION IN QUANTUM WELLS UNDER APPLIED ELECTRIC AND INTENSE LASER FIELDS

Within the framework of the effective mass approximation, we have theoretically investigated the linear intersubband optical absorption in a quantum well under external electric and intense laser field. Results obtained show that intersubband optical transition and energy levels in quantum wells can significantly be modified and controlled by the electric field and intense laser field. Generally there is a distinct feature for the case of the intersubband absorption compared with intersubband optical absorption in a quantum well with an applied electric field.

In-plane anisotropy of quantum elliptic heterostructures studied with symmetry-adaptedMathieu functions: an application to self-organized InAs quantum dots on InP

The problem of quantum-confined heterostructures with elliptic shape is studied withelliptic cylinder coordinates and a basis of symmetry-adapted Mathieu functions.The conventional effective mass Hamiltonian is written in two-dimensional andthree-dimensional geometries. The problem is not separable in radial-like and angular-likecoordinates. A variational method using a basis of symmetry-adapted Mathieu functions isproposed. Energy splitting, optical intersubband and interband transition spectra, andpolarization effects are analysed. A calculation for self-organized InAs and InAs/InAlAsquantum dots on InP is performed. Various island shapes and morphologies are observedfor such quantum dots, which may be important for potential applications in fibre optictelecommunication systems.

Intersubband optics in parabolic quantum dots

The infrared intersubband optical properties of parabolic cylinder and parabolic rotational quantum dots are studied using a single-band effective-mass theory. This allows us to study the impact of shape and size on the transition energies and polarization selection rules. Contrary to earlier work on spheroidal dots, we predict that the change in the polarization of intersubband optical transitions as a function of the parabolic quantum dot height and cross section is here due to level crossing.

Intersubband absorption in V-groove quantum wires

This article describes intersubband optical transitions in V-groove quantum wires and the resulting absorption. It is demonstrated that the optical absorption spectrum can be modified and controlled by varying the quantum wire dimensions. In addition, the importance of the conduction band nonparabolicity in calculating the absorption spectrum is pointed out. Finally, a possible application of this structure as a converter to the far-infrared wavelength range is discussed.

Midinfrared intersubband absorption in strain-compensated InGaP/InGaAs superlattices on (001) GaAs

Intersubband optical transitions in strain-compensated In0.32Ga0.68As–In0.32Ga0.68P superlattices grown using gas-source molecular-beam epitaxy on (001)GaAs are investigated by means of midinfrared absorption and low-temperature photoluminescence. Strong absorption corresponding to the transition from the first to second electronic subband is measured at wavelengths between 5.6 and 10.5 μm. The data indicate that the conduction band offset between the strained In0.32Ga0.68As and the strained In0.32Ga0.68P is 370 meV and the electron effective mass in the strained In0.32Ga0.68As well is 0.060m0. This material system is an interesting GaAs-based candidate for applications in midinfrared intersubband emitters and detectors.

Resonant second-order nonlinear optical processes in quantum cascade lasers.

We demonstrate an efficient intracavity nonlinear interaction of laser modes in a specially adapted quantum cascade laser. A two-wavelength quantum cascade laser structure emitting at wavelengths of 7.1 and 9.5 micrometer included cascaded resonant optical intersubband transitions in an intracavity configuration leading to resonantly enhanced sum-frequency and second-harmonic generation at wavelengths of 4.1, 3.6, and 4.7 micrometer respectively. Laser peak optical powers of 60 and 80 mW resulted in 30 nW of sum-frequency signal and 10-15 nW of second-harmonic signal, both in good agreement with theoretical calculations.

Self-assembled quantum dots: A study of strain energy and intersubband transitions

In this article we examine the strain energy and intersubband optical transitions in self-assembled dots on GaAs and InP substrates. On the GaAs substrate, in addition to the InAs/GaAs dots we examine strain compensated InAs/GaAsP dots on GaAs substrates. We find that the strain energy configuration profile shows that there is preference for certain dot sizes and shapes. Our calculated dot sizes agree well with experimental observations. We find that the addition of phosphorus in the covering matrix reduces the total strain energy of the system with little effects on the intersubband transition strength for the vertical incident light. The reduced strain energy should allow one to incorporate a large number of dot array stacks for devices such as lasers and detectors and thus increases the optical responses. Our studies for the InAs/InP system show that due to the lower strain mismatch there is no particular preference for dot sizes. The optical response for intersubband transitions is weaker and occurs at longer wavelengths in comparison to the InAs/GaAs dots.

Theoretical investigation of intersubband hole transitions in Si/SiGe/Si quantum wells

We study the effects of the pc–pc coupling in intersubband hole optical transitions in SiGe/Si quantum wells for x and z polarizations. We have used a k⋅p model taking into account both the p-like first conduction band and the s-like second conduction band. First, we have found a unitary transformation that block diagonalizes the 14×14 Hamiltonian (or 12×12 Hamiltonian) into two 7×7 (or 6×6) blocks that are real symmetric in the finite difference formulation. We find that pc–pc interaction plays a minimal role in intersubband optical transition for x and z polarizations. Moreover, our calculations clearly confirm that the pc–pv coupling favors intersubband transitions for an optical polarization parallel to the layer plane (x polarization). In addition, for z polarization, both s–pv and pc–pv interactions play an equal footing role in intervalence band transitions.

Digitally graded active region for optically pumped intersubband lasers and nonlinear wavelength convertors

A strategy is proposed for the realization of quantum-well structures optimized for devices based on intersubband optical transitions. It relies on the recently established techniques for the design of optimal smooth quantum-well profile by using the inverse spectral theory, followed by the design of an (almost) equivalent digitally graded structure, comprising just two different alloy compositions. Digital grading greatly simplifies the structure growth while essentially fully retaining the properties achieved in smoothly graded optimized structures. Example designs are presented for an optically pumped quantum-well laser and for a quantum well intended for the second-harmonic generation.

Optical intersubband transitions and femtosecond dynamics in Ag/Fe(100) quantum wells.

The optical intersubband transitions and femtosecond dynamics of electrons in quantum well states in Ag/Fe(100) are investigated by interferometric time-resolved two-photon photoemission. The quantum well wave functions and transition probabilities are evaluated from the two-photon photoemission resonance energies and intensities using an extended phase accumulation model. Direct femtosecond pump-probe correlation measurements elucidate the importance of interfaces in confined structures.

Ultra-broadband semiconductor laser.

The fundamental mechanism behind laser action leads in general only to narrowband, single-wavelength emission. Several approaches for achieving spectrally broadband laser action have been put forward, such as enhancing the optical feedback in the wings of the gain spectrum, multi-peaked gain spectra, and the most favoured technique at present, ultrashort pulse excitation. Each of these approaches has drawbacks, such as a complex external laser cavity configuration, a non-flat optical gain envelope function, or an inability to operate in continuous mode, respectively. Here we present a monolithic, mid-infrared 'supercontinuum' semiconductor laser that has none of these drawbacks. We adopt a quantum cascade configuration, where a number of dissimilar intersubband optical transitions are made to cooperate in order to provide broadband optical gain from 5 to 8 microm wavelength. Laser action with a Fabry-Pérot spectrum covering all wavelengths from 6 to 8 microm simultaneously is demonstrated with this approach. Lasers that emit light over such an extremely wide wavelength range are of interest for applications as varied as terabit optical data communications or ultra-precision metrology and spectroscopy.

Enhanced intraband transitions with strong electric-field asymmetry in stacked InAs/GaAs self-assembled quantum dots

Electronic properties and conduction intersubband optical transitions in vertically aligned double InAs/GaAs self-assembled quantum dots (SAD's) are studied theoretically by using an eight-band strain-dependent $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Hamiltonian. Because of the strain field extension into the barrier region between the stacked double SAD's, the conduction-valence band mixing induces a very different composition of the basis $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ wave functions in the conduction band states from those in single dots. This effect enhances the oscillator strengths of the transitions between the ground 1s states and excited 2p states in conduction band compared with single SAD's. We also show that the asymmetric dependence of the composition of the conduction band states on the applied longitudinal electric field would induce a strong field asymmetry of intraband transitions.

Infrared absorption inSi/Si1−xGex/Siquantum wells

The infrared intersubband optical transitions in SiGe/Si quantum wells is theoretically examined. We have used the $8\ifmmode\times\else\texttimes\fi{}8,$ $12\ifmmode\times\else\texttimes\fi{}12,$ and $14\ifmmode\times\else\texttimes\fi{}14$ $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Hamiltonians taking into account both the p-like first conduction band and the s-like second conduction band to calculate wave functions and energy dispersion of the valence band of ${\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}}_{0.8}{\mathrm{Ge}}_{0.2}/\mathrm{Si}$ quantum wells. We discuss intersubband absorption in the valence band and we show that the p-p interaction favors intersubband transitions for an optical polarization parallel to the layer plane $(x$ polarization). For z polarization, both s-p and p-p interactions play the same footing role in intervalence band transitions.

Enhanced intraband Stark effects in stacked InAs/GaAs self-assembled quantum dots

We present a theoretical study of the electronic properties and intersubband optical transitions in vertically aligned double InAs self-assembled quantum dots (QDs) which are subject to an electric field along their growth axis. The electron properties are calculated as a function of the applied electric field by using an eight-band strain-dependent k⋅p Hamiltonian. Transitions between ground s states and excited p states are found to be almost three times stronger than in single dot, with strong field anisotropy. The system also exhibits field tunable transitions between the bonding and antibonding s states, with polarization along the growth axis. Midinfrared photodetectors consisting of vertically coupled double-quantum-dot layers are expected to exhibit enhanced sensibility and voltage tunability, compared to devices using single-quantum-dot layer.

Dipole matrix elements of semiconductor intersubband quantum structures

A method for determining the dipole matrix element for an intersubband optical transition in multi-layered semiconductor quantum heterostructures is presented. The single-band effective-mass Schrödinger equation is solved by employing the argument principle method (APM) to extract the bound (B) and quasibound (QB) eigenenergies of the quantum heterostructure. The major types of optical transitions involving bound and QB states are defined and the corresponding dipole matrix elements are calculated for each type. The method presented incorporates the energy-dependent effective mass of electrons arising from conduction-band nonparabolicity. The performance and the accuracy of the method are evaluated for an asymmetric Fabry–Perot electron wave interference filter. The physical dimensions of the filter are varied to show their effect on the dipole matrix elements. Results with and without nonparabolic effects are presented and compared. Dipole matrix elements are also calculated for the filter with an applied electric field bias. In this case the eigenstate wavefunctions can be expanded as linear combinations of Airy and complementary Airy functions. In addition, results from the present method are compared to a Kronig–Penney and a multi-band model. The dipole matrix element values calculated by the present method are shown to be in excellent agreement with the values obtained from these models. Further, the present model is numerically efficient and easily implemented.

Intersubband absorption in Pöschl–Teller-like semiconductor quantum wells

The bound–bound, bound–free and free–free intersubband optical transitions are considered in semiconductor quantum wells derived from the Pöschl–Teller potential with a constant electron mass. To make this type of Hamiltonian effectively realizable in ternary (Al x Ga 1− x As) alloys, where the effective electron mass necessarily varies, just as does the potential, the coordinate transform method was employed to design variable-mass variable-potential structures realizable in Al x Ga 1− x As, with their Hamiltonians fully equivalent to the constant-mass Pöschl–Teller case. Applications of this special QW profile for photodetectors operating in the infrared are analyzed and the results are compared to those obtained in the simple square wells.

Time-resolved intersubband optical transitions in resonantly optically pumped semiconductor lasers

Using picosecond visible-pump mid-infrared-probe technique, we directly measured the photoexcited electron dynamics in the first conduction subband of a GaAs/AlGaAs quantum well laser, below and above its threshold. Our results clearly show that even at low temperatures, under optical excitation into the lowest heavy hole exciton resonance, GaAs/GaAlAs quantum well lasers lase in accordance with the common two-plasma model and not from an excitonic phase.