Intersubband Light Research Articles

Intersubband optical absorption in Gaussian GaAs quantum dot in the presence of magnetic, electrical and AB flux fields

We theoretically study the spectral properties of single electron two-dimensional (2D) Gaussian quantum dot (GQD) in the presence of applied magnetic, electrical field and along with an Aharonov–Bohm (AB) flux field. We have calculated the exact solutions for the normalized wave functions and energy levels by using the Nikiforov–Uvarov (NU) method within the effective-mass approximation and compared the results with parabolic potential (PP) model. Based on the calculated energy spectrum and the wave function, we have obtained the intersubband light absorption coefficient (K(ϖ)) and the value of absorption threshold frequency (ϖ). The main and important object of the present work is to study the effect of the GQD size (R) and the strength of the potential (V0) on the energetic spectrum and the absorption threshold frequency (ϖ). According to the present work results, the ground state (GS) energy and the (ϖ) shows that the size of the QD, depth of the potential and electric field plays an important role.

Determination of sign during phase correction of sign-variable modulation spectra of intersubband light absorption in GaAs/AlGaAs quantum wells

Modulation of intersubband light absorption in GaAs/AlGaAs quantum wells under external impact may contain different sign peaks. In this work, we show that fast Fourier transform may give mistakes in negative modulation intensity during the phase correction of obtained interferogram of absorption modulation. We have demonstrated that information about the spectrum sign could be extracted from the phase spectrum. Sign mistake could be eliminated using the phase correction with modified phase spectra during the fast Fourier transform.

Electric field influence on mid-infrared absorption and interband photoluminescence in tunnel-coupled GaAs/AlGaAs quantum wells

The influence of transverse and lateral electric field on the mid-infrared intersubband light absorption is experimentally investigated in tunnel-coupled GaAs/AlGaAs quantum wells. Observed absorption modulation in electric field is related to the electron redistribution between the quantum well states resulting in variation of space charge in the structure. This phenomenon in transverse electric field is connected with a change of potential profile, whereas in lateral field effect is caused by electron heating. Electron heating also leads to a modulation of near-infrared interband photoluminescence under lateral electric field.

Temperature shift of intraband absorption peak in tunnel-coupled QW structure

An experimental study of the intersubband light absorption by the 100-period GaAs/Al0.25Ga0.75As double quantum well heterostructure doped with silicon is reported and interpreted. Small temperature redshift of the 1–3 intersubband absorption peak is detected. Numerical calculations of the absorption coefficient including self-consistent Hartree calculations of the bottom of the conduction band show good agreement with the observed phenomena. The temperature dependence of energy gap of the material and the depolarization shift should be accounted for to explain the shift.

Intersubband light absorption by holes in InAsSb/AlSb quantum well heterostructures

The absorption coefficients of intersubband optical transitions in the valence band of the AlSb/InAs0.84Sb0.16/AlSb quantum wells in framework of the four-band Kane model. It is established that the light absorption by holes may lead to the laser generation breakdown. It is shown that we need to decrease the quantum well width to range a<6 nm to neutralize the negative influence of this effect.

Modulation of intersubband light absorption and interband photoluminescence in double GaAs/AlGaAs quantum wells under strong lateral electric fields

The effect of a lateral electric field on the mid-infrared absorption and interband photoluminescence spectra in double tunnel-coupled GaAs/AlGaAs quantum wells is studied. The results obtained are explained by the redistribution of hot electrons between quantum wells and changes in the space charge in the structure. The hot carrier temperature is determined by analyzing the intersubband light absorption and interband photoluminescence modulation spectra under strong lateral electric fields.

SiGe Nanomembrane Quantum-Well Infrared Photodetectors

SiGe quantum wells are promising candidates for the development of intersubband light emitters and photodetectors operating at mid- and far-infrared wavelengths. By virtue of their inherent compatibility with the Si microelectronics platform, these devices may be integrated seamlessly within complex optoelectronic systems for sensing and imaging applications. However, the development of high-quality SiGe intersubband active layers is complicated by the large lattice mismatch between Si and Ge, which limits the number of quantum wells that can be grown on bulk Si substrates before the onset of structural degradation due to inelastic strain relaxation. To address this issue, we investigate the use of lattice matched growth templates consisting of quantum-well nanomembrane stacks that were at one point free-standing, allowing for the internal stress to be relaxed via elastic strain sharing rather than defect formation. SiGe quantum-well infrared photodetectors (QWIPs) based on this approach are developed and characterized. Efficient current extraction from these ultrathin devices is obtained by bonding the nanomembranes directly on a doped Si substrate. Pronounced photocurrent peaks at mid-infrared wavelengths are measured, with improved responsivity compared to otherwise identical devices grown simultaneously on the supporting Si substrate.

Optical properties of GaAs/AlGaAs double quantum wells in lateral electric field

The influence of the lateral electric field on the mid-infrared intersubband light absorption and near-infrared interband photoluminescence is experimentally investigated in tunnel-coupled GaAs/AlGaAs quantum wells. Absorption and emission modulation in lateral electric field are related to the electron heating and redistribution of hot electrons between the quantum well states resulting in variation of the space charge in the structure.

Intersubband light absorption in double GaAs/AlGaAs quantum wells under lateral electric field

Spectra of variation of intersubband light absorption induced by lateral electric field and lattice temperature change were studied in the structure with double GaAs/AlGaAs quantum wells. The calculated positions of energy levels have been verified with interband photoluminescence measurements. The change in intersubband light absorption under lateral electric field was explained by the redistribution of hot charge carriers between the two lowest electronic states and by the subsequent emergence of space charge between quantum wells.

Effect of transverse electric field and temperature on light absorption in GaAs/AlGaAs tunnel-coupled quantum wells

The photoluminescence and intersubband absorption spectra are studied in GaAs/AlGaAs tunnel- coupled quantum well structures. The peak positions in the photoluminescence and absorption spectra are consistent with the theoretically calculated energies of optical carrier transitions. The effect of a transverse electric field and temperature on intersubband light absorption is studied. It is caused by electron redistribution between the size-quantization levels and a variation in the energy spectrum of quantum wells. The variation in the refractive index in the energy region of observed intersubband transitions is estimated using Kramers–Kronig relations.

Refraction index modulation induced with transverse electric field in double tunnel-coupled GaAs/AlGaAs quantum wells

Modulation of refraction index under transverse electric field was studied in structures with multiple tunnel-coupled GaAs/AlGaAs quantum wells in the spectral range corresponding to intersubband light absorption. The change of refraction index in electric field was calculated using Kramers-Kronig relation and experimentally determined spectra of intersubband light absorption in equilibrium conditions and under transverse electric field.

Intersubband light absorption in tunnel-coupled GaAs/AlGaAs quantum wells for electrooptic studies

Structures with doped multiple tunnel-coupled quantum wells GaAs/AlGaAs were grown. They were designed to investigate the intersubband absorption and modification of absorption spectra under transverse electric field. The samples were formed in mesa configuration. Preliminary results for the temperature modification of the intersubband absorption spectra were obtained.

Strain Engineered SiGe Multiple-Quantum-Well Nanomembranes for Far-Infrared Intersubband Device Applications

SiGe/Si quantum wells are of great interest for the development of Group-IV THz quantum cascade lasers. The main advantage of Group-IV over III-V materials such as GaAs is that, in the former, polar phonon scattering, which significantly diminishes the efficiency of intersubband light emission, is absent. However, for SiGe/Si multiple-quantum-well structures grown on bulk Si, the lattice mismatch between Si and Ge limits the critical thickness for dislocation formation and thus the number of periods that can be grown. Similarly, the use of composition-graded SiGe films as a lattice-matched substrate leads to the transfer of dislocations from the graded buffer substrate into the quantum wells, with a consequent decrease in light emission efficiency. Here we instead employ nanomembrane strain engineering to fabricate dislocation-free strain relaxed substrates, with lattice constants that match the average lattice constants of the quantum wells. This procedure allows for the growth of many periods with excellent structural properties. The samples in this work were grown by low-pressure chemical vapor deposition and characterized via high-resolution X-ray diffraction and far-infrared transmission spectroscopy, showing narrow intersubband absorption features indicative of high crystalline quality.

Short-Wavelength Intersubband Light Emission from Optically Pumped GaN/AlN Quantum Wells

AbstractDue to their large conduction-band offsets, GaN/Al(Ga)N quantum wells are currently the subject of extensive research efforts aimed at extending the spectral range of intersubband optoelectronic devices towards shorter and shorter wavelengths. Here we report our recent measurement of optically pumped intersubband light emission from GaN/AlN quantum wells at the record short wavelength of about 2 μm. Nanosecond-scale optical pulses are used to resonantly pump electrons from the ground states to the second excited subbands, followed by radiative relaxation into the first excited subbands. The intersubband origin of the measured photoluminescence is confirmed via an extensive study of its polarization properties and pump wavelength dependence.

Latest developments in GaN-based quantum devices for infrared optoelectronics

In this work, we summarize the latest progress in intersubband devices based on GaN/AlN nanostructures for operation in the near-infrared. We first discuss the growth and characterization of ultra-thin GaN/AlN quantum well and quantum dot superlattices by plasma-assisted molecular-beam epitaxy. Then, we present the performance of nitride-based infrared photodetectors and electro-optical modulators operating at 1.55 μm. Finally, we discuss the progress towards intersubband light emitters, including the first experimental observation of intersubband photoluminescence in nitride nanostructures.

Two-Electron Linear Intersubband Light Absorption in a Biased Quantum Well

We point out a novel manifestation of many-body correlations in the linear optical response of electrons confined in a quantum well. Namely, we demonstrate that along with the conventional absorption peak at a frequency omega close to the intersubband energy delta, there exists an additional peak at frequency h omega approximately = 2delta. This new peak is solely due to electron-electron interactions, and can be understood as excitation of two electrons by a single photon. The actual peak line shape is comprised of a sharp feature, due to excitation of pairs of intersubband plasmons, on top of a broader band due to absorption by two single-particle excitations. The two-plasmon contribution allows us to infer intersubband plasmon dispersion from linear absorption experiments.

Dynamics of the Intraband Light Absorption in Selectively Doped GaAs/AlGaAs Quantum Wells

Spectral and temperature dependences of equilibrium and non-equilibrium intersubband light absorption in the mid-infrared spectral range were studied in selectively doped asymmetrical tunnel-coupled GaAs/AlGaAs quantum wells. The temporal evolution of the absorption studied by means of a picosecond pump-probe technique was found to have a biexponential character. The fast decay times are determined by intersubband electron relaxation due to electron scattering by optical phonons and impurities. The presence of long decay times in transient mid-infrared absorption is probably connected with electron transitions from the states in barrier (X and L valleys as well as deep centers) to the states of the quantum well. Experimentally determined intersubband scattering times are compared with the calculated ones.

Pump-probe measurement of ultrafast all-optical modulation based on intersubband transition in n-doped quantum wells

Modulation of interband-resonant light (∼800 nm) by intersubband-resonant light (5–7 μm) was investigated in n-doped AlGaAs/GaAs multiple quantum wells by a two-color femtosecond pump-probe technique. Modulation with a recovery time of ∼1 ps is observed in a plainer-type modulation device at room temperature. The modulation of interband absorption coefficient is ∼1000 cm−1 when the energy density of the intersubband light pulse is ∼4 fJ/μm2. The modulation efficiency indicates that 99% modulation can be achieved with a control pulse energy of ∼1 pJ when a conventional waveguide-type device structure is utilized. The mechanism which determines the modulation speed is discussed in terms of carrier relaxation process. It is shown that the modulation speed is mainly determined by the inter- and intrasubband relaxation times, where the latter is influenced by hot phonon effects.

Ultrafast interband-light modulation by intersubband light (free-electron laser) in quantum wells

Ultrafast interband-resonant light modulation induced by an intersubband transition in GaAs/AlGaAs multiple quantum wells is observed by using a free-electron laser (FEL) for intersubband excitation. The FEL operates with a picosecond pulse width and a wide wavelength tunability. The dependence of the modulation depth on the wavelength of the intersubband-resonant light shows good agreement with the intersubband absorption spectrum of the multiple quantum wells measured by Fourier transform infrared (FTIR) spectroscopy. We also investigated the relaxation time of the intersubband transition by a pump–probe experiment and found that it was less than the FEL micropulse width (∼3 ps).

Ultrashort-pulse-controlled all-optical modulation by interband and intersubband transitions in doped quantum wells

The optimum condition for achieving highly efficient ultrafast all-optical pulse modulation by modification of the absorption of a pulsed interband light field by ultrasubband control-light pulses in a doped quantum well is investigated. The modulation efficiency in the femtosecond domain can be maximized by an intersubband control light with a pulse width (200 fs) that is comparable with the phase-relaxation time of the system and an intensity that is close to the intersubband saturation intensity.