Intraband Absorption Research Articles

Dynamics of Quantized Auger Recombination in CdSe Nanocrystals Studied by Femtosecond Intraband Pump–Probe Spectroscopy

The dynamics of quantized Auger recombination in CdSe nanocrystals was studied by femtosecond intraband pump–probe spectroscopy at room temperature. The intraband transient absorption signals increase linearly with excitation laser intensity, while the interband transient absorption signals saturate at high excitation intensities. We clarified that the excitation intensity dependence of the Auger recombination rate in nanocrystals is quantitatively explained by three-body photocarrier collision processes.

Absorptivity Transition in the 1.06 μm Wavelength Laser Machining of Structural Ceramics

This study presents absorptivity transitions in two-dimensional, low aspect ratio laser machining (cutting) of structural ceramics such as alumina (Al2O3), silicon nitride (Si3N4), silicon carbide (SiC), and magnesia (MgO) using a 1.06 μm wavelength-pulsed Nd:YAG laser. Because temperature measurement at high temperatures is difficult, thermocouples were used to measure temperatures in the low-temperature regime (700–1150 K). A thermal model was then iterated to obtain trends in absorptivity variation below the phase transition temperature for different ceramics. Following this, measured machined depths were used as a benchmark to predict absorptivity transitions at higher temperatures (>1150 K), using the developed thermal model. For temperatures below the phase transition, because of intraband absorption, the absorptivity decreases with an increase in temperature until the surface temperature reaches the melting point (in the case of Al2O3, Si3N4, and SiC) and the vaporization temperature (in the case of MgO). The absorptivity then continues to follow an increasing trend with increasing temperature because the physical entrapment of the laser beam in the cavity evolved during machining of the desired depth in the ceramic. Such a study would provide an efficient control over the input energy required to machine to a certain depth.

Room temperature intraband Raman emission and ultrafast carrier relaxation in GaN/AlN quantum dots

AbstractWe report on the intraband emission at room‐temperature from GaN/AlN quantum dots grown by plasma‐assisted molecular beam epitaxy as well as on the measurements of GaN quantum dot intraband relaxation using femtosecond spectroscopy. The quantum dots exhibit s‐pz intraband absorption peaked at 1.55 μm. The pz‐s intraband luminescence at λ = 1.48 μm is observed under optical excitation at λ = 1.34 μm. The population of the pz state arises from Raman scattering by GaN A1 longitudinal optical phonons. Based on the emission spectral shape, we estimate that the homogeneous linewidth of the s‐pz intraband transition is less than 4 meV.The carrier lifetime of the pz state (T1) is measured by femtosecond pump‐probe spectroscopy. T1 = 165 fs while the thermalization of electrons in the ground state proceeds in 1.5 ps. The saturation intensity of the s‐pz intraband absorption is estimated to be in the 15 to 137 MW/cm2 range taking into account the uncertainty on the pump beam waist. These results demonstrate the promising capabilities of GaN/AlN QDs for application to telecom intraband lasers as well as multi‐Tbit/s all‐optical switching devices. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

INTRABAND AUGER EFFECT IN QUANTUM DOT STRUCTURES

Intraband photocurrent and absorption measurements were performed on InAs / InGaAlAs / InP quantum dot structures. To identify the optical transitions, theoretical calculations were performed and showed that the observed PC is generated by a bound to bound transition, where the final state is about 200 meV deep below the conduction band continuum. The reported results strongly suggest that an Auger process plays a fundamental role in generating the observed intraband photocurrent.

Intraband Auger effect in InAs/InGaAlAs/InP quantum dot structures

Intraband photocurrent and absorption measurements were performed on InAs/InGaAlAs/InP quantum dot structures. A full three-dimensional theoretical model has been employed to identify the observed photocurrent as a bound to bound transition, where the final state is about 200 meV deep below the conduction band continuum. The reported results strongly suggest that an Auger process plays a fundamental role in generating the observed intraband photocurrent.

Luminescence and dielectric properties of nano-structured Eu 3+:K 2O–Nb 2O 5–SiO 2 glass-ceramics

In the present study, results concerning luminescence and dielectric properties of Eu 2O 3 (0.5 wt% in excess) doped nano-crystallized KNbO 3 containing transparent glass-ceramics obtained from glass of composition 25K 2O–25Nb 2O 5–50SiO 2 (mol%) by varied heat-treatment duration at 800 °C have been analyzed and reported. The formed crystallization phase, crystallite size and morphology have been examined through XRD, FESEM, TEM and FTIRRS measurements. The observed steep increase in the dielectric constant ( ɛ) of glass-ceramics over the as-prepared glass is attributed to the formation of ferroelectric nano-crystalline KNbO 3 in glass matrix. The absorption spectra of all the samples have revealed the characteristic 4f–4f intraband absorption transitions of Eu 3+ ions. The measured photoluminescence spectra have exhibited emission transitions 5D 0, 1 → 7F j ( j = 0, 1, 2, 3 and 4) of Eu 3+ ions. The excited level lifetimes have been determined from measured fluorescence decay curves. The rare earth ion site symmetry (nearly C v) has been understood based on the nature of the Stark splittings of emission bands detected in both Eu 3+: glass and Eu 3+: glass-ceramics.

Quantum-dot infrared photodetectors: a review

Quantum-dot infrared photodetectors (QDIPs) are positioned to become an important technology in the field of infrared (IR) detection, particularly for high-temperature, low-cost, high-yield detector arrays required for military applications. High-operating temperature (150 K) photodetectors reduce the cost of IR imaging systems by enabling cryogenic dewars and Stirling cooling systems to be replaced by thermo-electric coolers. QDIPs are well-suited for detecting mid-IR light at elevated temperatures, an application that could prove to be the next commercial market for quantum dots. While quantum dot epitaxial growth and intraband absorption of IR radiation are well established, quantum dot non-uniformity remains as a significant challenge. Nonetheless, state-of-the-art mid-IR detection at 150 K has been demonstrated using 70-layer InAs/GaAs QDIPs, and QDIP focal plane arrays are approaching performance comparable to HgCdTe at 77 K. By addressing critical challenges inherent to epitaxial QD material systems (e.g., controlling dopant incorporation), exploring alternative QD systems (e.g., colloidal QDs), and using bandgap engineering to reduce dark current and enhance multi-spectral detection (e.g. resonant tunneling QDIPs), the performance and applicability of QDIPs will continue to improve.

Size quantization effects in thin film Casimir interaction

We investigate the role of size quantization in the vacuum force between metallic films of nanometric thickness. The force is calculated by the Lifshitz formula with the film dielectric tensor derived from the one-electron energies and wavefunctions under the assumption of a constant potential inside the film and a uniform distribution of the positive ion charge. The results show that quantization effects tend to reduce the force with respect to the continuum plasma model. The reduction is more significant at low electron densities and for film size of the order of few nanometers and persists for separation distances up to 10-50 nm. Comparison with previous work indicates that the softening of the boundary potential is important in determining the amount of the reduction. The calculations are extended to treat Drude intraband absorption. It is shown that the inclusion of relaxation time enhances the size quantization effects in the force calculations.

Ultrafast relaxation and optical saturation of intraband absorption of GaN/AlN quantum dots

We report on the measurements of ultrafast relaxation and absorption saturation of the s-pz intraband transition at λ=1.55 μm in GaN/AlN quantum dot superlattice. The recovery time of the intraband absorption is assessed using degenerate pump-probe experiments at room temperature. Measurements reveal a multiexponential decay of the differential transmission with an ultrafast (∼160 fs) and a slower (∼1.5 ps) time constant attributed to the excited state lifetime and to the thermalization time in the ground state, respectively. The saturation intensity of the intraband absorption (<140 MW/cm2) is one order of magnitude smaller than the value measured in nitride quantum wells.

Investigation of Thermal Properties of Heavily Doped 4H-SiC Crystals by a Picosecond Transient Grating Technique

We applied a picosecond dynamic grating technique for investigation of thermal diffusivity, sound velocity, thermo-optic and photoelastic coefficients in heavily doped 4H-SiC substrates. Spatial modulation of thermal properties was achieved by intraband carrier absorption at excitation by an IR picosecond laser pulse (1064 nm) and subsequent carrier thermalisation. Decay of dynamic grating at its various periods and sample temperatures was monitored at 532 nm wavelength. The thermal diffusivity DT = 0.94 cm2/s, thermo-optic coefficient dn/dT = 3.610–5 K-1 and effective photoelastic constant = – 0.10 were determined at 300 K. DT value increased at lower temperatures and saturated below 100 K. Temperature dependences of thermo-optic coefficient and thermal expansion were found in good correlation with heat capacity peculiarities.

Efficient Multiple Exciton Generation Observed in Colloidal PbSe Quantum Dots with Temporally and Spectrally Resolved Intraband Excitation

We have spectrally resolved the intraband transient absorption of photogenerated excitons to quantify the exciton population dynamics in colloidal PbSe quantum dots (QDs). These measurements demonstrate that the spectral distribution, as well as the amplitude, of the transient spectrum depends on the number of excitons excited in a QD. To accurately quantify the average number of excitons per QD, the transient spectrum must be spectrally integrated. With spectral integration, we observe efficient multiple exciton generation in colloidal PbSe QDs.

Formation of absorbing heterogeneous plasma layer by femtosecond laser-induced melting and ablation of silicon

The experimental study of absorption in silicon in infrared and visible spectral ranges, where the photon energy is less or more than the bandgap width, is performed by means of the ultrafast interferometry technique. The exactly solvable model in the electromagnetic of heterogeneous lossy plasma layer was developed. The density of carriers, their frequency of collisions, absorbing depth of the probing waves, real and imaginary parts of dielectric function of nonuniform layer and their spatial gradients are determined from the reflectance data by means of this model subject to the pump fluence. The heterogeneity-induced effects are visualized due to comparison of obtained plasma parameters with those calculated in the framework of homogeneous plasma model It is shown that in the intensity range near thresholds of melting and ablation the absorption, occurring in both cases mainly within a thin (∼10 nm) absorbing layer (similarly to metals), is due to free carrier intraband absorption.

Optical properties, structural parameters, and bonding of highly textured rocksalt tantalum nitride films

Tantalum nitride is an interesting solid with exceptional properties and it might be considered as a representative model system of the d3s2 transition metal nitrides. In this work highly textured, stoichiometric, rocksalt TaN(111) films have been grown on Si(100) by pulsed laser deposition. The films were under a triaxial stress, which has been determined by the sin2 ψ method. The stress-free lattice parameter was found to be 0.433±0.001 nm, a value which has been also determined by ab initio calculations within the local spin density approximation. The optical properties of TaN have been studied using spectroscopic ellipsometry and detailed band structure calculations. The electron conductivity of TaN is due to the Ta 5dt2g band that intercepts the Fermi level and is the source of intraband absorption. The plasma energies of fully dense rocksalt TaN were found to be 9.45 and 9.7 eV based on the experimental results and ab initio calculations, respectively. Additional optical absorption bands were also observed around 1.9 and 7.3 eV and attributed to be due to crystal field splitting of the Ta 5d band (t2g→eg transition) and the N p→Ta d interband transition, respectively.

Intraband absorption in solar cells with an intermediate band

This article presents a thermodynamic treatment of an intermediate band solar cell that includes photoinduced electronic transitions between two distinct states of the intermediate band. The treatment also allows for two black-body sources, interband photoinduced electronic transitions, overlapping absorption coefficients, multiple electron-hole pair generation, and nonradiative processes. A schematic of the device’s thermodynamic configuration shows that the solar cell is composed of three particle engines operating in tandem. The authors present detailed-balance results where it is assumed that when there is the physical possibility of both photoinduced intra- and interband electronic transitions at intermediate levels that the latter predominates. Results indicate that as the intermediate band’s width increases, the efficiencies saturate to those of two-stack tandem solar cells while the band structures approach that of a material that should operate as a black body. The authors conclude that the assumption that interband transitions predominate over intraband transitions, which is equivalent to ignoring or excluding intraband transitions, may yield results inconsistent with physical reality. The larger the difference between the intermediate band width and the smallest band gap in the system, the more pronounced will be the inconsistency.

Ultrafast Dynamics of Polarons in Conductive Polyaniline: Comparison of Primary and Secondary Doped Forms

Time- and wavelength-resolved pump-probe measurements are performed on the conductive, primary and secondary doped, forms of polyaniline in solution to investigate the relaxation dynamics of photoexcited polarons. Contrasting dynamics observed in the two forms allow investigation of electronic (and structural) relaxation of this material. Pump pulse at 800 nm photoexcites an electron from the valence to polaron band, and subsequent relaxation dynamics are probed by a white light continuum pulse. Three distinct features are observed in the primary doped sample: (1) absorption near time zero in the 900-1025 nm probe wavelength region; (2) delayed absorption from 850 to 1025 nm; (3) pronounced oscillations with frequencies of 165 and 210 cm(-1). The first two features are associated with intraband absorptions to higher-lying states in the polaron band from the initial excited and conformationally changed intermediate states. The oscillations reflect torsional motions associated with photoexcitation and relaxation. In the secondary doped material, only bleaching and stimulated emission are observed throughout the whole spectral region; neither transient absorption signals nor oscillatory dynamics are observed. Kinetic modeling is performed to establish the mechanism of relaxation. We propose that the excited polaron relaxes nonradiatively to the ground state through an intermediate state(s) with a twisted geometry. Our measurements and analysis allow us to describe the structure of the polaron bands for primary and secondary doped polyaniline; they are in the small and large polaron limits, respectively, and are consistent with a bandgap for the primary doped form and without a bandgap (i.e., metallic) for the secondary doped material.

Second-Order Resonant Polaron in a Self-Assembled Quantum Dot

A compound system consisting of an electron in a quantum dot (QD) and longitudinal optical (LO) phonons has interesting features that cannot be observed in higher-dimensional semiconductor structures. Because of a relatively large size of the QD (compared to the lattice constant), only long wavelength phonon modes are coupled to the electron. Since the LO phonons are almost dispersionless in the vicinity of the Γ point, these effectively coupled modes can be represented as a system of discrete oscillators, rather than a continuum. As a result of the interaction between the confined electron and the set of LO phonon oscillators with a well-defined frequency, pronounced anticrossings are observed in the intraband absorption spectra of a single dot whenever the energy difference between electron levels matches a multiple of the LO phonon energy [1]. Such resonant polarons are an object of current interest, both from the point of view of their properties in various QD systems as well as of the formal methods that can be used to describe them [2–4]. The spectrum of a resonant polaron can easily be described for a single-phonon (first order) resonance [5]. However, in most self-assembled structures the energy spacing between the electron levels considerably exceeds the LO phonon energy. In fact, resonances observed in experiments performed at moderate magnetic fields occur at the crossing of the electron levels with twice the LO phonon energy. Contrary to the first-order resonance, modeling of two-phonon polaron states is less trivial, since the carrier–phonon interaction has no matrix elements directly coupling the anticrossing states. Thus, the effect

Intraband Auger effect in InAs∕InGaAlAs∕InP quantum dot structures

InAs quantum dot structures grown on InGaAlAs have been investigated for midinfrared photodetection. Intraband photocurrent and absorption measurements, together with a full three-dimensional theoretical modeling revealed that a bound-to-bound optical transition, where the final state is about 200meV deep below the conduction band continuum, is responsible for the photogenerated current. The reported results strongly suggest that an Auger process plays a fundamental role in generating the observed intraband photocurrent. Photoluminescence and interband photocurrent spectra of the same structures further support the reached conclusions.

Hybrid-phonon resonances in a quantum channel

Intraband absorption of electromagnetic radiation by electrons in a quantum channel in a transverse quantizing magnetic field is studied. In the calculation, the interaction of conductivity electrons with optical phonons is taken into account. An analytical expression for the coefficient of absorption of high-frequency electromagnetic radiation is derived. It is shown that the absorption spectrum exhibits asymmetric resonance peaks, and the positions of the peaks are determined. The dependence of the absorption coefficient on the magnetic field is studied.

Tuning of intraband absorption and photoresponse in self-assembled InAs∕GaAs quantum dots by thermal annealing

The effects of the thermal annealing on the conduction band states in self-assembled quantum dots (QDs) have been investigated using midinfrared absorption and photocurrent spectroscopies. We demonstrate a significant tuning of the intraband electron transition energies (∼15%) upon annealing due to the reduction in the confinement energy, observed both in intraband absorption and photocurrent spectra. More uniform QD size distribution in the growth direction after annealing results in narrowing of the absorption peak. We also observe additional intersublevel transitions arising after QD annealing.

Multiphoton intraband absorption of electromagnetic waves and stimulated scattering by optical phonons in the superlattice

Multiphoton intraminiband absorption of the electromagnetic wave and stimulated scattering by optical phonons in the semiconductor superlattice are considered. The absorption coefficient is found without using the quantum kinetic equation. The absorbed energy is plotted versus the electric field of the wave. It is found that, as the electric field grows, the absorbed energy reaches a maximum and then starts decreasing in an oscillatory manner. An explanation for such a run of the absorbed energy with electric field is given. It is shown that multiphoton absorption must be taken into account if the condition eEd ≥ ℏω is satisfied. Stimulated multiphoton scattering by optical phonons in superlattices is shown to produce harmonics of the scattered light at frequencies both higher and lower than those of the incident light. Scattering partial cross sections are found and plotted versus the incident wave intensity.