Interband Absorption Research Articles

Quadratic electro-optic Kerr effect in doped graphene

We theoretically investigate one of the third-order nonlinear optical (NLO) effects, namely, the quadratic electro-optic Kerr effect (EOKE), in doped graphene. To avoid the screening of an in-plane external dc electric field by the graphene’s electrons, we propose to use ‘low-amplitude’ ( cm−1) terahertz radiation pulses focused onto the graphene sample collinearly with a normally incident optical beam of frequency ω. Using the Dirac cone approximation for the π-electron energy bands of graphene, we calculate the real part of the effective third-order NLO susceptibility , describing the EOKE in doped graphene under the above conditions. The results obtained show that a large electro-optic modulation of the graphene’s refractive index n (up to ) can be achieved by proper tuning the Fermi level of charge carriers in the graphene sample via electrostatic gating. Furthemore, a change of sign of the electro-optic Kerr coefficient of doped graphene can occur in the spectral range below the photon energy threshold value of , corresponding to the onset of the fundamental (single-photon) interband absorption in the graphene. These theoretical findings open up new opportunities for practical exploitation of the EOKE in graphene-based NLO devices.

Recent Progresses in Mid Infrared Nanocrystal Optoelectronics

Over the past few years, colloidal nanoparticles have started to be investigated for their optical properties in the mid-infrared, past 3 μ m. Research on detector application has led to background limited detection and fast video imaging at 5 μ m. With further development, one could imagine that these new materials could vastly reduce the costs of infrared technology and this would lead to a trove of new applications for infrared imaging into our daily lives. This paper reviews the progress regarding the optical, transport, and photodetection properties of thin film based on these materials, and the three different ways by which infrared resonances have been realized with colloidal nanoparticles: interband absorption with small gap semiconductor quantum dots, intraband absorption in lightly doped quantum dots, and plasmonic resonances in heavily doped nanocrystals.

Two-channel conduction in YbPtBi

We investigated transport, magnetotransport, and broadband optical properties of the half-Heusler compound YbPtBi. Hall measurements evidence two types of charge carriers: highly mobile electrons with a temperature-dependent concentration and low-mobile holes; their concentration stays almost constant within the investigated temperature range from 2.5 to 300 K. The optical spectra (10 meV - 2.7 eV) can be naturally decomposed into contributions from intra- and interband absorption processes, the former manifesting themselves as two Drude bands with very different scattering rates, corresponding to the charges with different mobilities. These results of the optical measurements allow us to separate the contributions from electrons and holes to the total conductivity and to implement a two-channel-conduction model for description of the magnetotransport data. In this approach, the electron and hole mobilities are found to be around 50000 and 10 cm$^{2}$/Vs at the lowest temperatures (2.5 K), respectively.

The influence of resonance IR laser radiation on magnetoabsorption in quantum wires

The coefficient of interband absorption of a weak electromagnetic wave by quantum wires in a transverse magnetic field and an intense laser radiation field is calculated. It is shown that, if the laser radiation frequency is equal either to the size quantization frequency (dimensional infrared resonance) or to a hybrid frequency (magnetoinfrared resonance), laser illumination can determine the shape of absorption oscillations. In particular, it is shown that the second magnetoabsorption peak is split into two peaks, the half-widths of which and the distance between which depend on the intensity of resonance laser radiation. The influence of the polarization of IR radiation on the interband absorption in quantum wires is discussed. The dynamics of the frequency dependence of the optical absorption coefficient with increasing intensity of resonance laser radiation is studied.

Nonlinear UV absorption properties of bulk 4H-SiC

The intensity-dependent light absorption in bulk high-purity semi-insulating 4H-SiC at above band gap photon energies has been studied. In particular, 3.49 eV (355 nm) UV absorption of 160 μm-thick samples of varying recombination lifetimes in the intensity range of 1 mJ/cm2–30 mJ/cm2 is addressed. The effective absorption coefficient was found to vary up to 30% within this range. Assuming deep level trapping, interband absorption, and free carrier absorption as dominant processes, a four energy level model reproduces the experimentally observed absorption behavior. While nonlinearities in the optical absorption behavior of SiC have been studied previously as function of wavelength α(λ), temperature α(T) and, to a very limited extent, at below bandgap optical intensities, the presented elucidates the UV intensity-dependent nonlinear absorption behavior, α(I), of SiC at above bandgap photon energies.

Nanoparticles alloying in liquids: Laser-ablation-generated Ag or Pd nanoparticles and laser irradiation-induced AgPd nanoparticle alloying

Laser irradiation of a mixture of single-element micro/nanomaterials may lead to their alloying and fabrication of multi-element structures. In addition to the laser induced alloying of particulates in the form of micro/nanopowders in ambient atmosphere (which forms the basis of the field of additive manufacturing technology), another interesting problem is the laser-induced alloying of a mixture of single-element nanoparticles in liquids since this process may lead to the direct fabrication of alloyed-nanoparticle colloidal solutions. In this work, bare-surface ligand-free Ag and Pd nanoparticles in solution were prepared by laser ablation of the corresponding bulk target materials, separately in water. The two solutions were mixed and the mixed solution was laser irradiated for different time durations in order to investigate the laser-induced nanoparticles alloying in liquid. Nanoparticles alloying and the formation of AgPd alloyed nanoparticles takes place with a decrease of the intensity of the surface-plasmon resonance peak of the Ag nanoparticles (at ∼405 nm) with the irradiation time while the low wavelength interband absorption peaks of either Ag or Pd nanoparticles remain unaffected by the irradiation for a time duration even as long as 30 min. The nanoalloys have lattice constants with values between those of the pure metals, which indicates that they consist of Ag and Pd in an approximately 1:1 ratio similar to the atomic composition of the starting mixed-nanoparticle solution. Formation of nanoparticle networks consisting of bimetallic alloyed nanoparticles and nanoparticles that remain as single elements (even after the end of the irradiation), joining together, are also formed. The binding energies of the 3d core electrons of both Ag and Pd nanoparticles shift to lower energies with the irradiation time, which is also a typical characteristic of AgPd alloyed nanoparticles. The mechanisms of nanoparticles alloying and network formation are also discussed.

Density-functional theory study on the electronic properties of laves phase superconductor CaIr2**Project supported by the National Natural Science Foundation of China (Grant No. 11274084).

In this work we have used density-functional theory methods such as full-potential local orbital minimum basis (FPLO) and ELK-flapw to study the electronic structure of newly discovered Laves phase superconductor CaIr2. The calculation of density of states (DOS) indicates that the bands near Fermi level are mostly occupied by the d-electrons of iridium. The simulation of de Haas–van Alphen (dHvA) effect has been performed by using Elk code to check the Fermi surface topology. The results show that there exist four Fermi surfaces in CaIr, including two electron-type and two hole-type surfaces. The optical response properties of CaIr2 have been calculated in the dipole-transition approximations combined with including intra-band Drude-like terms. In the optical spectrum shows that the crossover from intra-band to inter-band absorption occur near 1.45 eV. Further analysis on the electron energy loss spectra (EELS) matches the conclusion from that of optical conductivity .

Near-field thermal radiation transfer between semiconductors based on thickness control and introduction of photonic crystals

We numerically investigate the spectral control of near-field thermal radiation transfer using interband absorption in semiconductors and the band-folding effect in photonic crystals (PCs) for highly efficient thermophotovoltaics. We reveal that the near-field coupling between two semiconductors (Si and GaSb) realizes frequency-selective thermal radiation transfer concentrated above their bandgap energy when their thicknesses are optimized considering their absorption coefficient spectra. Moreover, we elucidate the role of PC structures in the near-field thermal radiation transfer and demonstrate that the band-folding effect in PCs can further increase both the radiation power and frequency selectivity of the near-field thermal radiation transfer.

Carrier Lifetimes in aIII−V−NIntermediate-Band Semiconductor

We have used transient absorption spectroscopy to measure carrier lifetimes in the multiband band semiconductor GaPAsN. These measurements probe the electron populations in the conduction band, intermediate band and valance band as a function of time after an excitation pulse. Following photoexcitation of GaP0.32As0.67N0.01 we find that the electron population in the conduction band decays exponentially with a time constant 23ps. The electron population in the intermediate band exhibits bimolecular recombination with recombination constant r = 2 10^-8 cm-3/s. In our experiment an optical pump pulse excited electrons from the valance band to the intermediate and conduction bands, and the change in interband absorption due to absorption saturation and induced absorption was probed with a delayed white light pulse. We modeled the optical properties of our samples using the band anti-crossing model to extract carrier densities as a function of time. These results indicate that the minority carrier lifetimes are too short for efficient solar power conversion and that improvements in material quality will be required for practical applications of GaPAsN based intermediate band solar cells.

A theoretical study of the optical Stark effect in InGaAs/InAlAs quantum dots

In this paper, we examine the three-level optical Stark effect of excitons in InGaAs/InAlAs quantum dots using renormalized wavefunction formulation. The system was assumed to be irradiated by two lasers in which a strong laser dynamically couples electron-quantized levels, while a weaker laser probes interband absorption. Our results show that, in the presence of the resonant strong laser, two new absorption peaks of excitons appear in the absorption spectrum as a clear indication of the effect. In addition, we propose that the formation of the effect in low-dimensional structures could have connection to the splitting of electron levels. Furthermore, we seek to explain the essential dependence of the amplitude and position of two peaks on pump field detuning.

Evolution of Defective State of Aluminum Oxide Irradiated with Chromium Ions after Annealing in Oxygen Environment

The characteristics of interband and exponential optical absorption of leucosapphire and polycrystalline corundum (polycor) after irradiation with chromium ions and subsequent annealing in vacuum at 300–1800 K and in air at 300–800 K are studied. Contributions of defects with different thermal and chemical stability into optical parameters were established. The effect of intrinsic radiation defects, of substitutional defects and of complexes on base of oxygen and defects on formation the focal point in absorption spectra owing to fulfilment of the Urbach rule was determined. Heating in air of strongly defective material synthesized in surface layers of alumina by the ion–heat modification influences the characteristics of defects and the electronic structure of band gap negligibly.

Electronic States and Optical Transitions in an Asymmetric Quantum Dot Molecule

In the framework of adiabatic approximation the electronic states and direct interband absorption of light in the asymmetric double quantum dot molecule (QDM) having a shape of Cassini lemniscate revolution are discussed. Analytical expressions for the wave functions and energy spectrum of the electron in the QDM are derived. Nonmonotonic split and step-like behavior of the energy spectrum is revealed due to the possibility of the electron tunneling between quantum dots (QDs) in the molecule. The corresponding selection rules of quantum transitions for the direct interband absorption of light are obtained. The absorption edge characteristics depending on the QDM geometrical sizes are also revealed.

Interband Absorption Enhanced Optical Activity in Discrete Au@Ag Core-Shell Nanocuboids: Probing Extended Helical Conformation of Chemisorbed Cysteine Molecules.

Detailed understanding of the interaction between a chiral molecule and a noble metal surface is essential to rationalize and advance interfacial self-assembly of amino acids and metal-mediated anchoring of proteins. Here we demonstrate that individual Au@Ag core-shell nanocuboids can serve as a plasmonic reporter of an extended helical network formed among chemisorbed cysteine molecules, through generating an interband absorption enhanced, Ag-surface-exclusive circular dichroism (CD) band in the UV region. The observed unusual, strong CD response in the hybrid Au@Ag-cysteine system can be used to probe in real time conformational evolution and structural rearrangement of biomolecules in general and also monitor the interfacial interaction between a metal surface and an adsorbed molecule, opening up the possibility of using Ag nanostructures as promising stereochemically attuned nanosensors.

Interband Absorption Enhanced Optical Activity in Discrete Au@Ag Core–Shell Nanocuboids: Probing Extended Helical Conformation of Chemisorbed Cysteine Molecules

AbstractDetailed understanding of the interaction between a chiral molecule and a noble metal surface is essential to rationalize and advance interfacial self‐assembly of amino acids and metal‐mediated anchoring of proteins. Here we demonstrate that individual Au@Ag core–shell nanocuboids can serve as a plasmonic reporter of an extended helical network formed among chemisorbed cysteine molecules, through generating an interband absorption enhanced, Ag‐surface‐exclusive circular dichroism (CD) band in the UV region. The observed unusual, strong CD response in the hybrid Au@Ag–cysteine system can be used to probe in real time conformational evolution and structural rearrangement of biomolecules in general and also monitor the interfacial interaction between a metal surface and an adsorbed molecule, opening up the possibility of using Ag nanostructures as promising stereochemically attuned nanosensors.

Solar-selective absorbers based on semiconducting β-FeSi2 for efficient photothermal conversion at high temperature

Solar-absorbing multilayers with β-FeSi2-containing SiO2 composites were deposited on low-emissivity Mo base layers, and the optical reflectance spectra were obtained to investigate the solar–thermal conversion efficiency. Interband absorption of the β-FeSi2 provided a desirable selective absorbing behaviour with a steep transition curve from high solar absorption to low infrared absorption in the near-infrared region. The β-FeSi2 nanoparticles dispersed in the SiO2 matrices were stable even after annealing at 750°C, and the optical constant spectra of the composite films were controllable by adjusting the β-FeSi2 content. Although the interface between the β-FeSi2 and Mo base layer had low thermal stability because of the diffusion of Si, a thin SiO2 barrier at the interlayer was effective in suppressing the interaction. The above findings were integrated into the designed multilayer “SiO2/β-FeSi2+SiO2 composites/SiO2 barrier/Mo base”, for which the optical absorption spectrum achieved high conversion efficiencies of over 72.3% at high temperatures below 700°C.

Transparency of graphene and other direct-gap two-dimensional materials

Graphene and other two-dimensional materials display remarkable optical properties, including a simple light transparency of $T \approx 1 - \pi \alpha$ for light in the visible region. Most theoretical rationalizations of this "universal" opacity employ a model coupling light to the electron's crystal momentum and put emphasis on the linear dispersion of the graphene bands. However, such a formulation of interband absorption is not allowable within band structure theory, because it conflates the crystal momentum label with the canonical momentum operator. We show that the physical origin of the optical behavior of graphene can be explained within a straightforward picture with the correct use of canonical momentum coupling. Its essence lies in the two-dimensional character of the density of states rather than in the precise dispersion relation, and therefore the discussion is applicable to other systems such as semiconductor membranes. At higher energies the calculation predicts a peak corresponding to a van Hove singularity as well as a specific asymmetry in the absorption spectrum of graphene, in agreement with previous results.

Landau quantization, Aharonov–Bohm effect and two-dimensional pseudoharmonic quantum dot around a screw dislocation

We investigate the influence of a screw dislocation on the energy levels and the wavefunctions of an electron confined in a two-dimensional pseudoharmonic quantum dot under the influence of an external magnetic field inside a dot and Aharonov–Bohm field inside a pseudodot. The exact solutions for energy eigenvalues and wavefunctions are computed as functions of applied uniform magnetic field strength, Aharonov–Bohm flux, magnetic quantum number and the parameter characterizing the screw dislocation, the Burgers vector. We investigate the modifications due to the screw dislocation on the light interband absorption coefficient and absorption threshold frequency. Two scenarios are possible, depending on if singular effects either manifest or not. We found that as the Burgers vector increases, the curves of frequency are pushed up towards of the growth of it. One interesting aspect which we have observed is that the Aharonov–Bohm flux can be tuned in order to cancel the screw effect of the model.

Optical Absorption Peculiarities of Al-Si-N Coatings Produced by Magnetron Sputtering

In this paper the optical absorption characteristics of Al–Si–N coatings, deposited by magnetron sputtering on the substrates from silicon single crystals and glass were studied. The influence of conditions deposition on optical properties has been determined. The characteristics of interband and exponential absorption are determined by influence of localized states (LS) of defects continuously distributed in the band gap (BG) and reflect the influence on properties of crystalline and amorphous components material of coatings. The interband absorption is realized via the allowed indirect transitions through the optical gap 2.7–2.8 eV and direct transitions through the gap 3.8–3.96 eV. The interrelations between the parameters of interband and exponential absorption are typical for the naterials, whose properties are determined by a static and/or dynamic disordering of the crystal lattice by the defects of a different nature. Maximal average value of the BG width is 4.8–5.1 eV in the scope of semiclassical analysis applied to amorphous and strongly defective materials. The optical parameters of deposited coatings are changed in correlation with a changing of the mechanical characteristics in dependency on the nitrogen pressure.

Mid-Infrared Intersubband Absorption from p-Ge Quantum Wells Grown on Si Substrates

There are many applications in the mid-infrared part of the electromagnetic spectrum which include thermal imaging [1] and the unique identification of molecules through absorption spectroscopy [2]. For all these applications, sources of mid-infrared light and photodetectors are key to enable any application. III-V and II-VI semiconductor materials have dominated over the last decades with both interband and intersubband emission and/or absorption devices [1-3] but there is now significant interest in developing technology on silicon substrates to enable far cheaper systems for mass market applications in environmental sensing, personalised healthcare and security. SiGe quantum well (QW) intersubband photodetectors (QWIPs) have previously been demonstrated [4] but the number of QWs was limited by the SiGe critical thickness thereby limiting performance. Now Ge QWs have the potential to improve this performance significantly in the mid-infrared and the number of QWs can be increased using strain symmetrisation of the QWs and barriers to allow improved absorption in the longer wavelength mid-infrared. Here we demonstrate strain symmetrized growth of 500 periods of Ge QWs with Si0.5Ge0.5 barriers on Si0.2Ge0.8 virtual substrates grown on top of (001) Si. The samples were grown by low energy plasma enhanced chemical vapour deposition. Transmission electron microscopy, Raman spectroscopy and x-ray diffraction were used to characterize the heterolayer thicknesses, Ge contents, strain and defects. The layers demonstrate excellent quality with low defect densities. Three samples with QW thicknesses (barrier thicknesses) of 5.4 ± 0.4 nm (3.5 ± 0.4 nm), 8.1 ± 0.5 nm (5.2 ± 0.6 nm) and 9.2 ± 0.6 nm (6.1 ± 0.6 nm) have been fully characterized by Fourier transform infrared spectroscopy (FTIR) to understand the potential for mid-infrared QWIPs. An 8-band k.p modelling tool has been used to understand the results. For the lowest QW width, only z-polarised absorption is observed at ~ 7 µm wavelength from the light hole 1 (LH1) to LH2 transition. Despite surface-normal illumination, the z-polarized transition is observed since the heterolayers and substrate will scatter some of the light into the z-polarized mode. For the thicker QWs, only xy-polarized (surface normal) absorption is measured with absorption peaks centred at 7.5 µm and 7.8 µm corresponding to the heavy hole 1 (HH1) to LH2 transition. Temperature dependent FTIR measurements demonstrate virtually no shift in the wavelength of absorption with temperature as is typical for intersubband transitions. Photoluminescence (PL) demonstrates a completely different behavior to the absorption due to the population of states being different through the heating of the samples by the 532 nm PL laser. We will demonstrate how to design QWIPs using a HH1 to LH bound-to-continuum transitions that will enable surface-normal QWIPs to be designed with large pixel numbers and produced cheaply on a silicon substrate using any silicon foundry with SiGe growth capability. The research leading to these results has received funding from the European Unions Seventh Framework Programme under grant agreement no. 613055 and U.K. EPSRC (Project no. EP/N003225/1). [1] A. Rogalski, J. Appl. Phys. 93, 4355 (2003) [2] L. Baldassarre, E. Sakat, J. Frigerio, A. Samarelli, K. Gallacher, E. Calandrini, G. Isella, D. J. Paul, M. Ortolani, and P. Biagioni, Nano Lett. 15, 7225 (2015) [3] A. Rogalski, Infrared Phys. Technol. 38, 295 (1997). [4] R. P. G. Karunasiri, J. S. Park, Y. J. Mii, and K. L. Wang, Appl. Phys. Lett. 57, 2585 (1990) Figure 1

Optical spectroscopy of intermetallic compounds TbNi2Mn x (x = 0, 0.5, 1)

The optical properties of intermetallic compounds TbNi2Mnx (x = 0, 0.5, 1) have been investigated using the ellipsometric method in the spectral range from 0.22 to 16 μm. The specific features of the modification of the dispersions of spectral characteristics with a variation in the manganese concentration have been determined. The behavior of the frequency dependences of the optical conductivity in the interband absorption region has been discussed in terms of the available data on the electronic structure of these compounds. The concentration dependences of a number of electronic parameters have been calculated.