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Wavefunction engineering using intraband transition is the most versatile strategy for the design of infrared devices. To date, this strategy is nevertheless limited to epitaxially grown semiconductors, which lead to prohibitive costs for many applications. Meanwhile, colloidal nanocrystals have gained a high level of maturity from a material perspective and now achieve a broad spectral tunability. Here, we demonstrate that the energy landscape of quantum well and quantum dot infrared photodetectors can be mimicked from a mixture of mercury selenide and mercury telluride nanocrystals. This metamaterial combines intraband absorption with enhanced transport properties (i.e. low dark current, fast time response and large thermal activation energy). We also integrate this material into a photodiode with the highest infrared detection performances reported for an intraband-based nanocrystal device. This work demonstrates that the concept of wavefunction engineering at the device scale can now be applied for the design of complex colloidal nanocrystal-based devices. The field of wavefunction engineering using intraband transition to design infrared devices has been limited to epitaxially grown semiconductors. Here the authors demonstrate that a device with similar energy landscape can be obtained from a mixture of colloidal quantum dots made of HgTe and HgSe.

Luminescence upconversion nanocrystals capable of converting two low-energy photons into a single photon at a higher energy are sought-after for a variety of applications, including bioimaging 1 , 2 and photovoltaic light harvesting 3 . Currently available systems, based on rare-earth-doped dielectrics 4 , 5 , are limited in both tunability and absorption cross-section. Here we present colloidal double quantum dots as an alternative nanocrystalline upconversion system, combining the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. By tailoring its composition and morphology, we form a semiconducting nanostructure in which excited electrons are delocalized over the entire structure, but a double potential well is formed for holes. Upconversion occurs by excitation of an electron in the lower energy transition, followed by intraband absorption of the hole, allowing it to cross the barrier to a higher energy state. An overall conversion efficiency of 0.1% per double excitation event is achieved. Double-nanocrystal systems can be used as an alternative to rare earth-doped dielectric quantum dots for upconversion luminescence.

Many linked processes occur concurrently in strongly excited semiconductors, such as interband and intraband absorption, scattering of electrons and holes by the heated lattice, Pauli blocking, bandgap renormalization and the formation of Mahan excitons. In this work, we disentangle their dynamics and contributions to the optical response of a ZnO thin film. Using broadband pump-probe ellipsometry, we can directly and unambiguously obtain the real and imaginary part of the transient dielectric function which we compare with first-principles simulations. We find interband and excitonic absorption partially blocked and screened by the photo-excited electron occupation of the conduction band and hole occupation of the valence band (absorption bleaching). Exciton absorption turns spectrally narrower upon pumping and sustains the Mott transition, indicating Mahan excitons. Simultaneously, intra-valence-band transitions occur at sub-picosecond time scales after holes scatter to the edge of the Brillouin zone. Our results pave new ways for the understanding of non-equilibrium charge-carrier dynamics in materials by reliably distinguishing between changes in absorption coefficient and refractive index, thereby separating competing processes. This information will help to overcome the limitations of materials for high-power optical devices that owe their properties from dynamics in the ultrafast regime.

Linear and nonlinear optical properties in colloidal CdSe/CdS core/shell quantum dots with different sizes have been theoretically investigated in the framework of effective mass approximation. The electron states in colloidal CdSe/CdS core/shell quantum dots have been calculated using the finite element method. The intraband linear and nonlinear absorption spectra have been calculated for colloidal CdSe/CdS core/shell quantum dots with different sizes. In addition, the dependences of the linear and nonlinear refractive index change on the incident light energy have been calculated. In the last section of the paper the second- and third-order harmonic generation spectra have been presented.

Plasmonic excitations, consisting of collective oscillations of the electron gas in a conductive film or nanostructure coupled to electromagnetic fields, play a prominent role in photonics and optoelectronics. While traditional plasmonic systems are based on noble metals, recent work has established graphene as a uniquely suited materials platform for plasmonic science and applications due to several distinctive properties. Graphene plasmonic oscillations exhibit particularly strong sub-wavelength confinement, can be tuned dynamically through the application of a gate voltage, and span a portion of the infrared spectrum (including mid-infrared and terahertz (THz) wavelengths) that is not directly accessible with noble metals. These properties have been studied in extensive theoretical and experimental work over the past decade, and more recently various device applications are also beginning to be explored. This review article is focused on graphene plasmonic nanostructures designed to address a key outstanding challenge of modern-day optoelectronics – the limited availability of practical, high-performance THz devices. Graphene plasmons can be used as a means to enhance light–matter interactions at THz wavelengths in a highly tunable fashion, particularly through the integration of graphene resonant structures with additional nanophotonic elements. This capability is ideally suited to the development of THz optical modulators (where absorption is switched on and off by tuning the plasmonic resonance) and photodetectors (relying on plasmon-enhanced intraband absorption or rectification of charge-density waves), and promising devices based on these principles have already been reported. Novel radiation mechanisms, including light emission from electrically excited graphene plasmons, are also being explored for the development of compact narrowband THz sources.

Infrared HgSe quantum dots (QDs) enable mid-infrared and longer-wavelength infrared detection through intraband absorption, thereby expanding the selection range of traditional infrared detector materials, which holds promise for overcoming the challenges of complex fabrication processes and high costs. However, control of the size and distribution of HgSe QDs is a key factor limiting the performance enhancement of infrared detectors. Here, the reaction temperatures, growth periods, and reactant stoichiometries of the precursors were systematically regulated to achieve high-quality HgSe QDs with sizes ranging from 2.42 nm to 7.54 nm and excellent monodispersity. Further ligand exchange and film formation tests indicate that this HgSe QD film exhibits excellent flatness. Consequently, the high-quality mid-infrared HgSe QDs reported here are anticipated to facilitate subsequent advancements in associated domains.

Optoelectronic devices based on intraband or intersublevel transitions in semiconductors are important building blocks of the current THz technology. Large nanocrystals (NCs) of Mercury telluride (HgTe) are promising semiconductor candidates owing to their intraband absorption peak tunable from 60 THz to 4 THz. However, the physical nature of this THz absorption remains elusive as, in this spectral range, quantum confinement and Coulomb repulsion effects can coexist. Further, the carrier dynamics at low energy in HgTe NCs, which strongly impact the performances of THz optoelectronic devices, is still unexplored. Here, we demonstrate a broad THz absorption resonance centered at ≈4.5 THz and fully interpret its characteristics with a quantum model describing multiple intraband transitions of single carriers between quantized states. Our analysis reveals the absence of collective excitations in the THz optical response of these self-doped large NCs. Furthermore, using optical pump-THz probe experiments, we report on carrier dynamics at low energy as long as 6 ps in these self-doped THz HgTe NCs. We highlight evidence that Auger recombination is irrelevant in this system and attribute the main carrier recombination process to direct energy transfer from the electronic transition to the ligand vibrational modes and to nonradiative recombination assisted by surface traps. Our study opens interesting perspectives for the use of large HgTe NCs for the development of advanced THz optoelectronic devices such as emitters and detectors and for quantum engineering at THz frequencies.

A perovskite LaFe 1− x Mg x O 3 ( x  = 0, 1/3) compound was prepared by the sol–gel method and characterized by TG–DTA, XRD, SEM, and XPS. The infrared (IR) emissivity of the nanoscale LaFe 2/3 Mg 1/3 O 3 powder synthesized after heat preservation for 2 h at 1300 °C was as high as 0.93 in the range 0.2–2 μm, an increase of 121 % than that of undoped LaFeO 3 . The increase in the IR emissivity of LaFe 2/3 Mg 1/3 O 3 can be mainly attributed to the substitution of Mg 2+ with Fe 3+ in the LaFeO 3 lattice. The substitution of Mg 2+ with Fe 3+ introduced the energy level of Fe 4+ impurity and generated oxygen vacancies, thus increasing the impurity and oxygen vacancy absorption. The lattice distortion caused by doping strengthened the lattice vibration absorption. The polaron hopping of electrons between Fe 3+ and Fe 4+ also significantly strengthened the absorption properties within the corresponding spectrum area of Mg 2+ -doped LaFeO 3 . Graphical Abstract The absorptivity of LaFeO 3 was stable at ~0.94 in the range 200–500 nm and decreased drastically after 500 nm, and the absorption edge was ~600 nm. The absorption of Mg-doped LaFeO 3 sample was 0.93 in the range 200–2000 nm. Mg doping significantly improved the absorptivity of LaFeO 3 (0.75); moreover, the absorption edge red-shifted to the spectral region at ~2000 nm. This was caused by the introduction of Mg 2+ that increased the oxygen vacancy concentration in LaFeO 3 valence band and enhanced the intraband absorption, thus strengthening the absorptivity to some extent.

This paper studies the optical properties of the Mn1.8Co1.2Al alloy, whose composition is close to that of spin gapless Mn2CoAl semiconductor. Our investigation found the absence of intraband absorption contribution, which is an anomalous behavior of optical properties of the alloy. The presence of intense interband absorption in the IR range of spectrum indicates the existence of low-energy gap in the band spectrum of the alloy, which was found by electron structure calculations of the Mn1.75Co1.25Al alloy.

A microscopic analysis of the radiation intraband absorption mechanism by holes with their transition to a spin-split band for quantum wells based on InGaAsP/InP solid solutions is performed within the framework of the four-band Kane model. The calculation is made for two polarizations of the incident radiation: along the crystal growth axis and in the plane of the quantum well. It is shown that this process can be the main mechanism of internal radiation losses for quantum well lasers. It is also shown that the dependence of the absorption coefficient on the width of the quantum well has a maximum at a well width from 40 to 60 A.