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Quantum well infrared photodetectors
Published in Antoni Rogalski, Infrared and Terahertz Detectors, 2019
Figure 19.13 shows two detector configurations used in the fabrication of multicolor QWIP FPAs. The major advantage of the bound-to-continuum QWIP (Figure 19.13a) is that the photoelectron can escape from the QW to the continuum transport states without being tunneled through the barrier. As a result, the voltage bias required to efficiently collect the photoelectrons can be reduced dramatically, thereby lowering the dark current. Furthermore, since the photoelectrons are collected without having to tunnel through a barrier, the AlGaAs barriers can be made thicker without reducing the photoelectron collection efficiency. The multilayer structure consists of a periodic array of Si-doped (Nd ≈ 1018cm−3) GaAs QWs of thickness Lw separated by undoped-AlxGa1–xAs barriers of thickness Lb. The heavy n-type doping in the wells is required to ensure that freezeout does occur at low temperatures and that a sufficient number of electrons are available to absorb the IR radiation. For operation at λ = 7–11 μm, typically Lw = 40 Å, Lb = 500 Å, x = 0.25–0.30, and 50 periods are grown. In order to shift the intersubband absorption to longer wavelength, the x value is decreased to x = 0.15 and, in addition, in order to maintain the strong optical absorption and reasonably sharp cutoff line shape, the QW width is increased from 50 Å to 60 Å. This optimization allows the same bound state to excited continuum state optical absorption and efficient hot electron transport and collection. It appears that the dark current decreases significantly when the first excited state is decreased in energy from the continuum to the well top in a bound-to-quasi-bound QWIP (Figure 19.14 [74]), without sacrificing responsivity. In comparison with narrow response of bound-to-bound transitions, the bound-to-continuum transitions are characterized by a broader response. The simple QWIP structures shown in Figures 19.5a and 19.13a are based on the photoemission of electrons from the QWs. They are unipolar devices with contacts on both sides, which require typically 50 wells for sufficient absorption (although 10–100 have been used).
Harmonic Generation with Mie Resonant Nanostructures
Published in Costantino De Angelis, Giuseppe Leo, Dragomir N. Neshev, Nonlinear Meta-Optics, 2020
Costantino De Angelis, Luca Carletti, Davide Rocco, Andrea Locatelli, Lavinia Ghirardini, Marco Finazzi, Michele Celebrano, Lei Xu, Andrey Miroshnichenko
Recent theoretical predictions, as well as experimental realizations, demonstrated the huge impact that nonlinear optics in nanostructured materials can have in applications ranging from high-throughput sensing to small-footprint quantum optics. High-index dielectric nanostructures have demonstrated second-harmonic generation (SHG) [5–10] and third-harmonic generation (THG) [11–13,19] from an optical pump beam either at visible (VIS) or near infrared (NIR) frequencies with unprecedented record high conversion efficiencies in proportion to the structure footprint. Key points to achieve enhanced nonlinear optical processes at the nanoscale are material transparency at both the pump and emission wavelengths, large susceptibilities (χ(2), χ(3)) values, and the possibility to sustain not only electric but also magnetic resonances. As far as χ(2) is concerned, this is in contrast to plasmonic nanostructures, where the nonlinear optical response is mainly associated with the surface currents [20–22], high-index dielectric resonators may display a strong nonlinear response from the material volume thanks to their large bulk nonlinear coefficients and the sizable electric fields attained inside the structure. Magnetic resonances played a pivotal role in this context due to the strong electric field enhancement provided within the nanoparticle volume. Thus, the design of nanoparticles for enhanced harmonic generation sets its basis on the analysis of their basic linear optical properties such as resonant frequencies, bandwidth, field enhancement and radiation diagrams at fundamental and harmonic frequencies [3]. Engineering of the nanoparticle geometry has emerged as a very important aspect to increase efficiency; different strategies can be employed in this approach, but due to the close link with the observed physical features, multipole decomposition of the electric fields and currents is widely adopted [23,24]. This framework revealed that the interplay between strong electric and magnetic Mie-type resonances can result not only in constructive or destructive interference leading to beam shaping in the far-field, but also to resonant enhancement of electromagnetic fields in the nanoparticles, which opens up new possibilities for efficient nonlinear processes [25]. Along with this research line, recently it was suggested [26] that subwavelength nanoscale resonators can support localized states with high-Q factors (on the order of 102 in the visible and near-infrared spectrum), provided that their parameters are closely matched to a bound state in the continuum (BIC) [27] formed via destructive interference of two similar leaky modes [28,29]. Obviously, the BIC is a mathematical abstraction and its realization demands infinite size of the structure or either zero or infinite permittivity [30,31]; however, using this approach, high-index dielectric nanoparticles can exhibit high-Q resonances associated with the so-called supercavity modes [32] and these large Q factors can dramatically enhance nonlinear effects at the nanoscale [2,3,9].
Inner-shell photoabsorption and photoionisation cross-sections of valence excited states from asymmetric-Lanczos equation-of-motion coupled cluster singles and doubles theory
Published in Molecular Physics, 2021
Torsha Moitra, Sonia Coriani, Bruno Nunes Cabral Tenorio
Below the ionisation limit, the oscillator strength corresponds to the usual discrete strengths [51]. In the case of photoionisation, the discrete oscillator strengths are replaced by a continuous oscillator strength function , which can be calculated, in an analogous way as the discrete ones, from the integral between initial bound state, and a final continuum state . The latter is a continuum solution of the electronic Schrödinger equation with an energy ω above the ground state [51]. Unlike bound-state wavefunctions, the continuum wavefunctions are not square-integrable, i.e. the integral is infinite.
Achieving polarization control by utilizing electromagnetically induced transparency based on metasurface
Published in Waves in Random and Complex Media, 2022
Cheng-Jing Gao, Yuan-Zhe Sun, Han-Qing Dong, Hai-Feng Zhang
Metamaterials, as artificial materials, are well-known for their many novels and amusing electromagnetic (EM) properties. Moreover, the metamaterials have attracted increasing attention over the past decades and can be applied in invisibility cloak [10,11], negative refractive index [12,13], diffraction-unlimited imaging [14–16], and optical switching [17]. Based on the charming characteristics of the metamaterials, Chowdhury et al. [18] have experimentally and numerically studied the nature of the coupling between laterally paired terahertz (THz) metamaterial split-ring resonators well. Furthermore, compared with the traditional materials, some physical phenomena can be clearly observed in the metamaterials without rigorous experimental conditions of strong laser and lower temperature, including the EIT. Zhang et al. [19] achieved the EIT behavior by employing the EM metamaterials in 2008. Since then, increasing research groups have begun to deeply research EIT behaviors. Additionally, the polarization torsion about the metamaterials is greater than that of natural materials [20], which provides a novel idea and approach to operate the polarization state of the EM waves (PSEWs). And the PSEWs can be a significant feature in modern optics and information science research fields. The manipulation of the polarization is critical for imaging [21], sensitive detection, antenna, and other EM applications [22–24]. More importantly, the metamaterials can exhibit rich anisotropy. In 2021, Tan et al. [25] demonstrated that a nanometric dielectric or semiconductor layer can induce a dynamically controllable quasi-bound state in the continuum with ultrahigh quality factor in a symmetric metallic metasurface at terahertz frequencies, realizing a narrow-band terahertz filter / modulator with 200% intensity modulation and recovery time within 7 ps. Song et al. [26] used an anisotropic dielectric element reflection line to realize the cross-polarization in the THz regime, and the reflection coefficient after cross-polarization was as high as about 100% in 0.75-1.0 THz. Moreover, the energy loss was very small. Numerous methods have been explored to operate the amplitude and phase of the EM waves in reflection spectra, transmission, and absorption [27–30], birefringence effects [31], and Faraday effect [32]. Of course, there are also lots of research on the control of the THz metamaterials [33,34]. Nevertheless, since the thicknesses of conventional polarization controller devices are greatly larger contrasted with wavelength, it is comparatively troublesome for low-frequency applications [35,36] and the integration to the photonic system [37]. Furthermore, strong interference effects between the incident waves and the reflected waves can exist. Therefore, it is particularly indispensable to explore and design transparent polarization converters [38–41].