Region Of Interband Transitions Research Articles

Sub-picosecond biphasic ultrafast all-optical switching in ultraviolet band

Abstract Ultrafast all-optical control has been a subject of wide-spread attention as a method of manipulating optical fields using light excitation on extremely short time scales. As a fundamental form of ultrafast all-optical control, all-optical switching has achieved sub-picosecond switch speeds in the visible, infrared, and terahertz spectral regions. However, due to the lack of suitable materials, ultrafast all-optical control in the ultraviolet range remains in its early stages. We demonstrate sub-picosecond all-optical switching in the ultraviolet wavelength by designing a Si3N4-ITO Fabry–Pérot resonance aligns with the edge of the interband transition region of ITO. The response time of 500 fs achieved at a pump fluence as low as 0.17 mJ/cm2. Notably, unlike conventional binary switches (0, 1), this biphasic all-optical switch enables the modulation of optical intensity with positive, zero, and negative ΔR/R (0, 1, −1) at the same wavelength, all achieved with a switching speed of 680 fs at a pump fluence of 0.45 mJ/cm2. This work establishing a new pathway for all-optical control in the ultraviolet spectrum, the biphasic switch provides an extra degree of freedom for all-optical modulation.

Chiral biosensing at both interband transition and plasmonic extinction regions using twisted-stacked nanowire arrays.

Chiral metal nanostructures that exhibit strong chiroptical properties and enhanced light-matter interactions have recently attracted great interest due to their potential applications including chiral sensing and asymmetric synthesis. Most studies in this field focused on chiral sensing using circular dichroism (CD) responses at the plasmonic extinction region. In comparison, little is known about their CD responses at interband transition regions and their utility in chiral biosensing. Herein, we constructed a series of twisted-stacked silver nanowire arrays (TNAs) featuring CD signals at both the interband transition and plasmonic extinction regions and that are independently controllable. These TNAs are highly sensitive towards protein secondary structures. Proteins containing more β-sheets are more sensitive toward strong chiral plasmonic fields, whereas proteins rich in α-helices tend to generate larger CD shifts at the interband transition region of TNAs. The mutually independent optical activities at the interband transition and plasmonic extinction regions complement each other, providing more sensitivity and reliability in chiral biosensing.

Tunable Infrared Plasmonic Properties of Epitaxial Ti1-xMgxN(001) Layers.

Optical transmission and reflection spectra in combination with ellipsometry and transport measurements on epitaxial rocksalt structure Ti1-xMgxN(001) layers with 0.00 ≤ x ≤ 0.49 are employed to explore their potential as refractory infrared plasmonic materials. A red shift in the reflection edge ℏωe from 2.0 to 0.8 eV and the corresponding unscreened plasma energy ℏωpu from 7.6 to 4.7 eV indicate a linear reduction in the free carrier density N with increasing x. However, nitrogen vacancies in Mg-rich samples act as donors, resulting in a minimum N = 1.6 × 1022 cm-3 for x = 0.49. Photoelectron valence band spectra confirm the diminishing conduction band density of states and indicate a 0.9 eV decrease in the Fermi level as x increases from 0 to 0.49. The dielectric function ε = ε1 + iε2 can be divided into a low-energy spectral region where intraband transitions result in large negative and positive ε1 and ε2, respectively, and a higher energy interband transition region with both ε1 and ε2 > 0. The screened plasma energy Eps that separates these two regions red-shifts from 2.6 to 1.3 eV for x = 0-0.39, indicating a tunable plasmonic activity that extends from the visible to the infrared (470-930 nm). Electron transport measurements indicate a metallic temperature coefficient of resistivity (TCR) for TiN-rich alloys with x ≤ 0.26 but weak carrier localization and a negative TCR <60 K for x = 0.39 and <300 K for x = 0.49, attributed to Mg alloying-induced disorder. The plasmonic quality factor Q is approximately an order of magnitude larger than what was previously reported for polycrystalline Ti1-xMgxN, making Ti1-xMgxN(001) layers competitive with Ti1-xScxN(001).

Plasmonic mode coupling in graphene-based photonic crystals

Abstract Plasmonic bands in 1D graphene-based photonic crystals have recently been reported for transverse magnetic polarization in the low THz regime. In this work, we demonstrate that plasmon coupling, giving rise to plasmonic bands, is also present for transverse electric polarization at IR frequencies corresponding to the region of interband transitions in graphene. Due to the doping level dependence of graphene interband optical conductivity, plasmonic band region can be shifted in the IR frequency regime for different doping levels. We have analyzed graphene-based photonic crystals with a homogeneous and periodic doping level sequence along crystal growth direction in the unit cell; our results prove that plasmonic band opening is obtained for these two different graphene doping sequences, in the frequency region such that imaginary optical conductivity turns negative, displaying different levels of attenuation due interband absorption processes. Finally, we show that TE plasmonic modes in a finite multilayer can be excited by attenuated total reflection technique in Otto configuration.

The Structure of Electronic States in FeSb2 According to Optical Spectroscopy and Band Calculations

The electronic structure and optical properties of a binary intermetallic FeSb2 compound are studied. The calculations of the band structure, that demonstrate the existence of a narrow ∼0.3 eV gap in the energy spectrum of this material, are performed in the approximation of a local electronic density. Spectral characteristics are studied by the ellipsometric method within a 0.22–18 μm range of wavelengths. It is shown that, in the region of interband transitions, the experimental optical conductivity of the compound is satisfactorily interpreted within the performed calculations of the density of electronic states.

Структура электронных состояний в FeSb-=SUB=-2-=/SUB=- по данным оптической спектроскопии и зонных расчетов

The electronic structure and optical properties of the binary intermetallic compound FeSb2 have been studied. The band structure was calculated in the local density approximation, which showed the existence of a narrow ~0.3 eV gap in the energy spectrum of this material. The spectral characteristics were studied by ellipsometric method in the wavelength range of 0.22-18 μm. It is shown that the experimental optical conductivity of the compound in the region of interband transitions is satisfactorily interpreted in the framework of the calculations of the density of electronic states. The work was performed as part of the state assignment of the Ministry of Education and Science of the Russian Federation (theme Electron, N AAAA-A18-118020190098-5) with partial support from the Russian Foundation for Basic Research (project N 17-52-45056).

Electronic and Spectral Properties of RRhSn (R = Gd, Tb) Intermetallic Compounds

The investigations of electronic structure and optical properties of GdRhSn and TbRhSn were carried out. The calculations of band spectrum, taking into account the spin polarization, were performed in a local electron density approximation with a correction for strong correlation effects in 4f shell of rare earth metal (LSDA + U method). The optical studies were done by ellipsometry in a wide range of wavelengths, and the set of spectral and electronic characteristics was determined. It was shown that optical absorption in a region of interband transitions has a satisfactory explanation within a scope of calculations of density of electronic states carried out.

Электронные и спектральные свойства интерметаллических соединений RRhSn (R=Gd, Tb)

AbstractThe investigations of electronic structure and optical properties of GdRhSn and TbRhSn were carried out. The calculations of band spectrum, taking into account the spin polarization, were performed in a local electron density approximation with a correction for strong correlation effects in 4f shell of rare earth metal (LSDA + U method). The optical studies were done by ellipsometry in a wide range of wavelengths, and the set of spectral and electronic characteristics was determined. It was shown that optical absorption in a region of interband transitions has a satisfactory explanation within a scope of calculations of density of electronic states carried out.

Optical emission of graphene and electron-hole pair production induced by a strong terahertz field

We report on experimental observation of optical emission of graphene induced by an intense terahertz (THz) pulse. P-doped chemical-vapor-deposition graphene with an initial Fermi energy of about 200 meV was used; optical photons were detected in the 2.0--3.5 eV range. Emission started when the THz field amplitude exceeded 100 kV/cm. For the THz fields from 200 to 300 kV/cm, the temperature of optical radiation was constant, while the number of emitted photons increased by several dozen times. This fact clearly indicates multiplication of electron-hole pairs induced by an external field and not electron heating. The experimental data are in good agreement with the theory of Landau-Zener interband transitions. It is shown theoretically that Landau-Zener transitions are possible even in the case of heavily doped graphene because the strong THz field removes quasiparticles from the region of interband transitions for several femtoseconds, which cancels the Pauli blocking effect.

Magneto-optical Kerr spectroscopy of (Ga,Mn)(As,P) ferromagnetic layers: Experiments and k.p theory

We present a theoretical and experimental study of the magneto-optical properties of thin (Ga,Mn)(As,P) layers on GaAs substrates with varying phosphorus fractions. Using a 40-band k.p model as well as an antiferromagnetic (s, p)-d exchange interaction, we calculate the dielectric tensor, the Kerr rotation angle, and the ellipticity in the interband transition region. Our calculations are validated through a set of accurate comparisons with experimental results. The Kerr ellipticity peak is found to be 2 to 3 times larger than the Kerr rotation angle both experimentally and theoretically. This work will enable to optimize the magneto-optical effects in these layers for maximum sensitivity in ultra-fast magnetization dynamics and domain microscopy experiments.

Ultraviolet Analysis of Gold Nanorod and Nanosphere Solutions

The localized surface plasmon resonances of gold nanoparticles have been widely studied at visible and near-infrared frequencies but less so in the ultraviolet. The spectral extinction measurements of gold nanospheres at UV wavelengths reported here closely match calculated spectra down to 200 nm using the measured dielectric function of gold. The nanosphere volume attenuation coefficient, αv, is size-dependent in the 200–300 nm wavelength range where the dielectric function follows the Drude free electron model (as it does for wavelengths larger than 500 nm). In the interband transition region, the extinction is more closely proportional to mass and therefore has a value of αv that is largely independent of size. Gold nanorod solutions exhibit very strong UV extinction below 220 nm due to charge transfer to solvent (c.t.t.s.) excitations of the bromide counterion to the cationic surfactant. The gold nanorod UV properties depend on the alignment between the optical polarization and nanorod structure. For ...

Strong modulation of plasmons in Graphene with the use of an Inverted pyramid array diffraction grating.

An optical device configuration allowing efficient electrical tuning of surface plasmon wavelength and absorption in a suspended/conformal graphene film is reported. An underlying 2-dimensional array of inverted rectangular pyramids greatly enhances optical coupling to the graphene film. In contrast to devices utilising 1D grating or Kretchman prism coupling configurations, both s and p polarization can excite plasmons due to symmetry of the grating structure. Additionally, the excited high frequency plasmon mode has a wavelength independent of incident photon angle allowing multidirectional coupling. By combining analytical methods with Rigorous Coupled-Wave Analysis, absorption of plasmons is mapped over near infrared spectral range as a function of chemical potential. Strong control over both plasmon wavelength and strength is provided by an ionic gel gate configuration. 0.04eV change in chemical potential increases plasmon energy by 0.05 eV shifting plasmon wavelength towards the visible, and providing enhancement in plasmon absorption. Most importantly, plasmon excitation can be dynamically switched off by lowering the chemical potential and moving from the intra-band to the inter-band transition region. Ability to electrically tune plasmon properties can be utilized in applications such as on-chip light modulation, photonic logic gates, optical interconnect and sensing applications.

Far- and near-field optical properties of Al nanorod by discrete dipole approximation

We report a detailed theoretical investigation about the influence of the nanorod length and incident polarization on the far-field and near-field optical properties of Al nanorod using the discrete dipole approximation. The results show that Al nanorod has great tunability in the UV-Vis-NIR region and much stronger near-field intensity at UV and NIR wavelengths. The band position of transverse mode (TM) for the nanorod has a little blueshifted as the length increases, while that of the longitudinal mode (LM) is linearly redshifted with the size. In addition, we investigate how the incident polarization affects the extinction position and efficiency of the TM and LM. Finally, the near-field distribution dependences of the nanorod length for the TM and the LM have been analyzed. It is shown that much enhanced near-field always occur in the lateral surface of the nanorod for TM and the tip of the nanorod for the LM, respectively. The maximum local field enhancement factor of the TM is found to be about two times higher than that of the LM, which is due to close the region of interband transitions at about nm. These results would be of great potential application in localized surface plasmon resonance sensor and UV-plasmonic device.

Plasmon enhanced fluorescence studies from aligned gold nanorod arrays modified with SiO2 spacer layers

Here, we demonstrate that quasi self-standing Au nanorod arrays prepared with plasma polymerisation deposited SiO2 dielectric spacers support surface enhanced fluorescence (SEF) while maintaining high signal reproducibility. We show that it is possible to find a balance between enhanced radiative and non-radiative decay rates at which the fluorescent intensity is maximized. The SEF signal optimised with a 30 nm spacer layer thickness showed a 3.5-fold enhancement with a signal variance of &amp;lt;15% thereby keeping the integrity of the nanorod array. We also demonstrate the decreased importance of obtaining resonance conditions when localized surface plasmon resonance is positioned within the spectral region of Au interband transitions. Procedures for further increasing the SEF enhancement factor are also discussed.

Shape-Selective Formation of Monodisperse Copper Nanospheres and Nanocubes via Disproportionation Reaction Route and Their Optical Properties

Synthesis of stable and monodisperse Cu nanocrystals of controlled morphology has been a long-standing challenge. In this Article, we report a facile disproportionation reaction approach for the synthesis of such nanocrystals in organic solvents. Either spherical or cubic shapes can be produced, depending on conditions. The typical Cu nanospheres are single crystals with a size of 23.4 ± 1.5 nm, and can self-assemble into three-dimensional (3D) nanocrystal superlattices with a large scale. By manipulating the chemical additives, monodisperse Cu nanocubes with tailorable sizes have also been obtained. The probable formation mechanism of these Cu nanocrystals is discussed. The narrow size distribution results in strong surface plasmon resonance (SPR) peaks even though the resonance is located in the interband transition region. Double SPR peaks are observed in the extinction spectra for the Cu nanocubes with relative large sizes. Theoretical simulation of the extinction spectra indicates that the SPR band l...

Metal-Enhanced Fluorescence Platforms Based on Plasmonic Ordered Copper Arrays: Wavelength Dependence of Quenching and Enhancement Effects

Ordered arrays of copper nanostructures were fabricated and modified with porphyrin molecules in order to evaluate fluorescence enhancement due to the localized surface plasmon resonance. The nanostructures were prepared by thermally depositing copper on the upper hemispheres of two-dimensional silica colloidal crystals. The wavelength at which the surface plasmon resonance of the nanostructures was generated was tuned to a longer wavelength than the interband transition region of copper (>590 nm) by controlling the diameter of the underlying silica particles. Immobilization of porphyrin monolayers onto the nanostructures was achieved via self-assembly of 16-mercaptohexadecanoic acid, which also suppressed the oxidation of the copper surface. The maximum fluorescence enhancement of porphyrin by a factor of 89.2 was achieved as compared with that on a planar Cu plate (CuP) due to the generation of the surface plasmon resonance. Furthermore, it was found that while the fluorescence from the porphyrin was quenched within the interband transition region, it was efficiently enhanced at longer wavelengths. It was demonstrated that the enhancement induced by the proximity of the fluorophore to the nanostructures was enough to overcome the highly efficient quenching effects of the metal. From these results, it is speculated that the surface plasmon resonance of copper has tremendous potential for practical use as high functional plasmonic sensor and devices.

Effect of carbon doping on electronic transitions in Mn5Ge3

Mn5Ge3 is a ferromagnetic compound with high Curie temperature (Tc = 293 K), high spin polarization, and a good lattice match to germanium. Doping Mn5Ge3 with carbon increases Tc above room temperature and makes these compounds promising candidates for spin injectors for potential spintronics applications. The resistivity and magnetic susceptibility measurements show anisotropic behavior of these compounds. Optical spectroscopy is employed to measure near-normal reflectance of Mn5Ge3C0.89 in the frequency range from far-infrared to ultraviolet at three different temperatures (10, 200, and 300 K), and results are compared with those on pure Mn5Ge3. Both Mn5Ge3 and Mn5Ge3C0.89 have weak temperature dependence of the optical properties in the 10–300 K range, and both have similar electrodynamics responses with similar temperature trends. However, important differences in the region of interband transitions, indicating the electronic nature of the increased ferromagnetic stability of carbon doped compound, are observed.

Intraband optical transitions of holes in strained SiGe quantum wells

Optical absorption spectra of polarized infrared radiation in p-type SiGe quantum-well structures have been investigated. Three types of structures with different magnitudes and signs of biaxial elastic strains have been studied. The light absorption factor has been found to double in the region of interband transitions under the in-plane tensile strain of the quantum-well heterojunction. The results are attributed to a change in the order of light- and heavy-hole bands caused by the tensile strain and to the formation of the ground state of charge carriers in the light-hole band.

Study of Positive and Negative Refraction of Visible Light at the Cu/Air Interface

Light transmission at the Cu/air interface was measured in the visible region for a series of wedge-shaped Cu film samples. It is found that light refraction at the Cu/air interface changes from negative in the Drude region, passing through zero at about E g , to positive in the interband-transition region. Detailed discussion is given to exclude those mechanisms which have been arguably assigned as the cause of negative refraction at a metal/dielectric interface. Based on the spectra of the measured refractive index and modeled in the Drude region, the positive and negative refraction analyzed in this work are qualitatively in agreement with dispersion of the group refractive index, and may aid in understanding light transmission at the metal/dielectric interface as characterized by the law of refraction.

Measurement of III-V Compound Semiconductor Characteristics using the Contactless Electroreflectance Method

The electromodulation methods of photoreflectanceand the related technique of contactless electroreflectance(CER) are valuable tools in the evaluation of important device parameters for structures such as heterojunction bipolar transistors, pseudomorphic high electron mobility transistors, and quantum dots(QDs). CER is a very general principle of experimental physics. Instead of measuring the optical reflectance of the material, the derivative with respect to a modulating electric field is evaluated. This procedure generates sharp, differential-like spectra in the region of interband (intersubband) transitions. We conduct electric-optical studies of both GaAs layers and InAs selfassembled QDs grown by molecular beam epitaxy. Strong GaAsbandgap energy is measured in both structures. In the case of lnAs monolayers in GaAs matrices, the strong GaAsbandgap energy is caused by the lateral quantum confinement.