Large Optical Response Research Articles

Tunable and strong circular dichroism mediated with graphene-Weyl semimetal planar scheme

Strong circular dichroism (CD) has been observed in Weyl semimetals (WSMs) because of the extremely large gyrotropic optical response. However, traditional proposals cannot tune the wavelength of CD peaks continuously without changing the geometric parameters or external magnetic field, which limits the practical application. This study presents a lithography-free platform exhibits near-unity CD in a WSM-graphene-dielectric structure. It is theoretically shown that the proposal possesses different reflectance properties for right-hand circular polarization and left-hand circular polarization light in the mid-infrared range. The merits of the wide-angular chiral capacity of CP waves can also be maintained. The CD of the chiral structure can be tuned not only by adjusting the structural parameters, but also the chemical potential of graphene, which may also facilitate the design of more efficient chiral optical devices.

Thermally tunable broadband circular dichroism with Weyl semimetal/vanadium dioxide planar structure

Weyl semimetals (WSMs) have lately attracted significant interest because of the intrinsic large gyrotropic optical response and strong chiral anomaly. This study presents a mid-infrared (mid-IR) lithography-free platform exhibits broadband near-unity circular dichroism (CD) with WSM-vanadium dioxide (VO2) dual layers. The optical responses with mid-IR radiation have been explored with the generalized 4 × 4 transfer matrix algorithm. It is theoretically shown that the proposal possesses different absorption characteristics for right-handed circularly polarized (RCP) and left-handed circularly polarized (LCP) light. Theoretical results show that a CD in absorption reaching 0.936 and a g factor of 1.76 under 300 K temperature. The merit of the wide-angular chiral capacity can be maintained. The magnitude and resonance of CD can be tuned not only by adjusting the structural parameters, but also the insulator-to-metal transition (IMT) of VO2, which may facilitate the design of more efficient chiral optical devices.

Large nonlinear optical magnetoelectric response in a noncentrosymmetric magnetic Weyl semimetal

Weyl semimetals resulting from either inversion (P) or time-reversal (T) symmetry breaking have been revealed to show the record-breaking large optical response due to intense Berry curvature of Weyl-node pairs. Different classes of Weyl semimetals with both P and T symmetry breaking potentially exhibit optical magnetoelectric (ME) responses, which are essentially distinct from the previously observed optical responses in conventional Weyl semimetals, leading to the versatile functions such as directional dependence for light propagation and gyrotropic effects. However, such optical ME phenomena of (semi)metallic systems have remained elusive so far. Here, we show the large nonlinear optical ME response in noncentrosymmetric magnetic Weyl semimetal PrAlGe, in which the polar structural asymmetry and ferromagnetic ordering break P and T symmetry. We observe the giant second harmonic generation (SHG) arising from the P symmetry breaking in the paramagnetic phase, being comparable to the largest SHG response reported in Weyl semimetal TaAs. In the ferromagnetically ordered phase, it is found that interference between this nonmagnetic SHG and the magnetically induced SHG emerging due to both P and T symmetry breaking results in the magnetic field switching of SHG intensity. Furthermore, such an interference effect critically depends on the light-propagating direction. The corresponding magnetically induced nonlinear susceptibility is significantly larger than the prototypical ME material, manifesting the existence of the strong nonlinear dynamical ME coupling. The present findings establish the unique optical functionality of P- and T-symmetry broken ME topological semimetals.

Large nonreciprocal optical responses in antiferromagnets with broken space-inversion and time-reversal symmetries

Large nonreciprocal optical responses in antiferromagnets with broken space-inversion and time-reversal symmetries

Low-Cost and Fast Hydrogen Alarm System With Optical Readout Based on Suspended Palladium Film

A low-cost and high-efficiency hydrogen sensor can greatly promote the development of the hydrogen economy. It has been demonstrated that the light scattering induced by volume expansion of Pd film during hydrogenation can enable fast and high-contrast optical hydrogen sensing. Here, we present an optical hydrogen alarm system based on light scattering on a Pd film during hydrogenation. Two fibers are used to connect the sensing chip with the LED and photodiode in the alarm system, which separates the explosive detection area from the electronics and thus avoid the potential risk of spark induced by electric current. The sensing chip consists of a Pd/PMMA bilayer on a PDMS substrate with a micropillar array on the surface. This forms a partially suspended Pd/PMMA bilayer, which can reduce the constraint from the substrate on Pd film and improve the response rate. We optimize the structural parameters of the sensing chip and reduce the response time <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">90</sub> to about 2.2 s with a high reflectance contrast of about 960% at 650-nm wavelength when exposed to 4% H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mixed with air. Such a large optical response can reduce the requirements of the detection equipment and save cost. Meanwhile, the sensor exhibits high recyclability and hydrogen selectivity in the air, which shows great potential for practical hydrogen alarm applications.

Spiral Chiral Metamaterial Structure Shape for Optical Activity Improvements.

We report on a spiral structure suitable for obtaining a large optical response. We constructed a structural mechanics model of the shape of the planar spiral structure when deformed and verified the effectiveness of the model. As a verification structure, we fabricated a large-scale spiral structure that operates in the GHz band by laser processing. Based on the GHz radio wave experiments, a more uniform deformation structure exhibited a higher cross-polarization component. This result suggests that uniform deformation structures can improve circular dichroism. Since large-scale devices enable speedy prototype verification, the obtained knowledge can be exported to miniaturized-scale devices, such as MEMS terahertz metamaterials.

Strong nonreciprocal thermal radiation based on topological edge state in one-dimensional photonic crystal with Weyl semimetal

Kirchhoff's law is the theoretical basis for characterizing thermal radiation. The construction of nonreciprocal thermal radiation needs to violate Kirchhoff's law, in which its existing methods usually utilize the magneto-optical effect with an external magnetic field. However, natural materials' relatively weak magneto-optical response in the thermal wavelength range requires a strong or moderate magnetic field. Fortunately, the unique topologically nontrivial electronic state and inherent time-reversal symmetry breaking of Weyl semimetals can exhibit highly unusual and extremely large gyrotropic optical responses in the mid-infrared band without an external magnetic field, which provides a new way to violate Kirchhoff's law. This work theoretically investigates the nonreciprocity of a multilayer structure in which a Weyl semimetal is embedded in two one-dimensional photonic crystals. The results show that the absorptivity and emissivity of the structure almost completely violate Kirchhoff's law over a broad range of angles (7° ∼ 51°) and frequencies (56.4 THz ∼ 65.3 THz) without any external magnetic field. We also discuss the effects of the incident angle, the axial vector of the Weyl semimetal, and the thickness of the Weyl semimetal on the characteristics of nonreciprocal thermal radiation. Furthermore, the electric field distribution reveals that the structurally excited topological edge states significantly enhance the local electric field in the vicinity of the Weyl semimetal, providing positive conditions for the realization of perfect strong nonreciprocal thermal radiation. We hope this work may contribute to the design of strong nonreciprocal thermal emitters.

Broadband Optical Properties of Bi2Se3.

Materials with high optical constants are of paramount importance for efficient light manipulation in nanophotonics applications. Recent advances in materials science have revealed that van der Waals (vdW) materials have large optical responses owing to strong in-plane covalent bonding and weak out-of-plane vdW interactions. However, the optical constants of vdW materials depend on numerous factors, e.g., synthesis and transfer method. Here, we demonstrate that in a broad spectral range (290-3300 nm) the refractive index n and the extinction coefficient k of Bi2Se3 are almost independent of synthesis technology, with only a ~10% difference in n and k between synthesis approaches, unlike other vdW materials, such as MoS2, which has a ~60% difference between synthesis approaches. As a practical demonstration, we showed, using the examples of biosensors and therapeutic nanoparticles, that this slight difference in optical constants results in reproducible efficiency in Bi2Se3-based photonic devices.

Enhanced Diffractive Circular Dichroism from Stereoscopic Plasmonic Molecule Array

Artificial nanostructures with large optical chiral responses have been intensively investigated recently. In this work, we propose a diffractive circular dichroism enhancement technique using stereoscopic plasmonic molecule structures. According to the multipole expansion analysis, the z-component of the electric dipole becomes the dominant chiral scattering mechanism during the interaction between an individual plasmonic molecule and the plane wave at a grazing angle. For a periodical structure with the designed plasmonic molecule, large diffractive circular dichroism can be obtained, which can be associated with the Wood-Rayleigh anomaly. Such a diffractive circular dichroism enhancement is verified by the good agreement between numerical simulations and experimental results. The proposed approach can be potentially used to develop enhanced spectroscopy techniques to measure chiral information, which is very important for fundamental physical and chemical research and bio-sensing applications.

Electronic Spectrum and Optical Properties of Y-Shaped Kekulé-Patterned Graphene: Band Nesting Resonance as An Optical Signature

Employing tight-binding model we investigate the effects of a uniform Y-shaped Kekulé lattice distortion on the electronic spectrum and optical conductivity of graphene. We derive a low-energy effective Hamiltonian which is found to be in excellent agreement with one calculated from a diagonalization of the full tight-binding Hamiltonian. Then using the low-energy Hamiltonian and Kubo formula we obtain an analytical expression for the real part of the optical conductivity used to explore the effects of chemical potential, temperature and on-site and hopping energy deviations in details. In particular we find that Y-shaped Kekué-patterned graphene at finite chemical potential displays a large optical response called band nesting resonance. This effect is shown to be robust against increasing temperature, facilitating its detection as an optical signature for the Y-shaped Kekulé distortion even at room temperature.

The High Photoresponse of Stress‐Tuned MoTe2 Optoelectronic Devices in the Telecommunication Band

There are attractive prospects to achieve large photocurrent and high optical response in telecommunication wavelengths for optoelectronic devices. Inspired by the recent experiment [Nat. Photonics 2020, 14, 578], the photocurrent of the devices composed of noncentral inversion symmetric monolayer MoTe2 along the armchair direction under different linearly polarized light by the quantum transport calculation is investigated. The results demonstrate that the maximum photocurrent will increase under the biaxial tensile stress, and importantly, the peak of photocurrent will move toward the direction of lower photon energy owing to the reduction of the bandgap of monolayer MoTe2, which is opposite to the case of applying the bias voltage. Ultimately, it is proved that monolayer MoTe2 devices with large photoresponse can be tuned to the interesting telecommunication band via the imposition of biaxial stress and bias voltage, which makes the devices have potential applications in the field of silicon photonics.

Strong nonreciprocal thermal radiation by optical Tamm states in Weyl semimetallic photonic multilayers

The almost-complete violation of Kirchhoff's law possesses various potential applications, such as nonreciprocal thermal emitters, thermal routers, and other logic devices. Recently, Weyl semimetal materials have attracted wide attention. It exhibits a very unusual and extremely large gyroscopic optical response within the mid IR without an external magnetic field, which is due to its unique topologically nontrivial electronic state and inherent time-reversal symmetry breaking. Here, we show a planar multilayer structure composed of Weyl semimetals, which can achieve an almost-complete violation of Kirchhoff's law at an incident angle of 18° without applying any external magnetic field. By tuning the thickness of the ITO layer, the angle of incidence, and the axial vector of the WSM, an almost complete violation of Kirchhoff's law can persist over a wide range of angles (18°–52.3°) and wavelengths (6.6 μm–7.2 μm). Furthermore, through the study of the electric field distribution, it is revealed that this enhanced nonreciprocity is due to the local field-enhancing properties of TPPs. Our work may contribute a practical design scheme for constructing strongly nonreciprocal thermal emitters.

Mid-infrared broadband circular polarizer based on Weyl semimetals.

As a three-dimensional topological phase of matter, Weyl semimetals possess extremely large gyrotropic optical response in the mid-infrared region, leading to the strong chiral anomaly. This study proposes a circular polarizer design with a double-WSM-layer structure. It is theoretically shown that the proposed polarizer possesses a high circular polarization efficiency and high average transmittance in the wavelength region from 9 µm to 15 µm at incidence angles up to 50°. The modified 4 × 4 matrix method is used to calculate the circularly polarized transmittance of Weyl semimetals in thin-film or multilayer structures. The temperature dependence on the transmittance is also examined to demonstrate the flexibility of the proposed polarizer in a varying temperature environment. This study reveals the technological prospect that Weyl semimetals are promising candidates for high-performance circular polarizers in infrared spectroscopy and polarimetry.

Interplaying Role of Particle Size and Polymer Layer Thickness on the Large Tunable Optical Response of Polymer-coated Silver Nanostructures

The interplaying role of particle size and polymer layer thickness on the tunable optical response of polymer-coated Ag nanoparticles (NPs) has been studied experimentally and theoretically. A large redshift ( $$\sim$$ 38 nm) of surface plasmon resonance (SPR) peak position has been observed experimentally for Ag NPs of sizes in the range of 8−18 nm synthesized by polyol process with the varying concentration of metal precursor (AgNO $$_{3}$$ ) and using a surfactant (PVP) as a stabilizer. The observed large redshift of the SPR peak position of Ag NPs coated with PVP has been argued due to mainly change of NP size as well as change of local dielectric environment in the vicinity of the Ag NPs. The experimentally observed SPR peak shift has been explained by considering the system as a metal-core polymer-shell nanostructure. The change of local dielectric environment of the surrounding of the NPs is due to the change in the PVP layer on the NPs. Such observation has been confirmed through the theoretical studies considering the changes of NP size as well as the effective thickness of the PVP layers on the NPs. The SPR peak of the system with core-shell nanostructure has been found to vary linearly with the increase in radius of core and thickness of shell together. However, it changes exponentially with particle size alone. It has been found from a detailed study that the change in the ratio of radius of core and thickness of the shell is responsible for the observed redshift. Thus, the redshift of SPR peak position of Ag NPs cannot be justified by only considering the increase in particle size. Rather shell thickness has been found to play a prominent role in the SPR peak shift. This study can be used to understand the optical response of noble metal NPs coated with organic polymers.

Enhanced nonlinear optical effects based on strong coupling between epsilon-near-zero mode and gap surface plasmons

Optical nonlinear effect plays an important role in optical communication, optical detection, quantum information and other areas. However, it is constrained by the weakness of the nonlinear optical response of the common materials. The enhancement of the optical nonlinear response on a nanoscale becomes a critical challenge. Over the years, several ways to enhance the optical nonlinear effects have been suggested. In fact, these technologies can slightly enhance the optical nonlinear response. Recently, some research groups focused on the materials with vanished permittivity, which is called epsilon-near-zero (ENZ) material, showing that it can exhibit large optical nonlinearity due to the field enhancement in the material of this type. However, the ENZ material only holds a large optical nonlinear response in a limited spectral range. In order to overcome this limitation, here in this paper we report the ENZ mode which is excited by the ITO film and strongly coupled to the gap surface plasmons excited by the metal-dielectric-metal structure. To acquire the nonlinear refractive index &lt;i&gt;n&lt;/i&gt;&lt;sub&gt;2&lt;/sub&gt;, we first calculate the ITO permittivity through the Drude-Lorentz model and find the wavelength of the ENZ material. Then we calculate the time-dependent electron temperature and lattice temperature of ITO by the two-temperature model. According to the elevated electron temperature, we can calculate the plasma frequency &lt;inline-formula&gt;&lt;tex-math id="M1"&gt;\begin{document}${\omega _{\rm p}}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, and by taking it into the Drude-Lorentz model, we can obtain a new permittivity of ITO compared with the initial one. Finally, we can calculate the variation of the refractive index &lt;inline-formula&gt;&lt;tex-math id="M2"&gt;\begin{document}$ \Delta n $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M2.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M2.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, and the nonlinear refractive index &lt;inline-formula&gt;&lt;tex-math id="M3"&gt;\begin{document}$ {n_2} = \Delta n/{I_0} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M3.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M3.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;. In this paper, our coupled structure exhibits a broadband (～1000 nm bandwidth) enhancement of the nonlinear optical effect in the near-infrared spectrum, a maximum nonlinear refractive index &lt;i&gt;n&lt;/i&gt;&lt;sub&gt;2&lt;/sub&gt; as large as 3.02 cm&lt;sup&gt;2&lt;/sup&gt;·GW&lt;sup&gt;–1&lt;/sup&gt;, which is nearly 3 orders larger than the previously reported nonlinear refractive index of bare ITO film. As a result, it is possible to realize a dramatically large variation of nonlinear refractive index under a low-power optical field. It is expected to be used in the nano photonic devices such as optical storage, all-optical switches, etc.

Reversible Tuning of the Plasmoelectric Effect in Noble Metal Nanostructures Through Manipulation of Organic Ligand Energy Levels.

Ligand-controlled tuning of localized surface plasmon resonance (LSPR) properties of noble metal nanostructures is fundamentally important for various optoelectronic applications such as photocatalysis, photovoltaics, and sensing. Here we demonstrate that the free carrier concentration of gold triangular nanoprisms (Au TNPs) can be tuned up to 12% upon functionalization of their surface with different para-substituted thiophenolate (X-Ph-S-) ligands. We achieve this unprecedentedly large optical response (plasmoelectric effect) in TNPs through the selective manipulation of electronic processes at the Au-thiolate interface. Interestingly, thiophenolates with electron withdrawing (donating) groups (X) produce λLSPR blue (red) shifts with broadening (narrowing) of localized surface plasmon resonance peak (λLSPR) line widths. Surprisingly, these experimental results are opposite to a straightforward application of the Drude model. Utilizing density functional theory calculations, we develop here a frontier molecular orbital approach of Au-thiophenolate interactions in the solid-state to delineate the observed spectral response. Importantly, all the spectroscopic properties are fully reversible by exchanging thiophenolates containing electron withdrawing groups with thiophenolates having electron donating groups, and vice versa. On the basis of the experimental data and calculations, we propose that the delocalization of electrons wave function controls the free carrier concentration of Au and thus the LSPR properties rather than simple electronic properties (inductive and/or resonance effects) of thiophenolates. This is further supported by the experimentally determined work functions, which are tunable over 1.9 eV in the X-Ph-S-passivated Au TNPs. We believe that our unexpected finding has great potential to guide in developing unique noble metal nanostructure-organic ligand hybrid nanoconjugates, which could allow us to bypass the complications associated with off-resonance LSPR activation of noble metal-doped semiconductor nanocrystals for various surface plasmon-driven applications.

Intraband divergences in third order optical response of 2D systems

The existence of large nonlinear optical coefficients is one of the preconditions for using nonlinear optical materials in nonlinear optical devices. For a crystal, such large coefficients can be achieved by matching photon energies with resonant energies between different bands, and so the details of the crystal band structure play an important role. Here we demonstrate that large third-order nonlinearities can also be generally obtained by a different strategy. As any of the incident frequencies or the sum of any two or three frequencies approaches zero, the doped or excited populations of electronic states lead to divergent contributions in the induced current density. We refer to these as intraband divergences, by analogy with the behavior of Drude conductivity in linear response. Physically, such resonant processes can be associated with a combination of intraband and interband optical transitions. Current-induced second order nonlinearity, coherent current injection, and jerk currents are all related to such divergences, and we find similar divergences in degenerate four wave mixing and cross-phase modulation under certain conditions. These divergences are limited by intraband relaxation parameters and lead to a large optical response from a high quality sample; we find that they are very robust with respect to variations in the details of the band structure. To clearly track all of these effects, we analyze gapped graphene, describing the electrons as massive Dirac fermions; under the relaxation time approximation, we derive analytic expressions for the third order conductivities and identify the divergences that arise in describing the associated nonlinear phenomena.

Near-infrared third-order optical nonlinearity in CaCu3Ti4O12 thin films

We investigate the optical nonlinearity of CaCu3Ti4O[Formula: see text] (CCTO) thin films on MgO substrates using Z-scan technique at a near-infrared wavelength of 1064[Formula: see text]nm with 25[Formula: see text]ps duration pulses. The near-infrared nonlinear saturable absorption coefficient [Formula: see text] and the nonlinear refraction coefficient [Formula: see text] of the values [Formula: see text][Formula: see text]cm/GW and 0.66[Formula: see text]cm2/GW, respectively, were deduced from the study of the nonlinear response at different excitation intensities. The three-level model is proposed to interpret the large optical nonlinearity in CCTO films in near-infrared spectrum, by considering the intermediate energy levels due to the defects or grain boundaries. The large optical nonlinearity and fast response at picosecond time-domain indicate that CaCu3Ti4O[Formula: see text] thin films are promising candidates for applications in near-infrared nonlinear photonics devices.

Nonlinear optics: A surprise in store?

Nonlinear Optics At ultrafast data rates, the ability to use light to control things could speed information processing. However, photons tend not to interact with each other, and so a nonlinear optical material is needed and the response of such materials is typically weak. Alam et al. report a surprising finding: that indium tin oxide, a commercially available transparent conducting oxide widely used in microelectronics, exhibits a large nonlinear response. They used a wavelength regime where the permittivity of the material is close to zero and observed a large and fast nonlinear optical response. The finding offers the possibility that other, so far unexplored, materials may be out there for nonlinear optical applications. Science , this issue p. [795][1] [1]: /lookup/doi/10.1126/science.aae0330

Heterogeneous plasmonic trimers for enhanced nonlinear optical absorption

A dramatic enhancement of the thermally induced nonlinear optical response in compositionally heterogeneous plasmonic trimers is reported. It is demonstrated numerically that the nonlinear absorption performance of silver nanoparticle dimers under pulsed illumination can be enhanced by more than two orders of magnitude through the addition of only 0.1 vol. % of gold in the dimer gap. The nonlinear absorption performance of the resulting Ag-Au-Ag trimer exceeds the peak performance of isolated gold nanoparticles by a factor 40. This dramatic effect is enabled by cascaded plasmon resonance, resulting in extreme field concentration in the central nanoparticle of the trimer. The observed localized heat-generation, large optical response, and a predicted response time below 1 ns make these structures promising candidates for use in nonlinear optical limiting and optical switching.