Transfer Matrix Method-Compatible Model for Metamaterial Stacks

Mean-field theory-based effective refractive index models are widely used to design optical metamaterials and interpret their optical properties. However, emerging applications where metamaterials are embedded into layered device architectures require a detailed consideration of the metamaterial’s dispersive properties and interfacial boundary conditions, which are beyond the scope of the mean-field theory for homogeneous bulk media. Here, we describe an approach to calculate the optical transfer function for one-dimensional optical metamaterials that includes the dispersive properties of the effective index as well as the effective interfacial impedance. We address the boundary conditions at a metamaterial interface by a complex-valued effective interfacial impedance. Combined with the effective refractive index, the effective interfacial impedance enables a description of the optical transfer for 1D optical metamaterials with the transfer matrix method. This opens up scalable design of one-dimensional multilayered structures that include metamaterial layers. We illustrate the approach with the design of a metamaterial-based antireflection coating for a thin-film photodetector

Semiconducting [(Bi₄Te₄Br₂)(Al₂Cl_6–<i>x</i>Br_<i>x</i>)]Cl₂ and [Bi₂Se₂Br](AlCl₄): Cationic Chalcogenide Frameworks from Lewis Acidic Ionic Liquids

Lewis acidic organic ionic liquids provide a novel synthetic medium to prepare new semiconducting chalcogenides, [(Bi₄Te₄Br₂)­(Al₂Cl_5.46Br_0.54)]­Cl₂ (<b>1</b>) and [Bi₂Se₂Br]­(AlCl₄) (<b>2</b>). Compound <b>1</b> features a cationic [(Bi₄Te₄Br₂)­(Al₂Cl_5.46Br_0.54)]²⁺ three-dimensional framework, while compound <b>2</b> consists of cationic layers of [Bi₂Se₂Br]²⁺. Spectroscopically measured band gaps of <b>1</b> and <b>2</b> are ∼0.6 and ∼1.2 eV, respectively. Thermoelectric power measurements of single crystals of <b>1</b> indicate an n-type semiconductor

Possible <i>n–</i>type carrier sources in In₂O₃(ZnO)_k

Homologous compounds with the formula In₂O₃(ZnO)_k, where k is an integer, have potential applications as transparent conducting oxides and high temperature thermoelectric materials. In this study, we focus on the defect properties. Using the <i>k</i> = 3 phase as a prototype, we calculate with the first-principles method the defect formation energies and transition levels of the most probable <i>n</i>-type carrier producers, which include oxygen vacancy (V_O), indium antisite on zinc (In_Zn), indium interstitial (In_i), and zinc interstitial (Zn_i). The site-preference of these defects has been explored by comparing the total energies of defects at different sites. Under the <i>n</i>-type environment, In_Zn has a low formation energy and meanwhile a transition energy level close to the conduction band minimum (CBM); V_O also has a lower formation energy, however a deep transition energy level in the band gap; the cation interstitials have high formation energies, although their defect transition energy levels are quite shallow. Besides, we find that V_O and In_Zn tend to form a defect complex when the two isolated defects take the nearest-neighboring atomic sites in the same <i>ab</i>-plane. We conclude that In_Zn and its related defect-complex are the possible <i>n</i>–type carrier sources in In₂O₃(ZnO)_k. Besides, we found that V_O has a significant site-preference, which can modify the site-preference of In_Zn by forming defect-complexes. This may lead to high anisotropy in relaxation time, and then the experimentally reported strong anisotropy in electrical conductivities in In₂O₃(ZnO)₅

Semiconducting [(Bi₄Te₄Br₂)(Al₂Cl_6–<i>x</i>Br_<i>x</i>)]Cl₂ and [Bi₂Se₂Br](AlCl₄): Cationic Chalcogenide Frameworks from Lewis Acidic Ionic Liquids

Lewis acidic organic ionic liquids provide a novel synthetic medium to prepare new semiconducting chalcogenides, [(Bi₄Te₄Br₂)­(Al₂Cl_5.46Br_0.54)]­Cl₂ (<b>1</b>) and [Bi₂Se₂Br]­(AlCl₄) (<b>2</b>). Compound <b>1</b> features a cationic [(Bi₄Te₄Br₂)­(Al₂Cl_5.46Br_0.54)]²⁺ three-dimensional framework, while compound <b>2</b> consists of cationic layers of [Bi₂Se₂Br]²⁺. Spectroscopically measured band gaps of <b>1</b> and <b>2</b> are ∼0.6 and ∼1.2 eV, respectively. Thermoelectric power measurements of single crystals of <b>1</b> indicate an n-type semiconductor

CsSnI₃: Semiconductor or Metal? High Electrical Conductivity and Strong Near-Infrared Photoluminescence from a Single Material. High Hole Mobility and Phase-Transitions

CsSnI₃ is an unusual perovskite that undergoes complex displacive and reconstructive phase transitions and exhibits near-infrared emission at room temperature. Experimental and theoretical studies of CsSnI₃ have been limited by the lack of detailed crystal structure characterization and chemical instability. Here we describe the synthesis of pure polymorphic crystals, the preparation of large crack-/bubble-free ingots, the refined single-crystal structures, and temperature-dependent charge transport and optical properties of CsSnI₃, coupled with <i>ab initio</i> first-principles density functional theory (DFT) calculations. <i>In situ</i> temperature-dependent single-crystal and synchrotron powder X-ray diffraction studies reveal the origin of polymorphous phase transitions of CsSnI₃. The black orthorhombic form of CsSnI₃ demonstrates one of the largest volumetric thermal expansion coefficients for inorganic solids. Electrical conductivity, Hall effect, and thermopower measurements on it show p-type metallic behavior with low carrier density, despite the optical band gap of 1.3 eV. Hall effect measurements of the black orthorhombic perovskite phase of CsSnI₃ indicate that it is a p-type direct band gap semiconductor with carrier concentration at room temperature of ∼ 10¹⁷ cm^–3 and a hole mobility of ∼585 cm² V^–1 s^–1. The hole mobility is one of the highest observed among p-type semiconductors with comparable band gaps. Its powders exhibit a strong room-temperature near-IR emission spectrum at 950 nm. Remarkably, the values of the electrical conductivity and photoluminescence intensity increase with heat treatment. The DFT calculations show that the screened-exchange local density approximation-derived band gap agrees well with the experimentally measured band gap. Calculations of the formation energy of defects strongly suggest that the electrical and light emission properties possibly result from Sn defects in the crystal structure, which arise intrinsically. Thus, although stoichiometric CsSnI₃ is a semiconductor, the material is prone to intrinsic defects associated with Sn vacancies. This creates highly mobile holes which cause the materials to appear metallic

Switching of Photonic Crystal Lasers by Graphene

Unique features of graphene have motivated the development of graphene-integrated photonic devices. In particular, the electrical tunability of graphene loss enables high-speed modulation of light and tuning of cavity resonances in graphene-integrated waveguides and cavities. However, efficient control of light emission such as lasing, using graphene, remains a challenge. In this work, we demonstrate on/off switching of single- and double-cavity photonic crystal lasers by electrical gating of a monolayer graphene sheet on top of photonic crystal cavities. The optical loss of graphene was controlled by varying the gate voltage <i>V</i>_g, with the ion gel atop the graphene sheet. First, the fundamental properties of graphene were investigated through the transmittance measurement and numerical simulations. Next, optically pumped lasing was demonstrated for a graphene-integrated single photonic crystal cavity at <i>V</i>_g below −0.6 V, exhibiting a low lasing threshold of ∼480 μW, whereas lasing was not observed at <i>V</i>_g above −0.6 V owing to the intrinsic optical loss of graphene. Changing quality factor of the graphene-integrated photonic crystal cavity enables or disables the lasing operation. Moreover, in the double-cavity photonic crystal lasers with graphene, switching of individual cavities with separate graphene sheets was achieved, and these two lasing actions were controlled independently despite the close distance of ∼2.2 μm between adjacent cavities. We believe that our simple and practical approach for switching in graphene-integrated active photonic devices will pave the way toward designing high-contrast and ultracompact photonic integrated circuits

Microstructured Air Cavities as High-Index Contrast Substrates with Strong Diffraction for Light-Emitting Diodes

Two-dimensional high-index-contrast dielectric gratings exhibit unconventional transmission and reflection due to their morphologies. For light-emitting devices, these characteristics help guided modes defeat total internal reflections, thereby enhancing the outcoupling efficiency into an ambient medium. However, the outcoupling ability is typically impeded by the limited index contrast given by pattern media. Here, we report strong-diffraction, high-index-contrast cavity engineered substrates (CESs) in which hexagonally arranged hemispherical air cavities are covered with a 80 nm thick crystallized alumina shell. Wavelength-resolved diffraction measurements and Fourier analysis on GaN-grown CESs reveal that the high-index-contrast air/alumina core/shell patterns lead to dramatic excitation of the low-order diffraction modes. Large-area (1075 × 750 μm²) blue-emitting InGaN/GaN light-emitting diodes (LEDs) fabricated on a 3 μm pitch CES exhibit ∼39% enhancement in the optical power compared to state-of-the-art, patterned-sapphire-substrate LEDs, while preserving all of the electrical metrics that are relevant to LED devices. Full-vectorial simulations quantitatively demonstrate the enhanced optical power of CES LEDs and show a progressive increase in the extraction efficiency as the air cavity volume is expanded. This trend in light extraction is observed for both lateral- and flip-chip-geometry LEDs. Measurements of far-field profiles indicate a substantial beaming effect for CES LEDs, despite their few-micron-pitch pattern. Near-to-far-field transformation simulations and polarization analysis demonstrate that the improved extraction efficiency of CES LEDs is ascribed to the increase in emissions via the top escape route and to the extraction of transverse-magnetic polarized light