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Defects and disorder work as controlling parameters to alter the electronic structure of nanostructures and significantly influence their electronic, magnetic, and nonlinear optical (NLO) properties. In this study, we found that defect engineering is an effective strategy for tailoring the linear and nonlinear optical properties of Cu-doped titanium oxide (TiO2) flower-shaped nanostructures. The concentration of chemical doping of Cu in the TiO2 lattice creates intermediate defect states that impact electronic bandgap reduction and tunable defect luminescence. The estimation of the bandgap from density functional theory calculation follows the same trend of bandgap narrowing with Cu doping. The XPS study reveals that oxygen defects are responsible for bandgap narrowing and quenching of the PL intensity. A single-beam Z-scan technique with open and closed aperture configurations using ultrashort pulses centered at 532 nm excitation wavelength was used to study the NLO measurements. The open aperture reveals saturable absorption, whereas the closed aperture shows self-focusing behavior. The nonlinear absorption coefficient and refractive index extracted from NLO measurements demonstrate the linear dependence on the defect concentration and bandgap. The effects of heterogeneous dopants and lattice disorder on the nonlinear absorption behavior of these nanostructures are discussed in comparison with the figure of merit, non-linear refractive index, and absorption coefficient. The tunable NLO properties achieved by controlling such dopant-induced defects boost the scope of these nanostructures as optical limiting, optical switching, and optical photodiode applications. [ABSTRACT FROM AUTHOR]

We present an experimental investigation on third-order nonlinear optical properties of RF-sputtered WSe 2 thin films using a single-beam Z -scan technique around the optical communication window in a near-infrared spectral band. The nonlinear absorption coefficient was obtained from the open-aperture Z -scan transmission using ultrashort pulses centered at 1520 nm excitation wavelength. The results show a reverse saturable absorption signature for all the WSe 2 films due to two-photon absorption (TPA) mediated electronic transitions. The dependency of the TPA coefficient on film thickness is analyzed through the modification in an electronic band structure essentially caused due to defects induced in the samples. In addition, we also explored the impact of high laser intensity on the TPA process, which essentially pointed toward a significant contribution of free-carrier absorption at small laser intensities in WSe 2 thin films. This investigation provides a basis for optimally tailoring the nonlinear optical properties of transition-metal-dichalcogenides via bandgap engineering for improving photonic device functionality. [ABSTRACT FROM AUTHOR]

We present a detailed investigation on the intensity dependent third-order nonlinear optical (NLO) properties of RFsputtered WSe2 thin film using ultra short pulses. The investigation employed a single-beam Z-scan technique using nearinfrared (NIR) ultra short pulses centered at 1030 nm for ascertaining the NLO response. The results reveal a reverse saturable absorption (RSA) signature, which is primarily due to the contribution from two-photon absorption (TPA). We explored the impact of high optical fluence on the TPA process and observed the weakening of the nonlinear optical absorption in WSe2 thin film, which essentially pointed towards a significant contribution of free-carriers as well as bandfilling effect. This research elucidates the insight of photophysical properties of WSe2 thin films and shows the potential use as optical limiters. [ABSTRACT FROM AUTHOR]

In this work high density ZnO nanorods were grown on glass substrate by using a two-step method. In the first step, drop-casting method was used for growth of seed layer by taking Zinc Acetate Dihydrate as precursor. In the second step, ZnO nanorods were prepared through Chemical Bath Deposition process on this seed layer by taking Zinc Nitrate as precursor. Field Emission Scanning Electron Microscope study was done to study the morphology of grown ZnO nanorods. From this study the average diameter and length of nanorods were found to be 33 nm and 270 nm respectively. These nanorods are successfully used for second harmonic generation (SHG) of femtosecond laser pulses of central wavelengths like 1000 nm, 1100 nm and 1300 nm. Discussion is given on the significance of this result along with its potential application. [ABSTRACT FROM AUTHOR]

A refractive index sensor is proposed comprising one-dimensional photonic bandgap geometry in conjunction with a thin metal film. The refractive index sensor is based on the cavity mode formed in a thin cavity layer sandwiched between multilayers of SiO2/Ta2O5 and a metal film. The cavity structure traps light through excitation of the cavity mode and hence sharp resonance minima (maxima) are obtained in the reflection (transmission) spectrum. Our theoretical model investigates the impact of cavity thickness, nature of plasmon-active metal, the angle of incidence, and polarization of light on the sensitivity. It is found that the resonance wavelength increases linearly as the analyte refractive index increases. Additionally, the enhancement in the interaction between the analyte medium and micro-cavity mode-field further enhances the sensitivity of the proposed sensor. It is observed that as the cavity thickness increases from 200 nm to 900 nm, the sensitivity increases from 200 nm/RIU to 350 nm/RIU. The performance of the proposed photonic crystal based sensor exhibits strong polarization and angular dependences. Also, the sharp micro-cavity resonance results in improved detection accuracy as compared to surface plasmon resonance based schemes. The proposed structure advantage is an alternative to the conventional surface plasmon refractive index sensor. [ABSTRACT FROM AUTHOR]

We report here a set of triaryl pyrazole based B−N coordinated boron compounds (11‐17) synthesized by electrophilic aromatic borylation strategy. All the pyrazole boron compounds were thoroughly characterized using multinuclear NMR spectroscopy, LC–MS, and single crystal X‐ray diffraction analysis (for 12–17). The photoluminescence measurements of 11–17 revealed that the emission peak maxima were tuned based on the substitution on N‐phenyl. The photophysical and electrochemical properties were further supported by theoretical calculations. Z‐scan based investigations at 515 nm pump wavelength showed that B−N coordination led to enhancement of nonlinear absorption (two‐photon absorption (TPA)) in these compounds if an electron deficient moiety is attached. It has also been observed that an appropriate choice of moiety allows to optimally maneuver the molecular polarizability of the π‐system and consequently, assists in controlling the third‐order nonlinear optical response. [ABSTRACT FROM AUTHOR]

We report temperature-dependent photoluminescence (PL) in the temperature range between 77 K and 300 K, and room temperature nonlinear optical (NLO) properties of solution processed lead-free Cs2NaBiI6 (CNBI) and Cs2KBiI6 (CKBI) perovskite films. The de-convolution analysis of temperature-dependent PL spectra showed thermal quenching behavior of free-exciton (FX) emission, an unusual blue-shift of PL emission, and line broadening with increasing temperature as a consequence of strong exciton–phonon interaction. The nonlinear refractive index (n2) and nonlinear absorption coefficient (β) of both the CNBI and CKBI films are determined using a closed aperture (CA) and open aperture (OA) Z-scan technique, respectively. Both the CNBI and CKBI perovskites exhibited features of saturable absorption (SA) with β ∼ −6.23 × 10−12 cm W−1, and −1.14× 10−12 cm W−1, respectively. The CA measurements depicted a self-defocusing effect in both the samples with n2 values ∼−1.06 × 10−14 cm2 W−1 and −1.337× 10−14 cm2 W−1, respectively. With such emission and NLO characteristics, CNBI and CKBI perovskite films can be used for designing eco-friendly optoelectronic and NLO devices. [ABSTRACT FROM AUTHOR]

The objective of the proposed work is to design a biosensor that monitors hemoglobin (Hb) concentration using the combination of nanolayer, i.e., barium titanate (BaTiO3) and antimonene based on surface plasmon resonance (SPR) technique. Antimonene is used here as bio-recognition element (BRE) layer to attach the Hb analyte through physical adsorption due to its hydrophilic nature, higher adsorption energy and larger active surface area. The use of BaTiO3 adlayer (7 nm) just before antimonene is to enhance the refractive index (RI) sensitivity up to 1.90 times for the proposed SPR biosensor. The reason behind sensitivity enhancement is its high dielectric constant which enhances the electromagnetic field with in analyte medium. The performance of the biosensor is demonstrated with performance parameters namely sensitivity, detection accuracy (DA), figure of merit (FOM) and resolution. The proposed biosensor has potential to achieve much higher performance in terms of RI sensitivity of 303.83°/RIU, FOM of 50.39 RIU−1 and resolution of 0.021 g/l in comparison with reported biosensors in the literature for detection of Hb concentration. Thus, based on the obtained results one can say that the proposed work unlocks a reliable sensing in the field of medical science to detect hemoglobin-related diseases in human being. [ABSTRACT FROM AUTHOR]

We present a comprehensive study on the impact of phase transformations of metallophthalocyanine thin films on their third-order nonlinear optical (NLO) properties. The metallophthalocyanine thin films are prepared by thermally evaporating the commercially available Copper(II)2,9,16,23-Tetra-tert-butyl-29H, 31H-phthalocyanine (CuPc) and Zinc(II) 2,9,16,23-Tetra-tert-butyl-29H, 31Hphthalocyanine (ZnPc) powder on glass substrate. Thermal annealing causes a phase transformation which has a distinct signature in powder X-ray diffraction and UV-Vis-NIR spectroscopy. The NLO characteristics which include nonlinear refractive index n2, as well as nonlinear absorption coefficient (βeff), were measured by using a single beam Z-scan technique. An ultrashort pulsed fiber laser emitting femtosecond pulses (▵ ι≈250 fs) at 1064nm central wavelength is used as a source for the Z-scan experiment. The βeff values in as prepared thin films were ascertained to be smaller as compared to the annealed one due to the smaller value of saturation intensity (Is) which, in turn, is a consequence of ground-state bleaching in the thermally unstable amorphous state of the molecule. Interestingly, the nonlinear refractive indices bear opposite sign for CuPc and ZnPc. The variations in the third-order nonlinearity in CuPc and ZnPc are discussed in terms of molecular packing and geometries of metallophthalocyanine molecules. [ABSTRACT FROM AUTHOR]

Three-coordinated organoboron fluorophores bearing 3,5-diphenyl pyrazoles have been synthesized. The pyrazole anchored boron fluorophores show selective fluorescence quenching response to trinitrophenol (or) picric acid (PA) and have the ability to discriminate picric acid over other analytes. We investigated nonlinear optical (NLO) properties of these three-coordinated organoboron compounds (in solutions) in the presence and absence of PA. In absence of PA, the two-photon-absorption coefficient (β) of organoboron fluorophores exhibits a variation from 2 × 10−12 cm W−1 to 4 × 10−12 cm W−1. The results also reveal that the NLO characteristics of organoboron fluorophores exhibit a discernible variation with PA addition which has correlations with quenching observed in fluorescence measurements. [ABSTRACT FROM AUTHOR]

Complex aperiodic 1D dielectric distributions offer intriguing optical propagation features. For example, Thue–Morse (TM) sequence-based 1D photonic crystals (PCs) exhibit transmission spectra that are characterized by fractal band structure, i.e. they comprise traditional photonic band gaps (TPBGs) and fractal PBGs along with non-intuitive local resonances. These arise out of not-so-apparent positional correlations in the dielectric distribution. We identify inter-cell and intra-cell interaction between hidden dimer-like units in a TM-PC and present a relationship between the topological phase of lowest-order pass bands with reflection phase (φ) in the TPBGs. The analysis reveals that the dielectric filling factor (σ) of a TM-PC could serve as a parameter that determines the topological invariants for the geometry and could be tuned optimally in TM-PC-based heterostructures for exciting optical edge modes, which are disorder resistant (or topologically protected). We also discuss the robustness of optical edge modes. [ABSTRACT FROM AUTHOR]

In this study, we analyze the combined effect of TMD/Perovskite/2D Material on the performance of SPR biosensor using Kretschman configuration. In order to optimize the parameters, we compare the results of each material falls in the family of TMD/Perovskite/2D respectively and finally applied in the proposed sensor. It is observed that the combined structure based on black phosphorus, a 2D material, Barium Titanate (BaTiO3) as a Perovskite material and Tungsten disulfide (WS2) as a TMD material respectively shows excellent performance in terms of a performance parameter of the sensor. Incident light wavelength is fixed of 633 nm and radiative properties of a multi‐layered structure are calculated by transfer Metrix method. Essential performance parameters like Figure of Merit (FOM), Sensitivity, Detection Accuracy obtained by using the standard formula given in the paper. The reported figure of merit (FoM) and sensitivity are 62.51/RIU and 370. respectively. The results obtained in this paper expect to prove a significant role in the biosensing field like DNA hybridization as well as for the application of Perovskite materials in biosensing. [ABSTRACT FROM AUTHOR]

Intramolecular B ← N coordinated fluorophores have shown potential applications in optoelectronics and as sensors due to their unique photophysical properties. In this work, we report the synthesis and characterization of π-conjugated boron doped phenanthroimidazole dimers. All the π-conjugated B ← N coordinated phenanthroimidazole dimers exhibited high quantum yields in solution (up to 99%) and moderate quantum yields in the solid state (up to 51%). We investigated the nonlinear optical properties of phenanthroimidazole dimers and found that the measurement of two-photon-absorption cross-section is correlated with the conjugation length. [ABSTRACT FROM AUTHOR]

We report a compact, highly sensitive all SMF refractive index (RI) sensor based on a waist enlarged tapered (WET) fiber probe with a micro-air cavity (MAC) located at the tip. The fabrication of WET probe has been accomplished using a commercial splicer machine and the MAC has been realized using ultrashort pulsed laser based micromachining. The WET fiber section facilitates excitation of cladding modes while the MAC in conjunction with the curved tip, provides a convenient platform for achieving core mode as well as cladding mode interference within the probe. The probe, with an optimized WET fiber section, exhibited maximum refractive index (RI) sensitivity of 123.7 nm/RIU with a negligible temperature sensitivity of 15.34 pm/°C. [ABSTRACT FROM AUTHOR]

A series of triaryl pyrazole B ← N coordinate dimers with different π-conjugated (spacer) bridges (compound 4 (no spacer); compound 7 (2,5-bis(hexyloxy)benzene); compound 8 (9,9-dihexyl-9H-fluorene); compound 9 (9-hexyl-9H-carbazole); compound 10 (thiophene); compound 11 (2,2′-bithiophene)) has been synthesized from simple triaryl pyrazoles. Three different synthetic protocols were applied to synthesize the desired B ← N coordinate pyrazole dimers. All the B ← N coordinate pyrazole dimers exhibit absorption maxima between 337 and 399 nm with good molar absorption coefficients (12 000 to 22 100 M−1 cm−1 in CH2Cl2). They are highly emissive in the solution state with quantum yields of up to 0.94. Due to a substantial change in the excited state dipole moment, B ← N coordinate pyrazole dimers exhibit versatile and appreciably large nonlinear optical (NLO) properties. The dimer symmetry and the presence of donor or acceptor moieties affect the two-photon-absorption (TPA) cross-section substantially. In addition, the investigation reveals that the dimer π-conjugation length in all dimer variants has a remarkable impact on the NLO characteristics. [ABSTRACT FROM AUTHOR]

We present a novel scheme for generation of broadband mid-infrared (mid-IR) radiation using a quasiphase-matched difference-frequency-generation (DFG) process in a planar high index core symmetric Bragg reflection waveguide (BRW) geometry based on GaN/AlxGa1-xN system. By suitably tailoring the dispersion properties of the BRW so as to maintain the phase-matching condition over a broad range of pump wavelengths, we show that the pump acceptance bandwidth could be enhanced by nine to ten times for a DFG process. This manifests into a broadband mid-IR difference-frequency idler with greater than 280 nm bandwidth close to 4.35 μm wavelength. Since the design presented here exploits the dispersive features of the BRW; we can shift the broadband characteristics to any desired region of the mid-IR spectrum within the constraints imposed by transparency and nonlinearity of the constituent materials. The design also facilitates pumping by standard high-power solid-state laser sources. [ABSTRACT FROM AUTHOR]

In this work we have demonstrated the formation of continuous and non-continuous laserinduced periodic structures on silicon substrate (SiLIPS) using nanosecond (ns) laser. The continuous SiLIPS were generated in air by linearly polarized ~2 ns laser pulses. The optimized fluence, repetition rate and scanning velocity for generation of such structures are 0.4 J/cm2, 40 kHz and 0.5 mm/s respectively. The period of the SiLIPS (900 nm) was found to be close to the laser wavelength and the orientation was perpendicular to the polarization of the laser beam. Further, in this work we have also shown the appearance of non-continuous SiLIPS in localized region. Based on this observation, discussion is given on the mechanism of SiLIPS formation. [ABSTRACT FROM AUTHOR]

An aluminum-doped zinc oxide (AZO) coated photonic crystal fiber (PCF) is proposed and numerically analyzed for surface plasmon resonance-based refractive index (RI) sensing. A microchannel is incorporated in this PCF, which is proposed to be fabricated using a pulsed laser. Such a structure is expected to have less roughness compared to that fabricated by mechanical polishing. Furthermore, properties such as no island formation and absence of intraband transition are added advantages of AZO. Numerical simulation shows that the maximum wavelength sensitivity and the corresponding resolution of this sensor are 5000 nm/RIU and $2\times 10^{-5}$ RIU, respectively. Furthermore, the amplitude sensitivity is found to be as high as 167 RIU−1. The proposed structure can be easily fabricated and utilized for sensing applications as it does not require either coating of metals on the inner surface of holes of PCF nor filling of the voids of PCF and it requires a small amount of analyte for RI monitoring. [ABSTRACT FROM AUTHOR]

We propose a hybrid Tamm-plasmon-polariton (TPP) mode based sensing scheme for accurate determination of temperature. The sensing architecture is comprised of dual “metal-distributed-Bragg-reflector (DBR)” based structure, which sandwiches a common layer acting as a temperature sensing medium. Hybrid-TPP modes formed as a consequence of individual TPP mode-coupling is characterized by two distinct sharp reflectivity minima within the photonic bandgap of DBR for normal incidence of broadband source. The symmetric hybrid mode exhibits discernible variation in terms of change in reflectivity and resonance wavelength as the temperature of the sensing layer alters. On the other hand, the antisymmetric mode remains unchanged that gives rise to self-referenced temperature sensing scheme. We obtained a spectral sensitivity of 50 pm/°C by optimally choosing the thickness of sensing layer. In addition, the sharp TPP resonances gives rise to improved detection accuracy of such a sensing configuration as compared to competing optical sensing technologies. [ABSTRACT FROM PUBLISHER]

We present a detailed design principle and propagation characteristics of a channel plasmon-photonic-crystal-coupled-waveguide (PPCCW) for realizing terahertz (THz) waveguides and waveguide-based devices. The avoided-crossing behavior exhibited by dispersion curves of the supermodes due to modal interference leads to a sharp change in group velocity close to the phase-matching wavelength. As a consequence, the group-velocity dispersion (GVD) peaks to a maxima (~1.40 × 10 ps/km·μm) and dips to a minima (~−1.60 × 10 ps/km·μm), thus exhibiting extremely large GVD. The propagation length is obtained in the order of a few centimeters. The analysis is substantiated by the frequency dependence of mode field distribution and propagation loss suffered by the supermodes of PPCCW geometry. Also, the obtained result is in good agreement with FEM-based study. The proposed waveguide is aimed at facilitating enhanced performance THz-waveguide sensors and GVD compensation modules. [ABSTRACT FROM AUTHOR]

We present an experimental study on measurement of nonlinear refractive index ( n) of organic liquids when the thermo-optic effects manifest into large nonlinear phase shifts (Δ ϕ) in an open-aperture Z-scan configuration. Although we do not obtain the familiar peak-valley normalized transmittance curve as in the case of closed-aperture Z-scan technique, we use a theoretical model using Gaussian beam decomposition (GD) technique to estimate the value of n. Using this recipe, we obtain the nonlinear refractive index n = −(4.90 ± 1.20) × 10 cm/W for toluene (organic solvent) and n = −(10.60 ± 2.10) × 10 cm/W for an organic polymer solution (10 M in toluene). By carrying out absorption measurements directly with an unfocussed Gaussian beam, we found nonlinear absorptions β = (2.42 ± 0.20) × 10 m/W and β = (2.79 ± 0.24) × 10 m/W which are close to the expected results. [ABSTRACT FROM AUTHOR]

The article reports on the tailoring of the spectral features of extraordinary transmission (EOT) through sub-wavelength periodic hole array using a distributed-Bragg-reflector (DBR). Topics discussed include the new EOT created by the photonic stop-bands and high transmission regions in the DBR spectrum, and full-vectorial two-dimensional finite difference time domain (2D-FDTD) technique.

We present a comprehensive analysis for transverse electric (TE) and transverse magnetic (TM) polarized guided Tamm-plasmon polariton (TPP) mode at metal-heterostructure media interface. We explicitly show that the quarter-wavelength stack condition will not be satisfied for TE or TM polarized TPP mode due to the existence of null-point at metal-heterostructure media boundary. Therefore, we propose an alternate route to design TPP waveguide by solving the mode-dispersion relation for different geometrical parameters in a TiO2/SiO2 bilayer system. The guided TPP-modes (TE and TM) exhibit interesting dispersion characteristics which can be tailored as per the desired application. The group index of TM polarized TPP mode remains constant over a significant wavelength range which results into zero group-velocity dispersion (GVD) at λ ≈ 630 nm wavelength. Also, the propagation length for TM-polarized TPP modes vary between 25 μm to 50 μm in a 630-650 nm wavelength range. However, the variation of GVD for TE-modes exhibit a monotonic variation with an exceptionally large GVD ≈ -3 × 104 ps/km·nm around λ = 632.8 nm. [ABSTRACT FROM PUBLISHER]

We present a detailed study on waveguide design principle and propagation features of a channel Bragg-surface plasmon-coupled-waveguide (BSPCW). After proposing a convenient design route, we analyze the mechanism of coupling between bandgap-guided modes and surface plasmon polariton modes with respect to change in waveguide parameters such as type of metal, metal thickness and number of unit cells. In order to understand the physical mechanism behind the mode-coupling, we observe the redistribution of modal power of the BSPCW as wavelength changes. The interesting dispersion characteristics exhibited by the BSPCW supermodes opens a novel route to realize accurate sensors and efficient dispersion compensating modules in signal processing. Therefore, this comprehensive study could serve as a tool to optimize the performance of devices dictated by the application requirements in sensor technology and optical communications. The performance of the device in terms of sensing application is also being discussed. [ABSTRACT FROM PUBLISHER]

We propose a highly sensitive temperature sensor based on photonic crystal surface plasmon waveguides comprising different plasmonic active metals such as gold, silver, and aluminum, utilizing surface plasmon resonance phenomenon. We found that the resonance wavelength can be easily and substantially tuned over a broad spectral range by changing the temperature and also by judiciously choosing the different plasmonic metals. Employing coupled mode theory, we found that the proposed sensor can be used in harsh environment with sensitivity as high as ∼70 pm/K around telecommunication window. [ABSTRACT FROM AUTHOR]

A novel scheme for enhancing the bandwidth of optical parametric amplification using asymmetric Bragg reflection waveguides is presented. The unique dispersion characteristics exhibited by the Bragg reflection waveguides are exploited for countering the rapidly falling signal gain as one moves away from the phase-matching wavelength. The role of the periodic cladding on the gain and bandwidth of optical parametric amplification in a lithium niobate based Bragg reflection waveguide is analysed. Such an idea could be used in any nonlinear up conversion or down conversion process for enhancing the bandwidth significantly and for easing fabrication tolerances. [ABSTRACT FROM AUTHOR]

We propose a high-performance bimetallic surface plasmon resonance (SPR) sensor based on periodic-multilayer waveguide and analyze its performance with respect to SPR active metals, such as gold (Au) and aluminum (Al). We found that an ultra thin (\sim3 nm) layer of gold (Au) over aluminum (Al) protects aluminum (Al) from oxidation and exhibits better sensor performance. Using couple-mode theory, we show that the sensitivity and detection accuracy could be appropriately tailored for bimetallic (Au+Al) configuration. [ABSTRACT FROM PUBLISHER]

We present a detailed design principle and propagation characteristics of channel Bragg-plasmon-coupled-waveguide. We have found that there is a significant change in the slope of the phase-velocity dispersion curve leading to an ultranarrow interaction bandwidth (∼765 pm) and group-velocity dispersion (GVD∼±4.5×104 ps/km-nm) that is an appreciably large GVD using a waveguide mode-coupling geometry in any region of the optical spectrum. The effect of waveguide parameters such as channel width, number of bilayers, etc. on a mode-coupling mechanism is also studied with significant emphasis on the propagation loss suffered by the supermodes of the structure. The proposed waveguide exhibits sensitivity as high as 7500 nm/RIU, thereby opening a route for biochemical sensing. [ABSTRACT FROM AUTHOR]