Nonlinear optical properties, such as two-(or multi-) photon absorption (2PA), are of special interest for technologically important applications in fast optical switching, in vivo imaging and so on. Highly intense infrared ultrashort pulses probe deep into samples and reveal several underlying structural perturbations (inter-layer distortions, intra-layer crumpling) and also provide information about new excited states and their relaxation. naturally self-assembled inorganic-organic multiple quantum wells (io-MQWs) show utility from room-temperature exciton emission features (binding energies ~200-250 meV). These Mott type excitons are highly sensitive to the self-assembly process, inorganic network distortions, thickness and inter-layer distortions of these soft two-dimensional (2D) and weak van der Waal layered hybrids. We demonstrate strong room-temperature nonlinear excitation intensity dependent two-photon absorption induced exciton photoluminescence (2PA-PL) from these IO-MQWs, excited by infrared femtosecond laser pulses. Strongly confined excitons show distinctly different one-and two-photon excited photoluminescence energies: from free-excitons (2.41 eV) coupled to the perfectly aligned MQWs and from energy down-shifted excitons (2.33 eV) that originate from the locally crumpled layered architecture. High intensity femtosecond induced pL from one-photon absorption (1PA-PL) suggests saturation of absorption and exciton-exciton annihilation, with typical reduction in PL radiative relaxation times from 270 ps to 190 ps upon increasing excitation intensities. From a wide range of IR excitation tuning, the origin of 2PA-PL excitation is suggested to arise from exciton dark states which extend below the bandgap. Observed two-photon absorption coefficients (β ~75 cm/ GW) and two-photon excitation cross-sections (η 2 σ 2 ~ 110GM), further support the evidence for 2PA excitation origin. Both 1PA-and 2PA-PL spatial mappings over large areas of single crystal platelets demonstrate the coexistence of both free and deep-level crumpled excitons with some traces of defectinduced trap state emission. We conclude that the two-photon absorption induced pL is highly sensitive to the self-assembly process of few to many mono layers, the crystal packing and deep level defects. This study paves a way to tailor the nonlinear properties of many 2D material classes. Our results thus open new avenues for exploring fundamental phenomena and novel optoelectronic applications using layered inorganic-organic and other metal organic frameworks. High intensity ultrashort laser pulses interact with nanomaterials to produce fascinating parametric and nonparametric nonlinear optical phenomena such as two-photon (or multi-photon) absorption 1 , harmonic generation 2 , coherent anti-Stokes Raman scattering (CARS) 3 , excited state absorption and ultrafast charge carrier dynamics 4,5. Studies of two-(or multi-) photon absorption in layered materials (such as graphene oxide, MoS 2 type transition-metal dichalcogenides) 6-11 have given new in...

as compared to the dielectrics. [1][2][3] The enhanced nonlinear optical properties of metals are extensively explored either as metal-dielectric (MD) nanocomposites or plasmonic structures. [4][5][6][7][8][9][10][11] The inherent properties of strong resonant absorption and considerable local-field enhancement of metals are observed due to huge optical polarization associated with the free electron oscillations. These MD composites show many interesting applications in ultrafast and nonlinear photonic domains. However, accessing of optical activities from metals or metal composites in the visible region is rather difficult due to thickness restricted high optical attenuation. All dielectric 1D photonic crystals become attractive for enhanced optical field confinement, thus produce several orders of magnitude changes in both linear and nonlinear optical properties. [12,13] Unlike dielectric-dielectric wavelength ordered multilayers, MD multilayer shows strong optical response due to high refractive index contrast between the metal and dielectric layers. [14][15][16] Therefore, a transparent metal can be realized when thin metal layer is sandwiched between the wavelength-ordered dielectric layers to make 1D photonic bandgap structure. [17][18][19][20] Novelty of these MD structure is that the total amount of metal is increased to several times larger than the skin depths in net thickness and still remains transparent for optical pulse propagation. In such Bragg photonic structures, the resonant Fabry-Perrot (FP) cavity modes produce a tailor-made photonic stopband and the transmission split into several minibands on both sides of the stopband. [17,21] As a result, optical fields can easily be propagated and manipulated to much deeper of these structures than the normal skin depth restricted bulk metals in both resonant and nonresonant optical regions. [14,22] Thus, the propagation of high intense light can be hugely altered in such MD structures and can be designed for high optical nonlinearities, while retaining the photonic transmission/reflection features similar to dielectric photonic structures. [23][24][25][26][27] Such novel MD structures have many exciting applications such as optical switching, laser optical limiters for human eye and sensor protection, and transparent conducting device technology. [28,29] The nonlinear enhancement of these 1D photonic bandgap structures is generally explained in the context The giant nonlinear optical responses of photonic minibands of (Ag/SiO 2 ) 4 metal-dielectric multilayers are reported using high intense femtosecond laser pulses. Ag and SiO 2 alternative stack of layers form a series of coupled Fabry-Pérot resonators (Ag-SiO 2 -Ag) and the cavity modes are split into transmission minibands in the metal reflective spectral region. The strong saturation of two-photon absorption associated with multiphoton absorption (MPA) is observed at photonic miniband minimum (≈700 nm), whereas MPA is the strong dominant nonlinearity at peak maximum (≈725 nm). The metal-cavity indu...

Metallic nanostructures are highly attractive for refractive index sensing, as the evanescent field from the associated plasmonic resonances resides in close proximity to the adjacent analyte media. However, this benefit is often reduced due to broad plasmonic lineshapes producing poor quality factors. The rational design provides strategies for narrowing the plasmonic modes by incorporating photonic diffraction, which promotes surface lattice resonances . Due to the stringent parametric dependencies, these resonances in metallic lattices are not always feasible, particularly when a straightforward fabrication route with fewer process steps is desired. Herein, hybridized guided‐mode resonance in a 2D‐metallic photonic crystal slab (2D‐mPhCs) is introduced that ensures high‐quality hybrid modes while maintaining a simple fabrication methodology. In direct comparison to its constituent plasmonic and photonic modes, this concept is discussed for sensing applications. The “figure of merit (FOM)” is frequently regarded as a valid metric for measuring sensing performanceensuring high‐quality modes with an improved detection limit. The experimental results confirm enhanced FOM (three to six times) for the hybrid modes, in contrast to the constituent counterparts. For optoelectronic applications, such as photodetection and photocatalysis, these hybrid structures with high‐quality modes offer a promising platform to harvest light at the metal–semiconductor interfaces.

Synthesis, crystal structure, and optical properties of two-dimensional (2D) layered structurally slightly different inorganic–organic (IO) hybrid semiconductors (R–C6H4C2H4NH3)2PbI4 (R = CH3, Cl) are presented. They are naturally self-assembled systems where two (RNH3)+ moieties are sandwiched between two infinitely extended 2D layers of the [PbI6]4– octahedral network and treated as natural IO multiple quantum wells. While the former compound crystallizes into an orthorhombic system in the Cmc21 space group, the latter crystallizes into a monoclinic system in the space group P21/c. As a thin film, they are well-oriented along the (l00) direction. Both single crystals and thin films show strong room-temperature Mott type exciton features that are highly sensitive to the self-assembly and crystal packing. Linear (one-photon) and nonlinear (two-photon) optical probing of single crystals for exciton photoluminescence imaging and spectral spatial mapping provide deep insight into the layered re-arrangement and structural crumpling due to organic conformation. The strongly confined excitons, within the lowest band gap of inorganic, show distinctly different one- and two-photon excited photoluminescence peaks: free excitons from perfectly aligned 2D self-assembly and energy down-shifted excitons originated from the locally crumpled layered arrangement. Their structural aspects are successfully presented with proper correlation that emphasize various differences in physical and optical properties associated between these novel IO hybrids.

The paper describes synthesis and structural characterization of the whole series of two closely related lanthanide coordinated chromium or aluminum hexamolybdates (Anderson-Evans cluster) including twelve new members hitherto unreported: [Ln(H2O)7{X(OH)6Mo6O18}]·4H2O and [Ln(H2O)7{X(OH)6Mo6O18}Ln(H2O)7]{X(OH)6Mo6O18}·16H2O where X = Al or Cr and Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y. Crystal structures of all the solids were established by powder and single crystal X-ray diffraction techniques. The two series are dictated by a different aggregation of the same set of molecular species: Lighter lanthanides favor coordination interaction between lanthanide ions and molybdate cluster forming 1D chains (Series I) while the heavier lanthanides result in the stacking of a cation, a pair of lanthanide hydrates coordinating to the cluster, and an anion, the discrete cluster is further stabilized through a large number of water molecules (Series II). Crystallization with Er3+ and Tm3+ ions results in a concomitant mixture of Series I and II. Photoluminescence of single crystals of all the chromium molybdates was dominated by a ruby-like emission including those which contain optically active ions Pr, Sm, Eu, Tb, Dy, and Tm. In contrast, aluminum analogs showed photoluminescence corresponding to characteristic lanthanide emissions. Our results strongly suggest a possible energy transfer from f levels of lanthanide ions to d levels of chromium (III) causing the quenching of lanthanide emission when coordinated with chromium molybdates. Intensity measurements showed that the emission from chromium molybdates are almost two orders of magnitude lower than naturally occurring ruby with broader line widths at room temperature.