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...

A new series of “Push-Pull” meso-substituted trans-A2BC porphyrins, where A = mesityl, B = phenothiazine (push) and C = o/p-nitrophenyl moiety (pull) and M = 2H, Ni(II), Cu(II), and Zn(II)...

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The ultrafast absorption dynamics of photonic modes and enhanced features of electronic states have been demonstrated in a barium titanate (BaTiO 3 , BTO)-embedded onedimensional photonic crystal. The photonic structure has been realized as an optical microcavity, where a BTO central layer is sandwiched between two SiO 2 /TiO 2 -distributed Bragg reflectors. Angle-dependent transient absorption behavior reveals the excitedstate absorption dynamics of cavity-tuned BTO defect energies. Furthermore, the dynamic evolution of photonic minibands of both sides of the photonic cavity mode demonstrates the enormous cavity field confinement effect. The temporal evolution of nonlinear absorption dynamics is specific to cavity angle tuning and is evidenced by seven orders of two-photon absorption enhancement in BTO mid-bandgap energies. Overall, the results highlight the impact of strong optical field confinement within the BTO central layer. The simulations of spatial and angledependent localized optical field and energy deposition within the photonic structure further support the experimental findings. The enhanced features of ultrafast absorption dynamics and strong optical nonlinearities of the BTO-based active photonic structure offer a better understanding of electron−photon interaction, which paves the way for many novel nonlinear, hybrid optoelectronic, and photonic device applications.

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.

A series of β-TCBD (1,1,4,4-tetracyano-buta-1,3diene)-appended porphyrins, M-TCBD (M = 2H, Co(II), Ni(II), Cu(II), and Zn(II)), was synthesized from 2,3-diphenylethynyl-12nitro-meso-tetraphenylporphyrin, H 2 -PE 2 , and characterized by various spectroscopic techniques and electrochemical studies. The reaction proceeds via [2 + 2] cycloaddition and retroelectrocyclization reactions of tetracyanoethylene (TCNE) with H 2 -PE 2 . The observed unusual reduction potentials in the cyclic voltammograms of the synthesized porphyrins in the range of −0.06 to −0.10 V are the consequence of the TCBD moiety present at the β-position of the porphyrin macrocycle. Notably, these porphyrins exhibited three porphyrin ring-centered reductions due to extended π-conjugation. The higher nonlinear optical response exhibited by the M-TCBD series as compared to the precursor (H 2 -PE 2 ) was attributed to the existence of intramolecular charge transfer and enhanced polarization in the M-TCBD series. The single-beam femtosecond Z-scan measurements were performed to elucidate the third-order nonlinear optical properties, and the temporal response of these porphyrin molecules was investigated using optical pump−probe spectroscopy to study the excited state absorption dynamics. Zscan measurements revealed that Co-TCBD exhibited a higher nonlinear optical response as compared to free base porphyrins. The two-photon absorption coefficient (β) and the imaginary part of third-order nonlinear optical susceptibility (χ (3) ) were obtained from the open aperture experiment, whereas the close aperture experiment delivered the magnitude and the sign of the nonlinear refractive index (n 2 ) and the real part of χ (3) . Furthermore, the femtosecond transient absorption spectroscopy revealed a faster relaxation dynamics of various absorption processes in a picosecond timescale. The excellent optical limiting threshold (1.90−2.33 × 10 15 W/m 2 ) of the synthesized porphyrins makes them good materials for laser protection and high-power laser operation.

The photonic cavity-mediated precise control of femtosecond optical nonlinearity of several orders of magnitude enhancement is demonstrated in a novel nonlinear one-dimensional (1D) photonic crystal. The demonstrated photonic structure contains a highly nonlinear metal oxide, Bi 2 O 3 as a central defect layer within two SiO 2 /TiO 2 distributed Bragg reflectors. The nonlinear optical interactions of the electronic states of Bi 2 O 3 with the cavity mode and adjacent photonic minibands are closely monitored by femtosecond Gaussian laser beam propagation over a wide-range of spectral wavelengths, 350−1600 nm. Abnormal cross-over from positive (reverse saturation) nonlinear absorption (RSA, β = (+)12 × 10 −10 m W −1 ) to negative (saturation) nonlinear absorption (SA, β = (−) 11× 10 −10 m W −1 ) is witnessed when the confined optical fields are strongly coupled to the excitation laser and mid-band gap energies of Bi 2 O 3 , during effective cavity length tuning. The femtosecond laser pulse propagation at different wavelengths effectively probed the multiphoton-induced optical nonlinearities, which are distinctly different from lowand high-energy minibands compared to the cavity resonance and are manifold-enhanced relative to pristine Bi 2 O 3 . The photonic mode density-dependent pronounced two-/multiphoton absorptions are systematically analyzed with experiments and simulations. The novel photonic architecture can be utilized in optical switches, optical limiters, and ultrafast photonic device applications.

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