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Finding appropriate photosensitizers (PSs) for daylight photodynamic therapy (dPDT) applications is extremely challenging, even though heavy-atom-free photosensitizers (HAFPSs) such as thiocarbonyl-modified nucleobases have shown a ray of hope. Few attempts have been made to find alternative natural products for dPDT applications. Pteridine heterocycles consisting of a pyrazine ring and a pyrimidine ring, such as lumazine, which exhibit many structural similarities to the alloxazine ring of the flavin molecule, could be an option for HAFPSs. The photophysical and quantum mechanical studies of the thio-modified lumazines revealed that sequential thiomodifications in lumazine result in a bathochromic shift. Additionally, higher tissue penetration depths were observed for thiolumazines. The fluorescence quenching in the case of thiomodified lumazines was explained using triplet state formation, whereas the contribution from the photoinduced electron transfer process cannot be ignored. It was also noticed that a strong one-photon absorption influenced the two-photon absorption (TPA) process, leading to a self-focusing effect in the visible spectral region. The higher tissue penetration and larger TPA cross section are the hallmark characteristics of the thiolumazines to be considered as potential HAFPSs for dPDT applications.

The phenomenon of trapping a broad spectrum of light is known as “rainbow trapping” and is achieved by using all-dielectric, hybrid metallo-dielectric, or all-metallic configurations. The latter architectures allow strong confinement but exhibit very high ohmic losses. This results in practical lifetimes of trapped modes to less than 1 ps. Therefore, novel strategies are required to be devised for trapping and, subsequently, releasing broadband electromagnetic field with lifetime >1ps. We present a rainbow trapping configuration using the excitation of multiple optical Tamm (OT) modes in an one-dimensional chirped photonic crystal (CPC) designed for adiabatically coupling counterpropagating modes. In the geometry, the backscattered phase undergoes multiple discontinuities (=π), which enables excitation of many OT modes in the presence of a thin plasmon-active metal, which is placed adjacent to the terminating layer of CPC. All the OT modes are spatially separated in the CPC, and the strong modal confinement manifests into group velocities as low as 0.17c. The time-domain simulations depict mode-localization in the dielectric sections of CPC, which manifest into lifetimes ∼3ps.

We present an investigation to ascertain the existence of Tamm-plasmon-polariton-like modes in one-dimensional (1D) quasi-periodic photonic systems. Photonic bandgap formation in quasi-crystals is essentially a consequence of long-range periodicity exhibited by multilayers and, thus, it can be explained using the dispersion relation in the Brillouin zone. Defining a “Zak”-like topological phase in 1D quasi-crystals, we propose a recipe to ascertain the existence of Tamm-like photonic surface modes in a metal-terminated quasi-crystal lattice. Additionally, we also explore the conditions of efficient excitation of such surface modes along with their dispersion characteristics.

We report a continuous wave (cw) singly-resonant, single-frequency optical parametric oscillator (SRO) using 50 mm long congruently-grown 8-mol% MgO doped periodically poled LiTaO3(MgO:cPPLT). The SRO is pumped by a high-power green laser derived from a near-infrared (NIR) fiber laser source. The SRO is tunable in visible band from 740–755 nm (signal) and mid-infrared (MIR) band from 1810–1930 nm (idler). The MgO:cPPLT crystal is characterized by a strong optical absorption at the pump wavelength and relatively small thermal conductivity which leads to substantial rise in crystal temperature which is spatially confined in the region where the pump is present. This local rise in temperature facilitates longitudinal dephasing by virtue of a large thermo-optic coefficient for MgO:cPPLT crystal and its impact manifests through an early saturation effect in conversion efficiency. The impact of thermo-optic manifestations could be observed in the measurement of spatial beam qualities of the modes.

We present an efficient and tunable source generating multi-watt single-frequency red radiation by intra-cavity frequency doubling of the signal in a MgO-doped periodically poled LiNbO_3 (MgO:PPLN)-based singly resonant optical parametric oscillator (SRO). By optimally designing the SRO cavity in a six-mirror configuration, we generate ≈276  nm tunable idler radiation in mid-infrared with a maximum power of P_i=2.05  W at a pump power of P_p=14.0  W. The resonant signal is frequency doubled using a 10 mm-long BiB_3O_6 (BiBO) crystal which resulted in tunability of a red beam from ≈753 to 780 nm band with maximum power P_r≈4.0  W recorded at λ_r≈756  nm. The deployment of a six-mirror SRO ensures single-frequency generation of red across the entire tuning range by inducing additional losses to Raman modes of LiNbO_3 and, thus, inhibiting their oscillation. Using a scanning Fabry–Perot interferometer (FPI), nominal linewidth of the red beam is measured to ≈3  MHz which changes marginally over the entire tuning range. Long-term (over 1 h) peak-to-peak frequency fluctuation of the generated red beam is estimated to be about 3.3 GHz under free-running conditions at P_p=14.0  W. The generated red beam is delivered in a TEM_00 mode profile with M^2≤1.32 at maximum power in a red beam.

We present a study on the impact of phase transformation (α→ β) of tetra-tert-butylphthalocyanine (H2Pc) thin films on their third-order as well as fifth-order nonlinear optical (NLO) properties. The α-phase of H2Pc thin film is prepared by thermally evaporating the commercially available H2Pc powder on a glass substrate which is further annealed at 473 K to obtain the more stable β-phase. The structural modifications in H2Pc films have a distinct signature in UV–vis absorption spectra, powder X-ray diffraction, and Raman spectra which have been employed for identifying the transformation from α-phase to β-phase. The NLO characteristics which include nonlinear refractive indices such as n2and n4, as well as nonlinear absorption coefficient (βeff), were measured by using a single beam Z-scan technique. Using an ultrashort pulsed fiber laser at 1064 nm central wavelength in the Z-scan experiment, we obtained a smaller value of βeffin α-phase as compared to β-phase due to a smaller value of saturation intensity (Is) which, in turn, is a consequence of ground-state bleaching in thermally unstable amorphous state of the molecule. The impact of χ(5)nonlinearity in the Z-scan transmittance of α-phase was apparent, and interestingly, the optical phase changes due to χ(3)and χ(5)nonlinearities bear opposite sign. In addition, they alter their signs as the laser intensity increases. A dominant fifth-order NLO contribution is mainly a consequence of inherently large χ(5)for the films as well as two-photon-assisted nonlinear refraction as a result of excited state absorption.

The transverse magnetic (TM) polarized hybrid modes formed as a consequence of coupling between Tamm plasmon polariton (TM-TPP) mode and surface plasmon polariton (SPP) mode exhibit interesting dispersive features for realizing a highly sensitive and accurate surface plasmon resonance (SPR) sensor. We found that the TM-TPP modes, formed at the interface of distributed Bragg reflector and metal, are strongly dispersive as compared to SPP modes at optical frequencies. This causes an appreciably narrow interaction bandwidth between TM-TPP and SPP modes, which leads to highly accurate sensing. In addition, appropriate tailoring of dispersion characteristics of TM-TPP as well as SPP modes could ensure high sensitivity of a novel SPR platform. By suitably designing the Au/TiO_2/SiO_2-based geometry, we propose a TM-TPP/SPP hybrid-mode sensor and achieve a sensitivity ≥900  nm/RIU with high detection accuracy (≥30  μm^−1) for analyte refractive indices varying between 1.330 and 1.345 in 600–700 nm wavelength range. The possibility to achieve desired dispersive behavior in any spectral band makes the sensing configuration an extremely attractive candidate to design sensors depending on the availability of optical sources.

Highly sensitive, accurate and stable high temperature (800 K) sensor based on Photonic Crystal Surface Plasmon Waveguides is proposed. The PCW is based on widely used lithographic and nano-fabrication compatible materials like TiO2and SiO2. Gold has been used as a SPR active metal. Employing coupled mode theory, the sensitivity is found to be as high as ~ 66 pm/K.

We present a detailed analysis and recipe for designing a channel Bragg-plasmon coupled waveguide where the intermodal coupling between a bandgap-guided mode and surface plasmon mode substantially modifies the phase-velocity dispersion slope of supermodes. This leads to appreciably large group-velocity-dispersion with peak value of 4.33x104ps/km-nm close to the optical communication band as well as interaction bandwidth of about 765 pm for 1 cm long waveguide. We also found that bandgap guided Bragg modes exhibit stronger dispersive features than the plasmon modes around ~ 1.50 m which is exactly opposite to what is usually observed in visible region. The impact of waveguide parameters such as channel width etc. on mode-coupling mechanism is also studied with significant emphasis on the propagation loss suffered by the supermodes of the structure.

We propose a novel scheme for ultrabroadband midinfrared (mid-IR) generation using quasi-phase-matched difference-frequency generation (DFG) in a GaN/Al_xGa_1−xN based Bragg reflection waveguide (BRW). By optimally tailoring the phase- and group-velocity dispersion properties of symmetric BRWs, we show that the phase-matching condition for a DFG process could be maintained over a broad range of signal wavelengths. This could lead to generation of an ~700 nm broad idler close to 3.26 μm wavelength. Since the idea is based on dispersion compensation using photonic bandgap geometry, we can shift the broadband features to any desired spectral region and for any material system within the constraints imposed by the transparency of nonlinear materials. We also investigate the possibility of broadband mid-IR generation using pump sources with broad spectral width.

A detailed mathematical analysis along with a theoretical model for the modes supported at the interface of a metal and periodically stratified medium (Bragg structure) is presented. The modes that are supported at the interface of a plasmon active metal (such as gold) and a Bragg structure are commonly known as surface plasmon-Bragg modes. We found that these modes have effective indices lower than any of the material indices of the layers comprising the Bragg structure, and they are highly dispersive when compared to the conventional surface plasmon modes that are supported at the metal and dielectric interface. The plausible physical explanation behind the strong dispersive behavior of the surface plasmon-Bragg mode is provided. Finally, the comparison of dissipation loss for the surface plasmon-Bragg modes is investigated and it has been shown that there is more than fivefold enhancement in the magnitude of propagation lengths as compared to the conventional surface plasmon mode.

We report a compact and viable source of broadband, high-power, cw, mid-IR radiation based on a singly resonant optical parametric oscillator (SRO) pumped by a wide-bandwidth cw Yb fiber laser centered at 1060 nm. By exploiting the extended phase-matching bandwidth in a 50 mm crystal of MgO:PPLN and a ring SRO cavity, we obtain 5.3 W of broadband idler output for 25.5 W of pump at >80% depletion, transferring a pump bandwidth of 73.9 cm^−1 to an idler spectrum spread across an equal bandwidth centered at 3454 nm. By deploying output coupling of the signal, we generate 11.2 W of total power at 44% extraction efficiency with a pump depletion of >73% at the maximum available pump power. Measurements of transverse modal power confirm Gaussian distribution of signal and idler beams.

We report a high-power, cw, singly resonant optical parametric oscillator (SRO) using a simple, compact fiber pump laser architecture in the green. The SRO, based on MgO:sPPLT, is pumped by 9.6 W of single-frequency cw radiation at 532 nm obtained by single-pass second-harmonic generation (SHG) of a 30 W Yb fiber laser, also in MgO:sPPLT. Using two identical crystals of 30 mm length for SHG and SRO, we generate cw idler powers of up to 2 W over 855-1408 nm, with a peak-to-peak power stability <11.7% over 40 min, in a TEM_00 spatial mode with M^2<1.26. Using finite output coupling of the resonant wave, we extract 800 mW of signal power with peak-to-peak power stability <10.7% over 40 min, and a frequency stability <75 MHz over 15 min. The signal and idler output have TEM_00 beam profile with M^2<1.52 across the tuning range.

We present an analysis of a high index core symmetric Bragg reflection waveguide (BRW) design based on a GaN/Al_xGa_1−xN system for efficient quasi-phase-matched second-harmonic generation for broadband applications. By choosing the fundamental frequency to be a BRW mode and suitably tailoring the overall dispersion characteristics, the strong dispersion of the second-harmonic mode is partially canceled, leading to phase matching between the fundamental and second-harmonic over a broad range of wavelengths. The crucial interplay between the dispersive behavior of the fundamental and second-harmonic wave manifests as a broad acceptance bandwidth of ~33 nm accompanied with appreciable conversion efficiency (22.8%/W) for a 10 mm long waveguide. The impact of tailoring the dispersion characteristics on the conversion efficiency is also discussed.

We propose a broadband fiber optic parametric amplifier (FOPA) based on a near-zero ultra-flat dispersion profile with a single zero-dispersion wavelength (ZDW) by using a selective liquid infiltration technique. The amplifier gain and bandwidth is investigated for a variety of fiber lengths, pump power, and operating wavelengths. It is observed that sufficient peak gains and broader bandwidths can be achieved with a small negative anomalous dispersion (β_2≤0) and a positive value of the 4th-order dispersion parameter (+?β_4) around the pump. We can optimize an FOPA with a bandwidth of more than 220 nm around the communications wavelength.