Photophysical Studies of Metalloporphyrins for Sensitized Noncoherent Photon Upconversion

Noncoherent photon upconversion (NCPU) realized by means of triplet-triplet annihilation (TTA) is of significant recent interest because of the possibility of using this phenomenon for increasing the efficiency of dye-sensitized solar cells (DSSCs). Efficiencies can be achieved if the near-IR parts of the solar spectrum can be absorbed and used in TTA. However, realizing this potential is not trivial. Dual absorption and TTA usually populates a higher excited singlet state (Sn, n ≥ 2) of the absorber molecule and the fate of this state can be a critical factor controlling efficiencies and hence the potential efficiency improvements in DSSCs. With a motivation to understand the photophysical processes that decides the fate of the product state of TTA, the role of the S2 state of a metalloporphyrin and a fullerene in solution-phase NCPU has been investigated using photophysical techniques. TTA in the model porphyrin, zinc(II) meso-tetraphenylporphine (ZnTPP) realized by excitation with a green laser, was found to occur through a short-range Dexter-type energy transfer mechanism. It was also found, contrary to a previous suggestion, that a Förster-type energy transfer cannot occur from the short-lived S2 state of ZnTPP to an acceptor molecule. It was hypothesized that prior aggregation of ZnTPP and the acceptor molecule should exist to enable such an energy transfer. For blue emitter (BE) molecules with triplet energies lower than that of ZnTPP, a triplet-triplet energy transfer (TTET) from ZnTPP to the BE followed by TTA in the BE populates its S1 state. However, this is not possible for BEs with triplet energies significantly greater than that of ZnTPP. In this case, it is proposed that the triplet ZnTPP forms a triplet exciplex with a ground state BE. The triplet exciplex then annihilates with a second triplet ZnTPP to form the S1 state of the BE. For the studies of the NCPU in C60, the BEs chosen were having triplet energies similar or slightly lower than that of C60. The NCPU in these systems follows the well-established mechanism of TTET from C60 to the BE followed by TTA in the BE to produce its S1 state. However, for systems in which the triplet energies of C60 and BE are similar, the NCPU process is controlled by entropic factors which in turn can be controlled by the concentration of the BE. Compared to this system, NCPU in a system in which the triplet of C60 lies slightly higher than that of the BE was found to be more efficient. The involvement of the higher excited singlet states of C60 (Sn, n ≥ 2) can be considered as insignificant because of the proximity of these states to the S1 state and the large rates of internal conversion from these states to the S1 state. Because triplets can be quenched by molecular oxygen, the rate of oxygen diffusion in devices based on NCPU needs to be evaluated. With this objective, upconverted S2 emission from ZnTPP produced by TTA was used as a tool to measure the rate of oxygen diffusion in a thin polymer film. It was found that the oxygen permeability is controlled by the characteristics of the polymer matrix, including its water content and its distribution. C60 has been evaluated as a possible electron acceptor for the TTA-produced S2 state of ZnTPP. Quenching of the S2 and S1 fluorescence of ZnTPP by added C60, with more efficient quenching for the Soret-excited ZnTPP+C60 species, was demonstrated. Significant ground state aggregation between ZnTPP and C60 was proven. However this factor could not account for the observed difference in the rates of quenching of the S2 and S1 fluorescence of the excited porphyrin. The difference in the rates of quenching was attributed to differences in the free energies of electron transfer from the S2 and S1 states of ZnTPP to C60. Conclusive evidence for this was obtained from transient absorption studies carried out at the University of Melbourne, which demonstrated the formation of a long-lived charge transfer state upon Soret-excitation of the porphyrin-C60 complex. However, the involvement of short-range Dexter type energy transfer could not be ruled out in this system

A survey on fiber nonlinearity compensation for 400 Gbps and beyond optical communication systems

Optical communication systems represent the backbone of modern communication networks. Since their deployment, different fiber technologies have been used to deal with optical fiber impairments such as dispersion-shifted fibers and dispersion-compensation fibers. In recent years, thanks to the introduction of coherent detection based systems, fiber impairments can be mitigated using digital signal processing (DSP) algorithms. Coherent systems are used in the current 100 Gbps wavelength-division multiplexing (WDM) standard technology. They allow the increase of spectral efficiency by using multi-level modulation formats, and are combined with DSP techniques to combat the linear fiber distortions. In addition to linear impairments, the next generation 400 Gbps/1 Tbps WDM systems are also more affected by the fiber nonlinearity due to the Kerr effect. At high input power, the fiber nonlinear effects become more important and their compensation is required to improve the transmission performance. Several approaches have been proposed to deal with the fiber nonlinearity. In this paper, after a brief description of the Kerr-induced nonlinear effects, a survey on the fiber nonlinearity compensation (NLC) techniques is provided. We focus on the well-known NLC techniques and discuss their performance, as well as their implementation and complexity. An extension of the inter-subcarrier nonlinear interference canceler approach is also proposed. A performance evaluation of the well-known NLC techniques and the proposed approach is provided in the context of Nyquist and super-Nyquist superchannel systems.Comment: Accepted in the IEEE Communications Surveys and Tutorial

A Liquid Crystal Coated Tapered Photonic Crystal Fiber Interferometer

An experimental investigation on liquid crystal coated tapered photonic crystal fiber (PCF) interferometer is presented in this paper. The interferometer is fabricated by tapering a small section of a PCF by collapsing the air holes and thinning down the air hole collapsed region to a micron size. Several interferometers are fabricated from different types of photonic crystal fibers and the temperature dependence studies are carried out to select an intrinsically temperature insensitive interferometer that can be used with liquid crystal (LC) materials. The properties of the LC materials to be used with the tapered PCF interferometers are also discussed. Temperature tuning of the spectral response of the tapered interferometers with different waist diameters coated with high index and low index LC materials are carried out and the characteristics are presented

Knowledge, attitude, and preventive practices of leptospirosis affected populations in South Andaman, India: A cross-sectional study

Introduction: Leptospirosis is the most common reoccurring zoonosis worldwide. Climatic conditions in tropical and subtropical regions are optimal for Leptospira survival. The pathogen thrives in flood-prone slum settlements of underprivileged areas where waste, open sewers, and standing water are present. Methods: A descriptive cross-sectional study using universal sampling methodology was conducted to determine associationsbetween sociodemographic variables and knowledge, attitudes, and practices of leptospirosis-infected individuals compared with a control group from the South Andaman population. Results: Eight hundred and one (388 cases and 413 controls) subjects were included in the study. Overall, 61.5% of the participants were male, while the main occupation of 43.94% of the subjects was farming or agricultural work. Multilogistic regression assessing the likelihood of good knowledge about leptospirosis showed that leptospirosis-positive subjects were more likely to have good knowledge (adjusted odds ratio [AOR]: 3.5 [95% CI: 2.59–4.97], p < 0.001), better attitude (AOR: 97.30 [95% CI: 41.72–226.9], p < 0.001] than leptospirosis-negative subjects, male population groups were also more likely to have a good attitude (AOR: 3.03 [95% CI: 1.94–4.73], p < 0.001), and those whose main occupation is farming were more likely to have a good attitude (AOR: 3.59 [95% CI: 2.31–5.56], p < 0.001). The leptospirosis seropositive group was more likely to have good practices (AOR: 5.80 [95% CI: 3.58–8.73], p < 0.001), rural residents were 88% less likely to have good practice levels than urban residents (AR: 0.12 [95% CI: 0.07–0.20], p < 0.01). Conclusion: The infected group had better knowledge than the control group. The integration of knowledge and attitudes to maintain good practices, along with the provision of an adequate sanitation system, waste disposal system, and availability of essential personal protective equipment is necessary for disease control in these islands

Digital signal processing techniques for fiber nonlinearity compensation in coherent optical communication systems

The capacity of long-haul coherent optical communication systems is limited by the detrimental effects of fiber Kerr nonlinearity. The power-dependent nature of the Kerr nonlinearity restricts the maximum launch power into the fiber. That results in the reduction of the optical signal-to-noise ratio at the receiver; thereby, the maximum transmission reach is limited. Over the last few decades, several digital signal processing (DSP) techniques have been proposed to mitigate the effects of fiber nonlinearity, for example, digital back-propagation (DBP), perturbation based nonlinearity compensation (PB-NLC), and phase-conjugated twin wave (PCTW). However, low-complexity and spectrally efficient DSP-based fiber nonlinearity mitigation schemes for long-haul transmission systems are yet to be developed. In this thesis, we focus on the computationally efficient DSP-based techniques that can help to combat various sources of fiber nonlinearity in long-haul coherent optical communication systems. With this aim, we propose a linear time/polarization coded digital phase conjugation (DPC) technique for the mitigation of fiber nonlinearity that doubles the spectral efficiency obtained in the PCTW technique. In addition, we propose to investigate the impact of random polarization effects, like polarization-dependent loss and polarization mode dispersion, on the performance of the linear-coded DPC techniques. We also propose a joint technique that combines single-channel DBP with the PCTW technique. We show that the proposed scheme is computationally efficient and achieves similar performance as multi-channel DBP in wavelength division multiplexed superchannel systems. The regular perturbation (RP) series used to analytically approximate the solution of the nonlinear Schrödinger equation (NLSE) has a serious energy divergence problem when truncated to the first-order. Recent results on the transmission of high data-rate optical signals reveal that the nonlinearity compensation performance of the first-order PB-NLC technique decreases as the product of the transmission distance and launch power increases. The enhanced RP (ERP) method can improve the accuracy of the first-order RP approximation by partially solving the energy divergence problem. On this ground, we propose an ERP-based nonlinearity compensation technique to compensate for the fiber nonlinearity in a polarization-division multiplexed dispersion unmanaged optical communication system. Another possible solution to improve the accuracy of the PB-NLC technique is to increase the order of the RP solution. Based on this idea, we propose to extend the first-order solution of the NLSE to the second-order to improve the nonlinearity compensation performance of the PB-NLC technique. Following that, we investigate a few simplifying assumptions to reduce the implementation complexity of the proposed second-order PB-NLC technique

Deep Learning-Aided Perturbation Model-Based Fiber Nonlinearity Compensation

Fiber nonlinearity effects cap achievable rates and ranges in long-haul optical fiber communication links. Conventional nonlinearity compensation methods, such as perturbation theory-based nonlinearity compensation (PB-NLC), attempt to compensate for the nonlinearity by approximating analytical solutions to the signal propagation over optical fibers. However, their practical usability is limited by model mismatch and the immense computational complexity associated with the analytical computation of perturbation triplets and the nonlinearity distortion field. Recently, machine learning techniques have been used to optimise parameters of PB-based approaches, which traditionally have been determined analytically from physical models. It has been claimed in the literature that the learned PB-NLC approaches have improved performance and/or reduced computational complexity over their non-learned counterparts. In this paper, we first revisit the acclaimed benefits of the learned PB-NLC approaches by carefully carrying out a comprehensive performance-complexity analysis utilizing state-of-the-art complexity reduction methods. Interestingly, our results show that least squares-based PB-NLC with clustering quantization has the best performance-complexity trade-off among the learned PB-NLC approaches. Second, we advance the state-of-the-art of learned PB-NLC by proposing and designing a fully learned structure. We apply a bi-directional recurrent neural network for learning perturbation triplets that are alike those obtained from the analytical computation and are used as input features for the neural network to estimate the nonlinearity distortion field. Finally, we demonstrate through numerical simulations that our proposed fully learned approach achieves an improved performance-complexity trade-off compared to the existing learned and non-learned PB-NLC techniques