Ultrafast photo-induced carrier dynamics of perovskite quantum dots during structural degradation.

In this study, the ultrafast photo-induced carrier dynamics of red-emitting PQDs during structural degradation was investigated using time-resolved transient absorption spectroscopy. The spectroscopic analysis revealed how the carrier dynamics varied when PQDs were exposed to a polar solvent. Three decay modes (carrier trapping, radiative carrier recombination and trap-assisted non-radiative recombination) were proposed to analyze the carrier dynamics of PQDs. The light-emitting property of PQDs is primarily influenced by radiative carrier recombination. This study demonstrates that structural degradation induced halide migration within PQDs and the formation of defects within the crystal lattice, leading to a proliferation of carrier trapping states. The increased trap states led to a reduction in carriers undergoing radiative carrier recombination. Additionally, PQDs degradation accelerated radiative carrier recombination, indicating a faster escape of carriers from excited states. Consequently, these factors hinder carriers remaining in excited states, leading to a decline in the light-emitting property of PQDs. Nevertheless, increasing an excitation fluence could reduce the carrier trapping mode and increase the radiative carrier recombination mode, suggesting a diminishment of the impact of carrier trapping. These findings offer a more comprehensive understanding of structural degradation of PQDs and can contribute to the development of PQDs with high structural stability.

Comparative analysis of dermal collagen and lipids in cereblon ablated mice using a multimodal nonlinear optical system.

By utilizing a multimodal nonlinear optical system that combines coherent anti-Stokes Raman scattering and second harmonic generation to investigate biological characteristics of dermal tissues ex vivo, we demonstrate the potential feasibility of using this optical approach as a powerful new investigative tool for future biomedical research. For this study, our optical system was utilized for the first time to analyze lipid and collagen profiles in cereblon knockout (KO) mouse skin, and we were able to discover significant alterations in the number of carbon-carbon double bonds (wild-type vs. cereblon KO; NCC : 0.75 vs. 0.85) of skin fatty acids in triacylglycerides as well as changes in dermal collagen fibers (25% reduction in cereblon KO). By adopting our optical system to biological studies, we provide researchers with another diagnostic approach to validate their experimental results, which will significantly advance the state of biomedical research.

Adaptive demodulation by deep-learning-based identification of fractional orbital angular momentum modes with structural distortion due to atmospheric turbulence.

Since the great success of optical communications utilizing orbital angular momentum (OAM), increasing the number of addressable spatial modes in the given physical resources has always been an important yet challenging problem. The recent improvement in measurement resolution through deep-learning techniques has demonstrated the possibility of high-capacity free-space optical communications based on fractional OAM modes. However, due to a tiny gap between adjacent modes, such systems are highly susceptible to external perturbations such as atmospheric turbulence (AT). Here, we propose an AT adaptive neural network (ATANN) and study high-resolution recognition of fractional OAM modes in the presence of turbulence. We perform simulations of fractional OAM beams propagating through a 1-km optical turbulence channel and analyze the effects of turbulence strength, OAM mode interval, and signal noise on the recognition performance of the ATANN. The recognition of multiplexed fractional modes is also investigated to demonstrate the feasibility of high-dimensional data transmission in the proposed deep-learning-based system. Our results show that the proposed model can predict transmitted modes with high accuracy and high resolution despite the collapse of structured fields due to AT and provide stable performance over a wide SNR range.

Deep-learning-based high-resolution recognition of fractional-spatial-mode-encoded data for free-space optical communications.

Structured light with spatial degrees of freedom (DoF) is considered a potential solution to address the unprecedented demand for data traffic, but there is a limit to effectively improving the communication capacity by its integer quantization. We propose a data transmission system using fractional mode encoding and deep-learning decoding. Spatial modes of Bessel-Gaussian beams separated by fractional intervals are employed to represent 8-bit symbols. Data encoded by switching phase holograms is efficiently decoded by a deep-learning classifier that only requires the intensity profile of transmitted modes. Our results show that the trained model can simultaneously recognize two independent DoF without any mode sorter and precisely detect small differences between fractional modes. Moreover, the proposed scheme successfully achieves image transmission despite its densely packed mode space. This research will present a new approach to realizing higher data rates for advanced optical communication systems.

Fabrication-Method-Dependent Excited State Dynamics in CH3NH3PbI3 Perovskite Films.

Understanding the excited-state dynamics in perovskite photovoltaics is necessary for progress in these materials, but changes in dynamics depending on the fabrication processes used for perovskite photoactive layers remain poorly characterised. Here we report a comparative study on femtosecond transient absorption (TA) in CH3NH3PbI3 perovskite films fabricated by various solution-processing methods. The grain sizes and the number of voids between grains on each film varied according to the film synthesis method. At the low excitation fluence of 0.37 µJ cm-2, fast signal drops in TA dyanmics within 1.5 ps were observed in all perovskite films, but the signal drop magnitudes differed becuase of the variations in charge migration to trap states and band gap renormalisation. For high excitation fluences, the buil-up time of the TA signal was increased by the activated hot-phonon bottleneck, while the signal decay rate was accelerated by fluence-dependent high-order charge recombination. These fluence-dependent dynamics changed for different perovskite fabrication methords, indicating that the dynamics were affected by morphological features such as grain sizes and defects.

The Effect of Fluorine Substitution on the Molecular Interactions and Performance in Polymer Solar Cells.

Fluorine (F) substitution on conjugated polymers in polymer solar cells (PSCs) has a diverse effect on molecular properties and device performance. We present a series of three D-A type conjugated polymers (PBT, PFBT, and PDFBT) based on dithienothiophene and benzothiadiazole units with different numbers of F atoms to explain the influence of F substitution by comparing the molecular interactions of the polymers and the recombination kinetics in PSCs. The preaggregation behavior of PFBT and PDFBT in o-DCB at the UV-vis absorption spectra proves that both polymers have strong intermolecular interactions. Besides, more closely packed structures and change into face-on orientation of fluorinated polymers are observed in polymer:PC71BM blends by GIXD which is beneficial for charge transport and, ultimately, for current density in PSCs (4.3, 13.0, and 14.5 mA cm-2 for PBT, PFBT, and PDFBT, respectively). Also, the introduction of F atoms on conjugated backbones affects the recombination kinetics by suppressing bimolecular recombination, thereby improving the fill factor (0.41, 0.68, and 0.69 for PBT, PFBT, and PDFBT, respectively). Consequently, the PCE of PSCs reached 7.3% without any additional treatment (annealing, solvent additive, etc.) in the polymer containing difluorinated BT (PDFBT) that is much higher than nonfluorinated BT (PBT ∼ 1%) and monofluorinated BT (PFBT ∼ 6%).

All-optical THz wave switching based on CH3NH3PbI3 perovskites.

Hybrid structures of silicon with organic-inorganic perovskites are proposed for optically controllable switching of terahertz (THz) waves over a broad spectral range from 0.2 to 2THz. A 532-nm external laser was utilized to generate photoexcited free carriers at the devices and consequentially to control the terahertz amplitude modulation, obtaining a depth of up to 68% at a laser irradiance of 1.5 W/cm2. In addition, we compared the performances from three types of perovskite devices fabricated via different solution processing methods and suggested a stable and highly efficient THz switch based on a one-step processing. By this we demonstrated the possibility of perovskites as THz wave switching devices in addition to photovoltaics.

Effect of Fluorine Substitution on the Charge Carrier Dynamics of Benzothiadiazole-Based Solar Cell Materials.

The femtosecond transient absorption (TA) characterization of a new benzothiadiazole (BT)-based donor-acceptor conjugated copolymer, poly[(2,6-dithieno[3,2-b:2',3'-d]thiophene)-alt-(4,7-di(4-octyldodecylthiopen-2-yl)-2,1,3-benzo[c][1,2,5]thiadiazole (PBT), as well as its fluorinated derivatives, PFBT and PDFBT, is carried out. Additionally, bulk heterojunction (BHJ) films consisting of the copolymers and [6,6]-phenyl-C71 -butylic acid methyl ester (PC70 BM) are examined using TA spectroscopy. Both the singlet excited state dynamics in the copolymers and the charge transfer state dynamics in the BHJs are investigated in terms of fluorination dependency; the fluorinated copolymers exhibit less singlet exciton recombination rate than the fluorine-free copolymer, and the BHJs including the fluorinated copolymers display slower monomolecular recombination than the fluorine-free analogue. Furthermore, the excitation-intensity-dependent TA dynamics of the copolymers and BHJs is investigated, revealing that, when sufficiently high excitation intensity is used to induce annihilation processes, the fluorinated copolymers and BHJs incorporating the fluorinated copolymers show more rapid TA decay ascribable to morphological enhancement. These TA spectroscopic findings are found to correlate with the device characteristics with respect to fluorinated content in the polymer solar cells. In particular, both the short-circuit current density and fill factor of BHJ solar cells correspond closely with the fast decay parameters of the BHJ films under high excitation intensity.

Label-free imaging and quantitative chemical analysis of Alzheimer's disease brain samples with multimodal multiphoton nonlinear optical microspectroscopy.

We developed multimodal multiphoton microspectroscopy using a small-diameter probe with gradient-index lenses and applied it to unstained Alzheimer's disease (AD) brain samples. Our system maintained the image quality and spatial resolution of images obtained using an objective lens of similar numerical aperture. Multicolor images of AD brain samples were obtained simultaneously by integrating two-photon excited fluorescence and second-harmonic generation on a coherent anti-Stokes Raman scattering (CARS) microendoscope platform. Measurements of two hippocampal regions, the cornus ammonis-1 and dentate gyrus, revealed more lipids, amyloid fibers, and collagen in the AD samples than in the normal samples. Normal and AD brains were clearly distinguished by a large spectral difference and quantitative analysis of the CH mode using CARS microendoscope spectroscopy. We expect this system to be an important diagnosis tool in AD research

Extraction of optical constants using multiple reflections in the terahertz emitter-sample hybrid structure.

To extract optical constants of nontransmitted samples in the terahertz (THz) spectral region, we employ a THz emitter-sample hybrid structure where THz pulses are generated at the emitter surface and multiply reflected at the interface between the THz emitter and the sample. Since each THz electric field profile appears well separated in a time domain, we could obtain the amplitude and phase spectra for each pulse from the Fourier transform, and determine the optical constants of the sample numerically based on the Fresnel equations. We applied this technique for doped semiconductors, and found that obtained optical constants are in good agreement with the values determined by using other conventional THz techniques.

Speckle noise reduction on a laser projection display via a broadband green light source.

A broadband green light source was demonstrated using a tandem-poled lithium niobate (TPLN) crystal. The measured wavelength and temperature bandwidth were 6.5 nm and 100 °C, respectively, spectral bandwidth was 36 times broader than the periodically poled case. Although the conversion efficiency was smaller than in the periodic case, the TPLN device had a good figure of merit owing to the extremely large bandwidth for wavelength and temperature. The developed broadband green light source exhibited speckle noise approximately one-seventh of that in the conventional approach for a laser projection display.

Rapid diagnosis of liver fibrosis using multimodal multiphoton nonlinear optical microspectroscopy imaging.

A multimodal multiphoton nonlinear optical (NLO) microspectroscopy imaging system was developed using a femtosecond laser and a photonic crystal fiber. Coherent anti-Stokes Raman scattering (CARS) microspectroscopy was combined with two-photon excitation fluorescence and second-harmonic generation microscopy in one platform and the system was applied to diagnose liver fibrosis. Normal and liver fibrosis tissues were clearly distinguished with the great difference from CARS spectra as well as multimodal multiphoton NLO images. We expect the system to be a rapid diagnosis tool for liver fibrosis at tissue level with label-free imaging of significant biochemical components.

All-optical image switching in a double-Λ system.

The coherent control of optical images has garnered attention because all information embedded in optical images is expected to be controlled in a parallel way. One of the most important control processes is switch for information delivery. We experimentally demonstrated phase-controlled optical image switching in a double-Λ system where the transmission of the image through a medium was switched. Two independent laser sources were adopted for a double-Λ system such that images inscribed in two weak probe light beams were incoherent with each other. Arbitrary phase was added to the optical images to show that switching could be accomplished just with the relative phase difference between the probe pixels.

Direct reconstruction of an object from dual exposure Fourier intensity measurements.

The reconstruction of an object with a method using a dual exposure single inverse Fourier transform is investigated. The method calculates phase information in the Fourier plane to perform the inverse Fourier transform. The phase information in the Fourier plane is calculated from the intensity distributions formed by an object with and without a reference electric field. The method successfully reconstructs an object in a simple and fast manner. For the practical use of the method, the effects of the intensity digitization and the noise in the intensity distributions are examined in reconstructing an object.

Quasi-holographic solution to polarization-sensitive optical coherence tomography acceptable to nonlaboratory applications.

Experimental proof-of-concept is presented for a quasi-holographic solution to polarization-sensitive optical coherence tomography (PS OCT). Due to decoupling between the reference and sample beams by polarization, the solution seems acceptable to acquisition and communication of optical data in the nonlaboratory environment. The nonlab environment implies uncontrollable disturbances, e.g., temperature changes and mechanical effects happening under shop testing in industry or routine examinations in common clinics and hospitals. For mapping the collagen-related depolarization ratio of light backscattered from the human dermis, a phenomenological model is evolved from the theory of light depolarization in crystalline polymers. The model yielded a simplified intensity-based estimation algorithm. The design concept and the model rely on a submillimeter tumor thickness as a proofed prognostic factor and an important criterion for complementary functional diagnostics of skin cancers in their early phase. Choice of the model is inspired by similarity of structural and optical properties between liquid-crystal collagen fibers in the dermis and birefringent crystalline lamellae in some polymer materials. The model gives a plausible interpretation of a peculiarity of cumulative birefringence in the abnormal skin dermis. Following a top-down approach to design, the authors attempt to contribute to bridging the gap between practitioners' concerns and academic studies.

Electromagnetically induced transparency on GaAs quantum well to observe hole spin dephasing.

Electromagnetically induced transparency (EIT) was observed with transient optical response of exciton correlation in GaAs/AlGaAs quantum well structure. Decoherence of EIT was increased with temperature (12-60 K), which could be simulated by increasing non-radiation decay rate between coherently coupled ground states in Bloch equation for Lambda type three level. The non-radiation decay was mainly due to hole spin dephasing in the system for EIT via coulomb correlation. The hole spin dephasing rate was found with increasing lattice temperature and well accorded to the past results of time resolving method with n-doping material.

Bandwidth control of a Ti:PPLN Solc filter by a temperature-gradient-control technique.

We have demonstrated the bandwidth control of a Ti-diffused periodically poled LiNbO(3) (Ti:PPLN) Solc filter by a temperature-gradient-control technique. Up to 2.8 nm of filtering bandwidth was achieved with a simple temperature-gradient-control technique in a 78-mm-long of Ti:PPLN waveguide, which has a 0.2 nm filtering bandwidth at an uniform temperature. We have also analyzed the experimental results with the theoretical calculation which is derived from the codirectional coupled mode equations.

Femtosecond phase control of spatial localization of the optical near-field in a metal nanoslit array.

We demonstrate spatial control of optical near-fields by femtosecond phase shaping in one-dimensional plasmonic structures. The near-field images display striking temporal-phase dependence, switching between double- and single-peak images within one lattice constant. The change of the near-field distribution is studied in the time and spectral domain. The spectral composition change observed by varying the time delay between two phase-locked femtosecond pulses explains the spatial control of the near-field images. Modal expansion calculations of linear light transmission using the surface impedance boundary condition are in excellent agreement with experiments.

Dispersion and birefringence of irregularly microstructured fiber with an elliptic core.

We have investigated the dispersion and birefringence of an irregularly microstructured fiber with an elliptic silica core and irregular airholes. The polarization-dependent output power through the fiber reveals two well-defined principal-axis modes despite the irregularity of airholes. The dispersion of the fiber is measured in the range of 680 to 1000 nm using the Mach-Zehnder interferometric technique with sub-10 fs laser pulses, which yield two zero dispersion wavelengths at 683 and 740 nm for the two principal modes, respectively. The birefringence measured using the wavelength scanning method is about 0.0055 at 800 nm. It is also demonstrated that this irregularly microstructured fiber with high birefringence and short zero dispersion wavelength is useful for the one-octave-spanning supercontinuum generation suitable for an f-2f interferometric system.

Method of reconstructing wavefront aberrations from the intensity measurement.

A method for reconstructing wavefront aberrations from intensity measurements in the focal plane of a focusing optic is presented. This reconstruction method is simple, fast, and accurate in reconstructing wavefront aberrations because it uses the inverse Fourier transform of an intensity distribution in the focal plane with a reference electric field. The validity of the reconstruction method is demonstrated by computing a wavefront aberration from the intensity distributions in the focal plane.