Unlocking the Full Potential of Polymer-Based Solid-State Photon Upconversion

To harvest the full potential of polymer-based solid-state photon upconversion (UC) devices, we examined the effect of the molecular weight of a fluorescent polymer on the UC efficiency. With a high-molecular-weight polymer, a long triplet lifetime of 11.2 ms was achieved, which led to a characteristic threshold intensity of 67 mW cm⁻², considerably lower than those of previously reported polymer-based UC devices. Furthermore, the external quantum efficiency of our UC device was as high as ∼0.35%. Consequently, fluorescent conjugated polymers with long triplet lifetimes can serve as attractive candidates for efficient solid-state UC devices

Sensitizer–host–annihilator ternary-cascaded triplet energy landscape for efficient photon upconversion in the solid state

In this paper, we introduce a new strategy for improving the efficiency of upconversion emissions based on triplet–triplet exciton annihilation (TTA-UC) in the solid state. We designed a ternary blend system consisting of a triplet sensitizer (TS), an exciton-transporting host polymer, and a small amount of an annihilator in which the triplet-state energies of the TS, host, and annihilator decrease in this order. The key idea underpinning this concept involves first transferring the triplet excitons generated by the TS to the host and then to the annihilator, driven by the cascaded triplet energy landscape. Because of the small annihilator blend ratio, the local density of triplet excitons in the annihilator domain is higher than those in conventional binary TS/annihilator systems, which is advantageous for TTA-UC because TTA is a density-dependent bimolecular reaction. We tracked the triplet exciton dynamics in the ternary blend film by transient absorption spectroscopy. Host triplet excitons are generated through triplet energy transfer from the TS following intersystem crossing in the TS. These triplet excitons then diffuse in the host domain and accumulate in the annihilator domain. The accumulated triplet excitons undergo TTA to generate singlet excitons that are higher in energy than the excitation source, resulting in UC emission. Based on the excitation-intensity and blend-ratio dependences of TTA-UC, we found that our concept has a positive impact on accelerating TTA

Experimental Demonstration of Accurate Noncontact Measurement of Arterial Pulse Wave Displacements Using 79-GHz Array Radar

In this study, we present a quantitative evaluation of the accuracy of simultaneous array-radar-based measurements of the displacements caused at two parts of the human body by arterial pulse wave propagation. To establish the feasibility of accurate radar-based noncontact measurement of this pulse wave propagation, we perform experiments with four participants using a 79-GHz millimeter-wave ultra-wideband multiple-input multiple-output array radar system and a pair of laser displacement sensors. We evaluate the accuracy of the pulse wave propagation measurements by comparing the displacement waveforms that are measured using the radar system with the corresponding waveforms that are measured using the laser sensors. In addition, to evaluate the estimates of the pulse wave propagation channels, we compare the impulse response functions that are calculated from the displacement waveforms obtained from both the radar data and the laser data. The displacement waveforms and the impulse responses both demonstrated the good agreement between the results of the radar and laser measurements. The normalized correlation coefficient between the impulse responses obtained from the radar and laser data on average was as high as 0.97 for the four participants. The results presented here strongly support the feasibility of accurate radar-based noncontact measurement of arterial pulse wave propagation

Radar-Based Estimation of Human Body Orientation Using Respiratory Features and Hierarchical Regression Model

This study proposes an accurate method to estimate human body orientation using a millimeter-wave radar system. Body displacement is measured from the phase of the radar echo, which is analyzed to obtain features associated with the fundamental and higher-order harmonic components of the quasi-periodic respiratory motion. These features are used in body-orientation estimation invoking a novel hierarchical regression model in which a logistic regression model is adopted in the first step to determine whether the target person is facing forwards or backwards; a pair of ridge regression models are employed in the second step to estimate body-orientation angle. To evaluate the performance of the proposed method, respiratory motions of five participants were recorded using three millimeter-wave radar systems; cross-validation was also performed. The average error in estimating body orientation angle was 38.3$^\circ$ and 23.1$^\circ$ using respectively a conventional method with only the fundamental frequency component and our proposed method, indicating an improvement in accuracy by factor 1.7 when using the proposed method. In addition, the coefficient of correlation between the actual and estimated body-orientation angles using the conventional and proposed methods are 0.74 and 0.91, respectively. These results show that by combining the characteristic features of the fundamental and higher-order harmonics from the respiratory motion, the proposed method offers better accuracy.Comment: 5 pages, 4 figures. This work is going to be submitted to the IEEE for possible publicatio

Radar-Based Estimation of Human Body Orientation Using Respiratory Features and Hierarchical Regression Model

This letter proposes an accurate method to estimate human body orientation using a millimeter-wave radar system. Body displacement is measured from the phase of the radar echo, which is analyzed to obtain features associated with the fundamental and higher order harmonic components of the quasi-periodic respiratory motion. These features are used in body orientation estimation invoking a novel hierarchical regression model in which a logistic regression model is adopted in the first step to determine whether the target person is facing forward or backward; a pair of ridge regression models is employed in the second step to estimate body orientation angle. To evaluate the performance of the proposed method, respiratory motions of five participants were recorded using three millimeter-wave radar systems; cross validation was also performed. The average error in estimating body orientation angle was 38.3 ∘ and 23.1 ∘ using, respectively, a conventional method with only the fundamental frequency component and our proposed method, indicating an improvement in accuracy by a factor of 1.7 when using the proposed method. In addition, the coefficients of correlation between the actual and estimated body orientation angles using the conventional and proposed methods are 0.74 and 0.91, respectively. These results show that by combining the characteristic features of the fundamental and higher order harmonics from the respiratory motion, the proposed method offers better accuracy

Development of an Operating Strategy for On-Demand Earth Observation Missions of the Diwata-2 Microsatellite

Diwata-2 is the Philippines’ 2nd microsatellite developed by Tohoku University, Hokkaido University, University of the Philippines, and the Philippine Department of Science and Technology. Its primary purpose is gathering remote sensing data through imaging areas of interest for the Philippines. This paper presents the study of Diwata-2’s initial Earth observation pointing performance, investigation of its Attitude Determination and Control System, the tuning of its Star Tracker sensor parameters, the in-flight target pointing calibration, and the sequential scheduling of its components forming an operation strategy for an effective on-demand earth observation mission. This operation strategy has managed to improve the satellite’s pointing performance from the initial 2.88°±2.06° RMS pointing error to having an accuracy of 0.204°±0.12° RMS for its High Precision Telescope payload. This strategy has been implemented to the university-built microsatellite for over 400 successful Earth observation missions and has covered about 82.8% of the Philippine’s land area with its Spaceborne Multispectral Imager payload

Triplet sensitization via charge recombination at organic heterojunction for efficient near-infrared to visible solid-state photon upconversion

Realizing efficient near-infrared to visible photon upconversion in the solid state is pivotal for commercial applications in various fields. We previously reported a solid-state upconversion device which imitated the photovoltaic conversion mechanisms of organic solar cells. This leads to a significant improvement of up to 2.3% in the external quantum efficiency, which is two orders of magnitude higher than that of conventional devices. Here, we investigate the upconversion mechanism of this device. We examine exciton and charge dynamics using transient absorption spectroscopy and find that approximately 67% of incident photons are utilized owing to fast singlet exciton diffusion in the nonfullerene acceptor layer. Strikingly, triplet excitons are accumulated near the donor/acceptor interface, enabling accelerated triplet–triplet annihilation by a factor of more than 10