Browsing by Author "Chang, Chen-Yu"

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A curvature-tunable random laser
(2019-02-28)
Lee, Ya-Ju
;
Yeh, Ting-Wei
;
Yang, Zu-Po
;
Yao, Yung-Chi
;
Chang, Chen-Yu
The application of random lasers has been restricted due to the absence of a well-defined resonant cavity, as the lasing action mainly depends on multiple light scattering induced by intrinsic disorders of the laser medium to establish the required optical feedback that hence increases the difficulty in efficiently tuning and modulating random lasing emissions. This study investigated whether the transport mean free path of emitted photons within disordered scatterers composed of ZnO nanowires is tunable by a curvature bending applied to the flexible polyethylene terephthalate (PET) substrate underneath, thereby creating a unique light source that can be operated above and below the lasing threshold for desirable spectral emissions. For the first time, the developed curvature-tunable random laser is implemented for in vivo biological imaging with much lower speckle noise compared to the non-lasing situation through simple mechanical bending, which is of great potential for studying the fast-moving physiological phenomenon such as blood flow patterns in mouse ear skin. It is expected that the experimental demonstration of the curvature-tunable random laser can provide a new route to develop disorder-based optoelectronic devices.
Bending-induced tunable threshold in random laser
(2019-01-01)
Lee, Ya-Ju
;
Yeh, Ting-Wei
;
Yang, Zu-Po
;
Yao, Yung-Chi
;
Chang, Chen-Yu
We investigate the case that the transport mean free path of emitted photons within the disordered scatterers, which is composed by the ZnO nanowires, is tunable by a curvature bending applied on the flexible polyethylene terephthalate substrate underneath, thereby creating a unique light source which is able to be operated above and below the lasing threshold for desirable spectral emissions. (C) 2019 The Author(s)
Femtosecond time-evolution of mid-infrared spectral line shapes of Dirac fermions in topological insulators
(2020-06-17)
Yeh, Tien-Tien
;
Tu, Chien-Ming
;
Lin, Wen-Hao
;
Cheng, Cheng-Maw
;
Tzeng, Wen-Yen
Mid-infrared (MIR) light sources have much potential in the study of Dirac-fermions (DFs) in graphene and topological insulators (TIs) because they have a low photon energy. However, the topological surface state transitions (SSTs) in Dirac cones are veiled by the free carrier absorption (FCA) with same spectral line shape that is always seen in static MIR spectra. Therefore, it is difficult to distinguish the SST from the FCA, especially in TIs. Here, we disclose the abnormal MIR spectrum feature of transient reflectivity changes (Delta R/R) for the non-equilibrium states in TIs, and further distinguish FCA and spin-momentum locked SST using time-resolved and linearly polarized ultra-broadband MIR spectroscopy with no environmental perturbation. Although both effects produce similar features in the reflection spectra, they produce completely different variations in the Delta R/R to show their intrinsic ultrafast dynamics.
In Vivo Speckle Contrast Imaging Constructed by Threshold-tunable Random Lasers
(2019-01-01)
Yeh, Ting-Wei
;
Yang, Zu-Po
;
Yao, Yung-Chi
;
Chang, Chen-Yu
;
Tsai, Meng-Tsan
The application of random lasers has been restricted owing to the absence of well-defined resonant cavity, as the lasing action mainly depends on multiple light scatterings induced by intrinsic disorders of laser medium to establish the required optical feedbacks, and hence that increases the difficulty to efficiently tune and modulate random lasing emissions. In this study, we investigate the case that the transport mean free path of emitted photons within the disordered scatterers, which is composed by the ZnO nanowires, is tunable by a curvature bending applied on the flexible polyethylene terephthalate (PET) substrate underneath, thereby creating a unique light source which is able to be operated above and below the lasing threshold for desirable spectral emissions. The developed curvature-tunable random laser, for the first time, is implemented for in vivo biological imaging with much lower speckle noise when compared to the non-lasing situation through a simple mechanical bending, which is of great potential for studying the fast-moving physiological phenomenon like blood flow patterns in mouse's ear skins. As a result, we expect the experimental demonstration on the curvature-tunable random laser can open up a new route towards developing disorder based optoelectronic devices.
Mechanisms of the Asymmetric Light Output Enhancements in a-Plane GaN Light-Emitting Diodes With Photonic Crystals
(2014-12-01)
Li, Hsiang-Wei
;
Yin, Yu-Feng
;
Chang, Chen-Yu
;
Tsai, Chen-Hung
;
Hsu, Yen-Hsiang
The unique properties of nonpolar GaN light-emitting diodes (LEDs) have the advantages of generating polarized light emission. The employment of asymmetric 2-D photonic crystals (PhCs) can further enhance the light polarization ratio. In addition, it was generally recognized that the Purcell effect can increase the internal quantum efficiency of the LEDs with PhCs. In this paper, we study the properties of optical modes from different crystal planes. The Purcell effect is analyzed based on the PhCs and material crystal orientations. With different transition probability of the polarized photons in valence bands, the corresponding Purcell effect enhancement on the quantum efficiency varies.
Minimum Variance Run-to-Run Controller for General Stochastic Time-Series-Based Disturbances
(2020-08-01)
Lee, An-Chen
;
Huang, Ruei-Yu
;
Chen, Yu-Xian
;
Tsai, Te-Hsiu
;
Chang, Chen-Yu
Exponentially weighted moving average (EWMA) or double EWMA controllers are often employed to deal with various stochastic time-series disturbances for run-to-run control. If the disturbance model is known, then the best EWMA or double EWMA controller can be obtained by minimizing output variance with respect to controller parameters. However, from the theoretical view point, results are only sub-optimum because the control scheme may not be the best control scheme for the underlying stochastic disturbance. Therefore, investigating the best control scheme for the process disturbance following the general ARIMA time series process is worthwhile. In this paper, the predictive disturbance observer (PDOB) is developed based on minimum variance control for various ARIMA(p,r,q) stochastic disturbances. If the ARIMA(p,r,q) disturbance model is known, then the PDOB scheme generates one-step ahead prediction for this disturbance and then feeds it back to compensate the effect of stochastic disturbance on the system output; as a result, the system generates the minimum output variance or simply white noise variance.