Professor Ming-Hua Mao's Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We studied the transient behaviors of current-injection quantum-dot microdisk lasers at room temperature. Unique optical responses were observed, including the suppression of relaxation oscillations and fast turn-on. With the help of rate-equation modeling, the suppressed relaxation oscillations are attributed to the enhanced spontaneous emission factor in microdisk lasers. Short turn-on time, around 1 ns without pre-bias, results from the reduced carrier lifetime caused by the Purcell effect and increased nonradiative recombination rate due to higher surface/volume ratio. With short turn-on time, a large-signal direct modulation experiment at 1 Gbps is demonstrated. Modal transient behavior was also investigated under various temperatures from 100 to 300 K. Both of the transient lasing and steady state lasing from side modes are suppressed at temperatures higher than 250K. Therefore, the quantum-dot microdisk lasers show the potential of single-mode operation under high-speed modulation at room temperature. This work has been published in Opt. Express, Vol. 20, No. 3, pp. 3302, (2012).

Figure 1. The temporal optical responses measured from a QD microdisk laser.

 

Figure 2. The ideal data stream at 1 Gbps, the real electric pulse shape, and the optical responses of the QD microdisk laser with pre-bias.

 

Enhancing LED Light Extraction by Optimizing Cavity and Waveguide Modes in Grating Structures

Professor Yih-Peng Chiou's Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We provided an optimization of light extraction efficiency (LEE) from light-emitting diodes (LEDs). The optimization is achieved when the structural parameters results in good cavity modes in both planar and grating structures, and also when the propagation direction of the fundamental waveguide mode is significantly converted to the direction normal to the grating surface by the gratings. In this research, we use two simple equations, one for cavity modes and the other for waveguide modes, to predict the loci of the optimal structural parameters of gratings. We first obtain the rules by analyzing simple planar structures as illustrate in Fig. 1. (a). Fig. 1. (b) shows the resonant condition by the micro-cavity effect. Fig. 1. (c) shows that the local maximum LEEs appear periodically and are close to the cross of the Fabry-Pérot resonances and the antinodes. The conditions of the resonance at normal incidence and the anitnodes for normal waves are marked by the thick red tics on the bottom and the thick black tics to the right, respectively. Even with the optimization, the LEE of typical planar of LED can only reach up to 30%. To realize ultra-high LEE LED, the source is designed to locate close to the middle of GaN cavity to concentrate the non-radiation power on the fundamental mode and a periodic dielectric wavelength-scaled structure is etched on the mesa surface of LEDs to convert the fundamental guided wave into air mode as schematized in Fig. 2. (a). We modified the resonant condition of normal incident case for grating-assisted system and gave the constructive interference of the emitted waves:

 

From Fig. 2. (b-c), the local maxima of LEE appear near the intersections of the two equations. The maximum LEE is close to 90% which is triple of that of the planar surface LEDs. These results verified our prediction.

Fig. 1. (a) Scheme of optical modes in in thin-film vertical LED. (b) Resonant condition by the micro-cavity effect. (c) Simulation results of LEE. Thick red tics represent the thickness with Fabry-Pérot resonance appearing at normal incidence, and thick black tics express as the antinode factors.

 

Fig. 2. (a) Propagation of waveguide modes in a grating structure. (b-c) Simulation results of the maximum LEE for grating depth versus the GaN thickness for duty circle=0.3 and 0.5.

Reference: W.-L. Yeh, C.-M. Fang, and Y.-P. Chiou, “Enhancing LED Light Extraction by Optimizing Cavity and Waveguide Mode in Grating Structures,” IEEE Journal of Display Technology, Vol. 9, No. 5, pp. 359-364, May, 2013.

 

Professor Snow H. Tseng's Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Digitally time-reversed ultrasound-encoded light propagation through turbid media is simulated using numerical solutions of Maxwell’s equations. A virtual light source embedded within a turbid medium is modeled; specifically, we simulate the propagation of phase-conjugated light through a turbid medium and homing in the virtual light source to form an optical focus within the turbid medium. The numerical simulations enable qualitative and quantitative analysis of the effectiveness of directing light through turbid media.

Fig. 1. Comparison of the incident light and playback reconstructed light in vacuum. The phasor profiles of the (a) original illumination, and (b) the phase-conjugated playback wave are compared. Without scattering loss, the original light profile is reconstructed without observable difference.

 

Professor Guo-Dung J. Su's Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Microlens arrays are important components in many applications, such as Shack-Hartmann wavefront sensors, beam shaping for illumination, light gathering for solar concentrators and information displays, etc. Various techniques to fabricate MLAs have been developed. Inkjet-printed microlens arrays (MLAs) were fabricated using hydrophilic confinement by UV/ozone treatment in the Micro Optics Device Lab of National Taiwan University. The MLAs were made from negative photoresist SU-8 (n=1.63 at 530 nm). A film of 10-µm-thick SU-8 shadow mask was used to define UV/ozone-treated hydrophilic zones on an SU-8 photoresist base layer. An inkjet print head was used to jet SU-8 photoresist drops onto these zones. After UV-curing, MLAs with diameters of 150, 200, 400, 800 and 1000 µm were successfully fabricated. Contact angles of MLAs increased from 22° (on MLAs fabricated using an SU-8 photoresist base layer without any surface treatment) to 45.5°, 47.7°, 52.4°, 51.3° and 54.2°, for 150, 200, 400, 800 and 1000 µm lens diameters, respectively. Using hydrophilic confinement, MLAs with a wide range of contact angles can be fabricated with diameters from 150 µm to 1 mm. This method provides a simple, cost-effective fabrication process without need for etch-transfer.

Fig. 1. MLAs of 400 µm diameter with 4 different contact angles: 29.7°, 39.6°, 47.3°, and 52.4°.

 

Professor I-Chun Cheng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We investigate the nanoporous TiO₂ films sintered by atmospheric pressure plasma jets (APPJs) and their applications as photoanodes of dye-sensitized solar cells (DSSCs). A 30-s APPJ-sintered nanoporous TiO₂ layer exhibits an additional absorption band between 400 and 500 nm in wavelength, attributed to incomplete removal of the organic solvents in the pastes. For TiO₂ layers sintered by APPJs for 60 s and beyond, the absorption spectra are nearly identical to those of a conventional 15 min, 510°C calcined sample. The XRD and XPS results indicate similar characteristics for APPJ-sintered and furnace-sintered TiO₂ films. A DSSC with a 30-s APPJ-sintered TiO₂ photoanode shows poor cell efficiency with an extremely large TiO₂/dye/electrolyte electron transport interfacial resistance and a short carrier lifetime (Figure 1). As the APPJ treatment time reaches 60 s and beyond, the power conversion efficiencies become comparable to that of a sample with a 510°C conventionally calcined TiO₂ photoanode (Figure 2). Our experimental results verify that a 60-s APPJ sintering process is sufficient to replace a conventional 15 min, 510°C furnace calcination process for TiO₂ photoanodes of DSSCs. The ultra-short sintering process is made possible by the synergistic effect of the temperature and the reactivity of the APPJ, which can lower the fabrication cost.

Figure 1. (a) Nyquist plot of DSSCs with photoanodes sintered by furnace and APPJ under illumination. (b) The model equivalent circuit diagram and the corresponding parameters.

 

Figure 2. (a) Typical IV characteristics and (b) power conversion efficiency of DSSCs with furnace-sintered and APPJ-sintered TiO2 photoanodes.

© 2013 Elsevier B.V.
H. Cheng, Y.-J. Yang, H.-C Li, C.-C. Hsu, I-C. Cheng, J.-Z. Chen, “Preparation of nanoporous TiO₂ films for DSSC application by a rapid atmospheric pressure plasma jet sintering process,” Journal of Power Sources, vol. 234, pp. 16-22, 2013

 

Professor Jui-che Tsai

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We developed a microelectromechanical systems (MEMS) doubly decoupled gyroscope with a wide driving frequency range. The gyroscope (Fig. 1) increases the resonance bandwidths of both the drive and sense oscillators without sacrificing their Q factors. This bandwidth enhancement ensures good frequency matching between the drive and sense oscillators even if fabrication imperfections are present. Additionally, no in-operation frequency tuning mechanism is needed. The gyroscope can be driven at any frequency within the overlapping region of the resonance bands of the drive and sense oscillators. Experiments showed that the gyroscope driving voltage can be of any frequency within a ∼240-Hz bandwidth (Fig. 2). The wide bandwidth provides flexibility and ease of use. The device’s doubly decoupled structure minimizes the interference/coupling between the drive and sense units. Performance tests under rotation rates equal to or smaller than 18.71 rad/s showed that the gyroscope has a sensitivity of 4.28 mV/(rad/s).

Figure 1

 

Figure 2

© 2012 IEEE
C. W. Tsai, K. H. Chen, C. K. Shen, and J. C. Tsai, “A MEMS doubly decoupled gyroscope with wide driving frequency range,” IEEE Transactions on Industrial Electronics, vol. 59, no. 12, pp. 4921-4929, Dec. 2012.