GIPO newsletter

Simulating light transmission through scattering medium via numerical solutions of Maxwell’s equations

Professor Snow H. Tseng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Human cornea and sclera have very similar structures but distinctive optical characteristics. Here we model monochromatic light propagation through a macroscopic scattering medium by employing the pseudospectral time-domain (PSTD) simulation technique. We analyze the transparency; various factors are investigated. By analyzing the transmitted light intensity, the research objective is to decipher factors that contribute to the corneal and scleral optical characteristics.

Figure 1. Simulation analysis of the transmitted light intensity through scattering media of various cylinder spacing. Wavelength of the CW light is λ = 538 nm. (Upper row): the scattering medium consists of 200-nm-diameter dielectric cylinders. (Lower row): The light amplitude exiting the scattering medium. The penetrated light intensity decreases with increased cylinder diameter.

Biologically based artificial compound eye designed for a camera lens

Professor Guo-Dung J. Su

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

TAIPEI, TAIWAN — Researchers have designed a new camera module that uses a curved hexagonal microlens array and all spherical surfaces. Traditionally camera designs are difficult to assemble due to tight tolerance, because they consist of many separated lenses. If there is only one lens element in the camera, tolerance buildups can be avoided. The way to achieve a one-lens camera is through combining many lenses.

Researchers led by Prof. Guo-Dung J. Su from the Micro Optics Device Laboratory of the Graduate Institute of Photonics and Optoelectronics at National Taiwan University, Taiwan, experimentally demonstrated a one-lens camera design using a biologically inspired artificial compound eye with multiple focal lengths to avoid tolerance buildups. The artificial compound eye is a curved hexagonal microlens array arranged across a hemispherical photopolymer dome, wherein each microlens collects light with a small angular acceptance.

Fig. 1. SEM images of the one-lens camera module system using a curved hexagonal microlens array attached to a hemispherical lens. The oblique view after cutting with a dicing saw is used to confirm the fabricated curved microlens array is spherical and its center of curvature locates at the down surface of the glass substrate. Image taken by Wei-Lun Liang, National Taiwan University.

“This structure is integrated by the principles of both the human eye as well as an insect’s compound eye. It helps us to achieve a compact and wide field-of-view camera module.” said Wei-Lun Liang of the Micro Optics Device Laboratory, who was instrumental in designing the one-lens camera. “Most camera sensors are flat, due to the lithographic fabrication process. Our research showed that the field curvature aberration can be reduced by focusing on the best way to project a sharp image onto a flat surface.”

There are several benefits of using a hemispherical lens for a camera system, such as avoidance of coma aberration, wide field-of-view collection, and reduction of astigmatism aberration. However, a common optical problem of a hemispherical lens is known as field curvature aberration, and its image plane can be referred to as a curved Petzval surface. If the microlens array is arranged on the hemispherical surface, it can help with solving the problem.

To create the one-lens camera, the researchers used inkjet printing technology with the hydrophilic confinement effect to establish microlens shapes with different profiles on a planar substrate. The replication process can convert the planar array into a curved shape. Next, liquid photopolymer is filled into a deformed elastomer membrane and then cure when exposed to ultraviolet light. In the end, the spherical configuration of the hexagonal array of multiple focal lengths is accomplished by applying the template architecture to a reconfigurable surface shape.

To verify that the fabricated lens has hemispherical shape, the researchers observe scanning electron microscope (SEM) images of the lens sectioned by a dicing saw. “The sample is fed into a high-precision dicing saw, and the cutting blade accurately passes through the centers of the microlenses,” said Liang. “From the SEM images, we can check that the curved array is spherical and its center locates on the planar side of the lens.”

After an image is captured by the experimental setup of the camera module system which is built up of the one lens component, the researchers stitch every partial image formed by each microlens within its segmented channel into a combined image. “Tracing the path of a light beam emitted from a point source toward the image plane, the rays of the light beam pass through neighboring microlenses and produce several convergent beams of light.” said Liang. “This phenomenon leads to duplicate regions in neighboring sub-images, and these duplicate regions become a benefit for the subsequent image stitching.”

The researchers are now managing to convert their design into a commercial product. The funding support is from Taiwan Ministry of Science and Technology (MOST).

Exciton dynamics in tri-layer organic thin-film with exciplex sensitized triplet-triplet annihilation

Professor Jiun-Haw Lee

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Exciton dynamics in tri-layer organic thin-film with exciplex sensitized triplet-triplet annihilation (ESTTA) process was studied by transient photoluminescence (trPL) measurement, which consists of 4,4’,4’’-tris(N-3-methyphenyl-N-phenyl-amino)triphenylamine (m-MTDATA) acting as the electron donor, 1-(2,5-dimethyl-4-(1-pyrenyl)phenyl)pyrene (DMPPP) as a triplet-diffusion-singlet-blocking (TDSB) layer, and 9,10-bis(2’-naphthyl) anthracene (ADN). With increasing the thickness of DMPPP from 0, 5, 10, and 15 nm, fluorescence decay time increased which was closed to the ADN thin-film, because DMPPP effectively blocked the singlet quenching of ADN by DMPPP.

Fig. 1. (a) Energy diagram of tri-layer organic thin-film, and (b) trPL measurement of thin film with different DMPPP thicknesses.

Surface plasmons coupled two-dimensional photonic crystal biosensors for Epstein-Barr virus protein detection

Professor Jian-Jang Huang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The fundamental detection scheme of typical grating or photonic crystal (PhC) sensors relies on the correlation of the diffraction behaviors with the refractive index of target analyte. For biomaterial detection, a thin gold film is usually deposited on the sensor surface so that cross linkers can be anchored before functionalizing the device. The nanoscale metallic film suggests plasmonic resonances play a critical role in light-matter interaction. In this work, we demonstrated a compact two-dimensional (2D) hexagonal photonic crystal (PhC) biosensor with thin gold film coating. In such a device, surface plasmon polariton (SPP) on the metal structure and phase matching of optical wave with the PhC are considered when analyzing light-biomaterial interaction on the detector. Using Epstein-Barr nuclear antigen-1 (EBNA-1) antibody as an example, protein sensing along different lattice orientations of the PhC biosensor was investigated. Based on the dispersion relationship and phase matching conditions, we unravel the mechanisms involved in the wavelength shifts and sensitivities. The simulation results verify the phenomenon of tighter spatial confinement and higher local field intensity from the SPP, which helps detecting small variations of the refractive index on the sensor surface.

Fig. 1. Experimental setup for characterizing EBNA-1 antibodies

Fig. 2. (a) Sensitivity curves (cut-off wavelength shifts of various EBNA-1 antibody concentrations) along (a) G_k and (b) G_m directions

Fig. 3. Simulation results of a EM field within a single nanohole

In-situ CH₃NH₃PbI₃ Interface Treatment by Dielectric Barrier Discharge Plasma for Perovskite Solar Cells

Professor I-Chun Cheng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The surface diffusion dielectric barrier discharge (SDDBD) is a type of low-temperature plasma operated at regular pressure without the requirement of vacuum pumps and chambers. The plasma is formed by applying an AC voltage to produce a filamentous discharge. In this work, the effect of surface diffusion dielectric barrier discharge treatment on the properties of methylammonium lead iodide (CH₃NH₃PbI₃) perovskite thin films is investigated. The in-situ SDDBD-treated MAPbI3 perovskite thin films are then used as the absorber layers for planar n-i-p perovskite solar cells (PSCs). N-O emission system is observed in the plasma during the treatment, indicating the reactions of reactive nitrogen plasma species with the organics in the precursor films. Long SDDBD treatment duration (60 s) leads to the phase precipitation of PbI₂ and degrades the cell efficiency, whereas short SDDBD treatment duration (20 s) significantly improves the PSC efficiency by nearly 40%. Only slight performance enhancement is observed in the PSCs prepared by conventional thermal treatment at the same substrate temperatures as that for SDDBD treatment, implying that SDDBD provides the synergetic effect of the reactivity and plasma heating. Electrochemical impedance spectroscopy indicates that the charge recombination impedance increases and then decreases with SDDBD treatment duration. The shunt resistance also shows a similar trend, and it is consistent with the PSC performance. The PSC with the 20 s SDDBD-treated absorber layer shows energy conversion efficiency of 14.29%, open circuit voltage of 1.022 V, short circuit current density of 19.45 mA/cm², and fill factor of 0.72, whereas the corresponding values for the untreated counterpart are 10.32%, 0.976 V, 18.52 mA/cm², and 0.57. The result shows that short-duration low-temperature SDDBD treatment effectively enhances the performance of solution processed PSCs. [Appl. Surf. Sci. 473, pp.468 – 475, 2019.]

Fig. 1. (a) Evolution of OES of SDDBD plasmas during the treatment for perovskite layers. (b) J-V curves of PSCs with various SDDBD treatment conditions.