GIPO newsletter

Interferometry-based EUV spectrometer

Professor Sheng-Lung Huang’s laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

A compact and wavelength-calibration-free interferometric scheme was numerically and experimentally investigated using an extreme ultraviolet (EUV) source generated by a laser-produced plasma (LPP). The LPP-EUV source was excited by a 4-stage Yb³⁺-doped fiber MOPA system with a pulse energy of 1.01 mJ, a peak power of 161 kW, and a pulse width of 4 ns at a 20 kHz repetition rate. A Michelson-type interferometer with a common path, formed by a Si/Mo-multilayer-based beam splitter and mirror, was utilized to achieve system compactness. Based on the Wiener–Khinchin theorem, an accurate EUV spectrum was obtained by numerically analyzing the measured signal autocorrelation without performing wavelength calibration. Our theoretical results demonstrated that the employed Si/Mo multilayer structure did not distort the interferometric signal as long as the layer thickness was less than the center wavelength of the incident light. The achieved spectral resolution of 30 pm was comparable to those of flat-field spectrometers. Various high-oxidation states of Sn and residual O were identified. This compact and calibration-free spectroscopic tool could be useful for plasma characterization and mask evaluation in the EUV lithography range.

Fig. 1. Measured EUV interferometric signal.

Fig. 2. EUV spectrum computed using measured interferometric signal, where various forms of ionized Sn and residual O were identified.

Reference:

Y. Y. Li, Y. W. Lee, T. S. Ho, R. T. Wei, P. Y. Lai, K. S. Jao, I. C. Wu, S. H. Chen, and S. L. Huang, “Interferometry-based EUV spectrometer,” IEEE Photonics Journal, 9, No. 4, 3400108, 2017.

In Situ Monitoring of Chemical Reactions by Femtosecond Acoustics

Professor Chi-Kuang Sun

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Recently, we have applied our invented “femtosecond acoustics” imaging techniques, which is a femtosecond version of the photoacoustic tomography, to "see" the interfacial and sub-interfacial chemical reaction in a solid-water interface, for the first time with a sub-atomic resolution in situ. Our work entitled “In Situ Monitoring of Chemical Reactions at a Solid−Water Interface by Femtosecond Acoustics” was recently published in The Journal of Physical Chemistry Letters, which is the number one journal in the field of Physics Chemistry, on October 23, 2017. In this study, we demonstrate that “femtosecond acoustics” is a subatomic-level-resolution technique to be able to longitudinally monitor chemical reactions at solid–water interfaces and subsurface area under atmospheric conditions, by taking the photoelectrochemical (PEC) water splitting as our example. Furthermore, whenever the oxide layer thickness equals an integer number of the effective atomic layer thickness, the measured acoustic echo will show higher signal-to noise ratios with reduced speckle noise, indicating the quantum-like behavior of this coherent-phonon-based technique. As a result, femtosecond acoustics was proved to be able to provide the long-desired tool for investigating reactions occurring at solid−liquid interfaces and to provide a non-invasive imaging tool to explore the next-generation energy conversion devices, with a sub-atomic level resolution.

Figure 1. (a) Femtosecond-acoustics-measured thicknesses of the Ga₂O₃ layer and the etched n-GaN cap layer versus PEC water splitting time. (b) The signal-to-noise ratio (SNR) of the measured acoustic echoes versus PEC water splitting time. The black line is a regression line. When the thickness of Ga₂O₃ equals an integral number of the effective atomic-layer-thickness, the measured acoustic echo will show a higher SNR, indicating the quantized behavior of our coherent-phonon-based measurement. (c) Corresponding M-mode image showing the oxidation process at the water−anode interface during the PEC water-splitting reaction.

Parameters Design and Viewing Zone Analysis of 360-degree Cylindrical Autostereoscopic 3D Display with Novel View-pixels Arrangement

Professor Hoang-Yan Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

An autostereoscopic 3D (AS3D) display allows users to perceive 3D image without wearing the special glasses. The parallax barrier is one of the several techniques to achieve AS3D display which uses a mask with periodic slits to lead the light from different pixels on the screen to different directions, forming a row of viewing zones (VZs) in the horizontal direction. When viewers’ left eyes and right eyes locate in different VZs separately, they can obtain the depth cue of binocular disparity and perceive the 3D image. Since this technique only adds a layer of periodic slits, it possesses the advantages of low weight, low cost and easy fabrication.

The parallax barrier AS3D display with flat screen can be considered as a window to observe the 3D image. The observe direction is limited by the boundary of screen, e.g. viewers can not see the back of the 3D image. To create a more realistic 3D image, we can use the cylindrical display which allows viewers to see the image in 360 degrees.

In this research, we apply the parallax barrier technique on the static cylindrical display to achieve the 360-degree AS3D display. This idea becomes more practical due to the fact that recently flexible displays and displays with extremely fine pixels can be fabricated. The parameters design rules for cylindrical AS3D display is discussed in detail. In addition, we propose a novel pixel arrangement which provides VZs surrounding the display symmetrically with equal distances and areas. In this case, the field of view (FOV) on each design-eye-point (DEP) in different direction remains the same (Fig. 1). To verify the new pixel arrangement indeed produces such VZs, we used ray-tracing method to simulate the luminance distribution and examine the quality factors, the uniformity of luminance and the crosstalk.

(a)

(b) (c)

Fig. 1. (a) Schematic diagram of the cylindrical AS3D display which is built by connecting several convex displays. While each color represents a specific view, the occupied-range (a arc on display) of view-pixels and the VZ are drawn. (b) A novel pixel arrangement for cylindrical AS3D display, and the corresponding well-arranged VZs. (c) Magnification of the small area around the point (z = R, x = 0) to show that which pixel belongs to which view and the definitions of parameters. (c) The small area around the point (z = R*cos(b), x = R*sin(b)).

Reference:

Jhih-Yong Lai and Hoang Yan Lin, Parameters Design and Viewing Zone Analysis of 360-degree Cylindrical Autostereoscopic 3D Display with Novel View-pixels Arrangement, SID 2017.

Broadband VCSEL for Data Centers and High Speed Violet LD for Light Fidelity

Professor Gong-Ru Lin’s laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Multi-Mode VCSEL Chip with High-Indium-Density InGaAs/AlGaAs Quantum-Well Pairs for Data Center Network

An 850-nm multi-mode vertical cavity surface emitting laser (VCSEL) bare chip with high-indium-density InGaAs/AlGaAs quantum-well pairs is demonstrated for directly encoded QAM-OFDM transmission in multi-mode fiber (MMF). By directly encoding the 850-nm VCSEL bare chip with a pre-leveled 14-GHz 16-QAM OFDM data, >50-Gbit/s transmission over 100-m-long OM4 MMF can be realized without using data recovery circuit. Increasing the bias current of the VCSEL beyond 7.5Ith improves the signal-to-noise ratio (SNR) and bit error ratio (BER) of received QAM-OFDM data to 15.5 dB and 2.9´10⁻³, respectively. The 100-m-long OM4 MMF transmission degrades the SNR with its covered bandwidth reducing to 13 GHz. The OFDM subcarrier pre-leveling technique with a slope of 0.2 dB/GHz ensures the 16-QAM-OFDM data transmission with an error vector magnitude of 17.1% and a BER of 3.4´10⁻³.

Fig. 1 (a) The VCSEL bare chip on a commercial probe station; (b) experimental setup of the proposed VCSEL chip based 52-Gbit/s 16-QAM OFDM transmission system over 100-m OM4 MMF.

Fig. 2 The basic characteristic of the VCSEL chip including the output power, the compliance voltage and resistance, the optical spectrum, and the modulation throughput.

Fig. 3 The transmission performance of VCSEL chip carried 14-GHz 16-QAM OFDM data at 56 Gbit/s. before and after 100-m OM4 MMF transmissions.

Violet Laser Diode Enables Lighting Communication

Violet laser diode (VLD) based white-light source with high color rendering index (CRI) for lighting communication is implemented by covering with Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) or Lu₃Al₅O_12:Ce³⁺/ CaAlSiN₃:Eu²⁺ (LuAG:Ce/CASN:Eu) phosphorous diffuser plates. After passing the beam of VLD biased at 70 mA (~2I_th) through the YAG:Ce phosphorous diffuser, a daylight with a correlated color temperature (CCT) of 5068 K and a CRI of 65 is acquired to provide a data rate of 4.4 Gbit/s. By using the VLD biased at 122 mA (~3.5I_th) to excite the LuAG:Ce/CASN:Eu phosphorous diffuser with 0.85-mm thickness, a warm white-light source with a CCT of 2700 K and a CRI of 87.9 is obtained at a cost of decreasing transmission capacity to 2.4 Gbit/s. Thinning the phosphor thickness to 0.75 mm effectively reduces the required bias current by 32 mA to achieve the same CCT for the delivered white light, which offers an enlarged CRI of 89.1 and an increased data rate of 4.4 Gbit/s. Further enlarging the bias current to 105 mA remains the white-light transmission capacity at 4.4 Gbit/s but reveals an increased CCT of 3023 K and an upgraded CRI of 91.5.

Fig. 4 The white-lighting performances of the VLD illuminating YAG:Ce and LuAG:Ce/CASN:Eu phosphorous diffusers. (a) The images of the white light generated by exciting the YAG:Ce and LuAG:Ce/CASN:Eu fluorescent phosphors with the violet laser beam. (b) The CCTs response and the CRIs performance of the VLD illuminating phosphor doped diffusers at different DC biases. (c) The operation optimization of the phosphorous diffusers divergent VLD. The BERs, constellation plots and subcarrier SNRs of 16-QAM OFDM data carried by the VLD illuminating phosphor doped diffusers at different bias currents.

Utilizing Microcavity Enhanced Radiative Transition to Realize High Efficiency Orange-to-Red TADF Organic Light emitting Devices

Professor Chung-Chih Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Since the pioneering introduction of TADF emitters, considerable progresses have been made in EQEs of blue and green TADF OLEDs. In contrast, the development of high-efficiency orange-to-red TADF OLEDs remains far behind(with EQE of only up to 17.5%), mainly associated with numerous strict and sometimes even conflicted requirements, e.g., strong radiative transition rate and high F_PLs and yet also small DE_ST, which may be even more difficult for orange-to-red emitters (or long-wavelength emitters in general) generally suffering the increasing non-radiative internal conversion process governed by the energy gap law. In our study, we show that the optical microcavity effect could provide an additional/independent approach for boosting the radiative transition rates/emission quantum yields of TADF emitters in devices, and thus strict and conflicted material design requirements for effective TADF emitters may be relaxed and even materials initially with non-perfect properties (e.g. <100% F_PLs) may be turned into highly useful for efficient devices. By embedding appropriate and strong enough optical microcavity structure in the orange-to-red TADF OLEDs, the radiative fluorescence transition rate is found to increase by a Purcell factor of up to 1.7 and increase the less than ideal PLQY (internal quantum efficiency) of an orange-to-red TADF emitter by ~20% in the device configuration. As a result, very high EQE of up to 30% was realized for orange-to-red OLEDs for the first time, making it comparable to state-of-the-art blue and green TADF OLEDs.

Fig. 1. Achieving very high external quantum efficiencies for orange-to-red TADF OLEDs.