Congratulations to Ming-Shung Tsai and Kai-Hsun Chang for being awarded “Laser Display and Lighting Conference 2023 Student Award.”

Simultaneous oscillation of dual-wavelength optical parametric oscillators on monolithic c⁽²⁾ nonlinear photonic crystals

Professor Lung-Han Peng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Multi-wavelength visible laser sources are desirable to some research regimes, for example, multi-wavelength yellow orange laser can be used in stimulated emission depletion (STED) microscopy and in the colocalization analyses. However, to produce multi-wavelength visible laser through quasi-phase matched (QPM) nonlinear photonic crystals (NPCs), multi-wavelength near infrared (NIR) laser source would become a critical issue. The studies of multi-wavelength NIR optical parametric oscillators (OPOs) of aperiodic or apodized designs had been published, but the wavelength stability or tunability of such designs were not very well. The main reason is the gain competition of multi-wavelength OPOs, it would degrade the performance of multi-OPOs. To reduce the gain competition, we proposed a parallel design of multi-OPOs, i.e., the QPM periodicities were placed in transversal way of pump beam. In this way, the gain competition would be mitigated compared with the cascaded designs. In the following, the characteristics of a dual- or double-OPOs would be demonstrated, including the slope efficiency and wavelength tunability. Moreover, we will build a yellow-orange laser via sum frequency generation (SFG) or second harmonic generation (SHG) of dual-idlers of our dual-OPOs on monolithic NPC. Our design can not only facilitate simultaneous multi-wavelength oscillation in NIR band, but also had great potentials in generation of multi-wavelength lasers source in visible or even UV bands.

Ultra-broadband wavelength-swept crystal fiber laser

Professor Sheng-Lung Huang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Ultra-broadband wavelength-swept laser (WSL) was generated using glass-clad Ti:sapphire crystal fiber as the gain media. With the low signal propagation loss of the crystal fiber, the swept laser has a tuning bandwidth of 250 nm (i.e., 683 nm to 933 nm) at a repetition rate of 1200 Hz. The steady-state and pulsed dynamics of the WSL were analyzed. The 0.018-nm instantaneous linewidth corresponds to a 3-dB coherence roll-off of 7 mm. When using the laser for swept-source optical coherence tomography, an estimated axial resolution of 1.8 μm can be achieved.

Reference:
Y. C. Lin, T. I Yang, and S. L. Huang, “Ultra-broadband wavelength-swept Ti:sapphire crystal fiber laser,” Opt. Lett. 47, No. 11, pp. 2778–2781, 2022.

Revealing the interlayer van der Waals coupling of epitaxially-grown few layers MoS₂ by terahertz coherent phonon spectroscopy

Professor Chi-Kuang Sun

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Semiconducting 2D materials such as layered-stacking MoS₂ were widely considered as a versatile platform for electronics design and for novel physics research. Due to their excellent electronic and optical properties, the direct and large area synthesis method for few-layer MoS₂ is in demand. However, numerous studies have reported that the van der Waals (vdWs) interfaces are commonly defective, and thus it is crucial to have a non-destructive detection technique to monitor and ensure the high interfacial quality. In this work, we employed picosecond ultrasonics technique to observe the actual interlayer vdWs bonding strength. Since the observed physical quantity is determined by the electronic behavior of the 2D structure, it serve as a good indicator for the performance of these devices.

We first applied THz coherent phonon spectroscopy to optically probe the vibrational modes of the epitaxially-grown bi-layer and tri-layer MoS₂. The interlayer out-of-plane vibrations beyond 1 THz were stimulated and temporally retrieved by femtosecond laser pulses (see Fig. 1(a)), revealing both Raman-active and Raman-inactive modes in one measurement (see Fig. 1 (b)). With the complete breathing modes revealed, here we extend the linear chain model by considering the elastic contact with the substrate and the vdWs coupling of next-nearest-layer to analyze the effective spring constants. We further considered the intralayer stiffness as a correction term to acquire the actual interlayer vdWs coupling. Our THz phonon spectroscopy results provide the interlayer spring constants for bi-layer and tri-layer, and a softening effect was observed when layer number decreases. The extended model further suggests that a non-negligible substrate mechanical coupling and the next-nearest-neighbor vdWs coupling have to be considered, and both of them are responsible for the correction of the large deviation when comparing with the lowest Raman active mode (see Fig. 2).

Reference:

[1] P.-J. Wang, P.-C. Tsai, Z.-S. Yang, S.-Y. Lin, C.-K. Sun, "Revealing the interlayer van der Waals coupling of bi-layer and tri-layer MoS2 using terahertz coherent phonon spectroscopy," Photoacoustics 28 (2022)

Design and Analysis of Integral Imaging based 3D Light-Field Display

Professor Hoang-Yan Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The integral-imaging based 3D light field display can restore its light-field information, reconstruct different images in different depth planes, and realize the floating projection effect. In the metaverse, AR/VR wearable devices, HUD for smart cockpits, medical care, entertainment, art performance and other scenarios, naked-eye 3D display is one of the most crucial requirements for the coming future.

Using the Computer Generated Elemental Image (CGEI) algorithm, we realize the generation of elemental image from the 2D image and its depth map, and use the ray-tracing method to simulate and analyze the reconstructed planes of different depths. Finally, naked eye 3D display and VAC free can be achieved.

Fig. 1 shows the design principle of the CGEI algorithm. Using the reversibility of light, the floating image can be restored during the display process. We input an image of ABC, and set the depth of "A" to 200 mm, the depth of "B" to 105.37 mm, and the depth of “C” to 48 mm. Using the CGEI algorithm, the elemental image is calculated as follows in Fig. 2. Considering the real display environment, we use the Monte Carlo ray tracing method of the software LightTools^TM for simulation and analysis. We set the receiver depth to 20 mm, 48 mm (“C”), 80 mm, 105.37 mm (“B”), 150 mm, 200 mm (“A”), and the results are shown in Fig 3.

Reference:
Kai-Siang Hsu, Chia-Yuan Chang, and Hoang-Yan Lin, "Design and Analysis of Integral Imaging based 3D Light-Field Display," IDW 2022.

Photonic Inter-Layer Vias (photonic ILVs)- Three-Dimensional Inter-Layer Optical Signal Transmission Realized by a Monolithically Integrated Carrier Transport Structure

Professor Ming-Hua Mao's Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We proposed and demonstrated a brand new type of monolithic photonic devices which realizes the three-dimensional (3D) all-optical switching for inter-layer signal transmission. This device is composed of a vertical Si microrod which serves as optical absorption material within a SiN waveguide in one layer and as an index modulation structure within a SiN microdisk resonator lying in the other layer. The ambipolar photo-carrier transport property in the Si microrod was studied by measuring the resonant wavelength shifts under continuous-wave laser pumping. The ambipolar diffusion length can be extracted to be 0.88 μm. Based on the ambipolar photo-carrier transport in a Si microrod through different layers, we presented a fully-integrated all-optical switching operation using this Si microrod and a SiN microdisk with a pump-probe technique through the on-chip SiN waveguides. The switching time windows for the on-resonance operation mode and the off-resonance operation mode can be extracted to be 439 ps and 87 ps, respectively. This device shows potential applications for the future all-optical computing and communication with more practical and flexible configurations in monolithic 3D photonic integrated circuits (3D-PICs). This work has been published in Optics Express: Yu‑Chien Wei and Ming‑Hua Mao*, "Three-Dimensional Inter-Layer Optical Signal Transmission Realized by a Monolithically Integrated Semiconductor-based Carrier Transport Structure," Optics Express, 31, 11820, (2023).