The 2nd RAP Edge Photonics Seminar

Talk 1 

Title: Ultrafast optoelectronics in van der Waals heterostructures enabled by THz electronics

Speaker: Dr. Katsumasa Yoshioka (Basic Research Laboratories, NTT, Inc., Associate Distinguished Scientist)

Abstract:

Accessing ultrafast electrical and optoelectronic processes in van der Waals heterostructures requires platforms beyond conventional gigahertz electronics. We have developed on-chip terahertz (THz) electronics based on laser-triggered photoconductive switches, enabling sub-picosecond generation and detection of ultrashort electrical signals [1–5]. In this talk, I will present two applications of this platform. First, we demonstrate on-chip manipulation of THz electrical signals using graphene plasmons [2]. Ultrashort plasmonic wavepackets with durations of ~1 ps are generated and propagated, with velocity and dispersion controlled by carrier density. Second, we use THz electronics to directly resolve ultrafast optical-to-electrical conversion in graphene, black phosphorus, and WTe₂. A 220-GHz-bandwidth graphene photodetector reveals sub-picosecond hot-carrier extraction [3], while black phosphorus shows super-diffusive hot-carrier transport [4]. In WTe₂, ultrafast edge photocurrents from broken crystal symmetry are resolved, enabling competing generation mechanisms to be distinguished [5]. These results establish THz electronics as a platform for manipulating THz signals and directly probing photocurrent generation, transport, and extraction in van der Waals devices.

[1] Appl. Phys. Lett. 117, 161103 (2020).

[2] Nat. Electron. 7, 537 (2024).

[3] Nat. Photon. 16, 718 (2022).

[4] Nat. Commun., accepted.

[5] Nano Lett. 26, 96 (2026).

Talk 2

Title: Attosecond electron beams for visualizing electronic motion

Speaker: Dr. Yuya Morimoto (PRI & RAP, RIKEN, RIKEN-Hakubi team leader)

Abstract:

The advent of attosecond lasers has enabled the real-time observation of electron motion within matter. However, spatial resolution remains a significant challenge due to wavelength limitations. In this talk, I will discuss our recent progress toward achieving simultaneous attosecond and Angstrom-scale imaging using electron beams. First, I will describe the methodologies developed for the generation and characterization of attosecond electron pulses [1-3]. Second, I will present recent experimental results on the attosecond electron diffraction of single-crystalline membranes [4] and a theory for describing the scattering processes of attosecond electron beams [5]. Finally, I will introduce the newly developed attosecond electron beamlines at RIKEN and discuss the future perspectives of time-resolved imaging utilizing ultrashort electron beams.

[1] Y. Morimoto and P. Baum, Nat. Phys. 14, 252–256 (2018).

[2] Y. Morimoto and P. Baum, Phys. Rev. Lett. 125, 193202 (2020).

[3] Y. Morimoto, Microscopy 72, 2–17 (2023).

[4] Y. Morimoto and P. Baum, Phys. Rev. Lett. 132, 216902 (2024).

[5] Y. Morimoto and L. B. Madsen, New J. Phys. 26, 053012 (2024).