May 2025 Vol. 23 No. 5

Front-line Researchers

• Hiroshi Sawada, Senior Distinguished Researcher, NTT Communication Science Laboratories

  

  Abstract
  Optimization problems involve expressing the problem to be solved as a mathematical formula and finding the best solution from all feasible solutions. They are used to solve a wide range of problems, such as creating shift schedules that even out work frequencies and times of employees and determining the optimum production volume of a product to maximize profits, and there are a variety of approaches to solve them. As artificial intelligence (AI) becomes more prevalent, more opportunities to use AI to solve optimization problems will arise. Optimization problems are also applied to machine learning in AI. Hiroshi Sawada, a senior distinguished researcher at NTT Communication Science Laboratories, has been achieving new results by incorporating a long-standing optimization method into his research on two themes: audio source separation and a method for training hardware-oriented neural networks. We spoke with him about his research approach and results concerning these themes as well as the importance of building connections with other fields on the basis of one’s technical expertise.

Rising Researchers

• Yu Sasaki, Distinguished Researcher, NTT Social Informatics Laboratories

  

  Abstract
  We are now in an age where the Internet is not only used for communication between people, but also for connecting various devices. Information circulating on the Internet is diverse. Not only personal and confidential information, but also information emitted by devices is at increased risk of being intercepted and misused for criminal purposes. The field of symmetric-key cryptography prevents third parties from eavesdropping on communications. We spoke with Distinguished Researcher Yu Sasaki, who is researching lightweight cryptography for communication between IoT (Internet of Things) devices.

Feature Articles: The Frontiers of Ultrafast Optical Physics Research

• Prospects for the Frontiers of Ultrafast Optical Physics Research: Engineering Petahertz Waves

  

  Abstract
  The Nobel Prize in Physics in 2023 was awarded for the generation and measurement techniques of an attosecond (an attosecond is 1 × 10⁻¹⁸ second) pulse, which has a temporal duration in the attosecond range in wavelength typically from extreme ultraviolet to soft x-ray. The duration of an attosecond pulse is shorter than the period of typical light waves, and on this timescale, light can be reinterpreted as an electric field oscillating at sub-petahertz (PHz = 10¹⁵ Hz) frequencies. This article provides an overview of attosecond pulse generation techniques that have paved the way for the era of “attosecond science” and introduces the attosecond pulse laser technologies advanced by NTT Basic Research Laboratories, as well as the latest research and prospects of ultrafast optical physics focusing on petahertz electric field-electron interaction.

• Megahertz-repetition-rate 1.7-cycle Intense Pulse Sources for Future Bright Attosecond Spectroscopy

  

  Abstract
  Attosecond pulses are the shortest optical pulses available today and are a crucial technology for unveiling extremely ultrafast phenomena. However, the photon flux of attosecond pulses presents a significant limitation for various spectroscopic applications. In this article, we introduce the generation of attosecond driver sources for next-generation, unprecedentedly brighter attosecond spectroscopies.

• Topological High-harmonic Generation from Solids

  

  Abstract
  Topological light exhibits unique geometric spatial structures in the beam cross-section and is promising for various applications including optical communications, spectroscopy, laser processing, and optical tweezers, particularly in the infrared to visible range. Generating such exotic light through high-harmonic generation (HHG) enables extreme ultraviolet wavelengths and attosecond time precision, paving the way for next-generation ultrafast optical measurements. This article discusses the physical mechanisms underlying the generation of topological light via HHG and presents the first experimental demonstration of the fundamental physical principles behind this phenomenon.

• Attosecond Time-resolved Spectroscopy for Light-electron Interaction in Solid-state Materials

  

  Abstract
  Pump-and-probe attosecond time-resolved spectroscopy based on an isolated attosecond pulse source is the fastest measurement technique available to humans, which enables us to capture unprecedented ultrafast phenomena with a temporal resolution on the attosecond timescale. NTT Basic Research Laboratories has developed the most basic attosecond spectroscopic technique consisting of an isolated attosecond pulse source and time-resolved near-infrared-pump attosecond-probe absorption spectroscopy. In this article, an application of this technique to real-time measurement of electronic responses induced by lightwave fields in wide-gap semiconductors is presented.

• Exploring Ultrafast Electron Dynamics and Extreme Nonlinear Optics Induced by Intense Optical Field

  

  Abstract
  This article explores the physical limits of light-matter interactions, with a particular focus on ultrafast electron dynamics induced by high-intensity optical fields. In the presence of high-intensity light, we evaluated the transition rate of electrons due to the tunneling effect when a large-amplitude electric field is applied to semiconductors and insulators using quantum dynamics simulations. By using a theoretical model that includes electron-hole interactions, we demonstrated that electron interactions enhance this tunneling-transition rate and that this enhancement increases with the applied electric field.

Regular Articles

• Proposal of a Transdermal Iontophoresis Patch and Sheet Mask Using the Principle of Magnesium-air Batteries

  

  Abstract
  To develop an iontophoresis technology that does not require a large power supply, this study focused on magnesium-air batteries, which use atmospheric oxygen and magnesium, a metal with a low incidence of allergic reactions. We thus propose a transdermal iontophoresis patch and sheet mask using the principle of magnesium-air batteries. To verify the feasibility of the transdermal iontophoresis patch and sheet mask, we conducted an evaluation using time-of-flight secondary ion mass spectrometry. Ion mapping of the active ingredient and analysis of its line profile revealed that the use of our iontophoresis patches enhanced active-ingredient penetration by approximately twice that of patches without iontophoresis. These findings indicate that the patch and sheet mask technologies contribute to the development of a highly portable and user-friendly transdermal iontophoresis patch and sheet mask that can be easily used at home.

Global Standardization Activities

• Discussions on the Use Conditions of Wireless LANs in ITU-R and APT

  

  Abstract
  The amount of data communication using wireless local area networks (LANs) in smartphones, personal computers, audio/visual equipment, household appliances, and other devices has been increasing rapidly, and demand for expansion of the frequency bands available for wireless LANs to achieve higher speeds and higher quality is growing. In response to this situation, the International Telecommunication Union - Radiocommunication Sector (ITU-R), an international standardization organization, and the Asia-Pacific Telecommunity (APT), which consists of countries in the Asia-Pacific region, are discussing the regulatory aspects such as the use conditions of wireless LANs. Because these discussions are important to NTT’s business dealing with wireless LANs, NTT continuously participates in them as the Japanese delegation in charge of wireless LANs at each meeting. This article describes the status of recent discussions on wireless LANs at ITU-R and APT and NTT’s efforts.

Practical Field Information about Telecommunication Technologies

• OTDR Waveform Evaluation Tool Enabling Automatic Identification of Location and Cause of Faults

  

  Abstract
  An optical time-domain reflectometer (OTDR) is commonly used to check the condition of optical fiber cables. However, identifying the location and cause of faults in optical fiber cables using an OTDR requires advanced skills and a substantial workload to analyze the obtained waveform. Technical Assistance and Support Center, NTT EAST developed an OTDR waveform evaluation tool to automatically identify abnormal sections in optical fiber cables from waveforms obtained with an OTDR. This article presents an overview of this tool.

External Awards/Papers Published in Technical Journals and Conference Proceedings

• External Awards/Papers Published in Technical Journals and Conference Proceedings