Toyohashi University of Technology: Dr. Hiroto Sekiguchi

During my doctoral studies, I researched green and red LEDs using nitride semiconductor nanocolumn crystals under the guidance of Professor Katsumi Kishino, who is currently a guest professor at Sophia University. There was a demand for high-efficiency green and red LEDs using nitride semiconductors, but challenges arose due to crystal defects and internal electric fields at longer wavelengths. Our laboratory focused on nanocrystals, known as nanocolumns, which showed potential for fewer defects in crystals and suppression of internal electric fields. This led me to focus on fabricating light-emitting devices using these nanocrystals. One of my major achievements as a doctoral student was developing a technology to control the position and shape of nanocrystals, integrating RGB primary colors on the same substrate.�

While some time has passed since then, I am pleased that many researchers worldwide have referenced my papers. However, as an independent researcher, I feel the responsibility to establish valuable research themes. Although my research primarily focused on crystal growth, Professor Kishino encouraged me to consider the devices I wanted to fabricate and their practical applications. This guidance continues to influence my research approach, emphasizing the fabrication of usable devices with clear future goals.

After joining Toyohashi University of Technology in 2010, I conducted research under Professor Akihiro Wakahara (currently Director and Vice President of Toyohashi University of Technology) to develop a red LED by incorporating the rare earth element europium (Eu) into a nitride semiconductor. Additionally, gas lines and electrical equipment for Molecular Beam Epitaxy (MBE) were upgraded from those used for other material systems to facilitate crystal growth of GaN-based materials. Through the use of magnesium (Mg), we discovered that Eu in GaN emits strong luminescence, showing its potential for LED applications. Furthermore, we observed that incorporating Eu into nanocrystals revealed the potential to create devices with a higher luminescence intensity at elevated concentrations compared to thin film.

In my mid-thirties, I explored various avenues to broaden my research horizons. This included fabricating microLED arrays with a diameter of 30 µm by processing GaN-based LED wafers. Additionally, I contributed to the realization of monolithic integration with Si transistors through wafer bonding, exploring the fusion of polymer waveguides and LED arrays, and investigated integrated circuits using GaN-based transistors. Looking back, it was a time of searching for my own research theme and grappling with various challenges.

Since my student days, I have been fascinated by light, which is intangible but visible, and I have wanted to pioneer a new field of devices using visible light. During this time of seeking new possibilities as a researcher, I had a significant encounter that shaped my current research direction.

Around 2017, at my daughter’s kindergarten event, I discovered that a classmate’s father was a professor in the pharmacy department of another university. Our interaction was brief, but about six months later, I received an unexpected email from that professor asking if it would be possible to create LED devices for use in neuroscience research. Under normal circumstances, I might have declined such a request. However, I was actively exploring a different theme using visible light at that time, and I thought that we might leverage our university’s facilities for this purpose. This marked a significant turning point in my exploration of a new research field. Given the vast difference from my previous research, I believe I wouldn’t have pursued it if the timing had been slightly different, maybe a year or two earlier or later. It was a timely pursuit as I searched for a new research theme. This interaction became a significant catalyst for my exploration of new research fields.

My current research centers on developing an implantable biological tool to understand brain function mechanisms. Neural activity has historically been managed through medication or electrical stimulation. However, medication necessitates removing the previous dosage before administering the next. Additionally, electrical stimulation can inadvertently affect unintended areas. In contrast, light offers precise control over specific locations, enabling the study of the direct relationship between stimulation sites and neural activity. Optogenetic methods, prevalent in life science, provide this light-controlled neural activity regulation, thus creating a demand for devices capable of illuminating neurons at multiple points. We are actively working on developing microLED devices tailored for such applications.

Our research encompasses needle-like structures for deep tissue insertion, flexible microLED films for widespread neuron illumination, and evaluation techniques for ensuring device durability and effective temperature management during operation. Furthermore, I am actively involved in the development of electrocorticogram measurement devices, embedding them successfully in mice brains to acquire multisensory information simultaneously. The overarching goal is to integrate these devices into multifunctional tools capable of both manipulating and measuring neural activities accurately.