Meet Keshav Dani, an experimental physicist at the Okinawa Institute of Science and Technology (OIST) in Japan. Delve into his quest to uncover the mysteries of dark excitons – elusive particles that have evaded direct observation for decades. Discover how his innovative approach shattered the barriers to imaging dark excitons, opening new avenues for quantum science and technology.Explore his unique journey from India to the US to Japan, and learn how his work holds the potential to revolutionize quantum technologies through the manipulation of previously invisible particles.

Which wall does your research break?

About a hundred years ago, in the 1930s, two well-known physicists – Frenkel and Wannier, explained some emerging mysteries in semiconductor physics by postulating the existence of a particle called the exciton. It was known at the time that when a semiconductor absorbs light, negatively charged electrons and positively charged holes were created. Frenkel and Wannier explained that these oppositely charged particles could attract each other, creating a bound, two-particle state – the exciton. Over the decades, excitons have been vigorously studied in a variety of materials, and have proved to be essential to our understanding of how semiconductors respond to light, and how opto-electronic devices operate, such as solar cells and LEDs.
Yet, in all these years, it has also always been known that our experiments, which were based on optics, couldn’t directly access a certain class of excitons – the dark excitons. These are excitons that remained invisible to light due to basic conservation rules, like momentum-conservation. We knew they were there – through their indirect influence on the measurable bright excitons. But techniques or tools to probe the dark excitons directly didn’t exist. It was also speculated that their ‘darkness’ could afford them some protection from light, and hence make them suitable for certain quantum technology applications.
In 2020, we broke the wall to directly imaging the dark excitons. Since it’s their momentum coordinate that prevented them from being imaged by light, we developed a new experimental platform that allowed us to image them in momentum space itself! We saw, that in certain situations, they could vastly outnumber the bright excitons. The ones that we have been ‘seeing’ all these years, were just a fraction of what was going on behind the scenes.
It is my hope now, that with the ability to directly image the dark excitons, one could explore the various quantum mechanical properties that they could be imbibed with, and how one could pursue novel quantum science and technology with these previously invisible particles.

What inspired or motivated you to work on your current research or project?

I think it is the confluence of a few aspects of my personality and my experiences that attracted me to this particular problem.
As an experimentalist, I love the idea and beauty of visualising a phenomenon. Of being able to just directly see something, rather than having some indirect, less-than-clear evidence of the phenomena. Figuring out how to visualise something, especially if it has been ‘unseeable’ before – that excites me. So, you can see how the idea of imaging the hitherto invisible ‘dark excitons’ captivated my imagination.
Then, there’s the challenge. Taking on an ambitious challenge that has remained beyond human reach – that’s motivating. Arriving at OIST in 2011, I was lucky to be in a unique academic environment, where young researchers were encouraged, and provided with resources, to pursue creative, ambitious solutions to challenging scientific problems. Imaging dark excitons in momentum space was just such a hard problem – an experimental aspiration for a few generations. And finally – my PhD advisor, Daniel Chemla. He was a very well known and respected physicist – the director of two divisions at the Lawrence Berkeley National Lab, and one of the world’s foremost authorities on excitons in quantum confined structures. I remember a plaque from his colleagues at Bell Labs that showed him as a cartoon figure marching and hungrily calling out for excitons. Daniel and I ended up having a very special relationship – he had a stroke very early in my PhD that left him unable to talk, eat and drink. His health was in poor condition, but yet he was also holding on to life – maybe hoping to recover, maybe hoping to buy just enough time so that his students, such as me, could finish their work in the group, and find their next foothold in life. During my PhD, I would go over to his house – in the evenings, or sometimes on Sunday mornings, to get whatever guidance I could. We communicated through deliberately and slowly scribbled notes, or through hand gestures, and that’s where I learned about excitons from him. Those discussions were very special for both of us. They brought us both peace and joy outside of our existing struggles – his with his recovery, and mine with the unusual circumstances of my PhD. In time, and with Daniel’s support, I was able to complete my PhD. However, his health never quite recovered, and in 2008 he passed away. I miss him very much, and I think in making these discoveries about excitons, I get back a brief glimpse of those occasional Sunday mornings with him, where I tell him what I have newly learned about excitons!

In what ways does society benefit from your research?

Currently, there is a strong and renewed interest in understanding how quantum science could lead to new technologies that benefit society. While a number of potential applications have already been conceived, and numerous others are soon to be discovered, harnessing the full power of quantum science remains non-trivial due to a process called decoherence, which erodes the delicate nature of the quantum state. Dark excitons, due to their invisibility from light, could potentially be protected from one of the main culprits of decoherence – interaction with the blackbody radiation that pervades our universe. Thus, Dark excitons are a plausible platform for next generation quantum technologies.
Nonetheless, all these years, the very thing that protects them from decoherence, also makes them very hard to probe – their invisibility to light, since most experiments probing excitons relied on light. Now, our new techniques that directly enable us to image dark excitons in momentum space, could be used to see if it’s possible to manipulate and utilize dark excitons for quantum technology applications.

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