'Don't be afraid to step outside of your academic background'

When Pramit Ghosh left Bangladesh for Pennsylvania State University in 2022 to begin his doctoral studies in mechanical engineering, he had no plans of making headlines. However, within just three years, he now finds himself at the centre of a scientific development that challenges a 165-year-old physical law and places him among the co-authors of one of the most significant experimental breakthroughs in thermal radiation research.

Pramit, an alumnus of the Bangladesh University of Engineering and Technology (BUET), is currently a graduate research assistant at Penn State. His work forms part of a research team that has demonstrated the strongest experimental violation to date of Kirchhoff's law of thermal radiation. This law, introduced in 1860 by German physicist Gustav Kirchhoff, states that an object's ability to absorb thermal radiation must match its ability to emit it at the same wavelength and angle. It's a principle long treated as a cornerstone in thermodynamics.

Previous attempts at breaking the law have shown only marginal deviations, with small-scale effects recorded in highly controlled settings. But this Penn State group, under the supervision of Assistant Professor Linxiao Zhu, has experimentally achieved nonreciprocal thermal emission on an unprecedented scale. Their findings have already been published in Physical Review Letters and flagged as an "Editor's Suggestion", indicating the paper's novelty and potential impact.

"Our experiment presents a strong nonreciprocity of approximately 0.43 over a broad spectral range exceeding 10 micrometres," said Pramit Ghosh, stating that this is the highest reported figure so far - both in magnitude and bandwidth. "This strongest nonreciprocity over the broadest band and angles till date marks a significant breakthrough in the field that can push the boundaries of thermal radiation control."

The experiment involved a material system composed of five ultra-thin layers of semiconductors with slight compositional variation which is a structure known as a gradient-doped epsilon-near-zero material. The material, primarily made from indium gallium arsenide (InGaAs), is thinner than a strand of human hair but shows dramatically different behaviours depending on the direction of heat flow and emission. This directional dependence is what breaks the conventional rule of thermal reciprocity.

To validate the results, the team built a highly specialised measurement platform: a custom-designed angle-resolved magnetic thermal emission spectrophotometer. It took them over seven months to develop this system, which was capable of measuring emitted thermal radiation under varying angles and magnetic fields with the level of precision required to detect and confirm the violation.

Mr Ghosh emphasises the scale and complexity involved. "Designing such an intricate and highly precise experimental setup is exceptionally challenging," he said. "The idea and design had already been conceptualised by my supervisor Mr Linxiao Zhu even before I joined the group. Once the material and instrumentation were ready, the actual measurement phase progressed relatively quickly but getting to that point took years."

Linxiao Zhu led-team includes doctoral students Zhenong Zhang, Alireza Kalantari Dehaghi, and Pramit Ghosh himself - all now in their third or fourth years of PhD studies at Penn State. Their supervisor, Linxiao Zhu, earned his PhD from Stanford University and completed postdoctoral work at the University of Michigan, Ann Arbor.

Beyond the theoretical accomplishment, the implications of this research extend across a range of technologies. "Breaking thermal reciprocity opens a promising pathway toward reaching the thermodynamic limits of various energy systems," said Pramit Ghosh. Applications could include high-efficiency solar cells, thermophotovoltaic systems, radiative energy harvesting, and new techniques in heat flux control and thermal communication.

He pointed to existing theoretical studies suggesting that a nonreciprocal solar cell - one that redirects unused photons back into productive conversion - could reach efficiency levels as high as 93.3 per cent, close to the Landsberg limit, the theoretical maximum for any solar energy system.

While the research is still at a fundamental stage, its architecture allows for integration into real devices. The emitter can be transferred to other surfaces, making it viable for adaptation in commercial-scale systems, provided further research confirms its reliability and efficiency over time.

Mr Ghosh, who also completed his master's degree during the course of his PhD programme, plans to continue working in the domain of thermal photonics and heat transfer whether in academic or industrial settings. He remains focused on both theoretical and experimental approaches, noting that the next phase of their research will look deeper into the mechanisms of thermal nonreciprocity and how they might be enhanced or tailored for specific use cases.

His advice to students in Bangladesh pursuing advanced research in STEM is both pragmatic and reflective. "Securing funding should be your top priority when applying for a PhD position," he said. "But if you have multiple offers, don't choose just based on university rankings or reputation. Pay attention to the lab culture, the mentorship style, and speak with current members. These matter more in the long run."

Pramit also advises students not to be restricted by their academic background. "Although I come from mechanical engineering, most of my current work is rooted in applied physics," he said. "Interdisciplinary work can open new directions. If you're aiming for cutting-edge science, study your potential supervisor's research profile carefully. Try to join a group that is either leading in the field or trained by someone who is."

He added that research success is shaped by many variables and not all of them visible on paper. "Stay open-minded. Find an environment where you feel supported and inspired. That's what makes the difference."

For Pramit Ghosh, stepping outside the formal bounds of his academic discipline has not just led to the publication of a landmark study and it has placed him in the middle of a scientific advance that may change how we think about heat, light, and energy itself.

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