Session: Research Posters

Paper Number: 120226

120226 - On-Chip Measurement of Near Field Heat Transfer Between Sub-Wavelength Structures 

Planck’s law is no longer valid to model thermal radiation if the thermal characteristic wavelength is not much less than the separation gap between objects or the dimension of the heat-exchanging surfaces. There have been many studies measuring the near field thermal radiation, i.e., sub-wavelength gap distance, which indicates up to four orders of enhancement towards blackbody limit. Besides, far field radiation with sub-wavelength surfaces could also exceed blackbody limit. Recently, hundred-folds of enhancement towards blackbody limit is demonstrated between two sub-wavelength membranes. However, despite that many experiments have been done to measure near-field thermal radiation between large surfaces and far-field super-Planckian thermal radiation between sub-wavelength surfaces, the cutting-edge area, near-field thermal radiation between sub-wavelength surfaces is still less explored.

 

In this study, we measure the near field thermal radiation between co-planar silicon-nitride sub-wavelength membranes using custom-built on-chip devices. Besides enhancement towards blackbody, we further consider the effect of sub-wavelength dimensions in near-field heat transfer, by comparing our experiments with analytical calculations derived for two semi-infinite plates neglecting geometrical constraints of the surfaces.

 

The heat exchanging area of the coplanar membranes is 300-nm thick and 7-mm wide. The gap distance ranges from ~150nm to ~725nm. The silicon nitride membranes are suspended by removing the silicon substrate using potassium hydroxide (KOH) wet etching. To maintain good coplanarity between the two membranes, the width of the suspended region is confined to suppress the deflection of the membranes. Platinum serpentine heater is fabricated on the emitting membranes. Correspondingly, platinum thermometry sensor is fabricated on the absorbing membrane side to measure the temperature increase due to near field radiation. The sensor structure is specifically designed for easy calibration so that the absorbed heat flow can be directly calculated from the measured temperature variation. To validate thermal measurements, fluctuational-electromagnetic simulation is implemented between the membranes using fluctuational surface current (FSC) method.

 

We realize 20-fold enhancement towards blackbody at 30K temperature bias and ~150nm gap distance. With the small exchange area of the sub-wavelength structure, the measured maximum heat conductance of radiation is ~0.26 nW/K, which is smaller than the reported near field thermal measurements between parallel plates. Fluctuational-electromagnetic simulation and near field thermal measurements match with each other. Besides that, our calculation suggests that the near-field thermal radiation based on semi-infinite model is greater than that between sub-wavelength surfaces, but the difference becomes less for shorter separation gaps. The ratio between experiments and semi-infinite model increases from ~0.3 at ~725nm gap distance to ~0.9 at ~150nm gap distance. Such less heat transfer is attributed to less contributing polariton modes of large wavevectors and energy escaping from the cavity. We anticipate this study would inspire more studies in the cutting-edge area of sub-wavelength gaps and dimensions. With sub-wavelength structure widely available among nanofabricated devices, this work would also pave the way for developing on-chip near-field devices for thermal management and energy systems.

Presenting Author: Xiao Luo Carnegie Mellon University

Presenting Author Biography: PhD student at Carnegie Mellon University since 2020. My research interests lie in the energy transport between nanostructures. Specifically, I experimentally studied the near field thermal radiation between sub‑wavelength structures by designing and fabricating suspended micro device. I also developed organic polymer thin film and nanofiber for enhanced thermal conductivity and active control of heat transfer.

Authors:

Xiao Luo Carnegie Mellon University
Hakan Salihoglu Carnegie Mellon University
Zexiao Wang Carnegie Mellon University
Zhuo Li Carnegie Mellon University
Hyeonggyun Kim Carnegie Mellon University
Jiayu Li Carnegie Mellon University
Bowen Yu Carnegie Mellon University
Shen Du Carnegie Mellon University
Sheng Shen Carnegie Mellon University