Spatial and Frequency-Domain Multiplexing Approaches to Thermal Property Mapping in 2D and 3D

Thermal property mapping in 2D and 3D has diverse applications, from identifying buried voids to property screening for combinatorial materials development. Here I will describe two efforts in this regard, which are respectively based on the superposition of multiple heat forcings in space (i.e., multiple heating pixels in x,y) and in frequency (multiple heating frequencies, omega). First, we are developing the next generation of an all-optical “structured illumination, thermal imaging” (SITI) method [Q. Zheng et al., Appl. Phys. Rev. 9, 021411 (2022)] for spatially-multiplexed thermal measurements, here with simpler hardware, a larger field of view, and new algorithms for extracting the property map. Second, the thermal property variation as a function of depth into a sample (z direction) can be probed by varying the frequency of a periodic surface heat source and thus its thermal penetration depth. While this can be done by sequentially sweeping the frequency of a pure sinusoidal excitation, a more information-dense approach is to simultaneously apply multiple sinusoids of several different frequencies and study the superposed system response. To demonstrate the latter scheme we developed an electrothermal “3-omega” apparatus using a fine wire suspended horizontally in a shallow oil bath, which measured the depth of the oil with accuracy around ±18 microns [W. Hodges et al., Rev. Sci. Instrum. 90, 094903 (2019)]. A common theme across both studies is the need to condition the property fitting by thoughtfully incorporating prior knowledge about the allowed variabilities of the unknown thermal property field.