Fig. 1: Experimental details and spectral-temporal response of HPhP modes in hBN.
a, Illustration of the proposed mechanism and experimental measurement. A pump pulse (520 nm) heats a gold pad, while a subpicosecond tunable mid-IR probe pulse measures the modulated reflectivity response of the hBN patterned flake. After pulse absorption in the Au, both phonons and ballistic electrons spread from the hot spot in the Au, depicted by the small blue particles and the background waves emanating from the hot spot. Radiation from the hot electrons (red arrows) escapes and couples into the HPhP modes of hBN (pump and probe spot sizes not to scale). b, The sample geometry. The reliefs in the image show the position of Au excitation pads used for both s-SNOM characterization (Methods), as well as thermal HPhP launching. c, The measured thermoreflectance signal of the 116-nm hBN flake as a function of probe energy and pump–probe delay time for an incident pump fluence of 95.5 J m−2. The strong ΔR/R response within the Reststrahlen band (indicated by the span of the dotted lines) and near the TO phonon frequency of hBN shows the high thermal activity within the region that can be attributed to ultrafast heating from near-field radiation emitted by the Au pad. d, For reference, a similar pump fluence of an uncoated (no Au) hBN flake is provided, noting that in this case, no temporal thermoreflectance response is observed within the range of the hBN Reststrahlen band, illustrating the critical role of the Au pad as a thermal transducer in this experiment. The dark band that appears in the middle of the blank hBN contour is attributed to polymethyl methacrylate residue, a photo-resistive polymer in the lithographic patterning process. e, Waterfall plots of the data shown in c at a variety of pump–probe time delays (80–2,030 ps) following transient Au heating, indicating more clearly the ultrafast optical response surrounding the TO phonon mode and within the hBN Reststrahlen band (indicated by the span of the dotted lines).
