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(Color online) (a) Emission spectrum at RT of a laser device used for this work. The resonator length is . The spectrum was acquired at a resolution of . The laser was operated with pulses at repetition rate. The Fabry-Pérot modes are spaced of . Panel (b): SEM image of a typical device. Panel (c): Schematic of the a-SNOM setup (details in Ref. 14). The semitransparent gray rectangle corresponds to the typical area size scanned during a measurement run.
(Color online) Main panel: Light-current characteristics of a typical device. Red dots: Far-field emission from the laser facet. The laser was operated with pulses at repetition rate. Black dots: Near-field measurement (demodulated at the SNOM tip frequency .) The laser was operated with pulses at repetition rate. Note that the near- and far-field signals appear at the same injection current. Panel (a): Atomic force microscopy view of the laser ridge. The scanning range is , and it is identical for panels (a)–(d). The two lateral bright regions correspond to the lateral contact bands on top of the laser resonator, while the central, slightly darker region corresponds to the exposed semiconductor surface. The latter zone is where the evanescent electric field is expected to appear. Panel (b): Near-field measurement below threshold: only the thermal emission is present. Panel (c): Near-field measurement at threshold: the standing wave starts to emerge. Panel (d): Near-field measurement well above threshold. Panel (e): High resolution three-dimensional SNOM image of the standing wave. The same periodical structure is observed when demodulating the near-field signal at and . The measured field at the device surface is thus purely evanescent, which results in the absence of background contribution from scattering centers located along the tip shaft far from the apex. For the same reason, the standing wave at the exposed surface of the ridge is also clearly observed in images obtained by demodulating the optical signal at the laser-pulse repetition frequency , while the tip scans the surface.
(Color online) (a) Schematic view of the laser Fabry-Pérot resonator and its standing wave. A 2D simulation of the TM air-guided mode at is superimposed on the facet (the component is plotted). Note that only TM optical modes are expected, since the dipole of the ISB transitions is directed along . This is in contrast with diode lasers that typically operate on transverse-electric modes. (b) One-dimensional simulation of the laser mode, corresponding to a cross section marked by the red dashed line in panel (a). The a-SNOM detects the evanescent electric field circled in red.
(Color online) (a) a-SNOM signal as a function of the tip-sample distance. Black squares: SNOM signal demodulated at the tip frequency. Green triangles: SNOM signal demodulated at the laser repetition rate . Red circles: 1D numerical simulation ( is plotted). (b) 2D simulation of the electric field intensity ( is plotted) across the plane above the device surface. (c) Experimental near-field intensity measured by scanning the a-SNOM tip on the plane above the device surface. The agreement with the numerical simulation [panel (b)] is excellent.
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