Ultrafast near-field imaging of an operating nanolaser using free electrons

Abstract

Integrated opto-electronic devices have the potential to revolutionize information processing, with substantial increase in computing speed, seamless information transfer and reduction of energy consumption. A key missing unit for the successful implementation of compact functional devices are nanometer scale modular and tunable light sources. Monotonically grown semiconducting nanowire lasers (NWLs) fill this gap. However, NWLs operation improvement and optimization require the characterization of their near-field and its dynamics at the nanometer scale, which is hindered due to the light diffraction limit. Here we show how synchronous electron near-field and photon far-field time-resolved spectroscopies surpass this limitation and map a NWLs near-field with nanometer and sub-picoseconds temporal resolution. We quantitatively measured the evolution of the absolute number of stimulated photons $N_0(t)$ in the NWL cavity, measuring that up to 4x10$^5$ are present simultaneously in the cavity. We mapped the lasing cavity mode's near-field, showing that both whispering gallery and Fabry-Perot modes can participate in the lasing. Our results demonstrate how the near-field of a NWL under operation evolves in the sub-picoseconds and the nanometer scales. We anticipate that a direct observation of the near-field will help to elucidate the influence of materials heterogeneities (defects, chemical changes, contaminants, interface roughness, strain) in NWL operation.

Figures (16)

• Figure 1: Synchronous PINEM and $\mu$PL setup. a) Illustration of the experiment: a 250 fs laser excites a Cold-FEG tungsten tip, resulting in a 2 MHz pulsed electron beam. The third harmonic (THG) of the same fs-laser optically excites the nanolaser above the lasing threshold (here $P_{th}$ = 0.66 mW) via a parabolic mirror placed 200 $\mu$m above the sample. The photoluminescence is collected using the same parabolic mirror to track the nanolaser's emission. A motorized delay stage controls the delay between the optical pump and the pulse electron beam (probe). A spectrometer installed with a Timepix3 detector measures the scattered electron energy spectrum. b) Annular Dark Field and Bright Field STEM image of the NWL under study, the red dot showing the position of the electron beam. c) The NWL photoluminescence spectra, displaying the predicted peaks above the lasing threshold. d) The electron's energy spectrum after interaction at three distinct delays between the pump and the probe.
• Figure 2: PINEM of the stimulated near-field of a NWL. a), b) Two experimental datasets collected at a fixed position close to the nanolaser for pumping power levels below (P=0.25 mW) and above (P=0.66 mW) the lasing threshold. An electron energy spectrum is recorded every 33 fs delay between the laser pump and the electron probe. The data is normalized using the total intensity for each measurement. The noise visible only in the loss part of the spectrum is consistent with spontaneous electron losses standard in electron energy loss experiment. c) Simulation of b) taking with an onset time of 0.75 ps and a decay time of 0.76 ps for the laser mode and an electron chirp of 0.89 ps/eV.
• Figure 3: Number of photons in the cavity in the lasing regime a) The electron-near field coupling constant $g_{scatt}$ as a function of pump power at a fixed position. b) Evolution of the number of photons in the cavity, $N_0(t)$ deduced from $g_{las}(t)$, in the lasing regime as a function of the pump power. c) EELS spectrum acquired on the same position as the one where the delay scan of Figure \ref{['RawData']} have been acquired (same NWL). d) NWL emission spectrum taken simultaneously than the delay scan for the different pump-powers presented in a-b. The peak corresponding to the lasing mode (E = 3.3 eV) is fitted after removal of the background to extract $g_0$: the square root of the red area.
• Figure 4: Lasing mode spatial profile. a)Electron energy loss map (without laser pumping) at 200 keV using a monochromated Hermes Nion dedicated STEM on a typical NWL. Spectrum at the top and on the side of the NWL. We see also the fitted map at 3.3 eV b) PINEM on NWL1 : Map of coupling constant (g) along the nanolaser at 0 ps delay, with an excitation power of P = 0.34 mW and at t = 3.6 ps at P = 0.31 mW (just above the lasing threshold). The near-field of the lasing mode is here localized on the side of the NWL showing the expected behavior of a whispering gallery mode (WGM). c) PINEM on NWL2 : at t= 0 ps (P = 0.94 mW ) and at t = 2.1 ps ($P_{th}<P = 0.88$ mW and $P_{th}<P = 1.2$ mW). The electron coupling constant to the lasing mode (g) (t = 2.1 ps) is delocalized along the NWL showing the expected behavior of a Fabry-Perot mode. We can see that the neighboring NWL3 is not lasing. A zoom of the top of NWL2 shows the two expected lobe for a lasing FPM.
• Figure 5: (a)-(e) Process flow diagram for nanowire fabrication: (a) growth by metal-organic vapor phase epitaxy on a sapphire substrate; (b) resist spin-coating and photolithography to create 1 $\mu m$ diameter holes with a 10 $\mu m$ pitch; (c) metallization and lift-off to form a Ni hard mask; (d) dry-etching using Cl2-based ICP; (e) Ni removal with piranha solution followed by KOH wet-etching to smooth the sidewalls and further reduce wire diameters. (f) 30°-tilted SEM images of nanowires standing on the sapphire substrate