Development of complex integrated nanophotonic devices requires knowledge of the composition of optical modes within component nanostructures over a range of potential wavelengths. Within integrated devices, nanoscale waveguides will play an important role in transporting energy and information between various components. We are investigating waveguide modes inside gallium nitride (GaN) semiconductor nanowires (NWs) with triangular cross sections. We are specifically interrogating tapered NWs where the cross section decreases from one end of the NW to the other (Fig. 5b).
When laser light is focused onto the NW, the induced broadband emission can propagate along the NW axis in multiple longitudinal and transverse modes. Information about these modes is encoded in spectral characteristics of the light emerging from the NW and collected in the far field with an objective lens. To detect these spectral signatures, we developed a custom confocal microscope setup where a scanning mirror is inserted at a Fourier plane in the collection path (Fig. 5c), which allows the detection volume to be separated from the position of the laser focus. In this way, emission from different points along the NW can be directed to a fiber-coupled spectrometer for analysis. Scanning the mirror produces an off-axis confocal hyperspectral image where each 2D pixel contains spectral information for the emission collected from a particular position along the NW.
Using this technique, we use focused laser light (401-nm wavelength pulsed laser) to induce broadband emission from the NW, some of which can propagate along the NW axis in multiple guided modes. We find that when the transverse dimension of the tapered NW becomes too small to support a specific mode at a particular wavelength, some of the fluorescence “leaks” from the waveguide, leaving a spectral signature of that mode.
Figure 6 shows the collected emission along the length (Y) of a NW when the excitation laser is positioned near the larger (left) end of the NW. The top panel shows the emission intensity and the middle panel shows the collected spectrum where the pixels spanning the NW width were binned together at each position along the NW axis. Each binned spectrum was also self-normalized to reveal additional details that would otherwise be suppressed by large intensity variations. The diagonal spectral stripes are signatures of higher-order waveguide modes, whose simulated intensity profiles are plotted within the red boxes. Finite difference time domain (FDTD) simulations confirm the origin of the spectral stripes, including subtle variations in the slope of the stripes caused by structural defects in the tapered NW (data not shown).
Students Involved:
Maoji (Mogi) Wang, Physics PhD Student
Lauren Richey-Simonsen, PhD, Former Physics PhD Student
External Collaborators:
Jim Schuck, Columbia University (formerly LBNL)
Shaul Aloni, Molecular Foundry, LBNL
Tev Kuykendal, Molecular Foundry, LBNL