Far-field coupling in nanobeam photonic crystal cavities

We optimized the far-field emission pattern of one-dimensional photonic crystal nanobeams by modulating the nanobeam width, forming a sidewall Bragg cross-grating far-field coupler. By setting the period of the cross-grating to twice the photonic crystal period, we showed using three-dimensional finite-difference time-domain simulations that the intensity extracted to the far-field could be improved by more than three orders of magnitude compared to the unmodified ideal cavity geometry. We then experimentally studied the evolution of the quality factor and far-field intensity as a function of cross-grating coupler amplitude. High quality factor (>4000) blue (lambda = 455 nm) nanobeam photonic crystals were fabricated out of GaN thin films on silicon incorporating a single InGaN quantum well gain medium. Micro-photoluminescence spectroscopy of sets of twelve identical nanobeams revealed a nine-fold average increase in integrated far-field emission intensity and no change in average quality factor for the optimized structure compared to the unmodulated reference. These results are useful for research environments and future nanophotonic light-emitting applications where vertical in-and out-coupling of light to nanocavities is required. Published by AIP Publishing.

Fabrication defects and grating couplers in III-nitride photonic crystal nanobeam lasers (Conference Presentation)

Raphaël Butté, Jean-François Carlin, Nicolas Grandjean, Ian Michael Rousseau

We report a numerical and experimental investigation of fabrication tolerances and outcoupling in optically pumped III-nitride nanolasers operating near lambda = 460 nm, in which feedback is provided by a one-dimensional photonic crystal nanobeam cavity and gain is supplied by a single InGaN/GaN quantum well. Using this platform, we [1] and others [2] previously demonstrated single-mu W lasing thresholds due to the high beta Q-product inherent to the nanobeam geometry (beta is spontaneous emission coupling fraction into desired mode). In this work, we improved the fraction of emission emitted into our microscope's light cone by combining a redesigned photonic crystal cavity (c.f. [3]) with a cross-grating coupler with period approximately twice the photonic crystal lattice constant [4]. The samples were fabricated in epitaxial III-nitride layers grown on (111) silicon substrates using metal organic vapor phase epitaxy. The photonic crystal and output couplers were patterned using a single electron beam lithography exposure and subsequently transferred to the underlying III-nitride layers using dry etching. The nanobeams were then suspended via vapor phase etching of silicon in XeF2 (Figure 1) [5]. Scanning electron microscopy cross-sections revealed high-aspect ratio (>5), sub-70 nanometer diameter holes with near-vertical sidewalls. Fabrication-induced geometry errors were characterized by processing scanning electron micrographs with custom critical dimension software (Figure 2). Using UV micro-photoluminescence spectroscopy at room temperature, we measured the nanobeams' emission intensity, far-field profile, and quality factor. By comparing more than ten nominally identical nanobeams for each geometry with finite-difference time-domain simulations taking into account the geometrical deviations measured during fabrication [8], we characterized the role of fabrication-induced imperfections. Finally, we explored the trade-off between the quality factor and collected signal via lithographic variations of the output coupler grating amplitude. Our results demonstrate the robustness of III-nitrides for short-wavelength photonic crystal applications, such as photonic integrated circuits, optoelectronics, and cavity quantum electrodynamics.

Spie-Int Soc Optical Engineering2016

III-Nitride Semiconductor Photonic Nanocavities on Silicon

Ian Michael Rousseau

Optical nanocavities enhance light-matter interaction due to their high quality factors (Q) and small modal volumes (V). The control of light-matter interaction lies at the heart of potential applications for integrated optical circuits, including optical communication, quantum computation, biophotonics, and optical projection. Many of these applications could benefit from wide band gap III-nitride semiconductor material, which already forms the basis of industrial lighting technology based on blue light-emitting diodes. Cleanroom fabrication and optical spectroscopy comprise the laboratory work in this thesis. Air-suspended III-nitride nanocavities are fabricated using electron beam lithography, reactive ion etching, and vapor phase etching from thin (< 300 nm) III-nitride epilayers grown on silicon (111) substrates by metal-organic vapor phase epitaxy. An embedded single InGaN/GaN quantum well serves as an internal light source during micro-photoluminescence experiments. Strain and heating effects are analyzed by micro-Raman spectroscopy. A dedicated quantum optics laboratory is constructed in order to study nanocavities and quantum emitters in III-nitrides at short wavelengths (lambda < 500 nm). Photonic crystal nanocavities exhibit the highest figure-of-merit for light-matter interaction enhancement, Q/V. However, in contrast to silicon and gallium arsenide nanocavities working at telecommunication wavelengths, III-nitride based nanocavities underperform theoretical Q estimates by nearly three orders of magnitude at short wavelengths (lambda = 430 - 500 nm). This thesis accounts for this discrepancy by combined experimental and theoretical studies of nanobeam fabrication statistics. Surface state absorption is found to be the primary factor limiting Q at short wavelengths. Surface effects are studied in the microdisk resonator geometry. UV photoinduced desorption of oxygen gas from the III-nitride surface redshifts and broadens whispering gallery mode resonances over minute timescales. Oxygen desorption incurs additional optical absorption losses up to 100 cm^{-1}. The redshift and broadening is nearly reversible upon the introduction of oxygen into the measurement environment, implicating oxygen in passivation of the III-nitride surface. Finally, optimized surface passivation techniques using rapid thermal processing and conformal chemical vapor deposition more than double state-of-the-art Q of GaN-based microdisk resonators to beyond Q=10,000 in the blue spectral range. The results demonstrate the capabilities and limitations of III-nitride materials for optical nanocavities and photonic integrated circuits at short wavelengths.

EPFL2018

III-Nitride Semiconductor Photonic Nanocavities on Silicon

Ian Michael Rousseau

EPFL2018

Fabrication defects and grating couplers in III-nitride photonic crystal nanobeam lasers (Conference Presentation)

Raphaël Butté, Jean-François Carlin, Nicolas Grandjean, Ian Michael Rousseau

Spie-Int Soc Optical Engineering2016