Advances in III‐nitride semiconductor microdisk lasers

Abstract

This paper reviews the recent progress in the design and fabrication strategies of III-nitride semiconductor microdisk lasers on sapphire or Si substrates. Involving more accurate lithography methods in the fabrication boosts quality factors (Q-factors) due to the improved optical confinement of whispering-gallery modes (WGMs) at the rim of the microdisks with smooth sidewalls and perfect circularity. Quantum dots (QDs) as gain medium within the microdisks are investigated and promising for the demonstration of microlasers with super-high Q-factors and ultra-low thresholds. And these QD-cavity systems are also facilitating the research of cavity quantum electrodynamics (QED) in the strong coupling regime and cavity-enhanced single photon emission (SPE) for quantum computing. In this paper, we also report successful fabrication of transferred GaN free-standing microdisks with InGaN quantum wells (QWs) on conductive substrates, which achieves a significant step forward to realize electrically pumped high-quality microdisk lasers on target substrates. Strategies of applying flexible transparent electrodes such as graphene and metallic nanowires are potential effective solutions for realizing current-injection of nitride-based microdisks.ß 2015 WILEYÀVCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Over past few decades, group IIInitride compound semiconductors have been extensively studied and have emerged as indispensable materials for a wide range of optoelectronic devices. With a wide and direct energy bandgap, emissions from III-nitride semiconductors can cover the wavelength range from deep ultraviolet to the near-infrared, making them ideal candidates for incoherent and coherent light sources [1][2][3][4]. Moreover, as epitaxial growth techniques of III-nitride materials become mature, high quality quantum structures such as quantum wells (QWs) and quantum dots (QDs) can be readily achieved, greatly improving the emission efficiency via quantum confinement of the carriers within a small active region so that the possibility of radiative recombination increases, serving as gain medium for highly efficient light emitters [5][6][7]. In addition, the high binding energies of excitons in these wide bandgap materials motivate the research on photon-exciton (or polariton) dynamics and its coherent state polariton lasing in microcavities [8][9][10][11]. In fact, the research of nitride-based microcavity lasers has attracted much attention in past decades for both physical insight of lasing mechanisms in both the weak and strong coupling regimes and their widespread applications [12]. Until now,

GaN hemispherical micro-cavities

“…[1][2][3] Up to now, GaN optical microcavities of various geometrics have been studied, which generally include Fabry-Perot (F-P) cavities employing distributed Bragg reflectors (DBRs), 4 defective photoniccrystal (PC) structures, 5 and whispering-gallery mode (WGM) microcavities. 6 Amongst these options, WGM microcavities have been demonstrated as promising candidates for realizing low-threshold lasing with narrow emission linewidths, attributed to their low optical losses and modal volumes.…”

“…Improvements to the Q factor can be achieved by refining the surface roughness during the ICP dry-etching or chemical wet-etching processes. 2,30 And the spontaneous emission coupling factor (b) can be roughly estimated to be $0.28, taking the ratio of PL intensities below and above the threshold. Moreover, as the excitation energy densities gradually increase from $0.6E th to $1.1E th , a continuous blueshift in the peak wavelengths of the observed WGMs (P 1,2,3,4 ) can be identified.…”

Mathematical and Numerical Modeling of On-Threshold Modes of 2-D Microcavity Lasers with Piercing Holes

“…Therefore, it was also applied to the microcavity lasers, introduced in the 1990s, where the light is confined inside a transparent dielectric cavity of micrometer size. For arbitrary shapes, the mode analysis was performed usually with the aid of geometrical optics (see review papers [1][2][3][4][5] and references therein) and, for simple circular-disk shapes, with Maxwell equations (see reviews [6][7][8]). On the one hand, such analysis enabled one to explain the fact that the lasers emitted light on discrete frequencies, via the concept of natural modes of laser cavities, i.e., discrete eigenstates of the electromagnetic field as solutions to eigenvalue problems.…”

Demonstration of critical coupling in an active III-nitride microdisk photonic circuit on silicon

“…The main advantages of this material system are the possibility to have active monolithically integrated laser sources from the ultra-violet to visible (UV-VIS) spectral range and a large transparency window for energies smaller than 6 eV. There have been numerous demonstrations of microlasers [10][11][12][13][14][15] and high quality (Q) factor microresonators using III-nitrides [16][17][18] . Several reports have been made on passive microdisks evanescently coupled to bus waveguides in the UV-VIS spectral range [19][20][21] , but only very few demonstrations of active microlaser photonic circuits have been reported 22 , while critical coupling has been observed in passive photonic circuits in the IR [23][24][25][26] .…”