Abstract
Gallium nitride (GaN) has been established as a promising candidate for integrated electro-optic and photonic devices, aiming at applications from optical switching to signal processing. Studies of its optical nonlinearities, however, lack spectral coverage, especially in the telecommunications range. In this study, we measured the two-photon absorption coefficient (β) and the nonlinear index of refraction (n2) of GaN from the visible to the near-infrared by using femtosecond laser pulses. We observed an increase of β from (1.0 ± 0.2) to (2.9 ± 0.6) ×10−11 m/W as the photon energy approached the band gap from 1.77 up to 2.25 eV (700–550 nm), while n2 varied from (90 ± 30) ×10−20 up to (265 ± 80) ×10−20 m2/W within a broad spectral range, from 0.80 up to 2.25 eV (1550–550 nm). The results were modeled by applying a theory based on the second-order perturbation theory and the Kramers-Kronig relationship for direct-gap semiconductors, which are important for the development of GaN-based nonlinear photonic devices.
Highlights
The wide-bandgap direct-gap semiconductor gallium nitride (GaN) is well-known for its excellent electronic and linear optical properties [1,2,3] that have resulted in a large variety of electronic and optoelectronic applications, such as light-emitting diodes (LEDs) [4], field-effect transistors (FETs) [5,6], and high-temperature electronic devices [7]
The Gallium nitride (GaN) sample used in this study was grown epitaxially by MOVPE on a ~400 μm, double-side-polished 4 inch sapphire substrate
The ~10 μm thick GaN layer was unintentionally n-doped with impurity concentrations < 1017 cm−3
Summary
The wide-bandgap direct-gap semiconductor gallium nitride (GaN) is well-known for its excellent electronic and linear optical properties [1,2,3] that have resulted in a large variety of electronic and optoelectronic applications, such as light-emitting diodes (LEDs) [4], field-effect transistors (FETs) [5,6], and high-temperature electronic devices [7]. As integrated photonics emerged as an oncoming new technology to manipulate and process optical signals, GaN’s large second-order nonlinear optical coefficients have been exploited to produce second-harmonic-generation devices based on, for example, microdisks [14] and slab waveguides [15,16,17]. The design of such devices were supported by several studies on nonlinear optical characterization of GaN’s second-order susceptibility [18,19,20,21]. GaN-based devices using high-quality resonators [22] and ridge-waveguides [23] were recently demonstrated to exhibit third-order optical nonlinearities, manifested as four-wave mixing. The third-order susceptibility of GaN has not been thoroughly characterized yet, especially in the telecommunication spectral range and when excited with femtosecond pulses
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