Geometry-dependent two-photon absorption followed by free-carrier absorption in AlGaAs waveguides

Abstract

Nonlinear absorption can limit the efficiency of nonlinear optical devices. However, it can also be exploited for optical limiting or switching applications. Thus, characterization of nonlinear absorption in photonic devices is imperative for designing useful devices. This work uses the nonlinear transmittance technique to measure the two-photon absorption coefficients ( α 2 ) of AlGaAs waveguides in strip-loaded, nanowire, and half-core geometries in the wavelength range from 1480 to 1560 nm. The highest α 2 values of 2.4, 2.3, and 1.1 c m / G W were measured at 1480 nm for 0.8-µm-wide half-core, 0.6-µm-wide nanowire, and 0.9-µm-wide strip-loaded waveguides, respectively, with α 2 decreasing with increasing wavelength. The free-carrier absorption cross section was also estimated from the nonlinear transmittance data to be around 2.2 × 10 − 16 c m 2 for all three geometries. Our results contribute to a better understanding of the nonlinear absorption in heterostructure waveguides of different cross-sectional geometries. We discuss how the electric field distribution in the different layers of a heterostructure can lead to geometry-dependent effective two-photon absorption coefficients. More specifically, we pinpoint the third-order nonlinear confinement factor as a design parameter to estimate the strength of the effective nonlinear absorption, in addition to tailoring the bandgap energy by varying the material composition.

Harnessing microcomb-based parallel chaos for random number generation and optical decision making

“…Currently the microresonator in this work is critically coupled, and by using an over-coupled structure instead, higher conversion efficiency, wider optical spectrum and higher power per comb line could be obtained without degenerating the chaotic properties. Another factor which limits the chaotic bandwidth currently is the intra-cavity nonlinear loss 49 , 50 (discussed in Supplementary Note 8 ), which mainly comes from the three photon absorption and consequently free carrier absorption, leading to relatively high side lobes. This can be solved by employing an integrated PIN structure for the waveguide or working at a longer-wavelength band 51 .…”

AlGaAs Nonlinear Integrated Photonics

“…In this manuscript, we concentrate only on the third- and fifth-order nonlinear effects most studied in AlGaAs optical waveguides. Among these third-order effects, there are FWM [ 7 , 9 , 20 , 132 , 133 , 134 , 135 , 136 , 137 , 138 , 139 , 140 , 141 , 142 , 143 , 144 ], 2PA [ 13 , 19 , 23 , 114 , 145 , 146 , 147 , 148 ], SPM [ 23 , 32 , 114 , 133 , 134 , 146 , 147 , 149 , 150 , 213 ], XPM [ 32 , 133 , 146 , 149 ], SFWM [ 151 , 152 ], SCG [ 154 , 155 ], and Kerr frequency microcomb [ 8 , 156 ]. The -induced 3PA [ 13 , 19 , 27 , …”

“…Additionally, the optical field of the guided mode penetrates to the cladding regions in different waveguide geometries to a different extent. Hence, the measured nonlinear coefficient represents an intensity-weighted average over the different layers of the waveguide structure [ 148 , 215 ]. In Table 6 , we summarize the values of and measured in waveguides of various compositions, platforms, and geometries, together with some bulk-material values for comparison.…”

“… The spectra of the nonlinear coefficients of AlGaAs waveguides with guiding layer: ( a ) nonlinear refractive index, ( b ) 2PA coefficient, ( c ) 3PA coefficient, and ( d ) nonlinear FOM associated with 2PA and 3PA, as defined by Equations ( 10 ) and ( 11 ). References: • [ 9 ]; • , ▪ , and ∘ [ 13 ]; ▴ [ 22 ]; ▾ [ 23 ]; ▿ [ 28 ]; ∘ [ 36 ]; • and ▪ [ 39 ]; ⧫ , ◊ , and ⧫ [ 148 ]. …”