Third-Order Nonlinear Spectrum of GaN under Femtosecond-Pulse Excitation from the Visible to the Near Infrared

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.

Reduced Graphene Oxide Thin Film with Strong Optical Nonlinearity

“…It means that two‐photon absorption is only the main cause of the observed optical nonlinearity. According to theoretical model developed by Sheik‐Bahae et al for two‐photon absorption in the second‐order perturbation approach that can successfully be applied to model the dispersion of optical nonlinearities in wide‐gap dielectrics and direct‐gap semiconductors, the value of β as a function of the excitation energy E = ħω is given by Equation (7) [ 44 ] where F 2 ( x ) = (2 x − 1) 3/2 /(2 x ) 5 , n 0 is the linear refractive index, and E p is a nearly material‐independent energy which has a value of approximately 21 eV. The material‐independent constant K was theoretically calculated to be 1940 in units such that β is in cm GW −1 and E p and E g are in eV.…”

Ultrafast-nonlinear ultraviolet pulse modulation in an AlInGaN polariton waveguide operating up to room temperature

“…Values quoted for GaN are typically of order 10 −14 cm 2 W −1 for the near-infrared to visible spectral region 44 , 45 but values greater than 10 −12 cm 2 W −1 have been reported in the UV close to the band gap 46 . These results are orders of magnitude larger than can be expected from the well known material-independent scaling of n 2 with frequency 44 , 47 . This might be explained because they were measured using spatial nonlinear refraction at the pump spot position, which can be dominated by thermal effects or photo-generated free-carriers 45 , 48 .…”

Ultrafast Third-Order Nonlinear Optical Response Excited by fs Laser Pulses at 1550 nm in GaN Crystals

“…Some studies refer to the second harmonic generation in several GaN structures [16][17][18][19]. Other studies refer to the investigation of the third-order nonlinear optical response and to the measurement of the third-order nonlinear optical parameters by Z-scan and four-wave-mixing (FWM) in GaN [20][21][22][23][24][25][26][27][28][29][30]. For visible incident wavelengths, especially when high intensity laser beams are involved, these methods of nonlinear investigation may cause the inclusion of significant two-photon absorption, photo-generated free carriers, and thermo-optic effect contributions in the values of the third-order nonlinear parameters, in addition to the ultrafast electronic one [31].…”

“…For visible incident wavelengths, especially when high intensity laser beams are involved, these methods of nonlinear investigation may cause the inclusion of significant two-photon absorption, photo-generated free carriers, and thermo-optic effect contributions in the values of the third-order nonlinear parameters, in addition to the ultrafast electronic one [31]. Only very few studies of the third-order optical nonlinearity of GaN at the telecommunication wavelength of 1550 nm, by four-wave mixing (FWM) with ns pulses [23], and by Z-scan using ultrashort (femtosecond) laser pulses [25] were reported.…”