Microcavity-enhanced Kerr nonlinearity in a vertical-external-cavity surface-emitting laser

Abstract

Self-mode-locking has become an emerging path to the generation of ultrashort pulses with vertical-external-cavity surface-emitting lasers. In our work, a strong Kerr nonlinearity that is so far assumed to give rise to mode-locked operation is evidenced and a strong nonlinearity enhancement by the microcavity is revealed. We present wavelength-dependent measurements of the nonlinear absorption and nonlinear-refractive-index change in a gain chip using the Z-scan technique. We report negative nonlinear refraction up to 1.5⋅10 -11 cm 2 /W in magnitude in the (InGa)As/Ga(AsP) material system close to the laser design wavelength, which can lead to Kerr lensing. We show that by changing the angle of incidence of the probe beam with respect to the gain chip, the Kerr nonlinearity can be wavelength-tuned, shifting with the microcavity resonance.Such findings may ultimately lead to novel concepts with regard to tailored self-mode-locking behavior achievable by peculiar Kerr-lens chip designs for cost-effective, robust and compact fspulsed semiconductor lasers.The Kerr effect is at the basis of many important device concepts like all-optical switching 1 , optical limiting 2 and soliton mode-locking of lasers and microresonators 3,4 . The capability to accurately measure and model the nonlinear refractive index changes associated with the Kerr effect is crucial for improved device operation, where a specifically tailored nonlinear refractive index is required, e.g. for the intensity-dependent Kerr lensing. Several measurement schemes have been developed in the past for the characterization of the nonlinear refractive index 5-7 , with the Z-scan technique 8 being undoubtedly the most prominent one due to its simplicity and high sensitivity. This method continues to be of high experimental value with the rise of novel material classes like graphene and other two-dimensional semiconductors which often exhibit a very strong refractive nonlinearity 9-11 .Kerr-lens mode-locked Ti:Sapphire lasers have dominated the field of ultra-short high-power modelocked lasers since their initial discovery nearly three decades ago 12,13 . In these lasers, the intensitydependent refractive index of the gain crystal leads to self-focusing of the laser beam for high intensities. When part of the continuous-wave (cw) beam profile is suppressed by inserting a slit into C. Kriso et al. (2018) 2

Pulse Dynamics in SESAM-Free Electrically-Pumped VECSEL

“…As the laser switches from cw to pulsed operation, the D4-sigma beam diameters at the beam waist change from 389.7 µm to 390.5 µm in the horizontal direction and from 321.4 µm to 312.5 µm in the vertical direction. These small changes in beam diameters suggest that nonlinear Kerr-lensing effects are negligible in the presented EP-VECSEL setup, which is in agreement with previously reported measurements of the nonlinear refractive index in GaAs-based VECSEL chips [32][33][34][35]. It was possible to achieve close to single-mode beam characteristics (M 2 <1.1) by alignment of the output-coupler.…”

“…Similar effects have already been observed in OP-VECSELs and attributed to Kerr-lensing inside the VECSELs. However, the intracavity peak powers in the EP-VECSEL were more than four orders of magnitude lower than the peak powers required for Kerr-lensing in OP-VECSELs [21,32,35]. Furthermore, no nonlinear lensing effects were observed in the beam profile and M 2 -measurements, which showed no significant difference between cw and pulsed operation.…”

Large and negative self-defocusing optical Kerr nonlinearity in Palladium di-Selenide 2D films

“…For example, a negative Kerr nonlinearity can be used to self-compress ultrashort pulses in the presence of positive group-velocity dispersion [55,56]. Another application of a negative Kerr nonlinearity is mode locking of lasers using the Kerr mode-locking technique [53,57] as well as the possibility of achieving net parametric modulational instability gain under normal dispersion conditions [53,58].…”

Large 3rd Order Optical Kerr Nonlinearity in 2D PdSe2 Dichalcogenide Films for Integrated Nonlinear Photonic Chips

“…For example, a negative Kerr nonlinearity can be used to self-compress ultrashort pulses in the presence of positive groupvelocity dispersion. [37] Another application of a negative Kerr nonlinearity is mode locking of lasers using the Kerr modelocking technique [35,38] as well as the possibility of achieving net parametric modulational instability gain under normal dispersion conditions. [35,39] In addition, as shown in Figure 4(b), the absolute value of n2 initially decreases with laser intensity and then saturates at higher intensities.…”