Reflection z-scan measurements of the non-linear lens in VECSEL gain structures

We present reflection z-scan measurements of a quantum well VECSEL gain structure under pumped and unpumped conditions. The implications for the design of mode-locked cavities will be discussed; both in relation to SESAM mode-locked lasers and the possibility of self-mode-locking. Semiconductor disk lasers (SDLs), also known as vertical-external-cavity surface-emitting lasers (VECSELs), have been the subject of considerable interest in recent years due to flexibility in their design and low cost. This has led to steady improvements in power up to 106 W1 in continuous wave (CW) operation and kilowatt peak powers in mode-locked (ML) operation with sub-picosecond pulse durations typically at gigaHertz repetition frequencies. Most ML-SDLs have used semiconductor saturable absorber mirrors (SESAMs) to initiate mode-locking. Recently, there have been reports of SDLs operating in a pulsed regime without a saturable absorber in the cavity, so called self-mode-locking (SML). It has been suggested that Kerr lensing in the gain medium or saturable absorption in unpumped quantum wells (QWs) causes the observed pulsation. However, there has not yet been sufficient characterisation or modelling to determine what the causal effect is. To understand the observed lasing behavior of self-mode-locked SDLs it is important to characterize nonlinear lensing in a SDL gain structure. The magnitude of the non-linear lensing would influence the cavity design of SML-SDLs. Non-linear lensing may also need to be considered when designing SESAM ML-SDLs. SDL gain chips are multilayer structures with scope for large variation in composition, thickness and number of layers. The complexity of their design makes modeling the non-linear response challenging. The non-linear response of semiconductors is highly dependent on the photon energy of the incident light as compared to the band gap energy of the semiconductor. There are many different physical mechanisms that could affect lensing in the gain chip, including carrier and thermal effects, the strength of which are not well known and may occur on very different time scales. In order to obtain representative values of the lensing it is essential to replicate laser operating conditions when characterising the non-linear lens. The structure must be probed at a wavelength on resonance with the quantum well emission, where the SDL gain chip would naturally lase, and with a pulse duration that is comparable to reported self mode-locking (<1 ps). Previous measurements of non-linear lensing in a SDL gain structure indicate that there is a non-linear lens large enough to perturb a SDL cavity.19 However, the probe laser used in Ref.19 had long pulses compared to observed sub-picosecond SDL performance, and probed the sample below the band gap energy of the quantum wells. In this letter we present the first measurements of the non-linear lens in a SDL gain structure probing on resonance with the QW emission, with pulse durations typical of ML-SDLs. We observe a self-defocussing lensing effect that varies linearly with the average power of the probe