Continuous-wave, fourfold upconversion laser

Recent demonstrations 1-5 of room-temperature upconversion lasers have stirred interest in their potential as practical sources of short wavelength radiation for display and data storage applications, as well as for communications and ultrashort pulse generation at visible and ultraviolet wavelengths.6 To date however, little consideration has been given to the relative merits of various fundamental mechanisms for achieving upconversion, basic limits to achievable degrees of upconversion (ratio of upper laser level energy to incident photon energy), or to questions of stability of the highly nonlinear pumping processes involved in these devices. In this article we discuss two of these topics in the context of new results demonstrating a continuous-wave, fourfold upconversion laser. The important third issue of stability of solid state lasers in which nonlinear cooperative dynamics play an important role has been considered by Xie’ and will be published separately.’ Upconversion fluorescence observed in Er:LiYF4 due to irradiation with a continuous-wave (cw) NaCl laser at 1.5 pm at liquid-helium temperature is shown in Fig. 1 (a). The upconversion mechanisms responsible for fluorescent emissions near 850 and 550 nm have been studied in past work and arise from cooperative (multi-atom) energy transfer processes’ when cw excitation is restricted to 1.5 PmEmissions near 410, 650, 702 nm from higher lying states have not yet been studied as thoroughly, but are sufficiently intense to draw attention as potential laser candidates. The two near-ultraviolet lines at 407 and 413 nm arise from transitions with upper levels at roughly four times the incident photon energy, as indicated in Fig. 1 (b). The fluorescent emission at 702 nm arises from Stark components of the 2H9/2* 4I , ,,,transition. Laser experiments were performed in a three-mirror, astigmatically compensated’ cavity consisting of two 5 cm radius total reflectors and an output mirror with 97% reflectively at 702 nm. A 3-mm-thick crystal of 5% Er:LiYF4 inserted at Brewster’s angle within the focusing arm of the laser served as the gain medium. Its optic axis was oriented parallel to the crystal surface in the plane of incidence of horizontally polarized pump radiation and the crystal was suspended on a cold finger in vacuum. The laser emission spectrum at 701.5 nm, assigned to the 2H9/2 (1) -+4111,,(3) transition, lo and the variation of output power with input are shown in Fig. 2. The overall efficiency was 0.06% and observed slope efficiency was 0.09%. The excitation spectrum of laser emission revealed a one-to-one correspondence with erbium absorption wavelengths in the 1.5 pm region. This result is shown in Fig. 3 and is similar to earlier findings for pair’ and trio” lasers, but contrasts the restrictive wavelength dependence expected for multiphoton upconversion processes. It therefore has important implications for the inversion mechanism of the fourfold laser.12 For multiphoton absorption to be effective, several ground and excited state absorption (ESA) frequencies must overlap sharply. Hence, excited state resonances should appear in the excitation spectrum when this mechanism is operative. l3 For cooperative upconversion, only a single pump transition is relevant, terminating anywhere within the metastable manifold in which cooperative inter-