Fourfold Upconversion Laser: Operation and Enhancement Mechanisms, Lab Reports of Health sciences

Download Fourfold Upconversion Laser: Operation and Enhancement Mechanisms and more Lab Reports Health sciences in PDF only on Docsity! Continuous-wave, fourfold upconversion laser P. Xie and S. C. Rand Division of Applied Physics, 1049 Randall Laboratory University of Michigan, Ann Arbor, Michigan 48109-1120 (Received 1 July 1993; accepted for publication 23 September 1993) We report a new Er3+:LiYF4 cryog enic upconversion laser pumped by a fourfold upconversion process. Excitation at 1.5 ,um results in laser emission at 701.5 nm on a transition with an upper state at nearly four times the pump photon energy. Mechanisms and the concept of cooperative upconversion enhancement are examined. Recent demonstrations 1-5 of room-temperature upcon- version lasers have stirred interest in their potential as practical sources of short wavelength radiation for display and data storage applications, as well as for communica- tions and ultrashort pulse generation at visible and ultra- violet wavelengths.6 To date however, little consideration has been given to the relative merits of various fundamen- tal 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 in- volved 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 impor- tant 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 Pm- Emissions 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 can- didates. 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 com- ponents 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 wave- lengths 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 ex- pected for multiphoton upconversion processes. It there- fore has important implications for the inversion mecha- nism 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 mech- anism is operative. l3 For cooperative upconversion, only a single pump transition is relevant, terminating anywhere within the metastable manifold in which cooperative inter- ‘;; 1 .Z 3 2 4 z 0.5 z z 2 : E; 0 300 500 700 900 1100 1300 1500 Wavelength --4--- ‘G ,m h F : w 8 B L z k x 3 l .SBm 7-t excitation - -t - - - - 411sn *H 1u2 *s 32 (b) FIG. 1. (a) Upconversion fluorescence spectrum observed in Er3+:LiYF4 with excitation at &,= 1.5 /*m (T= 10 K), uncorrected for instrumental response. (b) Schematic diagram of Er3+ energy levels, showing emission at 410 and 701.5 nm from 2H9,2 at nearly four times the incident photon energy. 3125 Appl. Phys. Lett. 63 (23), 6 December 1993 0003-6951/93/63(23)/3125/3/$6.00 @ 1993 American Institute of Physics 3125 Downloaded 25 Mar 2002 to 141.213.8.164. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp 0-I 100 200 300 400 500 600 Pump Power (mW) FIG. 2. Output vs input power of the cw fourfold upconversion laser operating at 701.5 nm (T= 10 K). Inset: Laser emission spectrum, actions occur. In this case only ground state absorption resonances should appear in the excitation spectrum. The results in Fig. 3 therefore provide ample evidence that ab- sorption of light by excited ions does not occur and that the present fourfold laser operates by cooperative means. This argument also rules out mechanisms combining cooperative and ESA processes. For example, ESA of 1.5 pm light from 4S’,,, or 2H11,2 states populated by trio up- conversion” could in principle reach the high lying 2H9,2 state responsible for laser action in this work, but such transitions are off resonant by over 2 18 cm- ’ in LiYF, for all Stark levels” and are not present in the excitation spec- trum. ESA transitions from 4S3,2 to 4G11,2 are still farther off resonance. On the other hand, quartet upconversion can provide very nearly resonant excitation of the 4G,1,2 state lying immediately above the upper laser level [generating the fluorescence at 380 nm in Fig. 1 (a)]. The calculated energy defects on the quartet transitions 41,s,2 (3,3,3,4)+4G t&l) and 41ts,2(4,4,4,3)-r 4G11,2(4), for example, are only 4 and 7 cm-’ in LiYF,, respectively. The bracketed arguments of the initial state indicate Stark levels of the four interacting atoms, and the single argu- ment of the final state gives the Stark level of the acceptor. Taken together with the close match between the excita- tion spectrum and ground state absorption (Fig. 3), this argues strongly against any contributions by ESA pro- l.iO 1.52 Wavelength FIG. 3. Infrared excitation spectrum of fourfold upconversion laser emis- sion at 701.5 nm (T= 10 K, lower trace) and, for comparison, the Er3+ absorption spectrum ( T=9 K, inverted). no n, 13’ m l2> n “P n* nt /lb> IIS> FIG. 4. Mechanism for enhanced quantum efficiency in cooperative up- conversion systems. Following an initial excitation step (by direct pump- ing or upconversion), a cascade from higher to lower excited states occurs with branching ratios of vi at each step. Wiggly arrows indicate nonradi- ative decay. A given atom can emit more than one photon per no excita- tion if the upconversion rate exceeds the spontaneous decay rate of the lowest excited state (and detailed balance is satisfied). cesses to 2Hg,2 p o p ulation when 1.5 ,um pumping is used. The implication that laser operation may be sustained by a cooperative quartet process raises fundamental ques- tions as to just how effective higher order cooperative up- conversion processes can be in general. In this regard, we wish to point out that an enhancement mechanism exists for cooperative upconversion processes which can signifi- cantly extend their usefulness compared to multiphoton absorption. For emission terminating on or near levels in which strong cooperative interactions occur, no efficiency “penalty” is incurred for m-fold upconversion, where the integer m may be arbitrarily large. When upconversion emission terminates on an energy level which can renew cooperative upconversion, excited atoms are recycled with- out external pumping to enhance quantum efficiency by a factor of ( 1 ---r],71~r7s~Q -‘, where qUP is the m-fold coop- erative upconversion efficiency. vl, v2, and q3 are branch- ing ratios for the decay processes indicated in Fig. 4. En- ergy conversion between input wavelength iii, and output at 4,, can be much higher than the maximum value ~7~ =/2in//2out( 100/m ) % based on a picture in which only one of m atoms is upconverted following the absorption of m pump photons. Ignoring decay to the ground state, the energy efficiency takes on the enhanced value rl~=(~in~~oat)rl~rll(l-~??lrl2rl3rlup)-~ (1) and rll? sax when upconverted atoms reside long enough in their lowest excited state to participate repeatedly in upconversion. With recycling, energy efficiency can ap- proach 100% [for vi=1 (i=O,1...3) with a negligible en- ergy defect such that il,,,=ili,/( 1 -Q,)]. In practice, theoretical enhancement of quantum effi- ciency by cooperative dynamics must be mitigated by losses due to an increasing number of intervening (radia- tive) levels on real atoms with increasing m. Branching ratios are typically less than unity. Also a statistical de- crease is to be expected in the number of m-atom “clus- ters” with appropriate interatomic spacings, less than a nominal critical radius. However, the main conclusion is that a recycling mechanism exists whereby cooperative dy- namics can mediate unexpectedly efficient upconversion emission for arbitrarily high degrees (m) of upconversion. 3126 Appl. Phys. Lett., Vol. 63, No. 23, 6 December 1993 P. Xie and S. C. Rand 3126 Downloaded 25 Mar 2002 to 141.213.8.164. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp