laser sources available at wavelengths shorter than that of the argon-ion laser at 458 nm. Cw mode- locked dye lasers operate only to -500 nm,1 and solid-state ...
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L. Zhou and E.Y. Chen contributed equally to this work. Synthesis of ZnPc-(COOH)4. OH. HOOC. +. NC. NC. NO2. K2CO3. NC. NC. O. COOH n-pentanol.
Apr 23, 2008 - 4CNR-INFM CRS-SOFT, c/o Universita` di Roma ''La Sapienza,'' I-00185, Rome, Italy ... structurally similar to other group-IV oxide glasses.
variety of a double cover of a trigonal curve, and Pantazis showed in. [P] that the Prym varieties associated to the bigonal construction of. Donagi (c.f. [D1]) are ...
A = 0.5516 pm, corresponding to the 4S3/2(1)-"115 /2(4) transition with mixed o- and r polarization owing to. Kramer's degeneracy. This laser transition termi-.
where n1, n2, n3, n4, n5 are the fractions of erbium ions on different levels as shown in ... World Academy of Science, Engineering and Technology. International ...
May 17, 2017 - Scanning Laser Upconversion Microscopy offers the deep penetration ... intermediate state. c) Energy transfer upconversion. The presence of ...
Feb 9, 2013 - work  demonstrated the application of Gd2O2S: Yb, Er as another potential candidate ... hexagonal phase (JCPDS Card no.26-1422). The XRD results ... excitation in samples Gd2O2S:Yb(4.5)Ho(0.5) [GOS], La2O2S:Yb(8). Ho(0.5) [LOS] and
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Mar 10, 2014 - NaYF4:Yb3+/Tm3+ nanospheres (blue), UCPL of NaYF4:Yb3+/Er3+ nanospheres (yellow green), UCPL of NaYF4:Yb3+/Er3+ nanospheres through green and red ...... wavelength absorption pH probe bromothymol blue (BTB).363 ...... Lafferty, E. I.;
coating method using trifluoroacetic acid as a fluorine source. ... strong ultraviolet (UV) upconversion luminescence of the thin film was observed. To compare the .... sion of the powder in the 200â750 nm wavelength range under 980 nm ...
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The PEC cell consisted of fused optically flat silica possessing a cubic shape equipped with a Teflon cap where all the ... PEC linear sweep voltammetry (LSV) experiments were also performed to obtain photocurrent response as a function ..... Phys.,
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Above 75 K, the shallow trap excitation is thermally depleted, via the (quasi-) exciton band, to fast-trapping non-emissive sites. At liquid helium temperatures, the depletion of the highest-lying shallow traps is frozen out. At room temperature, the
Nov 24, 2013 - energy to defects, even in stoichiometric compounds with a high Yb3+ content (calculated as 98mol%). This allows us to generate an unusual four-photon-promoted violet upconversion emission from Er3+ with an intensity that is more than
First, studies were performed on curcumin in methanol, ethylene glycol, and chloroform to understand the effect of intermolecular hydrogen bonding on ESIHT. Second, micellar systems composed of sodium dodecyl sulfate (SDS), dodecyl trimethyl ammo
P. Xie and S. C. Rand Division of Applied Physics, 1049 Randall Laboratory Michigan 48109-1120
University of Michigan, Ann Arbor,
(Received 1 July 1993; accepted for publication 23 September 1993) We report a new Er3+:LiYF4 cryogenic 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 enhancementare examined. Recent demonstrations1-5of room-temperature upconversion lasers have stirred interest in their potential as practical sourcesof short wavelength radiation for display and data storage applications, as well as for communications and ultrashort pulse generation at visible and ultraviolet wavelengths.6To date however, little consideration has been given to the relative merits of various fundamental mechanismsfor achieving upconversion, basic limits to achievable degreesof upconversion (ratio of upper laser level energy to incident photon energy), or to questions of stability of the highly nonlinear pumping processesinvolved 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 fluorescenceobserved 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 com, ,,,transition. ponents of the 2H9/2* 4I 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, assignedto 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 observedslope efficiency was 0.09%.
The excitation spectrum of laser emissionrevealeda 3125
Appl. Phys. Lett. 63 (23), 6 December
one-to-one correspondencewith 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 dependenceexpected 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 metastablemanifold in which cooperative inter‘;;
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z z 2 : E;
Wavelength ‘G ,m
(b) *H 1u2 *s 32
h F : w 8 B L z k x 3 l.SBm excitation 7-t -
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
@ 1993 American Institute of Physics
no n “P
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 resonancesshould appear in the excitation spectrum. The results in Fig. 3 therefore provide ample evidence that absorption of light by excited ions does not occur and that the present fourfold laser operatesby cooperative means. This argument also rules out mechanisms combining cooperative and ESA processes.For example, ESA of 1.5 pm light from 4S’,,, or 2H11,2states populated by trio upconversion” 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 spectrum. ESA transitions from 4S3,2to 4G11,2are still farther off resonance.On the other hand, quartet upconversion can provide very nearly resonant excitation of the 4G,1,2state lying immediately above the upper laser level [generating the fluorescenceat 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 argument of the final state gives the Stark level of the acceptor. Taken together with the close match between the excitation spectrum and ground state absorption (Fig. 3), this argues strongly against any contributions by ESA pro-
Wavelength FIG. 3. Infrared excitation spectrum of fourfold upconversion laser emission at 701.5 nm (T= 10 K, lower trace) and, for comparison, the Er3+ absorption spectrum ( T=9 K, inverted). 3126
Appl. Phys. Lett., Vol. 63, No. 23, 6 December
FIG. 4. Mechanism for enhanced quantum efficiency in cooperative upconversion systems. Following an initial excitation step (by direct pumping or upconversion), a cascade from higher to lower excited states occurs with branching ratios of vi at each step. Wiggly arrows indicate nonradiative decay. A given atom can emit more than one photon per no excitation 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 questions as to just how effective higher order cooperative upconversion processescan be in general. In this regard, we wish to point out that an enhancement mechanism exists for cooperative upconversion processeswhich can significantly 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 without external pumping to enhance quantum efficiency by a factor of ( 1 ---r],71~r7s~Q -‘, where qUPis the m-fold cooperative upconversion efficiency. vl, v2, and q3 are branching ratios for the decay processesindicated in Fig. 4. Energy conversion between input wavelength iii, and output at 4,, can be much higher than the maximum value ~7~ =/2in//2out(100/m ) % basedon 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 (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 approach 100% [for vi=1 (i=O,1...3) with a negligible energy defect such that il,,,=ili,/( 1 -Q,)]. In practice, theoretical enhancement of quantum efficiency by cooperative dynamics must be mitigated by losses due to an increasing number of intervening (radiative) levels on real atoms with increasing m. Branching ratios are typically less than unity. Also a statistical decrease is to be expected in the number of m-atom “clusters” with appropriate interatomic spacings, less than a nominal critical radius. However, the main conclusion is that a recycling mechanism exists whereby cooperative dynamics can mediate unexpectedly efficient upconversion emission for arbitrarily high degrees(m) of upconversion. P. Xie and S. C. Rand
This has important implications not only for upconversion lasers, but also for other lasers in which cooperative nonlinear dynamics occur due to heavy rare-earth doping. One example is the 2.8 pm Er laser reported by Stoneman and Esterowitz. I4 These authors obtained an efficiency in practice which was marginally greater than the maximum theoretical quantum efficiency (ignoring cooperative dynamics). A calculation of enhanced quantum efficiency using the formula above, ignoring spontaneous radiative losses for simplicity, shows that for 980 nm pumping, complete recycling of terminal level population through pair upconversion results in a “renormalized” quantum efficiency of 2 (instead of 1) and a maximum energy efficiency of 70% (instead of 35% ). Similar but less dramatic improvements due to cooperative enhancement of quantum efficiency are expected for 2.8 pm lasers pumped by other wavelengths.8 Cooperative enhancement probably also accounts for the remarkable efficiency of the “self-terminating” Er trio laser at 855 nm.” In the present experiment, we note that the 701.5 nm upconversion laser line terminates on the 4Z11,2level of trivalent erbium. This level is known15 to be effective in pair upconversion to the 4F7,2 level, but is not the cooperative pumping level responsible for our inversion. The 4Z1312 level, not the 4Ztt,2 level, is responsible for cooperative pumping when excitation is tuned to 1.5 pm.9’1’This situation therefore resembles, though it is not identical to, the general enhancement scheme of Fig. 4, and we suggest that enhanced upconversion efficiency through cooperative recycling may help account for successful operation of this laser, with its unprecedented (fourfold) degree of upconversion. In summary, we have demonstrated operation of an upconversion laser in which population inversion is sustained in an upper laser level at nearly four times the incident photon energy,16through a fourfold upconversion process. The upconversion mechanism is thought to be exclusively cooperative in nature, arising from an atomic “quartet” interaction quite distinct from sequential energy transferI or multiphoton absorption. The general notion of recycling of excited state population has been introduced and discussed as a possible mechanism for enhancing mth
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order upconversion processesand accounting for successful operation of the present fourfold upconversion laser. Extension of this result to fourfold laser operation at 410 nm, 555 nm, and other wavelengths appears feasible. In the case of the 555 nm transition, the terminal level would be in the 4Z13/2 manifold, and especially strong cooperative enhancement of the efficiency may be possible. The authors gratefully acknowledge research support by the Air Force Office of Scientific Research (H. Schlossberg). P. Xie wishes to express gratitude to the CUSPEA program for graduate travel support. ’R. J. Thrash and L. F. Johnson, in Technical Digest of the Conference on Compact Blue-Green Lasers (Optical Society of America, Washington, DC, 1992), paper ThB3. 2 J. Y. Allain, M. Monerie, and H. Poignant, Electron. Lett. 26, 261 (1990). 3R. G. Smart, D. C. Hanna, A. C. Tropper, S. T. Davey, S. F. Carter, and D. Szebesta, Electron. Lett. 27, 1307 (1991). 4S. G. Grubb, K. W. Bennett, R. S. Cannon, and W. F. Humer, in Digest of the Conference on Lasers and Electra-Optics (Optical Society of America, Anaheim, CA 1992), paper CPDII. ‘D. S. Funk, S. B. Stevens, and J. G. Eden, IEEE Photon. Technol. Lett. 5, 154 (1993). 6P. Xie and S. C. Rand, Opt. Lett. 17, 1116 (1992); Opt. Lett. 17, 1822 (1992). ‘P. Xie, Ph.D. dissertation, University of Michigan, 1992. sP. Xie and S. C. Rand, J. Opt. Sot. Am. B (in press). 9P. Xie and S. C. Rand, Opt. Lett. 17, 1198 (1992). “M. R. Brown, K. G. Roots, and W. A. Shand, J. Phys. C Solid State Phys. 2, 593 (1969); S. M. Kulpa, J. Phys. Chem. Solids 36, 1317 (1975). “P. Xie and S. C. Rand, Appl. Phys. Lett. 60, 3084 ( 1992). 121n our earlier publications, unsaturated 4.S’,,2 emission intensity was shown to depend on the cube, not the fourth power, of incident intensity, thereby contradicting a sequential energy transfer model for visible upconversion with A,,= 1.5 pm in erbium requiring four transfers. Sequential pair processes and sequential multiphoton absorption processes are ruled out not only by our previous work on time dependence of upconversion fluorescence, but also by the excitation spectra which are not convolutions of ground and excited state resonances. 13F. Auzel, J. Lumin. 45, 341 ( 1990). 14R. C. Stoneman and L. Esterowitz, Opt. Lett. 17, 816 (1992). “H Chou, Ph.D. dissertation, Massachusetts Institute of Technology, Tech. Rep. 26, February 1989. 16A preliminary report of these results was first presented by S. C. R. at the Annual Meeting of the Optical Society of America, Albuquerque, New Mexico, Sept. 20-25 ( 1992). invited paper MJ2. Results at different output wavelengths have been reported by R. M. Macfarlane, E. A. Whittaker, and W. Lenth, Electron. Lett. 28, 2136 (1992).