Upconversion. How to get high-frequency laser light from longer wavelength sources?

Transcription

1 Upconversion How to get high-frequency laser light from longer wavelength sources? A K 3 Li 1.97 Nb 5.03 O (KLN) crystal converting 820 nm infrared light to 410 nm blue light by second harmonic generation M.Ouwerkerk, Adv. Mater. 3 (1991) 399

2 Need for upconversion? Research in the early 90s: How can we get compact-sized lasers that emit cw visible light at room temperature? Applications: High-density optical recording (DVD, Blu-Ray Disc) Barcode reading, laser printing, and many more Sources available: InGaAs (λ em 800 nm), AlGaAs (λ em 800 nm) laser diodes Idea: Laser-diode-pumped upconversion fiber laser (single mode) M.Oomen, Adv. Mater. 3 (1991) 404

3 Basic upconversion processes

4 Efficiency of basic upconversion processes Mechanism E Example sequential energy transfer 10 3 YF 3 :Yb 3+, Er 3+ two-photon absorption 10 5 SrF 2 :Er 3+ cooperative sensitization 10 6 YF 3 :Yb 3+, Tb 3+ cooperative luminescence 10 8 YbPO 4 second harmonic generation KH 2 PO 4 (KDP) two-photon excitation CaF 2 :Eu 2+ G. Blasse, B. C. Grabmaier, Luminescent Materials, Springer Verlag, Berlin 1994, ISBN

5 Upconversion materials Upconversion hosts need to be transparent at absorption and emission wavelenghts (problem in polymers) need to host upconversion-active materials (dopants or host itself) in required concentrations (dopants: problem in crystals) should not interact with dopants (phonon-electron interaction) but: sometimes interaction is required (relaxation to long-lifetime states) glasses and fibers made of heavy-metal-fluorides Upconversion dopants need of energy states resonant to absorption wavelength (with no alternate transitions in this energy region) no-emission energy states need to have a long lifetime and a sufficient effective absorption cross-section efficiency also dependent on laser power (photon flux), dopant concentration(s) and distibution, non-radiative relaxation trivalent rare-earth ions (Er 3+, Yb 3+, Nd 3+, Tm 3+, Pr 3+,...) but also works with U 3+, Np 4+, Ni 2+

6 Two-photon absorption (TPA) in Tm 3+ -doped glass Tm 3+ under excitation with λ ex = 650 nm first excitation to 3 F 2 relaxation to 3 H 4 (long-living excited state) second excitation to 1 D nm emission to 3 F 4 Tm 3+ under excitation with λ ex = 665 nm first excitation to 3 F 3 relaxation to 3 F 4 (or via the path shown above) Transition probability for each absorption: f second excitation to 1 G nm emission to 3 H 6 A. P. Otto et al., J. Non-Cryst. Solids 265 (2000) 176 G. Özen et al., J. Luminescence 63 (1995) 85

7 TPA plus energy transfer in TmP 5 O 14 Tm 3+ under excitation with λ ex = 659 nm 457 nm emission just as in the doped glass additional 365 nm excitation cross-relaxation between two Tm 3+ 1 G 4 3 H 5 ; 3 H 6 3 H 4 1 D 2 3 F 2 ; 3 H 6 3 H 4 1 G 4 3 H 4 ; 3 H 6 3 H nm emission is not observed 3 H 4 is much more populated energy transfer between two Tm 3+ to 3 P 1 level 347 nm emission to 3 F 4 energy transfer via Tm O P O Tm bonds X. B. Chen et al., J. Luminescence 69 (1996) 151, E. Bielejec et al., J. Luminescence (1997) 62

8 Upconversion efficiency in the Tm 3+ example Dependence on laser power two-step process (365 nm, 457 nm, 472 nm emission): F P 0 2 thee-step process (347 nm emission): F P 0 3 Dependence on concentration F population of excited state three-step process: r 10 dependence (only observed in TmP 5 O 14 ) 365 nm emission about 100 times stronger in Tm 0.1 La 0.9 P 5 O 14 compared to TmP 5 O 14

9 Two-photon absorption (TPA) in Er 3+ -doped glass Er 3+ under excitation with λ ex = 800 nm first excitation to 4 I 9/2 excitation to 4 S 3/2 (long-living excited state) via 2 G 9/2 or 4 F 5/2 with subsequent relaxation processes 550 nm emission ( 4 S 3/2 4 I 15/2 ) is dominant additional emission observed at 410 nm ( 2 H 9/2 4 I 15/2 ), 530 nm ( 2 H 11/2 4 I 15/2 ), and 660 nm (see next slide) M. Takahashi et al., J. Appl. Phys. 81 (1997) 2940 S. Tanabe et al., Phys. Rev. B 45 (1992) 4620 A. Brenier et al., J. Luminescence 69 (1996) 131

10 One-photon absorption plus energy transfer in Er 3+ Energy transfer in Er 3+ works even at low concentrations ( mol-%) F(550 nm) [Er 3+ ] (depends linearly on concentration) F(660 nm) [Er 3+ ] 2 (depends on distance of Er 3+ ions) M. Takahashi et al., J. Appl. Phys. 81 (1997) 2940

11 Er 3+ upconversion dependencies Er 3+ upconversion characteristics Luminescence at 530/550 nm... stays constant with excitation wavelength is quenched by any PO 4 2 content drops with rising temperature Other emission lines behave different! M. Takahashi et al., J. Appl. Phys. 81 (1997) 2940 S. Tanabe et al., Phys. Rev. B 45 (1992) 4620

12 Energy transfer upconversion by Yb 3+ /Er 3+ codoping Energy transfer Yb 3+ Er 3+ at λ ex = 970 nm First example of upconversion, reported in 1966 by Auzel (host material: CaWO 4 ) Yb 3+ acts as a sensitizer (excitation efficiency is much higher compared to Er 3+ ) Er 3+ acts as acceptor emission after two energy transfers Dependence on host lattice: C. J. DaSilva et al., Appl. Phys. B 70 (2000) 185 Host lattice Intensity α-nayf YF 3 60 BaYF 5 50 NaLaF 4 40 LaF 3 30 La 2 MoO 8 15 LaNbO 4 10 NaGdO 2 5 La 2 O 3 5 NaYW 2 O 6 5

13 Energy transfer upconversion by Yb 3+ /Tm 3+ codoping Energy transfer Yb 3+ Tm 3+ at λ ex = 970 nm Yb 3+ acts as a sensitizer (excitation efficiency is much higher compared to Er 3+ ) emission after three energy transfers to Tm 3+ F P 0 3 Yb 3+ also increases efficiency of 470 nm Tm 3+ emission under 683 nm excitation: [Yb 3+ ] (mol-%) Efficiency T. Miyakawa, D. L. Dexter, Phys. Rev. B 1 (1970) 70 G. Özen, J. Non-Cryst. Solids 176 (1994) 147

14 A more complicated upconversion scheme from Nd 3+, Yb 3+, Tb 3+ ground states, only Nd 3+ can be excited by 800 nm light Nd 3+ transferres energy to Yb 3+ which acts as a sensitizer for Tb 3+ note the cooperative sensitization process from Yb 3+ to the Tb 3+ : 5 D 4 level back energy-transfer from Tb 3+ : 5 D 4 to Nd 3+ states limits upconversion efficiency J. Qiu et al., J. Luminescence 86 (2000) 23

15 White light from IR upconversion Co-doping of Yb 3+, Er 3+, Tm 3+ in a special glass (30% AlF 3, 20% CaF 2, 22.6% YbF 3, 15% BaF 2, 11.4% MgF 2, % TmF 3, % ErF 3 ) with excitiation at 980 nm Good color simulation! J. E. C. da Silva et al., J. Alloys Comp. 344 (2002) 260

16 Upconversion A polemic résumé Upconversion is funny AlGaN / InGaN laser diodes developed nowadays (and GaP already used in DVD drives) will be much more efficient! Thanks to A. Osvet for providing me with specialized literature and to R. Müller for the textbook about luminescence materials. The sheets you have seen were intentionally left (almost) black & white!