1. Physics of, and requirements for laser crystals Blaž Kmetec Put together by: Blaž Kmetec prof. dr. Martin Čopič Supervisor: prof. dr. Martin Čopič Faculty of Mathematics and Physics, Ljubljana 21. 12. 2004

2. Physics of, and requirements for laser crystals2 Contents Foreword Introduction Interactions Material requirements material preparation Representative calculation: nonradiative energy transfer as a result of ion-ion electric dipole interaction Thermal effects in a crystal during laser operation Examples of laser crystals Nd,Cr:GSGG opposed to Nd:YAG 4 Nd:YAG and Nd:YVO 4 Summary

3. 21.12.2004Physics of, and requirements for laser crystals3 Foreword Introduction Interactions Material requirements material preparation Representative calculation: nonradiative energy transfer as a result of ion-ion electric dipole interaction Thermal effects in a crystal during laser operation Examples of laser crystals Nd,Cr:GSGG opposed to Nd:YAG 4 Nd:YAG and Nd:YVO 4 Summary

4. 21.12.20044 Foreword Laser inter eximia naturae dona numeratum plurimis compositionibus inseritur  The Laser is numbered among the most miraculous gifts of nature and lends itself to a variety of applications.  Plinius, Naturalis historia, XXII, 49 (first century A.D.) Kyrenaikan gold drachm showing Laser (Silphion) image

5. Look At Source, Erase Retina D a n g e r o u s, i n s t r u c t i v e, c h a l l e n g i n g Light Amplification by Stimulated Emission of Radiation Legal Amusement of Students, Engineers & Researchers

6. 21.12.2004Physics of, and requirements for laser crystals: Foreword 3/36 Foreword - continued Requirements for laser systems The demand for lower costs improved reliability long-term durability reduced operating costs The demand for improved beam quality The demand for shorter wavelengths the need for UV laser sources in the semiconductor chip industry The demand for shorter pulses Solid-state lasers high power output at relatively low power consumption and with high beam quality High stability and long life expectancy

7. 21.12.2004Physics of, and requirements for laser crystals7 Foreword Introduction Interactions Material requirements material preparation Representative calculation: nonradiative energy transfer as a result of ion-ion electric dipole interaction Thermal effects in a crystal during laser operation Examples of laser crystals Nd,Cr:GSGG opposed to Nd:YAG 4 Nd:YAG and Nd:YVO 4 Summary

8. 21.12.2004Physics of, and requirements for laser crystals8 Introduction Solid-state laser = laser system based on optically active centres (ions) in insulator host materials Components laser crystal = host crystal + active ions its optical spectroscopic properties are vital to its performance mechanism of optical pumping cavity configuration

9. 21.12.2004Physics of, and requirements for laser crystals9 Foreword Introduction Interactions Material requirements material preparation Representative calculation: nonradiative energy transfer as a result of ion-ion electric dipole interaction Thermal effects in a crystal during laser operation Examples of laser crystals Nd,Cr:GSGG opposed to Nd:YAG 4 Nd:YAG and Nd:YVO 4 Summary

10. 21.12.2004Physics of, and requirements for laser crystals: Interactions 1/310 Interactions Complex physical processes static electron-lattice interactions determine the types and position of the electronic energy levels electron-photon interactions determine the strengths of radiative transitions determine the fluorescence lifetime electron-phonon interactions determine the rates of nonradiative transitions determine the temperature-dependent widths and shifts of spectral lines ion-ion interactions cause energy-level splittings and energy transfer between ions Contributions to photon field in the cavity photons injected into the cavity by the pump source photons generated by the optically active ions through spontaneous emission processes stimulated emission processes

11. 21.12.2004Physics of, and requirements for laser crystals: Interactions 2/311 Interactions - continued Optical spectral properties of the laser crystal determined by the electronic transitions of the active ions in the local field environment of the host Types of ions that are useful for laser emission: transition-metal ions Cr 3+, Ti 3+ rare-earth ions Nd 3+, Er 3+ Efficient absorption of pump radiation  strong absorption transition at the of the pump radiation pump source can have broad or narrow emission spectra Generally the terminal state of the absorption is not the level from which laser emission occurs  transition absorbing the pump energy must result in populating the metastable state of the laser transition   requires efficient radiationless relaxation to the desired level without loss of excitation energy to other emission transitions

12. 21.12.2004Physics of, and requirements for laser crystals: Interactions 3/312 Interactions - continued Efficient emission of pump radiation strong laser transition at the of the desired laser output high quantum efficiency

13. 21.12.2004Physics of, and requirements for laser crystals13 Foreword Introduction Interactions Material requirements material preparation Representative calculation: nonradiative energy transfer as a result of ion-ion electric dipole interaction Thermal effects in a crystal during laser operation Examples of laser crystals Nd,Cr:GSGG opposed to Nd:YAG 4 Nd:YAG and Nd:YVO 4 Summary

14. 21.12.2004Physics of, and requirements for laser crystals14 Material requirements Material properties are determined by the properties of the host material, the properties of the optically active ions, and the mutual interaction between the host and the dopant ions The most fundamental requirement for a laser material is  that it can be easily and economically produced with high quality in large amounts and different sizes Stability with respect to local environmental changes such as temperature humidity stress thermal effects, thermal lensing It is possible to put 2 types of ions in the same host material nonradiative energy transfer from the sensitizers to the activators

15. 21.12.2004Physics of, and requirements for laser crystals15 Material requirements

16. 21.12.2004Physics of, and requirements for laser crystals16 Foreword Introduction Interactions Material requirements material preparation Representative calculation: nonradiative energy transfer as a result of ion-ion electric dipole interaction Thermal effects in a crystal during laser operation Examples of laser crystals Nd,Cr:GSGG opposed to Nd:YAG 4 Nd:YAG and Nd:YVO 4 Summary Cr 4+ :YAGNd 3+ :YAG

17. 21.12.2004Physics of, and requirements for laser crystals17 Material preparation Standard techniques pulling from the melt (Czochralski) melt growth (Bridgman-Stockbarger) Czochralski

18. 21.12.2004Physics of, and requirements for laser crystals18 Material preparation Even if the conditions for ideal crystal growth are known, accurate control of these conditions may be difficult Any variations in growth conditions can result in pieces with bubbles, multiple phases, and other defects that  scatter or distort optical beams passing through the crystal. Providing for optically active centres: should be uniformly distributed throughout the host crystal, otherwise spatial variations in lasing properties throughout the crystal occur Accurately knowing the dopant concentration and spatial distribution is one of the major challenges in characterising solid-state materials.

19. 21.12.2004Physics of, and requirements for laser crystals19 Foreword Introduction Interactions Material requirements material preparation Representative calculation: nonradiative energy transfer as a result of ion-ion electric dipole interaction Thermal effects in a crystal during laser operation Examples of laser crystals Nd,Cr:GSGG opposed to Nd:YAG 4 Nd:YAG and Nd:YVO 4 Summary

20. 21.12.2004Physics of, and requirements for laser crystals20 Nonradiative energy transfer as a result of ion-ion electric dipole interaction Photon energy absorbed by the sensitizer moves through the dipole-dipole interaction aided by surrounding lattice relaxation  to the activator (without radiation exchange). Main interaction = Coulomb interaction multipole expansion about the sensitizer-activator separation

21. spatial average

22. 21.12.2004Nonradiative energy transfer as a result of ion-ion electric dipole interaction 3/322 Nonradiative energy transfer as a result of ion-ion electric dipole interaction - continued : radiative decay time of the sensitizer metastable level : line-shape function of the sensitizer emission : absorption cross-section of the activator : refractive index of the host crystal : Förster radius; for good overlap, of range 2 nm – 4 nm

23. 21.12.2004Physics of, and requirements for laser crystals23 Foreword Introduction Interactions Material requirements material preparation Representative calculation: nonradiative energy transfer as a result of ion-ion electric dipole interaction Thermal effects in a crystal during laser operation Examples of laser crystals Nd,Cr:GSGG opposed to Nd:YAG 4 Nd:YAG and Nd:YVO 4 Summary

24. 21.12.2004Thermal effects 1/324 Thermal effects in a crystal during laser operation Diode-pumping of solid-state lasers has greatly reduced the proportion of wasted pump energy which is deposited as heat in the crystal end pumping (longitudinal)side pumping (transversal)  Diode laser prices decline  high pump power available  thermal distortion is again a critical issue in designing diode- pumped solid-state lasers (DPSSL)

25. 21.12.2004Thermal effects 2/325 Temperature gradients result in optical distortions in the rod, mostly through the refractive index variation attributable to deformations caused by thermal stress (photoelastic effect)

26. 21.12.2004Thermal effects 3/326 Thermal effects - continued The perturbation is equivalent to the effect of a spherical lens Optical pump beam cross- section should be larger than resonator beam cross-section (in the pumped region) A contribution to lensing power from the end-effects It is possible to lessen this impact by using composite rods

27. 21.12.2004Physics of, and requirements for laser crystals27 Foreword Introduction Interactions Material requirements material preparation Representative calculation: nonradiative energy transfer as a result of ion-ion electric dipole interaction Thermal effects in a crystal during laser operation Examples of laser crystals Nd,Cr:GSGG opposed to Nd:YAG Nd:YAG and Nd:YVO 4 Summary

28. 21.12.2004Examples of laser crystals 1/328 Er:YAG: lases at 2940nm Examples of laser crystals Nd,Cr:GSGG opposed to Nd(,Cr):YAG Nd:YAG (Nd 3+ :Y 3 Al 5 O 12 ) YAG host properties: hard, grown by Czochralski high thermal conductivity optically isotropic (cubic lattice) doping: Y 3+ is substituted by Nd 3+ the radii differ by 3%  strains occur at high doping how to increase the pump efficiency? idea: a second dopant, like Cr 3+  little improvement, however, achieved furthermore, low laser efficiency for pulsed applications due to (Cr 3+  Nd 3+ time)  (Nd 3+ decay time) High transfer efficiency possible in Nd,Cr:GSGG C O D O P I N G YAG=Y 3 Al 2 Al 3 O 12

29. 21.12.2004Examples of laser crystals 2/329 Codoping: Nd,Cr:GSGG and Nd,Cr:YAG Nd and Cr ions are separated by only 1 nm in Nd,Cr:GSGG  mostly usable with flashlamp pumping  Nd,Cr:GSGG exhibits stronger thermal focusing and stress birefringence GSGG YAG {Gd 1-x Nd x } 3 [(Sc,Ga )1-y Cr y ] 2 Ga 3 O 12

30. 21.12.2004Examples of laser crystals 3/330 Examples of laser crystals - continued Nd:YAG and Nd:YVO 4 Nd:YVO 4 large stimulated cross section the highest efficiency TEM 00 performance ever demonstrated naturally birefringent less sensitive to diode T:  21 (Nd:YVO 4 )   21 (Nd:YAG )  higher pulse rates required for Nd:YVO 4 Nd:YAG better for longer pulses

31. Summary The requirements for laser crystals Laser industryThe requirements for lasers Other industries Physics of laser crystals Interactions electrons light lattice optical sciences solid-state theory physics of deformations

32. Thank you for your attention! Which is Nd:YAG and which Ti:sapphire? Aleft: Nd:YAG, right Ti:sapphire Bleft: Ti:sapphire, right Nd:YAG CNd:YAG and Ti:sapphire spectra are equal