United States Patent (19) (11) 4,183,795 Bloom et al. (45) Jan. 15, 1980 (54) CREATION OF F2+ COLOR CENTERS Attorney, Agent, or Firm-Daniel D. Dubosky 75) Inventors: David M. Bloom, Holmdel; Linn F. 57 ABSTRACT Molenauer, Colts Neck, both of N.J. Production of F2+ color centers in alkalihalide crystals 73 Assignee: Bell Telephone Laboratories, with densities high enough for efficient use in lasers is Incorporated, Murray Hill, N.J. achieved by a two-step, highly selective photoioniza 21 Appl. No.: 908,774 tion mechanism. This mechanism is coupled with the use of suitable divalent metal ions as efficient and stable 22 Filed: May 23, 1978 electron traps to allow nearly 100 percent conversion of 51) Int. C.’................................................ B01J 1/10 F2 color centers to F2 color centers. The two-step 52 U.S. C. .................... 204/157.1 R; 204/DIG. 11; photoionization mechanism comprises photo-exciting 331/94.5F the F2 color center to the first bound state, which re 58 Field of Search ................................. 204/157.1 R quires lower energy than the corresponding transition for F color centers, then photoionizing the F2 color (56) References Cited center from the first bound state, and finally, capturing U.S. PATENT DOCUMENTS the electron thus liberated on the divalent metal ions. 3,970,960 7/1976 Mollenauer ..................... 331/94.5F Primary Examiner-Howard S. Williams 8 Claims, 7 Drawing Figures PUMP BEAM FOR LASER OPERATION CRYSTAL WITH W. F; CENTERS \\ \ N-PROCESSING BEAM U.S. Patent Jan. 15, 1980 Sheet 1 of 3 4,183,795 A/G. W. CONDUCTION / BAND 4 N-1-n ----|--- F2 F A/G.2 F+ 8065 F 8067 Fl U U3 2 plpo pupos 805 8056 806 8076 807 M*(F) U2 g M(F2) plpo, , 80 802 802 T use8034. - T 803 804. U.S. Patent Jan. 15, 1980 Sheet 2 of 3 4,183,795. A/G. 3 PUMP BEAM FOR LASER OPERATION WACUUM CRYSTAL WITH W. F; CENTERs \\\ N-PROCESSING BEAM A/G, 4 430 Nd:LASER YAG --- (i.e. E.440 40 402 420 403 40 f/G. 6 FILTER 540 ETHANOL 30 504 505 503 U.S. Patent Jan. 15, 1980 Sheet 3 of 3 4,183,795 AF/G. 5 NoF: OMnt SPECTRA AT 77°K F2 + F. CRYSTAL-0.8mm THICK BEFORE TWO-STEP PHOTOONZATION AFTER TWO-STEP PHOTOONIZATION ER-0 400 5 00 600 700 800 900 O00 OOO OO O 0. O OO 000 LASER POWER DENSITY (u/cm2) 4,183,795 1. 2 ity at temperatures in excess of ~200 K., would desta CREATION OF F2+ COLOR CENTERS bilize the admixture of color center populations. In particular, one would risk creating a variety of F3 BACKGROUND OF THE INVENTION color centers which tend to absorb radiation in the The invention pertains to the field of F2+ color cen 5 region of the F2 luminescence band. ter lasers. SUMMARY OF THE INVENTION The F2 color center in alkalihalide crystals has been shown to provide a nearly ideal gain mechanism for In accordance with the present invention, a method efficient, broadly tunable, optically pumped, CW and for creating F2+ color centers is provided which pulse lasers in the near infrared. This is because: 10 (a) causes an essentially complete conversion of F2 to (1) The small Stokes shift for the infrared transition F2 color centers, implies that the emission band of the F2 color (b) obtains the conversion in the presence of an exter center has the same high oscillator strength (f-0.2) nal electron trap density no greater than that of the as has the absorption band; original F2 color center population, and (2) There is strong evidence for a nearly 100 percent 15 (c) is selective in such a manner that substantially no quantum efficiency; positively ionized species other than the F2 color (3) Using the calculated energies of the H2 molecu centers are produced. lar ion, the splitting between the lowest lying even The method for creating the F2 color centers com parity excited state 2Sag and the 2Pou state can be prises: predicted to be much too large to allow for self-ab- 20 (1) incorporating, for external electron traps, a suffi sorption at the emission energy, and cient number of divalent metal ions, of a size that (4) Thus far there are no known color center species sets well in the crystal lattice, into an alkali halide foreign to the F2 that would absorb photons of crystal; the lower energy emission band other than one (2) creating anion vacancies and F color centers in center type which is probably a variety of F3 25 the alkali halide crystal when it is cooled suffi color center and is easily eliminated or otherwise ciently to prevent vacancy diffusion (such anion avoided. vacancies may be created by such mechanisms as The laser action does not seem to suffer from bleach radiation damage from electron beams, high inten ing or aging effects during normal operation as con sity gamma rays, or high intensity X-ray sources); trasted by such effects in the organic dyes. Also, the 30 required optical pump power at threshold is usually (3) warming the alkalihalide crystal to room temper many times less-than that required for the most efficient ature for a short time to allow F and F2 color dye lasers. centers to form therein; The F2t color center laser in alkalihalides will pro (4) cooling the crystal to laser-operating temperature; vide coverage for laser outputs in the range 0.85N52 35 and um. This region is of fundamental importance to molec (5) irradiating the alkalihalide crystal with the appro ular spectroscopy, pollution detection, fiber optic com priate radiation to provide the two-step photoion munications, and the physics of narrow-band-gap semi ization. conductors. One feature of this invention is that very large densi However, the production of F2+ color center densi ties (~ 1018/cm3) of F2+ color centers are provided. ties high enough for efficient use in lasers, especially in The F2+ color centers are accompanied by little else but those whose cavity modes are tightly focused, has been F color centers. a major source of difficulty. Another feature of this invention is that use with F2 color centers are typically converted to F2 color heavier alkali halides should extend the presently centers by subjecting the F2 color centers to ionizing 45 achieved tuning range of 0.82SAs 1.5 pum (obtained radiation. The conversion is permanent if suitable elec with hosts LiF, NaF and KF) beyond 2 um. tron traps have been provided for the excess electrons Yet another feature of this invention is that the F2 ejected from the F2 color centers. However, there is a color centers may be created with low power laser need for an alternative to the ordinary (single-step) beams. photoionization process for creating F2 color centers. 50 Yet another feature of this invention is that F2 color This need arises because the present methods of creat centers may be created with small inexpensive flash ingF2 or F2 color centers results in densities which are lamps or arc lamps equipped with long-pass filters to considerably outnumbered by accompanying F color avoid direct pumping of the Fabsorption band. centers. The ground states of the F and F2 color centers Yet another feature of this invention is that other lie approximately the same distance below the conduc 55 positively charged color centers such as F3 may be tion band in the crystal and the photoabsorption cross destroyed when the F2 color centers are created. sections and ionization efficiencies of these two color centers are comparable in the single-step photoioniza BRIEF DESCRIPTION OF THE DRAWING tion range. These conditions cause the fractional ioniza A complete understanding of the present invention tion of F2 color centers obtained using the single-step and of the above and other features thereof may be photoionization process to be low. The fractional ioni gained from a consideration of the following detailed zation may be improved by adding external electron description presented hereinbelow in connection with traps, but the density of such external traps must be the accompanying diagram in which: larger than the sum of all ionizable species. Such large FIG. 1 shows a diagram of the relevant energy levels densities of traps are very hard to achieve in practice 65 for the photoionization mechanism for F2 and F color and even if achievable, they would lead to other unde centers; sirable effects such as obtaining a large density of F+ FIG. 2 shows a diagram of the competing physical color centers. The F color centers, due to their mobil mechanisms in the crystal during photoionization; 4,183,795 3 4. FIG. 3 shows the embodiment in which the two-step the possibility of creating F2 centers has been omitted. photoionization mechanism was reduced to practice; This simplification does not substantially effect the re FIG. 4 shows the method for generating the laser sults. Also, the exclusion is justified on physical grounds beam used for the first reduction to practice with a NaF due to the fact that the F2 centers would be highly crystal; volatile at reasonable pumping conditions as well as due FIG. 5 shows the results of absorption spectra which to the fact that no evidence for their existance has been were obtained using a NaF crystal both before and after the two-step photoionization; found in absorption spectra performed on the crystals FIG. 6 shows one method for generating the laser used in the reduction to practice. beam used for the second reduction to practice with a O KF crystal; M + M= M (1) FIG.