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1 AMORPHOUS and glassy CHALCOGENIDES, HIGH TECH MATERIALS for FIBERS, MEMORIES, SENSORS, and OPTICAL SIGNAL PROCESSING M. Frumar, B. Frumarova 1, T. Wagner, P. Nemec and M. Hrdlicka Research Center and Dep. General and Inorg. Chemistry, University of Pardubice, Pardubice, Czech Republic, 1 Joint Lab. of Solid State Chem. of Inst. Macromol. Chemistry, Acad. Sci. of Czech Rep. and Univ. of Pardubice, miloslav.frumar@upce.cz Glass Tutorial Series: prepared for and produced by the International Material Institute for New Functionality in Glass An NSF sponsored program material herein not for sale Available at IMI Penn

2 Amorphous chalcogenides: high optical transmissivity in IR region, high index of refraction, high optical non-linearity, large free volume large photoinduced changes of structure and physico-chemical properties Many present and potential applications: in optics and optoelectronics: lenses, microlenses, filters, coatings (antireflection, ), planar optical and integrated circuits and devices, IR lasers, acousto-optic elements, optical signal processing (light up-conversion, signal couplers, frequency mixing, amplifiers, possibly all optical computing...) IMI Penn

3 chemical sensing and environmental monitoring -IR, chemical and ionic sensors (uncladed fibres - evanescent waves, membranes), fibers for remote sensing spectroscopy (chemistry, ecology), biomedical - X-ray and diagnostical sensors, surgical IR lasers, tissue removal, evanescent waves, in vivo measurements, eye safe radars, in communications fibers, lasers, signal processing,.. rare-earths doped chalcogenides - luminescence in NIR and MID IR spectral regions: light amplifiers, generators, eye-safe lasers, environmental and other sensors. fibres: telecommunications, power delivery, mechanical and temperature sensing, chemical and remote monitoring (medicine, in vivo spectroscopy, ) large photostructural effects: photoresists, waveguides, gratings, holography, phase-change optical and electrical memories, diffractive elements,photoinduced dissolution and diffusion of metals IMI Penn

4 large free volume: photostructural effects, doping by Ag +, Li + or other ions thin solid-electrolytes (batteries, ionic selective sensors). The phase-change memories based on Ge-Sb-Te and other telluride based films - commercially applied in DVD disks with capacity up to 50 Gb/disk, going to be increased. Ovshinsky universal (unified) memories (OUM), multilevel data storage IMI Penn

5 1. Preparation, bulk glasses, thin films, sputtering, pulsed laser deposition (PLD) - bulk glasses mostly direct synthesis in evacuated silica tubes thermal quenching, annealing - thin films, of binary, ternary or multinary systems, vacuum thermal evaporation (TE) is not suitable fractional evaporation or nonstoichiometry. possible solution: (PLD). Evaporation by (ns or fs) pulses of high intensity and energy, excimer lasers emitting in UV region, (e.g. KrF, 248nm, ~5eV 480kJ/mol., W/cm 2 ) nearly any material can be evaporated, every pulse evaporates all components, independently on their vapor pressure. The stoichiometry is often preserved. IMI Penn

6 The kinetic energy (temperature) of evaporated particles ~ kev the reactions among the particles or fragments are quicker. the structure of some ablated films can be closer to the structure of target glass new materials are obtained (contrary to thermal evaporation). IMI Penn

7 bulk glass Reduced Raman intensity / a. u. TE film PLD film Reduced Raman intensity / a. u. bulk glass TE film exposed annealed PLD film Raman shift / cm Raman shift / cm -1 Raman spectra of bulk As 2 Se 3 glass and As-Se thin films IMI Penn

8 The PLD films of As 2 Se 3 contain less wrong As-As and Se-Se bonds and As 4 Se 4, As 4 Se 3, Se 8 or Se n particles. probably due to higher kinetic energy of the plasma-plume particles: enables their interaction (Eqs. 1-3) densities of As 4 Se 4, As 4 Se 3, Se n are lowered, e.g. As 4 Se 4 + (2/n) Se n = 2 As 2 Se 3 (1) As 4 Se 3 + (3/n) Se n = 2 As 2 Se 3, (2) As-As + Se-Se = 2 As-Se (3) In As-S system: during evaporation - thermal dissociation, change of composition 2As 2 S 3 = As 4 S 4 + S 2, (4) para-realgar (As 4 S 4 ) is formed at high temperatures IMI Penn

9 Wavenumber (cm -1 ) PLD As 2 S 3 ~As 50 S 50 1 as-evap. 2 expos. 3 anneal. Reduced Raman intensity (a.u.) IMI Penn

10 The photoinduced processes are enabled by relatively large free volume. It is inversely proportional to the ω ω = M S δ = M S ( Ax / ρ ) ( Ax / ρ) ρ i i i i i Ax i i (5) low coordination number of chalcogens (mostly N c 2), and of pnictides, (e.g. N c (As) 3). Irreversible photoinduced changes: homogenization, chemical reactions among fragments formed by dissociation during evaporation, e.g. As 4 S 4 + S 2 = 2As 2 S 3 (6) IMI Penn

11 Exposure increases the rates of synthetic reactions, also some photolytic reactions. The reversible photoinduced changes are connected with changes of local bonding configurations. For As-S, As-Ga-S, As-Sb-S, and Sb 2 S 3,Ge-Sb-Sfilms and bulk glasses, a model has been proposed. 2 As S h ν (7), i I i As As + S S IMI Penn

12 supported by Raman spectroscopy results, As 2 S 3 as-evaporated exposed annealed Raman Intensity (a. u.) Raman shift (cm -1 ) IMI Penn

13 Sb 2 S 3 thin film (Khν) 1/2 (cm -1 ev) 1/ fresh evaporated exposed Energy [ev] IMI Penn

14 I rel I rel Raman shift (cm -1 ) Raman shift (cm -1 ) 1. As 42 S 58 fresh evaporated film 1. As S annealed film 2. mixture of 0.85As 40 S As 4 S 4 2. mixture of 0.9As 40 S As 4 S 4 3. mixture of 0.8As 40 S As 4 S 4 3. mixture of 0.85As 40 S As 4 S 4 IMI Penn

15 As 42 S 58 1 fresh 2 exposed 3 annealed 1 2 I rel frequency (cm -1 ) IMI Penn

16 Dependence of (Khν) 1/2 on photon energy in As 40 S 60 films. K is the absorption coefficient (cm -1 ), hν is photon energy As 40 S 60 film fresh evaporated film, 2 fresh annealed 3 fresh annealed exposed (khν) 1/2 [cm -1/2 ev 1/2 ] fresh annealed exposed annealed 5 fresh exposed E (ev) IMI Penn

17 T (%) bulk glass (d=2.45mm) As 38 S As 40 S As 42 S Wavelength [nm] IMI Penn

18 3 1 2 Reduced Raman spectra of thin films (d = 1000 nm) of 95As 2 S 3.5GaS. Raman intensity (a. u.) 1: annealed film, 2: film exposed after annealing, 3: film was annealed after exposure. The change of amplitudes corresponds to reversible structural changes, Raman shift (cm -1 ) the band (235 cm -1 ) with increased amplitude corresponds to As-As vibrations IMI Penn

19 In PLD films of As 2 Se 3, the exposure increases the index of refraction, and following annealing the index of refraction decreases densification and thermally-induced expansion of the amorphous layers TE film a Index of refraction, n, is different for bulk glasses, PLD and TE films, n can be larger for PLD films Singapore 11 04,, 19 Index of refraction PLD film exposed as-deposited annealed Wavelength / nm annealed exposed as-deposited b IMI Penn

20 Nonlinear properties signal processing The refractive index, n = n o + n 2 <E 2 >, n 0 and n 2 do not depend on intensity of light, n 0 >> n 2. The <E 2 > is the mean square of electric field. The fast part of nonlinear index of refraction, n 2, is connected with nonlinear electron polarizability, P NL P = χ (1) E + P NL, (8) P NL = χ (2) E 2 + χ (3) E 3. (9) The P is polarizability, χ (1) is the linear optical susceptibility, χ (2) and χ (3) are second- and thirdorder non-linear optical susceptibilities, respectively. IMI Penn

21 The linear optical susceptibility of isotropic medium is given by relation χ (1) = (n 2-1)/4π. (10) The dependence of (n 2-1) -1 vs. (ħω) 2 is often linear and this dependence can be described by single oscillator formula of Wemple and DiDomenico n 2 (ω) 1 = E 0 E d /(E 02 -(ħω) 2 ), (11) where E 0 is single oscillator energy, E d is so called dispersion energy and ħω is energy of light. IMI Penn

22 for long wavelengths, χ (1) = E d /4πE 0. For optically isotropic glasses, the χ (2) is zero. The χ (3), as a dominant nonlinearity in all glassy IMI Penn materials, produced by excitation in the transparent frequency region well below the band gap, E opt g. Only χ (3) contributes to the nonlinear behavior. IMI Penn

23 For the region far from resonance (Miller s rule) χ (3) A(χ (1) ) 4 x10-10 esu. (12) For single oscillator (Wemple and DiDomenico) χ (1) = 4π(E E d 2 0 E 0 ( hω) 2 ) (13) χ (3) = [E d E 0 /4π(E 02 ћω) 2 ] 4 x 10-10, (esu). (14) E 0 is single oscillator energy, E d is dispersion energy. For ћω 0, 4 ( ) 4 χ (3) A E = d A 2 = n 1 = 4.02x10-15 (E d /E 0 ) 4 (esu) (15) (4π ) E0 (4π ) A 1.7 x10-10 (for χ (3) in esu) and n 0 is the linear index of refraction. IMI Penn

24 The values of n 0 are high in chalcogenides and are significantly higher in PLD films in comparison with TE films. The χ (3) also higher. Using the formula (15) : As 2 S 3 glass: χ (3) = ( ) x10-12 esu GeS 2 glass, χ (3) = 1x10-12 esu, SiO 2 glass, χ (3) = 2.8x10-14 (esu) for λ= 1900 nm. larger χ (3) lower necessary power and shorter the interaction lengths Chalcogenides - promising candidates for optical switching and other applications IMI Penn

25 Silver containing chalcogenide glasses, photodoping. Exposure controlled dissolution of Me and diffusion of Me + Model, Applications: Lithography Diffractive optical elements, (diffractive gratings, ) IR components, waveguides, microlens arrays,...) Opticalstorage Nonlinear optics IMI Penn

26 Phase diffraction gratings Microlenses arrays IMI Penn

27 TE Sb-S + OIDD Ag b t = 20 [s] EXP 0,2 [mm] 250 Relative Diffraction Intesity [arb. u.] b amorphous layer(background) exposed dot (t EXP = 20 s) orthorhombic Sb 2 S 3 cubic AgSbS Θ[ ] IMI Penn

28 Eutectic tellurides, recently: Sn 16 Te 84, In 13 Te 87, Sb 11 Te 89. Melting t C Bulk samples- crystalline ablated films amorphous, good optical transmittance up to 18 µm, untill now- many droplets Excimer laser pulses crystallization, prevailing Te crystals IMI Penn

29 IMI Penn

30 RE 3+ doped glasses Intensive luminescence in NIR and MID-IR part of spectrum Low phonon energy: the number of phonons to bridge the energy gap between the electron levels of RE ion, Ε, is large multiphonon relaxation rate is low! The non-radiative decay rate, ω p, due to multiphonon relaxation, Miyakawa-Dexter: ω p = ω 0 exp(α Ε/ħω), (16) α = ln(p/g) - 1, p = Ε/ħω, g is the electron-phonon coupling strength, and ω o is a host dependent constant. IMI Penn

31 For a high luminescence efficiency also: high index of refraction, higher values of spontaneous emission probabilities, large emission cross-sections of radiative electron transitions between energy levels of RE 3+ ions. IMI Penn

32 luminescence spectra excitation wavelength 1064 nm 1 G 4-3 H 5 (Ge 30 Ga 5 Se 65 ) 99.5 (Pr 2 Se 3 ) 0.5 bulk glass thin film (PLD) Intensity / a.u. 3 F 3-3 H Wavelength / nm IMI Penn

33 luminescence spectra excitation wavelength 1064 nm (Ge 30 Ga 5 Se 65 ) 99.5 (Dy 2 Se 3 ) 0.5 bulk glass thin film (PLD) 6 F11/2, 6 H 9/2-6 H 15/2 Intensity / a.u. 6 F 9/2, 6 H 7/2-6 H 15/ Wavelength / nm IMI Penn

34 2) (As 2 S 3 ) 0.95 (In 2 S 3 ) 0.05 doped by Sm The luminescence, near IR region (bands near 1130, 1240, 1290, and 1490 nm), typical for Sm 3+ ions; Intensity [a.u.] TE PLD 6 F 9/2-6 H 5/2 and/or 6 F 11/2-6 H 7/2 6 F 11/2-6 H 9/2 and/or 6 F 9/2-6 H 7/2 and/or 6 F 7/2-6 H 5/2 6 F 11/2-6 H 11/2 and/or 6 F 9/2-6 H 9/2 and/or 6 F 7/2-6 H 7/ Wavelength [nm] IMI Penn

35 Judd-Ofelt intensity parameters were evaluated the measured oscillator strengths f cal (( S, L) J, ( S, L ) J ) 2 8π mυ = 3h (2J + 1) = 2 2 ( n + 2) () t Ω (, ) (, t S L J U S L ) 9n t= 2,4, 6 J 2 (17) f cal are calculated oscillator strengths, h is Planck s constant, m is electron mass, υ is mean wavenumber of the absorption band, J is the ground-state total angular momentum of Sm 3+ (J=5/2), n is the refractive index of the material, Ω t Judd-Ofelt phenomenological intensity parameters and the are the reduced matrix elements of the tensor operator, U (t) of rank t. are the () ( S,L) J U ( S,L )J t Values of the Judd-Ofelt intensity parameters, e. g. for Sm 3+ ions in Ge-Ga-Se glasses: Ω 2 = x cm 2, Ω 4 = x cm 2, Ω 6 = x cm 2. IMI Penn

36 From the Judd-Ofelt intensity parameters, Ω t : The spontaneous transition probability A of an electric-dipole transition: π e n n h (2J + 1) λ 9 [( ) ( ) ] ( ) () t, L J; S, L J = Ω ( S, L) J U ( S, L ) A S 2 t= 2,4,6 t J 2 (18) e is the electron charge and is the average wavelength of the transition. λ The value of A is high in studied glasses. IMI Penn

37 Conclusion: Chalcogenide glasses and films posses many interesting properties applicable in optics, optoelectronics, data storage, in chemistry, medicine and biology. Acknowledgements The support of Ministry of Education of the Czech Republic under the projects LC 523, KONTAKT ME 690, , is highly acknowledged. Thank you for invitation and your kind attention IMI Penn