FLUORIDE GLASSES – MATERIALS FOR BULK LASERS AND FIBRE OPTICAL AMLIFIERS Michał Żelechower, Silesian University of Technology, Katowice, Poland

1.What are fluoride glasses? 2.The role of rare earth elelments 3.Interaction of electromagnetic radiation with matter a. Scattering, absorption, spontaneous and stimulated emission b. Reconstruction of electron energy structure c. Radiative and non-radiative transitions 4.Real structure of fluoride glasses 5.Applications – advantages and disadvantages (drawbacks)

What is it? Fluoride glasses can be formed by total replacement of oxygen atoms in oxide glasses by fluorine atoms They are manufactured by melting of high purity single element fluorides mixture

HEISENBERG’S UNCERTAINTY PRINCIPLE  E~2· eV   t~1h  E~10 eV   t~ s FREE ATOM  SOLID ENERGY

Energy diagram showing two atoms encountering and resulting in a new molecule

DIELECTRICS VALENCE BAND FORBIDDEN BAND (ENERGY GAP) CONDUCTION BAND ENERGY E g > 2 eV EMPTY FULL EFEF

DOPED DIELECTRICS VALENCE BAND CONDUCTION BAND (EMPTY) DOPED IONS LEVELS USED IN LASER ACTION FOR INSTANCE RARE EARTH ELEMENTS IN GLASSES

RARE EARTH IONS IN CRYSTALS AND GLASSES

TABLE 1. CONVERSION FACTORS FOR ENERGY UNITS Unitjoule electron voltcm –1 joule × × electron volt × 10 – cm – × 10 – × 10 –4 1

EXAMPLE : CONVERSION OF ENERGY IN JOULES TO CM -1 Given: A HeNe laser photon has a wavelength of nanometers Find: (a) Photon energy in joules (b) Photon energy in cm –1 Solution :

THE INTERACTION OF RADIATION WITH MATTER Small no. of states -almost transparent Large no. of states -strongly absorbed Energy X-rays Ultraviolet Visible Infrared Microwaves Ionisation energy Rotation Vibration Electronic level changes Phototionisation Scattering

ATOM MUST RETURN FROM EXCITED STATE TO GROUND STATE. HOW?

SEVERAL WAYS TO RETURN TO GROUND STATE

QUANTUM YIELD OF LUMINESCENCE

SEVERAL WAYS TO RETURN TO GROUND STATE. LIFETIMES

FLUORESCENCE VERSUS PHOSPHORESCENCE

Spin multiplicity A state can be specified by its spin multiplicity (2S+1). No. unpaired electrons SMultiplicityState 0 S = 0 2S + 1 = 1singlet 1  S = 1/22S + 1 = 2doublet 2  S = 12S + 1 = 3triplet 3  S = 3/22S + 1 = 4quartet S 0 ground state singlet S 1, S 2 ……excited state singlets T 1, T 2 ….…excited state triplets SYMBOLS USED IN ATOMIC PHYSICS

Pr Eu Ho Er Tm Wavelength [nm] Wavenumber [cm -1 ] Absorbance REE ABSORPTION SPECTRA IN FLUORIDE GLASSES

EACH ABSORPTION LINE CORRESPONDS TO THE RESPECTIVE ELECTRON TRANSITION BETWEEN TWO ENERGY LEVELS (GROUND STATE AND EXCITED STATE) WE ARE ABLE TO RECONSTRUCT THE ELECTRON ENERGY STRUCTURE ON THE BASE OF ABSORPTION SPECTRA

Pr Eu Ho Er Tm RECONSTRUCTED ELECTRON ENERGY LEVELS IN FLUOROINDATE GLASSES Energy [cm -1 ]

SPONTANEOUS EMISSION

E3E3 E2E2 E1E1 P ij = P ji P 23 > P 13 >> P 12 INVERSION N 2 >> N 1  2 >>  3 THREE-LEVEL LASER (TRANSITION PROBABILITIES AND LIFETIMES)

STIMULATED EMISSION

Stimulated Emission Stimulated emission is the exact analogue of absorption. An excited species interacts with the oscillating electric field and gives up its energy to the incident radiation. Emission of Radiation Stimulated emission is an essential part of laser action. UU LL h LL h UU 2h

LIFETIMES OF EXCITED STATES

FOUR-LEVEL LASER (Cr 3+ doped ruby)

E3E3 E2E2 E1E1 E = h· = E 2 – E 1 THREE-LEVEL LASER (quantum amplifier) OPTICAL PUMPING s s

Time-schedule of laser action

To amplify number of photons going through the atoms we need more atoms in upper energy level than in lower. Amplification or loss is just N upper -N lower. N upper > N lower, more out than in N upper < N lower, fewer out than in

PRINCIPLE OF LASER ACTION

NUMBER OF PHOTONS ~ 2 N (N – ACTIVE ELEMENT CONTENT)

LASER RESONANCE SYSTEM

First commercial fluoride glass – about 1990 FLUOROZIRCONATE GLASS ZrF 4 -BaF 2 -LaF 3 -AlF 3 -NaF Acronym - ZBLAN FLUOROINDATE GLASS InF 3 -ZnF 2 -BaF 2 -SrF 2 -GaF 3 -NaF Acronym - IZBSGN Marcel & Michel Poulain and Jacques Lucas discovered first fluoride glass (Univ. Rennes, France) HISTORY Accidentally !!!

ADVANTAGES 1.Low phonon energy 2.Low absorption in IR range 3.Wide transmission band 4.High refraction index

Comparison of various glasses properties to those of silica glasses

A PIECE OF PHYSICS Phonons in a lattice Acoustic branch-wide frequency band Optical branch - almost constant frequency THIS FREQUENCY IS MUCH LOWER IN FLUORIDE GLASSES THAN IN SILICA GLASSES IR light absorbtion in fluoride glasses is much lower than in silica glasses

VIBRATIONS OF DIATOMIC CHAIN – OPTICAL PHONONS

Equation of motion (Newton’s second principle) Disperssion relations

Wavelength TRANSMISSION BAND FLUOROZIRCONATE GLASSES SILICA GLASSES FLUOROINDATE GLASSES

Wavelength [  m] Wavenumber [cm -1 ] Transmission[%] TRANSMISSION BAND – FLUOROINDATE GLASS 0 100

Pr Eu Ho Er Tm ELECTRON ENERGY LEVELS Energy [cm -1 ]

Wavenumber [cm -1 ] Wavelength [nm] Luminescence intensity [a.u.] LUMINESCENCE (IZBSGN) Ho 0.5 % mol. 6 % mol. 0.5 % mol. E [cm -1 ] 6 % mol. EMISSION

E [cm -1 ]0.5 % mol EMISSION (IZBSGN) Ho

Wavenumber [cm -1 ] Wavelength [nm] Luminescence intensity [a.u.] LUMINESCENCE (IZBSGN) Pr EMISSION

E [cm -1 ] EMISSION (IZBSGN) Pr

Wavenumber [cm -1 ] Wavelength [nm] Luminescence intensity [a.u.] LUMINESCENCE (IZBSGN) Er EMISSION

E [cm -1 ] Er EMISSION (IZBSGN)

Wavenumber [cm -1 ] Luminescence intensity [a.u.] Intensywność luminescencji [j.wzgl.] LUMINESCENCE (IZBSGN) Tm Tm + Tb EMISSION

EMISSION (IZBSGN) Tm E [cm -1 ]

EMISSION (IZBSGN) Tm - Tb

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Lifetime [ms] Dopant Level Concentr. [%mol] Experimental  m Computed  rad Quantum efficiency  =  m /  rad [%] LIFETIMES & QUANTUM YIELDS OF DOPED FLUOROINDATE GLASSES

DISADVANTAGES (DRAWBACKS) 1.Substrates are hygroscopic (built-in OH groups result in additional absorption band in IR range) 2.Difference of T X and T g is low (  C) 3.Crystallization susceptibility is high

T g – glass transformation temperature T X – crystallization temperature (beginning) T P - crystallization temperature (peak)  T = T x – T g  HRUBY PARAMETER H = (T X – T G ) / T G  SAAD PARAMETER : S = [(T X – T G ) (T P – T X )] / T G PARAMETERS OF STABILITY

Various dopants in fluoride glass CHARACTERISTIC TEMPERATURES OF FLUORINDATE GLASSES

GLOVE DRY PREPARATION BOX

GLOVE DRY MELTING BOX

Pr 3+ doped fluoroindate glass

REVERSE MONTE CARLO MODELLING (RMC) RIETVELD MODELLING STRUCTURE OF FLUORIDE GLASSES

VARIATION OF GIBBS FREE ENERGY DURING VITRIFICATION AND CRYSTALLIZATION liquid Overcooled liquid glass Single crystal Stable glass Range of structural order

STRUCTURE OF FLUOROZIRCONATE GLASS (ZBLAN) POULAIN & LUCAS 1974

PROJECTION OF THE RMC CUBIC BOX SHOWING THE 300 [MF 6 ] POLYHEDRA NETWORK. EXAMPLE OF RMC MODELLING (NaPbM 2 F 9 )

NaPbFe 2 F 9 [MF 6 ] octahedra are in blue; Na atoms in green and Pb atoms in red

Five [MF 6 ] polyhedra linked by edges as found in the RMC model NaPbM 2 F 9

EXPERIMENTAL VERIFICATION BY NEUTRON DIFFRACTION OR LOW ANGLE X-RAY SCATTERING

SiO 2 - crystalline I coordination zone – 3 at II coordination zone – 3 at III coordination zone – 6 at SiO 2 - amorphous I coordination zone – 3 at II coordination zone – 4 at III coordination zone – 4 at EXAMPLE

LEAST SQUARES FIT TO EXPERIMENTAL RESULTS (NEUTRON DIFFRACTION AND X-RAY SCATTERING) NaPbM 2 F 9 : neutron data for M = Fe

neutron data for M = V LEAST SQUARES FIT TO EXPERIMENTAL RESULTS (NEUTRON DIFFRACTION AND X-RAY SCATTERING) NaPbM 2 F 9 (M = Fe, V)

X-ray data for M = Fe LEAST SQUARES FIT TO EXPERIMENTAL RESULTS (NEUTRON DIFFRACTION AND X-RAY SCATTERING) NaPbM 2 F 9 (M = Fe, V)

REFERENCES