Luminescence properties of chlorine molecules in glassy SiO 2 and optical fibre waveguides

Glassy SiO2 is the basic material for optical fibre waveguides and manufacturing-induced Cl impurities reduce their transparency in UV spectral range. This work reports in-depth study/spectroscopic parameters of the near-infrared (1.23 eV) lowtemperature photo-luminescence (PL) of interstitial Cl2 molecules in SiO2. The zero-phonon line position was estimated at 2.075 eV on the basis of anharmonicity of Cl2 PL vibronic data. The vibronic sub-bands are broadened by coupling to phonons and by an additional contribution from the glassy disorder. The Huang‒Rhys factor is ≈13. The PL decay time is between 1 and 10 ms in the temperature range 100 K‒13 K and can be reproduced by 3 exponents. Cl2 PL retains relatively high quantum yield and its characteristic structured shape, when the temperature is increased from 13 K to the liquid nitrogen temperature. This allows using it conveniently as a high-sensitivity diagnostic tool for detecting Cl2 impurities in optical fibre waveguides. Timeresolved measurements of optical fibre waveguides indicate that the lower detection limit is below 10 Cl2/cm.


INTRODUCTION
* Synthetic glassy SiO 2 , also called silica glass, quartz glass, amorphous SiO 2 , is the key material for optical fibre waveguides, ultraviolet (UV), high power laser and radiation-resistant optics.For these applications, the purity of the material is of utmost importance and partsper-billion level has been reached for most impurities.One of the remaining exceptions is chlorine.It is left in glassy SiO 2 due to the synthesis process by oxidizing SiCl 4 or by the "drying" step where the silanol (bound hydroxyl ≡Si-O-H) groups are removed in Cl 2 ambient.Typical Cl concentrations are 1-1000 ppm.The "dry" (low silanol content) glasses tend to have larger concentrations of Cl compared to the "wet" SiO 2 glasses.Only few types of synthetic SiO 2 glass, obtained by sol-gel technology [1] or from siliconorganic raw-materials have Cl concentrations below 1 ppm (e.g., commercial types Suprasil UVL or Corning 7979, produced from octamethylcyclotetrasiloxane OMCTS, [(CH 3 ) 2 SiO] 4 ).
The main chemical form of chlorine in glassy SiO 2 is silicon chloride group (≡Si-Cl).It worsens the optical transparency of silica in the deep-UV and vacuum-UV spectral regions [2].The Cl impurities increase the irradiation-induced attenuation of optical fibres (e.g., ref. [3]).The low-Cl-content (<20 ppm) dry "KS-V4" silica [4] radiation-resistant fibres are considered among the best for visible spectral range optical plasma diagnostics in ITER reactor.In the presence of interstitial O 2 molecules Si-Cl groups can be transformed to interstitial Cl 2 molecules [5], which give rise to 328 nm (3.78 eV) optical absorption band [2].This absorption band is typically observed in many commercial optical fibre waveguide cables designed for the near-infrared (NIR) applications.
We have recently reported [6] that interstitial Cl 2 molecules in glassy SiO 2 give rise to a NIR-range photoluminescence (PL) band at 1.2 eV, distinguished by a characteristic vibronic structure due to a coupling to the Cl-Cl vibrations.The luminescence is observable at temperatures below 140 K.Since this PL: (1) retains a significant intensity at the liquid nitrogen temperature, 78 K; (2) has characteristic shape, distinguishing it from other broad and non-structured luminescence bands in glassy SiO 2 ; (3) has long lifetime (≥1 ms) enabling time-resolved separation of the emitted light, it could be possibly used as a high sensitivity tool to detect the presence of Cl 2 and its photochemical reactions in optical fibre waveguides.The purpose of the present work is to obtain further spectroscopic characteristics of interstitial Cl 2 molecules in glassy SiO 2 and to test their detection in optical fibre waveguides.

Samples
Conventional "bulk" samples of glassy SiO 2 and optical fibres were studied.The bulk samples were of size 5 mm × 10 mm × 12 mm 3 and had all faces optically polished.They contained 7 × 10 17 Cl 2 /cm 3 , as determined by the amplitude of the 328 nm optical absorption (OA) band, using the OA peak cross section for isolated Cl 2 molecules [7].Two 1 m long samples of multimode optical fibres were studied: UV-optimized fibre for 1200-200 nm region (core diameter 600 μ) and NIR-to-visible range optimized fibre (2200-400 nm, core diameter 400 μ).Both fibres had undoped silica core and fluorine doped cladding, numerical aperture was 0.22.UV fibre contained ≈800 ppm SiOH groups, while the NIR fibre contained less than 5 ppm SiOH.

Instrumentation
The PL of the bulk sample was measured in Herefrigerator cryostat in the conventional right-angle geometry.Excitation sources were 374.71 nm GaN diode laser (Oxxius OXV 375), 473 nm DPSS laser (Lasever) or 150 W Xe lamp (Oriel) through 200 mm double monochromator (AMKO LTI).A mechanical chopper was placed into the excitation beam for time-resolved measurements.CW-mode and time-resolved emission was measured with 300 mm spectrograph (Andor Shamrock 303i, 150 l/mm grating) and cooled Si CCD (Andor DU971-UVB).In time-resolved mode, the CCD exposure was synchronized with the chopper dark periods.Prior to the exposure, a CCD cleanout cycle had to be performed, introducing ≈4 ms jitter in readout timing relative to the chopper signal.PL decay kinetics was obtained using Hamamatsu R955 photomultiplier and FastComtec 7882 multichannel counter.
The PL of fibre-optic waveguides was measured in the back-scattering configuration (Fig. 1).For low-temperature measurement, ≈80 cm of the 100 cm long fibre was immersed in liquid nitrogen.A careful selection of the filters blocking the excitation laser light was necessary to reduce the admixture of their luminescence to the measured PL spectrum.
PL emission spectra at λ >850 nm were distorted by etaloning (light interference fringes) in the back-thinned CCD and by 2nd overtone absorption of SiO-H vibration at 930 nm in the optical fibre cable used to input light into the spectrograph.In the bulk sample PL measurements, these distortions were corrected by normalizing against the spectrum of a calibrated halogen lamp (Ocean Optics DH2000-CAL).In the case of optical fibre PL, this calibration was inefficient due to different patterns of CCD illumination during measurements and calibration, which yielded different positions of interference fringes on CCD.Therefore, these PL spectra are presented in uncorrected form.The attenuation of the optical fibres was measured using a CCD spectrograph (Hamamatsu 10082CAH, spectral resolution 1 nm) and a D 2 /halogen light source (Ocean Optics DH2000), both equipped by solarizationresistant 100 μ core fibre cables with SMA-1 connectors.The light was injected into, and extracted from the fibre under investigation, (core diameter 400 or 600 μ) by placing its ends nearly in contact with the centres of the end surfaces of SMA-1 connectors of both cables.After taking the spectrum, the middle fibre was removed, the two fibres were connected by SMA-1 adapter, and the reference spectrum through two fibres was recorded.

RESULTS
The optical absorption spectra of the NIR fibre and of the bulk sample show band at 3.78 eV with amplitudes 0.00311 and 0.17 cm -1 , (1.35 and 74 db/m), respectively (Fig. 2).This band is absent in the UV fibre.NIR fibre shows as well bands at 5.0 eV and 5.8 eV due to divalent Si atoms ("oxygen deficiency centres") and Si dangling bonds ("E centres") [8,9].
The inset of Fig. 3 (top curve) shows the PL spectrum of the bulk sample measured at T = 13 K and 473 nm excitation.The non-structured part of this spectrum can be approximated by a Gaussian (peak 1.215 eV, FWHM 0.39 eV).Subtracting it from the measured spectrum yields the pure vibronic component (Fig. 3 inset, bottom trace).The distances between the neighboring peaks are plotted against an arbitrary chosen sequential number of the vibronic band (Fig. 3, diamonds).The least squares linear regression line was calculated (Fig. 3, the solid line).) and 74 db/m (0.17 cm -1 ) for the bulk sample.The band at 5.0 eV is due to divalent Si ("SiODC"), the band at 5.8 eV is due to Si dangling bonds ("E centres").
PL excitation (PLE) spectra of the bulk sample are shown in Fig. 4. The PLE spectra, measured for each separate vibronic PL sub-band (inset of Fig. 3) nearly coincide (Fig. 4, top).The absence of correlation between the excitation and emission energies is further illustrated by the symmetric shape of the 2D excitation/ emission map (Fig. 4, bottom).Fig. 3. Photoluminescence spectrum of interstitial Cl 2 molecules in glassy SiO 2 , measured at T = 13 K and excitation 473 nm (inset, top curve), the approximation of its unstructured part by Gaussian (inset, middle curve) and the difference spectrum, showing pure vibronic sub-bands (inset, bottom).The hollow diamonds show the differences between the positions of the adjacent sub-bands.The crossing point of linear regression fit line with the line drawn at the Raman frequency corresponds to the vibrational quantum number v = 1 (upper X scale).This reference point is used to assign v values to the sub-bands in the inset.Figure 5 shows the PL decay kinetics of the bulk sample, measured at 13 K and 80 K with a 473 nm excitation, and the non-linear least squares fits by the sum of 3 exponents.Alternative fit models by 1 or 2 exponents or by stretched exponent gave worse results.Increasing the exponent count to 4 did not increase the accuracy.The proportion between the contributions of the 3 exponents remained roughly the same as the temperature was increased from 13 K-100 K.
The PL spectra of the "UV"-type optical fibre waveguide did not show any luminescence in NIR region both at room temperature and after its immersion in liquid N 2 (78 K).In contrast, "NIR" type fibre showed very strong NIR luminescence after cooling to 78 K (Fig. 6A).The time-resolved spectrum (Fig. 6B) and its difference to the CW spectrum (Fig. 6C) reveal that the emission shifts to larger energies at longer delays.

Anharmonicity and zero-phonon energy
The linear increase of the distance between the neighboring vibronic bands, evident in the plot of Fig. 3 is typical for the coupling of PL emission transition to slightly anharmonic vibration mode.In the Morse potential model [10], the energies E(ν) of the vibronic bands in PL spectrum are given by where v is the ground state vibrational quantum number, E 0 is the excited-state zero-phonon energy, ω is the ground-state harmonic vibration frequency (in cm -1 units) and χ is the anharmonicity constant.The slope of the fit, 2ωχ, is 6.2 cm -1 , giving anharmonicity parameter ωχ = 3.1 cm -1 .Extrapolating the fit on Fig. 3 up to the independently measured Raman frequency of interstitial Cl 2 in SiO 2 (546.2cm -1 [6]), gives the number of the corresponding sub-band (Fig. 3, bottom scale) as 12.98 ≈ 13.This sub-band has vibrational quantum number v = 1 and subsequently, v values can be assigned to the all lower energy sub-bands (Fig. 3, top scale and the inset).This assignment of v-numbers is similar to one made for Cl 2 molecule in Ar cryocrystal matrix [11].The natural isotope abundance of Cl 2 molecules is 35 Cl-35 Cl: 57.4%, 35 Cl-37 Cl: 36.7%, 37 Cl-37 Cl: 5.9%.If isotope-weighted averages of the positions of the sharp vibronic lines observed in [11] are calculated, their values differ less than 50 cm -1 from the respective positions of sub-bands measured for Cl 2 in glassy SiO 2 (Fig. 3).Zero-phonon (v = 0) line (ZPL) position, determined from the fit is at 16732 cm -1 (2.074 eV, 597.6 nm).ZPL is obviously unobservable, since the Huang-Rhys factor, the average number of vibrational quanta, created during relaxation to the ground state is S ≈ 13 and ZPL intensity is proportional to exp(-S).

Nature of the spectral broadening
Compared to the narrow-line PL spectra of Cl 2 in inert gas crystals (FWHM<8 cm -1 ) [11,12], the PL spectra of Cl 2 in SiO 2 are significantly broadened.Two origins of the broadening are possible: homogeneous broadening due to a strong interaction with SiO 2 matrix phonons and inhomogeneous broadening due to different Cl 2 interstitial sites in amorphous SiO 2 matrix.The homogeneous broadening surely plays a role, since the phonon energies in SiO 2 are much higher (up to 1300 cm -1 ) than in Ar crystals (<75 cm -1 [13]).At the first glance, the observed (Fig. 4) complete absence of any spectral shifts in PL emission when excitation photon energy is changed, could corroborate that assumption and indicate a relatively small inhomogeneous broadening.However, the excitation at energies above 3 eV goes to a repulsive state (A 1 Π u ) [14].The luminescence from the lower-lying triplet state (a 3 Π u ) can occur at this excitation only because Cl 2 is prevented from dissociation by the cage effect of the surrounding SiO 2 matrix [6].Under these conditions, any correlation between the excitation and emission energies, which is characteristic to inhomogeneously broadened luminescence, may be lost.
A clue for the relative size of inhomogeneous broadening can be found from the kinetics and timeresolved measurements.An analysis of the decay kinetics (Fig. 5) indicates the presence of several exponents or of a continuous distribution of decay constants.In contrast, in Ar matrix a single lifetime (τ = 76 ms) was found, independent of temperature or of the Cl isotopic species [12].The time resolved spectra (Fig. 6) show a clear blue shift of the spectra by ≈40 cm -1 at longer decay times, the difference spectrum (Fig. 6 C) is shifted by ≈80 cm -1 .These shifts are, however, relatively small compared to the halfwidth of the vibronic bands (240 cm -1 ).Evidently, the main broadening comes from electron-phonon coupling and ≈10-20% comes from different environments of Cl 2 in the interstices of glassy SiO 2 .

Detection threshold of Cl 2 in optical fibre waveguides
The PL spectrum of Cl 2 in optical fibre (Fig. 6A) was obtained for an optical fibre, containing 8 × 10 15 Cl 2 /cm 3 , as calculated from the amplitude of the 3.73 eV absorption band (Fig. 2) and Cl 2 absorption cross section, 2.58 × 10 -19 cm 2 [7].The intensity of the PL, obtained at the total exposure time of 18 s (30 × 0.6 s) was well over 10 3 times above the noise level.Further, the exposure time, the excitation power (1 mW) and the length of the cooled section of the fibre (0.80 m) all can be easily increased by a factor of 10.This gives an additional dynamic range of order 10 3 × 10 × 10 × 10 = 10 6 times as compared to the present measurement and gives the detectivity estimate of better than 10 10 Cl 2 /cm 3 .It is much better than the typical sensitivity (≈ 10 17 Cl/cm 3 ) of electron microprobe (EMPA) analysis.However, the PL technique is sensitive only to molecular Cl 2 , while EMPA detects all forms of chlorine.This can provide an opportunity to follow the early stages of formation of Cl 2 in photo-radio-chemical reactions [5] in the types of fibres initially containing chlorine as SiCl groups or in other chemical forms.

CONCLUSIONS
Following our recent work [6], this study provides further spectroscopic details on photoluminescence of interstitial Cl 2 in glassy SiO 2 .This PL represents one of the very few observations of vibronic structures in the luminescence of pure or light-element doped glassy SiO 2 .
The other instances comprise interstitial O 2 , dangling oxygen bonds [8], interstitial sulphur S 2 molecules [15] and not yet identified silicon-oxygen-related species in silica nanoparticles [16].The luminescence of Cl 2 is easily distinguished from other common PL bands in glassy SiO 2 [8] by its spectral position, shape and lifetime.
Given the importance of chlorine in the manufacturing processes of glassy SiO 2 and optical fibres, this PL has potential application for monitoring Cl-related processes in SiO 2 .

Fig. 1 .
Fig. 1.Setup for investigation of low-temperature near-infrared photoluminescence of optical fibre waveguides.

Fig. 2 .
Fig. 2. Optical attenuation spectra of studied samples.(A) UV-optimized optical fibre waveguide; (B) visible to nearinfrared range optimized optical fibre; (C) bulk sample of dry synthetic glassy SiO 2 .The amplitude of the 3.78 eV band for NIR-fibre is 1.35 db/m (0.00311 cm -1) and 74 db/m (0.17 cm -1 ) for the bulk sample.The band at 5.0 eV is due to divalent Si ("SiODC"), the band at 5.8 eV is due to Si dangling bonds ("E centres").

Fig. 4 .
Fig. 4. Excitation spectra of interstitial Cl 2 in bulk SiO 2 sample at 13 K, measured at different PL emission energies.Top: excitation spectra for individual vibronic bands (bands with v = 13 to v = 7 in the inset of Fig. 3).Bottom: excitation/emission 2D landscape map.

Fig. 5 .
Fig. 5. Photoluminescence decay kinetics at T = 13 K (top) and 80 K (bottom) measured with 473 nm excitation and integrated for the 650-850 nm emission region.Solid lines show fit by a sum of 3 exponents.

Fig. 6 .
Fig. 6.Comparison between the photoluminescence spectra of optical fibre waveguide ("NIR" type), measured in CW-mode (A) and time-resolved mode (B).The time-resolved spectrum is arbitrary normalized to match the CW spectrum.Difference spectrum (C) illustrates the blue-shift of the time-delayed emission.The fine spectral features at λ > 850 nm are experimental artefacts (see the Materials and methods section).