Investigation Of PbS Quantum Dot-doped Glasses And Glass Fibers With Broadband Near-infrared Emission

Fiber lasers are considered as one of the most important studies in the field of optoelectronics,due to their small size and high gain properties.Nowadays,the gain media of fiber lasers mainly concentrate on rare earth(RE)ions doped glass fibers.However,owing to the shielding of 6s and 5d orbits,the emission wavelength of RE ions is almost fixed and the emission bandwidth is very narrow(less than 80 nm).Thus,it is still a great challenge to find highly efficient broadband luminescent materials in near-infrared(NIR)regions using RE ions as activators.PbS quantum dots(QDs)can achieve strong quantum confinement effect,due to their 0.41 eV band gap and large exciton Bohr radii(18 nm),exhibiting intense tunable broadband NIR emissions.Herein,Pb S QD-doped glasses with tunable broad band NIR emissions were fabricated.We studied energy transfer process between QDs and temperature-dependent photoluminescence(PL)of QD-doped glasses.And optical amplification at 1550 nm was detected.By using“melt-in-tube”technique,PbS QD-doped glass fibers avoiding uncontrollable crystallization during fiber-drawing process were fabricated successfully.And under excitation of 808 nm laser,tunable broadband emission in the region from 1000-1700 nm were detected from PbS QD-doped glass fibers,which will lay the foundation for developing broadband fiber amplifiers and tunable fiber lasers.Main results of this paper are as follow:(1)PbS QD-doped silicate glasses were fabricated by using PbO and ZnS as the precursors.After heat treated at different temperature,Pb S QDs with diameter from 3 to 6 nm were precipitated in the glasses.Excited by 460 nm light,NIR emissions in the region from 900 to1700 nm were detected.PL spectra and PL lifetime of PbS QD-doped glass were measured,which indicated that the total emission was from 1S-1S state combined with trap state.Energy transfer process from smaller QDs with high energy to larger QDs with low energy was found by measuring the lifetime of different emission bands in one PL spectrum of PbS QD-doped glasses.Temperature-dependent PL spectra and the lifetime decay curves of PbS QD-doped glass with QDs sizes of 5.2 nm were studied.As the test temperature increased from 10 to 300K,the PL intensity and lifetime decreased along with PL peak blue shift.Moreover,the PL intensity from intrinsic state was independent with the test temperature,while the PL intensity from trap state dominated the whole temperature-dependent PL.Herein,a schematic temperature-dependent band gap structure of PbS QDs was illustrated the mechanism of temperature-dependent PL of PbS QD-doped glasses.The full width at half maximum(FWHM)of QD-doped glasses increased while life time decreased as the excitation energy increased.Finally,optical amplification at 1550 nm of PbS QD-doped glasses were detected,indicating that PbS QD-doped glasses will be a promising gain medium for fiber amplifiers and lasers.(2)While using rod-in-tube technique,the drawing temperature is dependent on the softening temperature of the cladding glass.And this drawing temperature is usually much higher than the crystallization temperature of QDs(>350 ~oC).The very rapid growth of QDs at the relative higher fiber-drawing temperature makes their sizes easily over their Bohr radii,resulting in high transmission loss and PL quenching of the fiber.So,the“melt-in-tube”technique was used to fabricated PbS QD-doped glass fibers.In this method,the precursor fiber was drawn at a temperature higher than the melting temperature of the glass core rod,namely the core rod was already melted when the glass-cladding tube was softened,making it possible to avoid the rapid uncontrollable growth of QDs that usually occurred in the fibers made by the conventional rod-in-tube method.Therefore,the QDs can be controllably precipitated in the glass fiber core successfully by the heat treatment.Firstly,PbS QD-doped silicate glass was used as the core glass and a commercial silica was used as the cladding glass.The precursor fibers were draw at 1830 ~oC.Nevertheless,we find that element-migration and volatilization of sulfur simultaneously happened during the whole fiber-drawing process,which eventually led to optical loss and PL quenching,because of the huge difference(>350 ~oC)between the melting temperature of the core glass and the fiber drawing temperature.Sequently,we developed a QD-doped borate glass with low melting temperature as the core glass and a borosilicate glass with low softening temperature as the cladding glass.The precursor fibers were drawn at 1000 ~oC.From elememtal mapping and Raman mapping,no obvious element migration between the core and cladding glass was observed and the precipitation of PbS QDs was well confined in the fiber core.Under excitation of 808 nm laser,tunable broadband NIR emission ranging in 1000 to 1700 nm was obtained.However,it was probable that the core glass would be under anisotropic stresses and even shattered during rapid cooling after the fiber-drawing and heat treatment,leading to high loss over 27 dB/m at 1530nm,due to the large difference in the thermal expansion coefficient between the core and cladding glass.At last,we designed a QD-doped borosilicate glass with smaller thermal expansion coefficient as the fiber core and a commercial K9 glass tube with larger thermal expansion coefficient as the fiber cladding.Therefore,the transmission loss of the fibers was reduced to 16 dB/m at 1530 nm.