Laser assisted processing of nanocrystalline (Ho 0.05 Y 0.95 ) 2 Ti 2 O 7 films for infrared photonics

. The existence of new highly thermally and chemically stable active optical materials is a challenging task for current photonics research targeted on high-power lasers. Holmium-doped titanates crystallizing in the pyrochlore lattice, represent a promising class of materials. However, their high processing temperature limits their applications in integrated optical devices. This weakness can be overcome by laser assisted processing as an alternative to common heat-treatment. The amorphous thin films were prepared by a sol-gel method followed by a dip-coating process and densified in a rapid thermal annealing furnace. The densified films were annealed by a CO 2 laser beam. The laser irradiation induced a crystallization process resulting in the formation of nanocrystalline (Ho 0.05 Y 0.95 )Ti 2 O 7 . The prepared film of a thickness 576 nm exhibited an optical transmission of 91.66% close to the maximum theoretical limit of a silica substrate. The film's refractive index at 632 nm was 2.219. The formation of the nanocrystals caused the activation of the electronic transition 5 I 7 → 5 I 8 at 2  m and the emission bands showed the distinct Starks splitting which is characteristic for (Ho 0.05 Y 0.95 )Ti 2 O 7 phosphors. The presented approach can be used to prepare transparent luminescence films with tailored optical properties by CO 2 laser treatment and together with direct laser writing can be used to prepared integrated optical waveguides as an alternative method to common heat-treatment processes.


Introduction
The lasers are one of the human breakthroughs that have changed the modern world [1].A great scientific effort has been dedicated to increasing the lasers power and shifting their operating wavelengths to infrared range.The existence of new highly thermally and chemically stable active optical materials; which are the heart of such a lasers; is therefore a challenging task for current photonics research.Holmiumdoped titanates have been widely investigated for their luminescence properties [2,3].Low-phonon pyrochlore lattice supports the radiative energy transfers improving the efficiency of high-power lasers and amplifiers operating around 2 m [3].However, their high crystallization temperatures limit the preparation of nanocrystalline coatings and their effective exploitation in integrated devices.A CO2 laser assisted treatment can solve this challenge representing a promising alternative to the common heat-treatment technologies.
In this contribution we present a versatile approach to the preparation of highly transparent films of (Ho0.05Y0.95)2Ti2O7by CO2 laser assisted treatment.We evaluate structural and luminescence properties of the films.The elaborated approach can be used to prepare active optical components operating at 2 m.

Experimental
The films were prepared by dip-coating of sol which was prepared according to previously demonstrated approach [3].Ten layers were subsequently dip-coated onto a silica glass substrate (Technical glass products, USA) by a withdrawing speed of 100 mm•min −1 .Each layer was densified in a rapid thermal annealing furnace AccuThermo AW410 (Allwin21 corporation) at 700 °C for 60 s under an oxygen flow of 5 l•min -1 and a heating rate of 10 °C•s −1 .The densified films were irradiated by a continuous CO2 laser SYNRAD 48-1 (Coherent) emitting at 10.6 μm.A laser beam was expanded to a 5mm circular spot, a power density of the laser beam was of about 20 mWmm 2 .The densified film was irradiated for 120 s.The X-ray diffraction (XRD) was performed on a Bruker D8 diffractometer operating with the Cu-K radiation.UV-VIS transmission spectra were recorded with a Lambda EZ 210 (Perkin-Elmer).The thickness of the films were measured on the optical profilometer NewView 7300 (Zygo).Luminescence spectra were recorded on a IHR320 monochromator (Horiba Jobin Yvon) equipped with PbS detector (EOS).The luminescence was excited by a laser diode emitting at 450 nm (OSRAM) chopped at a frequency of 15 Hz.The detected signal was analyzed by a lock-in amplifier SR510 (sensitivity 1 V, time constant 300 ms).

Results and discussion
The nanocrystalline structure must be precisely tailored to activate the radiative transition in the infrared region and minimize optical losses due to scattering of radiation on the nanocrystals.Fig. 1 shows the XRD patterns of the densified film and the film irradiated by CO2 laser for 120 s.The broad peaks located within the range 15-35 deg corresponded to the diffraction pattern of the fused silica glass substrate.The densified film did not exhibit any shape peaks revealing the existence of crystal lattice.The irradiated films exhibited a set of narrow peaks which was identified as a diffraction pattern of major Y2Ti2O7 according to ICCD data file record number 85-1584.The central positions of the diffraction peaks were slightly displaced suggesting the incorporation of Ho 3+ ions into the host lattice of Y2Ti2O7.The formation of the nanocrystals had a major effect on the films transmission spectra as can be seen in Fig. 2.After laser treatment, the number of interference patterns decreased, the pattern became better pronounced, the local extremes shifted to shorter wavelengths and the absorption edge shifted from 293 nm up to 325 nm.The transmittance maximum 93.24%, which corresponds to the value of 93.39% predicted for pure silica glass substrate [4], decreased to the value of 91.66% and the peak minimum decreased from the value of 82.32% to 70.13%.According to the Swanepoel's approximation [5], theses transmittances corresponded to the increasing of the refractive index at 632 nm from the value of 1.848 to 2.219.The films thickness decreased from 693 nm to 576 nm.The optical properties can be further adjusted by changing the laser irradiation parameters.Fig. 3 demonstrates the steady-state luminescence spectra of the irradiated films under excitation at 450 nm.Despite the densified amorphous films did not provide any detectable optical response, the formation of the nanocrystals caused the activation of the electronic transition 5 I7→ 5 I8 at 2 m.The emission bands showed the distinct Starks splitting which is characteristic for (Ho0.05Y0.95)2Ti2O7 phosphors [3].Fig. 3. Steady-state luminescence spectrum of (Ho0.05Y0.95)Ti2O7films irradiated by CO2 laser for 120 s under excitation at 450 nm.

Conclusions
A versatile approach to the preparation of highly transparent films of (Ho0.05Y0.95)2Ti2O7by CO2 laser assisted treatment was demonstrated.
We demonstrated the transparent nanocrystalline films can be prepared by CO2 laser irradiation.The laser treatment induced the formation of nanocrystalline phase of (Ho0.05Y0.95)2Ti2O7causing the refractive index raised up to 2.219.The nanocrystalline film exhibited a strong luminescence at 2 m which corresponds to electronic transition 5 I7→ 5 I8 of Ho 3+ ions.
The presented approach can be used to prepare transparent luminescence films with tailored optical properties by CO2 laser treatment and together with direct laser writing can be used to prepare integrated optical waveguides.

Fig. 1 .
Fig. 1.XRD records of the densified film and film irradiated by CO2 laser for 120 s.

Fig. 2 .
Fig. 2. Transmission spectra of the densified film and the very same film irradiated by CO2 laser for 120 s.
This work was supported by the Czech Science Foundation, contract No 22-17604S.