**4. Bioactive glasses**

the fabrication of integrated photonic devices by ULI has shown to be cost-effective and efficient so that a suitable configuration of the processing parameters may lead to waveguide

few microjoules in short period of time, of few femtoseconds, focused beneath the surface in micron-sized areas, results in nonlinear interaction processes. Such high intensities may be manipulated to be employed for modifying the dielectric focal volume, inducing from weak refractive index changes to ultrahigh pressures which lead to void generation. These permanent structural modifications depend not only on the laser peak power but also on the

The ultrashort laser-matter interaction process can be divided into three stages: generation of free carriers inside the material by non-linear processes such as multiphoton, tunnel ionization, or avalanche ionization, followed by energy relaxation processes and a subsequent modification of the material [3, 4]. The absorption process of ultrashort laser radiation for wide bandgap dielectric materials cannot be explained on the basis of linear absorption, since the photon energy of commonly used femtosecond laser pulses with wavelength between UV and NIR does not have sufficient energy to be linearly absorbed. On the contrary, multiphoton absorption can excite an electron from the valence to the conduction band as long as

, where *m* is the smallest number of photons for which the overall energy surpasses

extremely strong laser electromagnetic field. This mechanism in dielectrics permits electron from the valence band to tunnel to the conduction band in a period of time shorter than the laser pulse. However, in most dielectric multiphoton, ionization dominates the excitation processes [78, 79]. On the other hand, laser photons can be sequentially absorbed by electrons excited in the conduction band by means of free carrier absorption. When the energy of an electron in the excited state exceeds the bandgap energy, the ionization of another electron from the valence band can take place, resulting in two excited electrons at the conduction band minimum. These electrons can be excited again by free carrier absorption processes, and more valence electrons can be produced by the same mechanism, leading to the electronic avalanche [3, 4]. The requirement for avalanche ionization is the existence of seed electrons in the conduction band, which can be provided by multiphoton or tunneling ionization or by

Once the nonlinear photoionization and avalanche ionization create a free electron plasma, they transfer their energy to the lattice inducing three types of structural changes: a smooth refractive index modification for low pulse energies such as 100 nJ and 100 fs at 800 nm for 0.6 NA, a birefringent refractive index modification for pulse energies ranging 150–500 nJ and 100 fs at 800 nm for 0.6 NA, and microexplosions which result in void formation for pulse

energies higher than 500 nJ and intensities greater than 100 TW/cm<sup>2</sup>

. In addition, tunneling photoionization can also take place under an

, achieved delivering laser pulses of

[4, 78, 80], **Figure 1**. The

losses below 1 dB/cm, resulting in a low laser operation threshold.

focusing conditions, scanning speed, polarization and repetition rate.

**3. Ultrashort laser interaction with dielectrics**

The high power density, in the order of tens of TW/cm<sup>2</sup>

*mħω > E<sup>g</sup>*

the bandgap energy *Eg*

110 Advanced Surface Engineering Research

thermally excited impurity or defect states.

Formation of glasses with improved properties and manufacturability plays an important role in many technologies. Various noncrystalline materials such as silica glasses, amorphous halides, semiconductors and metals have been used in areas for many engineering applications including biology, communications, electronics, and so on. Glass formation consists basically in avoiding crystallization by cooling from the molten state. The ability to glass formation depends on the melt composition that defines some parameters as glass transition and liquidus temperatures, on the cooling rate which has to be higher enough to avoid the nucleation and growth of the crystalline phases and also on the sample size. As the solidification technique used limits at some extent the cooling rate, the first question in the development of glasses is to determine the composition ranges in which glasses can be obtained under certain cooling conditions (quenching, directional solidification, etc.).

It is known that for multicomponent systems, a strong tendency to glass formation exists near the eutectic points because they have the lower liquidus temperature. Glass formation and its relation to eutectic growth have been investigated for different metal and ceramic alloy systems in terms of the competition between the growth of crystalline phases and the formation of the amorphous phase. The maximum growth rate of a eutectic structure with two or more phases is lower than the maximum growth rate of a single crystalline phase. Therefore, the locations of glass formation in some oxide, halide or metal systems lie near the eutectic area compositions, even though the glass formation also depends on the cooling rate. High liquidus slopes, low mutual solubility of the phases (high eutectic range Cαβ), small diffusion in the melt and large solid/liquid interfacial energies favor the glass formation in competition with the eutectic formed through cooperative growth of the crystalline phases.

Bioactive glasses capable of forming tight chemical bonds to living tissues contain silicates and some of them also phosphates, the first providing a scarcely soluble matrix that compensates for the excess solubility of the latter. The ability of bonding to bone tissue is a result of their chemical reactivity of the surface in a physiological media. The formation of SiO<sup>2</sup> rich layer and calcium phosphate film on the surface of a bioglass implanted in a body and bond with living bone was reported by Hench [81]. Kokubo proposed the use of artificial body fluid as a simulated body environment to estimate the bioactivity of the glasses and ceramics [82].

De Aza et al. showed that glasses and ceramics in the systems containing CaO▬P<sup>2</sup> O5 ▬SiO<sup>2</sup> can be designed to optimize biological and mechanical response. They have studied different microstructures in the wollastonite (CaO.SiO<sup>2</sup> )-calcium phosphate TCP (3CaO.P<sup>2</sup> O5 ) system reporting the presence in the phase diagram of an invariant point at 1402°C and a composition of 80% mol W and 20% mol TCP [83]. It is possible to estimate the eutectic compositions from the values of melting temperature and heat of fusion of the components through the expressions [84]:

$$RT \ln X\_i^l = -\Delta H\_i^u \left(1 - \frac{T}{T\_i^u}\right) \quad i = 1, 2, 3, \dots, k \tag{5}$$

$$\sum\_{l=1}^{k} X\_l^l = \mathbf{1} \tag{6}$$

150 mm/h reported by Pardo et al. for the fabrication of a W-TCP eutectic glass rod of 3 mm in diameter [87]. A detailed description of this technique can be consulted elsewhere [88, 89]. This eutectic glass, produced by the laser floating zone technique, is chemically stable and has a high optical quality being able to be used as a matrix for luminescence active ions. This composition corresponds to an "invert" glass where the modifier content (Ca) is larger than the former content (Si + P). This term was introduced by Trapp and Stevels [90] because the

> and P<sup>2</sup> O5

in normal conditions; however, when the network modifying oxides are in majority on the molar basis, the glasses are structurally inverted compared to conventional glasses. This structural inversion is reflected in the properties of the glasses. In addition, the optical properties of this glass doped with Nd3+ and Er3+ ions have been assessed, resulting in emission crosssections and lifetimes similar to those reported for the best commercial alkaline-silicate glasses [87]. Furthermore, the incorporation of trivalent rare-earth (RE) ions can also be used as local ordering probe due to the close relation between their spectroscopic properties and the local structure and bonding at the ion site, since the spectroscopic properties of trivalent rare-earth ions depend on the chemical composition of the glass matrix, which determines the structure and nature of the bonds, and thus spectroscopic characterization allows studying the local structure surrounding the RE ion and the covalence of the RE-O bond [91, 92]. In particular, site-selective excitation and emission of Nd- and Eu-doped W-TCP glass ceramics provided information about the differences on the spectral features of amorphous and crystalline environments in this matrix, allowing the identification by means of Raman and LIBS of crystal-

SiO4

bioactive glasses. In both cases, the guiding structures were manufac-

well-known bioactive and biocompatible properties [93–95]. These results showed the potential applications of these materials obtained from the LFZ technique as luminescence bioprobes for in vitro applications and promote extended studies to other rare-earth ions, which can be used in biomedical applications such as multicolor bioprobes and biosensors among others.

Both stress-induced and depressed cladding waveguides have been studied in Nd-doped

tured by using the ULI technique. For this purpose, a tunable Ti:Sapphire laser emitting at the central wavelength of 795 nm with 120 fs laser pulses and 1 kHz repetition rate was used. **Figure 2** shows an example of double-line configuration and a depressed cladding waveguide obtained in this glass, (a) and (b), respectively. For the case of the stress-induced waveguide,

**5. Fabrication of photonic devices in W-TCP bioactive glasses**

form continuous molecular/ionic networks

**Calculated/Measured** 

113

**Teut (K)**

http://dx.doi.org/10.5772/intechopen.79577

1670/1675

) and apatite-like structures, both with

traditional network forming oxides SiO<sup>2</sup>

**A (K)**

**Tm B (K)**

Wollastonite TCP 1821 1943 36.844 156.057 80.25–20.61

**Table 2.** Calculated eutectic composition and temperature in W-TCP system.

**Hm A (J/ mol)**

**Hm B (J/ mol)**

**Calculated/Measured eutectic XA-XB (mol%)**

Ultrafast Laser Inscription of Buried Waveguides in W-TCP Bioactive Eutectic Glasses

80–20

**A B Tm**

line phases corresponding to dicalcium silicate (Ca<sup>2</sup>

CaSiO3

▬Ca3

(PO4 ) 2

where *Xl i* is the mole fraction of component *i* in the liquid and *Tm i* and *Hm i* are melting temperature and molar heat of fusion of component *i* in the liquid. The eutectic composition and the eutectic temperature can be calculated with the expressions indicated above if the melting temperature and heats of fusion of the components are known, as shown in **Table 2**.

On the other hand, in order to form an amorphous solid, the melt must be cooled sufficiently fast to prevent the precipitation of the competitive crystalline phases, defining a minimum cooling rate. Lu and Liu [85] used the dimensionless parameter γ = Tx/(Tg + Tl) as an indicator of the glass-forming ability of the system, with Tx, Tg and Tl the crystallization onset, glass transition and melting temperatures, respectively. The relationship between this parameter and the critical cooling rate (Rc) for glass formation is approximately given by the expression:

$$\mathbf{R}\_c = \mathbf{R}\_o \exp.\left[\left(-\ln \mathbf{R}\_o\right)\gamma/\gamma\_o\right] \tag{7}$$

where the constants Ro and γo are, for inorganic glasses, 8 × 1027 K/s and 0.421, respectively. Magallanes-Perdomo et al. described the devitrification and crystallization process of W-TCP eutectic glass, evidencing that the devitrification begins at 870°C with the crystallization of a Ca-deficient apatite phase [86]. Being the glass transition temperature 790°C, the γ parameter is 0.414 and the calculated critical cooling rate *Rc* is about 3 K/s. This rate corresponds to a solidification rate of about 100 mm/h in the Laser Floating Zone (LFZ) technique for an experimental axial thermal gradient of 105 K/m, in good accordance with the growth rate of


**Table 2.** Calculated eutectic composition and temperature in W-TCP system.

liquidus slopes, low mutual solubility of the phases (high eutectic range Cαβ), small diffusion in the melt and large solid/liquid interfacial energies favor the glass formation in competition

Bioactive glasses capable of forming tight chemical bonds to living tissues contain silicates and some of them also phosphates, the first providing a scarcely soluble matrix that compensates for the excess solubility of the latter. The ability of bonding to bone tissue is a result of their chemical reactivity of the surface in a physiological media. The formation of SiO<sup>2</sup>

layer and calcium phosphate film on the surface of a bioglass implanted in a body and bond with living bone was reported by Hench [81]. Kokubo proposed the use of artificial body fluid as a simulated body environment to estimate the bioactivity of the glasses and ceramics [82].

can be designed to optimize biological and mechanical response. They have studied different

reporting the presence in the phase diagram of an invariant point at 1402°C and a composition of 80% mol W and 20% mol TCP [83]. It is possible to estimate the eutectic compositions from the values of melting temperature and heat of fusion of the components through the expressions [84]:

)-calcium phosphate TCP (3CaO.P<sup>2</sup>

*<sup>m</sup>*) *<sup>i</sup>* <sup>=</sup> 1, <sup>2</sup>, 3, …,*<sup>k</sup>* (5)

*<sup>l</sup>* = 1 (6)

[(−ln Ro) γ/γo] (7)

is about 3 K/s. This rate corresponds to

are, for inorganic glasses, 8 × 1027 K/s and 0.421, respectively.

*i* and *Hm i*

De Aza et al. showed that glasses and ceramics in the systems containing CaO▬P<sup>2</sup>

*m* (<sup>1</sup> <sup>−</sup> \_\_\_*<sup>T</sup> Ti*

> *i*=1 *k Xi*

temperature and heats of fusion of the components are known, as shown in **Table 2**.

perature and molar heat of fusion of component *i* in the liquid. The eutectic composition and the eutectic temperature can be calculated with the expressions indicated above if the melting

On the other hand, in order to form an amorphous solid, the melt must be cooled sufficiently fast to prevent the precipitation of the competitive crystalline phases, defining a minimum cooling rate. Lu and Liu [85] used the dimensionless parameter γ = Tx/(Tg + Tl) as an indicator of the glass-forming ability of the system, with Tx, Tg and Tl the crystallization onset, glass transition and melting temperatures, respectively. The relationship between this parameter and the critical cooling rate (Rc) for glass formation is approximately given by the expression:

Magallanes-Perdomo et al. described the devitrification and crystallization process of W-TCP eutectic glass, evidencing that the devitrification begins at 870°C with the crystallization of a Ca-deficient apatite phase [86]. Being the glass transition temperature 790°C, the γ parameter

a solidification rate of about 100 mm/h in the Laser Floating Zone (LFZ) technique for an experimental axial thermal gradient of 105 K/m, in good accordance with the growth rate of

*<sup>l</sup>* = −∆*Hi*

is the mole fraction of component *i* in the liquid and *Tm*

microstructures in the wollastonite (CaO.SiO<sup>2</sup>

∑

R<sup>c</sup> = Ro exp.

and γo

is 0.414 and the calculated critical cooling rate *Rc*

*RT*ln *Xi*

112 Advanced Surface Engineering Research

where *Xl*

*i*

where the constants Ro

rich

O5 ▬SiO<sup>2</sup>

) system

O5

are melting tem-

with the eutectic formed through cooperative growth of the crystalline phases.

150 mm/h reported by Pardo et al. for the fabrication of a W-TCP eutectic glass rod of 3 mm in diameter [87]. A detailed description of this technique can be consulted elsewhere [88, 89].

This eutectic glass, produced by the laser floating zone technique, is chemically stable and has a high optical quality being able to be used as a matrix for luminescence active ions. This composition corresponds to an "invert" glass where the modifier content (Ca) is larger than the former content (Si + P). This term was introduced by Trapp and Stevels [90] because the traditional network forming oxides SiO<sup>2</sup> and P<sup>2</sup> O5 form continuous molecular/ionic networks in normal conditions; however, when the network modifying oxides are in majority on the molar basis, the glasses are structurally inverted compared to conventional glasses. This structural inversion is reflected in the properties of the glasses. In addition, the optical properties of this glass doped with Nd3+ and Er3+ ions have been assessed, resulting in emission crosssections and lifetimes similar to those reported for the best commercial alkaline-silicate glasses [87]. Furthermore, the incorporation of trivalent rare-earth (RE) ions can also be used as local ordering probe due to the close relation between their spectroscopic properties and the local structure and bonding at the ion site, since the spectroscopic properties of trivalent rare-earth ions depend on the chemical composition of the glass matrix, which determines the structure and nature of the bonds, and thus spectroscopic characterization allows studying the local structure surrounding the RE ion and the covalence of the RE-O bond [91, 92]. In particular, site-selective excitation and emission of Nd- and Eu-doped W-TCP glass ceramics provided information about the differences on the spectral features of amorphous and crystalline environments in this matrix, allowing the identification by means of Raman and LIBS of crystalline phases corresponding to dicalcium silicate (Ca<sup>2</sup> SiO4 ) and apatite-like structures, both with well-known bioactive and biocompatible properties [93–95]. These results showed the potential applications of these materials obtained from the LFZ technique as luminescence bioprobes for in vitro applications and promote extended studies to other rare-earth ions, which can be used in biomedical applications such as multicolor bioprobes and biosensors among others.

## **5. Fabrication of photonic devices in W-TCP bioactive glasses**

Both stress-induced and depressed cladding waveguides have been studied in Nd-doped CaSiO3 ▬Ca3 (PO4 ) 2 bioactive glasses. In both cases, the guiding structures were manufactured by using the ULI technique. For this purpose, a tunable Ti:Sapphire laser emitting at the central wavelength of 795 nm with 120 fs laser pulses and 1 kHz repetition rate was used. **Figure 2** shows an example of double-line configuration and a depressed cladding waveguide obtained in this glass, (a) and (b), respectively. For the case of the stress-induced waveguide,

parameters. As an example, **Figure 3** shows the near-field distribution intensity of TE mode in a double-filament waveguide inscribed 150 μm beneath the surface with 15 μm between

Ultrafast Laser Inscription of Buried Waveguides in W-TCP Bioactive Eutectic Glasses

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115

The absorption generated by color centers, appeared during the laser inscription process was reduced by means of isochronal heat treatments thus decreasing propagation losses [96–98]. This annealing process produced an improvement of up to a 29% in the transmitted power in the range of 300–350°C for the double-line waveguide, as shown in **Figure 4(a)**, whereas higher annealing temperatures led to a diminution of the stress-induced between filaments and therefore to a decrease in the refractive index change and consequently in the light confinement. In depressed cladding waveguides, the thermal treatment induced a continuous improvement of the transmitted intensity up to 650°C, based on a gradual removal of color centers and a partial recovery of the refractive index change caused by the release of the stress-induced by the ultrashort laser pulses. Annealing temperatures higher than 650°C reduced the optical barrier and the tunneling losses became non-negligible [99]. The analysis of the evolution of the modal intensity as a function of the thermal treatment temperature confirmed this mechanism, as shown in **Figure 4(b)**, revealing that the light confinement kept almost unaltered until 600°C,

tracks (a), and cladding waveguides (b) of 20 μm (left) and 30 μm (right) diameters.

whereas higher temperatures enlarged the modes hindering the light transmission.

waveguide's volume pointed out no significant modifications, **Figure 5**.

and both luminescence spectra and μL map of the 4

*n* = −*Δ nmax*

which:

bioactive glass [56, 60].

Luminescence characterization carried out in both types of waveguides revealed that spectroscopic properties were well preserved in the volume of the waveguides and that annealing treatment at high temperature recombined the damaged produced in the irradiated areas, resulting in a worsening the light confinement. Fluorescence lifetimes, around 240–250 μs,

The refractive index modification was estimated on the basis of the refractive index distribution generated by femtosecond laser pulses in double-filament waveguides [100, 101], for

> 1 − (*x*/*σx*)<sup>2</sup> \_\_\_\_\_\_\_

**Figure 3.** Near-field distribution intensity of TE mode in a double-filament waveguide with 15 μm between tracks (a) and in depressed cladding waveguides (b) 20 μm (left) and 30 μm (right) diameter produced in the W-TCP eutectic

<sup>1</sup> <sup>+</sup> (*x*/*σx*)<sup>4</sup> exp (*x*/*σy*)

F3/2→ <sup>4</sup>

I11/2 transition in the bulk and in the

<sup>2</sup> (8)

**Figure 2.** Double-line (a) and depressed cladding (b) waveguides produced in the W-TCP eutectic bioactive glass [56, 60].

the guiding characteristics were studied modifying the pulse energy between 0.42 and 1.68 μJ, the scanning speed at 25 and 50 μm/s and the distance between filaments at 15 and 20 μm. Also, in this case, the structures were inscribed at 150 and 250 μm beneath the surface of the sample. As shown in **Figure 2(a)**, the filament dimensions strongly depend on the energy delivered (*Ed* ) by the laser beam, between 8.4 and 67 μJ, achieving lengths from 16 up to 40 μm. For the case of the cladding structure, both the distance between filaments and the scanning speed were set constant at 3 μm and 500 μm/s respectively, studying the guiding properties with the diameter of the core, between 20 and 150 μm, and the depth from the surface, 300 and 600 μm. Worth mentioning is the fact that the fabrication of cladding structures required lower delivered energy than in the case of double-filament waveguides, since the filaments were very close to each other and an excessive amount of energy might create cracks on the irradiated area and hence to fracture the sample. Specifically, in this case, filaments of the cladding structure were produced delivering 1.44 μJ over each focal spot. In addition, the interaction of the ultrashort laser radiation with the glass sample gave rise to an affected zone in the surroundings of the tracks, caused by the generation of color centers. It is well known that high intensity ultrashort laser pulses may modify the properties of the material at atomic scale by the photophysical and photochemical interactions which take place after the laser absorption [3]. These color centers also may affect to both the guiding and luminescence properties.

The guiding properties were assessed by characterization of the near-field intensity distribution with a continuous wave He-Ne laser at 633 nm. Both types of waveguides presented TE and TM propagation. Furthermore, all double-filament waveguides inscribed were found to be monomode, whereas only 20 μm diameter depressed cladding waveguides presented this feature. In addition, for double-filament waveguides, the width found in TE and TM modes was shorter along the x-axis than in the orthogonal one, but the modal area decreased with the pulse energy thus increasing the light confinement. Moreover, the best confinement conditions were achieved for both the shortest track distance (15 μm) and depth (150 μm), and the highest scanning speed (50 μm/s). Concerning the cladding waveguides, there were no remarkable differences depending either on the guided polarization or processing parameters. As an example, **Figure 3** shows the near-field distribution intensity of TE mode in a double-filament waveguide inscribed 150 μm beneath the surface with 15 μm between tracks (a), and cladding waveguides (b) of 20 μm (left) and 30 μm (right) diameters.

The absorption generated by color centers, appeared during the laser inscription process was reduced by means of isochronal heat treatments thus decreasing propagation losses [96–98]. This annealing process produced an improvement of up to a 29% in the transmitted power in the range of 300–350°C for the double-line waveguide, as shown in **Figure 4(a)**, whereas higher annealing temperatures led to a diminution of the stress-induced between filaments and therefore to a decrease in the refractive index change and consequently in the light confinement. In depressed cladding waveguides, the thermal treatment induced a continuous improvement of the transmitted intensity up to 650°C, based on a gradual removal of color centers and a partial recovery of the refractive index change caused by the release of the stress-induced by the ultrashort laser pulses. Annealing temperatures higher than 650°C reduced the optical barrier and the tunneling losses became non-negligible [99]. The analysis of the evolution of the modal intensity as a function of the thermal treatment temperature confirmed this mechanism, as shown in **Figure 4(b)**, revealing that the light confinement kept almost unaltered until 600°C, whereas higher temperatures enlarged the modes hindering the light transmission.

Luminescence characterization carried out in both types of waveguides revealed that spectroscopic properties were well preserved in the volume of the waveguides and that annealing treatment at high temperature recombined the damaged produced in the irradiated areas, resulting in a worsening the light confinement. Fluorescence lifetimes, around 240–250 μs, and both luminescence spectra and μL map of the 4 F3/2→ <sup>4</sup> I11/2 transition in the bulk and in the waveguide's volume pointed out no significant modifications, **Figure 5**.

the guiding characteristics were studied modifying the pulse energy between 0.42 and 1.68 μJ, the scanning speed at 25 and 50 μm/s and the distance between filaments at 15 and 20 μm. Also, in this case, the structures were inscribed at 150 and 250 μm beneath the surface of the sample. As shown in **Figure 2(a)**, the filament dimensions strongly depend on the energy

**Figure 2.** Double-line (a) and depressed cladding (b) waveguides produced in the W-TCP eutectic bioactive glass

For the case of the cladding structure, both the distance between filaments and the scanning speed were set constant at 3 μm and 500 μm/s respectively, studying the guiding properties with the diameter of the core, between 20 and 150 μm, and the depth from the surface, 300 and 600 μm. Worth mentioning is the fact that the fabrication of cladding structures required lower delivered energy than in the case of double-filament waveguides, since the filaments were very close to each other and an excessive amount of energy might create cracks on the irradiated area and hence to fracture the sample. Specifically, in this case, filaments of the cladding structure were produced delivering 1.44 μJ over each focal spot. In addition, the interaction of the ultrashort laser radiation with the glass sample gave rise to an affected zone in the surroundings of the tracks, caused by the generation of color centers. It is well known that high intensity ultrashort laser pulses may modify the properties of the material at atomic scale by the photophysical and photochemical interactions which take place after the laser absorption [3]. These color centers also may affect to both the guiding and luminescence properties. The guiding properties were assessed by characterization of the near-field intensity distribution with a continuous wave He-Ne laser at 633 nm. Both types of waveguides presented TE and TM propagation. Furthermore, all double-filament waveguides inscribed were found to be monomode, whereas only 20 μm diameter depressed cladding waveguides presented this feature. In addition, for double-filament waveguides, the width found in TE and TM modes was shorter along the x-axis than in the orthogonal one, but the modal area decreased with the pulse energy thus increasing the light confinement. Moreover, the best confinement conditions were achieved for both the shortest track distance (15 μm) and depth (150 μm), and the highest scanning speed (50 μm/s). Concerning the cladding waveguides, there were no remarkable differences depending either on the guided polarization or processing

) by the laser beam, between 8.4 and 67 μJ, achieving lengths from 16 up to 40 μm.

delivered (*Ed*

[56, 60].

114 Advanced Surface Engineering Research

The refractive index modification was estimated on the basis of the refractive index distribution generated by femtosecond laser pulses in double-filament waveguides [100, 101], for which:

$$
\Delta \mathbf{n} = -\Delta \ln\_{\text{max}} \frac{1 - \left(\mathbf{x}/\sigma\_y\right)^2}{1 + \left(\mathbf{x}/\sigma\_y\right)^4} \exp\left(\mathbf{x}/\sigma\_y\right)^2 \tag{8}
$$

**Figure 3.** Near-field distribution intensity of TE mode in a double-filament waveguide with 15 μm between tracks (a) and in depressed cladding waveguides (b) 20 μm (left) and 30 μm (right) diameter produced in the W-TCP eutectic bioactive glass [56, 60].

processing technique, describing the main configurations for waveguide fabrication achieved to date, and the materials for which they have been applied. Also, a comparison to other techniques commonly used for waveguide fabrication has been included, pointing out the principal advantages of ULI when compared to these methods. Furthermore, the basis of the interaction between ultrashort laser pulses and dielectric media which may give rise to refractive index modification and hence, to waveguide fabrication has been outlined. In addition, this chapter briefly reviews the development of bioactive glasses related to their capability of forming tight chemical bonds to living tissues. This section is particularized to the develop-

applications as luminescence bioprobes in biomedical applications when doped with rareearth ions. Finally, this chapter reviews the fabrication of double-filament and depressed

▬Ca3

filament waveguides presented an increase of the refractive index in the region between damaged areas, the so-called "tracks," inscribed with the ultrashort laser pulses. In cladding waveguides, tracks were inscribed close to each other to surround a volume of bulk material. Both types of waveguides supported TE and TM polarization guiding under illumination of cw He-Ne laser radiation. In particular, only the cladding waveguide with the smallest core, 20 μm, presented monomode transmitted light. Color centers formation was observed in both types of waveguides as a consequence of the laser interaction with the glass. Isochronal thermal treatments allowed decreasing the absorption by color centers and for the highest annealing temperatures, a release of the stress-induced in the laser treated area. In addition, it was shown that spectroscopic properties were well preserved in the volume of the waveguides. Finally, the refractive index modification, *Δn*, was estimated in 5 × 10−3, in the same range of

Future work is expected to be done in the fabrication of integrated photonic devices in glasses

glass ceramics of this system are also bioactive. Therefore, we will ascertain and assess the optimal conditions for the fabrication of optical waveguides and their possible utilization as

Dr. Daniel Sola thanks the Ministry of Economy and Competitiveness of the State General Administration under the project MINECO FIS2016-76163-R and the PIT2 program of the

University of Murcia's own research plan for the financial support of his contract.

O5 ▬SiO<sup>2</sup> (PO4 )2

bioactive eutectic glass and glass ceramics, and to their potential

Ultrafast Laser Inscription of Buried Waveguides in W-TCP Bioactive Eutectic Glasses

bioactive eutectic glasses. Double-

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117

▬MgO system by the ULI technique. Glasses and

ment of CaSiO3

▬Ca3

(PO4 )2

cladding waveguides in Nd-doped CaSiO3

previously reported works by the ULI technique.

and glass ceramics of the CaO▬P<sup>2</sup>

**Acknowledgements**

**Conflict of interest**

laser sources for biomedical applications.

The authors declare no conflict of interest.

**Figure 4.** (a) Output power measured in double-filament and depressed cladding waveguides after isochronal heat treatment between room temperature (RT) and 750°C. (b) Evolution of the FWHM TE mode as a function of the annealing temperature between room temperature (RT) and 750°C for the 20 μm diameter cladding waveguide [56, 60].

**Figure 5.** Spatial variation of the Nd3+ fluorescence intensity for a double-filament waveguide at room temperature (a) and at 550°C (b) and emission of Nd3+ at 1060 nm under excitation at 808 nm for a 20 μm diameter depressed cladding waveguide in the core of the waveguide and in the bulk [56, 60].

where Δ*nmax* is the maximum reduction on the refractive index in the track, and *σ<sup>x</sup>* and *σ<sup>y</sup>* are the width of the damaged region along the horizontal and vertical directions respectively, resulting in a *Δn*~5 × 10−3, which was found to be in the same range of previously reported for other waveguides inscribed by femtosecond laser writing, such as Nd:YAG ceramics, Nd:YVO4 and fused-silica.

#### **6. Conclusions**

Fabrication of integrated photonic devices has been a hot topic in research since the first report on femtosecond laser written waveguides in glass by using the Ultrafast Laser Inscription Technique (ULI), in 1996. In this chapter, we have reviewed the fundamentals of this laser processing technique, describing the main configurations for waveguide fabrication achieved to date, and the materials for which they have been applied. Also, a comparison to other techniques commonly used for waveguide fabrication has been included, pointing out the principal advantages of ULI when compared to these methods. Furthermore, the basis of the interaction between ultrashort laser pulses and dielectric media which may give rise to refractive index modification and hence, to waveguide fabrication has been outlined. In addition, this chapter briefly reviews the development of bioactive glasses related to their capability of forming tight chemical bonds to living tissues. This section is particularized to the development of CaSiO3 ▬Ca3 (PO4 )2 bioactive eutectic glass and glass ceramics, and to their potential applications as luminescence bioprobes in biomedical applications when doped with rareearth ions. Finally, this chapter reviews the fabrication of double-filament and depressed cladding waveguides in Nd-doped CaSiO3 ▬Ca3 (PO4 )2 bioactive eutectic glasses. Doublefilament waveguides presented an increase of the refractive index in the region between damaged areas, the so-called "tracks," inscribed with the ultrashort laser pulses. In cladding waveguides, tracks were inscribed close to each other to surround a volume of bulk material. Both types of waveguides supported TE and TM polarization guiding under illumination of cw He-Ne laser radiation. In particular, only the cladding waveguide with the smallest core, 20 μm, presented monomode transmitted light. Color centers formation was observed in both types of waveguides as a consequence of the laser interaction with the glass. Isochronal thermal treatments allowed decreasing the absorption by color centers and for the highest annealing temperatures, a release of the stress-induced in the laser treated area. In addition, it was shown that spectroscopic properties were well preserved in the volume of the waveguides. Finally, the refractive index modification, *Δn*, was estimated in 5 × 10−3, in the same range of previously reported works by the ULI technique.

Future work is expected to be done in the fabrication of integrated photonic devices in glasses and glass ceramics of the CaO▬P<sup>2</sup> O5 ▬SiO<sup>2</sup> ▬MgO system by the ULI technique. Glasses and glass ceramics of this system are also bioactive. Therefore, we will ascertain and assess the optimal conditions for the fabrication of optical waveguides and their possible utilization as laser sources for biomedical applications.
