Effect of Ag Doping on Photobleaching in Ge 28 Sb 12 Se 60 Chalcogenide Films

: Chalcogenide glass is an optical material with excellent mid-infrared and far-infrared penetration properties. The silver-doped Ge 28 Sb 12 Se 60 (GSS) chalcogenide ﬁlms in this paper were deposited on a glass substrate by the co-evaporation technique. A continuous laser with different power outputs was then used to scan the glass material at a constant speed, and the photobleaching (PB) effects were observed using optical microscopy. The results show that silver doping can speed up the PB of GSS ﬁlm only under high-power laser irradiation. While silver doping helps to speed up the PB effect, it also increases the risk of ﬁlm damage. This study is beneﬁcial in the development of embedded optical waveguide structures.


Introduction
Chalcogenide glass is an inorganic non-oxide glass material based on sulfur elements.As an excellent nonlinear matrix platform, it has attracted great attention in the field of all-optical information processing.The unique photoinduced properties make chalcogenide an excellent material in the fields of optical information storage, lithography, and infrared waveguides [1][2][3].The participation of different elements also has different effects on the optical properties of the chalcogenide glass [4][5][6][7].Moreover, the optical, electrical, and structural properties of the chalcogenide change when exposed to band gap light [8,9].The most prominent photoinduced change in optical properties is the so-called photobleaching (PB) and photodarkening (PD), which correspond to photoinduced increases and decreases in the optical gap, respectively [10][11][12].
The PB phenomena in chalcogenides have been studied by many researchers as being highly prospective for practical applications, including GeSe [13], GeSbSe [14], and even Bi-doped GeSe-based chalcogenides [15].As a branch of chalcogenide study, Ag-doped chalcogenides have also been studied in former works due to the interesting properties in nonlinearity [16], phase transition [17], electrical conductivity [18], and structural evolution [19] aiming at the applications of optical storage, reversible NIR windows, and electronic devices.However, the PB effect in Ag-doped chalcogenides is seldom reported.
In this paper, we report the effect of the laser-induced PB of chalcogenide glasses with different silver concentrations by irradiating silver-doped Ge 28 Sb 12 Se 60 (GSS) films with a He-Ne laser beam.The silver-doped chalcogenide glass films in different Ag concentrations were fabricated using co-evaporation technology.The PB levels were characterized using transmitted optical microscopy images.The high-power density irradiation was controlled using a focal lens and continuous attenuator.The transmission nature of time-dependent PB by low-power irradiation was also studied.The results show that the phenomenon of PB became more obvious with the increasing silver concentration in silver-doped GSS thin films.

Experiments
The samples were acquired by using the co-evaporation technology to co-evaporate Ag (5N) and Ge 28 Sb 12 Se 60 materials onto glass substrates.A physical vapor deposition system (Nexdep physical vapor deposition platform, Angstrom Engineering, Canada) that integrates electron beam sources and current-induced thermal evaporation sources were used in the co-evaporation process.The electron beam source was used to evaporate Ag and the current-induced thermal source was used to evaporate GSS.The Ag (5N) particles were placed in the graphite crucible on the electron beam source, whereas the GSS was ground into a powder and placed in the S38 Alumina-coated open boat for thermal evaporation.The co-evaporation doping process was carried out by simultaneously evaporating Ag and GSS and depositing them onto the rotated substrate.During the deposition process, the chamber pressure was kept below 5 × 10 6 Torr, the substrate was rotated at a speed of 10 RPM at 20 • C. Energy dispersive X-ray spectroscopy (EDS) was used to check the elements in the samples.The result of the samples with different deposition rate ratios of Ag: GSS is shown in Table 1, where the sample name Ag x (x = 0, 1, 2, and 3) denotes the deposition rate ratio of Ag: GSS = x:10.The thickness of the thin films was 120 nm, which were controlled by the deposition rate and time of the deposition process, and confirmed using atomic force microscopy (AFM, SPA-400, SEIKO, Chiba, Japan) in tapping mode.The laser used for the irradiation of samples was a 40 mW He-Ne laser with a beam diameter of 2 mm and a wavelength of 632.8 nm.The PB characteristics of the irradiated samples were observed using an optical microscope with white light illumination.

Photobleaching in GSS Film
We first studied the PB effect in pure GSS film.The fully photobleached GSS film was achieved by irradiating the sample directly with the laser beam (the power density was 1.27 W/cm 2 ) for three hours.Figure 1a shows the transmission spectra of the asdeposited (black curve) and the fully photobleached (red curve) GSS film in the nearinfrared region.In general, the PB effect was characterized by a photoinduced shift in the energy band gap.Thus, we calculated the absorption coefficients [20] of the as-deposited and photobleached films, and the plot of the absorption coefficients versus photon energy is shown in Figure 1b.According to the Tauc Law [21], the band gap energy, E g shifts from 1.654 eV to 1.724 eV after exposure, which agrees with the PB effect illustrated in [14,15].Utilizing the transmission spectra, we calculated the refractive index of both samples using the Swanepoel method [22] and found that the refractive index of the photobleached GSS film was enlarged after exposure.The spectrum of the refractive index change is shown in Figure 1c.A refractive index change of approximately 0.15 was acquired, which is comparable to that in former reports [23].Owing to the refractive change property of the PB effect, GSS film can be utilized to fabricate embedded optical waveguides.

Photobleaching in GSS Films with Different Silver Concentrations
We studied the laser-induced PB effects under high power density irradiation using a focused laser to scan the samples on a moving stage, as shown in Figure 2. A CW He-Ne laser (operating at 632.8 nm) with a maximum power of 40 mW was focused to a small spot with a diameter of around 15 µm using a biconvex lens (MBCX106110, LBTEK Inc., Shenzhen, China).A continuous neutral-density attenuator was used to tune the output power.In the experimental process, the light spot was incident on the surface of samples (Ag 0 -Ag 3 ) which were placed on an electric translation stage (TSA300-BF, Zolix, Beijing, China).The samples were controlled to move perpendicularly to the laser beam at a speed of 1 mm/s.Due to such a high-power of the focused beam [24], the PB effect was also accompanied by the partial crystallization process, which is neglected in this work.Figure 3a-d shows the different PB levels for samples Ag 0 -Ag 3 scanned with a power density of 1.42 × 10 4 W/cm 2 on the sample surface.To facilitate the observation, grayscale optical microscopy images are normalized for comparison.Because the visible part of the transmission spectra (not shown) increases after exposure, the irradiated parts are brighter than the as-deposited region.It can also be observed more intuitively that the degree of bleaching is more obvious with the increase in silver concentrations from 0% to 40.7%.The optical contrast (OC) was utilized to characterize the PB efficiency [22], which can be defined as the difference in the normalized transmission of the nonirradiated film (as-grown) and the center of the irradiated region, namely OC = Normalized T center − Normalized T as−deposited = Normalized T center − 1 (1) Figure 3e shows the normalized spatial transmission profiles across the irradiated spot center extracted from the images in Figure 3a-d.From Figure 3e, the OC increases from 0.01 to 0.18 for Ag 0 -Ag 3 , indicating that the PB effects are more obvious for GSS films with a higher silver concentration under short-time-focused laser irradiation.

Laser-Induced Bleach Degree with Laser Power Density
To investigate the relationship between the PB phenomenon and the laser power density, chalcogenide film samples with different silver concentrations were scanned at different laser power densities with the same exposure time (same scanning speed), and the microscopy images of samples Ag 0 , Ag 1 , Ag 2 , and Ag 3 at the incident laser power density ranging from 1.19 × 10 4 W/cm 2 to 2.26 × 10 4 W/cm 2 are shown in Figure 4.
As can be seen from Figure 4, at the same doping concentration, the PB phenomena are more obvious with an increase in incident laser power density, as shown in Figure 4a.However, as shown in Figure 4b-d, for silver-doped chalcogenide glass, the black areas arose, indicating film damage during scanning.This is because silver doping improves the absorption of light (632.8nm wavelength) and the doped films were damaged due to heat.
For a more intuitive comparison and observation, Figure 5 shows the absorption for the 632.8 nm laser and the minimum film damage power with different silver concentrations.The corresponding absorption increases from 17%, 44%, 51%, to 55% with silver doping at concentrations of 0%, 18.6%, 31.4%, and 40.7%, respectively; the critical power density of the film damage decreased, indicating an increase in the risk of film damage.The reason for this is that silver possesses a high imaginary part of the refractive index that signifies the absorption of light in the material, thus the silver doping introduces a high imaginary part of the complex refractive index into the film and leads to the higher absorption of the samples.Thus, we can expect the silver-doped GSS with higher silver concentrations to absorb more laser energy, resulting in a stronger heating effect, which makes it easier to damage the film.This is also the reason for the increasing PB level in Figure 3a-d.

Effects of the Laser Irradiation Time
In order to study the exposure time-dependent characteristics of the PB effect in Ag 0 -Ag 3 , we investigated the transmittances of the samples with time under low laser power density to acquire a more distinguishable time dependence of the structure changes.As shown in Figure 6, the sample was directly illuminated with a laser beam of 2 mm diameter, with a power of 40 mW and without lens focus, i.e., the power density was about 1.27 W/cm 2 .To simulate the reaction kinetics of the photobleaching effects over time, we used the stretching exponential function to describe the PB.The transmission change can be expressed using the rate equation [18], as follows: Here, the subscript 'b' corresponds to the PB.∆T sb , τ, β, and t are the metastable part (saturated change in the rise of the transmittance), the effective time constant, the dispersion parameter, and the illumination time, respectively.The ultimate results are presented in Figure 7 with a solid line.It can be seen that the experimental data fit very well to the stretched exponential functional forms, as described in Equation ( 2), and the fitting parameters are also listed in Table 2. From Figure 7, we found that for all four samples, the normalized transmissions reach maximum values of 1.1 (i.e., the OC value is 0.1) after around 90 min of illumination, regardless of silver concentration.However, for the focused high-power density (1.42 × 10 4 W/cm 2 ) scanning, the OC value increases with increasing silver concentration.Therefore, it can be deduced that silver doping can speed up the PB of GSS film only under high-power laser irradiation.In addition, for the focused high-density scanning (1.42 × 10 4 W/cm 2 ), as shown in Figure 3, the OC value increases to a higher value of 0.18 as the silver concentration increases.This is because the transmission difference at 632.8 nm is smaller compared to that of the whole visible transmission spectra.

Conclusions
This study presents the effect of silver doping on the PB properties of GSS.Silverdoped chalcogenide thin films were prepared using the co-evaporation technique.Owing to the different silver concentrations, different PB phenomena were created.The results showed that the higher the silver-doped concentration, the faster the PB phenomenon for the (1.42 × 10 4 W/cm 2 ) high power density scanning.However, for the low power density irradiation of 1.27 W/cm 2 , the PB effect saturates regardless of silver concentration.Therefore, silver doping can speed up the PB of GSS film only under high-power laser irradiation.However, while silver doping helps to speed up the PB effect, it also increases the risk of film damage.This study is beneficial in the development of embedded optical waveguide structures.

Figure 1 .
Figure 1.(a) Transmission spectra for the as-deposited GSS film and the photobleached GSS film.(b) The plot of absorption coefficients versus photon energy.E g is the band gap energy.(c) The refractive index changes in GSS film induced by the PB effect.

Figure 2 .
Figure 2. The simple experiment devices for making a line on the sample.

Figure 3 .
Figure 3. Optic microscopic images of the (a) Ag 0 (b) Ag 1 (c) Ag 2 (d) Ag 3 thin films after laser irradiation at 25 mW.(e) Normalized transmittance of the irradiated samples.

Figure 5 .
Figure 5.The absorption and the critical power density of the samples doped with different silver concentrations.The red circles and green triangles indicates the absorption and threshold power density, respectively.

Figure 6 .
Figure 6.The light path of low power density irradiation.

Figure
Figure 7a-d shows the normalized transmittances of Ag 0 , Ag 1 , Ag 2 , and Ag 3 samples.The dashed line indicates the time at which the pump laser was turned on.Subsequently, the PB starts to grow, showing a significant change in the transmission with time.To simulate the reaction kinetics of the photobleaching effects over time, we used the stretching exponential function to describe the PB.The transmission change can be expressed using the rate equation[18], as follows:

Figure 7 .
Figure 7.The normalized transmittance of (a) Ag 0 , (b) Ag 1 , (c) Ag 2 , and (d) Ag 3 samples under low power density light exposure.The red dots and the black curves correspond to the measured and the fitted data.

Table 1 .
The deposition rate and the corresponding composition of the samples.

Table 2 .
Fitting parameters obtained from Equation (2) that correspond to the PB.