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Role of thermal and photo annealing on nonlinear optical response of Ge30Se55Bi15 thin films

In this article, we employed nanosecond Z-scan technique to demonstrate the nonlinear optical response in Ge30Se55Bi15 thin films after thermal and photo annealing. The intensity dependent open aperture Z-scan traces reveal that for all the samples, i.e. as-prepared, thermal and photo annealed thin films exhibit reverse saturable absorption (RSA). The experimental results indicate that both thermal and photo annealing can be efficiently used to enhance the nonlinear absorption coefficient compared to as-prepared sample. We further demonstrate that beta value of thermally annealed and as-prepared samples increase significantly at higher intensities. On the contrary, beta for photo annealed sample does not exhibit appreciable changes against the intensity variation.


INTRODUCTION
Amorphous chalcogenide glasses (ChG) stands apart from their counterpart optical materials because of their unique linear and non-linear optical properties. For example, they possess high linear refractive indices 1 (2.0-3.0 at 1.55μm) and high third order nonlinear refractive index (~1000 times that of silica) 2,3 . The later has attracted significant interest in theoretical and experimental investigations owing to their numerous applications in various fields which includes optical limiting 4 , optical switching 5 , multiphoton polymerization 6 , photo-dissolution 7 etc. Till date, most of the previous experiments exploring the non-linear optical properties have been conducted on Ge and As-based ChGs [8][9][10] . For example, Huang and co-workers observed strong enhancement in third order non linearities as a result of formation of defect like bonds in Ge-Sn-Se chalcogenide glasses 11 . On the other hand, Smektala and colleagues demonstrated a strong variation of the nonlinear refractive index of As2S3 and As2Se3 glasses with the input laser intensity 12 .
In spite of high photosensitivity of ChGs, their application get limited because of poor thermal stability. An efficient way to overcome such issue is doping a third impurity elements, such as Bi, Sb with more metallic character into the ChG matrix which can improve the thermal stability significantly with added functionalities 13,14 . For example, with the incorporation of metallic Bi, glass formation region gets expanded and stabilized for Ge-Se-Bi system 15 . The chemical durability and IR transmission also increases by the addition of Bi into Ge-Se system. Nevertheless, for practical applications, it is important to tune the linear and non-linear optical properties of Ge-Se-Bi system. Thermal and photo annealing are considered to be efficient techniques to alter the linear and non-linear optical constants of ChGs. Although, the effect of such annealing has been studied on the linear optical properties, the implications remain unknown for the non-linear optical response.
To gain new insights on the third order non-linear optical properties of Ge-Se-Bi ChGs upon thermal and photo annealing, we employed conventional open aperture Z-scan technique. Z-scan traces reveal that upon resonant excitation of 1064 nm, all the samples i.e. as-prepared, thermal and photo annealed samples undergo RSA. A comparative study reveals that non-linear absorption coefficient (β) increases significantly upon thermal and photo annealing.

II. EXPERIMENTAL
The bulk Ge30Se55Bi15 was prepared by using melt quench technique from the high pure elements Thin films of Ge30Se55Bi15 was deposited from the prepared bulk material by thermal evaporation technique on glass substrates at a base pressure of ~ 5 × 10 −5 Torr. The substrate temperature was maintained at room temperature and the deposition rate was fixed at 5 nm per second during the deposition process. The substrates were rotated slowly to get homogeneous and uniform film. We prepared films of thickness ~ 1 µm, confirmed from thickness crystal monitor. The films were thermally annealed for 2 hours at 120 0 C. For photo annealing, the sample was irradiated with 532 nm continuous wave laser for ~ 2 hours with illumination intensity of 50 mW cm −2 .
To explore the nonlinear optical properties of the films, we employed open aperture Z-scan technique which measures the total transmittance as a function of incident laser intensity. The schematic 4 diagram of open aperture Z-scan technique is presented in Fig.1. In this method, the thin films were illuminated with 1064 nm, 5 ns pulses of Nd:YAG laser. We used a fixed repetition rate of 10 Hz to avoid sample heating and photo-damage.

III. RESULTS AND DISCUSSIONS
First, we investigated the effect of thermal and photo annealing on the linear optical properties of as-prepared Ge30Se55Bi15 thin films by recording the optical absorption spectra of the samples as shown in Fig. 2a. The optical absorption coefficient (α) was calculated from the absorbance spectra from the following formula 16 where A and t are the absorbance and the thickness of the thin film, respectively. Then, we calculated the optical band gap of each sample by using Tauc equation 17 gives the value of optical band gap which is shown for all samples in Fig. 2 photodarkening process 19,20 and the effect of which is less than the thermal annealing. We believe that the tunability of the optical band gap 21 of the sample can be useful for many optical devices.
In spite of the promising linear optical properties of ChGs as active materials in optoelectronic devices, an extremely important issue to be addressed is the non-linear optical properties which makes them a favorable candidate to fabricate several non-linear optical devices. In this regard, we present an interpretation of the third-order nonlinear ( (3) ) optical properties of as-prepared, thermally and optically annealed Ge30Se55Bi15 chalcogenide thin films.
We employed conventional open aperture Z-scan technique to unveil the non-linear optical properties. In this context, Fig 3.  workers. 22 RSA has many potential applications in optical pulse compressor, optical switching, and laser pulse narrowing etc 22 . With the increase in laser intensity, the transmittance decreases gradually that is a typical behavior of optical limiting process which plays important role in protecting optoelectronic detectors as well as human eye from intense laser sources 23 . To quantify RSA process, we computed the non-linear absorption coefficient (β) and saturation intensity ( s) of all the samples from the following propagation equation 24 .
where α(I) is intensity dependent absorption coefficient and z is the propagation distance in the sample.
Here α (I) can be written as where 0 is the linear absorption coefficient, β is the RSA coefficient and Is is the saturation intensity.
However normalized transmittance as a function of position z for RSA can be expressed as: Where 0 = 0 1+ 2 0 2 and = (1− − ) Were I0, z0, L and the peak intensity at the focus (z = 0), the Rayleigh length, sample thickness, and linear absorption coefficient, respectively 23, 25 .
8 RSA coefficients were extracted from the best fit of normalized transmittance curve. The strength of RSA is determined by nonlinear absorption coefficient (β). To obtain an overall picture, we plotted the variation of β and Isat as a function of laser intensity for three different samples in Fig. 4(a) and 4(b) respectively, and the results are summarized in Table 1. It can be seen that for both as-prepared and thermally annealed samples, β increases with laser intensity, which is a true signature of RSA process. On the contrary, for photo annealed sample, β doesn't exhibit any significant changes against the intensity variation. Isat exhibits complementary behavior to β, i.e. decreases with laser intensity for as-prepared and thermally annealed samples and likewise remains invariant for optically annealed sample.  To quantify the effect of thermal and photo annealing on the non-linear optical properties, we compared the non-linear absorption coefficients β and Isat of all the samples at comparable peak intensity.
Precisely, we found that β is significantly increased in thermally and optically annealed films compared to as-prepared one, however, the enhancement is stronger by two times for thermally annealed samples compared to photo annealing. Thus, the result clearly indicates that the enhancement of could possibly be a direct consequence of band gap reduction. In literature 25,26 , it is shown that non-linear response (RSA) in the semiconductor follows an inverse cubic relation with band gap. i.e.
where ( ( 2ℎ ⁄ ) 5 ), depends on the photon energy and band gap, which will help to understand the band structure of compound. We found from Fig. 2(b) that band gap decreases for thermally and optically annealed samples compared to as-prepared sample which leads to the enhancement of β as predicted from the theoretical perspective.

IV. CONCLUSIONS
In conclusion, we demonstrated the non-linear optical properties of Ge30Se55Bi15 thin films by open aperture Z-scan measurements. We investigated the role of thermal and photo annealing on the non-linear optical response of the sample. All the samples exhibit RSA as a function of laser intensity. Our results indicate that RSA coefficient (β) increases after thermal and photo annealing with stronger effect is observed for thermal annealing. By comparing the linear and non-linear optical properties, we conclude that the significant increase in β is correlated with the reduction in optical band gap. The observed nonlinear optical absorbance is described on the basis of density of defect states and disorders present in the mobility gap. We believe that the remarkable enhancement of RSA holds tremendous potential applications in many optical devices like optical switching and limiting.