Nonlinear optical properties of chalcogenide hybrid inorganic/organic polymers (CHIPs) using the Z-scan technique

The linear and nonlinear optical behavior of novel sulfur based polymer materials are evaluated at the optical communication wavelength, 1550 nm. These polymers are attractive for near-IR (NIR) and mid-IR applications. The two photon absorption (TPA) coefficient (β) and second order refractive index (n2) of chalcogenide hybrid inorganic/organic polymers (CHIPs) from poly(sulfur-random-(1,3-diisopropenylbenzen) (poly(S-r-DIB)) are measured via the Z-scan technique. In this study, we investigated the linear and nonlinear optical behavior of two types of CHIPs where the weight percentage of sulfur is varied (poly(S50%-r-DIB50%) and poly(S70%-r-DIB30%)). The TPA coefficients for poly(S50%-r-DIB50%) and poly(S70%-r-DIB30%) obtained were 0.11 cm/GW and 0.063 cm/GW, respectively. The n2 for poly(S50%-r-DIB50%) and poly(S70%-r-DIB30%) was measured and determined to be 2.45 × 10 cm/W and 3.06 × 10 cm/W, respectively, and are in good agreement with Miller’s rule prediction. These materials exhibit low cost, low temperature processing, high transparency in the near to mid-IR range (except a few interval ranges) and relatively high refractive index, providing a unique set of properties for optics and photonics device applications. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (190.4400) Nonlinear optics, materials; (160.5470) Polymers; (160.4330) Nonlinear optical materials. References and links 1. S. A. Ray Hilton, Chalcogenide Glasses for Infrared Optics, 1st ed. (McGraw-Hill Professional, 2010). 2. V. F. Kokorina, Glasses for Infrared Optics (CRC Press, 1996). 3. A. B. Seddon, “Chalcogenide glasses: a review of their preparation, properties and applications,” J. Non-Cryst. Solids 184, 44–50 (1995). 4. A. Zakery and S. R. Elliott, Optical Nonlinearities in Chalcogenide Glasses and Their Applications, Springer Series in Optical Sciences (Springer Berlin Heidelberg, 2007). 5. A. Zakery and S. R. 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Introduction
In the last few decades, there has been strong interest in the use of chalcogenide glass materials for mid-IR applications [1][2][3]. High transmissivity, low loss and high nonlinear refractive index in the IR optical range [4,5] compared to fused silica, make chalcogenide materials a viable option for photonics devices, particularly for 1550nm applications. However, chalcogenide glass materials have toxic constituent elements, such as arsenic (As). These materials also require high temperature to process (~300°C) and accordingly are more difficult and costly to use for photonics components. We have developed a novel sulfur-based polymer material where polymeric chains of elemental sulfur (S 8 ) are crosslinked with an organic monomer, 1,3-diisopropenylbenzene (DIB) through the "inverse vulcanization" technique [6,7] in order to realize a processable IR material. Chalcogenide (e.g., S, Se) containing macromolecules prepared from inverse vulcanization are polymeric analogs to ChG's, where organic comonomer moieties replace the Group 3,4, or 5 atomic unit in these network solids, the first generation examples of which being poly(sulfur-random-(1,3diisopropenylbenzene)(poly(S-r-DIB) and selenium containing polymers. These high sulfur and selenium content materials are the first examples of chalcogenide hybrid organic/inorganic polymers (CHIPs), which we have demonstrated to exhibit among the highest refractive indices of any synthetic polymeric material (n ~1.75-2.10), excellent optical transparency and suitability for IR thermal imaging [6][7][8][9][10][11]. Poly(sulfur-random-1,3-(poly(Sr-DIB)) possesses a number of desirable material properties for use in IR optical and imaging applications, which include low cost, high IR transparency and favorable melt/solution processability. We have recently reported on the fundamental optical properties of CHIPs materials, however, investigation into the non-linear optical properties of these novel materials has not been explored. Herein, we present and discuss for the first time the linear and nonlinear optical properties of these polymers.

Linear optical properties
The CHIPs used in this study are fabricated with an inverse vulcanization reaction of S 8 and DIB as described in our previous work [7,9]. Due to the high content of elemental sulfur, poly(S-r-DIB) compounds have been demonstrated to have interesting linear optical properties in the NIR (particularly 0.75 µm to 1.65 µm and 1.8 µm to 2.1 µm) and mid-IR spectrum (specially 3.5 µm to 5 µm) [7,10]. For this study, we focus on two copolymers where the weight percent (wt%) of sulfur is varied, poly(S 50% -r-DIB 50% ) and poly(S 70% -r-DIB 30% ). Both compositions have relatively similar transmittance but different ultra-high refractive index in the IR regime.

Sample preparation
Several fabrication methods were explored to assemble relatively flat thick samples. Due to the high melt viscosity of fully cured poly(S-r-DIB) materials, it was found to be ideal to fabricate substrate supported flats by casting a prepolymer resin of poly(S-r-DIB) onto the substrate and completing the curing process in an oven. To achieve relatively large area samples with proper thickness that have good optical quality flatness, a mold was fabricated using 100 μm Corning Willow glass samples as shims attached to a glass substrate (also made of Willow glass) with double sided adhesive tape. The sulfur (0.70 g, 70 wt% and 0.50g, 50 wt% for poly(S 70% -r-DIB 30% ) and poly(S 50% -r-DIB 50% ), respectively) was melted in a vial at 180°C inside an oil bath until it turned orange. The DIB (0.30 g, 30 wt% and 0.50g, 50 wt%, respectively) was then added and stirred until a cherry red color was obtained (approximately 4 minutes). The resultant pre-polymer resin was poured into the mold which had been allowed to thermally equilibrate in a 180°C oven. A second piece of Willow glass, coated with a thin layer of cured polydimethyl siloxane (PDMS) (applied via spin coating), was placed on top of the mold containing the resin. Homogenous minimum pressure was applied for 27 mins, then the mold containing the sample was removed from the oven and allowed to reach room temperature before the PDMS coated top slide was removed. Figure  (1a) shows poly(S 50% -r-DIB 50% ) bulk compound materials and Fig. (1b) and Fig. 1(c) denote two flat samples of poly(S 50% -r-DIB 50% ) and poly(S 70% -r-DIB 30% ) respectively. Both samples presented high orange coloration, characteristic of the CHIPs materials, which does not affect the IR transmission.

Transmittance and linear refractive index
Previous studies have shown that poly(S-r-DIB) copolymers have high transparency in the NIR (0.75 µm to 1.65 µm and 1.8 µm to 2.1 µm) and mid-IR range (3.5 µm to 5 µm) [7,11]. In Fig. 2(a), the optical transmittance of poly(S 50% -r-DIB 50% ) and poly(S 70% -r-DIB 30% ) is shown from 0.5 μm to 2.7 μm as measured with a Cary 7000 spectrophotometer. Figure (2b) denotes the calculated attenuation in terms of dB/cm as the function of wavelength. Both compositions present excellent transparency in thin samples for the NIR and SWIR range. Strong absorption is observed below 0.5 μm in both cases. Compositions with higher sulfur wt% tend to be less transparent than those with lower values, however this difference is greatly reduced after correcting for Fresnel losses between the incident medium, the copolymer, and the substrate. At 1550 nm, the internal transmission is above 99.5% for 200 μm thick samples. The sulfur-containing polymers we have prepared, possess high refractive index (n>1.7) due to the high content of S-S bonds and the large polarizability of the sulfur electrons [12]. In the case of poly(S-r-DIB) compounds, the refractive index can be increased as the wt% of sulfur is increased [7,13]. In Fig. 3, the refractive index is shown as a function of wavelength. At 1550 nm, the measured refractive indices for poly(S 50% -r-DIB 50% ) and poly(S 70% -r-DIB 30% ) are n = 1.719 and n = 1.787, respectively.

Nonlinear optical properties
The nonlinear optical behavior of sulfur copolymer materials, and in particular the third order nonlinear optical susceptibility ( χ ), were determined using the Z-scan technique [14,15].
A simultaneous closed and open aperture Z-scan set-up was used in order to obtain the nonlinear refractive index (Re[ (3) χ ]), n 2 , and two photon absorption (TPA) coefficient (Im[ (3) χ ]), β, respectively (Fig. 4). These nonlinear coefficients were measured in the optical communication band at a wavelength of 1550 nm. The coherent source was an optical parametric amplifier (OPA) producing micro joules range energy per pulse with pulse widths of 187 femtoseconds (fs) at full width half maximum (FWHM) of the intensity profile. The OPA was pumped by a chirped pulse amplification (CPA) system including a regenerative amplifier (Spectra Physics, 800 nm, 100 fs, 1 kHz repetition rate) and the amplifier was also seeded with a mode-locked Ti:sapphire laser (Spectra Physics, Tsunami). The closed and open aperture transmissions in the far field were monitored as a function of sample position along the optical beam waist after the focusing lens using two photodiodes (PDs). The reference input laser beam was recorded by PD1 in order to capture the pulse-to-pulse fluctuations for normalization of closed and open aperture transmittances.
Here I is the peak irradiance, 0 α is the linear attenuation coefficient (including both scattering and absorption contributions), β is the TPA coefficient and ' z is the propagation depth in the sample [16]. Using the Z-scan technique the normalized transmission for the open aperture configuration can be written as: for poly(S 50% -r-DIB 50% ) and poly(S 70%r-DIB 30% ) concentration sulfur copolymer materials respectively. The nonlinear refractive of index can be evaluated from the closed aperture transmission data using the following equation: ( ± 10% of 2 n ) for poly(S 50% -r-DIB 50% ) and poly(S 70% -r-DIB 30% ) concentration sulfur copolymer materials respectively. Fig. 6. Z-scan closed aperture measurements. (a), (b) Measured normalized transmissions versus sample position for poly(S 50% -r-DIB 50% ) and poly(S 70% -r-DIB 30% ) respectively with nonlinear refractive index fits. The n 2 coefficients were calculated to be n 2 = + 2.45 × 10 −15 cm 2 /W and n 2 = + 3.06 × 10 −15 cm 2 /W for poly(S 50% -r-DIB 50% ) and poly(S 70% -r-DIB 30% ), respectively. The transmissions were measured by dividing the optical powers from 3 2 / . PD PD P P The peak irradiance at the focal point was roughly ~80 GW/cm 2 .

Comparison with Miller's rule prediction
The measured values for nonlinear refractive of index conform very well to the semiempirical Miller's rule relationship between the linear and nonlinear refractive index [17,18].
Here δ is the Miller's coefficient for chalcogenide glass and is equivalent to 2.7 × 10 −10 [19]. The reported value for the Miller's coefficient is consistent with other reports for most of optical crystals, chalcogenide glasses, oxide glasses [20,21] and ionic crystals [22]. Table 1 denotes the summary of predicted values for n 2 based on Miller's rule and obtained experimental values using the Z-scan method.

Comparison with other IR materials and applications
The sulfur copolymers and CHIPs materials have an intriguing set of optical properties, that occupies a unique range of refractive index (n 0 ~1.7-2.1) which is a new regime of materials that are intermediary to classical inorganic IR transmitting materials (e.g., ChG's, germanium) and low index organic polymers. The refractive indices of CHIPs the sulfur copolymers are not as high as chalcogenide glasses and germanium (Ge) semiconductors, however, CHIPs still retain the low cost and processability of polymeric materials (Fig. 7). Conversely, CHIPs exhibit profoundly higher refractive index relative to conventional organic polymers while retaining high IR transparency. Thus, these materials have the potential for use in applications as antireflection coatings or index matching materials in infrared optics. In addition, it would be possible to make compact polymer based photonic devices for infrared applications, especially mid-infrared optics, as high index contrast waveguide structures can be made. In high index contrast platforms e.g. silicon photonics, the relatively high refractive index of these polymers with respect to silica is advantageous to introduce better mode matching for light coupling from single mode waveguides to optical fibers and vice versa. Additionally, their nonlinear optical properties are ten times higher than silica due to the highly polarizable sulfur atoms; the properties of the sulfur copolymers are, in fact, seen to match those of silicon nitride quite well with respect to both refractive index and n 2 (Table. 2). Especially, the low temperature processing for these polymers makes them attractive for fabricating compact all optical photonic devices, such as all optical signal processors, all optical switches, all optical signal regenerators [29], super-continuum generators [30,31] and frequency combs [32][33][34].

Conclusion
We have evaluated the linear and nonlinear optical properties of a recently developed sulfurbased polymers. The experimentally measured values for the nonlinear refractive index for both poly(S 50% -r-DIB 50% ) and poly(S 70% -r-DIB 30% ) are in good agreement with the Miller's rule prediction for chalcogenide glasses. These sulfur-based polymers can potentially be made into films and bulk optics using conventional polymer processing techniques at relatively low temperature. One of the key advantage of our polymers in contrast to chalcogenide glasses, as mentioned before, is their low cost of processing. In addition, their unique transmission properties can enable more affordable infrared spectrometers, gas sensors, infrared cameras, and etc.