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Abstract
Laser transmission induced transparency (LTIT) has been observed in a polymer waveguide using commercial perfluorinated acrylate-based materials when a continuous-wave laser at 635 nm is injected. The transmitted optical power increases continuously and follows a non-linear curve with respect to the laser injection time. Loss reduction over 13 dB is observed within 60 min at a moderate laser power of 5 mW. While higher injection power leads to a quicker change of the waveguide transparency, this loss reduction tends to saturate at a level irrelevant to the injection power. Further experiments demonstrate that a laser injection at 635 nm can also slightly improve the transparency at near-infrared wavelengths from 1500 nm to 1600 nm which is also the target wavelength range for this material. The state after a certain laser injection dose of 635 nm proves to be stable and the transmission characteristics of the polymer waveguide can be maintained and will continue after being stored at room temperature over a long period of time. By baking the waveguide at 200 °C for 20 min, the transparency property can be reset and the waveguide will return to the original high-loss state of 635 nm. These unique properties can be attributed to the photo-induced generation and thermally induced recombination of free radicals in the organic material. Our discovery may trigger interesting applications of polymer waveguides in the development of optical memory, clock, and encryption devices, beyond their target applications in optical communication.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Polymer-based photonic platforms have experienced rapid development over the past decades. As the cornerstone of polymer photonic integrated circuits (PICs) [1–3], polymer waveguide technology can offer many advantages such as low-cost, simple technology for fabrication, low optical loss at customized wavelength windows, and high compatibility to integrate components from other material platforms [4–12]. Polymer waveguides can be fabricated on a variety of substrates by a spin-coating and curing process [13,14], and have shown great flexibility to integrate 2D materials [15,16], silicon nitride [10], and III-V semiconductor-based components [11,12] for different applications. Monomers in solution also open possibilities to develop active waveguide devices through various forms of doping [17]. Optical polymer materials typically have thermo-optic coefficients on the order of 10−4/°C and low thermal conductivity at the same time. Therefore, many thermo-optic devices have been developed on polymer waveguide platform [18], such as variable optical attenuators [19], thermo-optic switches [20–22] and tunable external cavity lasers [11,12].
In early explorations of polymer photonic devices, poly(methyl methacrylate) (PMMA), polystyrene (PS) and epoxy resins were commonly used as base materials [4]. However, as optical communication systems evolve toward more sophisticated configurations, the demand on the performance of the fundamental photonic components becomes high and much work has been focused on improving the stability and optical transparency of the polymer materials. Coming to the new generation of commercial polymer materials, the vibrational overtone absorption in near-infrared (NIR) communication band due to C-H bonds has been significantly improved by fluorine substitution of hydrogen atoms [23]. This class of perfluorinated acrylate polymer materials represented by ZPU series from ChemOptics exhibit low loss, low birefringence, low surface roughness, and good thermal confinement at 1550 nm and 1310 nm wavelengths [23–26]. However, until now little work has been done to investigate the transmission characteristics of the perfluorinated acrylate polymer-based waveguides in the visible wavelength range which is very important for many application fields beyond optical communication, e.g., astronomy and optical sensing.
In this work, we report the phenomenon of laser transmission induced transparency (LTIT) in polymer waveguides, stemming from a serendipitous experimental observation when aligning the fiber and the waveguide with the help of a red laser. It is found that the transmitted optical power through the polymer waveguide increases dynamically with the power of the injected 635 nm laser over time. In the following sections, we present the waveguide design and fabrication technology, followed by the experimental setup and procedures of referencing, ruling out factors from the measurement system. We then demonstrate that 635 nm laser injection with different powers results in a growth trend in the waveguide transmission at rates related to the laser power. It is discovered that the maximal loss-reduction is independent of the laser power and injection time, but only relevant to the waveguide itself. Waveguide transmission characteristics at NIR (1500-1630 nm) both after and during the 635 nm laser injection have been investigated, in which no significant change other than a slight improvement of about 0.2 dB is observed. The transparency state “written” by the 635 nm laser transmission proves to be retentive at room temperature over a long time but also resettable by baking the waveguide at 200 °C. These unique features of perfluorinated acrylate-based waveguides at 635 nm may be exploited for advanced photonic applications.
2. Waveguide design and fabrication
The schematic layout of the polymer channel waveguide is shown in Fig. 1(a). The core and cladding are fabricated from ZPU470 and ZPU450 commercial resins from ChemOptics ZPU12-RI series. As indicated by the polymer index, the refractive indices of the core and cladding at 1550 nm are 1.47 and 1.45, respectively. At 635 nm, the refractive indices for ZPU470 and ZPU450 measured by an ellipsometer are 1.483 and 1.465, respectively. The waveguide cross-section is 3 µm × 3.5 µm in parameters. The waveguides are taken from the same wafer as the multiport thermo-optic switches in our previous work [22].
Fig. 1. (a) 3D schematic of the polymer waveguide. The width and height of the polymer waveguide core are 3 µm and 3.5 µm, respectively. (b) Field profiles of the eigenmode supported by the waveguide at 635 nm. (c) The near-field profile at 635 nm from the waveguide facet captured by a CMOS camera resembles a Gaussian shape, indicating a ground mode behavior. The measured diameter of the Gaussian spot is 3.27 µm, similar to the simulation (3.13 µm).
Initially, ZPU12-RI materials were developed for applications in the communication O, C and L band. The waveguide design shown in Fig. 1(a) has a cut-off wavelength of around 1200 nm, below which the waveguide becomes multimode. At 635 nm, this waveguide supports in total 6 modes for each polarization state and the TE modes are listed in Fig. 1(b). In this work, single-mode fibers (S630-HP) at 635 nm have been used to couple light from the laser source to the waveguide. Since the fiber mode has a Gaussian-like symmetric profile, higher-order modes cannot be generated with high efficiency when the fiber and waveguide are central aligned. This has been verified by the near field imaging experiment, where light from the output facet of the waveguide chip is imaged onto a CMOS camera, as shown Fig. 1(c). The captured field profile resembles a Gaussian shape, indicating that the ground mode has been excited with little composition of higher-order modes. The diameter of the near-filed profile is measured as 3.27 µm which is very close to the simulated value of 3.13 µm.
The fabrication follows the same process as reported in our previous work [22]. The process is summarized in Fig. 2(a). The under-cladding and core polymers layers are cured on a 4-inch silicon wafer by spin coating and subsequent UV / thermal curing. The waveguide structures are defined by standard contact photolithography, followed by a lift-off process to pattern the thin metal (Ti) hard mask. Inductively coupled plasma (ICP) etching is used to fully etch the core polymer layer. The metal mask is then removed by wet chemical etching. The top view of the fabricated polymer channel waveguide before upper cladding is spin-coated and cured is shown in Fig. 2(b). Since both the core and cladding materials belong to the same Exguide series of perfluorinated acrylate resins, we assume the waveguide with and without upper cladding will exhibit similar LTIT effect. However, the coated polymer upper cladding protects the core from the environment, e.g., scattering caused by dust accumulation. It can also reduce the contrast of the refractive indices between the core and the cladding and increase the mode field area of the ground mode at 635 nm wavelength to match better with the chosen fiber. Finally, the wafer is diced into waveguide chips as shown in the inset of Fig. 2(b) with a standard sawing machine. The cross-sectional view of the channel waveguide facet without polishing is shown in Fig. 2(c). Since the polymer material is not as brittle as silicon and glass, the waveguide facets after dicing have already reached good smoothness without polishing [10,15,20]. Index matching fluid can further improve the coupling between the fiber and the waveguide.
Fig. 2. (a) Flowchart of the waveguide fabrication process. Microscopic images of the polymer waveguide. (b) Top view (inset: image of a fabricated waveguide chip) and (c) cross-sectional view.
3. Measurement results
For the waveguide characterization in the C and L band, standard cut-back measurement was performed using waveguides of different lengths. At 1550 nm, these waveguides show a propagation loss of 0.86 dB/cm and a coupling loss of 0.26 dB/facet to a reduced-core fiber [22]. The polarization dependent loss is negligible.
Figure 3(a) shows our waveguide testing bench. Light from a 635 nm laser (Daheng Optics GCI-0903) is coupled to S630-HP fiber and injected to the polymer waveguide. The laser has been pre-characterized and shows stable output with the standard deviation of power fluctuation below 0.3 dB during 24 hours at optical powers above 1 mW. On the output side, transmitted light is coupled to a visible light power meter (CNI PS100). Piezo controllers are used to fine adjust the fiber-chip edge-coupling position at 10 nm-step in X, Y and Z directions separately. A near-infrared (NIR) tunable laser kit (EXFO T00S-HP & CT440-PDL) and an optical spectrum analyzer (YOKOGAWA AQ6370C) are also equipped for further experiments. Details of the 6-axis fiber stages and the chip placement stage are shown in Fig. 3(b) together with the input and output fibers. Figure 3(c) shows a zoomed-in photo of a waveguide chip being injected with 635 nm laser and the red line indicating laser transmission in the waveguide can be clearly observed.
Fig. 3. (a) Photo of the experimental setup. (b) Photo of the fiber-chip-fiber alignment system. (c) Zoomed-in view of the fibers and the polymer waveguide chip.
In the first step, we took a chip containing a series of 1-cm-long waveguides and measured their transmission characteristics versus the injection time using the 635 nm laser. Figure 4(a) plots the measured transmission of different waveguides injected with 5, 10 and 15 mW power from the 635 nm laser source. We recorded power values from the visible light power meter every 3 minutes and normalized it to the reference, i.e., the previously obtained transmitted power of the fiber-to-fiber transmission without the chip. The results indicate that measured transmission curves all saturate around −7 dB after 60 minutes, which shows that the transmission limit is independent of the injection doses. However, the curves corresponding to higher laser powers have significantly steeper slopes during the initial period of injection (0-20 min). Solid curves in Fig. 4(b) represents the measured transmission of 0.5-cm-long, 1-cm-long and 2.5-cm-long waveguides all injected with 10 mW laser power at 635 nm wavelength. We find that both the transmission transparency tuning range (in dB) and the duration of high-rate transmission growth (in minutes) scale proportionally with the waveguide length, leaving longer waveguides with more pronounced LTIT effect. However, longer waveguides, e.g. > 3 cm, will result in significantly low transmitted power at the initial stage of rapid change, bringing much noise and uncertainty in the collected data. Therefore, we only used 1-cm-long waveguides for the subsequent experiments. Furthermore, data points of the three waveguide lengths at the start and after 60 minutes laser injection are obtained and analyzed using the cut-back method. The results show that the waveguide loss is sharply reduced from 16.47 dB/cm at the start, to 3.69 dB/cm after 60 minutes of laser transmission, presenting a decrease of more than 75%.
Fig. 4. (a) Relation between the transmission characteristics of the 1-cm-long waveguides and the injection time with different 635 nm laser powers. (b) Solid curves: relation between the transmission of 0.5-cm-long, 1-cm-long and 2.5-cm-long waveguides and the injection time under 635 nm laser injection. The laser power for all three tests is 10 mW. Crosses: tested data for waveguide of the three lengths at the start and after 60 minutes. Dashed lines: Fitting results of the cut-back method for waveguide losses at the start and after 60 minutes. Microscopic images of (c) the straight waveguide and (d) the bending waveguide at 0, 20, 40 and 60 min during 635 nm laser injection at 10 mW laser power. White line: indication of waveguide path.
Figure 4(c) shows optical microscopic images of the 1-cm-long waveguide taken at 0, 20, 40 and 60 min during the 635 nm laser injection with 10 mW power. The overall brightness of the waveguide near the output port increases prominently with the time, which is also consistent to the results in Fig. 4(a). Figure 4(d) shows optical microscopic images of the bend waveguide under 635 nm laser injection of 10 mW power at the same four time points. The 0.57-cm-long waveguide that consists of two S-bends belongs to a directional coupler from the same wafer and has the same cross-section design as the straight waveguides. This directional coupler was intended to work at 1550 nm wavelength. FDTD simulations show that the ground mode at 635 nm cannot efficiently couple across the 3.5-µm gap to the other waveguide of the directional coupler. Therefore, we assume that the guided ground mode at the wavelength of 635 nm propagates only in the bend waveguide, which has beenam verified experimentally. Similar to the results shown in Fig. 4(c), the increase in the brightness of the output port can still be clearly observed through the imaging time span. These results illustrate the LTIT effect in both straight and bend waveguides.
In the following step, visible and NIR systems were combined to explore the effects of the LTIT at 635 nm on other wavelengths. Figure 5(a) shows the measured and normalized transmission spectrum using the NIR system of the three waveguides that had been injected with 5, 10, and 15 mW laser power at 635 nm (corresponded to Fig. 4(a)). At the same time, a waveguide without LTIT implementation was also measured and presented as comparison. A slight increase in the transmission over the entire wavelength range of 1500-1630 nm occurs in waveguides that have been injected with a 635 nm laser beforehand. The slight periodical variation of the spectrum is caused by the Fabry-Perot effect at the facets of waveguides due to the mismatch of the (effective) refractive indices among the fiber mode, the waveguide mode, and the index matching liquid applied.
Fig. 5. (a) Measured transmission spectrum of NIR wavelengths (1500-1630 nm) of waveguides injected with 635 nm laser under different powers. (b) Relation between the transmission and the laser injection time of 635 nm at NIR wavelengths (1500, 1550 and 1620 nm). Input laser power for both 635 nm and NIR wavelengths are set to 10 mW.
Next, we established a system to simultaneously couple the 635 nm and NIR lasers into the waveguide and carried out the measurement in real time. To ensure low fiber loss at NIR wavelengths, SMF-28e fibers were used in the entire link. At the input, 635 nm light and NIR light are coupled into the input fiber through a fiber coupler. At the output, the optical fiber was directly connected to the optical spectrum analyzer to monitor the transmitted power of multiple wavelengths simultaneously. Figure 5(b) shows the dynamics of transmission measured at 635, 1500, 1550 and 1620 nm over 60 minutes, respectively. Data was acquired from the optical spectrum analyzer by power integration every 5 minutes. The increasing trend and the range of the transmission at 635 nm are very close to the measurement results by the visible light system shown in Fig. 4(a) and prove the repeatability of the induced transparency. Transmission at NIR wavelengths maintains basically stable after a very small increase within the initial time. Especially at 1620 nm, where the polymer material has strong absorption, its transmission did not change abnormally throughout the time section. The induced transparency by 635 nm laser injection is considered have little effects on the transmission characteristics at NIR wavelengths.
In the third experiment, to verify whether the LTIT effect at 635 nm has a long-lasting memory, the following experiments were designed. Another 1-cm-long waveguide was treated at 10 mW laser power (time span 60 min). The transmission vs. time at this initial injection stage is corresponding to the blue curve in Fig. 6 and is in good agreement with the measurement results under the same lab conditions as shown in Fig. 4(a). Accordingly, the experiments were conducted again under the same conditions 5 hours, 7 days and 15 days after the initial injection and measurement, each with a time span of 30 min. In order to ensure the accuracy of the results, the measured transmitted power is normalized with the data from fiber-to-fiber referencing at the corresponding test time point. Curves in Fig. 6 clearly demonstrate that the subsequently measured transmission basically continues to grow at a saturating rate, starting from the termination value of the previous measurement. These results demonstrate that the waveguide transmission at 635 nm is substantially stable and can be memorized over a long time at lab environment.
Fig. 6. Measured transmission at 635 nm of the same 1-cm-long waveguide at the initial laser injection and 5 hours, 7 days and 15 days after. The laser power for all measurements is set to 10 mW.
In the final step, we explored the effect of heating on waveguides after 635 nm laser injection. Transmission at 635 nm, again measured at the same laser power (10 mW), yielded a reduction of about 5 dB after being heated on a hotplate at 200 °C for 20 minutes, as shown in Fig. 7. This demonstrates that heating under certain conditions can restore the waveguide to its high-loss stage, at least to a certain extend. Continuous injection for the next 60 minutes again pushes the transmission to the pre-heating level. One such process was referred as a heating-injection cycle (HIC). In the subsequent 4 HICs, changes in transmission showed considerable reproducibility, allowing the transparency state to be thermally reset and rewritten by LTIT at 635 nm for multiple times. Subsequent experiments demonstrated that the thermal reset can be initiated when the temperature reached 80 °C. But in order to fully recover to the state before LTIT, the waveguide still needs to be baked at higher temperature (200 °C) for 20 min.
Fig. 7. Transmission at 635 nm of the same waveguide after heated at 200 °C for 20 min. For each measurement, the time duration is 60 min.
Additionally, we tested the same waveguide at NIR wavelengths after 5 HICs. A small but non-negligible increase in the transmission over the entire test wavelength range (1500-1630 nm) was observed as Fig. 8 shows. It is worth noting that the transmission at the material absorption peak of 1620 nm increased about 0.8 dB after HICs, which may be attributed to the denaturation of polymer materials under repeated heating and injections. These unique features of long-term stability and thermal resettability that introduced in Fig. 6 and Fig. 7 can be adopted to develop a store-and-erase process in memory applications. Moreover, the combination of the nonlinear time response of LTIT and its erasability may also provide a potential solution for burn-after-reading optical encryption.
Fig. 8. Measured waveguide transmission spectrum in the NIR (1500-1630 nm) before and after heating and injections with 635 nm laser for 5 cycles.
4. Discussion
Since commercial polymers with proprietary chemicals have been used to fabricate the waveguide, it raises some difficulty to explain the LTIT phenomenon from the specific molecular structure and electronic band diagrams. In this work, we attempt to lay out hypothesis both physically and chemically from the phenomenon, giving a logical speculation to the mechanism of LTIT.
First, we speculate that the thermal effect of laser transmission may cause changes to the physical properties of the waveguide, e.g., altered effective index of the guided modes and side-wall roughness of the waveguide core. For effective index change, it could be generated from variations in cross-sectional geometry. These variations may relate to the mode filed area of the excited waveguide ground mode and finally affect the facet coupling efficiency. Assuming that the length and width of the waveguide section are both increased by 10% due to thermal effects triggered by laser transmission, the coupling efficiency of the waveguide ground mode and fiber mode at 635 nm only increases 3.65% from numerical simulations, which is obviously not enough to explain the 75% reduction in waveguide loss of LTIT. Possible changes in sidewall roughness of the waveguide core are also believed to contribute very little but also not easily resettable. At the same time, changes at the physical level should affect transmission at a broad wavelength range. However, Fig. 5(a) and Fig. 8 both show the very weak effect of LTIT at near infrared. Therefore, the initiation of LTIT may not attribute to the change in waveguide physical properties.
Next, we focused our attention on the underlying photochemical mechanism of LTIT. Figure 9 shows the crosslinking process of the ZPU12-RI series polymers [25]. Rf in the monomer refers to a fluorinated group for refractive index adjustments [6]. UV-curable polymers, represented by poly(methyl methacrylate) (PMMA) and its derivative perfluorinated acrylates, require additives to make sure the polymerization process can be properly initiated [24–29]. This class of UV-sensitive additives or photoinitiators can generate strongly oxidizing free radicals under broad-spectrum ultraviolet irradiation. As indicated by Ro in Fig. 9, free radicals will attack the C = C bond in the polymer monomer molecule and initiate polymerization [25]. Since the degree of monomer crosslinking cannot reach 100%, these reacted / unreacted chemically active organic molecules are considered to be strongly associated with the transmission induced transparency occurring at 635 nm in polymer waveguides. In acrylate-based polymers, photo-induced refractive index change [30–32] and photochromism [33–35] triggered by additive components have been reported. Thus, although photons at 635 nm have slightly lower photon energy, we still have reason to believe that the extremely high light intensity (on the order of 1×1011 mW/m2) confined in the waveguide core may also induce the denaturation of photoinitiators and other additives. The photoinduced reaction can cause the molecules or groups with absorption peak at 635 nm to be decomposed into low-absorbing free radicals [36], which is manifested as an increase in transmission. As extracted from Fig. 3(a), the saturating trend of the waveguide transparency rate over long-time injection is also consistent with the basic characteristics of the chemical reaction: Stronger conditions (635 nm laser power) result in higher reaction rates, while the quantity of the reactants determines the degree of reaction (absolute increase in the transmission).
The thermally resettable properties of the waveguide transparency may likewise be explained by the behavior of free radicals. In the electronic domain, polymer transistors that use optical, electrical, and thermal signals for writing, reading, and erasing have been reported. Photo-induced decomposition of additionally dosed imidazoles may improve the transistor transfer characteristics, while the thermal recombination of free radicals which are decomposition products from laser injection allows the optically written information to be erased [36]. In-depth study of the phenomenon would require the exact material composition and will be carried out in our future work, in cooperation with the material developer.
5. Conclusion
To conclude, we have demonstrated the phenomenon of thermally resettable laser transmission induced transparency at 635 nm in polymer waveguides, fabricated using commercial perfluorinated acrylates with proprietary components under a mature and low-cost technology. When the 635 nm laser is coupled into the waveguide by fibers, the transmission at this wavelength increases significantly with time. Quantitative experiments reveal that the rate of transparency tunning is positively related to the laser power, while the tunable range is an intrinsic property of the waveguide independent of the laser power. The transmission increase at 635 nm has little improvement on the transparency at NIR wavelengths (1500-1630 nm). Through multiple tests the induced transparency state appears to be long-time stable in lab environment. Moreover, it is shown that the transparency state can be reset by thermal treatment. Baking at 200 °C for 20 minutes can reverse the transparency of the waveguide by about 5 dB towards the initial, high-loss state. The LTIT effect and thermally induced loss afterwards both exhibit good repeatability. In the end, we argue that the phenomenon is caused by the decomposition under laser illumination in a confined space (waveguide), i.e., at much higher intensity level compared to the conventional exposure in free space, and thermal resynthesis of organic photo-sensitive molecules in the polymer waveguide at elevated temperature.
As future work, we plan to investigate the LTIT effect at other wavelengths and systematically analyze the mechanisms of the phenomenon from the material perspectives. We believe this discovery will bring opportunities to develop polymer waveguides for advanced photonic applications such as memory and encryption devices.
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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