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Novel polymer thermo-optic switch arrays were successfully designed and fabricated based on dielectric-loaded surface plasmon polariton waveguide (DLSPPW) structure. Highly fluorinated low-loss photopolymers and organic–inorganic grafting materials were used as the waveguide core and cladding, respectively. The low absorption loss characteristics and excellent thermal stabilities of the core and cladding materials were obtained. The proposed DLSPPW model was included of fluorinated polymer ridge with 4 × 4 μm2 size loaded on 60-nm thin gold stripe electrode heaters, organic-inorganic grafting material cladding and PMMA substrate. The operation of the device at signal wavelengths is controlled via the thermo-optic effect by electrically heating the gold stripes of dielectric-loaded surface plasmon polariton waveguides. The optimized structural properties of dielectric-loaded surface plasmon polariton waveguides were provided. The propagation loss of a 4-μm wide straight DLSPPW was measured as 0.55 dB∕cm at 1550 nm wavelength. The insertion loss of the device was measured to be about 4.5 dB. The switching rise and fall time of the device applied by 200 Hz square-wave voltage were obtained as 287.5 μs and 370.2 μs, respectively. The switching power was about 5.6 mW, and the extinction ratio was about 13.5 dB. The flexible low-loss multi-functional waveguide switch arrays are suitable for realizing large-scale optoelectronic integrated circuits.
© 2015 Optical Society of America
1. Introduction
Photonics integrated devices and circuits are widely applied for light switching and routing system in the rapidly developing fields of broadband optical communications [1–5 ]. Such devices are traditionally based on the light propagation in optical waveguides consisting of dielectric core and cladding structures, due to the refractive index of core being larger than that of the cladding layer. Propagating electromagnetic radiation confined in the core by waveguide modes can be modulated with externally applied electrical signals via electro-, magneto-, and thermo-optic (TO) effects (depending on the dielectric properties and electrode configuration). It is noted that extra absorption loss is generated by metal controlling electrodes of the active device. The action of absorption could be minimized by increasing separation between electrode and waveguide, but that would decrease the response effects of the chip. The technique of selecting the right design positioning for metal electrodes of conventional waveguide modulators and switches is facing on a challenging problem [6,7 ]. Compared to optical waveguides, plasmonic devices can guide and manipulate optical signals on a sub-wavelength scale and below the diffraction limit of light. Among various surface plasmon polariton (SPP)-based waveguide configurations, dielectric-loaded SPP waveguides (DLSPPWs) represent an attractive alternative by virtue of being naturally compatible with different dielectric and industrial fabrication using large-scale UV lithography [8–10 ]. DLSPPWs satisfy the important requirements of strong mode confinement, relatively low propagation losses, and straightforward integration with control electrodes enabling TO effect. The main advantage of this plasmonic technology is that metal stripes can be used both as supports of DLSPPWs and electrodes, allowing thereby efficient heating of DLSPPW ridges and TO control of the DLSPPW mode index, since the mode field reaches its maximum at the metal-dielectric interface [11–13 ]. The ability to transmit light and electricity in the same component simultaneously opens interesting possibilities for hybrid electrical-optical integration [14,15 ].
The key issue in plasmonic technology is high attenuation related with metal-induced absorption. This impact can be minimized by integration with low loss dielectric waveguides. In this way, the small size and low power switching capabilities of plasmonic can be blended with the low loss of dielectric waveguides and processing capacity of electronics, to provide miniaturized and power efficient optoelectronic interconnect chips. Several material systems [16–18 ] have been used to realize dielectric waveguide, the notable being lithium niobate, silicon-on-insulator (SOI), InP and polymers. In recent demonstration of DLSPPWs heterointegrated with photonic waveguides, SOI [19] and polymer PMMA [20] DLSSWs have been attractive and reported as active waveguide platforms. As a multifunctional material system, polymers exhibit well-controlled refractive indices, highly flexible structures, and large thermo-optic (TO) and electro-optic (EO) coefficients [21–25 ], which can be advantageous to reduce manufacturing costs and open possibility of monolithic integration with functional devices such as lasers and detectors. To improve the performance of DLSPP-based switches, the direction towards the reduction of power consumption, footprint and losses has been identified by demonstrating the use of Cyclomer [26] or SU-8 [27] instead of PMMA as the polymer loading of DLSPPWs. The propagation loss factor of the Cyclomer-loaded DLSPP waveguide was also measured to be consistent with previous measurements of Si-DLSPP heterointegrated structures. Compared with the polymer Cyclomer material, fluorinated photopolymer [28] synthesized in our work has lower absorption loss. The reason realizing less absorbance is that the high overtone absorptions of the C-H bonds for Cyclomer or SU-8 can cause drastic optical loss in the optical communication wavelength regions around 1310 and 1550 nm. Using fluorine to replace hydrogen in polymers can theoretically reduce the intrinsic optical loss by about 5 orders of magnitude for fluorinated photopolymer because the heavier fluorine atoms shift the vibrational overtone absorption signals to longer wavelengths. Choosing the fluorinated photopolymer as dielectric-loaded material for DLSSWs is very beneficial to reduce the transmission loss of the device.
In this paper, novel TO waveguide switch arrays based on dielectric-loaded surface plasmon polariton waveguides were successfully designed and fabricated by the direct UV-written technique. Highly fluorinated photopolymer and organic-inorganic grafting material were composed as dielectric waveguide and cladding materials, respectively. The low absorption loss and excellent thermal stability of the core and cladding materials were obtained. Optical properties of the proposed TO switch were analyzed, simulated, and measured. The fabrication process of the device was described in details. Optimized structural characteristics of the dielectric waveguides and metal stripes were provided. The performances of the switch arrays were achieved.
2. Experimental
2.1 Waveguide material
The proposed highly fluorinated low-loss photopolymer was used as the dielectric waveguide material which is composed of fluorinated bis-phenol-A novolac resin (FSU-8) and fluorinated polycarbonate epoxy resin (FPER) [29]. Since the highly fluorinated photopolymer can be patterned by the direct UV written techniques, it is relatively superior to other fluorinated materials in integrated optical circuit application. To adjust the refractive index, the FSU-8 was synthesized and doped into FPER. Furthermore, the fluorine content of the mixture is improved significantly after doping, which is very conducive to the two-photon polymerization cross-linking reaction in the UV written process. The organic-inorganic grafting poly(methylmethacrylate) (PMMA) material [30] was used as the cladding material due to its low processing temperatures, good thermal stability, and compatibility with established silicon fabricating procedures. The SiO2-TiO2-PMMA cladding material was synthesized by hydrolysis and polycondensation of 3-methacryloxy-proyltrimethoxysilane (MAPTMS,KH570), (3-glycidoxypropyl) trimethoxy-silane (GPTMS, KH560), methylmethacrylate (MMA), epoxypropylmethacrylate (GMA), tetraethylorthosilicate (TEOS), and tetrabutyl titanate (Ti(OC2H5)4). The functional organic–inorganic network is realized by involving silica-based inorganic network into cross-linkable PMMA with grafting modification techniques. The stable SiO2-TiO2-PMMA films are resistant to organic solvent. The molecular structure of FSU-8 and organic-inorganic grafting PMMA are shown as Fig. 1(a) and 1(b) , respectively.
Fig. 1 Molecular structure of FSU-8 and organic-inorganic grafting PMMA (a) FSU-8 and (b) organic-inorganic grafting PMMA.
Figure 2 shows the near-infrared absorption spectrums of the FSU-8/FPER compared to commercial SU-8 photoresist. The absorption spectrums on both of different photopolymers were measured by UV- vis-NIR spectrophotometer (SHIMADZU UV3600) with slit width 2 nm. The photopolymers were spin-coated on the quartz substrate, respectively. The thin films were formed by UV during crosslinking. The standard quartz substrate without polymer films was firstly scanned as benchmark. As the same measuring process, the absorbance data of the photopolymers was obtained from ultraviolet to near infrared light region. Absorption spectrums were generated by fitting and analyzing the data by Origin software. Obviously there are certain absorption peaks for commercial SU-8 photoresist at the near-infrared region, while there is almost no absorption for the FSU-8/FPER. The low-loss characteristic greatly enables the mixture to be used in the photonic integrated chips which are contributed for optical communications, even for the visible light communication. As given in Table 1 , Tg is the glass transition temperature of uncross-linked polymers measured by differential scanning calorimetry (DSC); Td C and Td D are the onset temperature for 5% weight loss of uncross-linked and cross-linked polymers measured by thermal gravity analysis (TGA), respectively. It can be noted that the Tg of the two kinds of mixtures is above 150°C, which is higher than the fabricating temperature of the device. Furthermore, the very high Td C and Td D of the FSU-8/FPER are suitable to improve the thermal stability of the chip. For the hybrid cladding material, the Tg of the SiO2-TiO2 grafting PMMA measured using the modulated DSC is about 135°C, which is higher than pure PMMA, of which it is about 100°C. The thermo-gravimetric analysis (TGA) is nearly no decomposition at around 200°C. Altering the content of TiO2 could be used to adjust the refractive index of the product. Moreover, the active integrated waveguide devices with TiO2 component have great potential in biomedical applications [31,32 ].
Fig. 2 UV–vis–NIR absorption spectrums of FPER with Different FSU-8 compositions compared to commercial SU-8 photoresist.
Table 1. Thermal Properties of Fluorinated Photopolymer with Different Content of FSU-8
2.2 Theoretical analysis and simulation
Therm-optic waveguide switch arrays based on dielectric-loaded surface plasmon polariton waveguides were fabricated on PMMA substrate exploiting the proposed fluorinated low-loss photopolymer FSU-8/FPER as the core layer and the SiO2-TiO2-PMMA as the cladding material, respectively. The refractive index was measured by an M-2000 UI variable angle incidence spectroscopic ellipsometer. The 25% FSU-8 doped photopolymer and SiO2-TiO2-PMMA have the refractive index of 1.526 and 1.513 respectively at 1550-nm wavelength. The refractive index of gold is 0.559 + 11.5i at 1550-nm wavelength. The DLSPPW structure is formed by placing a FSU-8/FPER ridge with a cross section of 4 × 4 μm2 size as the dielectric waveguide on top of a 50-nm thick and 20-μm wide gold strip, which is given as Fig. 3(a) . This geometry is provided with full 2D confinement in the plane perpendicular to the propagation direction. The calculation of plasmonic mode properties for this geometry has been performed by a finite-element method implemented in the commercially available software COMSOL multiphysics. The efficient coupling of the dielectric photonic waveguide modes into DLSPPs has been found only with TM-polarized light. The SPP mode-field distribution calculated for this geometry at the operating wavelength of 1550 nm exhibits tight lateral confinement, the fundamental transverse-magnetic (TM) TM00 mode and first higher-order TM01 mode supported at operating wavelength are shown as Fig. 3(b) and 3(c). The fundamental mode effective index neff is 1.516. The proposed TO waveguide switch is designed as Mach–Zehnder interferometer (MZI) structure. For the refractive index difference between core and cladding, the bend radius of S bend waveguide based on Y branch of MZI structure is defined more than 3000 µm so that the bend loss could be low. In this work, sine and cosine functions are adopted to design the bending structure, smooth transition between the waveguide junctions could be realized and advantageous to reduce the scattering loss caused by the bending waveguides. Besides, the suitable length of the transition region is designed to reduce the splitting angle of Y branch. It can be effective to reduce the bending loss. The gold stripes could be used both as DLSPPW and electrodes allowing for heating (with electrical currents) one MZI arm that is electrically isolated from the other arm by 200 µm wide gaps. The operation of a thermo-optic MZI is based on changing the mode propagation constant in a heated arm, resulting in the phase difference of the DLSPP mode that interferes in the output Y junction. The length of the heated MZI arm required to ensure complete modulation, i.e. switch off the light in the Y junction, is related to the introduction of the phase difference π between the arms:
For exactly equal length of the arms, introducing the phase difference π can be realized by heating one of them because the refractive index is temperature-dependent:Fig. 3 Analysis and simulation of the DLSPP waveguide properties (a) schematic representation of the DLSPPW cross section; (b) fundamental TM00 mode and (c) first higher-order TM01 mode distribution of DLSPPW of optimal configuration, the TM00 mode effective index neff = 1.516; (d) thermal field distribution of electrode.
TO coefficients (/) of the materials are measured by an attenuated total reflection (ATR) setup [33] with its temperature precisely controlled. In the measurement, a collimated light beam from a laser passes through a polarizer and is then incident upon the interface between the prism and the material film with an appropriate angle. The reflected light is detected by a photodiode and averaged to reduce the noise. The various synchronization angles of the ATR spectra at different temperatures and laser wavelengths are measured. Based on eigenvalue mode equations [34], the refractive index of TM polarization with different temperatures at signal wavelength are calculated, TO coefficients of the materials are analyzed by fitting the slope of the data by Origin software. The TO coefficients of the 25% FSU-8 doped FPER material is obtained as −1.85 × 10−4 K−1 and that of the SiO2-TiO2-PMMA is equivalent to −1.65 × 10−4 K−1 respectively. The thermal conductivities of various materials are directly measured by a thermal conductivity detector (TC3010, Xian Xiaxi Electronic Technology Co., Ltd, Xian, China) at room temperature. The thermal conductivity of PMMA substrate, FSU-8/FPER and SiO2-TiO2-PMMA is obtained as 0.2, 0.25 and 0.28 Wm−1K−1, respectively. The organic-inorganic grafting PMMA cladding material with higher thermal conductivity is characterized by lower thermal resistances than that of the pure PMMA substrate, dependent on the SiO2-TiO2 crosslink network of the hybrid material. The thermal characteristics are calculated for the TO switch properties based on the thermal profile change in the DLSPPW cross-section. When Lπ as the interaction length is defined as 1cm at λ = 1550 nm, the temperature increment ΔT of the electrode needed to switch the light is obtained as 0.5 K. Thermal field distribution characteristics of electrode based on the thermal profile change in the waveguide cross-section are shown in Fig. 3(d).
2.3 Device fabrication and measurement
The combined schematic configurations of both the waveguide and electrode heater masks of the TO waveguide switch arrays are given in Fig. 4(a) . It can be noted that the effective interaction lengths of the devices are all about 1 cm. The devices are designed, including M-Z waveguide structures and arrayed electrode heaters with different widths. The structural diagram of the switch arrays is constructed, as shown in Fig. 4(b). The periodic intensity modulation of the novel TO DLSPPW switching arrayed device can be realized for fiber arrays.
Fig. 4 (a) The schematic configuration of both waveguide and electrode heater masks and (b) the structural diagram of the DLSPPW switch arrays.
The fabrication process for the TO DLSPP waveguide switch arrays is shown in Fig. 5 . Firstly, a 5-μm thin film of SiO2-TiO2-PMMA material was formed on the PMMA substrate as the bottom cladding layer by spin-coating. The sol was filtered through a 0.22-μm Teflon membrane filter. The coated films were dried in a vacuum oven at 125 °C for 1 h in nitrogen atmosphere to remove the residual solvent and to simultaneously achieve cross-linked organic-inorganic network. The gold stripes were patterned by photolithography and wet etching. Detailed fabrication process of the gold electrode heaters are given that the gold film was deposited onto the organic-inorganic grafting PMMA bottom cladding by vacuum evaporation technique. The deposition time is 1 min, the vacuum reaches 1.3 × 10-3 Pa. After that, the BP212 photoresist was spin-coated on the gold film and pre-baked at 85 °C for 20 min to remove solvent. The gold electrode heaters were directly patterned using BP212 photoresist by photolithography and wet etching. The pattern exposure was performed at a 365 nm wavelength and 350 mW Hg lamp power by ABM high resolution mask aligner and exposure system. 365 nm wavelength output intensity is 20 mW/cm2 and exposure time is 6 s. The BP212 photoresist on the exposure area is removed in 5‰ NaOH solution by wet etching process at room temperature. The gold electrode heaters were formed in I2 (1 wt%) + KI (4 wt%) developer at room temperature. The chemical reaction scheme is shown as
The BP212 photoresist on the gold electrode heaters was also exposed and removed in 5‰ NaOH solution at room temperature. The 25% FSU-8-doped FPER material was spin-coated on the gold electrode to form a 4-μm thick thin film as the core layer. After being baked at 110 °C for 1.5 h to make the solvent completely removed, the wafer was then exposed to a 350 mW Hg UV lamp power through a contact chromium mask to pattern the dielectric waveguide, and the exposure wavelength and exposure time were 365 nm and 300 s, respectively. This step was followed by post-baking at 120 °C for 30 min, which contribute greatly to achieve cross-linked epoxy. After that the wafer was developed in propylene glycol-monomethyl ether-acetate (PGMEA) for 5 s, rinsed in isopropyl alcohol, then into de-ionized water, and blown dry to form the desirable DLSPP waveguides. It is vitally important to cure the wafer at 120 °C for 30 min so that the adhesion between DLSPP waveguides and the cladding layer can be enhanced well. And 4 μm thickness SiO2-TiO2-PMMA thin film was formed as the upper cladding, followed by curing-bake at 125 °C for 1 h. The upper cladding regions on the double pins of lumped electrode arrays were removed by inductively coupled plasma etching technique. The novel material system, design approach and fabricating process of the devices could be useful in realizing low transmission loss and excellent TO response characteristic of the all-polymer DLSPPW switch arrays. The coefficient of thermal expansion (CTE), α sub of the PMMA substrate used is about 1.12 × 10−4 K−1, and the temperature-insensitive condition of polymer substrate could be well satisfied to prevent wavelength shift of the device based on Peltier thermo-controller equation [35,36 ]. This improves the noise immunity of the signal TO intensity modulation derived by the DLSPPW electrode heaters. The good flexible structure is also very suitable for lying in narrow space.The scanning electron microscope (SEM) is used to obtain the micrograph of the dielectric waveguide. Figure 6(a) gives the cross-section view of the input port of the dielectric waveguide structure. The smooth and vertical waveguide sidewall could be greatly helpful to reduce the scattering loss. Figure 6(b) shows the roughness of the surface of the gold stripes. The surface roughness less than 1.5 nm was measured by atomic force microscope (AFM). The measured total resistance is 150 Ω. The value of each resistance is changed with different widths of the arrayed DLSPPW electrode heaters. When the applied voltage is constant, the powers generated in the arrayed DLSPPW electrode heaters are proportional, and arrayed phase modulations of the light signals can be implemented.
Fig. 6 (a) SEM image of dielectric waveguide section fabricated with FSU-8/FPER: (b) AFM image of the gold stripe surface.
In the process of testing, the horizontal coupling mechanism is used for measuring the devices. Signal light is directly coupled to the photonic waveguide from the small-diameter tapered optical fiber. The efficient mode coupling between the photonic and DLSPP waveguides could been realized in the active region as one arm of the MZI switch. The output optical power of the device is coupled into the output fiber.
As shown in Fig. 7 , the propagation loss of a 4-μm-wide DLSPP waveguide is measured by the cutback method at 1550 nm, and found to be 0.55 dB/cm. The principle of the cut-back measurement is shown as
α is the propagation loss at 1550 nm wavelength, L is the length of straight waveguide. Based on the data of Loss (insertion) from different values of L, take advantage of linear fitting, y = ax + b, the propagation loss is the slope of the linear equation and Loss (coupling) is the intercept with the vertical axis.Schematic photographs of the proposed all-polymer TO DLSPPW switch arrays measured are shown as Fig. 8(a) . Figure 8(b) gives the near-field patterns of the device. Signal light at 1550 nm was launched to the input port of the MZI waveguide through a customized small-diameter polarization maintaining polymer optical fiber, which was tapered for the mode matching between the fiber and dielectric waveguide. The type of splitter/combiner used is Y branch structure in the MZI. When optical power of input fiber as 0 dBm (1mW) is coupled into the device, 4.5 dBm output power is measured by output fiber coupling into the optical power meter. The insertion loss of the device could be calculated and directly obtained as 4.5 dB. The insertion loss is derived from mode mismatching loss between fibers and waveguides, mode coupling loss between photonic and DLSPP waveguide, and transmission loss including bending, absorption, and scattering loss. Mode coupling loss between photonic and DLSPP waveguides and transmission loss of DLSPP waveguides may be the main impacting factors for the relatively high insertion loss.
Fig. 8 (a) Actual photographs of the proposed polymer all-polymer TO DLSPPW switch arrays measured. (b) Near-field guide-mode patterns of the device with signal light at 1550 nm wavelength.
In the process of measuring the DLSPP waveguide, using the high sensitivity galvanometer testing, we found vanishing current from both pins of the SPP electrodes. When signal optical power is coupled into the DLSPP waveguide, the current is generated immediately. After the optical power remains stable, the current will decay rapidly and disappear. As shown in Fig. 9 , It is indicated that 14.2 nA and 18.4 nA transient currents were obtained at 650 nm and 1550 nm wavelength as the same input optical power as 1 mW, respectively. The reasons for this phenomenon should be that the vanishing current is generated by Seebeck effect due to the gradient of temperature along the DLSPPW. The temperature gradient between both of the entrance and output waveguides could form thermo-electric power. Along the plasmonic waveguide, thermal current could be generated by the thermo-electric power. When heat diffusion leads to a flat temperature distribution along the sample, the thermal current will vanish. We also found that compared with that at 1550 nm wavelength, the response characteristic at 650 nm wavelength is relatively fast. The reason may be that as resonance wavelengths, SPP mode excitation is more sensitive and better able to meet the wave vector matching condition in the DLSPPW device at 650 nm than that at 1550 nm wavelength.
A square-wave voltage was applied to the electrode heater of DLSPPW with two needle-like probes. The output signal light from the switch was coupled into a photodiode detector and observed using an oscilloscope. Figure 10(a) shows the TO switching response observed by applying square-wave voltage at a frequency of 200 Hz. The rise and fall times were 287.5 and 370.2 µs, respectively, and the average switch on–off time was about 300 µs. Figure 10(b) shows the channel output intensity versus power consumption of the optical switch at 1550 nm for TM mode. The extinction ratio of the TO switch was about 13.5 dB. The applied electric power as the switching power was actually 5.6 mW, and the corresponding temperature increment of the electrode was 1.5 K. The MZI thermo-optic switch could realize an S-C-L optical band (1460 to 1625 nm) covering a wide-spectrum. In the scope of the bandwidth, the largest changes for the extinction ratio and insertion loss of the device are 0.5 dB and 0.8 dB, respectively. There is no significant effect on the switching power and the TO switching response. Comparison with key performance parameters of the other reports is shown as Table 2 . It could be observed that the proposed FSU8/FPER DLSPPW TO switches could achieve stable operation well with large TO thermo-optic coefficient, high extinction ratio, lower insertion loss and power consumption. The advantages of the performances from overall device can be obviously noted. In addition, the fabricating technique of the waveguide device using directly UV defined process is more convenient for realizing integrated photonic circuits.
Fig. 10 Performances of the device: (a) DLSPPW TO switch responses obtained by applying a square-wave voltage at frequency of 200 Hz, (b) actual channel output versus power consumption of the DLSPPW TO switch at 1550 nm for TM mode.
Table 2. Comparison with other published results for different material dielectric-loaded DLSSW TO switch.
3. Conclusion
In summary, DLSPPW TO switch arrays at 1550 nm have been successfully obtained based on the direct UV-written technique. Novel highly fluorinated low-loss photopolymer and organic–inorganic grafting materials were used as core and cladding, respectively. The thermal and optical properties of the core and cladding materials were investigated. The TO coefficients of the FSU-8/FPER material and SiO2-TiO2-PMMA were obtained as −1.85 × 10−4 °C−1 and 1.65 × 10−4 °C−1, and the glass transition temperatures were tested to be 158°C and 135°C, respectively. The transmission loss for a 4 μm wide DLSPPW was measured to be 0.55 dB∕cm by the cutback method. The electrode heaters of DLSPPW were theoretically designed to achieve arrayed phase modulations. The rise and fall times of the device applied 200 Hz square-wave voltage were 287.5 and 370.2 µs, respectively. The insertion loss of the device was directly measured to be about 4.5 dB. The extinction ratio was measured to be about 13.5 dB and the applied electric power as the switching power was about 5.6 mW. The phenomenon of the minute current generated makes the DLSPPW device be multi-functional and potential for biomedical applications. The low-loss DLSPPW integrated switch arrays are advantageous to improve stabilities and realize cross-connectors for the large-scale PICs, such as OXC and OADM systems.
Acknowledgments
The authors gratefully acknowledge financial support from National Natural Science Foundation of China (No. 61107019, 61475061, 61405070, 61177027, 61275033, 61205032, 61261130586), Ph.D. Programs Foundation of Ministry of Education of China (No. 20110061120054, 20130061120060), Science and Technology Development Plan of Jilin Province (No. 20130522151JH, 20140519006JH).The Fundamental Research Funds for the Central Universities (No. JCKY-QKJC08).
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