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Abstract
Tunability is a fundamental prerequisite for functional devices and forms the backbone of reconfigurable microwave photonic (MWP) signal processors. In this paper, we explore the use of indium tin oxide (ITO) thin films, notable for their combination of optical transparency and electrical conductivity, to provide tunability for integrated MWP devices. We study the impacts of post-thermal annealing on the structural, electrical, and optical properties of ITO films. The annealed ITO microheater maintains a low total insertion loss of just 0.1 dB while facilitating the tunability of the microring across the entire free spectral range (FSR) using less than half the voltage required by its non-annealed counterpart. Furthermore, the post-annealed ITO film exhibits a 30% improvement in response time, enhancing its performance as an active voltage-controlled microheater. Leveraging this advantage, we employed the post-annealed device to demonstrate continuous tunable radio frequency (RF) phase shifts from 0–330° across a frequency range spanning 15 GHz to 40 GHz with only 5.58 mW of power. The flexibility in modifying the ITO thin film properties effectively bridges the gap between achieving low-loss and high-speed thermo-optic based microheaters.
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1. Introduction
The field of microwave photonic (MWP) has evolved significantly over the last decade, driven by advancements in integrated photonics. These breakthroughs have enabled the integration of MWP devices with diverse functionalities condensed into a nanoscale footprint [1,2]. Out of the many integration platforms, silicon photonics has been a promising companion in the rapid growth of dense photonic integrated circuits due to its compatibility with CMOS technology [3]. The relentless pursuit of reconfigurable devices that can outperform their predecessors in terms of speed and power consumption whilst maintaining low optical loss has spurred ongoing efforts to seek new active materials capable of breaking the thresholds in existing silicon-based devices. The use of novel materials with CMOS compatibility is a critical consideration for technological adoption and co-integration of electronic and photonic circuitry on a single chip, which is especially relevant for MWP applications. This has prompted the hybridization of various silicon compatible materials to provide a more efficient tunability over the optical properties of the guided optical signal [4–7].
The large thermo-optic coefficient in silicon enables efficient thermal manipulation of the optical properties of light traveling through the silicon medium, rendering it an appealing tuning mechanism in silicon-based devices. This has been well demonstrated via the integration of metallic thin films [8–11] or doped silicon lines [12,13], which are placed at a safe distance away from the waveguide to avoid considerable optical loss associated with metal absorption effect or free carrier absorption in doped silicon. Minimizing the loss entails implementing critical design criteria such as precise positioning of the heat source or careful design of the doping distribution [14]. However, ensuring the effectiveness of these loss mitigation strategies often requires strict control over the fabrication accuracies. Additionally, alternative Joule heating elements incorporating 2D materials such as graphene demand specialized transfer processes [15,16].
Recently, the use of transparent conducting oxides (TCOs) to electrically manipulate light has been explored extensively due to their high transparency, good conductivity, and CMOS compatibility with silicon photonics [17,18]. In particular, the use of Indium Tin Oxide (ITO) has facilitated optical tunability and strong index modulation in various photonic applications [19,20]. The beauty of ITO film lies in the fact that its material properties can be tuned by modifying the deposition conditions and doping concentration to suit specific applications. Rigorous characterizations of ITO thin films have been demonstrated and optimized for a variety of applications, ranging from providing transparency at visible wavelengths to enabling operations under epsilon-near-zero conditions in the field of plasmonics [21]. In recent investigations, various nonlinear effects of ITO film such as nonlinear absorption [22,23] and Kerr nonlinearity [24], have also been studied. These nonlinear features position ITO thin films as ideal candidates for photonic applications, highlighting their potential suitability for various optical functionalities. ITO thin films have emerged as promising resistive contacts, harnessing their optical transparency and electrical conductivity to pave the way for low-loss thermo-optic phase shifters [25–27]. The tunability of ITO properties offers a vast spectrum for engineering and fine-tuning thermo-optic dependencies tailored to specific applications. Typically, post-deposition annealing is employed to enhance the crystallinity and bolster electrical conductivity in ITO films [28]. Further studies have also revealed that annealing induces changes in surface morphology and increases thermal conductivity [29]. However, the exploration of the necessity for thermally treated ITO film depositions, directed towards optimizing microheaters for MWP applications, remains largely unexplored, particularly in terms of the optical and electrical tunability within resonating structures such as microring resonators (MRRs). While previous studies have primarily focused on demonstrating the feasibility of using ITO thin films as microheaters, there is a gap between the material studies of ITO thin films and their impact on the overall performance of the final devices. In this work, we aim to bridge the gap by establishing a material-to-device understanding of how variations in the deposition of ITO film influence the performance of the final devices for MWP applications operating specifically in the near-infrared (NIR) region. We introduce, for the first time, tunable MWP devices incorporating thermally-annealed ITO microheater that leverages the optical transparency and electrical conductivity of these optimized ITO thin films to strike a balance between low insertion loss and fast tunability. This balance is crucial to advance the design and optimization of MWP devices with tunable capabilities, where the ability to manipulate and control microwave signals with precision is essential.
In this paper, we characterize the material properties of the ITO thin films employed as microheater elements for MWP phase shifter devices based on MRR by considering the optical transparency, electrical conductivity, and thermo-optic tunability of different ITO thin films. We investigate the influence of the ITO characteristics on the optical loss and electrical tunability of the MWP device by comparing both pre- and post-annealing conditions of the ITO film. Our results show that subsequent annealing of the ITO thin film resulted in a total insertion loss of only 0.1 dB, attributed to the ITO microheater. The low absorption loss of the ITO film reduces the need for a thick insulation layer and allows the heater source to be placed more closely to the waveguide. The post-annealed ITO thin film also shows an improvement in the average response time by 30%, providing the flexibility to modify layer properties to meet application-specific demands. Experimental results demonstrate the capability of the ITO-based microheater in achieving a continuous MWP phase shifting operation from 0 - 330° using only 5.58 mW of power. This provides tunability across the entire phase transient of the single MRR, thus enabling full RF phase control spanning a frequency range of 15 GHz to 40 GHz, with no additional amplitude variations introduced by the microheater.
2. Fabrication and characterization of ITO thin films
The schematic diagram in Fig. 1 shows the topology of a conventional MWP phase shifter system. Here, a tunable optical phase shifter (OPS) is employed to optically modify the phase shift of a single sideband (SSB) modulated RF signal, thereby inducing a corresponding RF phase shift in the microwave domain [30]. The ability to generate different RF phase shifts is empowered by the tunability of the OPS, which is based on an MRR fabricated on a commercial SOI platform, as illustrated in inset (a) of Fig. 1. An ITO film with a targeted thickness of 100 nm was first deposited on the substrate at room temperature via controlled sputtering of indium-oxide/tin-oxide (In2O3/SnO3) target in an Argon (Ar) + Oxygen (O2) plasma chamber, as depicted in inset (b) of Fig. 1. The chamber pressure was set to 2.2 × 10− 3 mTorr. The substrate was securely affixed to the sample holder using tape, and the distance between the target and the substrate was about 24 mm. The argon flow rate was 38.5 sccm, and the oxygen flow rate was 0.25 sccm. A radio frequency (RF) sputtering power of 35 W was used. Further post-processing thermal treatment is carried out to investigate the effects of annealing for our application. Considering the influence of different annealing temperatures on the ITO properties, the films are subjected to annealing at 350°C, 450°C, and 550°C for 30 minutes within the vacuum chamber, with a controlled flow of argon gas. The OPS, based on MRR, adopts a racetrack resonator design comprising low-loss 2 µm wide multimode straight waveguides and 500 nm wide single-mode bend waveguides. Spanning a total circumference of 850 µm, the MRR incorporates shallow-etched rib waveguides with a height of 110 nm. The foundation of its tunability resides in a 240 µm long ITO film, measuring 4 µm in width and 100 nm in height. The ITO film is deposited onto the straight waveguide segment of the MRR, following the specified deposition conditions. A 150 nm layer of Au/Ti contact pads is then patterned via e-beam evaporation to form the two electrodes at the opposing ends of the ITO microheater strip.
Fig. 1. Schematic diagram of the tunable MWP phase shifter system with ITO microheater where the ITO film is first deposited and later post-annealed in an RF sputtering chamber. LD: laser diode; PD: photodiode.
The deposition of the ITO offers the flexibility to tailor the optical and electrical characteristics of the material by controlling the number of free carrier concentration during the fabrication process via different deposition conditions [21]. In the near-infrared region, the ITO undergoes a quasi-metallic transition from dielectric to metallic. This shift in optical behavior is defined by the presence of free electrons and its relative permittivity, εITO. The optical properties of ITO in the near-infrared spectrum can be understood based on the Lorentz-Drude model given by [31]
Moreover, the electrical conductivity of the ITO film is a crucial factor influencing the efficiency of the thermal heating process. The ability to confine the thermal volume for efficient heating of silicon-based waveguides with fast response time without the addition of extra optical loss is important for enabling the seamless integration of microheaters with photonic devices.
The structural properties of the samples are studied using X-ray diffraction (XRD) technique with X-ray diffractometer (PANalytical X’pert Pro MRD) which uses Cu-Kα radiation source at a wavelength of 1.5406 Å and 2θ mode, where θ is the diffracted Bragg angle. Figure 2 shows the XRD patterns of the ITO thin film which was deposited onto the silicon substrate at ambient temperature and later post-annealed at different annealing temperatures. The as-deposited and the post-annealed ITO films at 350°C are found to be amorphous in nature as no crystallite peaks associated with ITO are observed. With the increase in the annealing temperature at 450 °C, the ITO samples start to exhibit a polycrystalline nature with evidently sharp peaks along the crystalline directions (211), (222), (400), (440), and (622) planes located at 2θ values of 21.5°, 30.5°, 35.4°, 50.97°, and 60.6°, where the (222) peak is verified as the peak of cubic bixbyite In2O3 [33]. More pronounced diffraction lines with increasing intensity are evident for the sample annealed at a higher temperature of 550°C. Diffraction peaks observed at other positions in all samples as marked by star signs belong to the underlying Si substrate.
Fig. 2. XRD patterns for ITO thin films deposited on silicon substrate. The curves are shifted vertically for clarity. The peaks marked by the star signs do not belong to the thin film ITO but may be from the silicon substrate.
The crystallites size, D, can be calculated using the Debye-Sherrer relationship as described by [34]
where λ is the wavelength of the incident X-ray (∼0.154 nm for Cu – Kα), and Δ is the full width at half maximum (FWHM) of the crystalline peak measured in radian. The measured FWHM at the highest peak (222) near 30.5° was analysed. Using these parameters in Eq. (5), crystallites sizes are estimated to be around 57 nm and 57.6 nm for annealing temperatures of 450°C and 550°C, respectively. The optical band gap energy is typically estimated by the dependence of the absorption coefficient on the photon energy and can be estimated using Tauc’s plot equation, which is expressed as [35] where a is a constant, Eg is the energy band gap of the films, hυ is the photon energy, and $w = 2\pi /{L_m}$0 is the linear absorption coefficient. The linear absorbance (Α) is calculated using the relation ${\alpha _0} = 2.303A/L$, where L is the thickness of the ITO thin film. It has been reported that as the thickness of the ITO thin film increases, the optical band gap decreases accordingly [36].Surface morphological properties of the different ITO thin film samples are characterized using Atomic Force Microscopy (AFM) (Bruker Icon) with a scan size of 2 × 2 µm. The AFM images of the four different samples: as-deposited and post-annealed at temperatures 350°C - 550°C are displayed in Fig. 3. The increase in annealing temperature favours the formation of crystalline structures and results in larger clusters as evident from the annealed samples at higher temperatures of 450°C and 550°C, thus confirming the effect of annealing on the samples. ITO thin films are reported to exhibit stability even beyond 1100 °C in air ambient, as noted in [37]. Consequently, the ITO thin films subjected to annealing temperatures in this study are anticipated to remain stable, considering that the applied temperatures are well below this threshold.
Fig. 3. AFM images of the various ITO thin films (a) as deposited and after annealing at (b) 350°, (c) 450°, and (d) 550°.
A spectroscopic ellipsometry (JA Woollam M2000) is used to determine the thickness and optical constants of the ITO films. The ellipsometry estimates the ITO film thickness to be around 102 nm, which is close to our targeted deposition thickness. The extinction coefficient (k), which provides an indication of the ITO film transparency, are subsequently extracted for the various ITO thin films to investigate the effects of annealing on the optical transmittance of the light in the infrared region, specifically around 1550 nm. Meanwhile, the electrical property is determined from the sheet resistance which is measured using the four-point probe method. Prior to annealing. a sheet resistance of 52 Ω/sq is achieved, which is significantly better than the reported values for ITO in [25]. The sheet resistance further decreases as the annealing temperature increase, as depicted by the blue line with circle markers in Fig. 4. This drastic reduction in resistance is correlated to the improvement in the conductivity of the ITO films with increasing annealing temperatures, thus indicating an increase in carrier concentration after thermal annealing. The increase in carrier concentration also causes the plasmonic wavelength to move towards shorter wavelength [38]. This relationship is elucidated in Eq. (2), where the Drude theory dictates that the reflection begins from shorter wavelengths with increasing carrier concentration. The extracted k value before annealing, which measures around 0.076 is considerably smaller than the extinction coefficient of Ti metals at 4.61 [39]. While a slight increase in the extracted k values is observed at different annealing temperatures, as depicted by the red line with diamond markers in Fig. 4 the overall extinction coefficient still remains comparatively small. This indicates a consistently lower absorbing film with substantially higher transparency compared to Ti metals.
Fig. 4. Measured sheet resistance and extracted extinction coefficient at 1550 nm for different annealing temperatures.
These observed behaviors underscore the substantial impact of post-annealing processes on the electrical and optical characteristics of the ITO films. Despite the slight increase in absorption, the significant reduction in sheet resistance from 52 Ω/sq to 13.2 Ω/sq signifies an enhancement in electrical conductivity of the ITO film. To further explore this trade-off for MWP applications, the effects of post-thermal annealing of ITO thin films on the optical and electrical performance of MWP devices are investigated next.
3. ITO thin films as microheaters for MWP devices
To investigate and compare the thermal performance of the ITO film with and without annealing, two sets of samples were fabricated. One chip employs the as-deposited ITO film as microheater, while the other chip undergoes further processing by post-annealing the ITO film at a chosen temperature of 450°C. To avoid the plasmonic effect, a thin layer of silicon dioxide is deposited via plasma enhanced chemical vapor deposition (PECVD) to form a cladding layer between the silicon waveguide and ITO film. In the case of metallic heaters, this layer is typically more than 1 µm thick in order to avoid the metal absorption effect [40]. However, due to the enhanced optical transparency of ITO, the thickness of this oxide layer was chosen to be 500 nm, which is reduced by half of what is typically used in devices with conventional metallic heaters. The microheater is electrically driven by applying voltages at the contact pads of the electrodes. Based on the induced heat flow from the ITO microheater under an applied voltage source, the light field travelling through the waveguide undergoes a phase shift due to the thermo-optic induced refractive index change of the Si waveguide [26].
The optical transmission spectrum of the MRR before and after deposition of the ITO film is measured for both chips to evaluate the impact of the deposited ITO film on the performance of the OPS. Figure 5(a) shows the optical notch resonance of one of the passive MRRs before ITO deposition in blue dashed line. Superimposing the optical spectrum of the MRR with the integration of the as-deposited ITO microheaters reveals an almost perfect overlap between the two spectra. This showcase the exceptional optical transparency of the as-deposited ITO microheater, as evidenced by the calculated additional optical loss of approximately 0.018 dB/cm, which is nearly negligible. The sample featuring the post-annealed ITO microheater shows a marginal difference of about 1 dB in the depth of the resonance notch, as depicted in Fig. 5(b). Nevertheless, the distinctive shape of the optical notch remains intact. The discrepancy in loss, before and after the deposition of the post-annealed ITO film was calculated to have a difference of 0.7 dB/cm. Accordingly, the estimated actual loss introduced by the ITO microheater strip is approximately 4.18 dB/cm. Given the microheater's length of 240 µm, the resulting insertion loss is only 0.1 dB, hence still effectively maintaining the optical transparency of the ITO microheater. This value is considerably lower than the loss observed in recent studies involving metallic microheaters, where the metal absorption losses were up to 0.2 dB for a microheater length of 100 µm, translating to 20 dB/cm [14]. Furthermore, our work demonstrates lower loss in comparison to other presented TCO materials, where a 0.5 dB insertion loss is reported for a microheater length of 10 µm [17].
Fig. 5. Optical transmission before and after ITO film deposition. (a) As-deposited (b) Post-annealed at 450°C.
The electrical control of the ITO is evaluated next by investigating the tunability of the OPS for both cases with as-deposited and post-annealed ITO-based microheaters. The DC bias voltage applied across the electrodes is varied to provide tunability to the microheaters. The subsequent wavelength shifts of the optical resonance responses are measured and displayed in Fig. 6 at different applied voltages and compared to its OFF state at 0 V (dashed line). Figures 6(a) and 6(b) illustrate the spectral tuning for the two microheaters based on as-deposited and post-annealed ITO films, respectively. Both instances demonstrate the efficacy of the fabricated ITO microheaters in controlling the OPS tunability across the full FSR. In the former case, a total of 19 V is required to shift 0.76 nm across the full FSR, while the latter accomplishes this with merely 8 V, less than half the voltage needed for almost the same tuning range of 0.74 nm. The difference in the voltage requirements arises from reduced resistivity after annealing: the as-deposited ITO microheater had a resistance of 7.36 kΩ, whereas the resistance of the post-annealed ITO microheater is reduced to 1.12 kΩ, indicating an improvement in conductivity due to thermal treatment. Considering the differences in resistance and voltages, both annealed and non-annealed ITO microheaters necessitate comparable power for tuning the optical spectrum across the entire FSR. This relationship is further illustrated in Fig. 6, delineating the power needed for each resonance wavelength shift throughout the FSR.
Fig. 6. Optical characteristics of the OPS at different bias voltage for (a) as-deposited and (b) post-annealed ITO microheaters. The dashed line shows the initial position of the resonance before applying voltage, the sold lines show the gradual shift in the resonances upon applying the respective voltages, and the red star signs indicate the tuning span of the resonances, which is almost an FSR apart.
To further assess the dynamic response of the ITO microheater, the tuning speeds of both the annealed and non-annealed device are evaluated. Given that the typical response time of the microheater is anticipated to be in the µs range, we employ 1 kHz and 10 kHz square-wave driving signals to assess both the response speed of the heaters and the repeatability of the response time under different driving signals. The laser wavelength is fixed at the linear region of the microring transient response and a small step signal of 0.3 Vpp is applied to the microheater for the MRR to maintain a swing within the linear region [41]. Figures 7(a) and (b) depict the normalized step response of the microheater for 1 kHz and 10 kHz input driving signal of the as-deposited ITO microheater where the rise/fall times are 9.91 µs/7.95 µs and 8.49 µs/8.29 µs, respectively. Using the same linear switching conditions, the step response of the post-annealed ITO film is also investigated. A rise/fall time of 6.18 µs/6.03 µs at 1 kHz and 5.63 µs/6.09 µs at 10 kHz were measured, where the switching responses are illustrated in Figs. 7(c) and (d), respectively. The response time, which is referred to as the average value of the measured rise time and fall time, is observed to be faster by 2.8 µs and 2.5 µs for both 1 kHz and 10 kHz input driving signals, respectively. This corresponds to about 30% improvement in the response time of the thermally treated ITO film, which could be due to the increase in thermal conductivity as a result of changes in the thermal transport of the ITO film with different surface morphologies [29]. The response time achieved in this work is comparable to previously reported ITO-based microheater which demonstrated a response time of 5.2 µs with a thinner cladding of 312 nm [26]. Note that the response time can be further improved by optimizing the geometry and position of the ITO microheater and incorporating more thermally conductive cladding materials [41].
Fig. 7. Step response of the ITO microheater operating in the linear region for the as-deposited ITO film at (a) 1 kHz and (b) 10 kHz, and the post-annealed ITO film at (c) 1 kHz and (d) 10 kHz.
By integrating the post-annealed ITO film to enable active, reconfigurable MWP applications, we incorporate the OPS featuring ITO-enabled thermo-optic control in the MWP phase shifter system presented in Fig. 1. The scanning electron microscopy (SEM) (Zeiss Crossbeam 550XL) image of the fabricated OPS is shown in Fig. 8(a). The optical features of the OPS are characterized by using the single sideband modulation technique to sweep across the notch passband of the resonance. The phase and amplitude responses of the OPS is illustrated in Figs. 8(b) and 8(c), respectively. The measured spectra of the OPS correspond to the typical characteristics of a MRR based on a single ring with a notch profile of about 9 dB extinction ratio and a maximum phase transient of about 330°. The OPS element is then employed to implement the MWP phase shifting operation based on the schematic diagram presented in Fig. 1. An optical carrier with a wavelength of 1550.0815 nm and a power of 10 dBm from the laser diode is fed into a dual-drive Mach Zehnder modulator (Sumitomo). An RF signal of 0 dBm generated from the vector network analyzer is modulated onto the optical carrier by applying a bias voltage of 0.6 V to produce an SSB modulated signal with an RF 90° hybrid coupler. The modulated light is then launched into the OPS where the optical carrier acquires a tunable optical phase shift facilitated by the ITO microheater. The beating of the optical carrier and optical SSB at the PD provides direct mapping between the optical and RF phase shifts. By varying the bias voltage applied to the microheater electrodes, different optical phase shift can be obtained in the optical domain which translates into a corresponding RF phase shift variation in the microwave domain.
Fig. 8. Microwave photonic phase shifter operation. (a) SEM image of the OPS with integrated ITO microheater. (b) Phase response of the OPS. (c) Amplitude response of the OPS. (d) Microwave phase shifts. (e) Microwave power variations. (f) Power consumption of the OPS.
Upon gradually tuning the biased voltage from 1 V to 2.5 V, Fig. 8(d) reveals a continuous RF phase shift spanning from 0 to 330° across a frequency range of 15 GHz to 40 GHz. This mirrors the complete optical phase tuning capacity of the single ring OPS. The accompanying RF power variations in Fig. 8(e) display a maximum fluctuation of approximately 9 dB, attributed to the direct mapping from optical to RF power [42]. As the optical carrier sweeps across the notch of the OPS, the RF power variations closely mirror the optical amplitude response of the OPS, which is consistent with the measured notch depth of the OPS. It is worth noting that the incorporation of the ITO film, which contributes to an additional insertion loss of only 0.1 dB, is significantly small in comparison to the intrinsic notch depth of the OPS. Figure 8(f) depicts the power consumption throughout the continuous RF phase tuning operation, demonstrating a maximum power requirement of only 5.58 mW to cover the complete achievable RF phase range. An external thermoelectric temperature controller (Newport) is used to minimize resonant drift resulting from variations in environmental temperature. For the MWP phase shifter operation, potential hindrance to the system performance may arise from the stability of the components, such as the laser source and the voltage control source. These instabilities can lead to wavelength drifts, subsequently causing phase drifts. The phase tunability control mechanism may also be affected by system noise. The presence of noise, which masks the integrity of the optical signal, will influence the accuracy of the optical-to-RF phase conversion. The main source of noise in our system is attributed to the fiber-to-chip coupling loss from a pair of vertical grating couplers, which is about 24 dB. This can be improved by adopting high-efficiency fiber-to-chip coupling designs to enhance the signal integrity, where coupling loss of less than 1 dB has been demonstrated [43].
Although the RF power variations and maximum attainable RF phase range are constrained by the optical amplitude and phase profiles of the single ring OPS, it is possible to extend this work by implementing reflective structures which allows the light to pass through twice and thus resulting in the phase to be doubled to achieve a full 360° phase range. The corresponding doubling of the amplitude on the other hand can be mitigated through amplitude compensation schemes such as the use of a tunable coupler to attenuate and equalize the amount of light passing through at different RF phase shifts [44]. Moreover, the RF power variations can also be minimized by employing optical gain equalization schemes which utilize the gain saturation of optical amplifiers to equalize the amplitude of the optical signal, ensuring that weaker signals experience a higher level of gain and vice versa [30].
The ability to understand and tailor the ITO material properties for tunable MWP applications provides an alternative approach to conventional metal-based microheaters [45–47]. Due to the ease of integration of the ITO-based microheaters as demonstrated in this work, the presented concept offers the opportunity to enhance the tunability performance of existing MWP signal processing schemes.
4. Conclusion
We investigate the optical and electrical properties of ITO thin films to facilitate the development of low-loss reconfigurable integrated MWP devices. Through thermal treatment, a discernible influence on the crystallite size of the ITO thin film is observed, as evidenced by the AFM data. This alteration in surface characteristics is accompanied by corresponding changes in the electrical and optical attributes of the ITO thin film. Whilst post-annealing is shown to increase the optical absorption of the ITO film, the total insertion loss of the ITO microheater remains a mere 0.1 dB. This modest insertion loss is eclipsed by the 30% enhancement in response time, rendering it an acceptable trade-off. Considering the relatively low absorption loss, these quasi-metal ITO thin films offer a transparent avenue for tuning the optical properties. We have demonstrated its successful use as a technological linchpin in MWP phase shifter systems, enabling continuous tuning of the RF phase shifts from 0 to 330° with only 5.58 mW power. The flexibility to further refine the properties of the ITO thin film deposition via enhanced fabrication techniques heralds a promising trajectory towards realizing high-performance MWP signal processors with low-loss and high-speed reconfigurability on hybrid ITO photonic integration platform.
Funding
Department of Defence, Australian Government.
Acknowledgments
The device was fabricated at the Research and Prototype Foundry, a core research facility at the University of Sydney, part of the NSW node of the NCRIS-enabled Australian National Fabrication Facility. The authors would also like to acknowledge Dr Yun Li from RPF for his valuable contribution to the deposition process, Dr Michelle Wood and Dr Samuel Duyker from Sydney Analytical for their assistance in film characterization, and Mr Yiming Yan for his help in the optical measurements.
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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