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Abstract
The Breakthrough Starshot Initiative, established in 2016, aims to propel an ultra-lightweight spacecraft to Alpha Centauri using radiation pressure from a high-power, ground-based laser. Nanopatterned silicon nitride has been proposed as a candidate material for the laser sail. In this work, we design and fabricate a silicon nitride photonic crystal with high reflectivity around a laser wavelength of 1064 nm. We demonstrate the ability to shift the resonant features of the laser sail using titanium dioxide coatings and increase the longwave infrared emissivity using polymer coatings. We also characterize the response of the sail to temperature and optical power.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
Corrections
Max R. Lien, Deming Meng, Zerui Liu, Mashnoon A. Sakib, Yongkui Tang, Wei Wu, and Michelle L. Povinelli, "Experimental characterization of a silicon nitride photonic crystal light sail: erratum," Opt. Mater. Express 13, 2594-2594 (2023)https://opg.optica.org/ome/abstract.cfm?uri=ome-13-9-2594
1. Introduction
Analogous to a sailboat being accelerated by the momentum of wind, a light sail is accelerated by the momentum of photons. The concept of light sailing is not new – spaceflight pioneers Konstantin Tsiolkovsky and Friedrich Zander hypothesized using radiation pressure from photons to propel a spacecraft in 1924 [1]. In the last half-century, efforts to experimentally demonstrate the concept of light sailing have been focused on using the Sun as the source of radiation pressure. In 2010, the Japan Aerospace Exploration Agency successfully demonstrated the IKAROS spacecraft, which used a broadband reflective coating to harness radiation pressure from the Sun [2]. Since then, other solar sails have been demonstrated, such as the NanoSail-D (NASA, 2010) and the LightSail-1 and-2 (The Planetary Society, 2015 and 2019) [3,4]. More recent work has explored the use of laser light sources for propulsion.
In 2016, the Breakthrough Starshot Initiative has proposed the use of an Earth-based laser array to accelerate an ultra-lightweight spacecraft to relativistic speeds [5]. Specifically, a phased laser array, emitting power on the order of gigawatts per square meter, would propel a light sail to a velocity that is approximately 20% of the speed of light [6]. To achieve this goal, one key engineering challenge is the design of the laser sail. An emerging body of literature has examined key principles of sail design. Multiple materials, such as silicon and silicon nitride, have been examined for their ability to enable an efficient exchange of momentum between photons and the sail [6–8]. Additionally, studies have explored the theoretical feasibility, and stability of, a rigid mass being propelled to relativistic speeds, at which the Doppler effect broadens the accelerating laser’s bandwidth [1,9–11]. Methods to leverage nonlinear effects, recent advances in metasurface technology, and spherical sail designs have been proposed to ensure the stability of the spacecraft in the laser’s beam [1,12–18]. Stability considerations must carefully consider both the impact of any beam fluctuations and tip/tilt perturbations to the sail. Moreover, the maximum impinging beam intensity will be a critical parameter to ensure thermal stability of the sail [19]. In light of these theoretical analyses, a major focus of ongoing research will be to fabricate, characterize, and test the stability of experimental laser sail prototypes [16,17,20,21].
In this work, we designed, fabricated, and characterized a silicon nitride photonic crystal for use as a light sail. The photonic crystal was designed to both reduce mass and increase reflectance at near infrared (NIR) wavelengths, two desirable properties for light sails. First, we fabricated a small-area sample using electron-beam lithography and reactive ion etching. We measured the sample’s basic optical properties, including the reflectance and transmittance spectra in the near infrared. We next demonstrated the ability to controllably shift the spectral features of the sample via thin film deposition of titanium dioxide. We characterized its response to a temperature increase to 80°C and did not observe a measurable shift in the spectral features. We then measured the emissivity of the sample at longer wavelengths (4 - 16 µm) and showed that the emissivity can be increased by adding a layer of polydimethylsiloxane (PDMS) polymer. Last, we studied the transmitted optical power as a function of the input power at a laser wavelength tuned to a photonic resonance mode of the sail. Our results provide valuable information for the further design and characterization of silicon nitride light sails.
2. Results and discussion
Figure 1(a) depicts the concept of the Breakthrough Starshot Initiative. A phased-array of Earth-based lasers accelerates a light sail towards the Alpha Centauri system, via radiation pressure. We investigate the use of a Si3N4 photonic crystal to address the laser propulsion requirements for the light sail. Si3N4 is a strong candidate for the laser-propelled spacecraft due to its low absorption and high refractive index at near IR wavelengths [6]. By patterning holes in the Si3N4 membrane, the mass can be reduced, which is desirable for creating a lightweight laser sail [6,7]. Moreover, the hole pattern can be chosen to create photonic resonances within the near IR, which can be used to boost reflectivity at specific wavelengths [22,23] and/or magnify nonlinear effects that may help to stabilize the sail against beam fluctuations [15]. While Si3N4 photonic crystals have previously been studied for other applications, such as optical sensing [24], experimental characterization of several properties relevant to laser propulsion has not been carried out. For this study, we fabricated a prototype Si3N4 light sail (SLS) and measured its spectral and thermal properties in the NIR, emissivity in the mid- and long-wave infrared (M/LWIR), and its power dependence at a resonance in order to understand the potential use of silicon nitride photonic crystals for the Breakthrough Starshot application.
Fig. 1. (a) Depiction of the Starshot sail being accelerated by an Earth-based laser array. (b) Silicon nitride photonic crystal laser sail design: a = period, d = diameter, t = thickness. (c) SEM image of the fabricated Si3N4 photonic crystal. The scale bar is 500 nm.
For our prototype SLS, we selected and fabricated a 2D square lattice photonic crystal consisting of etched holes. A schematic is shown in Fig. 1(b), and has period a, hole diameter d, and thickness t. Our prototype consists of structural parameters a = 1064 nm, d = 415 nm, and t = 690 nm, and we discuss these choices later in the text. An SEM image of the fabricated Si3N4 sail is shown in Fig. 1(c), and our fabrication process is summarized in the Methods section. To characterize the spectral response of our SLS, we measured its reflectance and transmittance in the NIR. Our procedure for measuring reflectance and transmittance, and all other subsequent measurements, is described in the Methods section. For reference, we also measured the reflectance and transmittance of an unpatterned Si3N4 membrane with the same thickness.
The optical response of a 2D photonic crystal at normal incidence is characterized by several general features [22]. For a non-absorptive material, the transmission spectrum has an overall Fabry-Perot background resulting from reflections off the top and bottom sides of the crystal. In addition, the spectrum exhibits narrower, Lorentzian lineshapes, corresponding to guided-resonance modes. At the extremely high incident intensities anticipated in the Breakthrough project (in excess of 10 GW/m2), the nonlinear properties of such guided-resonance modes may be useful for providing stability against spatial beam fluctuations [15].
We began by choosing the thickness of the slab to maximize broadband reflectivity in the near infrared. For an unpatterned dielectric slab, its thickness and refractive index will determine the location and width of reflective Fabry-Perot fringes, and maximizing the reflectance in the range of the ground-based laser is desirable for efficient transfer of momentum to the light sail. As the light sail accelerates, the laser wavelength will experience a Doppler shift Δλ/λ0 = v/c where Δλ is the is the Doppler shift in wavelengths, λ0 is the laser wavelength, and v is the velocity of the moving body [25]. The Doppler-broadened wavelength range for a light sail travelling at 0.2*c and illuminated by a laser at λ0 is approximately λ0 to 1.2*λ0 [6,7,13]. Thus, if the Earth-based laser operates at λ0 = 1064 nm, the incident laser radiation will redshift to 1.2*λ0 = 1277 nm as the light sail accelerates to its final velocity. We selected a thickness of t = 690 nm to produce a broadband Fabry-Perot fringe around these wavelengths. The measured reflectance and transmittance of the unpatterned slab are shown in Fig. 2(a). We assume that the Si3N4 is lossless; Reflectance is calculated as R = 1 – T, and the transmittance T is measured directly. The spectra exhibit clear Fabry-Perot fringes, and the reflectance peaks are close to 40% near 1.2 µm.
Fig. 2. (a) Measured transmittance T and calculated reflectance R = 1 – T of a bare Si3N4 membrane. (b) Measured transmittance and calculated reflectance of Si3N4 light sail and bare Si3N4 membranes. The bare (patterned) membrane’s transmittance and reflectance are the thinner (thicker) curves. (c) Simulated reflectance of the Si3N4 light sail from FDTD calculations compared to measured values. (d) Simulated and measured transmittance of the Si3N4 light sail.
The transmittance of the photonic crystal slab is shown as a thicker, black line in Fig. 2(b), and the transmittance of the unpatterned slab in Fig. 2(a) is shown again in 2b as a thinner line. The corresponding reflectance spectra are plotted assuming R = 1 - T and shown in red. The measured spectra show overall Fabry-Perot behavior with superimposed resonant features [22,26]. The Fabry-Perot features are shifted to lower wavelengths relative to the unpatterned slab, as expected from dielectric perturbation theory [26,27]. Resonances are created in the slab by patterning holes with a period of a = 1064 nm and diameter d = 415 nm. Resonant peaks are apparent and result from Fano interferences in the photonic crystal [28]. The exact location of the resonant peaks can be tuned by adjusting period and diameter, if desired [22].
We compared the measured transmittance spectra of our SLS to its simulated transmittance spectra (Finite-Difference Time Domain, FDTD). The measured and simulated reflectance and transmittance spectra are shown in Figs. 2(c) and 2(d), respectively. The resonances’ spectral locations are in good agreement. There is a significant decrease in the quality factor Q from the simulation to measurement, as is typical in experiment [29]. Q can be quantified as Q = f/Δf, where Q is the quality factor, f is the resonant frequency, and Δf is the full width at half maximum (FWHM) of the resonance. For the simulated and measured resonances around 1460 nm in Fig. 2(c), the simulated and measured Q are 155 and 85 respectively, which corresponds to decrease by a factor of approximately 1.8 from simulation to measurement. The decrease in Q can be attributed to imperfections in the fabrication process [30,31] and/or beam divergence in our measurement setup.
The spectral response of the light sail can be tuned by coating the photonic crystal with a thin dielectric film. We deposited TiO2 onto the surface of the SLS with atomic layer deposition and then recharacterized its spectra in the NIR. The 10-nm-thick TiO2 coating uniformly coats the surface of the SLS.
Respectively, Figs. 3(a) and 3(b) show the measured transmittance and reflectance of the SLS before and after depositing a 10-nm-thick layer of TiO2 on its surface; reflectance was obtained from transmittance as R = 1 - T. The TiO2 coating red-shifted the spectra of the uncoated sample. Figures 3(c) and 3(d) show the simulated transmittance and reflectance of the SLS after before and after depositing the same TiO2 film. Here, T and R were separately calculated in the FDTD simulation. It is apparent that the measured red-shift seen in Figs. 3(a) and 3(b) is consistent with the corresponding simulations. Additionally, the coated SLS retained the shapes of and distances between the resonant peaks of the uncoated SLS. Shifting the SLS’s resonant peak locations in a simple and controllable manner will enable finer control over the final lightsail design.
Fig. 3. (a) Measured transmittance of the Si3N4 light sail before and after applying a 10 nm thick layer of TiO2. (b) Reflectance (R = 1 - T) of the Si3N4 light sail before and after applying a 10 nm thick layer of TiO2. Simulated transmittance (c) and reflectance (d, R = 1 - T) of the Si3N4 light sail before and after applying a 10 nm thick layer of TiO2. The resonances red-shift after the deposition of TiO2.
In addition to its spectral properties, the thermal properties of the light sail play an important role in performance. As a light sail is propelled by incident radiation pressure, any consequent increase in temperature could create thermo-optic or expansion effects that change the SLS’s spectral response. To study this effect, we measured the transmittance spectra of our prototype SLS at low and high temperatures. The temperature of the SLS was controlled and maintained by a heated lens tube that has a maximum range of 23.5 to 80°C. Additional details of this measurement are available in the Methods section. The transmittance spectra of the uncoated and coated SLS at 23.5°C and 80°C are presented in Fig. 4(a) and 4(b), respectively. From both Figs. 4(a) and 4(b), we observe little to no spectral shift for a temperature increase of ∼56.5°C relative to room temperature.
Fig. 4. Transmittance spectra of the (a) uncoated and (b) coated Si3N4 light sails at 23.5°C (below) and 80°C (above). A spectral shift was not observable in this temperature range, which is emphasized by the dashed lines.
The observed lack of significant spectral shifts in this temperature range is consistent with previously reported temperature-dependent coefficients for CVD-grown Si3N4 films. The thermal expansion and thermo-optic coefficients are 3.27 × 10−6 °C−1 and 2.45 ± 0.09 × 10−5 (RIU/°C) respectively [32,33]. Using these values and assuming a temperature increase of ∼56.5°C, we can conclude that any temperature-dependent changes in the geometrical parameters (slab thickness, hole size, etc.) or refractive index are negligible over the range measured.
Thermal management of the spacecraft could be required to maintain the sail within a specified temperature range. For example, any on-board electronic components will have temperature specifications for their use [34], and improper thermal management can result in thermal runaway and structural damage [8,19]. Because the vacuum environment of space and lightweight design requirements make active thermal control of the spacecraft difficult, a passive, emission-based thermal management scheme is ideal for the Breakthrough mission [35]. The design of such a scheme requires detailed knowledge of the mid-wave and long-wave infrared emissive properties of the light sail. Moreover, techniques for increasing the emissivity in this range, while maintaining the optical properties of the sail at the operating wavelength of the laser, will lead to improved thermal management.
We have characterized the infrared emissivity of our sail both as fabricated, and with the addition of a PDMS coating layer. We chose PDMS because Si-O and Si-C bonds facilitate high emissivity in the M/LWIR, which can reach near unity for film thicknesses greater than 100 µm [36]. To increase the emissivity in the M/LWIR, we spin-coated PDMS layers of different thickness onto the uncoated SLS (8 and 16 µm). Afterwards, we recharacterized NIR spectra and measured M/LWIR emissivity with FTIR spectroscopy. The NIR transmittance/reflectance and M/LWIR emissivity spectra of the PDMS-coated and SLS are respectively shown in Figs. 5(a) and 5(b).
Fig. 5. (a) NIR Transmittance of the Si3N4 light sail and (b) Mid- and long-wave infrared emissivity (E = 1 - T - R) of the Si3N4 photonic crystal after spin-coating on different thicknesses of PDMS layers. The PDMS increases emissivity at longer wavelengths; some resonances are retained at shorter wavelengths.
Figure 5(a) reveals that spin-coating PDMS onto the SLS degrades and redshifts many of the resonances, although some are retained. However, Fig. 5(b) reveals that the PDMS films significantly increase emissivity at longer wavelengths, and that thicker films result in greater emissivity. These findings imply that the PDMS film thickness could be optimized to enhance emissivity while still retaining resonances, and that thicker films result in greater emissivity at the cost of adding mass to the sail.
We characterize our light sail by evaluating its acceleration distance, or the distance travelled before reaching the final velocity. Following [6], we calculate
Table 1. Total Mass mT and Acceleration Distance D for Four Light Sail Designsa
Finally, we characterized the SLS by measuring its input-output power dependence at the resonance near 1306 nm. The details of this measurement are described in the Methods section. From Fig. 2(a), the SLS has a resonance at 1306 nm. Transmittance from 1290 to 1320 nm, given by a photodetector readout, is shown in Fig. 6(a).
Fig. 6. (a) Transmittance spectrum of the uncoated Si3N4 light sail’s resonance at 1306 nm, measured by sweeping a tunable laser diode between 1290-1320 nm. The dashed red line is at 1306 nm. (b) Measured input-output power dependence of the light sail at 1306 nm with a fitted linear curve.
Next, we measured the input-output power dependence at 1306 nm. The laser was fixed to radiate at 1306 nm, and the input power was controlled with a fiber attenuator. Figure 6(b) shows a linear relationship between incident and transmitted power at 1306 nm. The linear relationship implies that our laboratory laser power is not sufficient to induce nonlinear effects.
3. Methods
3.1 Fabrication
Si3N4 membranes were fabricated onto Si wafers to create our SLS samples. First, a Si3N4 film (690 nm) was grown on a Si wafer with low-pressure chemical vapor deposition (Tystar Mini-Tytan). Then, a photoresist layer (AZ 5214E) was spin-coated onto the back side of the wafer and photolithography (SUSS MA/BA6 Gen4 mask and bond aligner) was performed with a custom photomask. The photoresist etch mask was transferred onto the back side of the wafer after exposure (85 mJ/cm2) and development (AZ 400K). By using reactive-ion etching (Oxford PlasmaPro 100), regions of Si3N4 were created through the back side of the wafer. After removing the residual photoresist with acetone, a Si3N4 mask remained on back side of the silicon wafer. Finally, we used potassium hydroxide wet etching to create the Si3N4 membranes (20% solution for 3.5 hours at 85°C).
To fabricate photonic crystals on the Si3N4 membranes, we used electron beam lithography, off-normal electron beam metal evaporation, and reactive-ion etching. 150 nm of polymethyl methacrylate (PMMA) was spin-coated onto the Si3N4 membranes, and e-beam lithography (Raith EBPG 5150) was used to expose the circular regions. We expose 200 × 200 unit cells (circles) for our experiments. After development (Methyl isobutyl ketone : isopropyl alcohol, 1:3), the PMMA in the exposed regions was removed. Then, four repetitions of off-normal (75°) metal evaporation (Temescal BJD 1800) were used to create a chromium etch mask that covered the unexposed regions. We used off-normal metal evaporation to avoid a lift-off process, protect the Si3N4 membranes, and reduce e-beam lithography time. Next, reactive-ion etching was used to etch through the uncovered Si3N4 regions. After soaking the sample in chromium etchant (Sigma-Aldrich) for 15 minutes, and then acetone for 3 hours, the photonic crystal samples were completed.
3.2 Spectral measurements
Reflectance and transmittance in the NIR were measured with a free-space optical bench setup. The setup is shown below in Fig. 7(a).
Fig. 7. (a) Optical setup for NIR transmittance and reflectance measurements. (b) Optical setup for measuring the input-output power dependence of the light sail at 1306 nm.
First, light from a broadband white light source (‘Lamp’, 360 - 2400 nm, Ocean Optics HL2000) was launched into free space from an optical fiber, collimated by L1, and then focused onto the SLS with an NIR objective lens (L2). Transmitted light is collected with an objective lens on other side of the sample (L3). Transmitted light then couples into an optical fiber via L4 so that the transmittance spectra could be measured with an NIR spectrometer (‘Spec.’, 900 - 1700 nm and 1.6 nm per spectral band, Ocean Optics NIRQuest 512). The beamsplitter BS1 is used to guide the Lamp’s illumination path to a visible wavelength CCD camera, which is used for free-space alignment only. The temperature dependent transmittance measurements in Fig. 4 were conducted in the same manner, except that the sample was mounted to a temperature-controlled lens tube (ThorLabs SM1L10HR). The stage’s temperature is maintained with a with a thermistor and PID controller. The sample and stage were maintained at each target temperature for 15 minutes before extracting spectra.
The measurement setup to study the input-output power relationship of the SLS is shown in Fig. 7(b). To characterize the resonance shown in Fig. 6(a), we illuminated the SLS with a continuous wave tunable laser diode and measured the transmitted power with a calibrated photodetector (ThorLabs PM100D Laser Diode, PDA10CS InGaAs Amplified Detector). We swept the tunable laser diode from 1290 nm to 1320 nm to locate the resonance wavelength with high accuracy. Once the resonance was located at 1306 nm, we fixed the laser diode to radiate at this wavelength and controlled its incident power on the sample with a fiber attenuator. The radiation from the laser was split with a 50:50 fiber splitter after the attenuator so that half of the power was sent to the sample and the other half was measured by an optical power meter (OPM, ThorLabs PM100D), which we used to extract the input power.
The emissivity measurements for Fig. 5 were performed with a Bruker Hyperion FTIR spectrometer.
3.3 Simulations
The simulations for this work were run with Lumerical FDTD on the Center for Advanced Research Computing supercomputer cluster at the University of Southern California. We simulated one unit cell of the SLS as a periodic structure, with a spatial resolution of 10 nm along the x, y, and z directions.
4. Conclusion
In this study, we have designed, fabricated, and charactered a prototype Si3N4 light sail that could be leveraged for the Breakthrough Starshot Initiative. We designed the light sail to create resonances and broadband reflectance in the NIR, and measured the resulting reflectance and transmittance spectra under different thin film and thermal conditions. We also demonstrated that the resonances can be easily shifted with thin film dielectric coatings. Additionally, we showed that the M/LWIR emissivity of our prototype light sail can be increased with PDMS films. Lastly, the input-output power dependence of a resonance at 1306 nm was observed to be linear. In conclusion, our work demonstrates the significant potential for silicon nitride as a light sail material for the Breakthrough Starshot Initiative.
Funding
National Aeronautics and Space Administration (80NSSC20K1161); Breakthrough Starshot Foundation.
Acknowledgments
The authors thank Raymond Yu for assistance with operating the 1306 nm laser diode.
ML was funded by NASA. Other authors were funded in part by the Breakthrough Starshot Foundation.
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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