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Abstract
Ionized air is used in various industries to control electrostatic charge. On the other hand, ionized air molecules can also cause various problems since electrification of materials can induce electrostatic discharges. Therefore, compact sensors that enable a quantitative detection of ionized air will help to improve industrial processes and safety. Here we report on the detection of negatively ionized air using a photonic crystal (PC) waveguide with a length of 800 µm. In this type of detector, the PC is exposed to a flux of air ions that transfers a part of the excess charges to the PC. The light transmitted through the waveguide is then attenuated by free carrier absorption due to the excess charges from the ionized air molecules. We show that the electron density in the PC can be estimated from the magnitude of the attenuation, and that this magnitude depends on the wavelength of the light propagating in the PC waveguide. Due to the wavelength dispersion of the group velocity, light at longer wavelengths is subject to stronger attenuation than light at shorter wavelengths. This property is useful for the development of ionized-air sensors with a variable detection range.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Silicon (Si) photonic crystal (PC) waveguides combined with nanocavities with high quality factors are used in the development of small optical devices such as photonic buffer memories [1], Raman Si lasers [2], detectors for telecom-band photons [3], multi-channel wavelength filters [4], and bio-sensors [5]. Many of these devices take advantage of the fact that a high-quality factor (high-Q) nanocavity can enhance light–matter interactions. A major advantage of Si PC devices is that mass production is possible using CMOS-compatible processes. Accordingly, Si PC devices that can be fabricated at commercial semiconductor foundries have been actively investigated [6–9]. For example, we demonstrated CMOS-compatible fabrication processes that enable mass fabrication of nanocavities with Q values larger than 1 million [10] and Raman Si nanocavity lasers with an ultra-low threshold [11].
Recently, we studied the detection of ionized air using high-Q nanocavities. We found that, in the case of a Raman Si nanocavity laser, the laser oscillation stops abruptly after exposure to a weak flux of ionized air [12,13]. Furthermore, we demonstrated that the emission from a nanocavity with a Q value on the order of tens of thousands decreases immediately when it is exposed to a flux of ionized air, and the emission intensity gradually recovers after the exposure [14]. These changes in the emission from nanocavities are caused by the charges transferred from the ionized air to the Si PC, as shown in the inset of Fig. 1(a): these charges reduce the photon density in the cavity by free carrier absorption (FCA) [15]. The decrease in the emission intensity has been confirmed for irradiation with both positive and negative air ions. Compared to existing detection methods, ionized-air detection by high-Q nanocavities has advantages in terms of size, weight, and accessibility. In addition, this method has a high resistance to electrostatic discharge (ESD) because passive nanocavity devices including Raman Si nanocavity lasers do not require a PiN structure.
Fig. 1. (a) Schematic of a Si PC slab exposed to a flux of negatively ionized air molecules. The ionized molecules generated at the metal tip move along the electric field lines. The inset illustrates the transfer of electrons from the ionized molecules to the Si slab. These electrons reduce the intensity of the light transmitted through the waveguide. (b)−(e) Phenomena and applications related to ionized air: lightning (b), plasmasphere (c), ionizer for semiconductor industry (d), and electrostatic coating (e).
The natural phenomenon of lightning illustrated in Fig. 1(b) can cause annual economic losses of several billion dollars or more. Lightning is caused by a separation of positively and negatively ionized air inside clouds [16]. Figure 1(c) illustrates the plasmasphere around the Earth [17], which is formed by ionized air molecules and electrons trapped in the Earth’s magnetic field. This plasma can electrify the electronics and solar panels of satellites, and thus can lead to device failure due to ESD [18]. On the other hand, ionized air is widely used in industry as shown in Figs. 1(d) and (e): ionizers are used in semiconductor plants to remove charges from semiconductor substrates, and electrostatic spray guns are usually used to apply paint to parts of cars. Sensors that can measure the density of ionized air with a high spatial resolution are required to develop technologies for the prevention of ESD and to optimize industrial equipment that uses ionized air.
In the case that the laser intensity of a nanocavity Raman laser is used as the detector signal, high-sensitivity detection should be possible due to the nonlinearity of the stimulated Raman scattering process [12]. In the case that the emission from a high-Q nanocavity is used as the detector signal, quantitative detection should be possible due to the linear response of the emission intensity [14]. However, it is also necessary to investigate whether the method of using only a PC waveguide as a sensor for ionized air can be advantageous.
In general, optical devices that only use Si PC waveguides have a simple design, are easy to fabricate, and they enable low-loss connections to functional devices using Si waveguides. Therefore, their application to two-photon absorption diodes [19], autocorrelators [20], optical modulators [21], and light sources for LiDAR has been investigated [22]. In addition, protein sensors [23], DNA sensors [24], and gas sensors based on Si PC waveguides have been studied [25]. In these investigations, the low group velocity of light in PC waveguides plays a key role in achieving high sensitivity, device miniaturization, or low power consumption [26].
An important aspect is that the group refractive index, which is often used as an indicator of the capability of slowing down light in a PC waveguide, varies with the wavelength. Since slow light implies more time for possible interactions with matter, the effective strength of the light–matter interaction in a PC waveguide can strongly depend on the wavelength. This property may be used to control the sensitivity of a sensor for ionized air. As indicated in Figs. 1(b)−(e), the density of ionized air molecules that needs to be detected depends on the application. Therefore, it is useful to develop a sensor with a variable detection range, which enables us to quantitatively detect both high and low densities of air ions. In this paper, we experimentally investigate the detection of ionized air by a Si PC waveguide and verify the concept of achieving variable sensitivity by a wavelength-tunable light source.
2. Sample structure
Figure 2 shows scanning electron microscope (SEM) images of the Si PC sample used in the experiment. The PC is defined by circular air holes with a radius of 120 nm. As shown in Fig. 2(a), the holes are arranged in a triangular lattice with a lattice constant of ax = 414 nm in the x-direction and ay = 710 nm in the y-direction. The width of the line-defect waveguide shown near the center of Fig. 2(a) is $1.05{a_y}$ ≈ 746 nm. Figure 2(b) clarifies that the thickness of the PC slab is about 220 nm.
Fig. 2. SEM images of the Si PC waveguide used in the experiment. (a) Top view of the waveguide at the waveguide edge. (b) Cross sectional view. (c) Image of air holes (the sample was tilted by 20°).
The PC pattern was formed on a 300-mm Si-on-insulator (SOI) substrate using ArF immersion photolithography and a CMOS-compatible process. We used the same SOI substrate as in [27], where we reported the fabrication of a Raman Si laser by photolithography. A chip with a size of 800 µm × 2000 µm was obtained by stealth dicing (the longer edge is parallel to the y-direction). The 3-µm-thick SiO2 layer underneath the Si slab was removed with 48% hydrofluoric acid to form a PC slab with an air-bridge structure. The details of the fabrication process are described in [28].
3. Experimental setup
Figure 3 shows the experimental setup used to measure the influence of ionized air on the light transmitted through the PC waveguide. A superluminescent diode (SLD, Thorlabs SLD1550P-A40) with a center wavelength of 1550 nm and a full width at half-maximum (FWHM) of 41 nm was used as the light source. We used a fiber isolator (Thorlabs IOT-G-1550A) to stabilize the emission spectrum and the intensity of the SLD. A wavelength-tunable optical filter (OTF, Santec OTF-350) and a wavelength meter were used to control the wavelength of the light coupled into the waveguide. The intensity was modulated by a mechanical chopper at about 1 kHz and then passed through a polarizer to obtain transverse electric (TE)-polarized light. This light was focused on the waveguide edge on the right-hand side of Fig. 3 by an objective lens with a numerical aperture (NA) of 0.4. The transmitted light at the opposite end of the waveguide was focused by another objective lens and passed through a polarizer that transmits TE-polarized light. Then, the intensity was measured by an InGaAs photodiode (Newport model 2011) and a lock-in amplifier system (NF LI5630). The transmitted intensity is important because, when the Si chip is irradiated with ionized air, the carriers that enter the PC slab absorb a part of the light propagating in the waveguide. We utilize the resulting reduction in the intensity of the transmitted light for the detection of ionized air (the results are described in Section 4).
Fig. 3. Experimental setup. NIR camera: near-infrared camera; OTF: tunable optical filter; SLD: superluminescent diode.
The method used to generate air ions is the same as that in [14]. Negatively ionized air was generated by a corona discharge using a copper needle connected to a high-voltage power supply (Green Techno Corporation, GT20) that controlled the potential of the metal tip (Vtip) relative to the ground. Vtip was manually controlled from −4 kV to −10 kV with a resolution of 1 kV. The tip radius of the copper needle was about 0.25 mm. The distance between the metal tip and the Si chip was about 1 cm. A thermally conductive adhesive sheet (electrical resistivity 0.5 /cm2) was used to fix the sample to a copper block, and the copper block was grounded by a wire. The copper block was fixed to the sample stage with stainless screws. By ensuring a path where the electrons can escape from the Si chip and the copper block, the reproducibility of the experiment is enhanced because an electrically grounded sample enables an almost constant potential difference between the Si chip and the metal tip during irradiation. All experiments were performed in ambient air at room temperature with a humidity of 26%–33% (dew point temperature 2.8–6.5 °C). The temperature of the sample stage was stabilized by a Peltier element.
4. Experimental results
For the characterization of the PC waveguide, we first measured the transmission spectrum of the waveguide by replacing the SLD, the isolator, and the OTF with a wavelength tunable laser (Santec TSL-510). To obtain a spectrum with a high resolution, the wavelength was scanned in steps of 25 pm (details of this measurement method are provided in [29]). Figure 4(a) shows the obtained transmission spectrum: it has a cutoff wavelength of about 1589 nm. The outline of the spectrum is similar to that of the spectra reported in [26,29,30]. Furthermore, Fabry–Perot (FP) oscillations due to the waveguide length of 800 µm are observed in the transmission spectrum. By defining the peak wavelength of the mth order as λm as shown in the inset of Fig. 4(a), the group refractive index (ng) of the waveguide at wavelength λm can be obtained by the following formula [26,31]:
Fig. 4. (a) Transmission spectrum of the PC waveguide. The inset shows several peaks of the FP oscillation. (b) Wavelength dependence of the group index of the PC waveguide. The insets show the transmission spectra near 1570 nm and 1550 nm.
Here, L is the length of waveguide. Figure 4(b) shows the wavelength dependence of the group index: from 1510 nm to 1550 nm, ng increases almost linearly with an average rate of about 0.035/nm. At wavelengths longer than 1550 nm, the group index increases nonlinearly. Near the cutoff wavelength, ng is larger than 20. The nonlinear increase in ng is a unique property of PC waveguides [26]. As shown in the two insets in Fig. 4(b), the oscillation period at 1570 nm is smaller than that at 1550 nm because the value of ng at 1570 nm is larger than that at 1550 nm. This means that light at 1570 nm requires more time to pass through the waveguide than light at 1550 nm. Due to this slow-light effect of the PC waveguide, the light–matter interaction in the waveguide for light at 1570 nm is enhanced compared to that for light at 1550 nm.
To investigate the influence of negatively ionized air on the light propagating in the waveguide, the center wavelength (λin) of the SLD and the bandwidth were set to 1550 nm and 3 nm, respectively. Figure 5(a) shows the temporal change of the transmitted light intensity I when the PC waveguide was exposed to negatively ionized air for a short time of 5 s (from t = 30 s to 35 s). The four curves show the temporal change of I for Vtip = −4 kV, −6 kV, −8 kV, and −10 kV. The intensities are normalized to the corresponding intensity at t = 0 s. Note that many commercial corona-discharge ionizers use a Vtip of ±7 kV and furthermore, most of them are equipped with several metal tips. Therefore, the magnitudes of Vtip used here are not very high.
Fig. 5. (a) The temporal change of the intensity of the light transmitted through the PC waveguide exposed to negatively ionized air. The four curves correspond to Vtip = −4, −6, −8, −10 kV. The gray region from 30 s to 35 s indicates the time during which the PC chip was exposed to ionized air. (b) The three illustrations show temporal changes of the transferred electrons (filled circles) in the nanocavity.
For the discussion, we divided the data into three different time regions [(i)–(iii)] and Fig. 5(b) illustrates the movement of the electrons from the ionized air molecules. In region (i), the PC is not exposed to ionized air and I is almost unity. In region (ii), electrons are transferred from the ionized air molecules to the Si PC slab. Due to FCA induced by the increasing number of electrons in the PC slab, the intensity of the transmitted light decreases rapidly. For each chosen Vtip, the minimum of I is reached at t = 35 s. We define the minimum value of each curve as Imin. During regime (iii), the transmitted light intensity recovers to the initial value since the carriers escape from the PC slab. We consider that most of the carriers will move to the ground.
As shown in Fig. 5(a), Imin decreases with the increase of Vtip. As Vtip increases, the flux of ionized air (or ion wind) increases. This increase results in an increase of the number of electrons transferred from the air ions to the Si PC. Accordingly, the electron density in the PC slab and the optical loss by FCA increase. Furthermore, the FP oscillation peaks shift due to the carrier plasma effect and the thermo-optic effect caused by the transferred carriers [32]. Compared to the case of a free-electron density close to zero, the refractive index of Si for a free-electron density of 1 × 1017 cm-3 is smaller by 8.8 × 10−5 [33], and it increases with a rate of 1.9 × 10−4/K if the temperature increases. These properties can be important, because a decrease of 1.0 × 10−4 in the refractive index leads to a blue shift of the FP peaks of about 40 pm at 1550 nm. However, since the chosen bandwidth of 3 nm is much larger than the period of the FP oscillation in this wavelength range, the contribution of the peak shift to the decrease of I is negligibly small. Such a weak influence of the carrier plasma effect and the thermo-optic effect is an advantage of a broad-light excitation source [14,29].
To enable a quantitative detection of ionized air, it is important to estimate the instantaneous density of excess electrons in the PC slab, NFCA. We define αFCA as the free-carrier absorption coefficient of the waveguide at t = 35 s. The relation between Imin and αFCA is given by the following equation:
where I0 is the intensity coupled into the waveguide for NFCA = 0 and n is the refractive index of Si at 1.55 µm. Here, we used I0 = 1 and n = 3.47. The term ng/n is the enhancement factor for light–matter interactions due to the slow-light effect. αFCA and NFCA have the following relation: where σFCA is the absorption cross section for FCA. Here we used σFCA = 7.27 × 10−18 cm2 for electrons [34]. From these equations, we estimated the dependence of NFCA on Vtip, and the result is shown in Table 1. NFCA increases by about an order of magnitude as Vtip is changed from −4 kV to −10 kV. Regarding the FP oscillation peaks, the Vtip-induced changes in NFCA lead to blue shifts of 30 to 260 pm due to the carrier plasma effect [33]. In a previous study on a nanocavity with a Q value of 34700, we estimated NFCA = 1.59 × 1017 cm-3 based on the observed decrease of the Q value for two-minute irradiation with Vtip = −5 kV [14]. Therefore, the NFCA values in Table 1 are consistent with our previous study.
Table 1. Imin, αFCA, and NFCA for the experiment in Fig. 5
The main point of this work is the utilization of the wavelength dependence of Imin in a PC-waveguide-based sensor for ionized air. Figure 6 shows the temporal change of I in the case of Vtip = −5 kV for λin = 1550 nm, 1570 nm, and 1585 nm. The bandwidth of the light was set to 3 nm and the duration of irradiation was 5 s. Since the irradiation conditions were the same in all three measurements, the NFCA values should be the same. We find that a longer wavelength results in a smaller Imin. The Imin for 1585 nm is smaller than that for 1550 nm because ng is 22.6 at 1585 nm while it is 6.13 at 1550 nm. In other words, since the group index is 3.69 times larger, the light–matter interaction for light at 1585 nm is enhanced compared to that for light at 1550 nm. Table 2 summarizes the NFCA values calculated from the values of Imin in Fig. 6. As expected, almost the same NFCA values are obtained. The slightly smaller NFCA for 1570 nm is probably due to a measurement error caused by the manual control of the irradiation time.
Table 2. IMIN, αFCA, and NFCA for the experiment in Fig. 6
Fig. 6. Time traces of the intensity of the light transmitted through the PC waveguide for different wavelengths (λin = 1550 nm, 1570 nm, and 1585 nm). The exposure time was 5 s and Vtip = −5 kV.
Figure 6 and Table 2 suggest that the use of a λin with a large group index (around 1585 nm in this study) is suitable for the detection of ionized air with a relatively low density. On the other hand, the use of a λin with a smaller group index (below 1550 nm in this study) is suitable for the detection of ionized air with a relatively high density.
5. Discussion and future prospects
In this study, it was possible to estimate NFCA from Imin by simple analysis. On the other hand, the estimation of NFCA from Imin was difficult in previous studies where the emission from a nanocavity was measured [12–14]. To compare the values of NFCA in Table 1 with those obtained by another reliable measurement, we consider that measurements of the surface potential can be useful. Furthermore, to increase the range of possible applications, it will also be important to estimate the density of the ionized air in the vicinity of the sensor (Nionized_air) from NFCA. If the estimation of the distribution of Nionized_air with a high spatial resolution can be achieved by placing several sensors at different locations in space, this technique should be useful in various industrial applications, for example, in enhancing the performance of ionizers and in the maintenance of large chemical plants.
For further development, we also need to consider that the exposure time in this work was only 5 seconds. Figure 7 shows the time traces of I for exposure times of 6 s, 10 s, and 20 s (Vtip = −5 kV and λin = 1550 nm). Each of these time traces seems to require more than one recovery time constant to explain the recovery of the transmitted intensity. Therefore, we need to consider that a longer exposure may alter the Si surface. For example, ozone can be generated during the corona discharge, and this may change the dominant escape path of the electrons [35]. To investigate the response to long irradiation with ionized air, it is important to improve the experimental setup. In addition, by increasing both the sampling rate and the control resolution of the high-voltage power supply, the time constants of the decay of the transmitted light and those of the recovery can be estimated using an equivalent electronic circuit model of Fig. 3.
Fig. 7. Time traces of the intensity of the light transmitted through the PC waveguide for different exposure times in the case of Vtip = −5 kV and λin = 1550 nm.
The results shown in Fig. 6 and Table 2 indicate that the detection sensitivity can in principle be adjusted by appropriately selecting the wavelength of the used light. In this study, a standard PC waveguide was used to prove the concept of variable detection sensitivity. The range of the detection sensitivity can be extended by using PC waveguides with a stronger slow-light effect [36,37]. For example, PC slow light waveguides may be applied in such ionized-air sensors. A compact detection system can be developed by using a fiber-coupled SLD and a fiber-connected tunable-wavelength filter.
The method presented in this study has other advantages compared to detection methods that use the emission from a high-Q nanocavity as a detector signal [12–14]. For example, the intensity of light transmitted through a waveguide is usually higher than the emission from a nanocavity, leading to a better signal-to-noise ratio. Additionally, the detection sensitivity can be simply enhanced by increasing the waveguide length. These results may help to develop sensors that can measure ionized air with a high spatial resolution.
Funding
Japan Society for the Promotion of Science (21H01373); Program for Creating STart-ups from Advanced Research and Technology (JPMJST2111).
Acknowledgements
Yuki Takahashi was supported by a fellowship from the ICOM 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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