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We report on the first application (to our knowledge) of an extended–wavelength (2.33 μm) multi-mode diode laser for simultaneous measurement of the concentrations of CH4 and CO in the ambient air. The signals identification and quantitative analysis are performed using correlation spectroscopy. A Herriott cell and the wavelength modulation spectroscopy technique with second harmonic detection are also utilized to improve the detection sensitivity of the system. The detection limits of the system are estimated to be about 81 ppbv and 31 ppbv for CH4 and CO, respectively. The accuracy, sensitivity, precision, and stability are also analyzed to confirm the potential of the system.
© 2016 Optical Society of America
1. Introduction
Methane (CH4) is the primary constituent of natural gas which is an attractive fuel because it is clean-burning and efficient. Energy from combustion of natural gas drives our everyday life, for example, cooking, heating, boiling water or drying clothes. Recently, it is being used as transportation fuels as well. As the world is heading towards an unprecedented large and potentially devastating global energy crisis, improvements in combustion efficiency are becoming very important. Detecting the concentration of CH4 in combustion gas is a useful way to monitor the energy utilization. As a very important species in combustion gas, carbon monoxide (CO) is a good indicator of combustion efficiency. The simultaneous detection of CH4 and CO can give more useful information for combustion efficiency. On the other hand, CH4 is the 2nd most significant long lived greenhouse gases [1], CO is a toxic pollutant that is regulated by stringent emission mandates [2], and both CH4 and CO become dangerously explosive when they reach a critical concentration in an enclosed environment. For these reasons, detection of CH4 and CO is crucial to environmental and industrial safety considerations regarding gas production management. Therefore, proper oversight and determination/implementation of essential precautions require the development and deployment of accurate CH4 and CO sensors to monitor CH4 and CO production and release, especially the sensors which can detect the two gases simultaneously.
Tunable diode laser absorption spectroscopy (TDLAS), known for its ability to provide non-intrusive, gas-specific, time resolved in situ measurements, has proven to be very suitable for some applications such as remote sensing of environmental gases and pollutants in the atmosphere [3–5 ]. The vast majority of current schemes employ single mode tunable lasers since their narrow linewidth provides the necessary high spectral resolution.
At present, the typical rapid-tuning range of the telecommunications quality single-mode DFB diode laser commercially available is 0 to 2 cm–1. Hence few of the sensors enable simultaneous measurements of multiple species and flow parameters along one line-of-sight by using a single DFB diode laser [6,7 ]. Usually multiple lasers are combined using a scheme for time or wavelength division multiplexing to detect the species separated by a wider frequency interval [8,9 ]. Such schemes are necessarily more complicated than those using a single laser and a single detector. Single mode lasers such as external cavity diode lasers (ECDLs) are capable of being tuned over a wide spectral range, although mode hops are expected and can therefore limit the spectral range covered in a single scan. A development of multi-mode absorption spectroscopy using well-behaved multimode diode lasers (MDLs) with rigorous parameter determination has recently been reported for multiple transition detection [10]. This technique relies on the coincidence of all the scanned longitudinal modes within the wide laser bandwidth over the range of one mode spacing and the absorption lines in the test sample. Such a multi-mode absorption spectroscopic method has the advantage of simple scheme and wide spectral coverage, but, most of the MDLs in the IR wavelength region are less well behaved and can therefore not be employed owing to mode instability [11]. Recently, a development of gas correlation spectroscopy (COSPEC) using low-cost multi-mode lasers as the light source has recently been reported [12,13 ]. Measurement using this technique relies on the correlations between the absorption signals measured in the measurement path and reference path. This method offers the potential advantage of low cost, high stability and ease of use due to the employment of cheap multimode diode lasers and the robustness of COSPEC [14,15 ].
In order to achieve high sensitivity and improved signal-to-noise ratios (SNRs), a simple and useful method is to increase the optical pathlength. Many embodiments have been developed to achieve it, such as White and Herriott cells [16,17 ], integrated cavity [18,19 ], integrating spheres [20], and gas-filled porous materials [21,22 ], etc. Each embodiment has its own advantages and can be used in different situations. As a mature technique, the Herriott cells have been used for many years to achieve long optical path lengths in TDLAS. Although there are some disadvantages such as the typically large size and volume of multipass cells, they are used widely due to some evident advantages such as easy to use, robust, adequate for all wavelength, and immune to the environment, etc. If combined with an advanced detection methods (e.g. wavelength modulation spectroscopy (WMS), frequency modulation spectroscopy (FMS)) simultaneously, the diode laser absorption spectroscopy measurements can be enhanced greatly [23].
Here we report probably the first simultaneous measurement of the concentrations of CH4 and CO using an extended–wavelength (2.33 μm) multi-mode diode laser. The chosen wavelength corresponds to an atmospheric transmission window where the absorption of the main interfering species (H2O and CO2) is weak. The lines intensities of the CH4 and CO absorption in this band are much stronger compared to the near infrared range. Furthermore, the room temperature extended-InGaAs detectors are mature and not expensive in this spectral range. The COSPEC is used for species identification and corresponding quantitative analysis of CH4 and CO. By using all the spectral lines in the selected absorption band rather than just a single transition, the combination of tunable multi-mode diode laser spectroscopy and COSPEC can overcome some drawbacks in TDLAS using single mode diode lasers, such as the drift of the central wavelength. In order to improve the detection sensitivity of the system, a Herriott cell and the WMS technique with second harmonic (2f) detection are also utilized. The accuracy and linearity, sensitivity, detection limit, precision and stability are evaluated to verify the ability of the system for simultaneous measurements of CH4 and CO.
2. Principles
Simply, a multimode laser could be considered as a collection of single-mode lasers [24,25 ], often with random intensity distribution. In wavelength modulation spectroscopy, the diode laser injection current is sinusoidally modulated with angular frequency to produce laser frequency modulation
where [cm−1] is average optical frequency and [cm−1] is the modulation amplitude.According to the “weak transition” assumption associated with wavelength modulation measurements (peak absorption should be less than ~0.1), the transmission coefficient for the nth mode of a MDL with n longitudinal modes through a uniform medium can be approximated as
where Itn and I0n are the transmitted and incident laser intensities of the nth mode, respectively, P [atm] is the total pressure, x is the mole fraction of the absorbing species, L [cm] is the path length, Sn(T) [cm−2 atm−1] is the line strength of the transition, φn [cm] is the line-shape function which is normalized such that , and T [K] is the gas temperature. It is a periodic even function in ωt and can be expanded in a Fourier cosine series:For harmonic detection of wavelength modulation spectroscopy, the second-harmonic Fourier component which is used most frequently could be expressed as
The magnitude of the absorption-based 2f signal, which measured by a lock-in amplifier, can be described as
where is the optical-electrical gain of the detection system, is the average laser intensity for the nth mode at .In optical correlation spectroscopy, two 2f signals are measured for a sample gas and a reference gas, respectively [26]. The concentration of the sample gas can be obtained by the relation between the two magnitudes of the absorption-based 2f signal and the concentration of the reference gas
where the indices M and R denote the parameters belonging to the measurement and the reference beams, respectively.3. Sensor development
3.1 Absorption band selection
In order to select an optimum absorption bands for simultaneous measurements of CH4 and CO, the absorption spectra of CH4 and CO are computed carefully based on the HITRAN2012 database [27]. Figures 1(a)-1(c) show the absorption transitions of CH4, CO, H2O and CO2 in the range of 1-5 µm. As shown in these figures, the transitions of CH4 and CO are overlapped near 2.33 µm. The lines intensities of the CH4 and CO absorption in this band are strong enough for the measurement here. Furthermore, there is no strong absorption of the main interfering species (H2O and CO2) in this region. Besides that, the mature and cheap optical elements and detectors are another advantages for this band. Finally, the transitions in 4284-4289 cm−1 are chosen for the simultaneous measurements of CH4 and CO based on their suitable line strength and proper separation, as shown in Fig. 1(d).
Fig. 1 The absorption transitions of (a) CH4, (b) CO, (c) H2O and CO2 in the range of 1-5 µm. Insert graph: (d) the CH4 and CO transitions in 4284-4289 cm−1.
3.2 Pressure selection
Because the correlated signals used for the concentration evaluation are selected based on the correlation between the reference signals and measured signals, the line shape discrepancies between the two signals must be as small as possible. It is known that the line shape of a spectrum is mainly depended on temperature and pressure, so it is important to select suitable temperature and pressure for reference cell and measurement cell.
In order to minimize the influence of temperature, the temperatures of the measurement and reference cells are kept equal to room temperature during the experiment. As for pressure, because the concentrations of the mixtures filled in the measurement and reference cells are different (usually a mixture with high concentration should be filled in the reference cell to obtain an accurate reference signal), an optimum pressure must be selected for the measurement and reference cells according the experimental conditions, respectively. Here we simulate the line-broadening width of two representative lines (a CH4 line at 4284.51 cm−1 and a CO line at 4288.29 cm−1) as a function of total gas pressure over the range of 0-1atm. For the reference signals, the simulation is performed for 3 different CH4-Air and CO-Air mixtures which contain 1%, 2%, and 3% CH4 and CO, respectively. For the measured signals, the simulation is performed for 5 different mixtures with the CH4 and CO concentrations of 1 ppm, 10 ppm, 100 ppm, 1000 ppm, and 10000 ppm, respectively. The simulated results are shown in Figs. 2(a) and 2(b) . Thanks to the similar values of air-broadened coefficient and self-broadened coefficient for each line, the discrepancies of the line widths are very small for different mixtures with same pressure but different concentrations. Because the sensitivity and selectivity of the measurement could be enhanced at low pressure, the pressures of ~0.2 atm are selected for both reference cells and measurement cell. Under this condition, discrepancies of the line widths due to different CH4 and CO concentration in reference cell and measurement cell are only 0.00369% and 0.00171%, respectively. Such small deviations could be neglected in the experiment.
Fig. 2 The simulated line-broadening width of two represent lines ((a) a CH4 line at 4284.51 cm−1, (b) a CO line at 4288.29 cm−1) as a function of total gas pressure over the range of 0-1atm for the reference and measured signals.
4. Experiment
The experimental setup used for the simultaneous measurement of CH4 and CO is shown in Fig. 3 . The multi-mode diode laser used in this work is an antimonide-based device fabricated on N-doped GaSb(Te) substrate by solid source molecular beam epitaxy system [28].
The emission wavelength of the diode laser is centered on 2.33 μm and the maximum output power is ~5 mW. The emission spectrum measured for an injection current of 90 mA at 23 °C is shown in Fig. 4 together with the absorption lines of CH4 and CO. As shown in this figure, the emission spectrum of the multi-mode diode laser overlaps well with some strong transitions of CH4 and CO in the selected region. The laser is placed in a home-made laser mount and driven with a modular diode-laser controller (ILX Lightwave LDC-3724). In order to cover a wide spectral range, the laser temperature is varied from 20°C to 30°C with a tuning rate of 0.5 °C /s by adding a step current to the temperature controller. Synchronously, the laser wavelength is driven by a 30 Hz triangle ramp summed in an adder with a 12 kHz sine wave to provide the wavelength modulation. The laser beam is collimated by a lens and divided into three beams by two beam splitters. The main one is directed across the measurement cell and the two minor beams are directed across two reference cells. The beam path is purged by high purity nitrogen so as to avoid the interference from ambient water vapor in room air. All transmitted beams are simultaneous detected by three same room temperature extended-InGaAs detectors (Thorlabs,PDA10DT-EC), respectively. All beam paths exposed in the ambient air are purged by high purity nitrogen so as to avoid the interference from the CH4 and CO in room air. The detector signals are sent through a low-pass analog filter before digital sampling by a multifunction data acquisition (DAQ) card (Wwlab, MP4221, 12-bit A/D conversion). The captured signals are demodulated by LabView-based software lock-in amplifier at the second harmonic of the modulation frequency.
Fig. 4 (a) The emission spectrum of the multimode diode laser measured for an injection current of 90 mA at 23 °C. (b) The absorption lines of CH4 and CO in this range are also shown.
In the experiment, the two same reference cells (50 cm long with a canted wedged (1.3°) window at one side to avoid residual etalon fringes) are filled with standard gas of 3% CH4-N2 mixture and 3% CO-N2 mixture, respectively. According to the pressure selection as mentioned above, the total pressure in the CH4 reference cell and CO reference cell are kept at ~0.2 atm. An astigmatic mirror multi-pass absorption cell (New Focus-5612) is employed as the measurement cell. The multi-pass cell is based on Herriott-type and has a base length of 50 cm. The total optical path is 100 m with 199 times of reflections. The measurement gases are CH4-CO-N2 mixtures with concentration range of 10 ppm to 10000 ppm. The total pressure in the multi-pass cell is also kept at ~0.2 atm to minimize the line shape discrepancies between the measurement and reference signals. The peak absorption of all the three gas cells is estimated to be less than 0.1 and the “weak transition” assumption associated with wavelength modulation measurements is satisfied. So there is a linear proportional relationship between the WMS-2f signal and the gas concentration. All the mixtures used in the experiment are prepared in a stainless steel tank. The prepared mixture is delivered into the cells via a stainless steel tube. The gas pressures in the cells are measured by a vacuum pressure gauge (Anliang, GP5A22) with an accuracy of ± 0.12% of reading. Before each measurement, the gas sample was allowed to stabilize thermally with enough time. In this way the temperatures of the three gas cells can keep at room temperature and the influence of temperature can be minimized.
5. Results and discussion
In order to validate the performance of the system for the simultaneous measurements of CH4 and CO concentrations, a series of experiments with controlled CH4-CO-N2 mixtures are performed. An example of the measured WMS-2f signals from the measurement cell and reference cells in a random ramp scan are shown in Fig. 5 . A third-order polynomial fit of the section without absorption in the raw signal, which is directly proportional to average laser intensity and detector gain, is used to normalize the WMS-2f signal.
Fig. 5 An example of the measured WMS-2f signals from the measurement cell and reference cells in a random ramp scan.
To confirm the accuracy of the measurement, only signal pairs with a high correlation coefficient between the measurement and reference WMS-2f spectra are used. The useful signal pairs are selected using a convenient and reliable signal processing method as mentioned in [29] and [30]. At first, some high correlated pairs at different CH4 and CO concentrations are selected manually to set up a model. Then the measured signals are analyzed to select useful signal pairs based on the model. The selected signal pairs could be used to derive the CH4 and CO concentrations in measurement cell from the normalized WMS-2f signal peak height according to Eq. (6). The measured concentrations are estimated by linear-regression between the measured and reference signal pairs based on a least-squares algorithm. As shown in Figs. 6(a) and 6(b) , the slope of the fitting line represents the concentration ratio of the gases in the measurement and reference cells after normalizing their cell lengths. The CH4 and CO concentrations in measurement cell are 90.7 ppm and 164.2 ppm calculated from the slopes of the fitting lines. The correlation coefficient represents the degree of correlation between the selected signal pairs. The high values confirm the useful of the signal processing method.
Fig. 6 Correlated data points (normalized intensities) of the measured and reference signal pairs and linear fitting of the data points for (a) CH4 and (b) CO.
5.1 Accuracy and linearity of the measurement
In order to evaluate the accuracy and linearity of the quantitative measurement, ten different CH4-CO-N2 mixtures with known concentrations at room temperature are measured. Figures 7(a) and 7(b) show the scattered plot of the measured concentrations of CH4 and CO versus the known values in the range of 10-10000 ppm. It can be seen from the figures that the measured CH4 and CO concentrations from the system are in good agreement with the known concentrations over the entire concentration range. Correlation of those measured points have square of the correlation coefficients R2 = 0.9986 and 0.9989 for CH4 and CO, respectively, indicating the good linearity of the measurements. The accuracies of the measurements of CH4 and CO are 0.74% and 0. 63%, which can be deduced from the linear fitted slopes of 1.0074 ± 0.0098 and 1.0063 ± 0.0088, respectively. The errors in the concentration measurements primarily come from uncertainties in the known CH4 and CO concentration measurements during the preparation of the mixtures (especially for the low concentrations), analysis of the measured spectroscopic data, the measurements of the optical path length, and the line shape discrepancies due to the pressures.
Fig. 7 Comparisons of the measured concentrations of (a) CH4 and (b) CO versus the known values in the range of 10-10000 ppm.
5.2 Sensitivity
The sensitivity of the simultaneous CH4 and CO detection system based on multi-mode absorption spectroscopy is determined by an Allan analysis [31]. Here the sample gas is the laboratory ambient air with relatively low pressure of ~0.2 atm. Because of the stable conditions during the estimated 30-minute experiment, the CH4 and CO concentrations in the measurement cell are assumed to be constant. Figures 8(a) and 8(b) show the Allan variance obtained from the continuous time series measurements for measurements of CH4 and CO, respectively. The Allan analysis in Figs. 8(a) and 8(b) show that a sensitivity of 34 ppb and 29 ppb can be achieved using a 350-s measurement time for CH4 and CO, respectively. The high sensitivity illustrate the high performance of the developed multi-mode absorption spectroscopy based CH4 and CO sensor.
Fig. 8 Allan variance from time series measurements of (a) CH4 and (b) CO sealed in the measurement cell.
5.3 Precision and stability
The precision of the system is evaluated by calculating the standard deviation of the average concentrations of CH4 and CO for a gas sample. Here the sample gas is also the laboratory ambient air with relatively low pressure of ~0.2 atm. The laboratory ambient air sealed in the multi-pass absorption cell is assumed to be invariant during a ~35-minute measurement series, as discussed in section 5.2. During this experiment, 30 successive measurements with 1 minute interval are performed for the same gas sample mentioned above, each measurement taking 10 s. The 30 successive measurements yield average concentration values of 2.600 ± 0.039 ppm (1 σ) and 1.807 ± 0.025 ppm (1 σ) for CH4 and CO, respectively, indicating the precisions of 39 ppb and 25 ppb for the concentration measurements of the system, as shown in Fig. 9 . The results of the measurements also indicate the nice stability of the system.
5.4 Detection limit
The detection limit is an important parameter for an optical gas sensing system. In order to evaluate the detection limit of the system, the concentration measurements of CH4 and CO are performed for the laboratory ambient air. Here the determination of the detection limit follows the procedure described in [32]. The strongest WMS-2f signals of CH4 and CO measured in the measurement and reference cells filled with laboratory ambient air are shown in Figs. 10(a) and 10(b) . The signals measured in the measurement cell are multiplied by 10 to make the peaks clearly. Both signals are measured with a 10 s acquiring time at a 30 Hz scan rate. With the noise level of 225 μV (1 σ) and 275 μV (1σ) in the base line and the peak heights of the WMS-2f signals from the measurement cell, the SNRs of ~32 and ~59 are obtained for CH4 and CO, respectively. According to the CH4 and CO concentrations of 2.600 ppm and 1.807 ppm measured in the laboratory ambient air above, the detection limits of ~81 ppbv (1 σ) and ~31 ppbv (1 σ) can be yielded for CH4 and CO from the measured concentrations and SNRs.
Fig. 10 The selected strongest WMS-2f signals of (a) CH4 and (b) CO measured in the measurement and reference cells filled with laboratory ambient air.
5.5 Test on long-duration running
In order to evaluate the long-duration running ability of the system, an experiment with flowing sample gas in the measurement cell is performed. In the experiment, ambient air is drawn through the Herriott cell using a mechanical pump. A pressure controller is used to maintain the cell pressure around 0.2 atm. In the middle of the three hours measurement, a CH4-CO-N2 mixture with ~25 ppmv CH4 and ~25 ppmv CO is released near the inlet of the cell to change the CH4 and CO concentrations in the sample gas. Figure 11 shows a set of concentration data recorded over a period of three hours. As shown in this figure, the CH4 and CO concentrations are near 2.6 ppm and 1.8 ppm, respectively, before the CH4-CO-N2 mixture is released. With release of the mixture near the inlet, the CH4 and CO concentrations increase gradually. Due to the dilution of the mixture by the ambient air, the measured maximum concentrations are only ~23 ppmv for both CH4 and CO. After the mixture is pumped and diffused, the concentrations decrease to the atmospheric concentrations again.
5.6 Comparison with other systems
Comparisons with other systems which have same functions which are reported in [13,33–36 ] are made to further verify the ability of the system. The comparison results are shown in Table 1 . In [33], CO and CH4 in volcanic gases are measured using a single–frequency distributed–feedback lasers emitting at 2.3 μm combined with cavity–enhanced absorption spectroscopy (the cavity length is 85 cm and the reflectivity of the mirrors is about 99.985%). Detection limits achieved for CO and CH4 are 16 ppb and 0.4 ppm, respectively. The detection limit for CO of our system is comparable with their value, and our detection limit for CH4 is lower than theirs because the methane absorption lines exploited in their experiment are not the most intense accessible with their laser. In [34], CH4 is measured employing a tunable multi-mode diode laser at 1.31 μm and wavelength modulation spectroscopy. A measurement sensitivity of 25 ppm and an accuracy of 0.27% are achieved by the system. In [13], CO is also detected using a multimode diode-laser-based correlation spectroscopy and wavelength modulation spectroscopy. Because the absorption in the overtone and combination bands of CO in 1.57 μm is weak, the detection limit and accuracy of the system is 200 ppm and 1%, respectively. In [35], McManus et al. review their recent results in development of high-precision laser spectroscopic instrumentation using midinfrared quantum cascade lasers (QCLs). In one of their instruments, CH4 and CO are detected simultaneously two pulsed, thermoelectrically cooled QCLs operating at 7.8 μm and 4.6 μm, respectively. The multipass absorption cell in this instrument has 76 m of path in a volume of 0.5 L. The demonstrated precision at 1Hz is 0.2 ppb for CO, 0.8 ppb for CH4 (1 part in 2000 of ambient). The detection limits at 100-s averaging time are 0.4 ppb and 0.1 ppb for CH4 and CO, respectively. In [36], Tao et al. develope a low-power mobile sensing platform with multiple open-path gas sensors. The sensing system can be used to measure CH4 and CO by using a vertical cavity surface emitting laser (VCSEL) at 1.65 μm and a QCL at 4.54 μm, respectively. The detection limit for CH4 is 5 ppb at 10 Hz with a Optical pathlength of 30 m. That value for CO is 3 ppb at 10 Hz with a optical pathlength of 15.8 m. The mid-infrared systems employing QCLs can yield a higher precision and lower detection limit, but disadvantages with regard to cost and system complexity must be considered. Compared with the mid-infrared systems, our system is relative cheap and simple. The detection limits for CH4 of 81 ppb and CO of 31 ppb can also meet the challenge in atmospheric trace-gas monitoring. As a valuable sensor based on optical spectroscopy, it is obvious that the system is not limited to the atmospheric trace-gas monitoring. It can be used in many other fields, such as analysis of combustion efficiency, prediction of CH4 and CO outburst in coalmine, continuous CH4 and CO monitoring in sealed cabin, and so on.
Table 1. Comparison of the Performance of Our System with Other Systems
6. Conclusions and perspectives
In this work, simultaneous measurements of the concentrations of CH4 and CO are demonstrated by means of an extended–wavelength (2.33 μm) multi-mode diode laser. Use of multimode laser permits measurement of the absorption spectrum in a larger interval than that which can be achieved with a single-mode laser. The correlation spectroscopy is used for signals identification and quantitative analysis between the measurement and reference signals. The optical-path and sensitivity are enhanced by using multi-pass Herriott cell and wavelength modulation spectroscopy. The validation of the system is conducted by a series of experiments with controlled CH4-CO-N2 mixtures. The accuracies of the measurements of CH4 and CO are 0.74% and 0. 63% in the range of 10-10000 ppm. According to the Allan analysis, a sensitivity of 34 ppb and 29 ppb can be achieved using a 350-s measurement time for CH4 and CO, respectively. By means of the detection of CH4 and CO in the laboratory ambient air, detection limits of 81 ppbv and 31 ppbv are yielded for CH4 and CO, respectively. Using 30 successive measurements with each measurement time taking ~10 s during 35 minutes, the precision of the system are estimated to be about 39 ppbv and 25 ppbv for CH4 and CO, respectively. A continuous measurement over a period of three hours confirms the long-duration running ability of the system.
Next the system will be used for a long term monitoring of CH4 and CO concentrations in ambient air. The results can be used to find the trend, seasonal and diurnal variations of atmospheric CH4 and CO. Because the simultaneous detection of CH4 and CO can give more useful information for combustion efficiency, the system can also be used for combustion diagnosis of the natural gas. For this purpose, some upgrading must be performed for the existing system. An open multi-pass absorption cell will be fabricated and replace the Herriott cell used in the system. Besides that, the scanning frequency of the laser should be increased to adapt the complex and transient process in the combustion.
Acknowledgments
The work is funded by the National Natural Science Foundation of China (NSFC) (No. 61475068, No. 11104237), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 15KJB140002), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). We thank in particular Dr. Guishi Wang, Prof. Xiaoming Gao, and Prof. Wei-dong Chen for the provision of devices. We also thank Prof. Jow-Tsong Shy for valuable discussions.
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