Open Access
Add to BibSonomy
Abstract
The measurement and diagnosis of electromagnetic fields are important foundations for various electronic and optical systems. This paper presents an innovative optically controlled plasma scattering technique for imaging electromagnetic fields. On a silicon wafer, the plasma induced by the photoconductive effect is exploited as an optically controlled scattering probe to image the amplitude and phase of electromagnetic fields. A prototype is built and realizes the imaging of electromagnetic fields radiated from antennas from 870MHz to 0.2 terahertz within one second. Measured results show good agreement with the simulations. It is demonstrated that this new technology improves the efficiency of electromagnetic imaging to a real-time level, while combining various advantages of ultrafast speed, super-resolution, ultra-wideband response, low-cost and vectorial wave mapping ability. This method may initiate a new avenue in the measurement and diagnosis of electromagnetic fields.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Electromagnetic (EM) wave functions as the important carrier of information and energy. Its measurement [1–4] and diagnosis [5,6] have brought revolutionized changes to a broad range of areas such as communications, radar and biomedicine, and have been constantly focused and improved. Recently, many inspiring advancements have been achieved based on various physical effects, such as dynamic EM field imaging with electron microscopy [1,7], atomic-resolution EM field imaging with scanning transmission electron microscopy [8,9], nondestructive EM field imaging with polarized neutrons [10], high-sensitivity EM field detection with Rydberg atom [2,11,12], nanoscale EM field detection [3,13,14] and imaging [14,15] with nitrogen-vacancy center, and live electrooptic EM field imaging with ZnTe [16–18]. These creative efforts explored the distributions of EM fields in all kinds of scenarios. However, due to the adaptation of the techniques mentioned above, the most widely used methods for EM fields are still scanning schemes with a single probe or probe arrays [4,19–22]. Meanwhile, the resolution and bandwidth are predetermined by the size of the probe, which is usually in forms of waveguide or antenna. The probe should be mechanically carried and scanned during the imaging process, therefore it usually takes tens of minutes or hours in a single task. For real-time EM fields imaging in the microwave band, the fast and accurate methodology with simple instruments has always been a persistent desire in EM applications.
In this paper, we exploited the well-known semiconductor photoconductive effect into a new EM field imaging technique. In this technique, optic-induced plasma on photoconductive semiconductor is utilized as a massless detection probe for EM field imaging. Based on the sequentially controlled laser scanning, the real-time imaging of the EM fields can be achieved simply by detecting scattered signals at the laser illuminated spot. The principle of the technology is described, and a prototype was built to demonstrate the real-time imaging of EM fields radiated from antennas. Experiments were taken with the prototype from radio frequencies up to terahertz. Based on the experiments, the advantages of this new technology compared to the existing mechanic probe scanning methods are discussed.
2. Principle and prototype
The feasibility of probing an electric field with a small metallic scatter was early demonstrated [23] and gradually developed into modulated scattering technique with various scatter probes, in the forms of dipoles [24,25], diodes [26] and electron materials [27]. Compared with the general scanning waveguide probe, the isolated small scatter probe may circumvent the errors caused by the flexing of the transmission line attached with the probe when moving in the scanning area. Even for the isolated small scatter probe, it still has to be carried by a mechanical scanning scheme to go through the area of the field. Compared to the limited performance improvement, the system complexity and stability affected the wide application of the modulated scattering technique in EM near-field measurements. The idea of taking the optic-induced plasma as a completely new type of probe will change the status quo.
It is well known that when a near-infrared laser is irradiated on the surface of a photoconductive semiconductor wafer, electron-hole pairs can be generated and diffuse around the illuminated area. These electron-hole pairs thus form a plasma region with notable conductivity variations [28–30]. This photoconductive phenomenon has been used in microwave areas as highspeed microwave switching since the 1970s [31], and have been utilized in microwave circuit devices [28,32,33] and antennas [34,35], showing great flexibility. Most of these applications are primarily based on the switching characteristics produced by the photoconductive effect. The response of this variational conductivity in the EM fields is also important. In fact, the photo-induced plasma region exhibits notable conductivity increments. These increments of conductivity will naturally change the EM field response from radio frequency to microwave frequency. Since the size of the plasma region is comparable to the microwave wavelength, it exhibits scattering properties to the EM fields. Hence, the optic-induced plasma can scatter EM waves in nearly the entire microwave frequency range and even up to terahertz, which is also verified in our experiments. This becomes the physical foundation of our new technique in this work, as shown in Fig. 1(a).
Fig. 1. Concept and setup of the prototype. (a) The concept of the new technique to image the EM field in real-time. A near-infrared laser beam is controlled to illuminate a photoconductive semiconductor wafer. The plasma is excited at the illuminated position, to form a conductive spot which scatters the EM field under-test. The amplitude and phase information of the EM field can be derived from the measured scattering signals at corresponding scanning positions. (b) The scheme of the prototype and experimental setup. The prototype consists of a photoconductive semiconductor wafer fixed at the position where the EM field is to be measured, a pointing laser source, an AUT connected to a vector network analyzer (VNA), the mirrors controlling and data processing units. The laser beam is guided by a set of laser scanning mirrors, which are synchronized with VNA, so that the dynamically generated plasma probe may precisely scan the area to be measured. The red lines indicate the pointing laser, and the black lines indicate the cables. The blue lines indicate the cables with information interaction.
In our previous study [36], the variations of optic-induced conductivity on a silicon wafer have been mapped with a microwave resonator near-field scanning microscope. As shown in the Supplement 1 Note S1, the size of generated plasma region is close to the diameter of the laser illuminating spot. Such a plasma region can be considered as a small “scattering plasma probe” for the microwave field. The probe scatters a portion of the field at the spot, which naturally carries amplitude and phase information. Since the “plasma probe” is massless, the laser beam scanning speed can be much faster than any substantial probe. In the following, this phenomenon of spot plasma scattering is exploited towards a new EM field measurement instrument.
A prototype is implemented for imaging experiments of the EM fields. The imaging of EM fields is the basis of the near-field antenna test, which has become the standard practice and most widely used approach of the antenna test [4,37]. To demonstrate the characteristics of this new imaging technology, EM fields of antennas are measured as an application example.
The scheme of the prototype and experimental setup is illustrated in Fig. 1(b). In the measurements, the antenna under test (AUT) operates as a monostatic transceiver. The AUT transmits the EM waves and receives the scattered waves by the optically controlled plasma. A thin photoconductive semiconductor wafer (Silicon, P111 intrinsic type, 300 µm in thickness) is used to produce the optic-induced plasma. A 100mw NIR laser module (FUZHE FU780AD100 Model, 780nm in wavelength) is used as the laser source, and a galvo mirror system (BLASER SC20, 20 kpps scanning speed) is used as the laser scanning mirrors to control the laser illuminating position on the silicon wafer. The VNA (Ceyear AV3672C modal, 10 MHz-43.5 GHz) is connected to the AUT and used as the signal transceiver of the system. A field programmable gate array (FPGA, ALTERA Cyclone IV EP4CE6F17C8) control card is connected to the laser source controller and the scanning mirrors controller, which enables the ultrafast scanning of the laser and the synchronized laser on/off states control. VNA records the backscattered signals and feeds them to the computer to retrieve the field distribution in the scanning area. The physical photo of the prototype and the experimental setup is shown in Fig. 2.
Fig. 2. Photo of the prototype and the experimental setup. The AUT is fixed at the measured distance from the wafer of the prototype. The control computer is connected to the prototype and VNA to control the measurement and get data from the VNA, then the distribution results of the amplitude and phase can be derived from these data.
The EM fields will be straightforwardly retrieved from the scattering signals based on the reciprocity principle [26,38]. As shown in Fig. 3, the point rp is an arbitrary field measuring point. Without excited plasma, the electric field at rp is E(rp), as a result of the AUT transmitting of excitation E0. In this case, the AUT received the signal Es which includes the response of the entire background environment. As the laser illuminates the location rp, the plasma probe is produced and scatters the local field E(rp), generating the scattered field ΔESp. Since the plasma probe is small and uniform at different positions, the scattered electric field ΔESp is proportional to E(rp) as αE(rp), where α is a complex constant reflecting the scattering characteristics of the plasma. The scattered electric field ΔESp is received by the AUT and measured by the VNA as a small variation in the received total signal, noted as ΔES.
Fig. 3. Schematic Diagram of EM Field Measurement. (a) The electric field to be measured at point rp is E(rp), which is caused by the radiation source E0. (b) When the plasma is excited at point rp, E(rp) is partially scattered and causes a field disturbance ΔESp. (c) This scattered disturbance is transmitted to the receiver port and measured as ΔES. According to the reciprocity principle, the proportion between the receiving and transmitting signal remains the same if the receiving and transmitting ports are switched. Therefore, it can be expressed as E(rp) / E0 = ΔES / ΔESp, which means the measured ΔES will leads to the acquisition of E(rp).
According to the reciprocity principle, the proportion between the receiving and transmitting signal remains the same if the receiving and transmitting ports are switched. In this case, there will be:
In this monostatic configuration, the measuring parameter of the VNA is the ratio of the received backscattered signal to the output source signal, recorded as S11. Its variation ΔS11 is,
It is clear that ΔS11 is simply proportional to the square of the electric field E(rp) to be measured. In practice, ΔS11 is simply the subtraction of the measured S11 signals with and without the presence of the plasma probe. Therefore, the amplitude and phase distribution of the EM field in the scanning region can be straightforwardly retrieved from the recorded ΔS11 point by point. It should be noted in this configuration that the AUT can be any reciprocal microwave device, such as all kinds of antennas or transmission lines. That means, this testing technique has a very broad adaptability.
3. Experiments and results
A series of experiments are taken to demonstrate the capability of this new imaging technique. EM fields from six different antennas operating at different frequencies ranging from 870 MHz to 200 GHz are imaged by the same single prototype. The corresponding experimental results are shown in Fig. 4.
Fig. 4. Measured EM field distributions from antennas. The measuring scenes are listed in the first column with the six antennas: (a) a dipole antenna at 870 MHz, (b) a flexible microstrip antenna at 2.4 GHz, (c) a patch antenna at 6.3 GHz, (d) a Ka-band 10 dB standard Gain antenna at 38 GHz, (e)a horn antenna at 90 GHz and (f) a horn antenna at 200 GHz. The corresponding measured amplitude and phase distributions are presented in the second and third columns.
The measurements are taken with a scanning area of 60 × 60 mm2 by 100 × 100 points. As shown in Fig. 4(a) and Fig. 4(b), the antennas are placed very close to the measuring plane, about 1mm away from the surface of the wafer. The measuring plane is in the near-field zone of the antenna radiation. The distributions clearly show the antenna structures and the local features of radiation. The working wavelengths of the antennas are 344 mm and 125 mm, respectively, which are significantly larger than the measuring point spacing and even larger than the whole image area. Under such super-resolution conditions, the edges of metallic structures can be easily recognized by the local concentration in the near-field images. The details in Fig. 4(a) and Fig. 4(b) clearly demonstrate the super-resolution near-field imaging ability of the technique.
In fact, the resolution of the prototype is only restricted by the size of the plasma, which is mainly affected by the diameter of the laser spot. The diameter of the plasma can be easily shaped to be less than 1 mm. As a nice result, the super-resolution condition can be fulfilled in the entire microwave band at least.
Figure 4(c) is a typical patch antenna that radiates by resonance along the edges of the metallic patch. The resonant state around the edges, which is determined by the patch perimeter, can be clearly observed. The interesting variation of the resonant state across different frequencies is revealed in the Visualization 1. Mechanical scanning methods are unable to implement measurement at such close positions where EM fields can only be obtained through calculation. The technique provides an intuitive and convenient method for analyzing the operating state of such antennas, which previously relied on numerical simulations.
The results of more horn antennas operating at higher frequencies of 38 GHz, 90 GHz and 200 GHz are shown in Fig. 4(d), Fig. 4(e) and Fig. 4(f). The measured results indicate that the optic-induced plasma in the wafer can scatter EM waves up to terahertz, demonstrating the ultra-wideband imaging ability for the EM fields. There is no existing antenna test technique that can achieve such wide band performance.
Another attractive feature of the technique is its ultra-fast field imaging speed. It is possible to measure the distribution of the EM field in real-time. Currently, the prototype can accomplish a typical field imaging with 100 × 100 sampling points within one second, while any mechanical scanning method takes tens of minutes or hours at least. As a demonstration, near real-time measurement movies of EM fields when the AUT moves around are displayed in the Visualization 2 and Visualization 3.
The imaging speed is mainly limited by the response time of the transceiver device. In the case of the VNA equipment in our prototype, one trans-receive signal cycle takes about 300 µs. The whole mapping process can be reduced to a few hundred milliseconds if a faster transceiver device is adopted. Therefore, this plasma scanning technique may realize near real-time observation of EM waves, allowing the researchers to “see” the instantaneous changes in EM fields.
To evaluate the accuracy of this imaging technique, the measurement results of the horn antenna at 38 GHz in Fig. 4(d) are compared with the simulation results. On the wafer aperture 40 mm away from the antenna, the measured field distributions agree very well with those of the simulation, as shown in Fig. 5. The concordance stands for both the amplitude and the phase, proving the high accuracy of this technique. With these accurately measured distributions of amplitude and phase, the far-field radiation pattern of the antenna can also be calculated instantly.
Fig. 5. Comparison between the measurement and simulation results. (a) Measured amplitude and phase distributions of a horn antenna at 38 GHz, 40 mm away from the antenna aperture. (b) The simulation results. Comparison of 1D (c) amplitude and (d) phase profiles along the x-axis where y is zero. The blue line is for the test result, and the red line is for the simulation result.
4. Discussion
From the results of the experiments, this new technology with direct laser scanning approach combines the characteristics of ultrafast speed, super-resolution, and ultra-wideband response, which reveals the distinct advantages over existing mechanical scanning methods. The detailed comparison with the scanning probe method can be found in Supplement 1 Note S2. The plasma probing approach can be operated wherever the silicon piece is positioned. Such close-range near-field imaging performance is usually challenging for existing conventional scanning probe measurements because of the resolution limit and non-ignorable disturbances caused by the probe in close range, not to mention the low efficiency. Thus, non-radiated close-range near-field super-resolution imaging of EM fields becomes easier than ever
As expected, the received backscattered signal is very weak and mixed with all the reflection signals from the surrounding environment, which are strong but static. The backscattered signal is retrieved by subtracting the received signals with the plasma probe exited or not. In the prototype, the maximum output power of the VNA instrument is about 20 dBmW, which limits the scattered signal strength to less than −60 dB, already close to the system ground noise. In order to improve the signal-noise ratio (SNR), besides using wave absorption and shielding to reduce the environment reflections, several efforts can be taken at the expense of time, such as the time average of multiple measurements and decrease IF bandwidth of the receiver in VNA. Repeating the background subtraction in the laser light-off state is another helpful approach to suppress time-varying environment reflections and extract the more accurate scattered signal. With the help of these approaches mentioned above, the imaging quality can be improved significantly, and the dynamic range of our prototype can be over 35 dB.
Compared with non-uniform errors introduced by the mechanical device in any probe scanning methods, the thin and uniform plate of the silicon wafer in this imaging system introduces no flatness deviation and little position error. Since the semiconductor wafer is basically dielectric and its thickness is much smaller than the wavelength, the influences of the wafer on the EM fields are tolerable. On the other hand, it is noticed that there are still small fluctuations all over the field, as shown in the comparison images in Fig. 5. The fluctuations are repeatable, therefore not caused by the random noise, but by reflected waves from the surroundings, including the multiple reflections from the wafer surface. Most of these environmental impacts could be further eliminated through more careful design of the instrument structures and environment control.
So far, the prototype developed in this work is in a simple monostatic transceiver configuration. It does not have to be restricted to the monostatic configuration, and other T/R configurations with flexible schematics will be explored. Separating the antenna under test and the receiving antenna might further improve the SNR. The area of the imaging plane can be enlarged as needed, simply by splicing wafer pieces. Another significance of this method is that it provides an optic-electric imaging approach without the need for complex devices and data processing methodologies. It has considerable potential to replace the currently widely used mechanical scanning methods.
5. Conclusion
A novel EM field imaging technology via scanning optic-induced plasma scattering is introduced in this paper. A prototype has been invented to demonstrate its characteristics. With this prototype, the near-field measurement experiments of various antennas from 870MHz to 0.2 terahertz have been carried out as an application case. The amplitude and phase distribution of the antenna field can be obtained within a second, realizing the real-time imaging of the EM fields. The resolution of the imaging results is about 1mm and significantly smaller than the corresponding EM wavelength, which presents the super-resolution characteristics covering the entire microwave band. The technology's accuracy is evaluated, and the distribution results of a horn antenna at 38 GHz agree very well with the simulations. From these experiments, it is validated that the plasma scanning technique with several significant advantages over existing mechanical scanning methods, including dramatically reduced imaging time, super-resolution imaging, close-range field testing ability, and a wide range of adaptable scenarios. This work is quite promising to open a new area of EM field imaging and will be of great value in EM applications, exerting a broad impact on antenna tests, microwave circuit diagnosis, EM compatibility analysis, radar imaging, and many other fields.
Funding
National Natural Science Foundation of China (NO. 61671032).
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.
Supplemental document
See Supplement 1 for supporting content.
References
1. A. Ryabov and P. Baum, “Electron microscopy of electromagnetic waveforms,” Science 353(6297), 374–377 (2016). [CrossRef]
2. J. A. Sedlacek, A. Schwettmann, H. Kübler, R. Löw, T. Pfau, and J. P. Shaffer, “Microwave electrometry with Rydberg atoms in a vapour cell using bright atomic resonances,” Nat. Phys. 8(11), 819–824 (2012). [CrossRef]
3. H. J. Mamin, M. Kim, M. H. Sherwood, C. T. Rettner, K. Ohno, D. D. Awschalom, and D. Rugar, “Nanoscale Nuclear Magnetic Resonance with a Nitrogen-Vacancy Spin Sensor,” Science 339(6119), 557–560 (2013). [CrossRef]
4. “IEEE Recommended Practice for Antenna Measurements,” IEEE Std 149-2021 Revis. IEEE Std 149-1977 1–207 (2022).
5. M. Li, R. Singh, M. S. Soares, C. Marques, B. Zhang, B. Zhang, S. Kumar, and S. Kumar, “Convex fiber-tapered seven core fiber-convex fiber (CTC) structure-based biosensor for creatinine detection in aquaculture,” Opt. Express 30(8), 13898–13914 (2022). [CrossRef]
6. Z. Wang, R. Singh, C. Marques, R. Jha, B. Zhang, B. Zhang, S. Kumar, and S. Kumar, “Taper-in-taper fiber structure-based LSPR sensor for alanine aminotransferase detection,” Opt. Express 29(26), 43793–43810 (2021). [CrossRef]
7. X. Fu, E. Wang, Y. Zhao, A. Liu, E. Montgomery, V. J. Gokhale, J. J. Gorman, C. Jing, J. W. Lau, and Y. Zhu, “Direct visualization of electromagnetic wave dynamics by laser-free ultrafast electron microscopy,” Sci. Adv. 6(40), eabc3456 (2020). [CrossRef]
8. N. Shibata, S. D. Findlay, Y. Kohno, H. Sawada, Y. Kondo, and Y. Ikuhara, “Differential phase-contrast microscopy at atomic resolution,” Nat. Phys. 8(8), 611–615 (2012). [CrossRef]
9. R. Ishikawa, S. D. Findlay, T. Seki, G. Sánchez-Santolino, Y. Kohno, Y. Ikuhara, and N. Shibata, “Direct electric field imaging of graphene defects,” Nat. Commun. 9(1), 3878 (2018). [CrossRef]
10. Y.-Y. Jau, D. S. Hussey, T. R. Gentile, and W. Chen, “Electric Field Imaging Using Polarized Neutrons,” Phys. Rev. Lett. 125(11), 110801 (2020). [CrossRef]
11. M. Jing, Y. Hu, J. Ma, H. Zhang, L. Zhang, L. Xiao, and S. Jia, “Atomic superheterodyne receiver based on microwave-dressed Rydberg spectroscopy,” Nat. Phys. 16(9), 911–915 (2020). [CrossRef]
12. S. Kumar, H. Fan, H. Kübler, A. J. Jahangiri, and J. P. Shaffer, “Rydberg-atom based radio-frequency electrometry using frequency modulation spectroscopy in room temperature vapor cells,” Opt. Express 25(8), 8625–8637 (2017). [CrossRef]
13. P. Wang, Z. Yuan, P. Huang, X. Rong, M. Wang, X. Xu, C. Duan, C. Ju, F. Shi, and J. Du, “High-resolution vector microwave magnetometry based on solid-state spins in diamond,” Nat. Commun. 6(1), 6631 (2015). [CrossRef]
14. B. Yang, M.-M. Dong, W.-H. He, Y. Liu, C.-M. Feng, Y.-J. Wang, and G.-X. Du, “Using Diamond Quantum Magnetometer to Characterize Near-Field Distribution of Patch Antenna,” IEEE Trans. Microwave Theory Tech. 67(6), 2451–2460 (2019). [CrossRef]
15. B. Yang, Y. Dong, Z.-Z. Hu, G.-Q. Liu, Y.-J. Wang, and G.-X. Du, “Noninvasive Imaging Method of Microwave Near Field Based on Solid-State Quantum Sensing,” IEEE Trans. Microwave Theory Tech. 66(5), 2276–2283 (2018). [CrossRef]
16. M. Tsuchiya, S. Takata, K. Ohsone, S. Fukui, and M. Yorinaga, “Nanoscopic live electrooptic imaging,” Sci. Rep. 11(1), 5541 (2021). [CrossRef]
17. M. Tsuchiya, S. Fukui, and M. Yorinaga, “Microscopic Live Electrooptic Imaging,” Sci. Rep. 7(1), 7917 (2017). [CrossRef]
18. K. Sasagawa, A. Kanno, and M. Tsuchiya, “Real-time digital signal processing for live electro-optic imaging,” Opt. Express 17(18), 15641–15651 (2009). [CrossRef]
19. J. Dyson, “Measurement of near fields of antennas and scatterers,” IEEE Trans. Antennas Propag. 21(4), 446–460 (1973). [CrossRef]
20. J.-C. Bolomey, B. J. Cown, G. Fine, L. Jofre, M. Mostafavi, D. Picard, J. P. Estrada, P. G. Friederich, and F. L. Cain, “Rapid near-field antenna testing via arrays of modulated scattering probes,” IEEE Trans. Antennas Propag. 36(6), 804–814 (1988). [CrossRef]
21. A. V. Diebold, M. F. Imani, T. Fromenteze, D. L. Marks, and D. R. Smith, “Passive microwave spectral imaging with dynamic metasurface apertures,” Optica 7(5), 527–536 (2020). [CrossRef]
22. S. Hisatake, Y. Tanaka, C. Belem, C. Luxey, F. Gianesello, G. Ducournau, and A. Hirata, “Near-field Measurement and Far-field Characterization of a J-band Antenna Based on an Electro-optic Sensing,” in 2020 14th European Conference on Antennas and Propagation (EuCAP) (2020), pp. 1–4.
23. R. Justice and V. Rumsey, “Measurement of electric field distributions,” IRE Trans. Antennas Propag. 3(4), 177–180 (1955). [CrossRef]
24. J. H. Richmond, “A Modulated Scattering Technique for Measurement of Field Distributions,” IEEE Trans. Microwave Theory Tech. 3(4), 13–15 (1955). [CrossRef]
25. A. L. Cullen and J. C. Parr, “A new perturbation method for measuring microwave fields in free space,” Proc. IEE - Part B Radio Electron. Eng. 102(6), 836–844 (1955). [CrossRef]
26. W. Liang, G. Hygate, J. F. Nye, D. G. Gentle, and R. J. Cook, “A probe for making near-field measurements with minimal disturbance: the optically modulated scatterer,” IEEE Trans. Antennas Propag. 45(5), 772–780 (1997). [CrossRef]
27. W. A. Vitale, M. Tamagnone, N. Émond, B. Le Drogoff, S. Capdevila, A. Skrivervik, M. Chaker, J. R. Mosig, and A. M. Ionescu, “Modulated scattering technique in the terahertz domain enabled by current actuated vanadium dioxide switches,” Sci. Rep. 7(1), 41546 (2017). [CrossRef]
28. W. Platte, “Optoelectronic Microwave Switching via Laser-Induced Plasma Tapers in GaAs Microstrip Sections,” IEEE Trans. Microwave Theory Tech. 29(10), 1010–1018 (1981). [CrossRef]
29. B. Boyer, J. Haidar, A. Vilcot, and M. Bouthinon, “Tunable microwave load based on biased photoinduced plasma in silicon,” IEEE Trans. Microwave Theory Tech. 45(8), 1362–1367 (1997). [CrossRef]
30. C. D. Gamlath, D. M. Benton, and M. J. Cryan, “Microwave properties of an inhomogeneous optically illuminated plasma in a microstrip gap,” IEEE Trans. Microwave Theory Tech. 63(2), 374–383 (2015). [CrossRef]
31. A. Johnson and D. Auston, “Microwave switching by picosecond photoconductivity,” IEEE J. Quantum Electron. 11(6), 283–287 (1975). [CrossRef]
32. W. Platte, “Spectral dependence of light-induced microwave reflection coefficient from optoelectronic waveguide gratings,” IEEE Trans. Microwave Theory Tech. 43(1), 106–111 (1995). [CrossRef]
33. A. J. Seeds and A. A. A. De Salles, “Optical control of microwave semiconductor devices,” IEEE Trans. Microwave Theory Tech. 38(5), 577–585 (1990). [CrossRef]
34. M. A. Collett, C. D. Gamlath, and M. Cryan, “An Optically Tunable Cavity-Backed Slot Antenna,” IEEE Trans. Antennas Propag. 65(11), 6134–6139 (2017). [CrossRef]
35. Y. Yashchyshyn, K. Derzakowski, P. R. Bajurko, J. Marczewski, and S. Kozłowski, “Time-modulated reconfigurable antenna based on integrated S-PIN diodes for mm-wave communication systems,” IEEE Trans. Antennas Propag. 63(9), 4121–4131 (2015). [CrossRef]
36. L. Ma, N. Leng, X. Z. Ye, M. Jin, and M. Bai, “The Distribution Measurement of the Photo-induced Plasma in Semiconductor by Near-field Scanning Microwave Microscopy,” in 2019 Photonics Electromagnetics Research Symposium - Fall (PIERS - Fall) (IEEE, 2019), pp. 169–172.
37. “IEEE Draft Recommended Practice for Near-Field Antenna Measurements,” IEEE P1720D2 April 2012 1–105 (2012).
38. G. Hygate and J. F. Nye, “Measuring microwave fields directly with an optically modulated scatterer,” Meas. Sci. Technol. 1(8), 703–709 (1990). [CrossRef]
