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Abstract
Polarization sensitive photo-detectors are the key to the implementation of the polarimetric imaging systems, which are proved to have superior performance than their traditional counterparts based on intensity discriminations. In this article, we report the demonstration of a superconducting nanowire single photon detector (SNSPD) of which the response is ultra-sensitive to the polarization state of the incident photons. Measurements carried out on a fabricated SNSPD show that a device efficiency of ~48% can be achieved at 1550 nm for the case of parallel polarization, which is ~420 times larger than that for the case of perpendicular polarization. While the reported polarization ultra-sensitive technique is demonstrated on a single-pixel SNSPD, it is also fully compatible with the multi-pixel SNSPD array platforms that emerged recently.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In light of its superior roles played in many applications such as quantum secure communication, fundamental quantum physics and others, superconducting nanowire single photon detector (SNSPD) [1–3] has experienced a rapid development over the past decade and entered a new era in term of technical performances. Some notable achievements accomplished may include SNSPDs with ultrahigh system efficiencies [4,5], SNSPDs with ultralow dark count rates and timing jitters [6–9], SNSPDs with reduced polarization sensitivities [10–13], and SNSPDs that can resolve photon numbers [14–16] and spatial locations [17–19]. In particular, with efforts being paid to solve the heat and layout problems associated with the readout circuitries, recently, two-dimensional scalable SNSPD arrays have been demonstrated [20,21]. These newly emerged two-dimensional SNSPD array platforms are expected to significantly improve the technical limits of many research and engineering fields such as optical imaging in astrophysics and biology, long haul laser ranging and deep-space optical communication links [20,21].
Viewing from a historic perspective, the most natural option for the applications of the two-dimensional SNSPD arrays might be the imaging application. While in traditional imaging systems intensity discrimination is used to generate the image results, it has been recognized for a while that polarimetric information can also be utilized to yield the image outputs [22,23]. Such a polarization related imaging technique is normally termed as the polarimetric imaging and has been shown to be capable of revealing various object surface features that are otherwise latent, i.e. shape, shading and roughness. Different from the traditional imaging system, in the polarimetric imaging system it is sometime preferred that the photodetector by itself is highly polarization sensitive, since such a polarization diversified technique enables the construction of polarimetric imaging systems with improved frame acquisition rates [23]. In order to make a SNSPD inherently polarization sensitive, a straightforward approach is to attach it with a micro-grating polarizer which acts as a polarization filter. We note that while this method is conceptually simple and practically effective, however, when the micro-grating is optimized to accept more photons with the preferred polarization (e.g. by decreasing the grating duty ratio) it also tends to reject insufficient photons with the unpreferred polarization [24,25], leading to a design tradeoff. In other words, it is difficult to optimize the device design in a way that the device efficiency for parallel polarization is maximized, while the device efficiency for perpendicular polarization is minimized, or vice versus [26]. The same tradeoff also applies to the case of the highly polarization sensitive SNSPD demonstrated by Guo et al. [27], where the high polarization extinction ratio is achieved by shrinking the width of the NbN nanowire at a cost of a reduced device efficiency for the case of parallel polarization.
In this article, we report the experimental demonstration of a SNSPD that is ultra-sensitive to the polarization state of the incident photons. The design framework involved follows that proposed by Zhen et al. [26], which has been theoretically shown to be free of the tradeoff problem mentioned above. Guided by such a framework, a polarization ultra-sensitive SNSPD operating at 1550 nm is designed, fabricated and measured, with a device efficiency of ~48% and a polarization extinction ratio of ~420 being achieved. The obtained preliminary results confirm the feasibility of the device design scheme proposed previously [26], and also open a route to the demonstration of polarization sensitive two-dimensional SNSPD arrays [20,21] for fast speed polarimetric imaging applications [23].
2. Principle and design
The schematic of the reported device is illustrated in Fig. 1(a). It consists of a Si substrate, a SiO2 layer within which a NbN meander and an Al grating are embedded, and an Au layer. In the absence of the Al grating, the device reduces to the case of a regular SNSPD that employs optical cavities for absorption efficiency enhancement [28,29]. The efficiency enhancement effect arises from the constructive interference between the incident wave and the wave reflected from the Au mirror. Since the reflection from the Au mirror exhibits little discrimination (<1%) with respect to the light polarization, the enhancement effect of the efficiency is therefore polarization insensitive. This leaves the size of the NbN nanowire as the primary factor that determines the polarization extinction ratio [29,30], for which the largest value reported to date is ~20 [27]. The polarization extinction ratio can be dramatically increased by placing an Al grating between the Au mirror and the NbN meander [26], which introduces a polarization dependence for the reflected wave. In an intuitive sense, the wave reflection can be thought as being resulted from the Al grating for the case of parallel polarization, but from the Au mirror for the case of perpendicular polarization. With proper arrangements for the separations between the Au, Ag and NbN elements, at a desired wavelength (1550 nm in this work), it is possible that the NbN meander may experience a constructive interference if the light is parallel polarized, but a destructive interference if the light is perpendicularly polarized. Such a situation is illustrated by Figs. 1(b) and 1(c), which plot the amplitude distributions of the electric fields for cases of parallel and perpendicular polarizations respectively. The huge contrast of the electrical field intensity at the NbN meander plane between these two cases results in a greatly increased value of the polarization extinction ratio.
Fig. 1 Illustration of the working principle of the reported device. (a) Unit cell of the reported device. (b) Distribution of the electric field amplitude for the case of parallel polarization. (c) Distribution of the electric field amplitude for the case of perpendicular polarization. The excitation wavelength for (b) and (c) is at 1550 nm.
We use numerical simulations to determine the optimized parameters for a device designed at 1550 nm. The numerical simulation is carried out using FDTD solutions, a software that is available off-the-shelf commercially. Owing to periodic conditions, the simulated object is the device’s unit-cell depicted in Fig. 1(a). We assume that the NbN meander has a pitch of 200 nm and the Al grating has a pitch of 400 nm. The unit-cell therefore has a pitch of 400 nm and contains one Al nanowire and two NbN nanowires inside. The pitch of the Al grating is chosen to be double of that of the NbN meander since a larger pitch allows easier fabrication of the Al grating. The widths of the NbN nanowire and the Al nanowire are assumed to be 100 nm and 200 nm respectively. The thicknesses of the NbN nanowire, the Al nanowire and the Au mirror are assumed to be 6 nm, 40 nm and 200 nm respectively. The refractive indices used in the simulation are from the software itself for all the materials involved except NbN, of which the refractive index is a Drude model fitted on room temperature based measurements [31].
Based on the numerical model described above, we proceed to determine the optimized positions of the Al grating, the NbN meander and the Si substrate. To this end, we first consider a unit-cell otherwise identical to that shown in Fig. 1(a) but in the absence of the NbN nanowires and the Si substrate. We illuminate such a unit-cell using parallel and perpendicularly polarized plane waves at 1550 nm respectively, and scan the position of the Al nanowire until the location of a constructive interference for the case of parallel polarization and the location of a destructive interference for the case of perpendicular polarization precisely overlap at some place underneath the Al grating. In this manner, the optimized position of the Al grating is determined. Next, the NbN meander and the Si substrate are included and the unit-cell returns to the form shown in Fig. 1(a). Note that the NbN meander is placed at the position of the aforementioned interference overlap and the position of the Si substrate is the only parameter that remains unknown. Finally, we numerically calculate the absorption efficiency of the NbN meander at 1550 nm for the case of parallel polarization, as a function of the position of the Si substrate. The optimized position value of the Si substrate is the one that maximizes the absorption efficiency. This completes the last step of the device design, of which the obtained optimal parameters are summarized in Fig. 1(a) for quick references.
Figure 2(a) shows the calculated absorption efficiencies of the optimized device, which are obtained by dividing the power absorbed by the NbN meander with respect to the power of the incident waves. The black curve is for the case of parallel polarization and has a maximum value of ~96.3%, while the red curve is for the case of perpendicular polarization and has a minimum value of ~0.017%. It can be observed that the wavelengths at which the black curve is maximized and the red curve is minimized are both positioned at 1550 nm, which is the desired device operating wavelength. Such a numerically obtained coincidence of these two wavelengths is in agreement with the theoretical prediction presented previously [26]. It indicates that our device design enables the absorption efficiencies for parallel and perpendicular polarizations to be maximized and minimized at the same time and therefore overcomes the tradeoff problem mentioned before. The peak value of the absorption efficiency for the case of parallel polarization is less than 100%. The missing portion of the absorption efficiency is due to the accompanying absorptions by the Au mirror (~0.05%) and the Al grating (~2.4%), and the residual reflection (~1.25%) by the device. Some minor improvement of the absorption efficiency can be attained by finely adjusting the width of the NbN nanowire to completely eliminate the reflected wave, in an analogy to the critical coupling effect encountered in microwave engineering [26]. The simulated polarization extinction ratio of the reported device is plotted in Fig. 2(b) using the symbols. The data set is obtained by calculating the ratio of the absorption efficiencies between cases of parallel and perpendicular polarizations. The obtained numerical result demonstrates an ultrahigh polarization extinction ratio of ~5.7 × 103 and exhibits a well-fitted Lorentz-type line shape, all in agreements with the theoretical model established previously [26].
Fig. 2 Numerically obtained spectral results of the reported device. (a) Absorption efficiencies as a function of the wavelength. (b) Polarization extinction ratio (PER) as a function of the wavelength.
3. Fabrication and measurement
Guided by the above design results, a polarization ultra-sensitive SNSPD operating at 1550 nm is fabricated. The fabricated device employs a 300 μm thick Si layer as the substrate, of which the front and back sides are thermally oxidized under a temperature of 1150°. The thicknesses of the two SiO2 oxidation layers are controlled by adjusting the pressure of the O2 and the oxidation time, and are determined to be 270 nm with the help of a nanoscale optical film thickness measurement system (NanoCalc, Ocean Optics). Note that such a choice of the oxidation thickness, i.e. 270 nm, is about a quarter of the desired operating wavelength inside the SiO2 medium and can help to reduce the coupling loss at the backside of the substrate. Moreover, since it also matches closely to the optimized thickness of the SiO2 buffer layer sandwiched between the NbN meander and the Si substrate, NbN films can be directly grown on top of the oxidized Si substrate by means of room temperature magnetron sputtering. A 6 nm thick NbN film is sputtered (Tc 7.2k), and is fabricated into a meander pattern by means of electron beam exposure and reactive ion etching. The formed NbN meander has a pitch of 200 nm and a duty cycle of 50%, and covers an effective area of 12 μm × 12 μm. On top of the NbN meander, a SiOx layer is grown using plasma enhanced chemical vapor deposition (PECVD) under a temperature of 350°. An Al grating is then subsequently fabricated using electron beam evaporation and liftoff. The formed Al grating has a pitch of 400 nm, a duty cycle of 50% and a thickness of 40 nm, and covers an effective area of 14 μm × 14 μm. Upon finishing the Al grating, a second layer of SiOx is grown using PECVD under a temperature of 50°. Finally, the device fabrication is completed by depositing a 120 nm thick Au film with the help of AC magnetron sputtering.
Figure 3(a) shows the transmission electron microscope (TEM) image of the fabricated device. From bottom to top, a Si substrate (dark grey), a SiO2/SiOx layer (light grey) within which 7 NbN nanowires and 3 Al nanowires are presented, and an Au layer (dark) can be identified. Note that the space between the NbN meander and the Si substrate is occupied by the SiO2 oxidation layer, which has a predetermined thickness of 270 nm. Using such a thickness as a reference for vertical length calibrations, the separation between the NbN meander and the Al grating, and as well the separation between the Al grating and the Au mirror, are measured to be 240 nm and 175 nm respectively. Comparing to the optimized parameters listed in Fig. 1(a), these numbers are larger and hence may dictate wavelength red shifts for the absorption efficiency curves. Some further comparisons between the TEM image and the unit cell also reveal that the Au mirror has a non-flat periodical arch structure and the Al grating is asymmetrically aligned with respect to the NbN meander. The effects caused by these two fabrication imperfections have been analyzed previously [26]. It is shown that the effect caused by the former one can be compensated by adjusting the position of the Au mirror, and the effect caused by the latter one is negligible.
Fig. 3 Preliminary results of the fabricated device. (a) TEM image of the fabricated device. (b) Measured photon counts as a function of the polarization angle. The excitation wavelength for (b) is at 1550 nm. A polarization extinction ratio of ~420 is achieved.
The fabricated device is optically packaged by attaching a fiber at the backside of its Si substrate [13], which is covered by a SiO2 oxidation layer for anti-reflections. The distance between the optical fiber and the Si substrate is set to be 1cm. Owing to the absence of any focusing lens and the long diverging distance of the beam, the illumination profile is much larger than the NbN meander, thereby resulting in a far-field alignment scenario [28]. Note that the illumination profile is determined by the diverging distance of the beam and the numerical aperture of the optical fiber as well, both of which are achromatic. It follows that the device coupling efficiency is wavelength independent, which is a virtue that helps to simplify the interpretation of the spectral domain measurement. The packaged device is mounted on a compact Gifford–McMahon cryocooler and cooled to 2.3 K. A bias-T is used to provide the device with a bias current and extract out the voltage signal for pulse counting after external amplifications.
We demonstrate the ultrahigh polarization sensitivity of the fabricated device by measuring the signal count rates as a function of the polarization angle. To carry out the measurement, a continuous wave semiconductor diode laser operating at 1550 nm is used as the light source. The optical power of the laser diode is attenuated to a level at which the maximum count rate is less than 106 counts per second. This ensures a non-saturated linear operating condition for the tested device, as being confirmed experimentally by a separate count-power linearity measurement. A serial combination of a 3-paddle polarization controller and a polarization synthesizer (N7786B, Keysight) is used to manipulate the polarization of the laser light. At first, a state of perpendicular polarization is attained by carefully twisting the 3-paddle polarization controller until a minimized photon count occurs. Following that, the polarization angle is rotated and scanned by the polarization synthesizer and the related photon counts are recorded. The obtained data set of the polarization angle dependent photon counts is plotted in Fig. 3(b) using the symbols, together with a well-matched sine-fit sketched by the line. The experimental result demonstrates a maximum count rate of 8.3 × 105 and a minimum count rate of 2 × 103. Taking into account of the effect of the dark counts, which is 100 for this measurement, the polarization extinction ratio is estimated to be ~420. When comparing to the numerical simulation, the experimental value of the polarization extinction ratio is considerably lower. Such a reduction or a difference is presumed to be primarily caused by the finite size effect of the fabricated device. In brief, due to the finite and comparable sizes of the Al grating (14 μm × 14 μm) and the NbN meander (12 μm × 12 μm), there exists a small yet non-negligible portion of the NbN nanowires near the meander’s edge position, where the reflected wave deviates from the ideal planar case and the destructive interference is not sufficient. Owing to such an edge effect, the absorption efficiency for the case of perpendicular polarization is increased and consequently the measured polarization extinction ratio is reduced.
To provide further information, spectral domain measurements on the device efficiencies are also carried out [13]. In performing the spectral domain measurements, a fiber-based and acousto-optically filtered supercontinuum is employed as the wavelength-tunable light source. An achromatic polarization controller (PC-FFb-1550, Thorlabs) is used to manipulate the polarization states of the delivered photons. The achromatic nature of the polarization controller helps to ensure that a desired polarization state, once is initially prepared at the device end, remains as unchanged during the wavelength scanning process. Figure 4(a) shows the experimental results of the device efficiencies for cases of parallel and perpendicular polarizations, obtained after excluding the contribution of the coupling efficiency [28], which is ~2.9 × 10−5 in our achromatic far-field packaging setup [13]. Comparing to the numerical result plotted in Fig. 2(a), it can be observed that other than a wavelength red shift, the experimental result exhibits a well-matched curve shape. As being stated before, the wavelength red shift is caused by the increased thicknesses of the two SiOx layers of the fabricated sample (as confirmed numerically). The measured device efficiency at 1550 nm is 48%. Such a moderate result of the device efficiency might be attributed to the fact that the quantum efficiency of the fabricated device is less than 100%, which can be inferred from the efficiency-bias curve shown in the inset of Fig. 4(a) [32]. We note that several methods can be used to increase the quantum efficiency, e.g. reducing the thickness of the NbN film and decreasing the NbN meander filling ratio from 50% to 33% to avoid the current crowding effect [33,34]. The experimental values of the polarization extinction ratios are obtained by dividing the device efficiencies for case of parallel polarization with respect to the device efficiencies for the case of perpendicular polarization. The results are plotted in Fig. 4(b) using the symbols, in together with a well-matched Lorentz line fit. The polarization extinction ratio at 1550 nm retrieved from this measurement is ~200, which is about a half of that retrieved from the measurement of Fig. 3(b). To explain such a reduction or a difference, we note that the light source used for Fig. 3(b) has a narrow linewidth of ~50 MHz, while the light source used for Fig. 4(b) has a broad linewidth of ~10 nm. The large amount of the unfiltered out-of-band spectral component for the latter case generates a significant background of the photon counts for the case of perpendicular polarization, which in turn results in the seemed reduction of the polarization extinction ratio.
Fig. 4 Experimentally obtained spectral results of the fabricated device. (a) Device efficiencies as a function of the wavelength. (b) Polarization extinction ratio (PER) as a function of the wavelength. The inset in (a) shows the bias dependence of the device efficiency at 1550 nm. The bias current is normalized using the critical current, which is 10.5 uA.
Before we conclude this article, it should be remarked that during the above numerical design and the experimental data analysis steps of the fabricated device, the effect of the position dependent local detection efficiency has not been considered [35]. In brief, we note that it has been experimentally found that photons absorbed near the edges of the NbN nanowires are more likely to successfully block the supercurrent and trigger a detection event [35]. Moreover, it is also well-known that for the case of perpendicular polarization, photons are not uniformly absorbed, with absorption probability being higher at the center of the NbN nanowires. Taking account of these two facts, it is believed that the effect of position dependent local detection efficiency helps to decrease the device efficiency for case of perpendicular polarization, and thereby helps to increase the experimental value of the polarization extinction ratio [26]. Further experiments carried out using the technique of differential polarization measurement [35] are needed to quantitatively determine the related contributions.
4. Conclusion
In conclusion, we have designed and fabricated a polarization ultra-sensitive SNSPD that operates at 1550 nm. Measurements carried out on the fabricated device reveal that a device efficiency of ~48% and a polarization extinction ratio of ~420 can be attained. The fabricated device is also validated by efforts spent in the spectral domain, where a good match of the experimental and numerical curves can be found. It is hoped that the reported polarization discriminating technique, when implemented on the SNSPD array platform in the future, may lead to the demonstration of an efficient SNSPD based fast speed polarimetric imaging system for dim objects.
Acknowledgments
This work was supported by the National Natural Science Foundation (Nos. 11227904, 61571105, 61471189), National Key R&D Program of China Grants 2017YFA0304002, the National Basic Research Program of China (973 Program) Grants 2014CB339804 and the National Basic Research Program of China (Nos. 2014CB339800).
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