High-Bandwidth White-Light System Combining a Micro-LED with

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A high-bandwidth white-light system combining a micro-LED with perovskite quantum dots for visible light communication Shiliang Mei, Xiaoyan Liu, Wanlu Zhang, Ran Liu, Lirong Zheng, Ruiqian Guo, and Pengfei Tian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17810 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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ACS Applied Materials & Interfaces

A high-bandwidth white-light system combining a micro-LED with perovskite quantum dots for visible light communication Shiliang Mei‡, Xiaoyan Liu‡, Wanlu Zhang, Ran Liu, Lirong Zheng, Ruiqian Guo* and Pengfei Tian* Institute for Electric Light Sources, School of Information Science and Technology, Fudan University, Engineering Research Center of Advanced Lighting Technology, Ministry of Education, Shanghai 200433, China KEYWORDS: Perovskite quantum dots, micro-LED, white-light system, free-space visible light communication, bandwidth, data rate

ABSTRACT: This work proposes a high-bandwidth white-light system consisting of a blue gallium nitride (GaN) micro-LED (μLED) exciting yellow-emitting CsPbBr1.8I1.2 perovskite quantum dots (YQDs) for high-speed real-time visible light communication (VLC). The packaged 80 μm × 80 μm blue-emitting μLED has a modulation bandwidth of ~160 MHz and a peak emission wavelength of ~445 nm. The achievable bandwidth of the white-light system is up to 85 MHz in the absence of filters and equalization technology. Meanwhile, the bandwidth of the YQDs as a color-converter is as high as 73 MHz with the blue GaN μLED as the pump

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source. A maximum data rate of 300 Mbps can be achieved by taking advantage of the high bandwidth of the white-light system using non-return-to-zero on-off keying (NRZ-OOK) modulation scheme. The resultant bit-error rate (BER) is 2.0 × 10-3, well beneath the forward error correction (FEC) criterion of 3.8 × 10-3 required for error-free data transmission. In addition, the YQDs which we proposed as a color-converter possess high stability for VLC. After half a year, the achievable bandwidths of the white-light system and the YQDs are still up to 83 MHz and 70 MHz, respectively. This study provides the direction of developing high-bandwidth white-light system for both high-efficiency solid state lighting (SSL) and high-speed VLC.

INTRODUCTION Visible light communication (VLC), an advanced wireless communication technology that can combine solid state lighting (SSL) and optical communications together, has attracted huge attention recently due to the advantages of high data rate, energy efficiency, enhanced security and no radio frequency interference.1-8 The commonly used white LEDs for SSL are composed of a blue-emitting broad-area LED and yellow-emitting phosphors. However, the phosphor suffers from low -3 dB electrical-to-optical (E-O) modulation bandwidth of ~2.5 MHz and the bandwidth of broad-area LEDs is usually less than 20 MHz which limits the maximum achievable data communication speed.9-10 Consequently, how to increase the bandwidth of the white light source for VLC has become a key issue, and great efforts have been made to increase the bandwidth of both the color converter and the blue LED.11-13 Regarding the color converter, quantum dots (QDs) have emerged as promising conversion materials in SSL due to their tunable emission, and narrow luminescent spectra and high quantum yield (QY).14-18 In particular, QDs have typical short fluorescent lifetime in the order of


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several or tens of nanoseconds, and thus high modulation bandwidth and high data rate may be achieved based on the QDs.12 Moreover, the solution-processability of the QDs provides flexibility in the fabrication of hybrid devices,19 e.g. combining the μLED and QDs in this work. Therefore, QDs are particularly suitable for color conversion to generate white-light for both SSL and VLC applications. Laurand et al. reported that CdSe/ZnS QDs were capable of generating white light when excited by a 450 nm blue LED, but the -3 dB bandwidth is only 1025 MHz.12 Green- or red-emitting QD-LED and QD-white LED based on CdSe/ZnS QDs for VLC have been proposed by Xiao et al., however, their modulation bandwidth is limited to be less than 3 MHz.13 Perovskite QDs exhibiting narrow emission and short fluorescent lifetime have attracted increasing attention for the SSL,20 but few work reported VLC application based on perovskite QDs.21 Dursun et al. obtained white light for VLC by designing solution-processed CsPbBr3 perovskite nanocrystals and conventional red phosphors excited by laser diodes (LDs), with much higher modulation bandwidth than CdSe/ZnS QDs.21 Nevertheless, for SSL using LDs, we need to address problems such as limited etendue, speckle noise and harm to human eyes.22 Thus, eye-safe blue LEDs can be employed, and high-bandwidth blue LED is also required to be combined with high-bandwidth perovskite QDs to develop a high-bandwidth white-light system. Recently, a single blue-emitting micro-LED (μLED), with typical size of tens of μm and the bandwidth of hundreds of MHz, was used to demonstrate VLC with data rate of several Gbps, much higher than broad-area LEDs.23 It is primarily ascribed to the intrinsic properties of the μLED such as smaller size, shorter carrier lifetime and higher sustainable current density, leading to significantly higher modulation bandwidths than broad-area LEDs. Up to now, the VLC based on a white-light system combining a μLED and perovskite QDs has not


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yet been achieved. Therefore, it is highly desirable to combine high-bandwidth μLED and highbandwidth perovskite QDs to establish eye-safe white-light system for both VLC and SSL. In this study, we proposed a full inorganic white-light system for VLC using a blue GaN μLED and inorganic yellow-emitting CsPbBr1.8I1.2 perovskite QDs (YQDs) as the colorconverter. The packaged 80 μm × 80 μm blue-emitting μLED has a maximum modulation bandwidth of ~160 MHz and a peak emission wavelength of ~445 nm. We achieved maximum 3 dB E-O modulation bandwidths of ~73 MHz and ~85 MHz for the perovskite QDs and the white-light system combining the μLED and the perovskite QDs, respectively. Furthermore, based on the high-bandwidth white-light system, a real-time data rate of 300 Mbps with a biterror rate (BER) of 2.0 × 10-3 below the forward error correction (FEC) of 3.8 × 10-3 was obtained using non-return-to-zero on-off keying (NRZ-OOK) modulation scheme. Moreover, the stability of our white-light system has been studied. The bandwidths of the YQDs and the whitelight system show little degradation after half a year, indicating that the white-light system exhibits a relatively high stability. EXPERIMENTAL SECTION High quality YQDs were synthesized by a modified hot-injection method based on our previous study.18 Materials. Cesium carbonate (Cs2CO3, 99%), lead bromide (PbBr2, 99.999%), lead iodide (PbI2, 99.999%), octadecene (ODE, 90%), oleylamine (OLA, 90%), and oleic acid (OA, 90%) were purchased from Aladdin. Epoxy resin (HS-302A) and hardener (HS-302B) was purchased from Shanghai Hennmas Fine Chemical Co. Ltd. No further purification was provided for all chemicals.


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Preparation of Cs-oleate. 0.08 g Cs2CO3 was loaded into a 100 mL three-neck flask along with 30 mL ODE and 2.5 mL OA, and then the Cs2CO3 can be completely dissolved after being heated under N2 to 150°C for 30 min. The mixture was stored at 100°C for the subsequent injection. Preparation of CsPbBr1.8I1.2 QDs. 0.0793 g PbBr2 and 0.0664 g PbI2 with 10 mL ODE, 1 mL OLA and 1 mL OA were loaded into a 50 mL three-neck flask and dried under vacuum for 30 min at 120°C. Then the reaction temperature was raised to 150°C under N2 and 0.8 mL Cs-oleate solution was quickly injected. Five seconds later, the reaction mixture was cooled down to room temperature by immersion in an ice-water bath. After a high-speed centrifugation (at 10000 rpm for 15 min) of the as-prepared YQDs, the supernatant was discarded and the particles were redissolved in toluene. Preparation of YQDs-epoxy resin film. 10 mL YQDs-toluene solution was added to epoxy resin/hardener (weigh ratio of 10) with vigorous stirring, and then the mixture was dropped on a clear glass slice and subsequently dried slowly at 80°C for 12 h in an oven to form the YQDsepoxy resin film. Characterization. The obtained YQDs were characterized by high-resolution transmission electron microscopy (HRTEM, JEM 2011, JEOL, Japan) and X-ray diffractometer (XRD, D8 Advance, Bruker, Germany). The photoluminescence (PL) spectra and ultraviolet and visible (UV-vis) absorption spectra were measured by F97XP fluorescence spectrophotometer and 759S UV-vis spectrophotometer, respectively. The PL decay curves were measured using Fluorescence Lifetime Spectrometers QM 40 (excitation wavelength: 463 nm). The PL QY of YQDs was calculated by using the relative quantum efficiency of the YQDs compared to the


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reference of Rhodamine 6G (RD6, QY = 95% dissolved in ethanol), using the following equation:24 QY=QYRD6 × (I

I RD6

ARD6

)×(

A

) × (n

2

n RD6

)

,

(1)

where QY and QYRD6 are the PL QY for the YQDs and the standard Rhodamine 6G, respectively; I and IRD6 represent the integral PL intensity of YQDs and the standard Rhodamine 6G at a specified wavelength; A and ARD6 show the absorption intensity at the same excitation wavelength, respectively; n and nRD6 are the refractive indices of the solvents for dissolution. To reduce the re-absorption impact, the absorbance of the YQDs solution was maintained less than 0.1 by dilution. Fabrication of the μLED device. The μLED was fabricated from commercially available GaNbased epitaxial structure on a patterned sapphire substrate grown by metal-organic chemical vapor deposition (MOCVD). The epitaxial structure consists of an n-GaN layer, an InGaN/GaN multiple quantum well with a center emission wavelength at ~445 nm, an AlGaN electron blocking layer and a p-GaN layer. During the μLED device processing, Ni/Au metal layers were deposited on p-GaN layer as the current spreading layer. We defined μLED mesas through etching away the Ni/Au and the GaN layers down to the n-GaN layer using reactive ion etching (RIE) and inductively coupled plasma (ICP) etching, respectively. Then, in order to form a better current spreading layer, the epitaxial structure deposited Ni/Au layer was thermally annealed in purified air. After that, a SiO2 isolation layer for p-track and n-GaN was deposited by plasma enhanced chemical vapour deposition (PECVD). Subsequently, apertures on μLED mesas were opened by wet etching with buffered oxide etch (BOE). Finally, Ti/Au metal layers were deposited as n-contacts and p-tracks at the same time to simplify the processing steps. The size


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of the fabricated μLED in this work is 80 μm × 80 μm. The device structure and fabrication processes are similar to our previous work, and the light emission was extracted from the sapphire side.25-28 The sapphire substrate was polished on the bottom side for light extraction. The top emitted light can be reflected by the Ti/Au on the top of the micro-LED mesa and most of the reflected light was collected for the SSL and VLC applications in this work. The μLED chip was bonded to a printed circuit board (PCB) connected to subminiature version A (SMA) connectors for following tests. The 2D schematic structure of the μLED is shown in Figure 1 and the corresponding optical microscope image is shown in the inset of Figure 1.

Figure 1. 2D schematic structure of the fabricated μLED. Inset: optical microscope image of the μLED. Measurements of VLC system. The YQDs-epoxy resin film was overlaid on a blue-emitting 80 μm × 80 μm μLED to obtain a white-light system. The experimental setup for measurements of the communication performance based on the proposed white-light system shown in Figure 2 was designed. Figure 2a illustrates the schematic diagram of the μLED based white-light realtime communication system using YQDs as a color-converter with NRZ-OOK modulation scheme. The pseudo-random binary sequences (PRBS) with peak-to-peak voltage (Vpp) of 2 V and pattern length of 27-1, which were produced by a pulse pattern generator (PPG) module from


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an Anritsu MP1800 signal quality analyzer, was combined with a direct current via a bias-tee to drive the μLED. An Agilent network analyzer was used to generate a clock signal for PPG. The blue light from the μLED excited YQDs overlaid on the surface of the μLED to generate white light. The light output from the white-light system was collimated by a transmitter lens and focused by a receiver lens (Tx and Rx lenses). In order to measure the VLC characteristics of the YQDs, a 495 nm long-pass optical filter was used to filter out any remnant of the blue light from the μLED. The optical signal from the transmitter was recorded and converted into electrical signal by a high-sensitivity APD (APD12702) or a high-speed 1.4 GHz PIN photodetector to further analyze the frequency response by an N5225A network analyzer or the BER by an error detector module from the MP1800 signal quality analyzer. The eye diagrams were captured by an 86100A wide-bandwidth oscilloscope. The corresponding picture of the experiment setup is shown in Figure 2b and c.


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Figure 2. (a) Schematic diagrams of the VLC link; experimental setup of (b) the transmitter and (c) the VLC link of the μLED based white-light communication using YQDs as a color-converter. RESULTS AND DISCUSSION Figure 3 shows essential characteristics of the synthesized YQDs. The TEM, HRTEM images and size distribution of the YQDs are shown in Figure 3a, b and c, respectively. It can be seen that the YQDs are monodispersed with a lattice space of ∼0.58 nm corresponding to the (110)


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lattice face of cubic phase29 and an average size of ~10.16 nm. The measured XRD patterns in Figure 3d display the phase of YQDs is cubic in contrast to the standard phase of CsPbBr3 (ICSD-29073) and CsPbI3 (ICSD-181288).30 The UV-vis absorption and PL spectra of the YQDs are shown in Figure 3e. The emission peak wavelength of the YQDs with high QY of 78% locates at 560 nm with narrow full width at half-maximum (FWHM) of 28 nm. The inset is the corresponding photograph of the YQDs solution sample under 365 nm UV light irradiation, from which bright yellow emission can be observed. Time-resolved PL measurement was further carried out to explore the fluorescence lifetime of the YQDs, as Figure 3f shows. The PL decay curve of YQDs can be fitted well by a double-exponential function.31 The corresponding fitting results (Table S1, Supporting Information) reveal that the average fluorescent lifetime of the YQDs is ~43.74 ns, much shorter than that (~μs to ms) of phosphors. These results suggest the potential of perovskite YQDs in applications of both SSL and VLC.


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Figure 3. (a) TEM, (b) HRTEM images, (c) size distribution and (d) X-ray diffraction patterns of the YQDs. (e) UV-vis absorption and PL spectra (λex=445 nm). Inset is photograph of the YQDs solution sample under 365 nm UV light irradiation. (f) Experimental and fitting PL decay curves monitored at 560 nm. To study the white light generated by utilizing YQDs for SSL, the YQDs were excited by the μLED (operating at 70 mA). Figure 4a shows the spectrum of the generated white light with a peak wavelength at ~445 nm from the μLED and a secondary peak wavelength at ~560 nm from the YQDs. The inset of Figure 4a demonstrates the white light generated by combining the μLED and the YQDs, and the corresponding Commission Internationale de L’Eclairage (CIE) color coordinates of (0.27, 0.30) is shown in Figure 4b. Multispectral combination of perovskite QDs with high QY (as high as 78%), narrow emission spectrum and color-tunable properties, can be adopted to improve the white-light system. Therefore, higher efficiency, tunable color temperature and higher color rendering index can be achieved, which shows great potential for SSL application. We have noticed that the maximal illuminance of our white-light system observed at the orientation angle of 90°is only 16 lux mainly owing to the small size and low light-output power of a single μLED. For practical applications in SSL, μLED arrays with high illuminance can be used as shown in our previous work,32 and high-speed parallel VLC using several μLEDs may be achieved.33 In addition, through optimizing optics of the white-light system to improve the light extraction efficiency, the illuminance can be further enhanced. Thus, our white-light system combining the μLED and the YQDs are suitable for potential application in SSL.


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Figure 4. (a) Spectrum of the white light generated using the μLED (operating at 70 mA) and the YQDs. Inset: photograph of generated white light. (b) The corresponding CIE coordinates of generated white light. The high-efficiency white-light system mentioned above is not only used for SSL but also VLC. Therefore, to investigate VLC applications and characteristics of the white-light system, we investigate modulation bandwidth characteristics of the white-light system with the YQDs as the color-converter. Figure 5 shows the frequency response characteristics and the extracted -3 dB modulation bandwidths at different currents from 5 mA to 80 mA. The overall frequency responses of the µLED, the μLED combined with the YQDs (µLED + YQDs) and the YQDs are characterized and shown in Figure 5a, b and c, respectively. The corresponding -3 dB modulation bandwidths are extracted as shown in Figure 5d. The high-speed PIN photodetector of 1.4 GHz was used to test the frequency response of the µLED, and the high-sensitivity APD of 100 MHz was employed to test the frequency responses of the µLED + YQDs system and the YQDs. It can be seen that the frequency responses and the modulation bandwidths are dependent on the bias current. Note that the bandwidth increases as the injection current increases, which


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may be attributed to the reduced carrier lifetime at higher currents.28 Figure 5d further summarizes the dependence of the -3 dB bandwidth on the injection current. At higher injection currents, the bandwidth of the µLED, µLED + YQDs and YQDs increases and saturates at ~160 MHz, ~85 MHz and ~73 MHz at 70 mA, respectively. It can be seen that the bandwidth of the YQDs is much higher than that of the conventional YAG: Ce phosphors (2.5 MHz),9 indicating the superiority of the high bandwidth of our white-light system. Meanwhile, it is found that the system bandwidth is significantly affected by the YQDs. The 85 MHz bandwidth of the whitelight system is limited by the relatively slow response of the YQDs (73 MHz), which is similar to the previous report.34 Therefore, by further optimizing the YQDs in the future, the bandwidth of the white-light system based on the μLED and the YQDs can be improved further.


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Figure 5. Frequency responses of (a) the μLED, (b) the white light from both the μLED and the YQDs, and (c) the YQDs as a function of the injection current of the μLED. The dashed line shows the -3 dB modulation bandwidth. (d) Comparisons of extracted -3 dB modulation bandwidths of the μLED, the μLED + YQDs system, and the YQDs as a function of the injection current of the μLED. The BER versus the data rates for the μLED combined with the YQDs (i.e. white-light system), and the YQDs were tested as shown in Figure 6. The dashed line represents the BER value of 3.8 × 10-3, which is the typical BER threshold that FEC can be implemented. With higher data rates, the BER increases. In Figure 6a, the maximum achievable data rate of the white-light system is 300 Mbps with a BER of 2.0 × 10-3, below the FEC threshold of 3.8 × 10-3. Using the 495 nm long-pass optical filter in front of the photodetector to filter the blue light, the BER characteristics of the YQDs are shown in Figure 6b. The maximum data rate is 190 Mbps and the resultant BER of 3.5 × 10-3 is below the FEC criteria. In general, the maximum data rate of the white-light system is relatively slow which is mainly ascribed to the limited bandwidth of the APD (100 MHz). Efforts will be made to further improve the data rate of the white-light system in our future study, e.g. Orthogonal Frequency Division Multiplexing (OFDM) multicarrier modulation scheme.


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Figure 6. BER at different data rates of (a) µLED + YQDs and (b) YQDs. The injection current of the µLED is 70 mA. The FEC threshold is marked in dash line. The eye diagrams of the µLED+YQDs system and the YQDs at an injection current of 70 mA are shown in Figure 7a and b, respectively. Figure 7a presents the eye diagrams of the whitelight system combining the µLED and YQDs at data rates of 100 Mbps and 280 Mbps, respectively. It can be seen that the eye diagrams are open and clear, which is attributed to the high bandwidth of the white-light system. The relatively noisy eye diagrams of the YQDs at 100 Mbps and 150 Mbps transmission speed can be seen in Figure 7b, which may come from the VLC channel noises and relatively low signal-to-noise ratio (SNR). As a result, when the bandwidth and light-output power of the YQDs decrease, the eye diagrams become less clear than those of the µLED combined with the YQDs.


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Figure 7. Eye diagrams versus data rates of (a) µLED + YQDs at 100 Mbps and 280 Mbps and (b) YQDs with a long-pass optical filter of 495 nm at data rates of 100 Mbps and 150 Mbps. The injection current of the µLED is 70 mA. It is well known that perovskite QDs have low environmental- and photo-stability due to the fast degradation when exposed to moisture and heat.35 Therefore, the stability is significantly important for white-light system using the perovskite YQDs as the color-converter for SSL and VLC applications. Here, we further tested the spectra characteristics of the YQDs and the modulation bandwidth of the YQDs-converted white-light after half a year, as shown in Figure 8. The corresponding CIE coordinates of generated white light move to (0.20, 0.24) because of the blue shift of the light emission spectra and the decrease of the emission intensity of the YQDs (Figure S1, Supporting Information). The intensity of the YQDs decreased by ~10.5%, but is


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probably relatively small compared with the high-bandwidth organic material for VLC.2, 3, 36, 37 It can be seen that in Figure 8b the bandwidths of the YQDs and the generated white light after half a year are still up to 70 MHz and 83 MHz, respectively, which suggests that the bandwidth of the white-light system was not significantly influenced. It has been analyzed that the light output power and modulation bandwidth are strongly related to the carrier recombination in the materials.26-28 The detailed mechanisms of the variations of bandwidth and emission intensity will be analyzed in our future study. As a result, the white-light system exhibits a relatively high stability especially regarding the key property of modulation bandwidth for VLC.

Figure 8. (a) Spectrum of the white light generated combining the μLED (operating at 70 mA) and the YQDs after half a year. (b) Frequency responses of the YQDs and the generated white light after half a year. The dashed line shows the -3 dB modulation bandwidth. CONCLUSIONS In conclusion, this work has demonstrated a high-bandwidth white-light system combining a blue GaN μLED with yellow CsPbBr1.8I1.2 QDs for VLC. A high modulation bandwidth of 85 MHz and a real-time data rate of 300 Mbps with a BER of 2.0 × 10-3 based on the white-light


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system with NRZ-OOK modulation scheme have been achieved. Moreover, the white-light system exhibits a relatively high stability as the bandwidths of the YQDs and the white-light system show little degradation after half a year. These results can help build up the white-light system for both high-efficiency SSL and high-speed VLC. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications Website at DIO: The double-exponential fitting results of the PL decay curve of YQDs; Evolution of spectra of the generated white light combining the μLED and YQDs after half a year. AUTHOR INFORMATION Corresponding Author *Pengfei Tian, Email: [email protected]; *Ruiqian Guo, Email: [email protected] Author Contributions ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC) (61705041, 61675049, 61571135 and 61377046), Shanghai Sailing Program 17YF1429100, and


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State Key Laboratory of Intense Pulsed Radiation Simulation and Effect Funding SKLIPR1607. REFERENCES (1) Pathak, P. H.; Feng, X.; Hu, P.; Mohapatra, P. Visible Light Communication, Networking, and Sensing: A Survey, Potential and Challenges. IEEE Commun. Surv. Tut. 2015, 17 (4), 2047-2077. (2) Wang, Z.; Wang, Z.; Lin, B.; Hu, X.; Wei, Y.; Zhang, C.; An, B.; Wang, C.; Lin, W. WarmWhite Light-Emitting Diode Based on a Dye-Loaded Metal-Organic Framework for Fast White-Light Communication. Acs. Appl. Mater. Inter. 2017, 9 (40), 35253-35259. (3) Sajjad, M. T.; Manousiadis, P. P.; Chun, H.; Vithanage, D. A.; Rajbhandari, S.; Kanibolotsky, A. L.; Faulkner, G.; O’Brien, D.; Skabara, P. J.; Samuel, I. D. W. Novel Fast Color-Converter for Visible Light Communication Using a Blend of Conjugated Polymers. Acs Photonics 2015, 2 (2), 194-199. (4) Shen, C.; Ng, T. K.; Leonard, J. T.; Pourhashemi, A.; Oubei, H. M.; Alias, M. S.; Nakamura, S.; Denbaars, S. P.; Speck, J. S.; Alyamani, A. Y. High-Modulation-Efficiency, Integrated Waveguide Modulator-Laser Diode at 448 nm. Acs Photonics 2016, 3 (2), 262-268. (5) Elgala, H.; Mesleh, R.; Haas, H. Indoor Optical Wireless Communication: Potential and State-of-the-Art. IEEE Commun. Mag. 2011, 49 (9), 56-62. (6) Tsonev, D.; Videv, S.; Haas, H. Towards a 100 Gb/s Visible Light Wireless Access Network. Opt. Express 2015, 23 (2), 1627-1637. (7) Liu, X.; Yi, S.; Zhou, X.; Fang, Z.; Qiu, Z.-J.; Hu, L.; Cong, C.; Zheng, L.; Liu, R.; Tian, P. 34.5 M Underwater Optical Wireless Communication with 2.70 Gbps Data Rate Based On a Green Laser Diode with NRZ-OOK Modulation. Opt. Express 2017, 25 (22), 27937-27947.


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