Open Access
Add to BibSonomy
Abstract
Organisms have evolved the ability to manipulate light for vision, as a means to capture its energy, to protect themselves from damage, especially against ultraviolet (UV) and other high flux radiation, and for display purposes. The makeup of the structural elements used for this manipulation often discloses novel pathways for man-made photonic devices. Iridocytes in the mantle of giant clams in the Tridacninae subfamily manipulate light in many ways, e.g., as reflectors, scattering centers, and diffusers. There is, however, a void in understanding the absorption and photoluminescence (PL) emission dynamics of these cells. In turn, a profound understanding of iridocytes’ photophysics can offer the prospect for a new generation of advanced optoelectronic materials and devices. Here, the structural and optical properties of the iridocytes embedded in the mantle tissue of the Tridacna maxima are investigated and their use as a high-speed color convertor for UV photodetection, well-suited to application in UV optical wireless communication, is demonstrated.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Many iridescent organisms advantageously change color during camouflage, social interaction, wooing mates, and fending off competition from rivals. This structural coloration process arises from an ability to tune color via manipulation of the periodicity of photonic crystal structures [1,2,11–14,3–10]. The eyes of spiders [9,15], eyes and scales of fish [5,7,8,16], and chameleon skin [2,10,17] are some examples in which light manipulation of this type is demonstrated. Giant clams of the Tridacninae subfamily, inhabiting Indo-Pacific coral reefs, are among the largest living bivalve mollusks and are unique in their symbiotic relationship with microalgae of the Symbiodinaceae family. The mantle tissue of the Tridacna maxima (T. maxima henceforth) giant clam is virtually covered by iridocyte cells, which consist of alternating layers (∼100-nm thickness) of high-index membrane-bound proteinaceous platelets and low index cytoplasm or extracellular space [18,19]. These Bragg-mirror-like structures serve as forward and sideways scattering sources for the photosynthetically productive part of the light spectrum (400–700 nm) and back-reflectors for other wavelengths [18,20]. This allows simultaneous enhancement of the photosynthetic rates of the symbionts while protecting the holobiont from harmful UV radiation [21]. Apart from sporadic reports on reflectivity and scattering phenomena related to T. maxima, absorption, and photoluminescence (PL) emission dynamics of these cells containing proteinaceous platelets have barely been studied. Here, we investigate the structural and optical properties of the iridocytes embedded in the mantle tissue of T. maxima and demonstrate their use as a high-speed color convertor for mid-deep UV photodetection, well-suited to application in mid-deep UV optical wireless communication (OWC).
With the recent surge in deployment of ‘Internet of Things’ smart devices, it is projected that there will be approximately 75.4 billion of such devices in use by 2025 [22]. OWC provides light-based internet opportunities that substantially add to this connectivity growth, leveraging license-free operation at high bandwidth and data-transmission rates across the near-infrared to ultraviolet (UV) spectral range for both terrestrial [23–26], and underwater domains [27–30]. In particular, and to enhance non-line-of-sight photodetection, mid-deep UV wavelengths are considered to have enhanced Rayleigh scattering that relieves the strict pointing, acquisition, and tracking (PAT) requirements for establishing a robust link [31,32]. The enhanced scattering is beneficial for short-distance optical signal broadcasting which relieves the strict PAT requirement for steady communication links needed for aircraft landing and secure communication [33,34]. Even though there are group-III-nitride based UV photodetectors (PDs) having better responsivities than that of Si PDs, epitaxy hardness leads to extra careful fabrication steps required to control cracking of high aluminum composition AlGaN-based PDs [35]. Recently, deep UV (DUV) photodetection research is gearing towards Ga2O3 semiconductor, which has high responsivity due to internal gain mechanisms [36,37], but not suitable for high-speed optical modulation due to a long carrier relaxation time that ranges from sub-milliseconds to seconds. The relaxation time of Ga2O3 depends on the phases i.e. α−, β-, ɛ-phases and the growth conditions of the materials. The relaxation time for the photodetectors based on β−Ga2O3 (sub-milliseconds: 0.3 ms) [38], ɛ−Ga2O3 (100 ms) [39], and α−Ga2O3 (89 µs) [40], shows that the PDs based on Ga2O3 are slow. Therefore, there is a genuine need to further research in the field of high-speed DUV photodetection to advance OWC. In addition, it should be noted that the status of the application of UV communication is still in its early stage and hence it needs a great deal of study to implement it practically in open areas.
Photon down-conversion is one of the techniques used in down-converting the UV photons to visible light wavelengths, where the responsivity of silicon photodetectors is the highest [41,42]. Continuing along this pathway to address UV photodetector design, we turn to nature for inspiration in identifying a suitable optical material for efficient mid-deep UV photodetection. We explore the use of giant clam based iridocytes for photon down-conversion from DUV to the visible spectrum, going beyond the prior research interest, which was solely focused in understanding the symbiotic relationship of iridocytes with symbiont algae for selective enhancement of the photosynthetically active radiation (PAR) [18,20,21,43]. We present promising results for this new platform as a high-speed color converter for efficient mid-deep UV photodetection well suited for OWC.
2. Experimental section
Time resolved PL (TRPL), temperature dependent PL (TDPL), and power dependent PL (PDPL) measurements were carried out using the third harmonic line (266 nm) of a 800-nm ultrafast (150 fs) Ti:Sapphire laser (Coherent, Germany) converted by HarmoniXX THG unit (APE, Germany) as the laser excitation source. Pulse repetition rate of 2 MHz was obtained by a pulse picker unit (APE, Germany) where the laser spot diameter was kept at ∼100 µm. The TDPL measurements were performed in the temperature window of 10–290 K using a closed-cycle cryostat. Emission from the sample was detected using a SpectraPro 2300 spectrograph (using a grating with 150-gr/mm groove density) attached to a Hamamatsu C6860 streak camera with a temporal resolution of ∼20 ps. The integration time was kept to 100 ms for 500 integrations. TRPL decay transients were fitted using a double exponential decay (Eq. (1)):
Power law fits are performed using Eq. (3):
where $IPL$ is the integrated PL, P is the laser excitation power density, and A and α are the fit parameters.We used an optical microscopy (Nikon measuring microscope MM-400) to capture the images of the iridocytes, using objectives in the range of 5× to 100× (Nikon Japan LU Plan ELWD). Transmission electron microscopy (TEM) samples were prepared using a standard protocol. Biopsy punches (approximately 1 mm2) of the T. maxima mantle tissue were fixed in 2.5% glutaraldehyde in 0.1 M Cacodylate buffer maintaining the temperature at 4 °C overnight. The tissue was then washed in 0.1 M (pH 7.2–7.4) Cacodylate buffer 3 times for 15 minutes each time. The post fixation step was performed in 1% osmium tetroxide in 0.1 M Cacodylate buffer in the dark for one hour. Afterwards, the tissue was washed in deionized water 3 times, keeping it for at least 15 minutes in water during each wash followed by dehydration with increasing concentration of ethanol (30, 50, 70, 90, and 100%) in water and 100% acetone. The tissue was then infiltrated with a mixture of acetone and Durcupan resin (3:1, 1:1, and 1:3 ratios), allowing 2 hours for each step before final infiltration in pure Durcupan resin overnight. Next, the tissue was embedded in pure Durcupan resin and polymerized in the oven at 65 °C for 48 hours. Leica Ultramicrotome EM UC7 (Leica, Germany) was used to cut 140-nm-thick sections from each resin-embedded tissue block. These sections were placed on 200 meshed TEM grids and stained with uranyl acetate and lead citrate following a standard protocol [21,45]. TEM images were then acquired at 300 kV using a Titan CT TEM (Thermo Fisher, USA).
For bandwidth modulation, a Nichia NDU4116, 375-nm laser diode (LD) mounted on a thermoelectric cooler integrated laser mount (Thorlabs, LDM56F/M) is used as the transmitter. The LD is then connected to the output channel of an electronic-calibrated (electronic calibration module: Agilent, 85093-60010) vector network analyzer (VNA) (Agilent, E5061B) for small-signal modulation. The direct current (DC) to the LD is set to 70 mA and the input channel of the VNA is connected to the avalanche photodiode (APD) (Thorlabs, APD430A2/M). For frequency response measurements using a 278-nm UVC LED (from LG Innotek), the APD and peripheral components were replaced by a photomultiplier tube (PMT) (Thorlabs, PMTSS) and corresponding peripherals. For the data rate measurement, the same 375-nm Nichia NDU4116 LD setup was connected to the bit-error-ratio tester (BERT) transmitter (Anritsu, ME522A), with a superimposed alternating current (AC) amplitude set to 3 V (peak to peak). On the receiver side, the modulated signal after the down-conversion process is received by the APD and connected to the BERT receiver through a linear amplifier (Tektronix, PSPL5865), which is not shown in the figure for clarity. The eye diagrams were captured using a digital communication analyzer (Agilent, 86100C Infinium DCA-J Wideband Oscilloscope).
3. Results and discussion
The micro- and nano-structures of the giant clam based iridocytes were characterized using optical and electron microscopy as shown in Fig. 1(a–e). Figure 1(d) shows, consistent with previous reports [18], a nearly spherical shape with a diameter of approximately 7 µm within which the alternating layers are evident. Figure 1(e) reveals their detailed structure with the highly regular platelets (variable lengths and width of ∼50 nm) separated by a center-to-center distance normal to the layers ∼150 nm. The intervening low refractive-index layers are significantly less ordered. Study suggests the platelets are formed by deposition of the proteinaceous material laid on cisternae [19,46]. The exact chemical composition of this material will be a subject of further investigation. It is this Bragg-mirror-like alternation of high- and low-refractive-index layers that allows the iridocytes to reflect harmful radiation [18,43]. The scattering of PAR deeper into and laterally within the mantle to benefit the symbiont algae will also depend on the lateral structure (normal to the plane of the TEM images) and the orientation of the iridocyte cells relative to the incident radiation direction. In the following, the PL characteristics of iridocytes are investigated, a topic that has not received much attention in the literature other than in the context of the red PL that many reef fish emit [47].
Fig. 1. Optical and electron microscopy images: (a) Photo of T. maxima specimen with blue mantle coloration (the red rectangle region on the mantle shows the region from which tissue was clipped); (b) Clipped T. maxima tissue ∼ 1×1 cm2 area; (c) 100× optical image of clusters of iridocytes observed using a Nikon microscope equipped with a LU Plan extralong working distance objective; (d) TEM image of a single iridocyte cell; (e) TEM image of the highlighted region in (d) showing layered proteinaceous nanostructures in a single iridocyte cell.
The sample used for optical characterization are not subjected to any chemical treatments nor were they chemically fixated. They were studied at room (RT ∼290 K) and low (LT ∼10 K) temperature. For PL, a 266-nm laser was used as the excitation source, at power densities ranging from 0.01 Wcm−2 to 38 Wcm−2. The distribution of iridocytes throughout the tissue was not homogeneous as shown inset of Fig. 2(a) and hence the PL measurement was carried out at different locations i.e., “Iridocytes + Tissue” (densely packed iridocytes region) and “Tissue” (few or almost no iridocytes cells) (see Fig. 2(a)). The PL spectrum from the Iridocytes + Tissue shows three distinct features whereas the PL spectrum from the tissue shows a dominant peak at 444 nm. The integrated PL (IPL) shows that the PL signal from the iridocytes is at least an order of magnitude higher than the PL signal solely from the tissue. The PL spectra taken at 3 different locations densely packed iridocytes (shown in Fig. 2(b)) shows that the PL peaks are consistent with slightly fluctuating intensity which can be attributed to the density of iridocytes. The bright blue light pictured in the image is the transmitted bluish-white light used to illuminate the tissue while taking optical images of the iridocytes in transmission mode. Previously, we performed the photoluminescence (PL) measurements at room temperature on the tissues taken from four (two blue and two brown) different T. maxima. Apart from the slight variation in the intensity of the PL spectrum when probed at different depth of the tissue layers, the PL emission peak-positions were found to be fairly similar irrespective of the color of the mantle tissue of T. maxima suggesting the iridocytes in each case are made up of similar proteinaceous material [21]. Fluorescent protein found in various organism are mostly guanine crystals, which we discussed in detail in the earlier section. Also, guanine has similar absorption and emission characteristics as observed in the proteinaceous material contained in iridocytes. However, further work is required to affirm the exact chemical composition of the proteinaceous material.
Fig. 2. Photoluminescence characterization of T. maxima iridocyte samples at room temperature: (a) Comparison of PL spectra measured at two different regions: densely packed iridocytes region, “Iridocytes + Tissue” (marked by purple circle) and “Tissue” (few or almost no iridocytes, marked by red circle). The inset at the top right corner is the image of the mantle tissue taken while performing µ-PL measurements, (b) PL spectra comparison of three different iridocytes packed regions.
The power dependent PL (PDPL) spectra measured at 10 K are plotted in semi-log format in Fig. 3(a); the spectra consist of three main features and extend from ∼280 to ∼750 nm, encompassing UV-B, UV-A and the whole visible spectrum. The first feature is a high energy band that peaks at ∼321 nm for the lowest excitation power density (0.01 Wcm−2), with a second peak emerging at 345 nm as the power increases. The second feature is a broad (full width at half maximum (FWHM) ∼195 nm) emission band spanning most of the visible range and peaking at ∼ 470 nm. These emission peaks falls in the PL emission regime of guanine aqueous solutions and frozen glasses, and is assigned accordingly [48]. The second feature dominates the PL spectrum at all powers and discernibly increases its relative contribution with increasing excitation power. The third feature is a sharp peak at 675 nm with weak shoulders at longer wavelengths. This latter emission matches that previously reported for algal chlorophyll [49]. The IPL power dependence shown in Fig. 3(b) is relatively linear under the conditions studied, consistent with the PL originating from neutral excitons [50,51]. In addition, TDPL was carried out across the temperature range from 10 K to 295 K (RT) at an excitation power density ∼1 Wcm−2. Selected TDPL spectra for a range of temperatures are shown in Fig. 3(c), with normalized IPL plotted against inverse temperature in Fig. 3(d). It is apparent that over this range, the IPL has its maximum value at LT (10 K), then decreases with increasing temperature up to ∼100 K, flattens off and then rises again at high temperatures (>200 K) reaching normalized IPL of ∼77% at RT. The key feature to note here is that these iridocyte samples are stable over a wide temperature range. The detailed interpretation of these behaviors will be the subject of a further study. In addition, comparison of respective PL and absorption spectra (Fig. 3(e–f)) shows that, as expected, the ∼675-nm contribution from the symbiont algae decreases significantly upon going deeper into the mantle tissue. Iridocytes in natural T. maxima tissue are found to be stable without any processing when stored in ambient conditions (air, 290 K) and are also not affected by relative humidity of 45–50% in the laboratory. To further explore their stability under optical pumping, we exposed samples mounted on sapphire substrates in ambient air to 266-nm laser (38 Wcm−2), whilst monitoring the IPL. Remarkably, the iridocytes showed only a modest 17% drop in PL intensity after 30 hours of continuous exposure and the PL spectra did not change (Fig. 4(a)), indicating no catastrophic material damage. The quantum yield (QY) of the photon down-conversion in case of the proteinaceous material found in iridocytes at 10 K is reported to be ∼ 39. 2 ± 4.2% [21]. However, we cannot deny the potential in the improvement of the quantum yield of the proteinaceous nanostructures if we are able to extract the proteinaceous material from the tissue in its purest form.
Fig. 3. Photoluminescence characterization of T. maxima iridocyte samples: (a) Semi-log plot of PDPL spectra at 10 K. (b) Double log plot for power law fits of IPL at RT (red dots) and 10 K (blue dots). (c) Selected TDPL spectra for temperatures between 10 and 290 K. (d) Normalized IPL plotted versus inverse temperature. These measurements are for mantle surface tissue samples. Comparison is also made for (e) PL and (f) absorption spectra obtained from two different tissue depths: outermost surface tissue (upper curve, red) and tissue at a depth of about 500 µm from the surface (lower curve, black).
Fig. 4. (a) Iridocyte sample photostability test when subjected to 38 Wcm−2 laser excitation power density at 266 nm. The data points are the IPL values recorded at 30-minute intervals over a 30-hour period. The red error bars represent the unavoidable laser fluctuations during the prolonged measurement. The inset presents selected PL spectra at t = 0, 15, and 30 hours; violet, green, and red symbols, respectively. (b) RT PL decay transient for 300 to 600 nm IPL: blue dots represent the measured data whereas the red line denotes the double exponential decay fit.
In addition to the steady state PL, the carrier dynamics is vital in understanding the photophysics of these iridescent cells. Luminescent nanoparticles and dyes with longer lifetime have been extensively used together with biomaterials for imaging and diagnostic purposes [52]. Long lifetime in biological imaging is used to eliminate short-lived auto-florescent background originated from the components of tissue (nucleobases). The lifetime of auto-florescent emission covers a wide range i.e. picoseconds (0.8 ps to > 100 ps) [53], sub-nanoseconds (0.7–0.9 ns) [54], to nanoseconds (1–4 ns) [55]. The ultrashort lifetime of nucleobases appears to be an intrinsic molecular property; however, it can be affected by their chemical environment to a certain extent [53]. In our case, the proteinaceous material is packed in iridocytes cells. Our aim here is to investigate the recombination process in the context of using such proteinaceous material found in iridocyte cells as a high-speed color converting material for optical modulation. The time resolved PL decay (shown in Fig. 4(b)), integrated over the range from 300 to 600 nm) was fitted using a double exponential decay equation, results in fast decay (0.26 ns; 56%) and slow decay (1.22 ns; 44%) components that yield an average decay time (${\tau _{\textrm{average}}}$) ≈ 1.02 ns. This PL decay time is, promisingly, shorter than the ∼ 4–5 ns for all-inorganic lead halide perovskite nanocrystals that have previously been studied for high-speed DUV photodetection [42,56].
As the differential PL decay time is inversely proportional to the frequency modulation bandwidth [57], the short decay time (∼1 ns) sets the stage for the optical bandwidth modulation tests (see Fig. 5(a)). The frequency response of iridocyte samples by modulated UV laser diode (LD) (375 nm) and UVC light-emitting diode (LED) (278 nm) is shown in Fig. 5(b). The frequency bandwidths of the iridocytes obtained by modulated UV LD (375 nm) are 56 MHz (–3 dB) and 220 MHz (–10 dB), respectively. This allowed 100-Mbit/s data-transmission rates using a return-to-zero on-off keying (RZ-OOK) communication scheme (Fig. 5(c)). The resulting BER of 1.6×10−3 lies below the standard 7%-overhead forward error correction (FEC) limit of 3.8×10−3, and the corresponding eye diagrams at 25 Mbit/s and 100 Mbit/s are shown as insets in Fig. 5(c). More importantly, we examined the frequency response of the iridocytes in free space by the modulated UVC LED (278 nm) and achieved frequency bandwidths of 22 MHz at –3 dB and 56 MHz at –10 dB, respectively. The obtained –3 dB modulation bandwidth (22 MHz) is adequate to establish a high-speed OWC link based on UVC light. This value is higher than the previously reported 11 MHz frequency response for all-inorganic halide perovskite nanocrystals [42]. Our initial attempt to directly test the data transmission rate for this 278-nm LED was, however, limited by the source power that can be modulated. Nevertheless, based on the data transmission rate (∼100 Mbit/s) and –3 dB bandwidth (∼ 56 MHz) measured for iridocytes modulated by 375-nm laser light, a data transmission rate in the tens of Mbit/s is expected for a higher power UVC LED, potentially comparable to or exceeding the recent development of halide perovskite based photodetectors [42]. It is also expected that the data transmission rates in each of the above cases can be improved further by employing more sophisticated communication schemes.
Fig. 5. Modulation bandwidth and data rate measurements to realize a stable UV communication link. (a) Experimental setup for measuring the modulation bandwidth of iridocytes using a 375-nm laser / APD or a 278-nm LED / PMT. Bandwidth and data rate experiments were performed using Agilent-E5061B and Anritsu-ME522A / Agilent-DCA-86100C, respectively. (b) –3-dB and –10-dB frequency bandwidth modulation of iridocytes using a 375-nm laser (blue line) and a 278-nm LED (red line). (c) Bit error ratio (BER) as a function of the data rate for the 375-nm laser source in the range from 20 Mbit/s to 100 Mbit/s. Eye diagrams are shown as insets for a return-to-zero on-off-keying (RZ-OOK) modulation scheme. The standard forward error correction (FEC) limit is shown by the dotted horizontal blue line.
4. Conclusions
In summary, we investigated the photoluminescence properties of iridocytes from the Red Sea giant clam, T. maxima and demonstrated the feasibility of using this material for deep ultraviolet optical wireless communications via high-speed color down-conversion. We observed a fast PL decay time (∼1 ns) that allowed a –3-dB modulation frequency bandwidth of 22 MHz for UVC LED (278 nm) modulation. This could support a data-transmission rate of tens of Mbit/s using a higher power modulated UVC source, which is promising, especially given the potential for optimization following a more detailed study of the iridocyte photophysics. Another encouraging finding was the photostability of the iridocytes, sustaining high-power density UVC laser (266 nm) irradiation for 30 hours without major material degradation. This contrasts with the rapid degradation typical of many other materials. Extracting pure iridocytes from the T. maxima tissue or artificially synthesizing such material will be the first step in our optimization strategy, seeking to enhance the PL intensity, frequency response, and OWC data-transmission rate. If successful, this would establish iridocytes from the mantle tissues of T. maxima as a novel material for a range of potential photonic applications.
Funding
King Abdullah University of Science and Technology (BAS/1/1071-01-01, BAS/1/1614-01-01); King Abdulaziz City for Science and Technology (KACST TIC R2-FP-008).
Acknowledgment
The authors further acknowledge access to the KAUST Imaging and Characterization Core Lab for electron microscopy measurements. Support from Coastal and Marine Resources (CMR) Core Lab in sample collection is also gratefully acknowledged. T.K.N. and B.S.O. gratefully acknowledge the funding support from King Abdulaziz City for Science and Technology (grant no. KACST TIC R2-FP-008).
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.
References
1. D. Gur, B. A. Palmer, S. Weiner, and L. Addadi, “Light manipulation by guanine crystals in organisms: biogenic scatterers, mirrors, multilayer reflectors and photonic crystals,” Adv. Funct. Mater. 27(6), 1603514 (2017). [CrossRef]
2. J. Teyssier, S. V. Saenko, D. van der Marel, and M. C. Milinkovitch, “Photonic crystals cause active colour change in chameleons,” Nat. Commun. 6(1), 6368 (2015). [CrossRef]
3. M. Lopez-Garcia, N. Masters, H. E. O’Brien, J. Lennon, G. Atkinson, M. J. Cryan, R. Oulton, and H. M. Whitney, “Light-induced dynamic structural color by intracellular 3D photonic crystals in brown algae,” Sci. Adv. 4(4), eaan8917 (2018). [CrossRef]
4. A. Levy-Lior, E. Shimoni, O. Schwartz, E. Gavish-Regev, D. Oron, G. Oxford, S. Weiner, and L. Addadi, “Guanine-based biogenic photonic-crystal arrays in fish and spiders,” Adv. Funct. Mater. 20(2), 320–329 (2010). [CrossRef]
5. D. Gur, Y. Politi, B. Sivan, P. Fratzl, S. Weiner, and L. Addadi, “Guanine-based photonic crystals in fish scales form from an amorphous precursor,” Angew. Chem. Int. Ed. 52(1), 388–391 (2013). [CrossRef]
6. M. Kolle, A. Lethbridge, M. Kreysing, J. J. Baumberg, J. Aizenberg, and P. Vukusic, “Bio-inspired band-gap tunable elastic optical multilayer fibers,” Adv. Mater. 25(15), 2239–2245 (2013). [CrossRef]
7. M. Kreysing, R. Pusch, D. Haverkate, M. Landsberger, J. Engelmann, J. Ruiter, C. Mora-Ferrer, E. Ulbricht, J. Grosche, K. Franze, S. Streif, S. Schumacher, F. Makarov, J. Kacza, J. Guck, H. Wolburg, J. K. Bowmaker, G. von der Emde, S. Schuster, H.-J. Wagner, A. Reichenbach, and M. Francke, “Photonic crystal light collectors in fish retina improve vision in turbid water,” Science 336(6089), 1700 (2012). [CrossRef]
8. D. Gur, B. Leshem, D. Oron, S. Weiner, and L. Addadi, “The structural basis for enhanced silver reflectance in koi fish scale and skin,” J. Am. Chem. Soc. 136(49), 17236–17242 (2014). [CrossRef]
9. K. P. Mueller and T. Labhart, “Polarizing optics in a spider eye,” J. Comp. Physiol., A 196(5), 335–348 (2010). [CrossRef]
10. O. Berger, E. Yoskovitz, L. Adler-Abramovich, and E. Gazit, “Spectral transition in bio-inspired self-assembled peptide nucleic acid photonic crystals,” Adv. Mater. 28(11), 2195–2200 (2016). [CrossRef]
11. D. Gur, B. Leshem, M. Pierantoni, V. Farstey, D. Oron, S. Weiner, and L. Addadi, “Structural basis for the brilliant colors of the sapphirinid copepods,” J. Am. Chem. Soc. 137(26), 8408–8411 (2015). [CrossRef]
12. V. E. Johansen, O. D. Onelli, L. M. Steiner, and S. Vignolini, “Photonics in Nature: From Order to Disorder,” in (Springer,, 2017), pp. 53–89.
13. Y. Zhang, J. Mei, C. Yan, T. Liao, J. Bell, and Z. Sun, “Bioinspired 2D nanomaterials for sustainable applications,” Adv. Mater. 32(18), 1902806 (2019). [CrossRef]
14. W. R. Farkas, T. Stanawitz, and M. Schneider, “Saturnine gout: lead-induced formation of guanine crystals,” Science 199(4330), 786 (1978). [CrossRef]
15. J. Marshall and S. Johnsen, “Fluorescence as a means of colour signal enhancement,” Philos. Trans. R. Soc., B 372(1724), 20160335 (2017). [CrossRef]
16. S. Armstrong, “Photonic crystals aid fish’s night vision,” Nat. Photonics 6(9), 575 (2012). [CrossRef]
17. G. H. Lee, T. M. Choi, B. Kim, S. H. Han, J. M. Lee, and S. H. Kim, “Chameleon-inspired mechanochromic photonic films composed of non-close-packed colloidal arrays,” ACS Nano 11(11), 11350–11357 (2017). [CrossRef]
18. A. L. Holt, S. Vahidinia, Y. L. Gagnon, D. E. Morse, and A. M. Sweeney, “Photosymbiotic giant clams are transformers of solar flux,” J. R. Soc. Interface. 11(101), 20140678 (2014). [CrossRef]
19. D. J. Griffiths, H. Winsor, and T. Van-Luong, “Iridophores in the mantle of giant clams,” Aust. J. Zool. 40(3), 319–326 (1992). [CrossRef]
20. A. Ghoshal, E. Eck, M. Gordon, and D. E. Morse, “Wavelength-specific forward scattering of light by Bragg-reflective iridocytes in giant clams,” J. R. Soc. Interface. 13(120), 20160285 (2016). [CrossRef]
21. S. Rossbach, R. C. Subedi, T. K. Ng, B. S. Ooi, and C. M. Duarte, “Iridocytes mediate photonic cooperation between giant clams (Tridacninae) and their photosynthetic symbionts,” Front. Mar. Sci. 7, 465 (2020). [CrossRef]
22. S. Lucero, “IoT platforms: enabling the internet of things,” IHS Technol. Whitepaper(March), 1–19 (2016).
23. R. Y. Mesleh, H. Haas, S. Sinanović, C. W. Ahn, and S. Yun, “Spatial modulation,” IEEE Trans. Veh. Technol. 57(4), 2228–2241 (2008). [CrossRef]
24. H. Elgala, R. Mesleh, and H. Haas, “Indoor optical wireless communication: potential and state-of-the-art,” IEEE Commun. Mag. 49(9), 56–62 (2011). [CrossRef]
25. B. Janjua, H. M. Oubei, J. R. D. Retamal, T. K. Ng, C.-T. Tsai, H.-Y. Wang, Y.-C. Chi, H.-C. Kuo, G.-R. Lin, J.-H. He, and B. S. Ooi, “Going beyond 4 Gbps data rate by employing RGB laser diodes for visible light communication,” Opt. Express 23(14), 18746 (2015). [CrossRef]
26. H. Chun, A. Gomez, C. Quintana, W. Zhang, G. Faulkner, and D. O’Brien, “A wide-area coverage 3.5 Gb/s visible light communications link for indoor wireless applications,” Sci. Rep. 9(1), 4952 (2019). [CrossRef]
27. F. Hanson and S. Radic, “High bandwidth underwater optical communication,” Appl. Opt. 47(2), 277–283 (2008). [CrossRef]
28. C. Shen, Y. Guo, H. M. Oubei, T. K. Ng, G. Liu, K.-H. Park, K.-T. Ho, M.-S. Alouini, and B. S. Ooi, “20-meter underwater wireless optical communication link with 1.5 Gbps data rate,” Opt. Express 24(22), 25502 (2016). [CrossRef]
29. H. M. Oubei, C. Li, K.-H. Park, T. K. Ng, M.-S. Alouini, and B. S. Ooi, “2.3 Gbit/s underwater wireless optical communications using directly modulated 520 nm laser diode,” Opt. Express 23(16), 20743 (2015). [CrossRef]
30. H. M. Oubei, J. R. Duran, B. Janjua, H.-Y. Wang, C.-T. Tsai, Y.-C. Chi, T. K. Ng, H.-C. Kuo, J.-H. He, M.-S. Alouini, G.-R. Lin, and B. S. Ooi, “4.8 Gbit/s 16-QAM-OFDM transmission based on compact 450-nm laser for underwater wireless optical communication,” Opt. Express 23(18), 23302 (2015). [CrossRef]
31. A. Vavoulas, H. G. Sandalidis, N. D. Chatzidiamantis, Z. Xu, and G. K. Karagiannidis, “A Survey on Ultraviolet C-Band (UV-C) Communications,” IEEE Commun. Surv. Tutorials 21(3), 2111–2133 (2019). [CrossRef]
32. Z. Xu and B. M. Sadler, “Ultraviolet communications: potential and state-of-the-art,” IEEE Commun. Mag. 46(5), 67–73 (2008). [CrossRef]
33. R. J. Drost and B. M. Sadler, “Survey of ultraviolet non-line-of-sight communications,” Semicond. Sci. Technol. 29(8), 084006 (2014). [CrossRef]
34. Z. Xu, H. Ding, B. M. Sadler, and G. Chen, “Analytical performance study of solar blind non-line-of-sight ultraviolet short-range communication links,” Opt. Lett. 33(16), 1860 (2008). [CrossRef]
35. E. J. Tarsa, P. Kozodoy, J. Ibbetson, B. P. Keller, G. Parish, and U. Mishra, “Solar-blind AlGaN-based inverted heterostructure photodiodes,” Appl. Phys. Lett. 77(3), 316–318 (2000). [CrossRef]
36. Y. Qin, L. Li, X. Zhao, G. S. Tompa, H. Dong, G. Jian, Q. He, P. Tan, X. Hou, Z. Zhang, S. Yu, H. Sun, G. Xu, X. Miao, K. Xue, S. Long, and M. Liu, “Metal-semiconductor-metal ɛ-Ga2O3 solar-blind photodetectors with a record-high responsivity rejection ratio and their gain mechanism,” ACS Photonics 7(3), 812–820 (2020). [CrossRef]
37. K. H. Li, C. H. Kang, J. H. Min, N. Alfaraj, J. W. Liang, L. Braic, Z. Guo, M. N. Hedhili, T. K. Ng, and B. S. Ooi, “Single-crystalline all-oxide α-γ-β heterostructures for deep-ultraviolet photodetection,” ACS Appl. Mater. Interfaces 12(48), 53932–53941 (2020). [CrossRef]
38. S. Nakagomi, T. A. Sato, Y. Takahashi, and Y. Kokubun, “Deep ultraviolet photodiodes based on the β-Ga2O3/GaN heterojunction,” Sens. Actuators, A 232, 208–213 (2015). [CrossRef]
39. Y. Qin, H. Sun, S. Long, G. S. Tompa, T. Salagaj, H. Dong, Q. He, G. Jian, Q. Liu, H. Lv, and M. Liu, “High-performance metal-organic chemical Vapor Deposition ɛ-Ga2O3 solar-blind photodetector with asymmetric Schottky electrodes,” IEEE Electron Device Lett. 40(9), 1475–1478 (2019). [CrossRef]
40. S. H. Lee, K. M. Lee, Y. Bin Kim, Y. J. Moon, S. Bin Kim, D. Bae, T. J. Kim, Y. D. Kim, S. K. Kim, and S. W. Lee, “Sub-microsecond response time deep-ultraviolet photodetectors using α-Ga2O3 thin films grown via low-temperature atomic layer deposition,” J. Alloys Compd. 780, 400–407 (2019). [CrossRef]
41. G. Tong, H. Li, Z. Zhu, Y. Zhang, L. Yu, J. Xu, and Y. Jiang, “Enhancing hybrid perovskite detectability in the deep ultraviolet region with down-conversion dual-phase (CsPbBr3-Cs4PbBr6) Films,” J. Phys. Chem. Lett. 9(7), 1592–1599 (2018). [CrossRef]
42. C. H. Kang, I. Dursun, G. Liu, L. Sinatra, X. Sun, M. Kong, J. Pan, P. Maity, E. N. Ooi, T. K. Ng, O. F. Mohammed, O. M. Bakr, and B. S. Ooi, “High-speed colour-converting photodetector with all-inorganic CsPbBr3 perovskite nanocrystals for ultraviolet light communication,” Light: Sci. Appl. 8(1), 94 (2019). [CrossRef]
43. A. Ghoshal, E. Eck, and D. E. Morse, “Biological analogs of RGB pixelation yield white coloration in giant clams,” Optica 3(1), 108 (2016). [CrossRef]
44. B. Xin, Y. Pak, S. Mitra, D. Almalawi, N. Alwadai, Y. Zhang, and I. S. Roqan, “Self-patterned CsPbBr3 nanocrystals for high-performance optoelectronics,” ACS Appl. Mater. Interfaces 11(5), 5223–5231 (2019). [CrossRef]
45. T. Sato, “A modified method for lead staining of thin sections,” J. Electron Microsc. 17(2), 158–159 (1968). [CrossRef]
46. Y. Kamishima and S. Kawaguti, “Iridoscent colorations on mantle tissue of giant clam, tridacna crocea,” in Toward the Desirable Future of Coral Reefs in Palau and the Western Pacific (2004), pp. 40–45.
47. N. K. Michiels, N. Anthes, N. S. Hart, J. Herler, A. J. Meixner, F. Schleifenbaum, G. Schulte, U. E. Siebeck, D. Sprenger, and M. F. Wucherer, “Red fluorescence in reef fish: a novel signalling mechanism?” BMC Ecol. 8(1), 16 (2008). [CrossRef]
48. J. W. Longworth, R. O. Rahn, and R. G. Shulman, “Luminescence of pyrimidines, purines, nucleosides, and nucleotides at 77 °K. the effect of ionization and tautomerization,” J. Chem. Phys. 45(8), 2930–2939 (1966). [CrossRef]
49. R. Pedrós, I. Moya, Y. Goulas, and S. Jacquemoud, “Chlorophyll fluorescence emission spectrum inside a leaf,” Photochem. Photobiol. Sci. 7(4), 498 (2008). [CrossRef]
50. D. D. C. Bradley, R. K. Friend, K. S. Wong, W. Hayes, H. Lindenberger, and S. Roth, “Radiative and Non-radiative Recombination Processes in Photoexcited Poly(p-phenylene vinylene),” in (Springer, Berlin, Heidelberg, 1987), pp. 107–112.
51. D. D. C. Bradley, R. H. Friend, and W. J. Feast, “Photoexcitation in poly(arylenevinylenes),” Synth. Met. 17(1-3), 645–650 (1987). [CrossRef]
52. K. H. Cheng, J. Aijmo, L. Ma, M. Yao, X. Zhang, J. Como, L. J. Hope-Weeks, J. Huang, and W. Chen, “Luminescence decay dynamics and trace biomaterials detection potential of surface-functionalized nanoparticles,” J. Phys. Chem. C 112(46), 17931–17939 (2008). [CrossRef]
53. H. Kang, K. T. Lee, B. Jung, Y. J. Ko, and S. K. Kim, “Intrinsic lifetimes of the excited state of DNA and RNA bases,” J. Am. Chem. Soc. 124(44), 12958–12959 (2002). [CrossRef]
54. A. V. Mamontova, I. D. Solovyev, A. P. Savitsky, A. Shakhov, K. A. Lukyanov, and A. M. Bogdanov, “Bright GFP with subnanosecond fluorescence lifetime,” Sci. Rep. 8(1), 13224–5 (2018). [CrossRef]
55. N. A. Nemkovich, Y. V. Kruchenok, A. N. Sobchuk, H. Detert, N. Wrobel, and E. A. Chernyavskii, “Subnanosecond spectrofluorimetry of new indolocarbazole derivatives in solutions and protein complexes and their dipole moments,” in Optics and Spectroscopy (English Translation of Optika i Spektroskopiya) (Springer, 2009), 107(2), pp. 275–281.
56. M. S. Kang, S. Do Sung, I. T. Choi, H. Kim, M. Hong, J. Kim, W. I. Lee, and H. K. Kim, “Novel carbazole-based hole-transporting materials with star-shaped chemical structures for perovskite-sensitized solar cells,” ACS Appl. Mater. Interfaces 7(40), 22213–22217 (2015). [CrossRef]
57. X. Xiao, H. Tang, T. Zhang, W. Chen, W. Chen, D. Wu, R. Wang, and K. Wang, “Improving the modulation bandwidth of LED by CdSe/ZnS quantum dots for visible light communication,” Opt. Express 24(19), 21577 (2016). [CrossRef]
