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Opto-thermal modulation in biological photonic crystals
Optics Communications 284 (2011) 1656–1660
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Opto-thermal modulation in biological photonic crystals Alain Haché ⁎,1, Guy-Germain Allogho Thin Films and Photonics Research Group, Department of Physics and Astronomy, Université de Moncton, Moncton, N.B., E1A 3E9, Canada
a r t i c l e
i n f o
Article history: Received 29 July 2010 Received in revised form 10 November 2010 Accepted 14 November 2010 OCIS codes: 190.3270 190.4710 050.5298 120.6810 140.5560
a b s t r a c t Photonic structures of biological origin have been well studied for their optical and morphological properties, but light-induced effects have not yet been explored. In this study, we report sizeable modulation in reflectance on iridescent areas of the wing of a butterfly Morpho didius. Chitin, the primary constituent of the sample, exhibits the large thermo-optic effect typically seen in biopolymers. Measurements yield a thermo-optic coefficient of dn/ dT = −4.7× 10− 4 °C− 1. Relatively low intensity levels (~0.05 W/cm2) are therefore required to induce measurable reflectance changes, which take place only in the iridescence spectral range and only in areas where chitin is periodically textured. This confirms that the effect originates from photonic band-shifting. For comparison purposes, Kerr nonlinearity is also investigated in chitin and chitosan films. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nature uses diffusion and interference effects to enhance coloration in living organisms, especially in the blue region of the spectrum where natural pigments are rare [1]. Discovered a long time ago in insects and birds [2], the phenomenon was thought to be limited to animals, but it was later found to exist also in plants [3]. Chitin is one of many organic and inorganic materials that compose natural photonic structures. The most abundant biopolymer after cellulose, chitin is the main constituent of outer shell of insects like butterflies and beetles. Iridescent coloration in these insects is the result of the interference of visible light in an intricate array of chitin fibers organized periodically at the nanometer scale [4]. Interestingly, polymers have been the object of much study for a wide variety of photonic applications. Among the many desirable properties they exhibit, the most cited are: wide bandwidth of operation, structural flexibility, low cost and compatibility with other materials [5]. Polymers also tend to exhibit large thermo-optic coefficients and electronic nonlinearity, thereby making efficient all-optical control possible in spite of their relatively low damage threshold. These reasons have motivated us to examine the possibility of lightinduced optical changes in chitin-based natural photonic crystals. For the purpose of this study, we chose to analyze Morpho didius specimen, a species belonging to a family of iridescent butterflies for which microstructural and optical properties have been measured and modeled in details (see Refs [6–9] for the M. didius species, Refs [9,10] for M. sulkowskyi and Ref [8] for M. rhetenor).
⁎ Corresponding author. Tel.: + 1 506 858 4938; fax: + 1 506 858 4541. E-mail address: [email protected] (A. Haché). 1 Canada Research Chair in Photonics. 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.11.043
If the scientific interest in understanding biological photonic structures is mostly academic for now, it may change in the future. How genetic information is used by living cells to unfold such intricate structures as a photonic crystal is a question that has yet to be answered. However, over the past years, advances in bioengineering suggest that it may be possible to drive living tissues into synthesizing desired structures based on selected genetic information [11]. If this becomes reality, a new avenue for assembling photonic devices will be open and the knowledge gained from biological systems will be valuable for application purposes. 2. Methods Light induces changes in the optical properties of a photonic crystal when it modifies the lattice constant and/or the refractive index of the constituent materials. Because of interferences, the resulting shift in the photonic bands modifies the optical properties of the medium to a much greater extent than would in a bulk, nonperiodic material. Thermal expansion and mechanical stress can deform the crystal lattice and shift the photonic band gap accordingly. Changes in the refractive index, on the other hand, may be induced thermally (the thermo-optic effect) or via the nonlinear susceptibility of the material, such as in the optical Kerr effect. If both mechanisms have similar effects on the index of refraction, they operate on vastly different timescales (10− 3 s versus 10− 15 s, respectively) and intensity levels. To investigate the possible nonlinearity of natural photonic crystals, we tested samples for the thermo-optic effect using a pump-probe technique. The method uses a laser beam to heat the sample and a beam of white light to probe optical reflection simultaneously. For comparison purposes, the nonlinear properties of chitin and chitosan in their
A. Haché, G.-G. Allogho / Optics Communications 284 (2011) 1656–1660
1657
synthetic forms were examined. Chitosan, the deacetylated form of chitin, exhibits similar optical properties but it has a much higher solubility in acidic media [12]. Third-order nonlinearity was measured with a pulsed laser beam using the standard z-scan technique [13].
chitosan films with 200 μm thickness were obtained by multiple dipping and drying. Z-scan was done with an amplified Ti:sapphire laser operating at 800 nm (Coherent Rega) producing pulses with 200 fs in duration, 4 μJ of energy and a repetition rate of 100 kHz.
3. Experimental
4. Results and discussion
Wing samples from two Morpho didius specimen were cut and analyzed in a spectrophotometer (Varian Cary 5000) for reflection and transmission measurements in the 200–2500 nm spectral range. An integrating sphere was used for measuring total and diffuse reflectance. For morphology and structural examination, samples were metalized on both sides with gold and analyzed with a scanning electron microscope (JEOL-5200) down to a resolution of 10 nm. Thermo-optic measurements were done by optical heating of samples with a Ti:sapphire laser (Coherent Vitesse) producing a train of pulses of 150 fs in duration with a repetition rate of 86 MHz. The laser spectrum is centered at 800 nm with a bandwidth of 25 nm, wavelengths that fall within the 600–900 nm spectral range where chitin exhibits the least optical absorption [12]. Since thermal effects occur on a time scale many orders of magnitude longer than the interval of time between two successive laser pulses (12 ns), the beam is effectively continuous. For probing reflection changes induced by the pump, a xenon/deuterium lamp producing a white beam of light was used. Fig. 1 shows the experimental setup for thermo-optic measurements. The 800 nm pump beam is modulated by a mechanical chopper and changes in reflectivity are monitored with the white light beam. Reflected white light is collected with a 4× microscope objective with a numerical aperture of 0.1 and sent to a silicon photodetector, the signal of which is analyzed with a lock-in amplifier. Band-pass filters with 10 nm bandwidth are used to study the effect at various wavelengths. Incident and reflected light beams are close to normal incidence. For third-order nonlinearity measurements, chitosan and chitin films were prepared by dipping a glass substrate into a solution of 1% acetic acid in water and 5% chitosan (or 0.5% chitin). Optical-grade
The microscopic structure and coloration of the Morpho didius wing are shown in Fig. 2. The top wing surface is covered with 80 μmwide scales containing regular arrays of chitin ridges. Studies using transmission electron microscopy have revealed a tree-like structure composed of ridges and several pairs cross-ribs [4,6–10]. The periodic structure, which thickness is the equivalent of 3–4 bi-layers of chitin and air, scatters blue and violet light. Interestingly, although the top and bottom surfaces of the wings are made of the same material and have similar morphology, they have different colorations: the top surface is metallic blue while the bottom one is brown for lack of long range structural order. As it was previously found [8–10], some features of the Morpho wing coloration cannot be explained with the simple interference (multi-layer) model. For example, some degree of irregularity in the ridge height is necessary to create iridescence over a wide range of viewing angles. In this study, we limit thermo-optic measurements to viewing angles near normal incidence, a regime where constructive interference in the periodic lamellar structure is the dominant effect. Here the relevant structural period is ~200 nm, thereby creating a reflection peak at 480 nm. In principle, opto-thermal effects should also be observed at wide viewing angle, but the experimental setup did not allow to investigate. Fig. 3 shows the reflection spectra of the top and bottom wing surfaces. The top surface shows a reflection (scattering) peak in the 450–525 nm range while the bottom surface does not. Reflectance is largely dominated by diffuse reflection; specular reflectance only account for b5% of the overall value. Transmission on both sides is also minimal, particularly in the visible spectral range. Thermo-optic results are shown in Fig. 4. Here the relative modulation of reflectance ΔR/R = (R − R0)/R0 is measured, where R
Fig. 1. Thermo-optic pump-probe setup. The Morpho didius sample is illuminated with a pulsed (but effectively continuous) laser beam at 800 nm. The beam is modulated with a mechanical chopper and changes in the reflection spectrum are measured on a silicon photodiode, the signal of which is processed with a lock-in amplifier (LIA). Band-pass filters (BPF) are used to measure the effect at various wavelengths. Incident and reflected light beams are close to normal incidence.
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Fig. 2. Electron microscopy of a Morpho didius wing. The top (A) and bottom (B) surfaces of the wing have similar structural arrangements but only the top one exhibits the metallic blue coloration (C); the bottom surface (D) is brown in appearance.
and R0 are the reflectance with and without pumping, respectively. Here the beam of white light arrives perpendicular to the plane of the wing (normal incidence), while the pump laser beam and the measured reflection are within 5° and 20° of normal incidence, respectively. Since the microscope objective probes an area of ~ 1 mm in diameter, measurements represent ensemble averages over many scales, each one being about 100 μm in size. The relative modulation peaks in the 480–520 nm wavelength range, i.e. near the photonic band edge. To within experimental error (b4 × 10− 4), no modulation
was detected on the bottom surface of the wing for the same pump intensity. This clearly shows that modifications in the photonic band gap are involved in the observed reflectance changes. The unfocussed 70 mW pump beam had an on-sample intensity of 0.28 W/cm2 and was modulated at 26 Hz. The dynamics of the reflection change is shown in Fig. 5. The chopping frequency was swept from 2 to 200 Hz and reflectance was monitored at 488 nm with constant pump power. A linear dependence exists for pump durations up to 1/f = 35 ms, followed by a
100 0.012
60
A
0.008
ΔR/R
Reflectance (%)
80
40
0.004 20
0
B 500
1000
1500
2000
2500
Wavelength (nm)
0
400
500
600
700
wavelength (nm) Fig. 3. Total optical reflectance spectra (diffuse and specular) of a Morpho didius wing. Unlike the top surface (A), the bottom surface (B) does not exhibit a strong scattering band in the visible region for lack of periodic arrangement of the chitin ridges. Reflectance is largely dominated (N95%) by diffusion.
Fig. 4. Relative modulation in reflectance of the top surface of a Morpho didius wing. The dashed line represents the maximum modulation measured on the bottom surface (i.e. within experimental noise level). Experimental error is of the order of 3–5%.
A. Haché, G.-G. Allogho / Optics Communications 284 (2011) 1656–1660
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60 0.02
20 oC
Reflectance (%)
ΔR/R
0.01 0.005
0.002
50 oC
40
100 oC 150 oC
20
0.001
0.001
0.01
0
0.1
400
pump period (s)
500
600
700
wavelength (nm) Fig. 7. Reflectance of a Morpho didius wing heated at constant temperature. Blue shifting of the photonic band is consistent with the lowering of the refractive index and/or thermal expansion of the structure.
saturation. The time scale of the effect confirms the thermal nature of the process. At 2 Hz, the lowest frequency attainable with the chopper, the relative reflectance changes of 3.5% are observed. In Fig. 6, the amplitude of reflectance change is measured as a function of pump intensity, with a constant chopping frequency of 7 Hz. A linear dependence is observed up to 0.15 W/cm2 and the maximum is reached at 0.25 W/cm2. Beyond that intensity level, a drop is observed, but it does not necessarily imply irreversible damage of the sample: the modulation recovers its original values when the intensity is lowered. To further investigate the effect of temperature on the optical properties of the Morpho didius wing, the sample was put in a controlled environment at various ambient temperatures while reflectance was simultaneously measured using a spectrophotometer (Varian Carry 5000). Blue-shifting of the reflection band was observed for temperatures up to 150 °C, as Fig. 7 shows. Since the reflection peak wavelength scales linearly with the optical thickness of a bi-layer, the blue shift suggests a lowering of the refractive index of chitin with increasing temperature. We can also estimate the thermo-optic coefficient of chitin from these results. With a shift of −7 nm over 75 °C, an estimated air gap thickness of 70 nm, a cuticle layer thickness of 110 nm and a refractive index of 1.54 for chitin (see Ref. [8]), we obtain the thermooptic coefficient dn/dT=−4.7×10− 4 °C− 1. Although comparison measurements on chitin or chitosan don't exist in the literature, the value is comparable to that of other polymers [14]. It should be noted that
thermal distortion (thermal expansion) would tend to red shift the reflection peak. Although chitin does retain its chemical integrity at temperatures up to 250 °C, our system did not allow to reach that point. Nonetheless, relative changes in reflectance of ~30% near 500 nm were measured, namely one order of magnitude larger than observed with optical pumping. We should then expect to observe larger reflectance modulations by using lower pump chopping frequencies. For the purpose of measuring third-order optical nonlinearity by z-scan, optical-grade chitosan layers with a thickness d = 200 μm deposited on a glass were made. However, no self-focusing effect (i.e. z-scan signal, closed or open aperture) was detectable to within experimental uncertainty for peak intensities up to 100 GW/cm2. Considering the nonlinear phase shift through the layer is Δϕ = k0dΔn, where k0 = 7.85 × 106 m− 1 is the free space wave vector of the pump beam and Δn is the refractive index change, and assuming the z-scan signal would be detectable if Δϕ N 0.1 (or λ/60 beam distortion, a conservative figure), this puts the upper limit on Δn at 6 × 10− 5. The nonlinear refractive index coefficient would then be limited to n2 b 6 × 10− 16 cm2/W, a level comparable to fused silica, but much lower than of pi-conjugated polymers [15]. Owing to its poor solubility and tendency to crystallize into highly diffusing films, chitin did not yield the possibility of z-scan measurements. Given the small polarization nonlinearity of chitosan and the low temperature damage threshold of chitin, one can reasonably expect that inducing changes in chitin photonic crystals via the optical Kerr effect would be difficult to achieve, even with single femtosecond laser pulses.
ΔR/R (a.u.)
Fig. 5. Dynamics of the reflectance changes at 488 nm with 0.28 W/cm2 pumping intensity. To better show the evolution over time, the graph is plotted as a function 1/f, with f the pump chopping frequency. A saturation occurs after ~ 200 ms, clearly establishing the thermal nature of the effect.
1.0
5. Conclusions
0.8
Many studies over the past decades have focused on the morphology and the linear optical properties of biological photonic structures, but none have explored their nonlinearity. This study showed that sizeable changes in photonic band properties can be induced optically. At the relatively modest intensity of 0.05 W/cm2, reflection changes of a few percents were observed in a Morpho didius wing in the violet-blue spectral region. The thermo-optic coefficient of chitin is estimated to be dn/dT = −4.7 × 10− 4 °C− 1, a value that is large but comparable to other polymers. Our findings point the way to further studies in that area, as many aspects have yet to be investigated. The pump wavelength used in this study is not optimal, as it not strongly absorbed by chitin; it would be interesting to pump at wavelengths in the UV. Nonlinear shifts in polarization and phase effects could also be explored. Like many iridescent species, the scattering of the Morpho didius wing is optically anisotropic due to the preferential orientation of chitin ridges.
0.6
0.4
0.2
0
0
0.1
pump intensity
0.2
0.3
(W/cm2)
Fig. 6. Dependence of the modulated reflectance at 488 nm on the pump intensity. The modulation frequency is constant at 7 Hz.
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The authors wish to thank Torriss Badr for chitosan sample preparation and Yves Poussart and Martin Chiasson for electron microscope images.
[5] [6] [7] [8] [9] [10] [11]
References
[12]
Acknowledgments
[1] [2] [3] [4]
P. Vukusic, J.R. Sambles, Nature 424 (2003) 852. A.A. Michelson, Philos. Mag. 21 (1911) 554. D.W. Lee, Nature 349 (1991) 260. S. Berthier, Iridescences: The Physical Colors of Insects, Springer, New York, 2007.
[13] [14] [15]
M.A. Uddin, H.P. Chan, J. Mater. Sci.-Mater. El. 20 (2009) 277. P. Vukusic, J.R. Sambles, C.R. Lawrence, R.J. Wootton, Proc. Biol. Sci. 266 (1999) 1403. L.P. Biro, Mater. Sci. Eng. B 169 (2010) 3. S. Yoshioka, S. Kinoshita, Proc. R. Soc. Lond. B 271 (2004) 581. S. Banerjee, J.B. Cole, T. Yatagai, Micron 38 (2007) 97. S. Kinoshita, S. Yoshioka, K. Kawagoe, Proc. R. Soc. Lond. B 269 (2002) 1417. M. Bedau, G. Church, S. Rasmussen, A. Caplan, S. Benner, A. Fussennegger, J. Collins, D. Dreamer, Nature 465 (2010) 422. G. Luna-Barcenas, B. Gonzalez-Campos, E.A. Elizalde-Pena, E. Vivakdo-Lima, J.F. Louvier-Hernandez, Y.V. Vorobiev, J. Gonzalez-Hernandez, Phys. Sat. Sol. 5 (2008) 3736. M. Sheik, A.A. Said, E.W. Van Stryland, Opt. Lett. 14 (1989) 955. Z. Zhang, P. Zhaoa, P. Lina, F. Suna, Polymer 47 (2006) 4893. R.W. Boyd, Nonlinear Optics, 2nd ed., Academic Press, Amsterdam, 2003, p. 194.
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Opto-thermal modulation in biological photonic crystals Alain Haché ⁎,1, Guy-Germain Allogho Thin Films and Photonics Research Group, Department of Physics and Astronomy, Université de Moncton, Moncton, N.B., E1A 3E9, Canada
a r t i c l e
i n f o
Article history: Received 29 July 2010 Received in revised form 10 November 2010 Accepted 14 November 2010 OCIS codes: 190.3270 190.4710 050.5298 120.6810 140.5560
a b s t r a c t Photonic structures of biological origin have been well studied for their optical and morphological properties, but light-induced effects have not yet been explored. In this study, we report sizeable modulation in reflectance on iridescent areas of the wing of a butterfly Morpho didius. Chitin, the primary constituent of the sample, exhibits the large thermo-optic effect typically seen in biopolymers. Measurements yield a thermo-optic coefficient of dn/ dT = −4.7× 10− 4 °C− 1. Relatively low intensity levels (~0.05 W/cm2) are therefore required to induce measurable reflectance changes, which take place only in the iridescence spectral range and only in areas where chitin is periodically textured. This confirms that the effect originates from photonic band-shifting. For comparison purposes, Kerr nonlinearity is also investigated in chitin and chitosan films. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nature uses diffusion and interference effects to enhance coloration in living organisms, especially in the blue region of the spectrum where natural pigments are rare [1]. Discovered a long time ago in insects and birds [2], the phenomenon was thought to be limited to animals, but it was later found to exist also in plants [3]. Chitin is one of many organic and inorganic materials that compose natural photonic structures. The most abundant biopolymer after cellulose, chitin is the main constituent of outer shell of insects like butterflies and beetles. Iridescent coloration in these insects is the result of the interference of visible light in an intricate array of chitin fibers organized periodically at the nanometer scale [4]. Interestingly, polymers have been the object of much study for a wide variety of photonic applications. Among the many desirable properties they exhibit, the most cited are: wide bandwidth of operation, structural flexibility, low cost and compatibility with other materials [5]. Polymers also tend to exhibit large thermo-optic coefficients and electronic nonlinearity, thereby making efficient all-optical control possible in spite of their relatively low damage threshold. These reasons have motivated us to examine the possibility of lightinduced optical changes in chitin-based natural photonic crystals. For the purpose of this study, we chose to analyze Morpho didius specimen, a species belonging to a family of iridescent butterflies for which microstructural and optical properties have been measured and modeled in details (see Refs [6–9] for the M. didius species, Refs [9,10] for M. sulkowskyi and Ref [8] for M. rhetenor).
⁎ Corresponding author. Tel.: + 1 506 858 4938; fax: + 1 506 858 4541. E-mail address: [email protected] (A. Haché). 1 Canada Research Chair in Photonics. 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.11.043
If the scientific interest in understanding biological photonic structures is mostly academic for now, it may change in the future. How genetic information is used by living cells to unfold such intricate structures as a photonic crystal is a question that has yet to be answered. However, over the past years, advances in bioengineering suggest that it may be possible to drive living tissues into synthesizing desired structures based on selected genetic information [11]. If this becomes reality, a new avenue for assembling photonic devices will be open and the knowledge gained from biological systems will be valuable for application purposes. 2. Methods Light induces changes in the optical properties of a photonic crystal when it modifies the lattice constant and/or the refractive index of the constituent materials. Because of interferences, the resulting shift in the photonic bands modifies the optical properties of the medium to a much greater extent than would in a bulk, nonperiodic material. Thermal expansion and mechanical stress can deform the crystal lattice and shift the photonic band gap accordingly. Changes in the refractive index, on the other hand, may be induced thermally (the thermo-optic effect) or via the nonlinear susceptibility of the material, such as in the optical Kerr effect. If both mechanisms have similar effects on the index of refraction, they operate on vastly different timescales (10− 3 s versus 10− 15 s, respectively) and intensity levels. To investigate the possible nonlinearity of natural photonic crystals, we tested samples for the thermo-optic effect using a pump-probe technique. The method uses a laser beam to heat the sample and a beam of white light to probe optical reflection simultaneously. For comparison purposes, the nonlinear properties of chitin and chitosan in their
A. Haché, G.-G. Allogho / Optics Communications 284 (2011) 1656–1660
1657
synthetic forms were examined. Chitosan, the deacetylated form of chitin, exhibits similar optical properties but it has a much higher solubility in acidic media [12]. Third-order nonlinearity was measured with a pulsed laser beam using the standard z-scan technique [13].
chitosan films with 200 μm thickness were obtained by multiple dipping and drying. Z-scan was done with an amplified Ti:sapphire laser operating at 800 nm (Coherent Rega) producing pulses with 200 fs in duration, 4 μJ of energy and a repetition rate of 100 kHz.
3. Experimental
4. Results and discussion
Wing samples from two Morpho didius specimen were cut and analyzed in a spectrophotometer (Varian Cary 5000) for reflection and transmission measurements in the 200–2500 nm spectral range. An integrating sphere was used for measuring total and diffuse reflectance. For morphology and structural examination, samples were metalized on both sides with gold and analyzed with a scanning electron microscope (JEOL-5200) down to a resolution of 10 nm. Thermo-optic measurements were done by optical heating of samples with a Ti:sapphire laser (Coherent Vitesse) producing a train of pulses of 150 fs in duration with a repetition rate of 86 MHz. The laser spectrum is centered at 800 nm with a bandwidth of 25 nm, wavelengths that fall within the 600–900 nm spectral range where chitin exhibits the least optical absorption [12]. Since thermal effects occur on a time scale many orders of magnitude longer than the interval of time between two successive laser pulses (12 ns), the beam is effectively continuous. For probing reflection changes induced by the pump, a xenon/deuterium lamp producing a white beam of light was used. Fig. 1 shows the experimental setup for thermo-optic measurements. The 800 nm pump beam is modulated by a mechanical chopper and changes in reflectivity are monitored with the white light beam. Reflected white light is collected with a 4× microscope objective with a numerical aperture of 0.1 and sent to a silicon photodetector, the signal of which is analyzed with a lock-in amplifier. Band-pass filters with 10 nm bandwidth are used to study the effect at various wavelengths. Incident and reflected light beams are close to normal incidence. For third-order nonlinearity measurements, chitosan and chitin films were prepared by dipping a glass substrate into a solution of 1% acetic acid in water and 5% chitosan (or 0.5% chitin). Optical-grade
The microscopic structure and coloration of the Morpho didius wing are shown in Fig. 2. The top wing surface is covered with 80 μmwide scales containing regular arrays of chitin ridges. Studies using transmission electron microscopy have revealed a tree-like structure composed of ridges and several pairs cross-ribs [4,6–10]. The periodic structure, which thickness is the equivalent of 3–4 bi-layers of chitin and air, scatters blue and violet light. Interestingly, although the top and bottom surfaces of the wings are made of the same material and have similar morphology, they have different colorations: the top surface is metallic blue while the bottom one is brown for lack of long range structural order. As it was previously found [8–10], some features of the Morpho wing coloration cannot be explained with the simple interference (multi-layer) model. For example, some degree of irregularity in the ridge height is necessary to create iridescence over a wide range of viewing angles. In this study, we limit thermo-optic measurements to viewing angles near normal incidence, a regime where constructive interference in the periodic lamellar structure is the dominant effect. Here the relevant structural period is ~200 nm, thereby creating a reflection peak at 480 nm. In principle, opto-thermal effects should also be observed at wide viewing angle, but the experimental setup did not allow to investigate. Fig. 3 shows the reflection spectra of the top and bottom wing surfaces. The top surface shows a reflection (scattering) peak in the 450–525 nm range while the bottom surface does not. Reflectance is largely dominated by diffuse reflection; specular reflectance only account for b5% of the overall value. Transmission on both sides is also minimal, particularly in the visible spectral range. Thermo-optic results are shown in Fig. 4. Here the relative modulation of reflectance ΔR/R = (R − R0)/R0 is measured, where R
Fig. 1. Thermo-optic pump-probe setup. The Morpho didius sample is illuminated with a pulsed (but effectively continuous) laser beam at 800 nm. The beam is modulated with a mechanical chopper and changes in the reflection spectrum are measured on a silicon photodiode, the signal of which is processed with a lock-in amplifier (LIA). Band-pass filters (BPF) are used to measure the effect at various wavelengths. Incident and reflected light beams are close to normal incidence.
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Fig. 2. Electron microscopy of a Morpho didius wing. The top (A) and bottom (B) surfaces of the wing have similar structural arrangements but only the top one exhibits the metallic blue coloration (C); the bottom surface (D) is brown in appearance.
and R0 are the reflectance with and without pumping, respectively. Here the beam of white light arrives perpendicular to the plane of the wing (normal incidence), while the pump laser beam and the measured reflection are within 5° and 20° of normal incidence, respectively. Since the microscope objective probes an area of ~ 1 mm in diameter, measurements represent ensemble averages over many scales, each one being about 100 μm in size. The relative modulation peaks in the 480–520 nm wavelength range, i.e. near the photonic band edge. To within experimental error (b4 × 10− 4), no modulation
was detected on the bottom surface of the wing for the same pump intensity. This clearly shows that modifications in the photonic band gap are involved in the observed reflectance changes. The unfocussed 70 mW pump beam had an on-sample intensity of 0.28 W/cm2 and was modulated at 26 Hz. The dynamics of the reflection change is shown in Fig. 5. The chopping frequency was swept from 2 to 200 Hz and reflectance was monitored at 488 nm with constant pump power. A linear dependence exists for pump durations up to 1/f = 35 ms, followed by a
100 0.012
60
A
0.008
ΔR/R
Reflectance (%)
80
40
0.004 20
0
B 500
1000
1500
2000
2500
Wavelength (nm)
0
400
500
600
700
wavelength (nm) Fig. 3. Total optical reflectance spectra (diffuse and specular) of a Morpho didius wing. Unlike the top surface (A), the bottom surface (B) does not exhibit a strong scattering band in the visible region for lack of periodic arrangement of the chitin ridges. Reflectance is largely dominated (N95%) by diffusion.
Fig. 4. Relative modulation in reflectance of the top surface of a Morpho didius wing. The dashed line represents the maximum modulation measured on the bottom surface (i.e. within experimental noise level). Experimental error is of the order of 3–5%.
A. Haché, G.-G. Allogho / Optics Communications 284 (2011) 1656–1660
1659
60 0.02
20 oC
Reflectance (%)
ΔR/R
0.01 0.005
0.002
50 oC
40
100 oC 150 oC
20
0.001
0.001
0.01
0
0.1
400
pump period (s)
500
600
700
wavelength (nm) Fig. 7. Reflectance of a Morpho didius wing heated at constant temperature. Blue shifting of the photonic band is consistent with the lowering of the refractive index and/or thermal expansion of the structure.
saturation. The time scale of the effect confirms the thermal nature of the process. At 2 Hz, the lowest frequency attainable with the chopper, the relative reflectance changes of 3.5% are observed. In Fig. 6, the amplitude of reflectance change is measured as a function of pump intensity, with a constant chopping frequency of 7 Hz. A linear dependence is observed up to 0.15 W/cm2 and the maximum is reached at 0.25 W/cm2. Beyond that intensity level, a drop is observed, but it does not necessarily imply irreversible damage of the sample: the modulation recovers its original values when the intensity is lowered. To further investigate the effect of temperature on the optical properties of the Morpho didius wing, the sample was put in a controlled environment at various ambient temperatures while reflectance was simultaneously measured using a spectrophotometer (Varian Carry 5000). Blue-shifting of the reflection band was observed for temperatures up to 150 °C, as Fig. 7 shows. Since the reflection peak wavelength scales linearly with the optical thickness of a bi-layer, the blue shift suggests a lowering of the refractive index of chitin with increasing temperature. We can also estimate the thermo-optic coefficient of chitin from these results. With a shift of −7 nm over 75 °C, an estimated air gap thickness of 70 nm, a cuticle layer thickness of 110 nm and a refractive index of 1.54 for chitin (see Ref. [8]), we obtain the thermooptic coefficient dn/dT=−4.7×10− 4 °C− 1. Although comparison measurements on chitin or chitosan don't exist in the literature, the value is comparable to that of other polymers [14]. It should be noted that
thermal distortion (thermal expansion) would tend to red shift the reflection peak. Although chitin does retain its chemical integrity at temperatures up to 250 °C, our system did not allow to reach that point. Nonetheless, relative changes in reflectance of ~30% near 500 nm were measured, namely one order of magnitude larger than observed with optical pumping. We should then expect to observe larger reflectance modulations by using lower pump chopping frequencies. For the purpose of measuring third-order optical nonlinearity by z-scan, optical-grade chitosan layers with a thickness d = 200 μm deposited on a glass were made. However, no self-focusing effect (i.e. z-scan signal, closed or open aperture) was detectable to within experimental uncertainty for peak intensities up to 100 GW/cm2. Considering the nonlinear phase shift through the layer is Δϕ = k0dΔn, where k0 = 7.85 × 106 m− 1 is the free space wave vector of the pump beam and Δn is the refractive index change, and assuming the z-scan signal would be detectable if Δϕ N 0.1 (or λ/60 beam distortion, a conservative figure), this puts the upper limit on Δn at 6 × 10− 5. The nonlinear refractive index coefficient would then be limited to n2 b 6 × 10− 16 cm2/W, a level comparable to fused silica, but much lower than of pi-conjugated polymers [15]. Owing to its poor solubility and tendency to crystallize into highly diffusing films, chitin did not yield the possibility of z-scan measurements. Given the small polarization nonlinearity of chitosan and the low temperature damage threshold of chitin, one can reasonably expect that inducing changes in chitin photonic crystals via the optical Kerr effect would be difficult to achieve, even with single femtosecond laser pulses.
ΔR/R (a.u.)
Fig. 5. Dynamics of the reflectance changes at 488 nm with 0.28 W/cm2 pumping intensity. To better show the evolution over time, the graph is plotted as a function 1/f, with f the pump chopping frequency. A saturation occurs after ~ 200 ms, clearly establishing the thermal nature of the effect.
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5. Conclusions
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Many studies over the past decades have focused on the morphology and the linear optical properties of biological photonic structures, but none have explored their nonlinearity. This study showed that sizeable changes in photonic band properties can be induced optically. At the relatively modest intensity of 0.05 W/cm2, reflection changes of a few percents were observed in a Morpho didius wing in the violet-blue spectral region. The thermo-optic coefficient of chitin is estimated to be dn/dT = −4.7 × 10− 4 °C− 1, a value that is large but comparable to other polymers. Our findings point the way to further studies in that area, as many aspects have yet to be investigated. The pump wavelength used in this study is not optimal, as it not strongly absorbed by chitin; it would be interesting to pump at wavelengths in the UV. Nonlinear shifts in polarization and phase effects could also be explored. Like many iridescent species, the scattering of the Morpho didius wing is optically anisotropic due to the preferential orientation of chitin ridges.
0.6
0.4
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0
0
0.1
pump intensity
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Fig. 6. Dependence of the modulated reflectance at 488 nm on the pump intensity. The modulation frequency is constant at 7 Hz.
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A. Haché, G.-G. Allogho / Optics Communications 284 (2011) 1656–1660
The authors wish to thank Torriss Badr for chitosan sample preparation and Yves Poussart and Martin Chiasson for electron microscope images.
[5] [6] [7] [8] [9] [10] [11]
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