Low-threshold blue lasing from silk fibroin thin filmsStefano Toffanin, Sunghwan Kim, Susanna Cavallini, Marco Natali, Valentina Benfenati, Jason J. Amsden,

David L. Kaplan, Roberto Zamboni, Michele Muccini, and Fiorenzo G. Omenetto Citation: Applied Physics Letters 101, 091110 (2012); doi: 10.1063/1.4748120 View online: http://dx.doi.org/10.1063/1.4748120 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/101/9?ver=pdfcov Published by the AIP Publishing

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

192.167.162.115 On: Thu, 20 Mar 2014 17:32:52

Low-threshold blue lasing from silk fibroin thin films

Stefano Toffanin,1,a),b) Sunghwan Kim,2,a) Susanna Cavallini,1 Marco Natali,1

Valentina Benfenati,1 Jason J. Amsden,2 David L. Kaplan,2 Roberto Zamboni,3,b)

Michele Muccini,1,b) and Fiorenzo G. Omenetto2,4,b)

1Consiglio Nazionale delle Ricerche (CNR), Istituto per lo Studio dei Materiali Nanostrutturati (ISMN),via P. Gobetti 101, I-40129 Bologna, Italy2Department of Biomedical Engineering, Tufts University, 4 Colby Street, Medford 02155, USA3Consiglio Nazionale delle Ricerche (CNR), Istituto per lo Sintesi Organica e Fotoreattivit�a (ISOF),via P. Gobetti 101, I-40129 Bologna, Italy4Department of Physics, Tufts University, 4 Colby Street, Medford 02155, USA

(Received 2 July 2012; accepted 13 August 2012; published online 29 August 2012)

Silk is a natural biocompatible material that can be integrated in a variety of photonic systems and

optoelectronic devices. The silk replication of patterned substrates with features down to tens of

nanometers is exploited to realize highly transparent, mechanically stable, and free-standing

structures with optical wavelength size. We demonstrate organic lasing from a blue-emitting stilbene-

doped silk film spin-coated onto a one-dimensional distributed feedback grating (DFB). The lasing

threshold is lower than that of organic DFB lasers based on the same active dye. These findings pave

the way to the development of an optically active biocompatible technological platform based on silk.VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4748120]

In recent years intensive studies have been devoted to or-

ganic materials and advanced device structures to develop

applications in the field of optics and photonics.1–4 Organic

materials offer a number of attractive properties like, for

example, ultrafast nonlinear optical response, electronic and

photonic multifunctionality, compatibility with a variety of

technological platforms, large active area, mechanical flexibil-

ity, and cost-effective fabrication.5 In particular, organic lasers

are of great interest due to the ability of molecules and poly-

mers to provide efficient lasing in the solid state.6,7 Further-

more, the compatibility of organics with natural biomaterials

makes organic photonics suitable to develop biocompatible

and biodegradable devices.8

Silk fibroin, the natural protein produced by the Bombyxmori caterpillar, is an attractive material for applications in

organic photonics and electronics.9,10 Silk has shown out-

standing photonic properties, which include high transpar-

ency and easy structuring at the optical wavelengths. We

have previously demonstrated various optical components

such as microlens arrays, waveguides,9,11 and diffraction

gratings.12 As a biopolymer, silk fibroin is biocompatible

and easily dopable with biological carriers providing func-

tionalization of engineered devices.13 Moreover it has been

demonstrated that silk supports the adherence and neurite

outgrowth of neurons, preserving neuronal functions.14,15

By extending the scope to organic lasers, the demon-

strated suitability of silk for use in functionalized active opti-

cal components for biophotonics is further expanded.

Stilbene S-420 from Coherent, Inc. (molecular structure

in Fig. 1(a)), a subgroup of azo dyes, has been shown to have

high emission quantum yield (QY) and a low-threshold for

optical gain narrowing. Combined with these optical proper-

ties, its water-solubility makes it ideally compatible for blend-

ing with silk fibroin in water. In biology and chemistry, stilbene

derivatives are a well-known class of natural phytochemicals,

applicable to human health for cancer prevention and antia-

ging.16 In this letter, we report on silk one-dimensional (1D)

distributed feedback (DFB) laser using stilbene as laser dye.

Lasing with an optical pump threshold intensity of 180 lJ/cm2

(45 kW/cm2) is achieved at 427 nm wavelength.

Figure 1(b) shows the schematics of the DFB fabrica-

tion. A silicon dioxide film with a thickness of 1.5 lm and a

refractive index of 1.46 was deposited on a silicon wafer

using a plasma enhanced chemical vapor deposition system.

1D grating patterns with 40% and 50% duty cycle were

generated by electron-beam lithography onto PMMA resist

spin-coated on the SiO2 film. Reactive ion etching tools

transferred the grating pattern onto the SiO2 film. The pitch

of the grating was determined according to the Bragg condi-

tion, mkL ¼ 2nef f K, in order to achieve overlapping of the

lasing wavelength kL with the gain spectrum of the stilbene

dye. Here, m is the order of the Bragg diffraction and K is

the lattice constant. The effective index of the active layer,

neff, was considered as 1.5. To obtain surface-emission, we

designed a second-order gratings with lattice constant vary-

ing between 265 nm and 275 nm. Figure 1(c) shows a high

resolution scanning electron microscope (SEM) image of the

detached silk grating. Stilbene/silk mixture with a concentra-

tion of 5 wt. % (thickness� 1.5 lm) was spin-coated onto the

fabricated SiO2 grating.

For lasing emission measurements, the sample was

mounted in vacuum-chamber (10�6 mbar) and pumped using

the third harmonic from a Nd:YAG laser (355 nm, 4 ns pulse

width, and 10 Hz repetition rate). An objective lens focused

the pumping beam to a 80 lm spot size, at 20� with respect

to the grating normal. The emitted light was detected using

an optical multichannel analyzer along the normal to the gra-

ting substrate (Fig. 2(a)). Figure 2(b) shows the absorption

and photoluminescence (PL) spectra of stilbene in water

a)S. Toffanin and S. Kim contributed equally to this work.b)Authors to whom the correspondence should be addressed. Electronic

addresses: s.toffanin@bo.ismn.cnr.it, roberto.zamboni@isof.cnr.it,

m.muccini@bo.ismn.cnr.it, and fiorenzo.omenetto@tufts.edu.

0003-6951/2012/101(9)/091110/4/$30.00 VC 2012 American Institute of Physics101, 091110-1

APPLIED PHYSICS LETTERS 101, 091110 (2012)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

192.167.162.115 On: Thu, 20 Mar 2014 17:32:52

solution and in stilbene-doped silk film. To characterize the

gain material, two stilbene-doped silk films (600 nm thick

and 800 nm thick) were deposited on the SiO2 substrates.

The absorbance has a maximum at 355 nm and the PL has a

maximum at around 450 nm. With respect to the water solu-

tion, stilbene molecules dispersed in silk show a slightly red-

shifted absorption spectrum and a structured PL emission

profile. This behavior is consistent with the possible confor-

mational stabilization of stilbene molecules in the solid

matrix.17

The photoluminescence QY was measured by exciting

the samples at 375 nm. In order to check the reliability and

reproducibility of the measurements, the QY was measured

four times for each sample and the values were found to be

highly reproducible, as shown in Table I. The high QY is

most likely due to the stilbene being confined in the silk ma-

trix such that the stilbene molecule is planarized and not able

to freely rotate like in solution. Indeed the well-structured

emission spectrum of stilbene in silk corroborates this hy-

pothesis. Thus, it is expected a decrease of the amount of the

available non-radiative deactivation paths with consequent

increase of the emission efficiency.

In Figure 3 we report the lasing characterization of the

stilbene-doped silk film onto DFB gratings. For pumping

above the lasing threshold (Fig. 3(a)), a narrow peak emerges

at 427 nm with the full width at half maximum (FWHM)

value approaching the limit of the spectrometer resolution

(about 1 nm). The emission spectrum excited below the lasing

threshold is broad (40 nm FWHM) and partially modulated

by internal reflections in the SiO2/silk/air waveguide slab.

As reported in Table II, we observed that stilbene lasing

wavelength was blue-shifted when the lattice constant of the

grating decreased regardless the duty cycle. This evidence is

well in accordance with the Bragg’s law even though vari-

ability of lasing wavelengths correlated to the specific investi-

gated grating is present. Moreover, the pump energy needed

for laser oscillation tends to increase as the laser wavelength

FIG. 1. (a) Molecular structure of the stilbene S-420 lasing dye. (b) Sketch

of the procedure for the fabrication of silk blue DFB laser. (c) SEM micro-

photograph of the DFB grating obtained by nanolithography on Si/SiO2

substrate.

FIG. 2. (a) Schematics of the setup imple-

mented for exciting the lasing emission in

dye-doped silk film deposited onto Si/SiO2

DFB grating. (b) Absorption (dashed line)

and photoluminescence (solid line) of stil-

bene dispersed in diluted water solution

(black) and in silk film at 5 wt. % (red).

TABLE I. Photoluminescence quantum yields of stilbene-doped silk films

with different thickness.

600 nm thick (%) 800 nm thick (%)

1st 95.59 97.61

2nd 94.85 97.12

3rd 94.26 97.06

4th 95.13 98.06

Average 94.96 97.46

091110-2 Toffanin et al. Appl. Phys. Lett. 101, 091110 (2012)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

192.167.162.115 On: Thu, 20 Mar 2014 17:32:52

moves away from the peak of the gain spectrum. Finally, we

report the calculated refractive index for the SiO2/silk wave-

guide, which shows a good agreement among values obtained

from different gratings.

In Figures 3(b) and 3(c) are reported the plots of the

emission peak FWHM and intensity output values as a func-

tion of pump intensity, respectively. A lasing threshold at

180 lJ/cm2 (45 kW/cm2) is clearly evident. Exciting the

stilbene-doped silk film outside of the SiO2 grating region

resulted in a linear increase of the output intensity as a func-

tion of the input intensity. Therefore, one can conclude that

the observed threshold behavior is not simply due to ampli-

fied spontaneous emission (ASE).

The stilbene/silk DFB laser provides a lasing threshold

almost an order–of–magnitude lower than those reported for

stilbene DFB lasers18 where stilbene is dispersed within an

UV-curable resin, which is then molded with a grating struc-

ture. This result is even more interesting considering that

imprinted optically active polymeric structures typically ex-

hibit better performances with respect to optically active

polymers spin-coated onto silica DFB patterns.19

Thus, it is evident that the specific chemical-physical

interaction between silk and stilbene salt-derivative plays a

fundamental role in determining the photonic properties of

the silk-based lasing system.

Indeed, it has been shown that other biocompatible poly-

mers implemented as dispersing matrix for organic lasing

dyes, e.g., DNA-CTMA based thin-film,20 present a lasing

threshold 2 orders of magnitude higher than that found in

silk, when measured in the same experimental conditions.

The dye stabilization in silk is related to the viscosity of

the matrix. It is well-known that viscosity can have a dra-

matic effect on the optical and spectroscopic features of fluo-

rophores such as stilbene derivatives.21 Typically, rotation

around the double bond in the molecule exited state, which

is responsible for the fluorescence quenching, is expected to

be hindered with increasing matrix viscosity.

We also mention that the repulsive electrostatic interac-

tion between the negatively charged stilbene moieties (origi-

nated by the SO3� anion) and the chains of the silk fibroin,

endowed with both acid an basic functional groups,22 might

also favorably affect the molecular rearrangement of the las-

ing dye.

From the application point of view, the possibility to

incorporate in a silk matrix biocompatible organic and inor-

ganic dyes can be useful for the realization of label-free opti-

cal detection. Indeed, the variation of the silk matrix

refractive index upon exposure to specific environment or

analytes induces the variation of the silk laser characteristics.

In perspective, the full implementation of silk-based pho-

tonic devices in portable and high throughput lab-on-a-chip

devices can be preferentially obtained by patterning directly

the photonic lattice on silk thin films.23

Finally, it is worth mentioning that nanopatterned DFB

silk thin-films can be implemented as biological lasers24

compatible with the support, adherence and outgrowth of

cells (i.e., neurons15), thus enabling novel non-linear detec-

tion and imaging schemes.

In conclusion, we have realized a low threshold stilbene/

silk DFB laser by combining two biocompatible optical

materials. The stilbene/silk film shows high QY and a sharp

lasing peak. These findings may open perspectives for appli-

cations of optically active silk in biophotonics and biological

sensors.

This work was supported by MIST E-R through Pro-

gramma Operativo FESR 2007-2013 della Regione Emilia-

Romagna – Attivit�a I.1.1., by MIUR through project PRIN

2009-2009AZKNJ7 – “Biosensori elettronici ed elettrochimici,”

and EU through project FP7-ICT-248052 (PHOTO-FET). We

thank E. T. C. s.r.l. for the use of the Nd:YAG nanosecond laser.

FIG. 3. (a) Photoluminescence spectra of stilbene-

doped silk collected in reflection at excitation pump

intensity below (black line) and above (red line) the

lasing threshold pump intensity. At low pump inten-

sity it is evident the modulation of the photolumi-

nescence spectrum due to the presence of the

grating. Dependence of the photoluminescence full

width at half maximum (b) and intensity (c) on the

excitation pump intensity. The lasing threshold

pump intensity is indicated by the dashed lines.

TABLE II. Dependence of stilbene lasing wavelength (kL) on the grating

periodicity (K). The grating duty cycle is 50%. The effective refraction

index values (neff) calculated from the Bragg law are reported, given that the

diffraction order is m¼ 2.

K (nm) kL (nm) neff

275 430, 427 1.56 (60.01)

270 424, 423 1.57 (60.01)

265 419 1.58

091110-3 Toffanin et al. Appl. Phys. Lett. 101, 091110 (2012)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

192.167.162.115 On: Thu, 20 Mar 2014 17:32:52

1M. Muccini, W. Koopman, and S. Toffanin, Laser Photonics Rev. 6, 258

(2012).2M. Muccini, Nat. Mater. 5, 605 (2006).3M. O’Neil and S. M. Kelly, Adv. Mater. 23, 566–584 (2011).4R. Capelli, S. Toffanin, G. Generali, H. Usta, A. Facchetti, and M.

Muccini, Nat. Mater. 9, 496 (2010).5S. R. Forrest, Nature (London) 428, 911 (2004).6I. D. W. Samuel and G. A. Turnbull, Chem. Rev. 107, 1272 (2007).7S. Toffanin, R. Capelli, T.-Y. Hwu, K.-T. Wong, T. Ploetzing, M. Forst,

and M. Muccini, J. Phys. Chem. B 114, 120 (2010).8A. J. Steckl, Nat. Photonics 1, 3 (2007).9F. G. Omenetto and D. L. Kaplan, Nat. Photonics 2, 641 (2008).

10R. Capelli, J. J. Amsden, G. Generali, S. Toffanin, V. Benfenati, M.

Muccini, D. L. Kaplan, and F. G. Omenetto, and R. Zamboni, Organ.

Electron. 12, 1146 (2011).11B. D. Lawrence, M. Cronin-Golomb, I. Georgakoudi, D. L. Kaplan, and F.

G. Omenetto, Biomacromolecules 9, 1214 (2008).12H. Perry, A. Gopinath, D. L. Kaplan, L. Dal Negro, and F. Omenetto, Adv.

Mater. 20, 3070 (2008).13S. T. Parker, P. Domachuk, J. Amsden, J. Bressner, J. A. Lewis, D. L.

Kaplan, and F. G. Omenetto, Adv. Mater. 21, 2411 (2009).

14V. Benfenati, S. Toffanin, R. Capelli, L. M. Camassa, S. Ferroni, D. L. Kaplan,

F. G. Omenetto, M. Muccini, and R. Zamboni, Biomaterials 31, 7883 (2010).15V. Benfenati, K. Stahl, C. Gomis-Perez, S. Toffanin, A. Sagnella, R. Torp,

D. L. Kaplan, G. Ruani, F. G. Omenetto, R. Zamboni, and M. Muccini,

Adv. Funct. Mater. 22, 1871 (2012).16A. M. Rimando and N. Suh, Planta Med. 74, 1635 (2008).17V. Strehmel, C. W. Frank, and B. Strehmel, J. Photochem. Photobiol. A:

Chem. 105, 353 (1997).18K. Yamashita, M. Arimatsu, M. Takayama, K. Oe, and H. Yanagi, Appl.

Phys. Lett. 92, 243306 (2008).19M. Salerno, G. Gigli, M. Zavelani-Rossi, S. Perissinotto, and G. Lanzani,

Appl. Phys. Lett. 90, 111110 (2007).20L. Sznitko, J. Mysliwiec, P. Karpinski, K. Palewska, K. Parafiniuk, S.

Bartkiewicz, I. Rau, F. Kajzar, and A. Miniewicz, Appl. Phys. Lett. 99,

031107 (2011).21J. R. Lakowicz Principles of Fluorescence Spectroscopy, 2nd ed.

(Springer-Science Business Media Inc., 2004)22F. P. Seib, M. F. Maitz, X. Hu, C. Werner, and D. L. Kaplan, Biomaterials

33, 1017 (2012).23H. Tao, D. L. Kaplan, and F. Omenetto, Adv. Mater. 24, 2824 (2012).24M. C. Gather and S. H. Yun, Nat. Photonics 5, 406 (2011).

091110-4 Toffanin et al. Appl. Phys. Lett. 101, 091110 (2012)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

192.167.162.115 On: Thu, 20 Mar 2014 17:32:52