Optofluidic evanescent dye laser based on a distributed feedback
circular grating
Wuzhou Song,
1
Andreas E. Vasdekis,
1,a兲
Zhenyu Li,
2
and Demetri Psaltis
1
1
Optics Laboratory, School of Engineering, Swiss Federal Institute of Technology Lausanne (EPFL),
CH-1015 Lausanne, Switzerland
2
Electrical Engineering, California Institute of Technology, 1200 East California Boulevard, Pasadena,
California 91125, USA
共Received 20 February 2009; accepted 2 April 2009; published online 24 April 2009兲
We demonstrate an optofluidic evanescent laser based on a solid circular distributed feedback
grating with the dye solution acting as the cladding layer. The laser mode is confined within the
grating and experiences optical gain via the interaction between its evanescent component with the
dye solution. Above a pump energy of 9.5
J/ pulse, the laser exhibited single mode operation at
571 nm. Stable, narrow-linewidth emission was observed for a wide range of fluid refractive indices,
even for those lower than of polydimethylsiloxane. We attribute this property to the evanescent
coupling of the laser mode with the fluidic gain. © 2009 American Institute of Physics.
关DOI: 10.1063/1.3124652兴
Optofluidics have recently attracted substantial attention
due to the potential of integrating optics with microfluidics
and thus enabling cost-effective methods for lab-on-a-chip
applications and noninvasive diagnostics.
1,2
As such, optof-
luidic dye lasers are particularly attractive for spectroscopic
applications due to the wide choice of emission wavelengths
and the possibility of avoiding photobleaching by flowing
the gain medium. To this end, various cavity configurations
have been demonstrated, such as Fabry–Perot,
3
microdroplets,
4
microcavities,
5
Bragg gratings,
6,7
and photo-
nic crystal fibers.
8
In all aforementioned structures, the
guided mode is confined inside the fluid gain medium and
thus the refractive index of the solution determines the emis-
sion wavelength. Although this enables the tuning of the las-
ing wavelength by fluidic mixing, in practice stable and
single mode operation require high control of the refractive
index of the fluid and moderate flow rates across the
waveguides. In addition, due to the relatively high index of
polydimethylsiloxane 共PDMS兲共n
PDMS
=1.4218兲, several
buffers cannot be directly employed for waveguiding in
PDMS based optofluidic lasers.
To overcome this, evanescent field dye lasers have been
proposed in the past.
9,10
In these lasers, the liquid gain me-
dium surrounds a solid waveguide and is optically excited.
The excited chromophores in the near-field of the waveguide
are evanescently coupled to the laser mode providing the
optical gain. To date, several evanescent optofluidic dye la-
sers have been demonstrated, primarily based on whispering
gallery mode resonators such as infilled silica capillaries or
fibers embedded in dye solutions.
11–13
These structures are
usually characterized by multimode emission spectra. More
recently, evanescent lasers operating in the telecommunica-
tion wavelength range have found applications in the field of
silicon photonics.
14
In this letter, we demonstrate an optofluidic evanescent
dye laser, exhibiting single mode operation. It comprises of a
solid second order circular distributed feedback 共DFB兲 grat-
ing and a PDMS chamber filled with dye solution. The thin
layer of the solution serves as the cladding and covers the
entire surface of the solid DFB cavity. Due to the high modal
confinement in the solid core, the mode selection and lasing
wavelength are primarily determined by the solid DFB cav-
ity. In comparison to liquid core dye lasers,
7
stable and single
mode operation can be achieved for a wide range of refrac-
tive indices of the dye solution.
In Fig. 1, a cross-sectional schematic of the circular DFB
resonator is illustrated. It is made of the negative photoresist
SU-8 共refractive index n
SU-8
=1.59兲 patterned on top of sili-
con dioxide layer 共n
SiO
2
=1.46兲 on a silicon substrate. The
PDMS 共n
PDMS
=1.4128兲 forms a microfluidic chamber which
envelops the entire surface of the solid DFB cavity. By in-
filling the dye solution into the PDMS chamber, a thin layer
of liquid gain medium is formed on top of the grating serving
as the upper cladding of the cavity. In comparison to linear
gratings, the circular DFB cavity has a large surface area and
enables an enhanced interaction area of the evanescent tail
with the gain medium. The second order DFB grating pro-
vides both optical feedback and output coupling via second
and first order diffraction, respectively, thus forming a sur-
face emitting optofluidic laser.
The fabrication process is illustrated in Fig. 2. The grat-
ing was realized by electron beam lithography 共Vistec
EBPG5000, 100 kV兲. The electron beam resist was a 500 nm
thick film of the negative photoresist SU-8 共GM-1030, Ger-
steltec Inc., 8
Cb/ cm
2
兲 ona5
m wet oxide layer on a
a兲
Electronic mail: andreas.vasdekis@epfl.ch. FIG. 1. 共Color online兲 The schematic cross section of the dye laser chip.
APPLIED PHYSICS LETTERS 94, 161110 共2009兲
0003-6951/2009/94共16兲/161110/3/$25.00 © 2009 American Institute of Physics94, 161110-1
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silicon wafer. SU-8 was selected for its optical transparency
in the visible spectrum and its high electron beam sensitivity,
thus allowing rapid patterning of large area structures. The
grating period was 370 nm in order to satisfy the second
order Bragg condition ⌳ = / n
eff
, where n
eff
is the effective
refractive index of the waveguide mode, ⌳ is the grating
period and the emission wavelength. A scanning electron
microscopy 共SEM兲 image of the grating at a tilt angle of 30°
is shown in Fig. 3. The width of the grating grooves was
approximately 130 nm. The PDMS structure for the micro-
fluidic channel was prepared by replica molding. The master
mold for the microfluidic channels was defined using UV
lithography on a 4
m thick SU-8 film 共GM-1040, Ger-
steltec Inc.兲 on a silicon wafer. After oxygen plasma treat-
ment 共15 W, 20 s兲, the PDMS layer and the substrate with the
circular grating were aligned under a microscope and
brought in contact to permanently bond. As shown in Fig. 2,
the PDMS layer had no structure in the area corresponding to
the grating cavity, except for the two channels that enabled
fluidic connection. The dye solution could flow and fill the
clearance between the grating and PDMS layer because of
the elasticity of PDMS and the absence of bonding between
PDMS and SU-8. In this way, the circular grating was cov-
ered entirely with a thin fluidic layer. This microfluidic con-
figuration was implemented to minimize the dye solution
volume and thus to decrease the background fluorescence
which would hinder the detection of stimulated emission.
Such configuration proved adequate for the evanescent dye
laser operation due to the near-field nature and thus very
short penetration depth of the evanescent waves into the
fluid. A rhodamine 6G solution was used at a concentration
of 1 mg/ml. The solvent was prepared by mixing water and
dimethylsulfoxide at a ratio of 54:46 and had a refractive
index of 1.401 lower than that of SU-8.
The optofluidic evanescent dye laser chip was optically
pumped with a Q-switched Nd:YAG laser 共532 nm, 4.5 ns
pulses, and 100 Hz repetition rate兲. The experimental setup is
shown in the inset of Fig. 4共a兲. The pump laser beam was
focused through a 10⫻ magnification objective lens, re-
flected by a dichroic mirror 共Chroma Inc.兲 in front of the
chip and focused on the laser cavity. The laser emission was
normal to the cavity surface. After passing through the same
dichroic mirror, it was collected by a fiber coupled charge
coupled device spectrometer 共Ocean Optics HR4000, 1.2 Å
resolution兲. A typical single mode laser spectrum above
threshold is shown in Fig. 4共b兲. The lasing mode appeared at
the wavelength 571.09 nm and had a linewidth of less than
0.2 nm. A plot of the laser output versus the pump energy is
shown in Fig. 4共c兲. The threshold was estimated to be
9.5
J/ pulse. This value is higher than previous reports on
optofluidic lasers.
6,7,13,15–17
We attribute this primarily to the
short absorption length of the pump light due to the top
pumping configuration, but also to the reduced overlap of the
laser mode with the gain medium.
From the emission wavelength and the grating period
we deduced the waveguide effective index to be n
eff
=1.546.
Based on a one-dimensional 共1D兲 slab waveguide model, the
refractive index of SU-8 was calculated to be n
core
=1.585.
18
In addition, the emission spectrum was found to be stable
both in intensity and wavelength. There was no detectable
spectral shift in the lasing wavelength during the experiment,
even when there was rapid dye flow through the cavity. The
fluctuation of the output intensity was also below 10%, com-
parable to the fluctuation of the pump laser.
Due to the high index contrast between the fluid clad-
ding and the solid core, this dye laser maintained single
mode operation over a wide range of the dye solution’s re-
fractive index. To explore the effect of the fluid refractive
index on the emission wavelength, dye solutions of different
refractive indices were employed on the same laser chip
共⌳ =370 nm兲. We observed a decrease in the emission wave-
length for decreasing fluidic indices, as plotted in square dots
in Fig. 5共a兲. Such behavior is expected due to the depen-
dence of the effective mode index on the cladding refractive
FIG. 2. The fabrication process of the evanescent dye optofluidic laser.
FIG. 3. The SEM image of the circular grating at tilt angle of 30°.
FIG. 4. 共Color online兲共a兲 The close-view of experimental set-up. 共b兲 A
typical laser emission spectrum above threshold and the input-output rela-
tionship 共c兲.
161110-2 Song et al. Appl. Phys. Lett. 94, 161110 共2009兲
Downloaded 24 Apr 2009 to 128.178.84.114. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
index. The experimental results were found to agree with
calculations based on a 1D slab waveguide model 关straight
line in Fig. 5共a兲兴, further confirming that the laser mode is
confined within the solid waveguide. The variation in the
cladding index also induces a change in the confinement fac-
tor of the laser mode. However, this was not manifested in
the laser threshold in our experiments due to the simulta-
neous tuning of the emission wavelength and thus the differ-
ent gain that rhodamine 6 G exhibits at these wavelengths. In
addition, we did not observe laser emission from dye solu-
tions with refractive indices below n =1.35, although wave-
guide modes are supported for these indices. This is attrib-
uted to the high concentration of water in the solution, which
has been shown to degrade the gain properties of rhodamine
6G.
19
Finally, we investigated the tuning of the emission
wavelength by changing the period of the circular DFB grat-
ing. The emission spectra are plotted in Fig. 5共b兲 for the
grating periods of 358, 362, 366, and 370 nm, denoting a
tuning range of approximately 20 nm.
In summary, we demonstrated an optofluidic evanescent
dye laser based on a solid circular DFB grating. Under opti-
cal excitation, gain was provided by the interaction of the
evanescent component of the waveguide mode with the liq-
uid cladding. The integration of a strongly wavelength selec-
tive periodic structure enabled single mode operation. Due to
the evanescent coupling between the laser mode and the
fluid, single mode operation was maintained for a wide range
of refractive indices of the fluid. This evanescent laser was
realized by constructing an optofluidic circuit on a pretreated
silicon wafer. Further integration of electrodes, or absorbing
layers can be implemented on top of the cladding layer for
passive or active modulation, since in such evanescent wave
coupled gain lasers these integration steps would induce
minimal modal perturbation.
11,14,20
In future work, we will
investigate ways to reduce the lasing threshold by modifying
the pumping geometry and employing first order gratings,
which we have shown in the past to reduce the lasing
threshold.
16
The research was partially supported by the DARPA cen-
ter for optofluidic integration.
1
D. Psaltis, S. R. Quake, and C. H. Yang, Nature 共London兲 442,381
共2006兲.
2
C. Monat, P. Domachuk, and B. J. Eggleton, Nat. Photonics 1,106共2007兲.
3
B. Helbo, A. Kristensen, and A. Menon, J. Micromech. Microeng. 13,307
共2003兲.
4
S. X. Qian, J. B. Snow, H. M. Tzeng, and R. K. Chang, Science 231,486
共1986兲.
5
H. Yokoyama, Science 256,66共1992兲.
6
M. Gersborg-Hansen and A. Kristensen, Appl. Phys. Lett. 89, 103518
共2006兲.
7
Z. Y. Li, Z. Y. Zhang, T. Emery, A. Scherer, and D. Psaltis, Opt. Express
14, 696 共2006兲.
8
A. E. Vasdekis, G. E. Town, G. A. Turnbull, and I. D. W. Samuel, Opt.
Express 15, 3962 共2007兲.
9
E. P. Ippen and C. V. Shank, Appl. Phys. Lett. 21, 301 共1972兲.
10
G. Pendock, H. S. Mackenzie, and F. P. Payne, Electron. Lett. 28,149
共1992兲.
11
H. J. Moon, Y. T. Chough, and K. An, Phys. Rev. Lett. 85, 3161 共2000兲.
12
X. S. Jiang, Q. H. Song, L. Xu, J. Fu, and L. M. Tong, Appl. Phys. Lett.
90, 233501 共2007兲.
13
S. I. Shopova, H. Y. Zhou, X. D. Fan, and P. Zhang, Appl. Phys. Lett. 90,
221101 共2007兲.
14
B. R. Koch, A. W. Fang, O. Cohen, and J. E. Bowers, Opt. Express 15,
11225 共2007兲.
15
S. Lacey, I. M. White, Y. Sun, S. I. Shopova, J. M. Cupps, P. Zhang, and
X. D. Fan, Opt. Express 15, 15523 共2007兲.
16
W. Z. Song, A. E. Vasdekis, Z. Y. Li, and D. Psaltis, Appl. Phys. Lett. 94,
051117 共2009兲.
17
C. Peroz, J. C. Galas, L. Le Gratiet, Y. Chen, and J. Shi, Appl. Phys. Lett.
89, 243109 共2006兲.
18
R. G. Hunsperger, Integrated Optics 共Springer, Berlin, 2002兲.
19
K. Igarashi, M. Maeda, T. Takao, M. Uchiumi, Y. Oki, and K. Shimamoto,
Jpn. J. Appl. Phys., Part 1 34, 3093 共1995兲.
20
E. B. Namdas, M. Tong, P. Ledochowitsch, S. R. Mednick, J. D. Yuen, D.
Moses, and A. J. Heeger, Adv. Mater. 共Weinheim, Ger.兲 21,799共2009兲.
FIG. 5. 共Color online兲共a兲 The lasing wavelength vs the refractive index of
the dye solution. The square dots represent the experimental data and the
line indicates the simulation result. 共b兲 The emission spectra of evanescent
optofluidic dye laser of different grating periods.
161110-3 Song et al. Appl. Phys. Lett. 94, 161110 共2009兲
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