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Microphotonic parabolic light directors fabricated by two-photon lithography

October 2011
Applied Physics Letters 99(15):151113-151113-3

October 2011
99(15):151113-151113-3

DOI:10.1063/1.3648115

Authors:

Jackson Atwater

University of California, Santa Cruz

Pierpaolo Spinelli

AMOLF

Show all 9 authorsHide

We have fabricated microphotonic parabolic light directors using two-photon lithography, thin-film processing, and aperture formation by focused ion beam lithography. Optical transmission measurements through upright parabolic directors 22 μm high and 10 μm in diameter exhibit strong beam directivity with a beam divergence of 5.6°, in reasonable agreement with ray-tracing and full-field electromagnetic simulations. The results indicate the suitability of microphotonic parabolic light directors for producing collimated beams for applications in advanced solar cell and light-emitting diode designs.

(a) SEM image of array of parabolic reflectors coated with silver; (b) SEM image of single paraboloid; (c) plan view of single paraboloid with light transmission aperture visible at bottom center; and (d) close up of light aperture etched with a focused ion beam.

…

(Color online) Optical transmission images taken at different heights for parabolic reflector and 1.3 l m reference aperture etched into substrate 100 l m away with 20 x objective NA 1⁄4 0.75. Images (a)–(e): Parabo- loid images with objective at heights z 1⁄4 0, 25, 50, 75, and 100 l m above the substrate. Images (f)–(j): Reference aperture images with objective at heights z 1⁄4 0, 25, 50, 75, and 100 l m above the substrate.

…

(Color online) Directivity of transmission for parabolic light directors and reference apertures. FWHM of optical intensity profiles versus objective lens height from transmission images taken at different heights for parabolic reflector (red curve, at left) and reference aperture (blue curve, at right). Dashed line indicates beam divergence half-angle from predictions of ray tracing simulations. Solid line is the divergence half-angle obtained from boundary element method full wave simulations.

…

(Color online) Electric field intensity j E(x,z) j 2 profile at k 1⁄4 514 nm for a radial cross section emerging from a Ag parabola illuminated by a plane wave from bottom. Inset indicates field intensity in vicinity of the parabola with 1.3 l m diameter aperture.

…

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Microphotonic parabolic light directors fabricated by two-photon

lithography

J. H. Atwater, P. Spinelli, E. Kosten, J. Parsons, C. Van Lare et al.

Citation: Appl. Phys. Lett. 99, 151113 (2011); doi: 10.1063/1.3648115

View online: http://dx.doi.org/10.1063/1.3648115

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Microphotonic parabolic light directors fabricated by two-photon lithography
J. H. Atwater,
1,a)
P. Spinelli,
1
E. Kosten,
2
J. Parsons,
1
C. Van Lare,
1
J. Van de Groep,
1
J. Garcia de Abajo,
3
A. Polman,
1
and H. A. Atwater
2
1
Center for Nanophotonics, FOM-Institute AMOLF, Science Park 104, Amsterdam 1098XG, The Netherlands
2
Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena,
California 91125, USA
3
IO-CSIC, Serrano 121, Madrid 28006, Spain
(Received 8 August 2011; accepted 20 September 2011; published online 13 October 2011)
We have fabricated microphotonic parabolic light directors using two-photon lithography, thin-ﬁlm
processing, and aperture formation by focused ion beam lithography. Optical transmission
measurements through upright parabolic directors 22 lm high and 10 lm in diameter exhibit strong
beam directivity with a beam divergence of 5.6
!
, in reasonable agreement with ray-tracing and
full-ﬁeld electromagnetic simulations. The results indicate the suitability of microphotonic
parabolic light directors for producing collimated beams for applications in advanced solar cell and
light-emitting diode designs.
V
C
2011 American Institute of Physics. [doi:10.1063/1.3648115]
Planar micro- and nano-fabrication has enabled the fab-
rication of a wide array of planar integrated microphotonic
components, including mirrors,
1–3
lenses,
4
splitters,
5
array
waveguide grating ﬁlters,
6
and other integrated compone nts.
7
Microphotonic components which control of out-of-plane
beam propagation have been designed using planar fabrica-
tion methods, such as Bragg mirrors for use in conjunction
with vertical cavity surface emitting lasers.
8
Recently, plas-
monic out-of-plane beam steering and focusing structures
9–11
have also been designed using planar fabrication methods. In
many cases, however, the direct fabrication of three-
dimensional microphotonic components offers greater design
ﬂexibility for out-of-plane beam control.
Direct laser write lithography is a promising method to
fabricate three dimensional micro- and nano- photonic struc-
tures.
12
It has previously been used for fabrication of three
dimensional nanophotonic components such as infrared and
optical frequency metamaterials,
13
but less attention has
been devoted to design of three dimension microphotonic
components for beam opt ics. Three dimensional structure
fabrication by direct laser writing takes advantage of the
ability to expose a well-deﬁned nanoscale voxel using two-
photon processes that occur at the waist of a tightly focused
infrared beam in a photosensitive resist medium.
In this paper, we report the fabrication of three dimen-
sional microphotonic parabolic beam directors in the form of
compound parabolic concentrators via direct laser write li-
thography. Microphotonic parabolic light directors employ the
well-known focusing properties of compound parabolic con-
centrators, which have previously only been fabricated at the
macroscopic scale.
14–16
Arrays of microphotonic compound
parabolic concentrator structures could be used in low-proﬁle
format on top of solid-state photonic devices for a wide vari-
ety of optoelectronic applications such as collimation of light
emitted from light-emitting diodes or improving the efﬁciency
of solar cells by collimating the collected light in order to
reduce the entropy changes associated with light emission.
17
In order to create three-dimensional compound para-
bolic concentrator structures which redirect transmitted light,
a Nanoscribe photonic professional two-photon lithography
system was used.
18
The lithography system focuses an
expanded 785 nm laser beam to a diffraction-limited spot at
a point where it catalyzes a chemical change in a photoresist
(IP-L). This allows the user to write a single voxel instead of
a column, allowing three-dimensional structures to be writ-
ten. During fabrication, a glass substrate coated with opt ical
coupling ﬂuid on one side to facilitate index matching and
IP-L two photon negative photoresist on the other is placed
at the beam focus and the structure is written in the exposed
IP-L by moving the glass substrate. The exposed resist is
then developed in 2-propanol, a process during which all
unwritten IP-L and all optical coupling ﬂuid is disso lved. Af-
ter development, only the parabolic structures bonded to the
glass substrate remained. The structures are then coated with
FIG. 1. (a) SEM image of array of parabolic reﬂectors coated with silver;
(b) SEM image of single paraboloid; (c) plan view of single paraboloid with
light transmission aperture visible at bottom center; and (d) close up of light
aperture etched with a focused ion beam.
a)
Author to whom correspondence should be addressed. Electronic mail:
jhatwater@gmail.com.
0003-6951/2011/99(15)/151113/3/$30.00
V
C
2011 American Institute of Physics99, 151113-1
APPLIED PHYSICS LETTERS 99, 151113 (2011)
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20 nm of chromium and 380 nm of silver using a plasma

sputter coater. Following metal coating, optical apertures

with diameters 0.7, 1.1, and 1.3 lm were etched into the bot-

toms of the paraboloids using focused ion beam lithography.

The end result is an array of micron-scale, silver-surfaced

three-dimensional parabolic structures anchored to a silver-

coated substrate at the base of the paraboloids.

During the process of creating microphotonic parabolic

structures, several discoveries regarding optimum write tech-

niques were made. Paraboloids written with a wall thickness

of 1 voxel (approximately 80-100 nm) were found to be

unstable during post-exposure processing. To overcome this

problem, paraboloids with thicker walls were fabricated by

writing ﬁve overlapping paraboloids consisting of a center

paraboloid and four others offset in the x and y directions by

þ/#250 nm. The thickness of the metal ﬁlms used to coat the

paraboloids was also found to be an important parameter. In

fact, thin Ag ﬁlms yielded a non-uniform coating of the

parabolas from top to bottom, resulting in signiﬁcant losses

of light due to partial transparency of the metal ﬁlm at the

bottom. Thick Ag ﬁlms, on the other hand, resulted in

increased roughness on the inside and outside walls. The opti-

mal Ag thickness was found to be between 150 and 380 nm.

Figure 1(a) is an electron micrograph of silver-coated

22 lm high parabolic light directors spaced by 50 lm. Figure

1(b) is an enlarged view of a single paraboloid which illus-

trates the paraboloid wall thickness resulting from the over-

lap of exposed voxels in the writing process, visible as

concentric rings on the paraboloid, as well as the additional

surface roughness associated with the silver coating. Figure

1(c) is a top view of a single paraboloid that illustrates the

1.3 lm wide light aperture visible at the bottom center of the

structure. At the rim of the paraboloid, the wall thickness is

evident, as are the ripples created by the overlap of exposed

voxels during the multiple write process used to thicken the

paraboloid walls. Figure 1(d) is an enlarged view of the light

aperture which was etched in the bottom of the paraboloid

with focused ion beam nanolithography. It indicates that the

depth of the light aperture hole is approximately 1 lm and

extends into the glass layer.

Optical transmission measurements were performed on

parabolic light directors and reference apertures in the metal-

coated planar substrate with the same size as the ones in the

paraboloids. The light source was a broadband supercontin-

uum laser with approximately uniform intensity in the

spectral range between 450 nm and 1500 nm. A single-

wavelength source from an Ar ion laser operated at

k ¼ 514 nm was also used to study the emission of monochro-

matic light. Paraboloids and reference apertures were illumi-

nated from below through the glass substrate using a 25%

microscope objective with NA ¼ 0.5. The excitation spot was

FIG. 2. (Color online) Optical transmission images taken at different

heights for parabolic reﬂector and 1.3 lm reference aperture etched into sub-

strate 100 lm away with 20 x objective NA ¼ 0.75. Images (a)–(e): Parabo-

loid images with objective at heights z ¼ 0, 25, 50, 75, and 100 lm above

the substrate. Images (f)–(j): Reference aperture images with objective at

heights z ¼ 0, 25, 50, 75, and 100 lm above the substrate.

FIG. 3. (Color online) Directivity of transmission for parabolic light direc-

tors and reference apertures. FWHM of optical intensity proﬁles versus

objective lens height from transmission images taken at different heights for

parabolic reﬂector (red curve, at left) and reference aperture (blue curve, at

right). Dashed line indicates beam divergence half-angle from predictions of

ray tracing simulations. Solid line is the divergence half-angle obtained

from boundary element method full wave simulations.

151113-2 Atwater et al. Appl. Phys. Lett. 99, 151113 (2011)

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defocused to uniformly illuminate both the paraboloids and
the reference apertures. Images were taken from the top with
a 20% microscope objective with numerical aperture
NA ¼ 0.75 and a working distance of 1 mm. Figure 2 shows
514 nm intensity images taken at heights above the substrate
surface ranging from z ¼ 0–100 lm, for light transmitted
through a paraboloid with a 1.3 lm aperture (panels (a)–(e))
and through a reference 1.3 lm aperture in the substrate (pan-
els (f)–(j)). Care was taken to record an unsaturated image at
the z ¼ 0 position in Figs. 2(a) and 2(f), and identical illumi-
nation conditions were maintained for Figs. 2(b)–2(e) and
2(g)–2(j) as the distance between the substrate and objective
was varied. These images illustrate the ability of paraboloids
to collimate light transmitted through the 1.3 lm aperture in
the bottom into a tight beam in the far ﬁeld. These results
clearly indicate that the microphotonic paraboloid light direc-
tors can be used to create collimate beams from micron-scale
optical apertures.
Figure 3 shows a plot of the full width half maximum
(FWHM) proﬁle obtained from line cuts at the center of the
spots shown in Fig. 2 as a function of height above the sub-
strate. The red curve corresponds to the FWHM of the spot
imaged above the paraboloids while the blue curve corre-
sponds to the FWHM of the spot imaged above reference
aperture. The black dashed line in Figure 3 corresponds to
the 5.6
!
angular spread of light emitted from the paraboloids
predicted by ray tracing calculations. The black solid line
represents the emission angle obtained from boundary ele-
ment method (BEM) calculations. Single-wavelength trans-
mission measurements were also taken at wavelength
k ¼ 514 nm and showed similar results to those seen from the
white light source in Figs 2 and 3. The data in Fig. 3 clearly
show that the emission of light from the parabola is highly
directional and in good qualitative agreement with ray-
tracing and BEM calculations.
Simulations of the electric ﬁeld for a dipole emission
from the parabola were made using the boundary element
method. Figure 4 is an image of the calculated electric ﬁeld
intensity jEj
2
for emissi on from a circularly polarized dipole
situated at the inner bottom of a parabolic silver mirror. The
calculations suggest that the ﬁeld intensity at the top of the
parabola is quite spatial uniform inside the parabola and
remains conﬁned in a spatially uniform beam up to a height
of approximately 50 lm, in agreement with the results of
Figs. 2(b) and 2(c).
The authors are grateful for technical assistance from G.
Vollenbroek and M. Vrucinic. The Caltech researchers are
supported by the Light-Matter Interactions Energy Frontier
Research Center, an EFRC program of the Ofﬁce of Science,
United States Department of Energy under Grant No. DE-
SC0001293 (EDK and HAA). Researchers of the Center for
Nanophotonics at AMOLF are supported by the research
program of FOM which is ﬁnancially supported by NWO.
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FIG. 4. (Color online) Electric ﬁeld intensity jE(x,z)j
2
proﬁle at k ¼ 514 nm
for a radial cross section emerging from a Ag parabola illuminated by a
plane wave from bottom. Inset indicates ﬁeld intensity in vicinity of the pa-
rabola with 1.3 lm diameter aperture.
151113-3 Atwater et al. Appl. Phys. Lett. 99, 151113 (2011)
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Feb 1976

Roland Winston

The CPC is a nonimaging light funnel that derives its characteristic optical properties from the specific shape of the external wall, which is made specularly reflecting. The diameters of these structures are generally large relative to the wavelength of light. The author also investigated models of certain visual receptors, which strikingly resemble the compound parabolic shape and wherein the specularity was achieved by total internal reflection. In these examples, chosen to simulate the conditions found in nature, a fraction of the light rays leaks out because they fail to satisfy the condition for total internal reflection. The novel suggestion in this communication is the observation that for certain values of parameters (of considerable practical importance), the interface between the CPC and the surround becomes a perfect total internal reflection mirror that permits no leakage of radiation.

Direct laser writing

Article

Jan 2010

Nadya Anscombe

Start-up company Nanoscribe has developed table-top systems that can write intricate 3D structures not possible through other lithographic technologies. Nadya Anscombe finds out how the company was founded and what its plans are for the future.