Holographic printing

4 | IEEE nanotechnology magazine | september 2014 1932-4510/14©2014IEEE
OOVER THE last two decades,
we have witnessed an unprecedented
technological advance in nanotechnology.
By engineering at quantum-realm scales,
devices and material performances can be
optimized within a single characteristic
wavelength of an electron wave or opti-
cal wave to reach their ultimate potential,
which is limited only by the laws of phys-
ics. As such, products and services based
on nanotechnology, as well as the nano-
manufacturing technology that produces
them, are becoming increasingly impor-
tant to our economy.
At present, a large number of fabri-
cation approaches have been used to
produce functional nanostructures
and devices. They can be loosely clas-
sified as top-down and bottom-up
approaches [1], [2]. The bottom-up
approaches seek to build complex
structures through self-assembly
[2] using more fundamental nano-
components such as nanoparticles.
On the other hand, the top-down
photocourtesyoffreeimages.com/mailsparky
DI XU, ZSOLT POOLE, YUANKUN LIN, and KEVIN P. CHEN
Holographic Printing
of Three-Dimensional
Photonics Structures
Digital Object Identifier 10.1109/MNANO.2014.2326968
Date of publication: 11 July 2014
A very large-scale integration approach.

september 2014 | IEEE nanotechnology magazine | 5
approaches, such as deep ultraviolet (UV)
or e-beam lithography, sculpt out nano-
structures using bulk materials in a more
predictable way. Both the top-down and
the bottom-up approaches have yielded
great successes on planar surfaces for two-
dimensional (2-D) nanostructure fabrica-
tion. Nanostructures with a feature size of
10–45 nm can be produced commercially
at the 15-in wafer scale, such as cutting-
edge very large-scale integration (VLSI)
computer chips.
However, the fabrication of three-
dimensional (3-D) nanostructures
remains one of the greatest challenges for
the modern manufacturing industry.
Three-dimensional nanostructures with a
feature size less than a single wavelength
of an optical wave have found vast appli-
cations in energy, optical communication,
and sensing. Engineering 3-D structures
at the length scale, that is, a fraction of a
wavelength of characteristic quantum
waves, has yielded some amazing engi-
neering possibilities such as fundamental
alternation of blackbody radiation spec-
trum [3] for high-efficient thermal and
photovoltaic power conversation, nega-
tive index metal materials [4], ultrahigh
Q optical resonators for lasers [5], and
ultrasensitive optical sensors. All of these
applications are hinged on the develop-
ment of a reliable, low-cost, large-scale,
and tunable 3-D nanomanufacturing
technique for the low-cost, large-scale
production of nanostructures.
Currently, 3-D nanofabrication can be
addressed by both the top-down and the
bottom-up approaches. For example,
the construction of 3-D photonic crys-
tal structures can be deterministically pro-
duced using a top-down, layer-by-layer
lithography [1], [3], [6]–[13]. Alternative-
ly, the nanosphere self-assembly approach,
a bottom-up approach [2], [14]–[17], can
be used to construct 3-D nano-optical
structures with a length scale smaller than a
single wavelength of a plasmonic electron
wave. However, structures through such
construction rely mainly on the intersphere
interaction, which cannot be easily recon-
figured and optimized.
HOLOGRAPHIC LITHOGRAPHY
Another prominent 3-D nanofabrication
approach is achieved through multibeam
laser holographic lithography [18], which
is considered to be one of the most
attractive approaches due to its control-
lability, flexibility, and, most importantly,
scalability. Laser holographic lithography is
generally considered to be a top-down fab-
rication scheme. Three-dimensional nano-
structures are first defined in photoresist,
and functional materials, such as silicon,
are then produced through various infiltra-
tion and inversion processes [2], [17].
In contrast to the top-down, layer-by-
layer lithography processes, the laser
holographic fabrication has clear advan-
tages in terms of low complexity and low
manufacturing cost. While the layer-by-
layer approach often requires more than
ten laser exposures to lithographically
produce two to five unit cell structures,
the laser holographic fabrication can pro-
duce structures with identical symmetry
in one or two laser exposures.
Compared with bottom-up self-
assembly processing, the laser holo-
graphic approach can also be superior in
terms of scalability, controllability, and
configurability. It can produce 3-D
nanostructures with all symmetries
defined by the Bravis lattices, inscribe
functional defects, and be scaled up to
produce 3-D structures over a large area
in one or two laser exposures.
Generally speaking, the laser holo-
graphic lithography approach is achieved
through the angles, amplitudes, and
phase controls of multiple coherence
laser beams to form interference pat-
terns. The current technology to per-
form holographic lithography is not
without shortcomings. Traditionally,
multibeam interference was realized by a
large number of bulk optical components
such as mirrors, beam splitters, and lens-
es. These optical setups are among the
most difficult to align and are highly
susceptible to thermal and mechanical
variations. Furthermore, using these bulk
optical elements to produce holographic
patterns is incompatible with the photo-
lithography process used for most opto-
electronic chip fabrication, which often
uses one optical element such as a metal
mask. This incompatibility brings the
major challenge of integration of 3-D
photonic structures with other functions
in integrated circuit units.
Given these challenges, researchers
around the world have made concerted
efforts since 2004 to develop a holo-
graphic lithography technique that is sim-
pler, more robust, more flexible, and,
most importantly, VLSI compatible
[19]–[23]. These research efforts have
led to the development of holographic
lithography techniques using one optical
element and one laser exposure.
This article details these exciting
advances in laser holographic lithography.
In the last decade, various approaches
have been reported on the development
of laser holographic lithography using a
single optical element. Most of these
approaches use either a diffractive optical
element or a deflective optical element. In
this article, two examples are used to illus-
trate the essence of these fabrication
approaches and to highlight the associated
challenges. On the basis of the discussion
of the current state of the art, we will
present a solution for improving the fabri-
cation flexibility and robustness of the
laser lithography processes using adaptive
optics technology.
USING DIFFRACTIVE OPTICAL
ELEMENTS
Diffractive optical elements, such as surface
relief gratings on a glass plates, have long
been used for various optical applications.
Gradient structures can be
obtained by designing super-cells with
different gray levels in the SLM.

6 | IEEE nanotechnology magazine | september 2014
Diffractive optical elements have also been
used to produce interference fringes for the
purpose of laser fabrication. A prominent
example is the phase masks used in fiber
Bragg grating fabrication.
Figure 1 shows SEM images of rectan-
gular and hexagonal 2-D phase masks
fabricated in SU8 photoresists and their
optical diffraction patterns. The multiple
beams produced by the phase masks can
be used to construct 3-D interference
patterns. However, the fabrication of 3-D
photonic structures using a phase mask,
such as those shown in Figure 1, is not
straightforward. It is relatively easy to
generate multiple diffractive laser beams
with the desired angles and amplitude
using a diffractive optical element. How-
ever, the controls of phases among dif-
fractive beams are rather challenging, and
phase controls are critical to produce 3-D
photonic structures. Here, we use the
2-D phase mask shown in Figure 1(a) to
highlight this challenge.
The 2-D phase mask we used for the
3-D fabrication has a rectangular lattice
[Figure 2(a)]. The diffraction angle and
diffraction efficiency of four first-order
diffraction laser beams labeled (1,0),
(–1,0), (0,1), and (0, –1) relative to those
of (0, 0) order central beam were mea-
sured to be 20° and 10%, respectively.
The four second-order diffraction beams
had a much lower diffraction efficiency at
1.5%. The self-interference of five beams
produced by the 2-D phase mask shown
in Figure 2(b) yields face-centered-cubic
or FCT structures. The FCT structures
generated in photoresist are not intercon-
nected and will be destroyed after the
photoresist development. Furthermore,
the FCT structures do not possess a
sufficient symmetry to produce a large
photonic bandgap. A diamondlike lattice
structure is more desirable.
Figure 2(a) and (b) illustrates the path
for lattice translation from FCT to dia-
mondlike structures, which have the larg-
est bandgap among all photonic crystal
structures. The diamondlike structures can
be viewed as the superposition of two FCT
structures [20]. This can be achieved by
two exposures through the 2-D phase
mask on the same photoresist sample with
the phase mask displaced along the [201]
direction for a quarter of the FCT unit cell
diagonal length between exposures. The
phase mask offsets xT and zT [shown in
Figure 2(a)] between two exposure pat-
terns are . a0 5 and . ,c0 25 respectively,
where a and c are the lattice constants of
the FCT structure in the x and z direc-
tion, respectively.
Acc.V Spot Magn Det WD Exp
10.0 kV 3.0 6632x SE 29.2 1
5 mm
10.0 kV 3.013129x SE 28.7 1
2 mm
(a) (b)
(c) (d)
Figure 1 Scanning electron microscope (SEM) images and diffraction patterns of (a) and (b) rectangular and (c) and (d) hexagonal 2-D phase masks [3].

These displacements were performed
with high accuracy by three-axis high-
precision motorized linear stages with a
resolution of 150 nm. The simulated
isointensity surfaces of the first FCT, sec-
ond shifted FCT, and final superimposed
structures are shown in Figure 2(b).
The 3-D template was fabricated in a
thick SU8 film sample of 20 μm.
Figure 2(c) and (d) shows the SEM top
view of the diamondlike structures recorded
in SU8 [23]. The enlarged view of the sur-
face feature in Figure 2(d) is consistent with
the simulation result of the (001) plane of a
diamondlike structure. The cross-linking
between two FCT structures formed by the
two laser exposures produces a stable 3-D
template for further inversion processes to
create high-index contrast structures. Fig-
ure 2(e) shows the photonic band structure
for / .c a 1 5= for the silicon inverse struc-
ture. The filling faction of silicon to achieve
such an optimal 27% band gap, as shown in
Figure 2(e), is approximately 18.4%.
As we can see from this work, the cum-
bersome bulk optical elements for holo-
graphic lithography are all but eliminated.
Three-dimensional photonic crystal structure
templates can now be produced using one
diffractive optical element as shown above.
First Exposure
First FCT
Structure
Second FCT
Structure
Final
Diamondlike
Structure
Second Exposure
2-D Phase Mask
Photoresist
(1, –1)
(0,–1)
(–1, 0)
(–1, 1)
(0, 1)(1, 1)
(1, 0)
X
Z
Y
(a) (b)
(d)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Z
Z
U
T
X R P
Q
Q QP R RX T TU UZ Γ
Γ
Γ
Bloch Vector
a/λ
27%
3.8%
Inverse Diamondlike
Structure
Band 2
Band 3
Band 8
Band 9
Inverse FCT Structure
(e)
(c)
(–1,–1)
Central
Beam
Figure 2 (a) A sketch of the propagation of light through an orthogonal 2-D phase mask. (b) The diamondlike structure constructed by double
exposures with one face-centered-tetragonal (FCT) pattern shifted by .x 0 5aT = and . .z 0 25cT = An SEM (c) top view and (d) enlarged view of
the fabricated structures. (e) A photonic band diagram for the inverse FCT structure and FCT-based diamondlike structure [3].

However, this phase mask configura-
tion still involves two laser exposures and a
very precise mechanical movement of the
mask to produce the needed phase delay
to generate two FCT structures with a
proper displacement. This is because phase
masks based on surface gratings cannot set
phases for different diffractive beams. The
need for two laser exposures and a precise
mechanic movement is still difficult for
VLSI-style production.
USING DEFLECTIVE OPTICAL
ELEMENTS
Another way to produce interference
patterns from one optical element is to
use deflective optical elements. One such
optical device is a simple prism.
Deflective optical elements, such as
top-cut prisms, can also be used to pro-
duce and combine multiple laser beams
for holographic fabrication. We have
explored this technique for one laser
exposure 3-D fabrication.
The single optical element used to con-
struct the five-beam interference pattern is a
top-cut, four-sided prism, as shown in
Figure 3(a). One laser beam is incident
from the bottom side of the prism. After
being totally internally reflected at four lat-
eral surfaces of the prism, beams 2–5 refract
through the top surface of the prism and
recombine with beam 1 to form interfer-
ence patterns.
The key to producing the diamondlike
pattern is to control the phase of one
incoming laser beam. To perform the phase
modulation, a thin microscope glass cover
slide with a uniform thickness is inserted
into beam 5. By rotating the glass slide, the
phase of beam 5 can be adjusted continu-
ously. Figure 3(b) shows the variation of
unit cell lattices for the five-beam interfer-
ence pattern as the phase change Td evolves
from 0 to r in 0.2r increments. The evolu-
tion of the phase change Td transforms the
interference pattern from face-centered-
cubic or FCT structure into interconnected
structures. A perfect diamondlike network
can be formed with .Td r=
To experimentally validate the phase
tuning and structure controlling, 3-D pho-
tonic crystal templates were fabricated in
photoresist. Figure 4(a) shows an SEM
image of the photonic crystal template
formed in photoresist with diamondlike
structures by one laser exposure. As pre-
dicted by the simulation in Figure 4(b), the
fine phase tuning transforms the photonic
crystal template from the FCT to a dia-
mondlike symmetry. The interference pat-
tern locked in photoresist clearly shows
diamondlike interlaced structures in
Figure 4. The surface of the photoresist
film shown in Figure 4 is not completely
perpendicular to the normal incident laser
beam but, instead, is cut through the (001)
surface of the diamondlike structure at a
small angle. These results in surface
topology represent diamondlike structures
at different depths. The comparison
between the simulation and the experiment
shown in Figure 4 confirms this specula-
tion. Figure 4(g)–(j) shows the computed
five-beam interference pattern, where one
beam has a phase retardation of π relative
to the other four beams and its selected
cross-section planes along the height (z)
direction. The fabricated topographic
images shown in Figure 4(c)–(f) match
well with those simulation planes.
The results shown in Figures 1–4
demonstrated that the fabrication of
highly complex 3-D nanostructures
(e.g., diamondlike) can be accom-
plished with a single laser beam using a
diffractive/deflective optical element.
These works promise a truly scalable
nanofabrication technique for 3-D struc-
ture construction.
However, the nanofabrication tech-
niques displayed in the figures are not
without disadvantage. Neither a diffractive
optical element nor a deflective optical ele-
ment can provide full control of the phase,
amplitude, and divergence of individual
laser beams. This deficiency severely limits
the applicability of the diffractive/deflec-
tive optical elements in the laser holo-
graphic fabrication. Two techniques are
being studied to expand the reconfigu-
rability and applicability of the one-optical-
element hologram approach without
losing its intrinsic merits.
Rotation Stage
Top-Cut Prism
Glass Slide
4
2
1
3
θ
Beam Five Has
∆δ Phase Shift
Five-Beam Interference Region
0 0.2π 0.4π
0.6π 0.8π π
Low High
(a) (b)
Figure 3 (a) The experimental setup of the five-beam interference with one beam modulated by a glass slide. (b) The isosurface of the unit cells
of the phase modulated five-beam interference pattern Imod. The phase change Δδ varies from 0 to π in 0.2π increments [4].

PHASE MASKS WITH
BUILT-IN PHASE CHANGE
The phase mask approach is an attrac-
tive technique to fabricate 3-D photonic
structures because of its potential to be
complementary metal-oxide-semiconduc-
tor (CMOS)-compatible. The diffractive
optical elements used for 3-D photonic
structure construction can be incorporat-
ed into the photolithography amplitude
masks used in optoelectronic circuit fab-
rications and, thus, enable a full integra-
tion of 3-D photonic structures on-chip.
However, surface relief phase grating
cannot control the relative phase among
diffractive laser beams as demonstrated
earlier. This deficiency of the phase con-
trol has limited the phase mask approach
from the direct fabrication of some of
most important 3-D photonic structures
such as woodpile or diamondlike pho-
tonic crystal structures. The fabrication
of interconnected woodpile structures
was accomplished by double laser expo-
sures. The phase change, which is criti-
cal for the formation of interconnected
structures, was introduced by a precise
displacement (1/4 of the woodpile lat-
tice) of the recorded interference pat-
tern between two laser exposures. This
elaborated fabrication scheme reduces
the CMOS compatibility of the phase
mask approach.
To address this challenge, we have
developed a multilayer diffractive optical
element to reduce two laser exposures to
one exposure, which completely removes
the need for the phase mask displacement.
The phase mask used in this work is a
two-layer phase grating. Two phase grat-
ings with desired orientations are separat-
ed by a spacer layer. The desired phase
changes among different diffractive laser
beams can be controlled by the thickness
of the spacer layer. By doing so, the need
for precise phase mask displacement is
eliminated. Interconnected 3-D photonic
crystal structures, such as woodpile struc-
tures, can be directly fabricated in photo-
resist using one laser exposure [24]. This
critical improvement enables the fabrica-
tion of complex 3-D photonic structures
by a single laser exposure through a single
optical element.
10.0 kV 3.0 3500x SE 32.2 1
10 µm
0.9
0.7
0.5
0.3
0.1
2
2
3
1
1
0 0
1
2
Unit: µm
(c) (d)
(f)(e)
(a)
(b) (i) (j)
(e) (f)
(c) (d)
(g) (h)
Figure 4: (a) SEM images for the photoresist templates of the interconnecting diamondlike structure produced by the five-beam interference with
a phase retardation. (b) The computed five-beam interference pattern and its selected cross-section planes along the height (z) direction [4].

Figure 5(a) depicts the two-layer phase
grating fabrication process in SU8. The
polydimethylsiloxane (PDMS) grating
mold was first used to imprint grating pat-
terns on SU8-2035 photoresist under a
flood UV irradiation source. The exposed
SU8 was partially polymerized by postbak-
ing at 65 °C for 10 min. The PDMS mold
is then peeled off from the SU8.
To produce the second grating layer,
an SU8 thin film was coated directly on
the PDMS mold by a spin-coating pro-
cess as shown in Figure 5(a). The spin
speed determines the film thickness,
which is important to the “built-in”
phase delay of the mask. After the second
laser exposure on the SU8 film coated on
the PDMS mold, both SU8 layers are
brought into contact to form a two-layer
structure. The two-layer mask was bond-
ed at 95 °C for 20 min under 50 kPa
pressure [25]. The mask was further
hardened by a hard-baking process at
200 °C for another 20 min. The interme-
diate layer between two gratings is about
22 μm thick, which is produced by the
spin-coating process at 2,000 r/min. An
SEM image of a bound two-layer phase
mask is shown in Figure 3(b). A two-layer
grating with orthogonal orientations is
clearly visible with a spacer layer.
When a single beam goes through the
first layer of grating, it produces diffractive
beams in the x–z plane as shown in
Figure 5(b) labeled (0, 0), (1, 0), and
(−1, 0) orders. The (0, 0) beam incurs a
different phase from (±1, 0) beams
through the intermediate layer due to the
propagation path difference. The second
layer of grating further diffracts the beams
in the y–z plane to form nine diffractive
beams labeled (0, 0), (0, ±1), (±1, 0), and
(±1, ±1), respectively. Figure 5(c) shows
the diffraction pattern of the grating. Uni-
form diffractive beams were found, and
the intensity ratios were measured as 50%:
10%: 1.5% for the (0, 0) order: (±1, 0) and
(0, ±1) orders: (±1, ±1) orders, respective-
ly, which is perfectly consistent with the
simulation result.
The highest-order beams (±1, ±1) have
a much weaker intensity as measured and
have negligible effects on interference pat-
terns. Therefore, the two-layer phase masks
will produce five-beam interference pat-
terns by (0, 0), (±1, 0), and (0, ±1) beams.
When a plane wave propagates through the
top layer of the phase mask, phases for
beams (0, 0) and (0, ±1) are ( / )21d r m=
·n·d and /1 1d d= ,cosi respectively,
where n and d are the index of refraction
and the thickness of the spacer layer,
respectively. The (0, 0) order beam was
PDMS Grating Mold
Plasma Surface Bonding
Glass Substrate
Glass Substrate
Bottom SU8
Phase Grating
SU8 Gratings
Bonding
Orthogonal
Grating
OrientationsIntermediate SU8 Layer
Upper SU8
Phase Grating
Imprinting Spin-Coating
UV Flood
Exposure
UV Flood
Exposure
x
z
y
Intermediate Layer
Incident Beam
22 µm
δ = 0
δ2
θ
δ2δ1∆δ = δ2 – δ1
10.0 kV 3.0 1600x SE 26.1 1
20 µm
0th : 1st: 2nd = 50%: 10%: 1.5%
(a) (c)
(b)
Figure 5 (a) The fabrication processes of the two-layer phase mask. (b) An SEM image of a bonded SU8 two-layer mask with orthogonal grating
orientations. (c) The diffraction pattern of the two-layer phase mask.

further split into (0, 0) and (0, +/–1)
beams after propagating through the sec-
ond layer of the phase mask. Since the peri-
ods for both layers of grating are identical,
the phase difference between the (0, ±1)
and the (0, 0), (±1, 0) beams remains
2 1Td d d= - after the second layer of the
phase mask.
If the built-in phase delays Td for five
beams are zero or integral multiples of ,r
the generated interference pattern has
FCT symmetry, as shown in the 3-D sim-
ulation in the inset of Figure 6(a). When
Td are odd integral multiples of / ,2r
10, the 3-D template will evolve into the
interconnected woodpile structures
shown in Figure 6(b).
Figure 6(a) and (b) shows SEM imag-
es of the 3-D photonic crystal by using
the thermal controlled two-layer phase
mask. Figure 6(a) reveals FCT structures
when the phase delay 2 1Td d d= - was
not well-controlled. Since FCT structures
are not interconnected, only one layer of
period structures was left on the glass
slide after the photoresist development.
The fabricated structures match the simu-
lated structure with zero phase delay
.02 1Td d d= - = When the phase
delays are approaching those values of
odd multiples of / ,2r thicker and inter-
connected 3-D structures start to develop.
Figure 6(b) shows a woodpilelike 3-D
multilayer structure. The surface mor-
phology closely matches the simulation
results shown in the inset with a phase
delay of ./22 1Td d d r= - =
RECONFIGURABLE PHASE
MASKS USING ADAPTIVE
OPTICS ELEMENTS
In holographic fabrication, the above-
mentioned single diffractive optical
element or defection optical element
is considered a static mask, which
provides a fixed interference pattern.
Using an electronically programma-
ble spatial light modulator (SLM) as a
digitally tunable phase mask, 3-D holo-
graphic interference patterns can be
changed to yield a truly reconfigurable
and scalable 3-D fabrication tool, as
shown in Figure 7.
When a laser beam is incident onto the
SLM, the optical phase of each pixel of a
liquid crystal array can be adjusted in real
time to arbitrarily sculpt the wave front of
the incoming laser beam on the fly.
Through relay Fourier optics, the phase
information can be used to control the
phase, amplitude, and divergence angles
of the entire or part of the laser beam.
Figure 7(a) shows a phase control
approach that we have used. As an exam-
ple, a hexagon is divided into six equal-
area and rotationally symmetric sections
with each section having a different gray
level as shown in the inset of Figure
7(a). Such a hexagon pattern works as a
phase mask because different gray levels
correspond to different phases. When an
incident laser is reflected by such a mask,
we can obtain six first-order side beams
and one central beam, and other beams
are blocked by the filter. These beams
can form an interference pattern. After
demagnification and laser exposure, we
can obtain 3-D structures, as shown in
Figure 7(b). We can obtain 3-D struc-
tures with small feature size using large
demagnification optics. The desired
defects can be obtained by incorporating
intrinsic defects in the phase pattern dis-
played in the SLM. In general, the defect
in the interference pattern can be
obtained by assigning a constant gray
into the desired defect area. By
10.0 kV3.0 15000x SE 33.1 1
2 mm Acc.V Spot Magn Det WD Exp
10.0 kV 3.0 15000x SE 33.1 1
2 mm
FCT Woodpile
(a) (b)
Figure 6 SEM images of fabricated structures in the photoresist through an orthogonal two-layer phase mask with symmetries of (a) FCT and
(b) a woodpile structure. The insets are the simulation structures for comparison with parameter setting (a) 0Td = and (b) / .2Td r=

overlapping the zeroth-order diffracted
beam with the first-order diffracted
beams, 3-D photonic structures and a
positive defect line can be formed and
fabricated as shown in Figure 7(b).
We can also build up a phase pattern in
the SLM for 3-D woodpile structures and
gradient structures. As shown in
Figure 8(a), a checkerboard phase pattern
consists of square unit cells with each unit
cell divided into four square (8 # 8 mm2
)
pixels (the pixel size of a commercial
SLM). When the laser beam is diffracted
by the checkerboard pattern, one can get a
zeroth-order and four first-order laser
beams. Other diffraction orders can be fil-
tered out by a Fourier filter. The phases of
the diffracted first-order beams can be
digitally tuned by the gray levels in the
square unit cell. We can fabricate woodpile
structures as shown in Figure 8(b) when
the phase of two first-order beams is phase
delayed by /2r relative to two other first-
order beams by setting up gray levels of
30, 255, 30, and 255 for dark gray, light
gray, dark gray, and light gray pixels.
Gradient structures can be obtained
by designing super-cells with different
gray levels in the SLM. As shown in
Figure 8(a), there are 3 # 3 unit cells
inside the yellow or red squares. The unit
cell in the solid yellow square has gray
levels of 30, 94, 30, and 94 for dark gray,
light gray, dark gray, and light gray pix-
els, while the gray levels are 30, 255, 30,
and 255 for unit cells inside the red
squares. Figure 8(c) shows the recorded
interference pattern using the charge-
coupled device camera when the phase
pattern is illuminated by a 532-nm laser
through a 4f imaging system and a Fou-
rier filter [26].
For a comparison, we draw squares
around the interference pattern with the
same structures as shown in Figure 8(b).
The orientation of squares in the inter-
ference pattern is rotated by 45° com-
pared with the square orientation in the
phase pattern. The interference patterns
isolated by the yellow, blue, and red
squares in Figure 8(c) are determined
correspondingly pixel by pixel by the
phase pattern surrounded by the yellow,
blue, and red squares in Figure 8(a),
(a) (b) (c)
1 2 31 2 3Unit Cell
Figure 8 (a) A synthesized phase pattern consisting of two types of super cells with each super cell having 3 # 3 unit cells. (b) An atomic force
microscope image of fabricated woodpile structures. (c) A charge-coupled device image of the interference pattern formed by the diffracted
zeroth-order and four first-order beams from the phase pattern in (a).
Lens
SLM
Phase Mask
Incident Laser
Fourier Plane Filter
Objective Lens
Sample
Lens
(a) (b)
Figure 7 (a) The schematics of one of the spatial light modulator (SLM) setups. The phase mask is displayed in the SLM for dynamic wavefront
engineering. Interference patterns are formed after the second lens and its demagnification is realized through an objective lens. (b) An SEM image of a
fabricated sample using SLM.

respectively. Inside the yellow and red
squares in Figure 8(c), the formed struc-
tures are similar to the simulated struc-
tures in Figure 3(b) (0 and 1p phase
shift), respectively. Thus, gradient struc-
tures can be obtained using the gradient
phase patterns displayed in the SLM.
CONCLUSION
This article describes the transformation-
al changes that have occurred in the last
few years in the field of laser holograph-
ic lithography. As the nanotechnology
industry demands a scalable manufactur-
ing solution for 3-D nanostructures and
devices, researchers around the world are
answering the call. The persistent efforts
and ingenuities of scientists and engi-
neers have transformed laser holographic
lithography into a one-optical-element
and one-laser-exposure process that is
amendable into the existing VLSI fab-
rication scheme. Leveraged by advances
in other fields of optics technology such
as adaptive optics, the laser holographic
lithography will continue to evolve into
a simple, robust, flexible, and scalable
manufacturing tool for sophisticated 3-D
nanostructures and devices.
ACKNOWLEDGMENTS
This work was supported by the U.S.
National Science Foundation under grants
CMMI-1300273, CMMI-0900564,
CMMI-0923006, CMMI-1109971,
CMMI-1266251, and DMR-0722754.
Support was also received from the U.S.
Air Force.
ABOUT THE AUTHORS
Di Xu (xudipitt@gmail.com) earned his
B.S. and M.S. degrees in physics from
Wuhan University, China, in 2001 and
2004, and his Ph.D. degree in electrical
and computer engineering from the Uni-
versity of Pittsburgh in 2010. After gradu-
ation, he worked as a scientist at Stanford
Linear Accelerator Center National Lab-
oratory. He is currently a senior optical
etrology engineer at Intel Corporation.
Zsolt Poole (zlp2@pitt.edu) is currently
pursuing his Ph.D. degree at the Univer-
sity of Pittsburgh in electrical engineering
while conducting research on the fabrica-
tion and applications of nanoengineered
subwavelength photonic materials.
Yuankun Lin (yuankun.lin@unt.
edu) earned his B.S. and M.S. degrees in
physics from Nankai University, China,
in 1991 and 1994, respectively, and his
Ph.D. degree in physics from the Univer-
sity of British Columbia in 2000. He is
now an associate professor in physics and
electrical engineering at the University of
North Texas.
Kevin P. Chen (pec9@pitt.edu) earned
his B.S. degrees in physics and control
science from Xiamen University, China, in
1994, his M.Sc. degree in physics from the
University of British Columbia in 1998,
and his Ph.D. degree in electrical engi-
neering from the University of Toronto
in 2002. He is an associate professor and
the Paul E. Lego Faculty Fellow in electri-
cal engineering and bioengineering at the
University of Pittsburgh.
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