of 17
Reviewers' comments:
Reviewer #1 (Remarks to the Author):
The manuscript titled ‘An optical metasurface planar camera’ by Arbabi et al, details theoretical and
experimental investigations into the development of a miniature camera, dimensions1.6 x 1.6 x 1.7
mm, employing a ‘doublet metasurface’ to perform imaging
rather than using a conventional lens.
They claim it to be ‘nearly diffraction limited’ over a 60 x 60 degree field
-
of
-
view, which to a large
extent they evidence in their experimental results. This work is an extension on previously reported
work by the
authors (most recently ‘Multiwavelength polarization
-
insensitive lenses based on dielectric
metasurfaces with meta
-
molecules, Optica, 2016), however here the authors report a modified design,
utilizing two metasurfaces, optimized for 850nm wavelength light
.
Demonstration of a working camera of this sort, albeit with very limited wavelength operability, is
novel and the results of interest to researchers involved in experimental optics, imaging, and applied
physics among other fields.
The manuscript is cl
early written and the results are well presented. The results appear to be valid and
the methodology is appropriate.
I have some concerns about some of the claims the authors make regarding the impact of
metasurfaces in their intended applications, since
they do not make clear what the scale of the cost
saving to be compared to more conventional imaging technologies. Or if instead it is the small form
factor that is the key strength, how small can these ‘metasurface planar cameras’ be?
There are a few s
pecific issues the authors should address by making modifications to the manuscript
or by clarifying in their response, after which I would consider this work suitable for publication in
Nature Communications.
Detailed comments:
-
page 3, paragraph 2: t
he authors state that the metasurfaces are polarization insensitive yet on
page 4, paragraph 3 they describe the focusing efficiency as polarization sensitive. This needs better
clarification.
-
page 4, paragraph 2: based on the results shown in Fig 3b, o
ne could argue that the at 30 degrees
the focused spot does not look diffraction limited, but instead closer to 20 degrees, but in comparison
to the singlet design there is certainly a significant improvement. Therefore, what criteria did the
authors use t
o determine when exactly the spot is not considered diffraction limited?
-
page 4, paragraph 3: to determine the focusing efficiency the authors use the ratio of focused power
to incident power, but do not explain if a mask is used to determine the area o
f interest for this
measurement. Please can they clarify this and provide detail as necessary.
-
page 5, paragraph 2: the authors indicate that a spectral filter is employed subsequent to the
metasurface, but in later investigations whereupon the metasurf
ace is in close proximity to the sensor,
the filter is placed after the illumination before the metasurface. Can the authors comment on whether
they would expect any variation in their results, specifically regarding the MTF.
-
page 5, paragraph 2: the au
thors should provide another sentence or two when they introduce
modulation transfer functions, to clearly outline their subsequent use within the manuscript, or at least
provide further details in the supplementary materials.
-
page 6, paragraph 1: the a
uthors indicate that a smaller pixel size of 0.4 micrometers would lead to
improved image quality based on the MTFs, theorizing that eventually pixels will reach this scale. The
authors must be aware that this is an order of magnitude smaller than today’s
miniature sensors,
which are themselves struggling for SNR due to reduced light gathering power compared to the noise
floor. I recommend a cautionary sentence to add to this paragraph, which at least acknowledges this.
-
In neither the introduction nor di
scussion to put their work into context, do the authors make any
reference to other, more conventional but comparatively smaller cameras, which arguably do not have
such limited operational spectral bandwidth (e.g. https://www.fraunhofer.de/en/press/resear
ch
-
news/2011/march/camer as
-
out
-
of
-
the
-
salt
-
shaker.html). Perhaps they should consider highlighting
briefly the ‘state of the art’ in miniature cameras, or detail the specific limitations that metasurface
based devices will potentially overcome.
Reviewe
r #2 (Remarks to the Author):
Dielectric metasurfaces for the visible spectral range are promising candidates for novel integrated
devices and optical elements. Here, Arabi et al. present a similar concept for realizing a metalens as
recently published b
y Khorasaninejad et al. in Science. However, the work distinguishes slightly as it
does not use the Berry phase effect for generating the phase pattern but a propagation effect to
obtain the desired phase. A further difference is the material system. The a
uthors used silicon
nanostructures in their study which are easier to handle in the fabrication process and would also have
advantages for a real commercial application by easier integration with silicon based cameras. I found
the idea and demonstration of
the entire imaging system with the camera chip very impressive. In my
opinion this is the first demonstration of a real application with metasurfaces. Such demonstration of
integrated devices was not obtained by the Harvard group.
The work nicely demonst
rates the potential of such dielectric metasurfaces and the realization of a
doublet lens system underlines the power of such approaches. Therefore, I will recommend the
publication of the manuscript in Nature Communications. I personally believe that the
presented
approach here will have a greater impact on real imaging systems for particular applications than the
recently published work by Khorasaninejad. Overall, the manuscript is well written and the details for
the fabrication process are extensively e
xplained. The discussion of the transfer function gives a good
inside into the performance of the device. However, I would recommend to move part of the
fabrication and the measurement procedure to the supplementary file.
There are only a few comments for the authors which they should take into account for a revision:
On page 6 it is stated that ‘The intensity of the image formed by a camera only depends on the NA of
its lens.’ This statement is in my opinion wrong. The in
tensity of the image is given by the ration of
the diameter of the entrance pupil to the focal length, which is the inverse f
-
number.
The distance between the metalens and the CCD chip seems to be important and it was taken care
about that in the design a
s demonstrate in the manuscript. However, I could not find any statement of
how the authors did this precise alignment for the measurement? Here it would be helpful to add
some information.
It seems like the simulations were performed only with a real par
t of the refractive index. What is the
influence of the imaginary part here? Why can it be neglected?
Supplementary Figure 2a shows the laser spectrum. Why is there such a strong modulation in the
spectrum that looks like an interference effect?
Revie
wer #3 (Remarks to the Author):
Authors report in this paper a compact camera that utilizes a flat metasurface doublet lens to deliver
nearly diffraction
-
limited performance within the field
-
of
-
view of 60 degree x 60 degree. The overall
dimensions of the
camera (including the image sensor) are 1.6 mm x 1.6 mm x 1.7 mm. The meta
surface doublet lens consists of one metasurface corrector plate and one metasurface focusing lens.
The phase profiles of both metasurface lenses have been optimized to collectivel
y reduce the
monochromatic aberrations. As the results, the performance of the metasuface doublet has been
significantly improved in comparison with the singlet lens. This is a significant step in developing a
high performance flat lens for the purpose of
optical imaging, as opposite to the focusing being
demonstrated before. Additionally, the metasufrace doublet has been conveniently fabricated on the
both side of 1mm thick quartz substrate with the alignment accuracy of 2 um. It eliminates the post
-
fabric
ation alignment procedure and thus, makes it possible for potential vertical integration using the
well
-
established micro
-
fabrication capabilities. Just for the curiosity, can author explain of the whether
the functions of the two metasurface lenses can be
combined into one metasurface lens with
aspherical phase profile? Furthermore, the title of “An optical metasurface planar camera” is not very
accurate. The demonstrated camera has the shape close to a cubic so it is hard to define it as a “flat
camera”.
The lens being used is the flat one though. Overall, the manuscript is well written and the
reported work is of the great interest to the readers. I would recommend the paper to be accepted for
publication with minor revision.
Reviewer #4 (Remarks to t
he Author):
The authors describe in their paper a planar single
-
layer and double
-
layer lens based on optical
metasurfaces. Furthermore they combine their proposed metasurface doublet lens with a commercial
CMOS image sensor.
The used approach for produ
cing the metasurface lenses is not restricted to laboratory prototypes. A
high
-
volume fabrication with thin
-
film production line is possible, which might yield to a high economic
impact of the proposed lenses.
Additionally the authors combine their propo
sed lens with a commercial CMOS image sensor.
Compared to the state
-
of
-
the
-
art they replace a "classical" lens with their proposed one. However the
authors should more clearly explain the advantages of their lens for the camera module. From a
production po
int of view for the camera module I do not see a significant advantage. The assembly of
the lens together with the CMOS sensor still needs to be done in the backend with a very similar
process.
A topic not mentioned in the paper is the influence of the a
lignment of the nano
-
posts of the lens and
the bayer pattern of the CMOS sensor.
Some further remarks:
-
Fig. 2b: what's the reason for the low transmission values
-
Non consistent wording for the substrate of the lens: "fused silica" vs. "glass" might
be a bit
confusing
-
p. 7: "high
-
throughput nano
-
fabrication techniques" is in my opinion a bit misleading. The fabrication
of the lens together with the CMOS sensor in the frontend is in my opinion not feasible. Instead both
have to be produced separate
ly in different frontend processes and then combined in the backend
All in all the paper describes a novel and interesting approach for planar lenses which the possibility
for a high impact. In addition the authors describe a planar camera module with th
eir planar lenses.
This approach is also interesting, but the advantages of the proposed solution are not completely clear
for me.
Our
response
to
the
reviewers’
comments
and
the
corresponding
text
from
the
manuscript
are
presented
below
in
blue
and
green
fonts,
respectively.
Reviewer
#1
comment:
The
manuscript
titled
‘An
optical
metasurface
planar
camera’
by
Arbabi
et
al,
details
theoretical
and
experimental
investigations
into
the
development
of
a
miniature
camera,
dimensions1.6
x
1.6
x
1.7
mm,
employing
a
‘doublet
metasurface’
to
perform
imaging
rather
than
using
a
conventional
lens.
They
claim
it
to
be
‘nearly
diffraction
limited’
over
a
60
x
60
degree
field
of
view,
which
to
a
large
extent
they
evidence
in
their
experimental
results.
This
work
is
an
extension
on
previously
reported
work
by
the
authors
(most
recently
‘Multiwavelength
polarization
insensitive
lenses
based
on
dielectric
metasurfaces
with
meta
molecules,
Optica,
2016),
however
here
the
authors
report
a
modified
design,
utilizing
two
metasurfaces,
optimized
for
850nm
wavelength
light.
Demonstration
of
a
working
camera
of
this
sort,
albeit
with
very
limited
wavelength
operability,
is
novel
and
the
results
of
interest
to
researchers
involved
in
experimental
optics,
imaging,
and
applied
physics
among
other
fields.
The
manuscript
is
clearly
written
and
the
results
are
well
presented.
The
results
appear
to
be
valid
and
the
methodology
is
appropriate.
Our
response:
We
thank
the
reviewer
for
carefully
reading
and
summarizing
the
manuscript.
Reviewer
#1
comment:
I
have
some
concerns
about
some
of
the
claims
the
authors
make
regarding
the
impact
of
metasurfaces
in
their
intended
applications,
since
they
do
not
make
clear
what
the
scale
of
the
cost
saving
to
be
compared
to
more
conventional
imaging
technologies.
Our
response:
The
metasurface
camera
lenses
we
present
in
the
manuscript
have
several
advantages
over
conventional
bulk
lenses,
namely
high
imaging
quality
with
small
and
flat
form
factor,
small
f
number,
high
scalability
of
the
fabrication
process
allowing
for
batch
fabrication
of
tens
of
thousands
of
camera
lenses
on
a
same
wafer,
and
elimination
of
post
fabrication
alignment
and
assembly
steps
required
for
fabrication
of
camera
lenses.
We
are
unable
to
provide
an
accurate
cost
saving
figures
at
this
stage
of
the
project,
but
we
expect
the
proposed
camera
lens
to
be
more
cost
effective
than
the
conventional
solution
with
the
similar
specifications
because
the
wafer
level
fabrication
significantly
benefits
from
the
economy
of
scale.
We
have
now
further
clarified
the
advantages
of
the
metasurface
doublet
over
conventional
solution:
“The
metasurface
enabled
camera
we
reported
here
has
a
flat
and
thin
form
factor,
small
f
number,
exhibits
nearly
diffraction
limited
performance
over
a
large
field
of
view.
From
a
manufacturing
standpoint,
the
metasurface
doublets
have
several
advantages
over
conventional
lens
modules.
Conventional
lens
modules
are
composed
of
multiple
lenses
which
are
separately
manufactured
and
later
aligned
and
assembled
together
to
form
the
module.
On
the
other
hand,
the
metasurface
doublets
are
batch
manufactured
with
simultaneous
fabrication
of
tens
of
thousands
of
doublets
on
each
wafer,
and
with
the
metasurfaces
aligned
to
each
other
using
lithographic
steps
during
fabrication.
Furthermore,
the
assembly
of
the
conventional
lens
modules
with
the
image
sensors
has
to
be
done
in
a
back
end
step,
but
the
metasurface
doublet
can
be
monolithically
stacked
on
top
of
image
sensors.”
Reviewer
#1
comment:
Or
if
instead
it
is
the
small
form
factor
that
is
the
key
strength,
how
small
can
these
‘metasurface
planar
cameras’
be?
To
address
the
reviewer’s
question
regarding
the
size
scaling
of
these
cameras,
we
added
the
following
to
the
Discussion
section
of
the
manuscript:
“The
metasurface
doublet
lens
and
camera
can
be
further
miniaturized
by
reducing
the
thickness
of
the
substrate,
the
diameters
of
the
metasurface
lenses,
the
focal
length
of
the
lens,
and
the
distance
to
the
image
sensor
by
the
same
scale
factor,
while
using
the
same
nano
post
metasurface
design
presented
in
Fig.
2.
For
example,
a
10×
smaller
camera
(160
μ
m
×
160
μ
m
×
170
μ
m)
can
be
designed
and
fabricated
using
a
similar
procedure
on
a
100
‐μ
m
thick
fused
silica
substrate.
Such
a
camera
would
have
10×
larger
bandwidth
compared
to
the
miniature
camera
presented
here,
the
same
image
plane
intensity,
but
with
10×
smaller
image
and
100×
lower
number
of
distinguishable
pixels
(94×94
pixels
instead
of
940×940).
Reviewer
#1
comment:
There
are
a
few
specific
issues
the
authors
should
address
by
making
modifications
to
the
manuscript
or
by
clarifying
in
their
response,
after
which
I
would
consider
this
work
suitable
for
publication
in
Nature
Communications.
Detailed
comments:
page
3,
paragraph
2:
the
authors
state
that
the
metasurfaces
are
polarization
insensitive
yet
on
page
4,
paragraph
3
they
describe
the
focusing
efficiency
as
polarization
sensitive.
This
needs
better
clarification.
Our
response:
The
metasurfaces
are
polarization
insensitive
upon
normal
incidence.
This
is
similar
to
transmission
through
the
planar
interface
between
two
materials
which
is
polarization
insensitive
upon
normal
incidence,
but
depends
on
the
polarization
for
non
zero
incident
angles.
We
refer
to
our
metasurface
design
as
polarization
insensitive
to
distinguish
them
from
metasurfaces
which
work
only
with
one
polarization
(such
as
the
ones
which
use
geometric
phase).
For
clarification,
we
added
the
following
to
the
manuscript:
“The
metasurfaces
are
polarization
insensitive
at
normal
incidence,
but
their
diffraction
efficiency
depends
on
the
polarization
of
incident
light
for
non
zero
incident
angles.”
Reviewer
#1
comment:
page
4,
paragraph
2:
based
on
the
results
shown
in
Fig
3b,
one
could
argue
that
the
at
30
degrees
the
focused
spot
does
not
look
diffraction
limited,
but
instead
closer
to
20
degrees,
but
in
comparison
to
the
singlet
design
there
is
certainly
a
significant
improvement.
Therefore,
what
criteria
did
the
authors
use
to
determine
when
exactly
the
spot
is
not
considered
diffraction
limited?
Our
response:
We
thank
the
reviewer
for
bringing
up
this
point
and
we
agree
with
them
that
a
quantitative
measure
should
be
used
for
determining
nearly
diffraction
limited
focusing.
A
widely
accepted
metric
for
the
focusing
quality
is
Strehl
ratio
which
is
the
ratio
between
the
volume
under
the
2D
MTF
of
a
lens
to
the
volume
under
the
2D
MTF
of
the
diffraction
limited
lens
with
the
same
NA.
We
computed
the
Strehl
ratio
for
the
doublet
and
singlet
and
added
them
as
Supplementary
Fig.1.
We
chose
a
threshold
of
0.9
for
the
Strehl
ratio
to
refer
to
a
focal
spot
as
nearly
diffraction
limited.
With
this
criterion,
as
the
Supplementary
Fig.
1
shows,
the
designed
doublet
is
nearly
diffraction
limited
up
to
more
than
25°
(but
less
than
30°).
In
addition
to
adding
the
Supplementary
Fig.
1,
we
revised
the
manuscript
and
added
the
following
explanation:
“The
doublet
lens
has
a
nearly
diffraction
limited
focal
spot
for
incident
angles
up
to
more
than
25°
(with
the
criterion
of
Strehl
ratio
of
larger
than
0.9,
see
Supplementary
Fig.
1)
while
the
singlet
exhibits
significant
aberrations
even
at
incident
angles
of
a
few
degrees.”
Reviewer
#1
comment:
page
4,
paragraph
3:
to
determine
the
focusing
efficiency
the
authors
use
the
ratio
of
focused
power
to
incident
power,
but
do
not
explain
if
a
mask
is
used
to
determine
the
area
of
interest
for
this
measurement.
Please
can
they
clarify
this
and
provide
detail
as
necessary.
Our
response:
We
used
a
mask
when
measuring
the
focusing
efficiency
and
the
procedure
was
detailed
in
the
Methods
section
(second
and
third
paragraphs
of
the
“Measurement
procedure
and
data
analysis”
subsection):
“The
focusing
efficiency
for
the
normal
incidence
(zero
incident
angle)
was
measured
by
placing
a
15
μ
m
diameter
pinhole
in
the
focal
plane
of
the
doublet
lens
and
measuring
the
optical
power
passed
through
the
pinhole
and
dividing
it
by
the
power
of
the
incident
optical
beam.”
Reviewer
#1
comment:
page
5,
paragraph
2:
the
authors
indicate
that
a
spectral
filter
is
employed
subsequent
to
the
metasurface,
but
in
later
investigations
whereupon
the
metasurface
is
in
close
proximity
to
the
sensor,
the
filter
is
placed
after
the
illumination
before
the
metasurface.
Can
the
authors
comment
on
whether
they
would
expect
any
variation
in
their
results,
specifically
regarding
the
MTF.
Our
response:
Placing
the
filter
between
the
objective
and
the
tube
lens
did
not
create
any
detectable
aberrations
because
the
objective
lenses
used
in
the
measurements
where
infinity
corrected.
We
added
the
following
to
the
Methods
section
to
clarify
this:
“The
placement
of
the
filter
between
the
objective
and
the
tube
lens
did
not
introduce
any
discernible
aberrations
to
the
optical
system.”
Reviewer
#1
comment:
page
5,
paragraph
2:
the
authors
should
provide
another
sentence
or
two
when
they
introduce
modulation
transfer
functions,
to
clearly
outline
their
subsequent
use
within
the
manuscript,
or
at
least
provide
further
details
in
the
supplementary
materials.
Our
response:
Per
reviewer’s
request,
we
expanded
the
explanation
of
the
MTF
by
adding
the
following
to
the
manuscript:
“Any
imaging
system
can
be
considered
as
low
pass
spatial
filter
whose
transfer
function
varies
across
the
field
of
view.
For
incoherent
imaging
systems,
the
transfer
function
for
each
point
in
the
field
of
view
can
be
obtained
by
computing
the
Fourier
transform
of
the
focal
spot
intensity.
The
modulus
of
this
transfer
function
is
referred
to
as
the
modulation
transfer
function
(MTF)
and
represents
the
relative
contrast
of
the
image
versus
the
spatial
details
of
the
object.”
Reviewer
#1
comment:
page
6,
paragraph
1:
the
authors
indicate
that
a
smaller
pixel
size
of
0.4
micrometers
would
lead
to
improved
image
quality
based
on
the
MTFs,
theorizing
that
eventually
pixels
will
reach
this
scale.
The
authors
must
be
aware
that
this
is
an
order
of
magnitude
smaller
than
today’s
miniature
sensors,
which
are
themselves
struggling
for
SNR
due
to
reduced
light
gathering
power
compared
to
the
noise
floor.
I
recommend
a
cautionary
sentence
to
add
to
this
paragraph,
which
at
least
acknowledges
this.
Our
response:
In
contrast
to
the
reviewer’s
comment,
we
do
not
theorize
or
predict
that
eventually
pixels
will
reach
0.4
μ
m.
The
part
of
the
manuscript
the
reviewer
is
referring
to
read
as:
“The
camera's
image
quality
is
reduced
by
the
nonuniform
sensitivity
of
the
image
sensor
pixels
to
the
850
nm
light
due
to
the
color
filters,
and
by
its
larger
than
optimal
pixel
size.
Therefore,
the
image
quality
can
be
improved
by
using
a
monochromatic
image
sensor
with
a
smaller
pixel
size
(which
is
0.4
μ
m
based
on
the
MTFs
shown
in
Fig.
4d).
Thus,
the
miniature
camera
benefits
from
the
current
technological
trend
in
pixel
size
reduction.”
In
the
above
statement
we
claim
that:
1.
The
optimum
pixel
size
for
the
miniature
camera
is
0.4
μ
m
2.
The
miniature
camera
benefits
from
reducing
pixel
size
3.
There
is
a
technological
trend
in
pixel
size
reduction
We
agree
with
the
reviewer
that
there
are
technological
challenges
in
pixel
size
reduction,
however;
we
note
that
the
pixel
sizes
of
today’s
miniature
sensors
are
not
an
order
of
magnitude
larger
than
0.4
μ
m.
For
example,
the
CMOS
image
sensor
we
used
in
our
study
has
pixel
size
of
1.4
μ
m,
or
the
pixel
size
for
the
Samsung’s
S5K3L2
image
sensor
which
is
used
in
current
cell
phone
cameras
is
equal
to
1.12
μ
m.
We
revised
the
statement
to
eliminate
any
potential
ambiguity:
“Therefore,
the
image
quality
can
be
improved
by
using
a
monochromatic
image
sensor
with
a
smaller
pixel
size
(the
optimum
pixel
size
for
the
miniature
camera
is
0.4
μ
m
based
on
the
MTFs
shown
in
Fig.
4d).
Thus,
the
miniature
camera
benefits
from
the
current
technological
trend
in
pixel
size
reduction.”
Reviewer
#1
comment:
In
neither
the
introduction
nor
discussion
to
put
their
work
into
context,
do
the
authors
make
any
reference
to
other,
more
conventional
but
comparatively
smaller
cameras,
which
arguably
do
not
have
such
limited
operational
spectral
bandwidth
(e.g.
https://www.fraunhofer.de/en/press/research
news/2011/march/cameras
out
of
the
salt
shaker.html).
Perhaps
they
should
consider
highlighting
briefly
the
‘state
of
the
art’
in
miniature
cameras,
or
detail
the
specific
limitations
that
metasurface
based
devices
will
potentially
overcome.
Our
response:
The
main
focus
of
the
manuscript
and
its
main
contribution
is
not
to
realize
the
smallest
camera,
but
to
demonstrate
how
monochromatic
aberrations
of
metasurface
lenses
can
be
effectively
eliminated
only
by
using
two
cascaded
metasurfaces.
In
addition
to
its
small
size
and
the
potential
for
further
miniaturization,
the
metasurface
doublet
has
several
advantages
over
conventional
lens
modules
that
we
have
now
explained
more
clearly
in
the
discussion
section
of
the
manuscript:
“The
metasurface
enabled
camera
we
reported
here
has
a
flat
and
thin
form
factor,
small
f
number,
exhibits
nearly
diffraction
limited
performance
over
a
large
field
of
view.
From
a
manufacturing
standpoint,
the
metasurface
doublets
have
several
advantages
over
conventional
lens
modules.
Conventional
lens
modules
are
composed
of
multiple
lenses
which
are
separately
manufactured
and
later
aligned
and
assembled
together
to
form
the
module.
On
the
other
hand,
the
metasurface
doublets
are
batch
manufactured
with
simultaneous
fabrication
of
tens
of
thousands
of
doublets
on
each
wafer,
and
with
the
metasurfaces
aligned
to
each
other
using
lithographic
steps
during
fabrication.
Furthermore,
the
assembly
of
the
conventional
lens
modules
with
the
image
sensors
has
to
be
done
in
a
back
end
step,
but
the
metasurface
doublet
can
be
monolithically
stacked
on
top
of
image
sensors.”
Nevertheless,
because
the
miniature
size
is
one
of
the
attractive
features
of
the
metasurface
cameras,
we
also
added
the
following
to
the
Discussion
section
and
compared
features
of
the
metasurface
doublet
and
other
state
of
the
art
miniature
lenses:
“Compared
to
other
miniature
lenses
reported
previously
[24
27],
the
metasurface
doublet
offers
significantly
smaller
f
number
and
better
correction
for
monochromatic
aberrations
which
lead
to
brighter
images
and
higher
resolution,
but
they
have
larger
chromatic
aberration
and
narrower
bandwidth.”
Reviewer
#2
comment:
Dielectric
metasurfaces
for
the
visible
spectral
range
are
promising
candidates
for
novel
integrated
devices
and
optical
elements.
Here,
Arabi
et
al.
present
a
similar
concept
for
realizing
a
metalens
as
recently
published
by
Khorasaninejad
et
al.
in
Science.
However,
the
work
distinguishes
slightly
as
it
does
not
use
the
Berry
phase
effect
for
generating
the
phase
pattern
but
a
propagation
effect
to
obtain
the
desired
phase.
A
further
difference
is
the
material
system.
The
authors
used
silicon
nanostructures
in
their
study
which
are
easier
to
handle
in
the
fabrication
process
and
would
also
have
advantages
for
a
real
commercial
application
by
easier
integration
with
silicon
based
cameras.
I
found
the
idea
and
demonstration
of
the
entire
imaging
system
with
the
camera
chip
very
impressive.
In
my
opinion
this
is
the
first
demonstration
of
a
real
application
with
metasurfaces.
Such
demonstration
of
integrated
devices
was
not
obtained
by
the
Harvard
group.
Our
response:
We
thank
the
reviewer
for
carefully
reading
the
manuscript
and
for
comparing
it
to
a
recent
work
in
this
field.
Reviewer
#2
comment:
The
work
nicely
demonstrates
the
potential
of
such
dielectric
metasurfaces
and
the
realization
of
a
doublet
lens
system
underlines
the
power
of
such
approaches.
Therefore,
I
will
recommend
the
publication
of
the
manuscript
in
Nature
Communications.
I
personally
believe
that
the
presented
approach
here
will
have
a
greater
impact
on
real
imaging
systems
for
particular
applications
than
the
recently
published
work
by
Khorasaninejad.
Overall,
the
manuscript
is
well
written
and
the
details
for
the
fabrication
process
are
extensively
explained.
The
discussion
of
the
transfer
function
gives
a
good
inside
into
the
performance
of
the
device.
Our
response:
We
are
glad
that
the
reviewer
appreciates
the
contributions
of
the
current
manuscript.
Reviewer
#2
comment:
However,
I
would
recommend
to
move
part
of
the
fabrication
and
the
measurement
procedure
to
the
supplementary
file.
Our
response:
The
details
of
fabrication
and
measurement
procedures
are
currently
not
part
of
the
main
text
and
are
included
in
the
Methods
section
to
comply
with
the
Nature
Communications
format
requirements.
Reviewer
#2
comment:
There
are
only
a
few
comments
for
the
authors
which
they
should
take
into
account
for
a
revision:
On
page
6
it
is
stated
that
‘The
intensity
of
the
image
formed
by
a
camera
only
depends
on
the
NA
of
its
lens.’
This
statement
is
in
my
opinion
wrong.
The
intensity
of
the
image
is
given
by
the
ration
of
the
diameter
of
the
entrance
pupil
to
the
focal
length,
which
is
the
inverse
f
number.
Our
response:
The
statement
is
correct
because
f
number
and
NA
of
a
lens
corrected
for
coma
and
spherical
aberrations
are
related:
f
number=1/(2NA)
[equation
(9.3)
W.
Smith
“Modern
Optical
Engineering,”
4th
edition,
p.
184,
McGraw
Hill,
2008.]
We
modified
the
manuscript
to
clarify
this:
“The
intensity
of
the
image
formed
by
a
camera
only
depends
on
the
NA
of
its
lens
(it
is
proportional
to
1/f
number
2
=4NA
2
[22]).”
Reviewer
#2
comment:
The
distance
between
the
metalens
and
the
CCD
chip
seems
to
be
important
and
it
was
taken
care
about
that
in
the
design
as
demonstrate
in
the
manuscript.
However,
I
could
not
find
any
statement
of
how
the
authors
did
this
precise
alignment
for
the
measurement?
Here
it
would
be
helpful
to
add
some
information.
Our
response:
The
metasurface
doublet
was
mounted
on
a
3
axis
translation
stage
during
the
measurements.
To
adjust
the
distance
between
the
image
sensor
chip
and
the
doublet,
a
far
object
was
imaged
and
distance
was
adjusted
until
the
image
was
brought
into
focus.
We
added
the
following
explanation
to
the
Methods
section:
“The
metasurface
doublet
was
mounted
on
a
3
axis
translation
stage
during
the
measurements.
To
set
the
distance
between
the
image
sensor
chip
and
the
doublet,
a
far
object
was
imaged
and
the
distance
was
adjusted
until
the
image
was
brought
into
focus.”
Reviewer
#2
comment:
It
seems
like
the
simulations
were
performed
only
with
a
real
part
of
the
refractive
index.
What
is
the
influence
of
the
imaginary
part
here?
Why
can
it
be
neglected?
Our
response:
The
imaginary
part
of
the
amorphous
silicon
refractive
index
is
smaller
than
10
4
at
850
nm
and
is
neglected
in
the
simulations.
The
significantly
smaller
absorption
loss
of
hydrogenated
amorphous
silicon
compared
to
crystalline
silicon
is
due
to
its
larger
bandgap.
We
added
the
following
to
the
Methods
section
for
clarification:
“The
imaginary
part
of
the
refractive
index
of
amorphous
silicon
is
smaller
than
10
4
at
850
nm
and
was
ignored
in
the
simulations.”
Reviewer
#2
comment:
Supplementary
Figure
2a
shows
the
laser
spectrum.
Why
is
there
such
a
strong
modulation
in
the
spectrum
that
looks
like
an
interference
effect?
Our
response:
The
laser
diode
used
in
the
characterization
is
multimode
and
different
peaks
observed
in
the
measured
spectrum
of
the
laser
diode
correspond
to
different
Fabry
Perot
modes
of
the
laser.
We
added
the
following
to
the
caption
of
Supplementary
Figure
2
to
clarify
this:
“Different
peaks
observed
in
the
spectrum
correspond
to
different
Fabry
Perot
modes
of
the
laser
cavity.”
Reviewer
#3
comment:
Authors
report
in
this
paper
a
compact
camera
that
utilizes
a
flat
metasurface
doublet
lens
to
deliver
nearly
diffraction
limited
performance
within
the
field
of
view
of
60
degree
x
60
degree.
The
overall
dimensions
of
the
camera
(including
the
image
sensor)
are
1.6
mm
x
1.6
mm
x
1.7
mm.
The
meta
surface
doublet
lens
consists
of
one
metasurface
corrector
plate
and
one
metasurface
focusing
lens.
The
phase
profiles
of
both
metasurface
lenses
have
been
optimized
to
collectively
reduce
the
monochromatic
aberrations.
As
the
results,
the
performance
of
the
metasuface
doublet
has
been
significantly
improved
in
comparison
with
the
singlet
lens.
This
is
a
significant
step
in
developing
a
high
performance
flat
lens
for
the
purpose
of
optical
imaging,
as
opposite
to
the
focusing
being
demonstrated
before.
Additionally,
the
metasufrace
doublet
has
been
conveniently
fabricated
on
the
both
side
of
1mm
thick
quartz
substrate
with
the
alignment
accuracy
of
2
um.
It
eliminates
the
post
fabrication
alignment
procedure
and
thus,
makes
it
possible
for
potential
vertical
integration
using
the
well
established
micro
fabrication
capabilities.
Our
response:
We
thank
the
reviewer
for
summarizing
the
manuscript,
and
we
are
glad
that
realize
the
impact
of
the
manuscript
on
imaging
using
metasurfaces.
Reviewer
#3
comment:
Just
for
the
curiosity,
can
author
explain
of
the
whether
the
functions
of
the
two
metasurface
lenses
can
be
combined
into
one
metasurface
lens
with
aspherical
phase
profile?
Our
response:
The
singlet
metasurface
lens
that
we
used
for
comparison
is
aspheric
and
the
only
possible
design
with
no
spherical
aberration.
As
we
showed
in
the
manuscript
a
metasurface
lens
corrected
for
spherical
aberration
(i.e.
the
aspheric
singlet)
has
significant
coma,
so
it
is
not
possible
to
make
a
singlet
which
is
corrected
for
both
spherical
and
coma
aberrations.
Reviewer
#3
comment:
Furthermore,
the
title
of
“An
optical
metasurface
planar
camera”
is
not
very
accurate.
The
demonstrated
camera
has
the
shape
close
to
a
cubic
so
it
is
hard
to
define
it
as
a
“flat
camera”.
The
lens
being
used
is
the
flat
one
though.
Our
response:
The
term
“planar”
in
the
title
refers
to
the
planar
metasurface
lenses
made
using
planar
fabrication
technology,
and
does
not
mean
that
the
camera
is
infinitesimally
thin.
To
eliminate
the
confusion
and
to
make
the
title
more
descriptive,
we
changed
the
title
to:
“Miniature
optical
planar
camera
based
on
a
wide
angle
metasurface
doublet
corrected
for
monochromatic
aberrations”
Reviewer
#3
comment:
Overall,
the
manuscript
is
well
written
and
the
reported
work
is
of
the
great
interest
to
the
readers.
I
would
recommend
the
paper
to
be
accepted
for
publication
with
minor
revision.
Our
response:
We
thank
the
reviewer
for
providing
constructive
feedback
and
recommending
the
manuscript
for
publication.
Reviewer
#4
comment:
The
authors
describe
in
their
paper
a
planar
single
layer
and
double
layer
lens
based
on
optical
metasurfaces.
Furthermore
they
combine
their
proposed
metasurface
doublet
lens
with
a
commercial
CMOS
image
sensor.
The
used
approach
for
producing
the
metasurface
lenses
is
not
restricted
to
laboratory
prototypes.
A
high
volume
fabrication
with
thin
film
production
line
is
possible,
which
might
yield
to
a
high
economic
impact
of
the
proposed
lenses.
Additionally
the
authors
combine
their
proposed
lens
with
a
commercial
CMOS
image
sensor.
Compared
to
the
state
of
the
art
they
replace
a
"classical"
lens
with
their
proposed
one.
Our
response:
We
thank
the
reviewer
for
carefully
reading
the
manuscript
and
summarizing
its
results.
Reviewer
#4
comment:
However
the
authors
should
more
clearly
explain
the
advantages
of
their
lens
for
the
camera
module.
From
a
production
point
of
view
for
the
camera
module
I
do
not
see
a
significant
advantage.
The
assembly
of
the
lens
together
with
the
CMOS
sensor
still
needs
to
be
done
in
the
backend
with
a
very
similar
process.
Our
response:
From
a
production
point
of
view,
the
metasurface
doublet
lenses
have
several
advantages
over
conventional
lens
modules
with
similar
degree
of
corrections
for
monochromatic
aberrations.
A
conventional
lens
module
is
made
of
multiple
lenses
which
are
separately
manufactured
and
later
aligned
and
assembled
together.
The
metasurface
doublets
are
batch
manufactured
with
the
potential
for
simultaneous
manufacturing
of
tens
of
thousands
of
doublet
lenses
on
the
same
wafer,
and
the
two
lenses
of
the
doublet
are
aligned
using
a
single
lithographic
step
during
fabrication.
Furthermore,
the
assembly
of
the
conventional
lens
modules
with
the
image
sensor
has
to
be
done
in
a
back
end
step,
but
the
metasurface
doublet
has
the
potential
for
monolithic
integration
with
the
image
sensor.
We
added
the
following
explanation
to
the
Discussion
section
of
the
manuscript
to
further
emphasize
these
advantages:
“From
a
manufacturing
standpoint,
the
metasurface
doublets
have
several
advantages
over
conventional
lens
modules.
Conventional
lens
modules
are
composed
of
multiple
lenses
which
are
separately
manufactured
and
later
aligned
and
assembled
together
to
form
the
module.
On
the
other
hand,
the
metasurface
doublets
are
batch
manufactured
with
simultaneous
fabrication
of
tens
of
thousands
of
doublets
on
each
wafer,
and
with
the
metasurfaces
aligned
to
each
other
using
lithographic
steps
during
fabrication.
Furthermore,
the
assembly
of
the
conventional
lens
modules
with
the
image
sensors
has
to
be
done
in
a
back
end
step,
but
the
metasurface
doublet
can
be
monolithically
stacked
on
top
of
image
sensors.”
Reviewer
#4
comment:
A
topic
not
mentioned
in
the
paper
is
the
influence
of
the
alignment
of
the
nano
posts
of
the
lens
and
the
bayer
pattern
of
the
CMOS
sensor.
Our
response:
There
is
no
need
for
aligning
the
Bayer
filter
pattern
on
the
image
sensor
and
the
nano
posts.
Generally,
the
lens
module
of
a
camera
does
not
need
to
be
aligned
with
the
Bayer
pattern
on
the
image
sensor.
This
applies
to
the
metasurface
doublets
as
well,
because
the
nano
posts
collectively
form
lenses
which
function
similar
to
conventional
glass
lenses.
Furthermore,
the
metasurface
doublet
is
designed
for
a
single
color
and
should
be
used
with
a
monochrome
image
sensor.
Reviewer
#4
comment:
Some
further
remarks:
Fig.
2b:
what's
the
reason
for
the
low
transmission
values
Our
response:
The
periodic
array
of
nano
posts
exhibits
distributed
resonances
for
the
diameter
values
corresponding
to
low
transmission.
We
added
the
following
to
the
caption
of
Fig.
2
to
clarify
this
point:
“The
diameters
with
low
transmission
values,
which
are
highlighted
by
two
gray
rectangles,
correspond
to
distributed
resonances
of
the
periodic
array
of
nano
posts,
and
are
excluded
from
the
designs.”
Reviewer
#4
comment:
Non
consistent
wording
for
the
substrate
of
the
lens:
"fused
silica"
vs.
"glass"
might
be
a
bit
confusing
Our
response:
To
eliminate
any
potential
confusion,
we
replaced
all
the
instances
of
“glass”
with
“fused
silica”
in
the
revised
manuscript.
Reviewer
#4
comment:
p.
7:
"high
throughput
nano
fabrication
techniques"
is
in
my
opinion
a
bit
misleading.
The
fabrication
of
the
lens
together
with
the
CMOS
sensor
in
the
frontend
is
in
my
opinion
not
feasible.
Instead
both
have
to
be
produced
separately
in
different
frontend
processes
and
then
combined
in
the
backend
Our
response:
The
“high
throughput
nano
fabrication
techniques”
applies
to
the
fabrication
of
the
metasurface
doublet
and
not
to
its
integration
with
the
image
sensor.
We
believe
that
the
metasurface
doublet
can
be
bonded
to
the
cover
glass
wafer
and
serve
as
both
the
cover
glass
of
image
sensors
and
the
imaging
optics.
Nevertheless,
in
response
to
the
previous
comment
of
the
reviewer
on
advantages
of
the
doublet
over
conventional
lens
module,
we
revised
the
manuscript
and
clarified
this
point:
“From
a
manufacturing
standpoint,
the
metasurface
doublets
have
several
advantages
over
conventional
lens
modules.
Conventional
lens
modules
are
composed
of
multiple
lenses
which
are
separately
manufactured
and
later
aligned
and
assembled
together
to
form
the
module.
On
the
other
hand,
the
metasurface
doublets
are
batch
manufactured
with
simultaneous
fabrication
of
tens
of
thousands
of
doublets
on
each
wafer,
and
with
the
metasurfaces
aligned
to
each
other
using
lithographic
steps
during
fabrication.
Furthermore,
the
assembly
of
the
conventional
lens
modules
with
the
image
sensors
has
to
be
done
in
a
back
end
step,
but
the
metasurface
doublet
can
be
monolithically
stacked
on
top
of
image
sensors.”
Reviewer
#4
comment:
All
in
all
the
paper
describes
a
novel
and
interesting
approach
for
planar
lenses
which
the
possibility
for
a
high
impact.
In
addition
the
authors
describe
a
planar
camera
module
with
their
planar
lenses.
This
approach
is
also
interesting,
but
the
advantages
of
the
proposed
solution
are
not
completely
clear
for
me.
Our
response:
We
are
glad
to
see
that
the
reviewer
has
a
positive
opinion
about
the
work
and
realizes
its
high
impact.
The
main
advantages
of
the
are
the
high
imaging
quality
with
small
and
flat
form
factor,
high
scalability
of
the
fabrication
process
allowing
for
batch
fabrication
of
a
large
number
of
camera
lenses
on
a
same
wafer,
and
elimination
of
post
fabrication
alignment
and
assembly
steps
required
for
fabrication
of
camera
lenses.
The
advantages
of
the
metasurface
doublet
over
conventional
design,
and
the
broader
impact
of
the
vertical
integration
approach
introduced
in
the
manuscript
are
now
explained
more
clearly
in
the
last
paragraph
of
the
revised
manuscript:
“The
metasurface
enabled
camera
we
reported
here
has
a
flat
and
thin
form
factor,
small
f
number,
exhibits
nearly
diffraction
limited
performance
over
a
large
field
of
view.
From
a
manufacturing
standpoint,
the
metasurface
doublets
have
several
advantages
over
conventional
lens
modules.
Conventional
lens
modules
are
composed
of
multiple
lenses
which
are
separately
manufactured
and
later
aligned
and
assembled
together
to
form
the
module.
On
the
other
hand,
the
metasurface
doublets
are
batch
manufactured
with
simultaneous
fabrication
of
tens
of
thousands
of
doublets
on
each
wafer,
and
with
the
metasurfaces
aligned
to
each
other
using
lithographic
steps
during
fabrication.
Furthermore,
the
assembly
of
the
conventional
lens
modules
with
the
image
sensors
has
to
be
done
in
a
back
end
step,
but
the
metasurface
doublet
can
be
monolithically
stacked
on
top
of
image
sensors.
More
generally,
this
work
demonstrates
a
novel
vertical
on
chip
integration
architecture
for
designing
and
manufacturing
optical
systems,
which
is
enabled
through
high
performance
metasurfaces.
This
architecture
will
enable
low
cost
realization
of
conventional
optical
systems
(e.g.
spectrometers,
3D
scanners,
projectors,
microscopes,
etc.),
and
systems
with
novel
functionalities
in
a
thin
and
planar
form
factor
with
immediate
applications
in
medical
imaging
and
diagnostics,
surveillance,
and
consumer
electronics.”