Novel phenomena in macroscopic photonic crystals
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Zhen, Bo, Song-Liang Chua, Jeongwon Lee, Wenjun Qiu,
Alejandro W. Rodriguez, Xiangdong Liang, Steven G. Johnson,
John D. Joannopoulos, Ofer Shapira, and Marin Soljacic. “Novel
Phenomena in Macroscopic Photonic Crystals.” Edited by
Ganapathi S. Subramania and Stavroula Foteinopoulou. Active
Photonic Materials V (September 11, 2013). © 2013 Society of
Photo-Optical Instrumentation Engineers (SPIE)
As Published
http://dx.doi.org/10.1117/12.2023928
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SPIE
Version
Final published version
Accessed
Wed May 25 22:40:56 EDT 2016
Citable Link
http://hdl.handle.net/1721.1/88506
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Invited Paper
Novel phenomena in macroscopic photonic crystals
Bo Zhen, Song-Liang Chua, Jeongwon Lee, Wenjun Qiu, Alejandro W.Rodriguez, Xiangdong
Liang, Steven G.Johnson, John D.Joannopoulos, Ofer Shapira, and Marin Soljacic
Research Laboratory of Electronics
Massachusetts Institute of Technology
77 Massachusetts Avenue, Cambridge, MA 02139, USA
ABSTRACT
Photonic crystals provide superb opportunities for tailoring of the photonic density of states. This ability can in turn be
explored to control radiation into far-field, enhance fluorescent light emission, as well as optimize laser emission. In
order to make these phenomena useful for large macroscopic devices, large-area nano-fabrication techniques have to be
successfully implemented. In this talk, I will present some of our recent theoretical and experimental progress in
exploring these opportunities.
Keywords: Photonic Crystals, Fano states, Surface emitting lasers
1. INTRODUCTION
Photonic crystal (PhC) slabs have been widely used in the experimental study of light manipulation by sub-wavelength
structures, largely due to their relative simple fabrication process and the ability to intimately integrate them with other
on-chip devices. Defects were introduced to form resonances, demonstrating intriguing lasing behavior and cavity
quantum electrodynamics phenomena, owing to the strong confinement of light in small volumes with high quality
factors. Uniform PhC slabs have been monolithically integrated on top of light emitting devices to enhance the
extraction of light while similar structures have been used to reshape the emission of thermal sources. These structures
have recently been shown theoretically to allow for low threshold laser operation due to the high quality factor dark Fano
resonances they support: here, we show experimental realizations of these promising phenomena.
2. OBSERVATION OF HIGH-Q FANO RESONANCES IN MACROSCOPIC PHOTONIC
CRYSTAL SLABS
Although this has been shown to be theoretically possible, experimental observation of high-Q Fano resonances over
macroscopically large areas has eluded researchers due to structural perturbation inherent to fabricated structures. The
physical origin of Fano resonances in PhC slabs lies in the coupling between the guided modes supported by the slabs
and external plane waves. This coupling is due to Bragg diffraction that occurs because of the periodic modulation of the
dielectric constant. Remarkably, in the case of perfect infinite periodic PhC slab, due to symmetry considerations, some
of the Fano resonances are completely decoupled from the external world. Therefore, in a perfect structure, the radiative
quality factor (Qrad) theoretically becomes infinite despite lying above the light line, and hence its radiation losses into
the far-field vanish. In practical structures however, in addition to limits imposed by material absorption, fabrication
imperfections partially break the symmetry which results in coupling of those Fano resonances to radiating modes, hence
limiting their maximal attainable Qtot. Moreover, while the existence of Fano resonances in a uniform PhC slab is due to
the nano-structured periodic dielectric profile, the mode itself needs to extend over a macroscopic area in order to
support ultra-high Qtot, posing a significant fabrication challenge. Here, we experimentally demonstrate the existence of
Fano resonances for visible wavelengths in a PhC slab (Figure 2), fabricated using interference lithography (Figure 1)
over macroscopic area (~cm2) and exhibiting Qtot as high as 104 (Figure 3). We confirm numerically and via symmetry
arguments that for modes with k vector normal to the PhC plane (Γ point) the radiative lifetimes of some modes of an
ideal structure can be infinite. Through angle resolved spectral measurements, we display the behavior of these modes in
k-space, close to the Γ point where large Qrad occur. Using temporal coupled-mode theory (Figure 4) we study the
measured Fano line shapes and discuss the effect of the change in the mode symmetries away from the Γ point on the
lifetimes of these resonances.
Active Photonic Materials V, edited by Ganapathi S. Subramania, Stavroula Foteinopoulou,
Proc. of SPIE Vol. 8808, 880814 · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2023928
Proc. of SPIE Vol. 8808 880814-1
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(a)
(b)
Figure 1. (a) Tilt-view SEM of the fabricated PhC. This uniform pattern was produced over the entire area of the sample. (b)
Side-view SEM of the same PhC. The top layer is the 250 nm Si 3N4 layer with periodic 55 nm deep cylindrical air holes,
and the underlying layer is the 5μm thick SiO2 layer. RIE produced a vertical and clear sidewall profile. The average period
of the pattern is 320 nm, the average hole diameter is 160 nm, and the average hole depth is 55 nm. Top inset: Image
obtained from AFM analysis. Bottom inset: Image of the large sample.
(a)
(b)
(c)
560
570
580
590
=
600
610
0.5
1
1.5
2
0.5
0
1
1.5
20
Angle (degrees)
Angle (degrees)
0.5
Reflectivity
560
0.5P
570
o
I
580
1
590
600
610
0.5
1
1.5
An le (de rees)
(d)
2
0
0.5
1
1.5
An le (de rees)
(e)
2
0
0.5
Reflectivit
(f)
Figure 2.(a),(d)Band diagram of the eight lowest energy leaky modes (measured at the Γ point) of the PhC obtained from
finite deference time domain (FDTD) simulation. Modes excited externally by odd (even) polarized source with respect to
the x-axis are colored purple (green) and their Ez field profiles at the center of the Si3N4 layer at at k = (0.01, 0) ∙ (2π/a) are
shown, other modes are colored gray. Contour of the hole is shown with black dashed circle. The inset depicts a schematic
of the unit computational cell used in the simulation. (b),(e) Reflectivity measurements of the PhC with Ey (Ex) polarized
beam. The Inset shows a schematic of the experimental setup. (c),(f) A slice of the reflectivity measurement results at 1.8°.
Proc. of SPIE Vol. 8808 880814-2
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8
10
-Qrad(kx); TE -like high -Q modes
-Qrad(kx); TE -like low -Q modes
-Qrad(kx); TM -like high -Q modes
-Qrad(kx); TM -like low -Q modes
4
10
0.005
o
0.015
0.01
0.02
kx (2ar/a)
Figure 3.Simulation results for radiative quality factors of different modes are shown with respect to different kx between 0
and 0.02 ∙ (2π/a). The two TE-like high-Q modes are plotted with blue solid lines, while the two TE-like low-Q modes,
which are doubly degenerate modes at Γ point, are shown with blue dotted lines. The two TM-like high-Q modes are shown
with red solid lines, while the two TM-like low-Q modes are shown with red dotted lines.
1
8
0.4
° measured data
o
- background fitting
fano fitting
0.3
0
1
O.6
51
o,á
0
-c
d 0.2
9
w
4.4
1
0.2°
0.1
o
O,s°
áo
583
585
585
586
fitted Qtotai
o
584
584
(nm)
o,i
h-
583
3
586
10
0
0.2
0.4
0.6
0.8
Angle (degrees)
(a)
(b)
Figure 4: (a) The reflectivity spectra of the sample’s lower frequency TM-like high Qrad mode measured at a range of angles
of 0.1o to 0.8o from the normal of the PhC slab. (b) Qtot (red solid dots) retrieved by fitting to the measured data in (a). Inset
shows an example of the curve fitting process applied on the data measured at 0.2 o shown in (a). A good match between the
fitted lineshape (green line) and measured data (red dots) is attained.
Proc. of SPIE Vol. 8808 880814-3
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3. ENABLING ENHANCED EMISSION AND LOW THRESHOLD LASING OF
ORGANIC MOLECULES USING SPECIAL FANO RESONANCES OF
MACROSCOPIC PHOTONIC CRYSTALS
The nature of light interaction with matter can be dramatically altered in optical cavities, often inducing non-classical
behavior. In solid state systems, excitons need to be spatially incorporated within nanostructured cavities to achieve such
behavior. While fascinating phenomena have been observed with inorganic nanostructures, the incorporation of organic
molecules into the typically inorganic cavity is more challenging. Here we present a novel optofluidic platform
comprising organic molecules in solution suspended on a photonic crystal surface (Figure 5), which supports
macroscopic Fano resonances and allows strong and tunable interactions with the molecules anywhere along the surface.
We develop a theoretical framework of this system and present a rigorous comparison with experimental measurements
showing dramatic spectral and angular enhancement of emission (Figure 6&7). We then demonstrate that these
enhancement mechanisms enable lasing of only 100nm thin layer of diluted organic molecules solution with
substantially reduced threshold intensity (Figure 8), which has important implications to organic light emitting devices
and molecular sensing.
Spectrometer
Singlet
stimulated
N3
1 emission
spontaneous
emission
NZ
Pump
absorption
1 reabsorption
non- radiative
Nt
ir transitions
Na
vibrational
relaxation
So
(a)
(b)
Figure 5: Optofluidic platform of organic molecules coupled to Fano resonances of the macroscopic photonic crystal. (a)
Schematic drawing of the two lowest singlet energy levels of a dye molecule and transitions it undergoes during
fluorescence emission. (b) Schematic drawing of the experimental setup of the angle-resolved fluorescence measurements of
Rhodamine 6G (R6G) dissolved in methanol at 1 mM concentration placed on top of the PhC. The grey substrate is the
macroscopic PhC slab. The orange spheres are schematic drawings of the R6G molecules in solution. The blue surface
represents the equal energy density surface of the Fano resonance. Fluorescence spectra of the organic solution for both
cases were recorded using a high-resolution spectrometer placed close to the normal of the PhC. By tuning the position of
the spectrometer, fluorescence spectra of the molecules along  to X and to M were measured.
Proc. of SPIE Vol. 8808 880814-4
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6000
X
5000
4000
.-.
On- resonance; PhC
3000
* 0.1
2000
0 1000
10
5
A
Angle (degrees)
300
0
200
100
Blank slab
YII
Off -resonance: PhC
Pr#M4r,
0
550
560
580
570
590
Wavelength 2,(nm)
Figure 6: Significantly enhanced fluorescence emission from R6G molecules. Comparison of fluorescence spectra of R6G
molecules measured in the normal direction, among on the PhC (solid lines) both pumped on-resonance (blue) and oresonance (red) as well as on a uniform unpatterned slab (dashed green line). By comparing the spectra, we obtain the
excitation, extraction, and total enhancement factors, which are compared with the theoretical predictions. The inset of the
figure shows FDTD calculation results of the band structure from which the incident angle for on-resonance coupling is
determined (the=10.0O), showing a good agreement with the experiment (exp=10.02O).
X MF
F
X
25
565
20
---- 570
15
575
10
°)
580
5
585
0
0
1
2
Angle (degrees)
4
3
-1
0
1
2
3
4
Angle (degrees)
8000
Figure 7: Comparison between theoretical model and experimental results
(e) of the enhancement mechanisms. (a) The band
structure of the PhC along  to M and  to X directions. (b) Angle-resolved fluorescence measurements of R6G solution
suspended on top of the PhC. The correspondence between the color and number of photons (arbitrary units) is given in the
1
6000
color bar on the
side.
1
cg
a.)
4
U
g 4000
L
2000
Proc. of SPIE Vol. 8808 880814-5
1
20
0.5
1
150
0.5
1
15
2
3
Angle (degrees)
Angle
(degrees)
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on 07/22/2014
Terms
of Use: http://spiedl.org/termsAngle (degrees)
100
10
_ - Model
Meas. (spectrum)
Meas. (power)
o
x103
-2
10
-0.79x10'
''10
ó
104
O
O
- 1.15x10'
(nJ.cm')
5
0.6
5
=
w
0
4t
Oy~
576
0
0.4
578 580 582
c°
CD
X (nm)
10 -6
-5 0.2
cl,
8 0
10
o
0
cP
2
4
6x
0°
Pump energy (n7.cm 2)
-8
1
103
104
105
Pump energy (nJ.cm 2)
Figure 8: Low threshold lasing of 100 nm thin layer of R6G molecules in solution. Input-output energy characteristics of
lasing through mode 4 in Figure 7a (580 nm) under pulsed excitation. The solid lines are analytic predictions from our lasing
model while red circles are energies measured using the spectrometer. Green circles are data measured with a power meter.
The jump in output power clearly indicates the onset of lasing. The lower inset shows the same results in linear scale, where
the output grows linearly with the pump energy beyond threshold. Top inset is the measured power spectrum of emission
from the PhC slab at normal incidence below (blue) and above (red) the lasing threshold. Single-mode lasing is attained at
approximately 9 J/cm2 (corresponding to the intensity of 1.8 kW/cm2).
REFERENCES
[1] Jeongwon Lee, Bo Zhen, Song-Liang Chua, Wenjun Qiu, John D. Joannopoulos, Marin Soljacic, and Ofer Shapira.
Phys. Rev. Lett. 109, 067401, (2012).
[2] “Enabling Enhanced Emission andLow Threshold Lasing of Organic Molecules Using Special Fano Resonances of
Macroscopic Photonic Crystals” Bo Zhen, Song-Liang Chua, Jeongwon Lee, Alejandro W.Rodriguez, Xiangdong
Liang, Steven G.Johnson, John D.Joannopoulos, Marin Soljacic, and Ofer Shapira. PNAS, in press.
Proc. of SPIE Vol. 8808 880814-6
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