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The ability to manipulate light propagation at the nanoscale is of vital importance for future integrated photonic circuits. In this work we exploit the high contrast in the optical properties of the phase change material Ge2Sb2Te5 to control the propagation of surface plasmon polaritons at a Au/SiO2 interface. Using grating couplers, normally incident light at λ = 1.55 μm is converted into propagating surface plasmons on a Au waveguide. Single laser pulses (λ = 975 nm) are applied to a thin film of Ge2Sb2Te5 placed on top of the device, which, upon transition from its amorphous to crystalline structural phase, dramatically increases both its refractive index and absorption coefficient, thus inhibiting propagation of the plasmonic mode. This effect is investigated for different interaction lengths between the phase change material and the Au waveguide, and contrast values in the transmitted intensity up to several tens of percents are demonstrated.Keywords: phase change materials; surface plasmon; nanophotonics; nonvolatile; chalcogenides
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Active Control of Surface Plasmon Waveguides with a Phase Change
Material
Miquel Rudé,*
,
Robert E. Simpson,
Romain Quidant,
,
Valerio Pruneri,
,
and Jan Renger
§
ICFO - The Institute of Photonic Sciences, Mediterranean Technology Park, Avenida Carl Friedrich Gauss 3, 08860 Castelldefels
(Barcelona), Spain
Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Road, Singapore 487372, Singapore
ICREA - InstitucióCatalana de Recerca i Estudis Avançats, Passeig Lluís Companys 23, 08010 Barcelona, Spain
§
Photonics Laboratory, ETH Zürich, Hönggerbergring 64, 8093 Zürich, Switzerland
*
SSupporting Information
ABSTRACT: The ability to manipulate light propagation at
the nanoscale is of vital importance for future integrated
photonic circuits. In this work we exploit the high contrast in
the optical properties of the phase change material Ge2Sb2Te5
to control the propagation of surface plasmon polaritons at a
Au/SiO2interface. Using grating couplers, normally incident
light at λ= 1.55 μm is converted into propagating surface
plasmons on a Au waveguide. Single laser pulses (λ= 975 nm)
are applied to a thin lm of Ge2Sb2Te5placed on top of the
device, which, upon transition from its amorphous to
crystalline structural phase, dramatically increases both its refractive index and absorption coecient, thus inhibiting propagation
of the plasmonic mode. This eect is investigated for dierent interaction lengths between the phase change material and the Au
waveguide, and contrast values in the transmitted intensity up to several tens of percents are demonstrated.
KEYWORDS: phase change materials, surface plasmon, nanophotonics, nonvolatile, chalcogenides
In recent years there has been an ongoing eort to develop
novel photonic circuits with high processing speed and
robustness against fabrication tolerances. Optical data commu-
nication already outperforms their electronic counterparts in
terms of speed and transmission losses. However, due to the
barrier imposed by the diraction limit of light,
1
conventional
optical components usually have dimensions larger than the
wavelength of light and are quite sensitive to changes in their
geometry arising from fabrication tolerances, thus making it
dicult to achieve both high eld connement and robustness.
Surface plasmon polaritons (SPPs) emerged as a promising
candidate for solving these drawbacks of conventional photonic
circuits.
24
SPPs are hybrid modes that are bound at the metaldielectric
interface when a light wave is coupled to the oscillation of the free
electrons present in the metal. These waves are conned to the
interface with an electric eld that decays exponentially in both
surrounding materials. Thus, the light is strongly conned, which
provides a platform to guide it in compact devices
5,6
that are
smaller than conventional optical components, such as electro-
optic modulators or optical bers.
One of the main challenges of plasmonic-based circuits is
nding a way to control SPP propagation. This problem has been
tackled by changing the optical properties of the surrounding
environment, for instance by exploiting the electro-optic eect,
7,8
using quantum dots,
9
or by externally pumping photochromic
molecules.
10
However, the majority of such proof-of-principle
experiments employ modulator designs that weakly eect the
SPP propagation and therefore exhibit a limited SPP modulation
depth. More recently, photonic switches and modulators using
low-dimensional materials such as graphene have also been
demonstrated
11
that can achieve high modulation frequencies in
the GHz regime but that require the continuous application of
electric elds and are more prone to long-term instabilities due to
interface eects.
Here, we use the unique features of the prototypical phase
change material (PCM) Ge2Sb2Te5(GST)
12
to perform
nonvolatile control of SPP propagation in Au waveguides.
PCMs on the GeTe-Sb2Te3pseudobinary tie-line exhibit an
extraordinarily large contrast between their two structural
phases. The covalently bonded amorphous phase of GST
corresponds to a disordered material with short-range atomic
order and low electrical conductivity and optical absorption
(ñamorph = 4.7 + 0.2iat λ= 1.55 μm). In contrast, the resonantly
bonded crystalline phase can be seen as a low-band-gap
semiconductor and exhibits an electrical conductivity that is 3
orders of magnitude greater than the amorphous phase and has a
larger optical absorption (ñcryst =7+2iat λ= 1.55 μm).
1317
The
large refractive index of the crystalline phase is due to the
Received: February 5, 2015
Letter
pubs.acs.org/journal/apchd5
© XXXX American Chemical Society ADOI: 10.1021/acsphotonics.5b00050
ACS Photonics XXXX, XXX, XXXXXX
existence of resonant bonds, which can be easily polarized by an
external electric eld.
13
Moreover, switching between the two
phases can be triggered externally by applying electrical or laser
pulses.
The transition from the amorphous state to the crystalline
state typically requires pulses with a duration of hundreds of
nanoseconds, although picosecond-order crystallization times
have been reported for GST.
18,19
While PCMs have been
Figure 1. (a) Schematic of the devices used to probe the propagation length of SPPs in Au/PCM hybrid waveguides. (b) Intensity images of three
devices with dierent values of LG(160, 100, and 80 μm) with the GST in the amorphous phase. (c) Logarithm of the normalized intensity at the output
port as a function of the distance between gratings with the GST in the amorphous (black squares) and crystalline (red circles) phases. The output value
is normalized by the intensity scattered at a single groove (A) placed at a xed distance from the input grating.
Figure 2. (a) Schematic of the cross section in the interaction region (not to scale). (b) Optical microscope image of the plasmonic waveguide with a
GST strip in the middle. The insets show the GST area before and after crystallization using a single laser pulse. (c) Principle of operation of the device. A
probe laser at λ= 1.55 μm is used to convert free space light into propagating SPPs that interact with an 80 nm GST lm. An independent pump laser at λ
= 975 nm is used to trigger the amorphous to crystalline phase transition in this lm.
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B
extensively studied for their application in optical data storage
and phase change random access memories (PCRAM),
2022
these properties also make them a promising candidate to control
and manipulate light in photonic circuits in a nonvolatile manner,
an application area that has started to receive an increasing
amount of attention.
2325
Indeed GSTs large change in optical properties, fast switching
ability, high cyclability, and ability to retain its structural phase for
years without any input energy make it ideal for programming
recongurable photonic circuits. Herein, we experimentally
demonstrate and simulate the use of GST to perform nonvolatile
switching of telecom frequency SPPs in plasmonic waveguides.
The change in GSTs optical properties upon phase transition
can strongly aect the propagation length of the plasmonic mode
and hence allows for a change in SPP transmission because of the
dierent attenuation for the amorphous and crystalline phase.
Figure 1a illustrates the geometry employed to measure the
dierence in the propagation length in the Au/SiO2/GST/
PMMA/air waveguides for the two phases. Here, the switch is
achieved by the phase change in the 30 nm thin GST layer, which
was separated by a SiO2layer of roughly 150 nm from Au and
covered by a 2 μm thick PMMA layer. The PMMA layer was used
to tune the number and properties of the waveguide mode(s)
and the SiO2to thermally isolate the GST from the Au as well as
to optimize the sensitivity to the phase transition without
substantially sacricing SPP propagation too much, as explained
in the Supporting Information (Figures S1 to S4). The dierence
of the real part of the modes propagation constant (cf. Figure
S2) enables the selective excitation of the SPP mode using the
appropriate spacing of the grooves of the coupling grating in the
metal. Additionally, a small groove-like line defect placed 20 μm
away from the excitation grating (marked by the label A in Figure
1b) is used to probe the SPP intensity, which allows for
normalization and elimination of changes in the incoupling
eciency. Similarly, line defects and outcoupling gratings have
been engineered at dierent LG. The resulting transmission is
measured by imaging the light at λ= 1550 nm by an InGaAs
camera, as shown in Figure 1b. Figure 1c shows the logarithm of
the intensity at the output port normalized by the input
scattering line defect at point A obtained from the intensity
images at various distances LG. The same measurement is
repeated for crystallized GST areas. As shown in the graph in
Figure 1c, the intensity along the waveguide decays exponentially
for both cases. In the case of crystalline GST, the decay is
stronger, which originates from the increased absorption in the
GST layer. For distances greater than 80 μm the losses are too
high and no transmitted intensity could be measured. This strong
dierence in the SPP waveguide-mode decay length can be used
to control optical signals at customized contrast simply by
choosing the adequate length of the active region.
The design of our nonvolatile plasmonic switch, sketched in
Figure 2a and c, consists of two grating couplers to convert
propagating waves into waveguide modes or SPPs at the input
port and vice versa at the output port. The two ports are
connected by a narrow (2 μm) SPP waveguide made of gold, as
illustrated in Figure 2b. Additionally, part of the SPP waveguide is
covered by the active PCM material. This allows to control the
transmitted intensity by changing from the low-loss amorphous
phase to the crystalline phase, which has higher propagation
losses. The same scheme can be used to electrically drive the
phase transition in the GST layer using the tapered design with
two electrodes. This design increases the electrical current
density at the region where the GST and the Au waveguide cross
over, thus providing a hybrid platform to optically or electrically
switch SPPs.
Figure 3. (a, b) Intensity images of the scattered and transmitted laser light at λ= 1.5 μm as seen by the imaging camera (the dashed lines are an outline
of the actual device). The intense spot on the left side corresponds to the focus placed on the in-coupling grating converting propagating light into
waveguide modes and SPP modes. The light propagates along the waveguide nearly radiationless and is out-coupled and detected. The intensity at the
out-coupling grating changes strongly upon changing the GST phase, leading to a strong dierential signal (c).
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C
The devices shown in Figure 2b have been fabricated using
multiple lithographic steps. The metallic pads containing the
gratings and waveguide parts have been structured by rst
employing a positive tone e-beam resist. Subsequently, 30 nm
thick Ti and then 60 nm thick Au lms are deposited by thermal
evaporation. The Ti layer serves as an adhesion layer and
suppresses the coupling of SPPs at the lower interface between
the Au and the SiO2substrate, which would otherwise contribute
to the transmitted intensity. After lifting-othe residual
photoresist at the areas unexposed by the electron beam, a top
layer of 150 nm of SiO2was radio frequency (RF) sputtered from
a high-purity target in an Ar atmosphere of 0.5 Pa. The second
lithographic step denes the GST strips in the center of the
waveguides, where we induce the phase change. The 80 nm thick
GST lms were deposited by RF co-sputtering from stoichio-
metric GeTe and Sb2Te3targets. Energy dispersive X-ray (EDX)
measurements performed on GST lms that were prepared
under the same conditions veried that the composition was
equal to the stoichiometric Ge2Sb2Te5, with an error smaller than
5 at. %. Additionally, X-ray diraction (XRD) measurements
conrmed the as-deposited lms to be in the amorphous phase.
Finally a 20 nm thick capping layer of Si3N4was deposited by dc
sputtering from a Si target in a reactive Ar:N2atmosphere at 0.5
Pa. The Si3N4cap is necessary to protect the GST from oxidation
and to conne heat in the structure during the switching process.
In a nal step, the remaining resist was removed and the whole
device was covered with a 2 μm layer of PMMA.
The optical micrograph of a device employing a 2 μm wide
SPP waveguide is shown in Figure 2b. The in- and out-coupling
gratings have been placed on larger gold pads to reduce the direct
transmission of the incident probe laser and to funnel the SPPs to
the narrow SPP waveguide.
26,27
The light-to-SPP conversion can
be controlled using the number of grooves and their shape.
28
Linearly polarized light (λ= 1.55 μm) from a laser diode is
focused using a 20×objective onto the in-coupling grating on the
left in Figure 3. The grating periodicity, G=1μm, is designed to
couple the incident light, which has no in-plane momentum, into
SPPs by fullling the condition kSPP =2π/iG, where iis an
integer.
29
The actual value of kSPP was found by numerically
solving the eigenvalue problem for the stratied media using
OpenMaXwell.
30
The out-coupling grating on the right is used to
convert the transmitted SPPs into far-eld radiation, where it is
collected using a 50×objective and imaged using an InGaAs
camera, as shown in Figure 3. The SPP waveguide width is 2 μm
and its length is 25 μm, which is smaller than the SPP
propagation length for Au at λ= 1.55 μm(ΛSPP 75 μm for Au
embedded in a dielectric slab).
The principle of operation is as follows. A 5 μm wide GST strip
is structured on top of the SPP waveguide and separated by a
SiO2layer in order to force the SPP to sense the presence of the
GST. The thickness of the SiO2lm (150 nm) is chosen to obtain
a strong mode overlap with the 80 nm GST lm, to improve heat
trapping in the GST layer, and to prevent chemical reactions
between the Au and the GST. In the initial amorphous state, the
lm has low absorption and behaves like a dielectric, allowing the
SPP to propagate along the waveguide, while in the crystalline
state the SPPs are reected and attenuated due to the strong
absorption present in this layer. The structural phase transition in
GST is triggered using a control laser (λ= 975 nm) focused on
the GST area down to a spot size of 4 μm using the same
objective as the probe laser. The focused laser spot size is larger
than the GST-Au crossover area to ensure that the GST lm
which is directly above the SPP waveguidecompletely
crystallizes for widths below 4 μm, as can be seen in the inset
of Figure 2b. By applying single laser pulses (tfwhm = 300 ns, trise =
80 ns, P= 23 mW), the temperature inside the lm increases
above the crystallization temperature of the PCM, which for this
heating rate is expected to be Tcryst > 625 K,
17,31
and a
polycrystalline region is obtained, corresponding to the brighter
area in the inset in image Figure 2b due to the increased refractive
index and absorption. At λ= 975 nm the absorption of GST is
high (k= 1) and the penetration depth of the laser at this
wavelength is approximately damor = 77 nm, which is smaller than
the lm thickness. Thus, most of the incident laser power will be
absorbed by the 80 nm thick lm, which will then crystallize.
Although not shown in this work, for a full switching cycle a
remorphization process is also present, which is much more
dicult to achieve. In this case the penetration depth of the laser
in the crystalline GST is dcryst =40nm(k=2atλ= 975 nm), and
even more energy will be absorbed. However, much higher
temperatures and cooling rates are necessary in order to melt the
crystalline phase and quench the resulting liquid state to obtain
the amorphous phase, and this could be achieved using the Au
waveguide as a heat-sinking structure to rapidly cool the molten
GST.
The transmitted λ= 1.55 μm light before and after the GST
phase change is shown in Figure 3a and b. After irradiating the
GST, the transmitted signal is visibly and quantiably less
intense, as shown in Figure 3b. To quantify the contrast due to
the GST phase change, we integrated the intensity in the
rectangular area on top of the out-coupling grating, as indicated
by the white rectangle in Figure 3a and b for the amorphous
phase Iaand after crystallization Icrys. The data have been
background corrected by measuring the thermal and readout
noise in the close vicinity of the grating, as indicated by the yellow
rectangle in Figure 3a and b, resulting in values for Ibg
aand Ibg
crys.As
the insertion loss of the device depends only on the coupling
eciency of the input grating and is thus independent of the
phase of the GST lm, these values can be used to calculate the
contrast of the device, which is given by
=
−−
C
II II
II
()()
crys bg
crys abg
a
abg
a
(1)
The strong SPP contrast by the GST phase transition is readily
visualized by taking the dierence of the camera images as shown
in Figure 3c. The majority of the image pixels (nearly) cancels,
and a large dierence in the intensity is found only at the position
of the out-coupling grating, which supports our intention to
inhibit SPP propagation by inducing a phase change in the GST
layer. In this case the contrast of the device is approximately 31%.
The experimental error, obtained by calculating the dierence in
the average intensity of the same image taken at two dierent
times, is smaller than 0.5% for both phases.
Using a uence of 1.8 W m2, the total energy required for the
switching process is 6.9 nJ, which is comparable to other
plasmonic switches.
10
Other approaches to switch/modulate
SPPs have been shown to need less energy;
79
however these
schemes are not nonvolatile and require a continuous external
stimulus, thus making it dicult to design recongurable
plasmonic circuits.
For future modulating devices based on the same scheme it
would be necessary to perform several crystallization/ream-
orphization cycles. Typically for optical data storage applications
GST can withstand 105cycles, while for electrical data storage the
cyclability is even higher (107cycles).
17
Interestingly, the number
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of cycles increases with decreasing energy required to trigger the
phase transition.
32
The GST volumes used in this work are huge
(0.080.8 μm3), and we expect cycleabilities smaller than 105
cycles. With regard to the maximum frequency achievable by the
device a full modulation cycle would include both an
amorphous/crystalline and a crystalline/amorphous transition.
The minimum time required to trigger these transitions will
ultimately limit the modulation speed of the device. In our
devices crystallization is achieved using 300 ns pulses. Ream-
orphization of GST usually requires fast quench rates on the
order of 109K/s; thus cooling the material from the melting
temperatue (1000 K) to room temperature (300 K) would take
approximately 700 ns. A full modulation cycle could be achieved
in approximately 1 μs, obtaining a modulation frequency of 1
MHz.
The contrast is based on the fact that the SPPs have
signicantly dierent absorption lengths for the two GST
phases, which allows one to engineer the contrast. In order to
evaluate the inuence of the GST area on the performance of the
device, the same experiment was repeated using dierent GST
widths (0.5, 1, 1.5, 2, 2.5, and 3 μm) for a xed Au waveguide
width of 2 μm (Figure 4) . For each PCM length the
measurement has been performed on three dierent devices.
The experimental points in Figure 4 indicate the mean value of
the contrast, and the error bars its standard deviation. For the
wide GST strip (w=5μm), full crystallization could not be
achieved due to the spot size of the laser being smaller than the
overlap area, obtaining an experimental contrast smaller than the
simulated one. As expected, the contrast increases for wider
active regions of GST due to a higher interaction length, which
leads to larger cumulative absorption of the light propagating in
the SPP waveguide mode.
In conclusion, we have demonstrated inhibition of SPP
propagation in a Au/SiO2interface by exploiting the high
contrast in the optical properties of the phase change material
Ge2Sb2Te5. The attenuation in the transmitted intensity strongly
depends on the interaction length between the Au waveguide and
the GST lm, obtaining a contrast of more than 30% for
suciently wide GST strips, using low switching energies to
inhibit SPP propagation in a nonvolatile manner. The results
demonstrate that the combination of surface plasmon polaritons
with phase change materials, such as GST, allows for designing
and fabrication of novel active devices such as plasmonic switches
or recongurable optical circuits.
Finally, we expect that larger contrasts could be achieved either
by using larger overlap areas or working at other wavelengths
where GST exhibits even higher absorption and contrast, while
an improved thermal design could reduce the crystallization time
from hundreds of nanoseconds down to the picosecond time
scale, thus reducing the switching energy. Moreover, smaller
devices that incorporate highly ecient phase change material
structures and new switching paradigms, such as coherent
phonon excitation,
33
could present a viable way to reduce the
amount of energy deposited in the structure and extend the
cycling endurance of the device.
ASSOCIATED CONTENT
*
SSupporting Information
Simulation of the characteristic eigenmodes in the plasmonic
waveguide for the armophous and crystalline phases of GST.
Inuence of the SiO2and PMMA thicknesses in the propagation
constants of the dierent modes. The Supporting Information is
available free of charge on the ACS Publications website at DOI:
10.1021/acsphotonics.5b00050.
AUTHOR INFORMATION
Corresponding Author
*E-mail: miquel.rude@icfo.es.
Notes
The authors declare no competing nancial interest.
ACKNOWLEDGMENTS
We acknowledge nancial support from the Spanish Ministry of
Economy and Competitiveness (MINECO) and the Fondo
Europeo de Desarrollo Regional (FEDER) through grant
TEC2013-46168-R, the European Research Council through
grant 259196 (PLASMOLIGHT), and FundacióPrivada
CELLEX.
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... No external power supply is required for GST to maintain its phase. GST is currently widely used in a variety of reconfigurable optical devices due to its switchable dielectric properties [19][20][21][22][23][24][25]. ...
Preprint
We introduce non-Hermitian plasmonic waveguide-cavity structures based on the Aubry-Andre-Harper model to realize switching between right and left topological edge states (TESs) using the phase-change material Ge$_2$Sb$_2$Te$_5$ (GST). We show that switching between the crystalline and amorphous phases of GST leads to a shift of the dispersion relation of the optimized structure so that a right TES for the crystalline phase, and a left TES for the amorphous phase occur at the same frequency. Thus, we realize switching between right and left TESs at that frequency by switching between the crystalline and amorphous phases of GST. Our results could be potentially important for developing compact reconfigurable topological photonic devices.
... Another application is to employ modulated light intensity to achieve optical switching function. 69,351 For instance, a plasmonic switch is achieved when GST is applied on a waveguide to control the propagation of SPP at an Au/SiO 2 interface (Figure 17c(i). By using a laser pulse to reversibly control the phase of GST transition from its amorphous to crystalline structural phase, the refractive index, transmission, and absorption coefficient of SPPs are modulated, thus enabling optically switching of SPP. ...
Article
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The alluring electronic, optoelectronic, and photonic properties of low‐dimensional materials have allowed brain‐inspired electronics to evolve in unprecedented ways. With highly efficient neuromorphic devices and architecture being concocted lately, an understanding of the underlying device mechanisms has emerged. The question of what types of materials and physical mechanisms will be used in future neuromorphic hardware is still open for debate. Herein, a critical review of the mechanisms among various configurations in state‐of‐the‐art low‐dimensional neuromorphic devices is presented. The factors are also reviewed that influence the working paradigm of low‐dimensional neuromorphic devices under different stimuli. Finally, a forward‐looking outlook on the challenges and perspectives in analyzing the mechanisms in this emerging research direction to drive next‐generation neuromorphic computing is provided. Brain‐inspired neuromorphic devices have attracted growing research interest for enabling next‐generation intelligent and autonomous systems. Underlying working mechanism in neuromorphic devices based on low‐dimensional materials is heavily based on device architecture and their electrical, optical, and photonic operating modes. This article presents an overview of the mechanisms among various configurations in state‐of‐the‐art low‐dimensional neuromorphic devices.
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Nonvolatile phase change materials owing to their robust stability and reversibility have shown significant potential in nanophotonic switches and memory devices. However, their performances deteriorate as the thickness decreases below 10 nm due to the local deformation induced by the phase change, which makes them less compatible with plasmonic nanogaps. Here, we address this issue by photothermally modulating the refractive index of germanium antimony telluride (GST) placed in plasmonic nanogaps, which tunes plasmon resonances in the visible region below the melting point of GST, making such optical switching highly reversible at a rate of up to hundreds of ∼kHz. They are also demonstrated to modulate the waveguiding efficiency of propagating surface plasmons, which is based on the photothermal modulation of plasmons with the assistance of GST. Such hybrid nanoplasmonic system with cost-effective fabrication and efficient operation method provides a promising route towards integrated nanophotonic chips.
Article
We introduce non-Hermitian plasmonic waveguide-cavity structures based on the Aubry-Andre-Harper model to realize switching between right and left topological edge states (TESs) using the phase-change material Ge 2 Sb 2 Te 5 (GST). We show that switching between the crystalline and amorphous phases of GST leads to a shift of the dispersion relation of the optimized structure so that a right TES for the crystalline phase, and a left TES for the amorphous phase occur at the same frequency. Thus, we realize switching between right and left TESs at that frequency by switching between the crystalline and amorphous phases of GST. Our results could be potentially important for developing compact reconfigurable topological photonic devices.
Article
The important research content of modern communication systems is to realize high-speed, stable, and intelligent information transmission and processing. All-optical neural networks based on the silicon integrated technology and phase change materials (PCMs) can realize picosecond-level modulation speed, faster processing speed, and lower energy consumption compared with the traditional electrical communication system. The photonic synapse is the core component of the all-optical neural network module, but the existing photonic synapses based on PCMs require a modulation distance (MD) of several micrometers or even ten micrometers to achieve a large output intensity range. In this paper, we propose an ultra-compact nonvolatile photonic synapse, in which MD can be shortened to 1 μm, breaking the record of the minimum signal MD of the silicon photonic synapse using the PCMs. At the same time, the output intensity range of our synapse is almost twice that of the existing research. Based on this photonic synapse, we analyze the relationship between the output response and incident wavelength, which can help to design an ultra-compact photonic convolutional neural network. This work has great potential in future photonic computing and photonic communication technologies.
Preprint
Full-text available
Optical phase-change materials are highly promising for emerging applications such as tunable metasurfaces, reconfigurable photonic circuits, and non-von Neumann computing. However, these materials typically require both high melting temperatures and fast quenching rates to reversibly switch between their crystalline and amorphous phases, a significant challenge for large-scale integration. Here, we present an experimental technique which leverages the thermo-optic effect in GST to enable both spatial and temporal thermal measurements of two common electro-thermal microheater designs currently used by the phase-change community. Our approach shows excellent agreement between experimental results and numerical simulations and provides a non-invasive method for rapid characterization of electrically programmable phase-change devices.
Article
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Materials with exceptionally strong optical activity with negative refractive index called negative index metamaterials (NIMs) or left-handed materials (LHMs) were predicted, and are now possible to fabricate, allowing an efficient light manipulation. By combining negative index materials (NIMs) with phase change materials (PCMs) like vanadium dioxide (VO2), which exhibits an insulator-to-metal transition at 341 K leading to a change in the material’s complex refractive index, one can be able to control the propagation, the amplitude, the phase, and the polarization state of light for applications in photonics.
Article
We propose an all-dielectric grating paradigm comprising an optical-phase-change-material (O-PCM), functional in the 5 µm to 10 µm spectral range. This system leverages the capabilities of a newly-discovered O-PCM [Nat. Comm. 10, 4279 (2019)], Ge2Sb2Se4Te1, which can be reliably switched between amorphous and crystalline phases at larger thicknesses close to 1 µm, while exhibiting a high-refractive-index shift of about 1.5 and no optical loss in this spectral range. The amorphous-O-PCM grating predominantly responds as an effectively homogeneous slab, letting light through without perturbing its path. The crystalline-O-PCM grating supports leaky Floquet-Bloch modes, which, at certain wavelengths, can simultaneously interfere destructively into the primary light path and constructively into the back-bent diffraction channel, in transmission. This “accidental” interference effect steers the incident beam in the negative direction. At a slightly detuned wavelength, the output power can be evenly split between the primary light path and the back-bent diffraction channel. Hence, our all-dielectric O-PCM-based metagrating can function as a platform for non-volatile reconfigurable beam steering and splitting. We have designed the metagrating paradigm and predicted its reconfigurable behavior with a semi-analytical calculation method and then verified it with a numerical first-principles experiment. We believe these results are relevant to MWIR/LWIR applications, but can also inspire new means for programmable and reconfigurable photonics across the spectrum as new O-PCMs are being developed. © 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement.
Article
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An optical switch operating at a wavelength of 1.55 μm and showing a 12 dB modulation depth is introduced. The device is implemented in a silicon racetrack resonator using an overcladding layer of the phase change data storage material Ge2Sb2Te5, which exhibits high contrast in its optical properties upon transitions between its crystalline and amorphous structural phases. These transitions are triggered using a pulsed laser diode at λ = 975 nm and used to tune the resonant frequency of the resonator and the resultant modulation depth of the 1.55 μm transmitted light.
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The unprecedented ability of nanometallic (that is, plasmonic) structures to concentrate light into deep-subwavelength volumes has propelled their use in a vast array of nanophotonics technologies and research endeavours. Plasmonic light concentrators can elegantly interface diffraction-limited dielectric optical components with nanophotonic structures. Passive and active plasmonic devices provide new pathways to generate, guide, modulate and detect light with structures that are similar in size to state-of-the-art electronic devices. With the ability to produce highly confined optical fields, the conventional rules for light–matter interactions need to be re-examined, and researchers are venturing into new regimes of optical physics. In this review we will discuss the basic concepts behind plasmonics-enabled light concentration and manipulation, make an attempt to capture the wide range of activities and excitement in this area, and speculate on possible future directions.
Article
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Surface plasmons are waves that propagate along the surface of a conductor. By altering the structure of a metal's surface, the properties of surface plasmons—in particular their interaction with light—can be tailored, which offers the potential for developing new types of photonic device. This could lead to miniaturized photonic circuits with length scales that are much smaller than those currently achieved. Surface plasmons are being explored for their potential in subwavelength optics, data storage, light generation, microscopy and bio-photonics.
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Article
Phase-change materials integrated into nanophotonic circuits provide a flexible way to realize tunable optical components. Relying on the enormous refractive-index contrast between the amorphous and crystalline states, such materials are promising candidates for on-chip photonic memories. Non-volatile memory operation employing arrays of microring resonators is demonstrated as a route toward all-photonic chipscale information processing.
Article
The further integration of optical devices will require the fabrication of waveguides for electromagnetic energy below the diffraction limit of light. We investigate the possibili- ty of using arrays of closely spaced metal nanoparticles for this purpose. Coupling between adjacent particles sets up coupled plasmon modes that give rise to coherent propagation of energy along the array. A point dipole analysis predicts group velocities of energy transport that exceed 0.1c along straight arrays and shows that energy transmission and switching through chain networks such as corners (see Figure) and tee structures is possible at high efficiencies. Radiation losses into the far field are expected to be negligible due to the near-field nature of the coupling, and resistive heating leads to transmission losses of about 6 dB/lm for gold and silver particles. We analyze macroscopic analogues operating in the microwave regime consisting of closely spaced metal rods by experiments and full field electrodynamic simulations. The guiding struc- tures show a high confinement of the electromagnetic energy and allow for highly variable geometries and switch- ing. Also, we have fabricated gold nanoparticle arrays using electron beam lithography and atomic force micros- copy manipulation. These plasmon waveguides and switches could be the smallest devices with optical functionality.
Article
The large optical contrast between crystalline and amorphous phases of phase change memory materials is shown to arise from a large difference in the optical matrix elements. These are enhanced in the crystal by aligned rows of resonantly bonded p orbitals. Amorphous phases have normal-sized matrix elements due to an absence of this order, irrespective of coordination number. This is a more general description of local order differences between the crystalline and amorphous phases, which applies even when coordinations in the amorphous phases exceed the 8−N value.
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We present a theoretical investigation of the local launching of surface plasmon polaritons (SPPs) by means of metal nanostructures. Efficient conversion of a propagating plane wave into interface-bound SPPs is achieved through grooves in the surface of bulk metals or slits in thin metal films. The incident light excites not only SPPs at the metal surface but also the electromagnetic modes inside the groove, which behaves like a cavity so that its width and depth have a strong impact on the SPP excitation efficiency. The light-SPP coupling can furthermore be improved by several grooves interfering constructively. Tuning the width of a slit in a thin metal film allows us to preferentially excite either one of the two fundamental SPP waveguide modes of the system, namely either the mode propagating at the upper metal interface or its counterpart at the lower interface. Maximum conversion efficiencies of more than 50% can be achieved with optimized geometries.