All-Optical Logic Gates Bad on Nanoscale Plasmonic Slot Waveguides
Yulan Fu,Xiaoyong Hu,*Cuicui Lu,Song Yue,Hong Yang,and Qihuang Gong*
State Key Laboratory for Mesoscopic Physics&Department of Physics,Peking University,Beijing100871,People’s Republic of China
interference effect
台江步行街ecently,nanoscale all-optical logic gate devices have attracted enormous attention becau of their important applications infields of optical computing and ultrahigh speed information processing.The basic idea is to form constructive/ destructive interference between two light signals in a photonic crystal,a photonic microstructure having photonic bandgap,to obtain output logic states“1”and“0”,where the pha difference is introduced through third-order optical non-linearity.1−3However,to date,few experimental results of nanoscale integrated all-optical logic gate devices have been reported becau of small third-order nonlinear susceptibility of conventional materials.4Linear interference can also be ud to realize such logic gate devices,where the logic operation depends on the relative optical pha difference of two input signals.5Owing to the difficulty in precily controlling the optical pha difference,this method suffers from an inherent inst
ability and low intensity contrast ratio between output logic states“1”and“0”.6Until now,only a few experimental results have been reported.6It is possible to reduce the inherent instability by device miniaturization and monolithic integra-tion.7Plasmonic microstructures can confine light into subwavelength-scale regions and exhibit strong plasmonic enhancement,which provides an approach to scale down photonic devices further and to realize direct integration with solid-state chips.8,9Recently,Wei et al.have reported veral all-optical logic gates bad on linear interference between surface plasmon polariton(SPP)modes in a silver nanowire network.10,11The length of silver nanowires was about20μm, and the achieved intensity contrast ratio of output logic states “1”and“0”was only6dB.10,11
Here,we propo and realize nanoscale integrated all-optical XNOR,XOR,NOT,and OR logic gates bad on the linear interference of SPP modes in plasmonic slot waveguides, consisting of air slots etched in a thin goldfilm.The structures can provide transver-magnetic(TM)-like SPP modes,also called quasi-TEM SPP modes,strongly confined at the interface of dielectric waveguide and metalfilms.12The plasmonic slot waveguide exhibits both ultralong range propagation of over veral tens of micrometers and strong spatial confinement of light into a subwavelength scale region.13 SPP modes propagate in the direction parallel to goldfilm, which is suitable for on-chip inte
gration applications.The miniature device featuring lateral dimensions of less than5μm, precily controlled optical pha differences obtained through exact patterning of logic gates using focud ion-beam(FIB) etching technology,and quasi-monochromatic SPP modes excited by an830nm continuous-wave(CW)lar beam ensure a high intensity contrast ratio of24dB between output logic states“1”and“0”.The compact logic devices are stable, robust,and free from environmental impact,meeting the requirements of practical on-chip integration applications. The schematic structure of the plasmonic slot waveguide is shown in Figure1a.It consists of an air slot with width w of100 nm etched in a goldfilm with thickness h of100nm on a silicon dioxide substrate.The dispersion relation of the plasmonic slot waveguide was calculated by thefinite element method(using a commercial software package Comsol Multiphysics),14and the calculated results are depicted in Figure1b.The refractive index of air and silicon dioxide was t at1and1.5,respectively.The wavelength-dependent complex
凶猛近义词Received:August20,2012
Revid:October15,2012
Published:November1,2012请输入验证码
refractive index of gold was obtained from ref 15.The plasmonic slot waveguide can provide wideband guided SPP modes,which has been con firmed by Dionne ’s calculations.13To further con firm the guided SPP modes,we also calculated the power density pro file of a 530nm guided mode by the finite element method,and the calculated results are shown in Figure 1c.The guided mode is con fined mostly in the slot region and extends slightly into the adjacent silicon dioxide and air regions.The maximum intensity is at the gold −air interface in the slot,which is in agreement with Dionne ’s calculations.13The e ffective refractive index of the plasmonic slot waveguide for a 530nm SPP mode was calculated to be 1.57+0.0106i ,which indicates that the e ffective propagating length of this SPP mode is 11μm.Gold films with a thickness of 100nm were prepared using a lar molecular beam epitaxy (LMBE)growth system (model LMBE 450,SKY Company,China).As the excitation light source,we ud the beam (with a wavelength of 248nm,a pul width of 25ns,and a pul repetition rate of 5Hz)output from an excimer lar system (model ComPexPro 205,Coherent Company,USA).A FIB etching system (model DB235,FEI Company,USA)was employed to prepare th
e logic gate patterns.All widths of the plasmonic slot waveguides were 100nm.To excite and collect the needed SPP mode,we also etched a coupling grating connected with an air groove with a triangular con figuration at the input port of each plasmonic slot waveguide.We also etched a grating in the output port to help couple the SPP mode to free space for the purpo of measurement.The scanning electron microscopy (SEM)images of the fabricated logic gate samples are shown in Figure 2.The lateral dimension of the logic gate devices was
less than 5μm,reduced 4-fold compared with tho ud in previous reports.10,11The all-optical logic functions were measured using a scanning near-field optical
microscopy Figure 1.Characteristics of the plasmonic slot waveguide.(a)Schematic structure.(b)Dispersion relation.(c)Power density pro file of a 530nm SPP
mode.Figure
2.SEM images of all-optical logic gate samples.(a)For the OR校园暴力心得体会
gate.(b)For the NOT gate.(c)For the XNOR gate.(d)Lar spectrum.
system.The input coupling grating was normally illuminated from the back side using a p-polarized CW Ti:sapphire lar beam with a wavelength of 830nm.The incident lar beam was focud into a spot with a radius of about 2.5μm,ensuring that the lar beam covers all input gratings.The spot center was t at the perpendicular bictor line of the connection line between two input coupling gratings.The patterned area of each input coupling grating was about 1μm ×1μm.This ensures a nearly equal average excitation intensity for all of the input coupling gratings.The spectrum of the 830nm lar beam is shown in Figure 2d.The line width of the lar spectrum curve was only 1.9nm,which ensured that only the needed quasi-monochromatic 530nm SPP mode can be excited in the plasmonic slot waveguide.The optical-thick gold film can prevent the direct transmission of the lar beam.The incident lar was coupled into a 530nm SPP mode through the input grating.The SPP mode propagating through the plasmonic slot waveguide was scattered by the decoupling grati
ng in the output port.The scattered light was collected by a long working distance objective (Mitutoyo 20,NA =0.58)and then imaged onto a charge coupled device (CCD).To con firm the propagation properties of the SPP mode in the plasmonic slot waveguide,we also etched a straight waveguide,a 90°circular arc waveguide with a radius of 900nm,and a cosine arc-shaped waveguide,as shown in Figure 3a −c.The width and length of each of the three kinds of plasmonic slot waveguides were 100nm and 11μm,respectively.It is very clear that,under excitation of an 830nm CW lar,there is a distinctive signal output from the decoupling grating for each kind of waveguide,as shown in Figure 3d −f.This con firms the excellent propagation properties of the 530nm SPP mode.The di fferent sizes of lar spot in Figure 3d −f are related to the di fferent excitation intensity.The XNOR gate has three input waveguides,as shown in Figure 2c,the bending regions of which having a circular arc shape with a radius of 900nm to reduce propagation loss.16There is an optical path di fference of 795nm between waveguides B and C,and 265nm between waveguides A and C.Therefore,a destructive interference can be reached in the output waveguide for the 530nm SPP modes propagating through waveguides A and C,or waveguides B and C,while a constructive interference can be reached for the 530nm SPP modes propagating through waveguides A and B.Waveguide C was ud as the reference waveguide.To perform the logic operation of “0XNOR 0=1”,we etched a coupling grating connected with a triangular air groove in the input port
of
Figure 3.SEM images of plasmonic slot waveguides with a width of 100nm and a length of 11μm for a straight waveguide (a),a 90°circle arc waveguide with a radius of 900nm (b),and a cosine arc-shaped waveguide (c).CCD images of plasmonic slot waveguide under excitation of an 830nm CW lar for the straight waveguide (d),the 90°circle arc waveguide with a radius of 900nm (e),and the cosine arc shape waveguide (f).The red arrow indicates the position of the coupling grating in the input port of the plasmonic slot waveguide.The green arrow indicates the position of the decoupling grating in the output port of the plasmonic slot waveguide.
waveguide C,as shown in Figure 4a.The 530nm SPP mode can only be excited in waveguide C.No SPP mode can be excited in waveguides A and B.The measured CCD image obtained under excitation of an 830nm CW lar is shown in Figure 4b.There is strong signal with an intensity of 2.09au output from the decoupling grating.This corresponds to output logic “1”.The simulated H z field distribution when the SPP mode is incident in waveguide C is shown in Figure 4c.A strong signal output can be obtained in the output port,thus con firming the measured results.To perform the
logic operation of “1XNOR 0=0”,we etched a coupling grating connected with a triangular air groove in the input port of both waveguides A and C,as shown in Figure 4d.The 530nm SPP mode can be excited in waveguides A and C simultaneously.The measured CCD image obtained under excitation of an 830nm CW lar is shown in Figure 4e.There is a very weak signal
with an intensity of 0.00725au output from the decoupling grating.This corresponds to output logic “0”.The reason is that a destructive interference is reached in the output waveguide for SPP modes propagating through waveguides A and C becau the optical path di fference is a half of the wavelength of the SPP mode.The simulated H
z field distribution when SPP mode is incident in waveguides A and C is shown in Figure 4f.No signal can be obtained from the output waveguide.This con firms the logic operation of “1XNOR 0=0”.To perform the logic operation of “0XNOR 1=0”,we etched a coupling grating connected with a triangular air groove in the input port of both waveguides B and C,as shown in Figure 4g.Thus,the 530nm SPP mode can be excited in both the waveguides simultaneously.The measured CCD image obtained under excitation of an 830nm CW lar is shown in Figure 4h.There is a very weak signal output from the decoupling grating with an
intensity of 0.00916au.This corresponds to the output logic “0”.The reason is that a destructive interference is reached at the output waveguide for SPP modes propagating through waveguides B and C becau the optical path di fference is
one
Figure 4.Logic operation of XNOR gate.The SEM image of sample (a),measured CCD image under excitation of an 830nm CW lar (b),and simulated results (c)for the logic operation of “0XNOR 0=1”.The SEM image of sample (d),measured CCD image under excitation of an 830nm CW lar (e),and simulated results (f)for the logic operation of “1XNOR 0=0”.
The
SEM
image
of sample
(g),measured兰亭集序王羲之
CCD image under
excitation of an 830nm CW lar (h),and simulated results (i)for the logic operation of “0XNOR 1=0”.
The SEM image of sample (j),measured CCD image under excitation of an 830nm CW lar (k),and simulated results (l)for the logic operation of “1XNOR 1=1”.The red arrow indicates the position of the coupling grating in the input port of the plasmonic slot waveguide.The green arrow indicates the position of the decoupling
用醋洗脸的好处grating in the output port of the plasmonic slot waveguide.
and a half of the SPP wavelength.The simulated H z field distribution when the SPP mode is incident in waveguides B and C is shown in Figure 4i.No signal can be obtained from
the output waveguide.This con firms the logic operation of “0XNOR 1=0”.To perform the logic operation of “1XNOR
1=1”,we etched a coupling grating connected with a triangular air groove in the input ports of all waveguides A,B,and C,as shown in Figure 4j.The SPP mode can be excited in all three waveguides simultaneously.The measured CCD image obtained under excitation of an 830nm CW lar is shown in Figure 4k.There is a strong signal output from the decoupling grating with an intensity of
2.193au.This corresponds to the output logic “1”.Here,the total intensity of SPP mode propagating through waveguides A and B is double that in waveguide C.Although a part of the SPP energy in the output waveguide is reduced through destructive interference,the remainder can still be obtained from the output waveguide.The simulated H z field distribution when SPP modes are incident in waveguides A,B,and C simultaneously is shown in Figure 4l.A strong signal can be obtained from the output waveguide.This con firms the logic operation of “1XNOR 1=1”.The intensity contrast ratio between the output logic “1”and “0”can be calculated from 10·log(P 1/P 0),where P 1and P 0are the signal intensities of logic
“1”and “0”,respectively.4,5The intensity contrast ratio between output logic “1”and “0”was measured to be 24.8dB,which is enhanced by 4-fold compared with previously experimental reports.10,11When the guided SPP mode propagates in the direction of Y -axis,as shown in Figure
1a,the guided SPP mode can be described in the form of品牌女裤
17⎯⇀⇀=⎯⇀βω−H r t H x y e (,)(,)i y t 0()(1)where βis the propagation constant and ωthe circular frequency of the guided SPP mode.Therefore,the H z field distribution of the guided SPP mode takes on a standing-wave-like pattern (Figure 4c,f,i,and l)due to the modulation action of the term e i (βy −ωt ).18
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The XOR gate has an asymmetric Y-shape con figuration,as shown in Figure 2b,the bending region of which has a circular arc shape
with a radius of 900nm to reduce propagation loss.16There is an optical path di fference of 265nm between waveguides A and B,which indicates that a destructive interference can be reached in the output waveguide for 530nm SPP modes propagating through waveguides A and B.When a signal light was input in one of the input waveguides,a strong signal was obtained in the output port.Therefore,the logic operations of “1XOR 0=1”(Figure 5a,b)and “0XOR 1=1”(Figure 5d,e)were realized.There is a strong signal with an intensity of 5.043au output from the decoupling grating,which corresponds to output logic “1”.When signal light was input in waveguides A and B simultaneously,no signal was obtained in the output port becau a destructive interference was reached in the output waveguide.Therefore,the logic
operation of “1XOR 1=0”(Figure 5g,h)was realized.There is a very weak signal output from the decoupling grating with an intensity of 0.0342au,which corresponds to output logic “0”.The measured intensity contrast ratio reached 21.7dB.
The
Figure 5.Logic operation of XOR gate.The SEM image of sample (a),measured CCD image under excitation of an 830nm CW lar (b),and simulated results (c)for the logic operation of “1XOR 0=1”.The SEM image of sample (d),measured CCD image under excitation of an 830nm CW lar
(e),and simulated
results
(f)
for
the logic
operation
of “0
XOR
1=1”.The SEM image of sample (g),measured CCD image under
excitation of an 830nm CW lar (h),and simulated results (i)for the logic operation of “1XOR 1=0”.The red arrow indicates the position of the
coupling grating in the input port of the plasmonic slot waveguide.The green arrow indicates the position of the decoupling grating in the output port of the plasmonic slot waveguide.
