IEEE PHOTONICS JOURNAL, VOL. 17, NO. 5, OCTOBER 2025 4800107
Imp o ing Limi -o -De ec ion in High-Sensi i i y
Plasmo-Pho onic Mach-Zehnde In e e ome e
Re ac i e Index Senso s
S elios Simos , Lamp ini Damakoudi, Dimos henis Spasopoulos, Kons an inos Fo iadis ,
E aggelia Cha zianagnos ou , Jose Ca ei a, Gab iele Na ickai e, Michael Geiselmann, Juan A ocas,
Jean-Claude Weebe , Dimi is. V. Bellas, Ele he ia Lampada iou, Ele e ios Lido ikis, Kons an inos Vy sokinos ,
and Nikos Ple os
Abs ac —This wo k p esen s a dual plasmo-pho onic b anch
Mach-Zehnde in e e ome e e ac i e index senso in eg a ed
on a Si3N4pla o m, wi h bo h he senso and e e ence a ms
hos ing iden ical aluminum plasmonic me al s ipes. Expe imen-
al e alua ion o he con igu a ion e eals a bulk sensi i i y up
o 8801 nm/RIU and an inc eased en i onmen al noise esilience
compa ed o s a e-o - he-a plasmo-pho onic MZI senso s wi h he
plasmonic sec ion esiding only on he senso a m. The enhanced
noise esilience along wi h he high sensi i i y esul s in an expe -
imen ally ob ained low de ec ion limi alue o 4.4 ×10−6RIU,
im-p o ed by an o de o magni ude compa ed o he de ec ion
limi o 5 ×10−5RIU ha was measu ed in a e e ence MZI senso
wi h a plasmonic s ipe only a i s senso a m.
Index Te ms—Aluminum, MZI, Pho onic in eg a ed ci cui s,
plasmonic senso s, e ac i e index, SiN.
I. INTRODUCTION
PHOTONIC and plasmonic senso s a e ad anced op ical
sensing echnologies ha le e age ligh -ma e in e ac ion
o highly sensi i e de ec ion o biological, chemical, and en-
i onmen al applica ions [1]. Pho onic senso s mainly ely on
Recei ed 24 July 2025; accep ed 4 Augus 2025. Da e o publica ion 11
Augus 2025; da e o cu en e sion 22 Augus 2025. This wo k was suppo ed
by he Eu opean p ojec AMBROSIA unde G an 101093166. (Co esponding
au ho : S elios Simos.)
S elios Simos, Kons an inos Fo iadis, E aggelia Cha zianagnos ou, and Nikos
Ple os a e wi h he Depa men o In o ma ics, A is o le Uni e si y o Thessa-
loniki,54124Thessaloniki,G eece,andalsowi h heCen e o In e disciplina y
Resea ch and Inno a ion (CIRI-AUTH), Balkan Cen e , Buildings A & B, 57001
Thessaloniki, G eece (e-mail: [email p o ec ed]).
Lamp ini Damakoudi and Kons an inos Vy sokinos a e wi h he Cen e
o In e disciplina y Resea ch and Inno a ion (CIRI-AUTH), Balkan Cen e ,
Buildings A & B, 57001 Thessaloniki, G eece, and also wi h he Depa men o
Physics, A is o le Uni e si y o Thessaloniki, 54124 Thessaloniki, G eece.
Dimos henis Spasopoulos is wi h he Depa men o In o ma ics, A is o le
Uni e si y o Thessaloniki, 54124 Thessaloniki, G eece, also wi h he Cen e
o In e disciplina y Resea ch and Inno a ion (CIRI-AUTH), Balkan Cen e ,
Buildings A & B, 57001 Thessaloniki, G eece, and also wi h he EOS Tech-
nologies, OX4 1 JB Ox o d, U.K..
Jose Ca ei a, Gab iele Na ickai e, and Michael Geiselmann a e wi h he
LIGENTEC SA, EPFL Inno a ion Pa k, CH-1024 Ecublens, Swi ze land.
Juan A ocas and Jean-Claude Weebe a e wi h he Pho onic Depa men , ICB
UMR 6303, Uni e si é de Bou gogne, 21078 Dijon, F ance.
Dimi is. V. Bellas, Ele he ia Lampada iou, and Ele e ios Lido ikis a e wi h
he Depa men o Ma e ials Science and Enginee ing, Uni e si y o Ioannina,
45110 Ioannina, G eece.
Digi al Objec Iden i ie 10.1109/JPHOT.2025.3597420
wa eguides [2],[3], ing esona o s [4],[5],[6], o Mach-
Zehnde in e e ome e s (MZI) [7],[9] o acili a e ligh -ma e
in e ac ion and sensing, o e ing ce ain ad an ages such as high
sensi i i y [10], label- ee de ec ion [11] and mass-p oduc ion
c eden ials h ough he use o CMOS echnology. Howe e ,
pho onic senso con igu a ions mainly u ilize e anescen wa e
sensing and equi e a he leng hy s uc u es in o de o achie e
high sensi i i y alues. Plasmonic senso s, on he o he hand,
can ha e e en he whole op ical mode in e ac ing wi h ma e
by exploi ing su ace plasmon esonances (SPRs) in me allic
nanos uc u es o su ace plasmon pola i ons (SPPs) in me al-
dielec ic in e aces. This allows o ul a-sensi i e de ec ion a
he nanoscale [12],[13] wi h eal- ime moni o ing capabili ies
[14], su e ing, howe e , om highe op ical losses and bulky
I/O s uc u es.
The loss and I/O in e ace d awbacks can be o e come by
con e ging plasmonics wi h pho onics in o plasmo-pho onic
senso con igu a ions. Me ging he bes o bo h wo lds can
combine he low-loss passi e ci cui y om he pho onic do-
main, like in e e ome y, op ical I/O in e aces, il e ing, phase
shi ing and powe balancing, wi h he ul a-high sensi i i y
p ope ies o minia u ized plasmonic s uc u es, pa ing he way
o he ealiza ion o compac and powe ul senso con igu a-
ions. Va ious plasmo-pho onic a chi ec u es using esona o s
o in e e ome ic s uc u es ha e been p oposed such as ing
esona o s [15],[17], bimodal in e e ome e s [18],[19] and
MZIs [20],[24]. The MZI con igu a ion p obably o e s he
mos powe ul plasmo-pho onic senso layou , as i o e s a high
deg ee o pa ame iza ion oge he wi h expe imen ally epo ed
sensi i i y alues up o 4764 nm/RIU [23] and a simula ed and
heo e ically o mula ed po en ial o up o 60000 nm/RIU [22],
[25]. Almos all plasmo-pho onic MZI a chi ec u es epo ed so
a employ non-iden ical wa eguide b anches, [21] esul ing o a
di e en ial phase change be ween he wo b anches e en when
he same en i onmen al change is expe ienced by bo h a ms.
This inc eases i s sensi i i y o en i onmen al pe u ba ions and
ob iously deg ades i s Limi -o -De ec ion (LoD) pe o mance
cha ac e is ics.
In his pape , we p esen expe imen ally a balanced plasmo-
pho onic MZI e ac i e index senso ha employs iden ical
plasmonic wa eguides a bo h i s e e ence and senso a ms,
© 2025 The Au ho s. This wo k is licensed unde a C ea i e Commons A ibu ion 4.0 License. Fo mo e in o ma ion, see
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4800107 IEEE PHOTONICS JOURNAL, VOL. 17, NO. 5, OCTOBER 2025
Fig. 1. (a) Concep ual 3D design o he dual plasmonic MZI senso , (b) c oss-sec ion o he ape ed Si3N4wa eguide, (c) c oss-sec ion o he aluminum plasmonic
s ipe, (d) mic oscope pho o o he ab ica ed chip and (e) simula ed ansmission alues conce ning he e ical o se .
allowing o bo h high bulk sensi i i y and imp o ed LoD pe -
o mance. We ex end ou p e ious wo k epo ed in [26] and
alida e expe imen ally i s imp o ed LoD pe o mance agains
s a e-o - he-a imbalanced plasmo-pho onic MZI senso a chi-
ec u es ha use only a single plasmonic wa eguide a hei
sensing a m. The p oposed balanced MZI de ice is in eg a ed
onaSi
3N4pla o m wi h an iden ical aluminum (Al) s ipe
placed on bo h a ms o he MZI. Expe imen al e alua ion o
he sensi i i y pe o med wi h di e en wa e -based solu ions
e eals a high bulk sensi i i y o 8801 nm/RIU, inc eased by
almos 84% compa ed o s a e-o - he-a plasmo-pho onic MZI
senso s [23]. I s noise esiliency is expe imen ally compa ed
agains an op imized e e ence plasmo-pho onic MZI senso ha
has a single plasmonic s ipe a i s senso a m and an e en highe
sensi i i y o 12,093 nm/RIU, e ealing a LoD o 4.4 ×10−6
ha is imp o ed by one o de o magni ude compa ed o he LoD
o 5 ×10−5measu ed by he e e ence MZI senso . Gi en he
signi ican ly imp o ed noise esiliency and LoD pe o mance
o he p oposed senso con igu a ion e en when ope a ing as
a ba e die wi hou any he mal s abiliza ion mechanism, i is
expec ed ha his could allow o LoD alues well below 10−7
when p oduced in a packaged and he mally s abilized se up.
These enhancemen s will educe po en ial ex e nal noise ac o s,
leading o a dec eased noise s anda d de ia ion imp o ing he
LoD o he p oposed senso .
II. SENSOR LAYOUT AND PRINCIPLE OF OPERATION
A 3D schema ic o he p oposed senso layou is illus a ed
in Fig. 1(a).Fig.1(b) shows he c oss-sec ion o he ape ed
Si3N4pho onic wa eguide along wi h he dimensions o he
ma e ials used, while Fig. 1(c) p esen he Aluminum (Al)
plasmonic wa eguide pa o he con igu a ion. Fig. 1(d) depic s
a mic oscope pho o o he ab ica ed chip on which se e al
a ia ions o he p oposed con igu a ion a e implemen ed. The
p oposed senso a chi ec u e comp ises a plasmo-pho onic MZI
in eg a ed on a Si3N4pla o m om Ligen ec. The MZI consis s
o 800 ×800 nm Si3N4wa eguides. A 1 ×2 Mul i-Mode
In e e ence (MMI) spli e was used o spli he inpu ligh
equally be ween he wo b anches o he MZI. Tape ed Si3N4
wa eguides wi h a size o 7.5 μm×800 nm we e used o
achie e he coupling o he pho onic modes wi h he plasmonic
modes o he Al s ipes ha we e implemen ed a he wo MZI
b anches. Two iden ical Al s ipes we e deposi ed in espec i e
e ched ca i ies wi h a Ve ical O se (V.O) o 350 nm. The
hickness and wid h o he me al s ipes we e se o 80 nm and
7μm, espec i ely. The plasmonic leng h o he s ipes (Lplasm)
was se o 70 μm, while longe plasmonic s ipes can lead o
highe sensi i i ies he losses o he de ice will also inc ease.
The 70 μm o plasmonic leng h p o ide a high sensi i i y alue
while he losses o he de ice a e wi hin easonable limi s. An
in e ed ape was used o couple he plasmonic mode back
in o a pho onic mode, so ha he wo spa ially sepa a ed ligh
beams p opaga e h ough he Si3N4wa eguides o he wo MZI
b anches and a e o ced o in e e e ia a 2 ×2 MMI couple .
A pho onic di e en ial leng h o ΔL was added in one o he
wo MZI a ms o de ine he F ee Spec al Range (FSR) alue o
he in e e ome e . Iden ical he mo-op ical phase shi e s we e
also employed a he wo MZI b anches in o de o spec ally
une he in e e ome e esonance wi hin a de ec able spec al
window. One o he wo iden ical Al plasmonic s ipes se e
as he senso a m, whe e di e en liquid analy es a e used
o co e he plasmonic egion and he SPP mode is en i ely
exposed o he o e lying analy e. The second Al s ipe se es as
he e e ence a m and is always co e ed by he same aqueous
solu ion h oughou he en i e measu emen p ocess, so ha he
guided SPP mode is cons an ly exposed o he same su ounding
medium. Hence, any di e ence in he cons i u ion be ween
he liquid analy es ha co e he senso and e e ence a ms,
espec i ely, will esul in di e en e ac i e indices o he wo
analy es and will be ansla ed in o a di e en ial phase change
o he wo SPP modes guided a he wo b anches. This will
SIMOS e al.: IMPROVING LIMIT-OF-DETECTION IN HIGH-SENSITIVITY PLASMO-PHOTONIC MACH-ZEHNDER 4800107
TABLE I
SIMULATED FSR AND SENSITIVITY VALUES FOR DIFFERENT ΔL
cause a MZI esonance wa eleng h shi ha is di ec ly ela ed
o he e ac i e index di e ence be ween he wo MZI b anches.
Taking in o accoun ha he en i onmen al condi ions can be
conside ed iden ical a he wo MZI b anches, a noise esilien
pe o mance equi es ha he di e en ial phase change and as
such he esonance wa eleng h shi emains cons an as long
as he wo liquid analy es emain he same, i espec i e o any
en i onmen al changes.
III. DESIGN ANALYSIS
A design analysis has been ca ied ou o op imizing sen-
so pe o mance wi h espec o i s bulk sensi i i y and noise
esiliency pe o mance using he FDE, 3D FDTD and INTER-
CONNECT modelling ools o he Ansys Lume ical so wa e
sui e. Fig. 1(b) and (c) depic he c oss-sec ions o he Si3N4
pho onic and plasmonic wa eguides, espec i ely. We in oduce
he Ve ical O se (V.O.) pa ame e ha co esponds o he
heigh o he bo om oxide laye ha needs o be e ched away o
o m he ca i y whe e he plasmonic wa eguide is deposi ed in
o de o achie e bes in e ace coupling. Fig. 1(e) illus a es he
powe coupling be ween he pho onic and plasmonic wa eguide
o a V.O. anging om 0 nm up o 1000 nm, showing ha a
maximum ansmission o ∼0.4 is obse ed a a V.O. o 350 nm.
This ansmission alue was hen employed as he in e ace loss
in a MZI ci cui le el simula ion analysis, whe e also di e en
pho onic ΔL alues o he e e ence wa eguide a m we e
u ilized o allow o MZI senso s wi h di e en FSR alues.
Taking in o accoun he well-known dependence o he MZI
senso sensi i i y on i s FSR alue [25], a selec ion o a he
small ΔL alues anging om 5–16 um was used leading o
FSR alues be ween 70 and 186 nm and o espec i e sensi i i y
alues anging om 3037 o 9323 nm/RIU, as shown in Table I.
The sensi i i y pe o mance o each di e en ΔL was cap u ed
ia he ci cui le el simula ions by applying di e en e ac i e
indices on he senso a m while e aining a cons an e ac i e
index alue he a he e e ence a m.
A simila p ocedu e has been ollowed o modelling also
he e e ence plasmo-pho onic MZI senso ha con ains only
a single plasmonic s ipe placed a i s sensing a m, wi h i s
e e ence a m employing a pho onic wa eguide. The same V.O.
o 350 nm was assumed also he e o op imal coupling be ween
he pho onic and plasmonic wa eguide sec ions. Conside ing
he ela ionship be ween sensi i i y and FSR, a FSR alue o
310 nm was op ed in o de o allow o a ai compa ison wi h
he balanced MZI senso wi h he highes sensi i i y, leading o
aΔL alue o 20.4 μm.
IV. FABRICATION
The plasmonic senso s we e ab ica ed on Ligen ec’s AN800
(all ni ide 800 nm hick Si3N4) pla o m [27]. The pho onic
s ackwas ab ica edon100mmSiwa e andconsis so abo om
he mal silicon oxide, 800 nm hick Si3N4, and a silicon dioxide
op cladding. In addi ion, aluminum-based hea e s a e in eg a ed
in he s ack. Local openings abo e he pads a e de ined allowing
o elec ical p obing. The plasmonic senso is de ined in wo
p ocesses: i s , a local opening o he cladding is e ched down
o he a ge V.O. (350 nm below he SiN bo om le el). Nex ,
an aluminum s ipe is de ined a he bo om o he ca i y using a
li -o echnique. The li -o p ocess p e en s aluminum om
deposi ing on he ca i y sidewall, which would con ibu e o
coupling losses a he pho onic-plasmonic in e ace. The li -o
pa e n was de ined ia s eppe li hog aphy, as i o e s high
alignmen accu acy ( educing die- o-die a ia ion) and high-
h oughpu , pa ing he pa h owa ds scalabili y.
In iew o CMOS compa ibili y aluminum is he p ima y
me al o in e es in his s udy. Howe e , i is wo h o no e
ha o he plasmonic me als can be deposi ed as well on he
pa e ned chips ea u ing he pho onic ci cui y and local open-
ing in he cladding. In his con ex an o e layed PMMA-based
elec on-beam li hog aphy p ocess exploi ing alignmen ma ks
e ched in he cladding has been success ully demons a ed. In
pa icula , gold and aluminum s ipes ha e been deposi ed by
li -o p ocess a he bo om su ace o sensing a eas. This e -
sa ile chip-le el p ocess allows o a posi ioning o me al s ipes
ends wi h espec o inpu and ou pu Si3N4wa eguides wi h
an accu acy down below 100 nm in spi e o he elec on-beam
exposu e being pe o med in a se e al mic on-deep openings.
A close-up image and a SEM image o he ab ica ed senso a e
p esen ed in Fig. 2(a) and (b) espec i ely. A o al o 11 dies
o igina e om di e en wa e s ha e been e alua ed and p esen
a ab ica ion yield o 100%.
V. EXPERIMENTAL RESULTS
The expe imen al se up used o cha ac e ize and alida e he
p oposed con igu a ion is shown in Fig. 3(a). Two San ec Tun-
able Semiconduc o Lase s (TSL) 570 ha e been u ilized. One
o he TSLs ope a es in he ange o 1355 nm–1485 nm while he
second TSL ope a es in he ange be ween 1480 nm o 1640 nm.
The TSL ou pu s a e connec ed o a San ec Op ical Swi ch Uni
(OSU) ia a Pola iza ion Main aining Fibe (PMF) in o de o
allow unabili y o e a spec al window o 285 nm. A Lensed
PMF (LPMF) was used a he ou pu o he OSU. The LPMF is
placed on a 562 Newpo XYZ s age along wi h a ibe o a o o
align he inpu LPMF wi h he Spo Size Con e e s (SSC) o he
chip. The ibe o a o is used o ensu e ha TM pola ized ligh
is injec ed in o he chip. A Pel ie module is placed oge he wi h
a he mis o below he DUT holde o con ol he empe a u e
on he chip. The The mo-Elec ic Con olle (TEC) is con olled
by an ILX LDT 5910C he moelec ic empe a u e con olle . A
Single Mode Fibe (SMF) and a second XYZ s age we e used o
align he SMF o he ou pu SSC o he chips and collec he ligh .
The SMF is connec ed o a San ec MPM 210 op ical powe me e
(PM). The expe imen al se up is placed on a ib a ion-isola ion
4800107 IEEE PHOTONICS JOURNAL, VOL. 17, NO. 5, OCTOBER 2025
Fig. 2. (a) Close-up image o he sensing a ea (SiN ph-WG: pho onic wa e-
guide and Al pl-WG: aluminum plasmonic wa eguide), (b) SEM image o he
ab ica ed senso .
able along wi h a humidi y senso loca ed close o he chip o
measu e he en i onmen al condi ions.
Fig. 3(b) depic s he spec al esponse o he dual plasmonic
MZI senso con igu a ion when bo h plasmonic s ipes a e ex-
posed o ai as he su ounding medium. The measu ed FSR o
he de ec ed spec um is 180.3 nm, which is in good ag eemen
wi h he 186 nm FSR alue (186 nm) p edic ed by he simula-
ions. A high ex inc ion a io (ER) o up o 22.5 dB was also
measu ed.
In he same s uc u e, we apply ol age o he he mo-op ic
phase shi e s (PS) loca ed on he a ms o he MZI o cha ac e ize
hei pe o mance. The PS allow us o shi he esonance o he
s uc u e in o he egion o ou op ical window. By using he PS,
we can use con igu a ions wi h high FSR alues and hus a oid
he p oblem o he na ow op ical window. Fig. 3(c) shows he
spec al esponse o he MZI when di e en ol age le els a e
applied a i s phase shi e (PS). As can be obse ed, sweeping
he ol age le el o he PS o ces he MZI esonance o une,
allowing in his way obeing he MZI wi hin ade ec able spec al
window e en when MZIs wi h high FSR alues a e employed. A
2πphase shi was ob ained by applying powe equal o 312 mW.
R.I. expe imen s we e subsequen ly pe o med in o de o
e alua e he bulk sensi i i y o he p oposed senso con igu a-
ion. A d ople o dis illed wa e was placed on he plasmonic
s ipe o he e e ence a m o he MZI con igu a ion and se ed
as a e e ence liquid. In pa allel, ano he d ople o he same
dis illed wa e was also placed on he plasmonic s ipe o he
senso a m, so ha bo h he e e ence and senso a m hos he
TABLE II
EXPERIMENTAL FSR AND SENSITIVITY VALUES FOR DIFFERENT ΔL
same o e lying liquid medium. As a esul , he wo spa ially
sepa a ed op ical beams expe ience again he same op ical pa hs,
as was also he case when bo h a ms we e exposed o ai
en i onmen , allowing in his way o ha e an unchanged di e -
en ial phase change and he same spec al loca ion o he MZI
esonance. The sensi i i y pe o mance was e alua ed by using
hen di e en wa e -based solu ions wi h di e en e ac i e
indices anging be ween 1.3322–1.3411, which we e used o
co e he senso a m while he e e ence a m was cons an ly co -
e ed wi h pu e dis illed wa e . The ob ained spec al esponses
and sensi i i y cu es o 3 di e en MZI senso s uc u es a e
shown in Fig. 4(a)–( ).Fig.4(a) and (b) illus a e he acqui ed
spec a o di e en aqueous solu ions and he sensi i i y cu e,
espec i ely, o he MZI 1 s uc u e ha has an expec ed FSR
alue o 186 nm, e ealing a bulk sensi i i y o 8801 nm/RIU
wi h a i ed R-squa ed o 0.99. The spec a o MZI 2 ha
has an expe imen ally measu ed FSR o 99.7 nm a e depic ed
in Fig. 4(c), while he achie ed sensi i i y o 3248 nm/RIU
is illus a ed in Fig. 4(d). The spec a o MZI 3 ha has an
expe imen ally measu ed FSR o 63.8 nm and he espec i e
sensi i i y cu e a e shown in Fig. 4(e) and ( ), espec i ely,
decla ing a sensi i i y o 2603 nm/RIU.
The expe imen al esul s o he h ee di e en MZI s uc u es
a e summa ized in Table II. The measu ed bulk sensi i i y alues
a e in good ag eemen wi h he co esponding sensi i i y alues
p edic ed by simula ions and p esen ed in Table I.Thisisalso
e i ied in Fig. 5, which shows he sensi i i y e sus FSR cu e
ob ained bo h h ough simula ions and expe imen s o he h ee
MZI s uc u es, e ealing a close ma ching be ween simula ions
and expe imen s bo h wi h espec o he absolu e sensi i i y
alues as well as o he slope o he cu es. An e o analysis
has also been pe o med on MZI 1 wi h FSR alue o 186 nm.
The analysis on he expe imen al esul s o he sensi i i y es s
p esen s a s anda d de ia ion o 286.1 nm/RIU and a ela i e
e o o he simula ion and expe imen al sensi i i y calcula ed
app oxima ely a 9.41%.
To e alua e he he mal s abili y p ope ies o he MZI senso ,
he spec al ou pu o he MZI 1 wi h an FSR close o 186 nm
was cons an ly moni o ed o a o al du a ion o 30 minu es a
oom empe a u e wi hou using a TEC. A zoomed-in pic u e
o 20 di e en eco ded spec a o e his 30 min imespan is
shown in Fig. 6(a), e ealing ha almos no esonance shi
akes place wi hin his ime pe iod. The same expe imen was
hen epea ed when empe a u e condi ions a e con olled by
means o a TEC placed below he pho onic chip holde . Fig. 6(b)
shows he eco dedspec al esponseswhen hechip empe a u e
inc eases g adually om 28.9 o 42.3 °C, depic ing again a mino
SIMOS e al.: IMPROVING LIMIT-OF-DETECTION IN HIGH-SENSITIVITY PLASMO-PHOTONIC MACH-ZEHNDER 4800107
Fig. 3. (a) Expe imen al se up used o e alua e he ab ica ed senso s, (b) expe imen al ans e unc ion o bo h ou pu po s o he MZI senso wi h FSR alue
o 186 nm and (c) MZI senso ans e unc ion wa eleng h shi by applying ol age o he mos-op ic phase shi e s.
Fig. 4. (a) Re ac i e index expe imen spec ums in MZI 1 wi h expec ed FSR 186 nm, (b) cap u ed esonance shi s and linea i o MZI 1 spec ums,
(c) Re ac i e index expe imen spec ums in MZI 2 wi h expec ed FSR 100 nm, (d) cap u ed esonance shi s and linea i o MZI 2 spec ums, (e) Re ac i e
index expe imen spec ums in MZI 3 wi h expec ed FSR 70 nm and ( ) cap u ed esonance shi s and linea i o MZI 3 spec ums.
Fig. 5. Sensi i i y e sus FSR acqui ed h ough simula ion and expe imen al
measu emen s.
a ia ion in he esonance dip wi h a maximum wa eleng h shi
o Δλmax =1.3 nm and a s anda d de ia ion o 0.508 nm o e
almos 13.5 °C empe a u e. This p o es ha he empe a u e
a ia ions esul o almos iden ical he mo-op ically induced
phase shi s a he wo MZI b anches, since he MZI exploi s a
balanced con igu a ion wi h almos iden ical plasmo-pho onic
b anches, allowing o high ole ance o empe a u e d i s.
The expe imen al cha ac e iza ion o he LoD was ca ied
ou o he MZI senso wi h he highes bulk sensi i i y o
8801 nm/RIU and elies on he use o he well-known o mula
LoD =3σsenso
S(1)
whe e σsenso is he s anda d de ia ion o he esonance and S
is he bulk sensi i i y o he senso .
Based on he expe imen al esul s ob ained o a du a ion o
30 min a oom empe a u e when no he mal con ol is used,
4800107 IEEE PHOTONICS JOURNAL, VOL. 17, NO. 5, OCTOBER 2025
Fig. 6. (a) Spec um esonance o e ime in oom empe a u e and (b) cap u ed
spec ums while inc easing he empe a u e.
he s anda d de ia ion o he esonance d i was calcula ed o
be σ =0.022 nm. This measu emen includes he esonance
d i expe ienced due o he mal a ia ions bu also includes
he d i s due o ibe I/O coupling ib a ions and addi ional
noise ac o s ha o igina e om he cha ac e iza ion se up o he
non-packaged de ice. In o de o emo e hese addi ional noise
componen s ha would no be p esen in case a ibe -pig ailed
and packaged de ice is employed, a simila spec al esponse
moni o ing p ocess was epea ed o a simple s aigh Si3N4
wa eguide by measu ing he spec al shi o he esonances
gene a ed due o he bi e ingence o he wa eguide. The s an-
da d de ia ion σI/O o hese spec al a ia ions was ound o be
0.018 nm and includes he noise o he ibe in e ace ib a ions
and any addi ional noise ac o s ha a ec he single wa eguide
esponse. Using hen he o mula:
σ2
=σ2
senso +σ2
I/O (2)
he noise o igina ing om he measu emen se up can be
sub ac ed and he senso noise is calcula ed o equal
σsenso =0.0129 nm. By applying S =8801 nm/RIU and
σsenso =0.0129 nm o (2) he LoD o he p oposed con-
igu a ion was calcula ed o be 4.4 ×10-6 RIU. The same
p ocedu e was also epea ed o he e e ence plasmo-pho onic
TABLE III
COMPARISON OF DUAL PLASMONIC MZI AND SINGLE PLASMONIC MZI
TABLE IV
PLASMO-PHOTONIC SENSORS COMPARISSON TABLE
MZI con igu a ion whe e only a single plasmonic s ipe was
employed a he MZI senso b anch. The sensi i i y was ound
o be 12093 nm/RIU and he s anda d de ia ion o he e e ence
MZI noise was ound o be 0.2024 nm, esul ing o a LoD o
5×10-5 RIU, i.e., one o de o magni ude lowe han he LoD
ob ained in he case o he balanced plasmo-pho onic MZI. The
ade-o o he enhancemen o he LoD alue o he dual
plasmonic con igu a ion is he inc eased ab ica ion complexi y
which leads o a highe sensi i i y o ab ica ion e o s. Table III
summa izes he pe o mance cha ac e is ics o he balanced and
non-balanced e e ence MZI senso s. A compa ison able o he
s a e o he a o he plasmo-pho onic senso s is p esen ed in
Table IV.
VI. CONCLUSION
We demons a e a no el, balanced plasmo-pho onic MZI e-
ac i e index senso con igu a ion on a Si3N4pla o m. The
p oposed con igu a ion u ilizes wo iden ical plasmonic me al
s ipes one in each a m o he MZI, and he phase changes due
o en i onmen al pe u ba ions a e he same in bo h a ms, com-
pensa ing o ex e nal noise. This imp o es he s abili y o he
senso and inc eases i s esilience o en i onmen al pe u ba ion.
The p oposed plasmo-pho onic MZI con igu a ion achie es he
highes epo ed bulk sensi i i y o up o 8801 nm/RIU and can
be u he imp o ed by changing he FSR o he a chi ec u e.
Addi ionally, he senso has a e y good LoD o 4.4 ×10−6
RIU. The a chi ec u e and he CMOS compa ible ma e ials used
in he design can lead o mass p oduc ion senso chips capable
o pe o ming mul iple examina ions on a single die.
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