本书将从原理和应用两个方面详细介绍多种环境下的等离激元增强荧光。基于局域表面等离激元共振的等离激元能提供荧光所需要电子和能量，在表面等离激元增强荧光，针尖增强荧光光谱，表面等离激元增强上转换荧光材料，表面等离激元增强的共振荧光能转移的过程中发现等离激元能选择性增强分子或材料体系的荧光。

Chapter Ⅰ
Introduction

The optically generated collective electron density waves on metaldielectric boundaries known as surface plasmons have been of great scientific interest since their discovery［1］. Being electromagnetic(EM) waves on gold or silver nanoparticle’s surface, localized surface plasmons (LSPs) can strongly enhance the EM fields［2］. These strong EM fields near the metal surfaces have been used in various applications like surfaceenhanced spectroscopy (SES), plasmonic lithography, plasmonic trapping of particles and plasmonic catalysis,etc［36］. Resonant coupling of LSPs to fluorophore can strongly enhance the emission intensity, the angular distribution and the polarization of the emitted radiation and even the speed of radiative decay, which is socalled plasmonenhanced fluorescence (PEF)［7］. As a result, more and more reports on surfaceenhanced fluorescence have been reported, such as surface plasmon amplification by stimulated emission of radiation (SPASER), plasmonassisted lasing, singlemolecule fluorescence measurements, surface plasmoncoupled emission (SPCE) in biological sensing, optical device designs,etc［810］. In this book, we focus on the principles and recent advanced reports on plasmonenhanced fluorescence.


All the color figures please scan the QR code.



References



［1］RITCHIE R H. Plasma losses by fast electrons in thin films［J］. Physical Review, 1957, 106(5): 874881.

［2］MOSKOVITS M. Surfaceenhanced spectroscopy［J］. Review of Modern Physics, 1985, 57(3): 783826.

［3］NIE S, EMORY S R. Probing single molecules and single nanoparticles by surfaceenhanced Raman scattering［J］. Science, 1997, 275(5303): 11021106.

［4］ESPINHA A, DORE C, MATRICARDI C, et al. Hydroxypropyl cellulose photonic architectures by soft nanoimprinting lithography［J］. Nature Photonics, 2018，12： 343348.

［5］JUAN M L, RIGHINI M, QUIDANT R. Plasmon nanooptical tweezers［J］. Nature Photonics, 2011, 5(6): 349.

［6］ZHANG Z, FANG Y, WANG W, et al. Propagating surface plasmon polaritons: towards applications for remoteexcitation surface catalytic reactions［J］. Advanced Science, 2016, 3(1): 1500215.

［7］GEDDES C D. Surface plasmonenhanced photochemistry［M］.//Metalenhanced fluorescence. New York: John Wiley & Sons, Inc., 2010: 769800.

［8］OULTON R F, SORGER V J, ZENTGRAF T, et al. Plasmon lasers at deep subwavelength scale［J］. Nature, 2009, 461(7264): 629632.

［9］PINILLA D H, MOLINA P, HERAS C D L, et al. Multiline operation from a single plasmonassisted laser［J］. Acs Photonics, 2017, 5(2)： 406412.

［10］FANG Y, SUN M. Nanoplasmonic waveguides: towards applications in integrated nanophotonic circuits［J］. Light Science & Applications, 2015, 4: e294.




Chapter Ⅱ

Physical Mechanism of PlasmonEnhanced Fluorescence


2.1Introduction

Since Prof.Ritchie first predicted that the existence of selfsustained collective oscillations at the surface of metallic nanostructures by consideration of energy losses characteristic of fast electrons transmitting across optically thick metallic films in 1957［1］,the collective electron density waves on metal surface known as surface plasmons (SPs) have been of great scientific interest in theoretical and experimental studies ［29］. Largely enhancement of the local electromagnetic (EM) field induced by surface plasmon resonance(SPR) plays an important role in studying SES, including PEF and SERS (surfaceenhanced Raman scattering)［1013］.It has been found that the characteristics of SPR (position and intensity) critically depend on geometrical parameters of the nanostructures, such as the size and shape, as well as the composition of the nanostructures, and the dielectric constant of the surrounding environment［1418］.

With the help of excitation of SPs, the electronphoton interaction and molecular photonic state can be modulated. As a result, by using plasmonic metallic nanostructures, the variety of responsive variables gives us a potential solution to study the new phenomenon appeared in many fields. Therefore, the study on the coupling effect among plasmonic metallic nanostructure substrate and surfaceenhanced fluorescence have been one of the most active and important topics, particularly since the first classical observation of the effect of environment on the emission characteristic of a fluorophoreexcited electronic state by Drexhage［19］. Although the enhanced effect is a systematic contribution from physical and chemical aspects, now it is widely recognized that the localized EM field generated at metal surface plays a key role for PEF effect. The SP distribution and the fluorescence emission from the fluorophores located near the metallic substrate can be modified by controlling the dielectric properties and shape of the metallic nanostructures. As a result, manipulation of optical nanostructure substrate with plasmonic properties is considered as the most promising direction in the PEF research area.

Particularly, better understanding of coupling and interconversion mechanism among free electrons, surface plasmons, photons and fluorophores based on metal substrate with various shape configurations in PEF effect is still highlighted［10,11,20，21］. In this review, we focus on the recent advancement of the effect of plasmonic nanostructures towards PEF. First, we introduce the concepts and principles of PEF. Then it is dedicated to PEF for fluorophores coupled to SPP excited on periodical metallic nanostructure substrate, to PEF for fluorophores coupled to localized enhancement of the EM field existed on nonperiodical metallic nanostructure substrate, and to PEF for the distance and wavelength dependence factor. And then, it will be demonstrated that tipenhanced fluorescence spectroscopy (TEFS) can even be conducted with subdiffraction limit spatial resolution. Finally, the recent progress on PEF from the rareearthdoped UC and DC nanoparticles is also demonstrated.

2.2The principle of PEF

Fluorescence as well as photoluminescence (PL), is the property of some luminescent center, such as organic molecules and Rareearth ions, to absorb photon with high energy and to emit a photon with low energy subsequently. The processes which occur between the absorption and the emission of photon could be demonstrated by a Jablonski diagram, which illustrates the electronic transition processes of fluorophores occurred in the excited states［22］. It is widely accepted that the excited electron transition from the upper energy level to the ground level, often by two methods, one is emission of photon, and the other is relaxation of energy into phonon［22］. With the condition of weak excitation, that is, far from saturation of the excited state, the fluorescence spontaneous emission rate γ0em can be regarded as the excitation from ground state to excited state and the subsequent relaxation back to the ground state via fluorescence emission［10,22］ i.e.


γ0em=γ0excQ0i(21)


where Q0i and γ0exc is the quantum yield and the excitation rate respectively, and the superscripts “0” specify that the molecule is in free space and does not couple to the local environment. The subscript “i” indicates that the quantum yield is defined by the intrinsic properties of the molecule.

As indicated before,Q0i is the probability of relaxing from excited to ground states by emitted a fluorescence photon. Here, the radiative and nonradiative decay rates can be defined as γ0r and γ0nr respectively where r means radiative,and nr means nonradiative, so the intrinsic quantum yield with more general definition for Q0i could be defined as below


Q0i=γ0r/(γ0r γ0nr) (22)


If the local environment of the fluorophore has been changed, the excitation and decay rates will be changed correspondingly. Then the Eqs. (21) and (22) can be modified as


γem=γexcQ(23)


and


Q=γrγr γnr=γrγr γ0nr γabs γm(24)


Here,γabs accounts for dissipation of heat in the environment and γm accounts for coupling to nonradiative EM modes, such as the emitted energy is transferred into heat through the interaction between the electron and the lattice. The whole decay rate γ=γr γnr defines the lifetime τ=1/γ of the excited state. In general, the fluorescence emission not only depends on the molecular properties but also on the external parameters accounting for the local environment of the fluorophore.

For a conventional fluorescence technique measurement, pursuing the brighter and more stable signals of the fluorophore performed at the minimization of the internal and environmentally conditioned nonradiative processes, increasing higher spontaneousemission rate mainly depends on its nature properties［22］. Though the fluorescence is known as the best optical methods for the detection of biological and chemical species, typical disadvantage for the conventional fluorescence technique is the relatively low signaltonoise (background) ratio which restricts its application in important areas of medical diagnostics, food control, security and, particularly realization of the purpose of the detection of a target fluorescent molecules on single molecule level. So exploring a proper solution to overcome the disadvantage mentioned before has attracted great attention because of their various application in many fields.

Since the pioneering work of Purcell［23］, many researches have demonstrated that the excited atomic state lifetime is critically dependent on the inner properties of the atom and its environment from both experimental and theoretical aspects［13,22］. Due to the interaction of the fluorophore with its environment nearby, the fluorescence processes including both the excitation and the emission processes, can be modulated through modifing its local EM field. As a result, obtaining the limit sensitivity detection of fluorescence signal, in order to overcome the disadvantage of the conventional fluorescence technique, can be performed by control of the local EM field around the fluorophores. With the help of the coupled effect of light with localized surface plasmons (LSPssupported by nanoparticles or nanoperodical, typed metallic nanostructure) and surface plasmon polaritons (SPPstraveling along periodicaltyped metal nanostructure), which can provide strong confinement of EM field intensity. These confined EM fields can interact with fluorophores at their absorption λab and emission wavelengths λem which alter respective transitions between the ground state and higher excited states (Fig.21). It is reported that the internal conversion process relies critically on the electronic configuration of the fluorophore through the overlap of its wavefunctions［10］. Thus, it is important to investigate the energy transfer process between the fluorophore and the metallic nanostructure, which will help us to study the contributed effect of the modified relaxation rate of fluorophore towards the radiative emission rate.



Figure 21Schematic of confined field of SPPs and LSPs modes coupled with a fluorophore excited with external EM field.



Based on the EM mechanism, the cross section of PEF from the metal nanostructures can be defined as σPEF(λL,λ,dav), where λ is the emission wavelength, λL is the excitation wavelength, dav defines as the average distance of fluorescent molecule from metal substrate. With the coupled effect between the plasmon resonance with the incidence wave and fluorescence emission, the total enhancement factor (EF) can be expressed as following equation［24,25］
σPEF(λL,λ,dav)=MEM(λL,λ,dav)2×σFL(λL,λ)Md(λ,dav)2

=Eloc(λL,dav)Ein(λL)2×Eloc(λ,dav)Ein(λ)2×σFL(λL,λ)Md(λ,dav)2

=M1(λL,dav)2M2(λ,dav)2×σFL(λL,λ)Md(λ,dav)2(25)
Here, MEM2 is total EF, while M12 and M22 are EFs related to the coupled effect between SPR with the incident wave and fluorescence emission respectively. σFL(λL,λ) and σPEF(λL,λ) are the scattering cross sections of fluorophore in the free space and local field respectively, and Mdav2 is the factor that depicts the energy transfer from fluorophore to metal substrate.

From Eq. (25), the fluorescenceenhanced effect shows a balance between several processes including the excitation rate increasing by the local EM field, a radiative decay rate enhancement by SPCE, and quenched effect due to the nonradiative energy transferring from the fluorophore center to the metallic substrate, all of which are critically distancedependent between the fluorophore center and the metallic substrate［26］.

From the point of view above, PEF is a multiprocess system, and the enhanced effect depends on many critical factors. In this chapter, we reviewed the recent progress of enhanced fluorescence effect from the periodicalar, nonperiodicaltyped plasmonic nanostructures, such as nanograting, nanohole arrays and so on, then the selection of excitation condition and distancedependent effect are also commented.


References


［1］RITCHIE R H. Plasma losses by fast electrons in thin films［J］. Physical Review, 1957, 106(5): 874881.

［2］PITARKE J M, SILKIN V M, CHULKOV E V, et al. Theory of surface plasmons and surfaceplasmon polaritons［J］. Reports on Progress in Physics, 2006, 70(1): 187.

［3］RINGE E, ZHANG J, LANGILLE M R, et al. Effect of size, shape, composition, and support film on localized surface plasmon resonance frequency: a single particle approach applied to silver bipyramids and gold and silver nanocubes［J］. Mrs Proceedings, 2009, 1208: 2.

［4］RINGE E, LANGILLE M R, SOHN K, et al. Plasmon length: a universal parameter to describe size effects in gold nanoparticles［J］. Journal of Physical Chemistry Letters, 2012, 3(11): 1479.

［5］HUANG Y, FANG Y, ZHANG Z, et al. Nanowiresupported plasmonic waveguide for remote excitation of surfaceenhanced Raman scattering［J］. Light Science & Applications, 2014, 3(3): e199.

［6］FANG Y, SUN M. Nanoplasmonic waveguides: towards applications in integrated nanophotonic circuits［J］. Light Science & Applications, 2015, 4: e294.

［7］ZHANG Z, FANG Y, WANG W, et al. Propagating surface plasmon polaritons: towards applications for remoteexcitation surface catalytic reactions［J］. Advanced Science, 2016, 3(1): 1500215.

［8］DONG J, WANG J, MA F, et al. Recent progresses in integrated nanoplasmonic devices based on propagating surface plasmon polaritons［J］. Plasmonics, 2015, 10(6): 18411852.

［9］AND P K J, ELSAYED M A. Surface plasmon resonance sensitivity of metal nanostructures: physical basis and universal scaling in metal nanoshells［J］. Journal of Physical Chemistry C, 2007, 111(47): 1745117454.

［10］FORT E, GRSILLON S. Topical review: surface enhanced fluorescence［J］. Journal of Physics D Applied Physics, 2008, 41: 33.

［11］BAUCH M, TOMA K, TOMA M, et al. Plasmonenhanced fluorescence biosensors: a review［J］. Plasmonics, 2014, 9(4): 781799.

［12］CALDAROLA M, ALBELLA P, CORTS E, et al. Nonplasmonic nanoantennas for surfaceenhanced spectroscopies with ultralow heat conversion［J］. Nature Communications, 2015, 6: 7915.

［13］MOSKOVITS M. Surfaceenhanced spectroscopy［J］. Review of Modern Physics, 1985, 57(3): 783826.

［14］SUN M, ZHANG Z, WANG P, et al. Remotely excited Raman optical activity using chiral plasmon propagation in Ag nanowires［J］. Light Science & Applications, 2013, 2(11): e112.

［15］SUIJIT K G, TARASANKAR P A L. Interparticle coupling effect on the surface plasmon resonance of gold nanoparticles: from theory to applications［J］. Chemical Reviews, 2007,107(11): 4797862.

［16］XU H, WANG X H, PERSSON M P, et al. Unified treatment of fluorescence and Raman scattering processes near metal surfaces［J］. Physical Review Letters, 2004, 93(24): 11945.

［17］EM V S L, DECKERTGAUDIG T, MANK A J, et al. Catalytic processes monitored at the nanoscale with tipenhanced Raman spectroscopy［J］. Nature Nanotechnology, 2012, 7(9): 583586.

［18］SUN M, ZHANG Z, CHEN L, et al. Plasmondriven selective reductions revealed by tipenhanced Raman spectroscopy［J］. Analytical Chemistry, 2015, 1(5): 9328.

［19］DREXHAGE K H. Interaction of light with monomolecular dye layers［J］. Progress in Optics, 1974, 12: 163232.

［20］CHRISTIANE H, LUKAS N. Exploiting the lightmetal interaction for biomolecular sensing and imaging［J］. Quarterly Reviews of Biophysics, 2012, 45(2): 209255.

［21］CHEN K, LEONG E S P, RUKAVINA M, et al. Active molecular plasmonics: tuning surface plasmon resonances by exploiting molecular dimensions［J］. Nanophotonics, 2015, 4(1): 186197.

［22］LAKOWICZ J R, MASTERS B R. Principles of fluorescence spectroscopy［M］. 3rd Edition. New York: Plenum Press, 1983.

［23］PURCELL E M. Spontaneous emission probabilities at radio frequencies［J］. Phys Rev, 1995, 69(11): 839.

［24］GALLOWAYC M, Etchegoin P G, Le R E. Ultrafast nonradiative decay rates on metallic surfaces by comparing surfaceenhanced Raman and fluorescence signals of single molecules［J］. Physical Review Letters, 2009, 103(6): 63.

［25］JOHANSSON P, XU H, K M, et al. Surfaceenhanced Raman scattering and fluorescence near metal nanoparticles［J］. Physical Review B, 2005, 72(3): 0335427.

［26］ITOH T, IGA M, TAMARU H, et al. Quantitative evaluation of blinking in surfaceenhanced resonance Raman scattering and fluorescence by electromagnetic mechanism［J］. Journal of Chemical Physics, 2012, 136(2): 1667.

CONTENTS PlasmonEnhanced Fluorescence： Principles and Applications CONTENTS Chapter ⅠIntroduction References Chapter ⅡPhysical Mechanism of PlasmonEnhanced Fluorescence 2.1Introduction 2.2The principle of PEF References Chapter ⅢPlasmonEnhanced Fluorescence 3.1Introduction 3.2PEF from periodical metallic plasmonic nanostructures 3.2.1PEF from nanograting substrate 3.2.2PEF from nanohole arrays substrate 3.2.3PEF from nanoparticle arrays substrate 3.2.4PEF from nanorod arrays substrate 3.3PEF from nonperiodical metallic plasmonic nanostructure 3.3.1PEF from metallic silver island substrate 3.3.2PEF from metallic fractallike substrate 3.3.3PEF from deposited metallic nanoparticle substrate 3.4The wavelength and spacer effect towards the fluorescence enhancement 3.5Conclusion and prospect References Chapter ⅣTipEnhanced Fluorescence 4.1Introduction 4.2Experimental works on tipenhanced fluorescence 4.3Theoretical calculations on tipenhanced Raman spectroscopy 4.4Results and discussion 4.5Conclusion and outlook References Chapter ⅤPlasmonEnhanced Upconversion Photoluminescence: Physical Mechanism and Applications 5.1Introduction 5.2Mechanism model of upconversion fluorescence 5.3Plasmonenhanced upconversion 5.3.1Plasmonenhanced upconversion photoluminescence from periodic plasmonic nanostructures 5.3.2Plasmonenhanced upconversion photoluminescence from nonperiodic plasmonic nanostructures 5.4Plasmonenhanced from single rareearthdoped nanoparticles 5.5The applications of plasmonenhanced UC luminescence 5.6Conclusion References Chapter ⅥTimeResolved PlasmonEnhanced Fluorescence for ExcitonPlasmon Interaction 6.1Introduction 6.2Two methods for the excitonplasmon coupling 6.2.1The first method 6.2.2The second method 6.3Conclusion References Chapter ⅦPlasmonEnhanced Fluorescence Resonance Energy Transfer 7.1Introduction 7.2Fluorescence resonance energy transfer 7.2.1The definition and physical mechanism of FRET 7.2.2Methods to measure FRET efficiency 7.2.3Applications of FRET 7.3Plasmonenhanced fluorescence 7.3.1The principle of PEF 7.3.2Principle of PEFRET 7.3.3Application of PEFRET 7.4Summary References ACKNOWLEDGEMENTS