Multiscale Optical Imaging of Complex Fields Based on the Use ofAzobenzene NanomotorsJ. Plain,† G. P. Wiederrecht,‡ S. K. Gray,‡ P. Royer,† and R. Bachelot*,†

†Laboratoire de Nanotechnologie et d’Instrumentation Optique LNIO-CNRS UMR 6279, Universite de Technologie de Troyes,Troyes, France‡Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States

ABSTRACT: Imaging approaches that enable visualization of electromagnetic fields overlength scales ranging from micrometers to nanometers are increasingly important as opticaltechnologies and scientific studies continue to push to smaller lengths scales to reveal newlight−matter interactions. In this Perspective, we review work on the use of azobenzenemoieties that are covalently attached to a polymeric backbone and function as opticallydriven nanomotors. The light-induced molecular movement has proven to be a very valuable approach for a wide range ofopportunities in subwavelength imaging and nano- to microscale control of mass transport. This approach is distinguished fromother types of optical exposure processes for imaging through vectorial transport. This feature opens new avenues for trulyunderstanding the governing processes of field formation and dynamic evolution in three dimensions in the condensed phase. Areview of modeling results is also provided in order to understand the mass-transport process beginning with light−moleculeinteractions.

The ability to image the optical near-field intensities thatresult when nanostructured materials and devices are

exposed to light is essential for both advancing scientificknowledge and enabling applications. Because ordinary opticalmicroscopy cannot resolve beyond about half of a wavelengthof visible light, other approaches that go beyond this diffractionlimit must be used, such as scanning near-field opticalmicroscopy (SNOM)1 and photoemission electron microscopy(PEEM).2 Typical achieved resolutions are in the 10−20 nmrange (∼λ/50). Whereas SNOM brings a probe tip in nearcontact with the sample and PEEM involves electronic opticsand ultrahigh vacuum conditions, we have been pursuing a“noninvasive” approach based on the mass-transport propertiesof poly(methylmethacrylate) (PMMA) functionalized withazobenzene derivatives.3,4

Eighteen years ago, the first observation of surface reliefgrating formation in azo dye polymers were reported.5,6 Visiblelight can excite the trans−cis isomerization in azobenzene-likemolecules7 grafted to PMMA, Figure 1. Upon absorbing aphoton, an isomerization cycle produces a “worm-like” motion8

that leads to a displacement along the transition moment axisthat could be on the order of a nanometer. Two-photonabsorption is also possible, enabling the use of infrared light.9

Many repeated isomerization cycles ultimately lead to morelarge-scale transport, which reflects the multiscale nature of thephenomenon. Imagine now a thin layer of such an azo-containing polymer being deposited over metal nanoparticlesthat have a strong plasmon resonance in the visible such thatthe near-field intensities are quite large in certain places. After

Received: March 27, 2013Accepted: June 13, 2013

The ability to image the opticalnear-field intensities that resultwhen nanostructured materialsand devices are exposed to lightis essential for both advancingscientific knowledge and ena-

bling applications.

Figure 1. The photosensitive copolymer. (a) Photoizomerization ofazobenzene molecules that present two different conformations (transand cis isomers). (b) DR1 molecules grafted onto a PMMA backbone.(c) The typical absorption spectrum of DR1-co-MMA. It defines therange of use of our molecular probe, that is, from 400 to 600 nm. Bluespectrum: DR1 in solution; red spectrum: PMMA-DR1 thin film.

Perspective

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exposure to the light, an atomic force microscope (AFM) canbe used to measure the resulting topography that will be relatedto the near-field intensity pattern to be characterized.The azobenzene-like molecules are thus nanomotors that

push and pull at the polymer material, leading to topographicalfeatures after exposure that reflect the underlying near-fieldintensities. This topography is therefore a reflection of the lightto be analyzed. This Perspective will discuss both ourexperimental studies of imaging complex near-field intensitypatterns utilizing optical nanomotors and our efforts attheoretically understanding the multiscale dynamics responsiblefor such results. The presented approach can be placed in thebroad and exciting framework of molecular and nanoscalemachines.10,11 The specificity that we would like to emphasizehere is that the nanomachines are driven by evanescent stronglylocalized optical fields supported by, in particular, metalnanostructures. The resulting displacement results in aninduced surface topography that is, in turn, exploited toinvestigate nanooptics of the metal nanostructures.Principle and Materials and Properties. In this Perspective, we

present a method to map complex electromagnetic (EM) fieldsbased on a molecular nanomotor. The principle is very simple,although data interpretation may be nontrivial, as we will show.A copolymer is spin-coated onto the metal nanostructures.Then, the nanostructures are simply irradiated using a suitablepolarized laser. The copolymer is a “self-developing” polymer.The concept of self-developing means that mass transport isinduced under light irradiation. Finally, an image is obtained,which corresponds to a projection of the EM field along thedifferent polarization components, using an atomic forcemicroscope (AFM).From the material point of view, the copolymer used in these

studies is composed of two components. The first componentis a polymeric backbone, in our case, PMMA. This matrixenables material manipulation. The second component is themolecular nanomotor, which is grafted onto the backbone. Inour case, the molecular motor is the disperse red 1 (DR1)molecule, part of the azobenzene derivative family. Azobenzenederivatives form a family of molecules made from two phenylgroups linked by a double bond NN, as shown in Figure 1a.These molecules are photoisomerizable, that is to say they

present two different conformations (trans and cis isomers), asshown in Figure 1a. Depending on the substituting moleculesin positions 4 and 4′ (see Figure 1a), the optical properties ofthe two different isomers will be modified. DR1 is a pseudo-stilbene. Pseudo-stilbenes are characterized by a substitution ofposition 4 (or 4′) by an electron-donor group and position 4′(respectively 4) by an electron-acceptor group. In this case, theabsorption spectrum is the same for both trans and cisconformations, allowing for multiple isomerization cycles with asingle incident wavelength. The probe used in our experimentsis a copolymer called DR1-co-MMA. It corresponds to DR1molecules grafted onto a PMMA backbone, as shown Figure1b. The typical absorption spectrum of DR1-co-MMA is shownFigure 1c. This spectrum defines the range of use of ourmolecular probe, that is, from 400 to 600 nm. It should bementioned that azo molecules can be grafted to matrixes otherthan polymers. For example, PMMA can be replaced by a sol−gel silica material,12 which could be advantageous when organicmaterials should be avoided.The photoinduced motion is highly dependent on the local

field polarization, as has been extensively demonstrated.4,13,14

Thus, the obtained image reflecting the EM field containsinformation on the local polarization state, which is animportant asset relative to, for example, photopolymeriza-tion.15,16 In order to interpret correctly this information, an in-depth understanding of the behavior of the mass motion and ofthe nanomotor at the molecular level is very important. Inorder to understand them, numerous models and numericalsimulations have been reported.17−19 Most of them aredescribed and commented on in ref 20. Each model fails togive a complete view of the problem. In order to give a generalmodel for the azobenzene molecules under light irradiation, wedeveloped the photoinduced molecular diffusion model (orPIMD model).20−22 Our model is based on three fundamentalhypotheses concerning the displacement of the azobenzenemolecules:(i) Each dye molecule must absorb a photon to undergo an

isomerization. The absorption probability is given by Pabs ∝|μ·E|2 ∝ |E|2 cos2 ϕ, with ϕ being the angle between thedirection of the local incident field, E, and the molecular dipole,μ.

Figure 2. PIMD model. (a) inchworm translation model. (b,c) Example of such a calculated topography. (b) Calculated local EM field obtained byFDTD. The field was calculated in the vicinity of a metal bowtie antenna. Red arrows illustrate the resulting molecular displacement. (c)Experimental AFM image of the topography obtained after irradiation at 514 nm. (d) Optically induced topography calculated from (b). (e) Zoom-out of the AFM image showing the position of images b−d. The white arrow represents the incident polarization direction, and the red arrowsillustrate the molecular displacement. From refs 4 and 20.

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(ii) The resulting motion of a dye molecule occurs along theaxis of the molecule (identified as the direction of the moleculardipole), as described by the inchworm translation modelproposed by Lefin et al.8 and schematically represented inFigure 2a.(iii) A dye molecule jumps from a hole in the polymer matrix

to another hole. The hole concept is defined between twolimiting values of the local density of the polymer film. If thelocal density is higher (lower) than a certain value dhigh (dlow),the translation process is impossible. We assume that theprobability distribution is defined by a Lorentzian function ofthe local density. Then, a molecule is only allowed to move to anew position according to a joint probability PA→B = PA × PBformed from a probability for it to leave its current position PAand one for it to occupy the new position PB. Several additionalhypotheses are taken into account concerning the nature of thepolymer matrix and the forces experienced by the azobenzenemolecules.(iv) Short-range interactions (e.g., van der Waals inter-

actions) are accounted for with a random reorientation of thenearest neighbors in the direction of the moving molecule.(v) Long-range interactions of azobenzene molecules on the

same chain are accounted for as follows. Each time anazobenzene group moves, a random set of n molecules graftedin the same polymer chain, within a sphere of radius R, isselected. These molecules are then randomly reoriented in thedirection of the moving molecule. The grafting density iscorrelated with n, and the polymer chain length and radius ofgyration are correlated with R.

The PIMD model allows one to calculate topographies afterirradiation.20,21 In the case of a resonant metal nanoparticleacting as a localized light source, we coupled the PIMD modelwith the finite difference time domain (FDTD) method forcalculating EM fields. This PIMD-FDTD approach allowed usto self-consistently account for the dynamic changes of thedielectric environment as the material motion occurs.22 Anexample of such a calculated topography is shown in Figure 2for the case of a complex EM field configuration including farand near fields. Figure 2b presents a calculation of the local EMfield intensity obtained by FDTD at λ = 514 nm. Figure 2bshows the isocontours of the optical intensity in the vicinity of asilver bowtie antenna made of two triangles. The “1” and “2”labels are the high-intensity regions inducing the massmigration (red arrows) leading to the formation of atopography maximum. Figure 2c is the corresponding AFMimage of the topography taken after 10 min of irradiation at 514nm (incident polarization perpendicular to the bowtie axis),and panel d presents the calculated topography based on themodel presented above. There is an excellent agreementbetween the experimental topography and calculated top-ography. In this example, at first approximation, there isextensive mass transport from regions of high near-fieldintensity toward regions of low near-field intensity. Figure 2e

is a zoom-out of Figure 2c, highlighting the zone of interest inFigure 2b−d. More details can be found in ref 20. (The precisegeometry of the bowtie will be shown in Figure 5b.) Afterexposure to the light, the resulting topography is essentially the“negative image” of the near-field intensity. This effect is well-known. It has been observed at the microscale in the case of useof interferometric patterns and focused beams.23,24 We willcome back to this point later.From the Microscale to Molecular Scale Optical Imaging. Here,

we present and comment on some significant examples thathighlight the assets of the imaging method, especially itsmultiscale and vectorial aspects. Figure 3 shows examples of the

photoinduced topography at the micrometer scale. In thesecases, no evanescent fields are involved, and classical optics areused to generate incident optical fields. Figure 3a illustrates thecase of a grating obtained from two-beam interferometry.25

Despite the apparent simplicity of the configuration, it has beenintensively used by many groups for understanding the processof topography formation. This surface relief grating (SRG)approach allows one to easily retrieve the photorecordedpatterns in terms of period and orientation. Additionally, thecapability to record a grating constitutes a first step towardholography. Other far-field micrometer-scale optical fields havebeen recorded. Figure 3b−d shows three examples oftopographical patterns obtained from focused fields presentingmore and more complex features in terms of intensity andpolarization distribution. Panels b and c in Figure 3 correspond

The azobenzene-like moleculesare thus nanomotors that pushand pull at the polymer material,leading to topographical featuresafter exposure that reflect theunderlying near-field intensities.

Figure 3. Examples of photoinduced topography at the micrometerscale. (a) Case of a grating obtained from two-beam interferometry.Reprinted with permission from ref 25. Copyright 1998, AmericanChemical Society. (b) Case of simple linearly polarized focused beamof 3 μm diameter and (c) a circularly polarized focused beam; matterescapes from the light and accumulates along the incident polarizationdirection. (b) and (c) reprinted with permission from ref 26.Copyright 1999, American Institute of Physics. (d−f) Case of alongitudinal (out-of-plane) field generated at a focus plane of a highnumerical aperture (NA = 1.45) objective lens that was initiallyilluminated with a linearly polarized field. (d,e) Topography obtainedwith two perpendicular incident fields. (f) Calculated intensities: totalintensity |E|2 (left), in-plane intensity |Et|

2(middle), out-of planeintensity |Ez|

2 (right). (g,h): Case of a focused Laguerre−Gauss beam.(d) and (f) reprinted with permission from ref 27. Copyright 2006,The Optical Society. (g) Schematics of two examples of vortex beamoptical fields used. (h) Example of obtained topography (AFM 6 × 6μm2 image). (g) and (h) reprinted with permission from MacmillanPublishers Ltd.: Nature Communications (Ref 28), copyright 2012.

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to a simple linearly polarized focused beam of 3 μm diameterand a circularly polarized focused beam, respectively.26 Thefields involved are purely of in-plane nature. In the case of thelinearly polarized beam, the initial field’s high degree ofsymmetry is broken; matter escapes from the light andaccumulates along the incident polarization direction, leadingto a two-lobe topographical structure. This example illustratesthe possibility to “conduct” matter along a preferential distancein a controlled way just by remote tuning of the polarizationdirection.Figure 3d−f shows a more complex case.27 A longitudinal

(out-of-plane) field was generated at a focus plane of a highnumerical aperture (NA = 1.45) objective lens that was initiallyilluminated with linearly polarized light. The intensity of thislongitudinal (z-polarized) field, Ez

2, forms two lobes parallel tothe incident field (see the right part of Figure 3f). It is 10 timesweaker than the in-plane intensity Et

2 (the left part and middleof Figure 3f). Nonetheless, its effect on topographicalmodification dominates over in-plane effects. Figure 3d,ecannot be explained by lateral diffusion of the polymertriggered by transverse components where a central depletionqualitatively similar to the spatial distribution of Et would beexpected. Instead, we observed high-level topography where theEz intensity is maximum (see the comparison between Figure3d and the right part of Figure 3f). Additionally, the two hightopography lobes rotate with incident polarization (Figure3d,e), as predicted by numerical calculation of the Ez

2 map.This example illustrates another mode of matter displacement.For the case of a thin film, interfacial longitudinaldiscontinuities (glass−polymer−air) are present, and free-space volumes need to be taken into account, leading to astrong sensitivity to the longitudinal field component even if itsamplitude is low compared to the in-plane field, as is the case inFigure 3d−f. As a result, the longitudinal field effect dominates,producing a raised topography where the intensity is high(positive image of the intensity distribution), which is theopposite topography produced by in-plane fields. This is animportant point because optical evanescent near fields aregenerally characterized by significant out-of-plane polarization.Figure 3g,h shows a more complex example.28 The azobenzene-containing polymer was exposed to focused Laguerre−Gaussbeams presenting helical wave fronts, an optical vortex at theiraxis, and a doughnut-shape intensity profile. The obtained spiralreliefs have been shown to result from interference of thelongitudinal and transverse components. This example opensup the door to nanolithography of complex structures.From the Micrometer World We Now Zoom-in to the

Nanometer World. Optical fields at the nanoscale can begenerated through diffraction/scattering of subwavelengthscaled objects. The resulting fields are characterized by richangular spectra (including high in-plane wave vectors),evanescent waves, and complex polarization states. In particular,strong in-plane → out-of-plane depolarization is characteristicof near-field scattering. A first example is shown in Figure 4. Itconcerns the topography induced by a local optical field that isconfined and enhanced at the extremity of a metal tip underlaser illumination, through the optical “lightning-rod effect”.This effect refers to an electrostatic phenomenon where theelectric charges on the surface of a conductive material arespatially confined by the shape of the structure. For a conductorwith a varying shape, the surface charge density σ varies frompoint to point along the shape. In the regions of high curvature,σ is locally increased, resulting in a large electric field just

outside of the material. A similar phenomenon can be observedin nanometer-sized metallic structures (presenting shapesingularities) excited by electromagnetic radiation. This pointof view was discussed in detail by Van Bladel.29 This effect is ofincreasing interest to the near-field optics community because itallows for production of efficient optical nanosources. Figure 4ashows a calculated field distribution at a plane placed below theend of a 30 nm cobalt tip in air illuminated by p-polarized light(component parallel to the plane of incidence). P-polarizationis actually suitable for exciting an electromagnetic singularity inthe vicinity of metal tips as a consequence of the metal−dielectric boundary condition of the electric field.30

Metal tips were used for inducing photodisplacements inazobenzene-containing polymer thin films. An example of theresulting sub-50 nm size dot cone is shown in Figure 4b. Itpresents a “positive” topography due to the presence of a strongfield parallel to the tip axis (the intensity shown in Figure 4acorresponds to a field that is mainly vertically oriented). Thelightning rod effect is actually characterized by longitudinalfields. This approach enabled extensive study of the effect. Inparticular, the tip field enhancement was evaluated as a functionof the illumination condition (state of polarization, angle ofincidence, ...) and the tip’s features (radius of curvature,material, ...).30 As a significant example, Figure 4c shows theinfluence of the incident polarization state; the height of theobtained near-field spot turns out to be very sensitive to theincident angle of polarization, confirming that the intensity ofthe local tip field is enhanced gradually from s-polarization to p-polarization. These experiments stimulated interesting dis-cussions on the nature of the optical response of the molecule.Especially, it was confirmed that while lateral field componentstend to make molecules escape from light along the polarizationaxis, longitudinal components lift the matter vertically (drawing

Figure 4. Case of the lightning rod effect. (a) The calculated intensityat the end of a laser-illuminated cobalt tip. (b) Obtained topography inthe case of incident p-polarization (800 × 800 nm2 AFM image). (c)Height of the obtained cone as a function of incident polarizationdirection. Reprinted with permission from ref 30. Copyright 2003,American Institute of Physics.

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matter into the regions of higher light intensity), as aconsequence of the free space requirement of the molecules.Aperture-based optical near-field probes were used as well.31 Inthat case, an optical nanosource is generated through near-fielddiffraction by a small hole of subwavelength size. Again, theinfluence of local field components has been discussed, andoptical near-field lithography has been implemented.

Plasmonic structures have been imaged by this uniqueapproach. Over the past decade, the optical properties of metalnanoparticles have been the subject of increasing researchefforts that are motivated by potential applications in manyareas, including chemical/biological sensing,32 energy harvest-ing,33 light generation,34 data storage,35 near-field imaging,30,36

and nanoscale photochemistry.37 This area of research,generally denoted as “plasmonics”, has derived clear benefitsfrom intensive improvements in electromagnetic modeling30,38

and nanofabrication and characterization methods.39 Theimaging of plasmons still constitutes considerable challengesdue to the subwavelength spatial dimensions of surfaceplasmons and is an area that azobenzene nanomotors canaddress. Figure 5 shows examples of imaging plasmonic

nanostructures through this approach. Figure 5a concernshemispherical silver nanoparticles made by e-beam lithography.The inset shows an electronic image of the structures. The topimage is an AFM image of the structure covered with PMMA-DR1. The bottom image shows the same zone after a 10 minillumination using circularly polarized light at normal incidence.This illustrates the unique capability of azobenzene nanomotorsto nanostructure matter using a remote optical input.The case of Figure 5b is of particular interest.4 Silver bowtie

nanoantenna (see the SEM image in the inset) have beenimaged using linearly polarized light either parallel orperpendicular to the antenna axis. The interaction betweenlight and such a complex object is characterized by a strong in-plane → out-of plane depolarization, field confinement andenhancement, and coupling effects. This study highlights theoptically induced vectorial molecular mass transport of thelight-sensitive polymer. The experimental and theoreticalresults clearly showed that this method can map the threespatial components of the optical near field of complex metalnanostructures. The top image of Figure 5b shows the AFMimages of such structures taken before laser illumination, whilethe bottom row shows AFM images of the same structure afterillumination (λ = 514 nm, polarization perpendicular to thebowtie axis). In this way, a clear near-field snapshot was taken.Precise analysis of such images can lead to valuable information.In particular, it was found that longitudinally polarizedplasmons are confined at the top of the metallic bowtie actingas an optical cavity for propagating surface plasmons.Furthermore, the intricate optical near fields in the polymerled to molecular trapping regions at intensity minima in thevicinity of the antenna.

The imaging of plasmons stillconstitutes considerable chal-

lenges due to the subwavelengthspatial dimensions of surfaceplasmons and is an area that

azobenzene nanomotors can ad-dress.

Figure 5. Examples of azobenzene nanomotor imaging of plasmonic nanostructures. (a) Hemispherical silver nanoparticles made by e-beamlithography. (Top) AFM image of the structure covered with PMMA-DR1. (Inset) SEM image of the structures. (Bottom) The same zone after a 10min illumination using circularly polarized light at normal incidence. (b) Silver bowtie nanoantenna. (Top) AFM images of such structures takenbefore laser illumination. (Inset) SEM image. (Bottom) AFM images of the same structure after illumination (λ = 514 nm, polarization perpendicularto the bowtie axis). (c) Gold nanocube obtained by colloidal chemistry. (Top) AFM image of the structure covered with PMMA-DR1. (Inset) TEMImage. (Bottom) AFM images of the same structure after illumination. (Inset) Zoom of the differential AFM image obtained by subtracting theimage taken before exposure from the image taken after exposure. The cube was covered with a sub-10 nm thick PMMA-DR1 film. From refs 3, 4,and 14.

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Figure 5c illustrates an ultimate use of the method.14

Extremely confined longitudinal fields were characterized at thesurface of gold nanocubes at different localizations. Thethickness of the deposited polymer film is a crucial parameter;it imposes the effective field depth that is investigated throughmolecular displacement. In the case of Figure 5a and b, thetypical thickness was in the 50−100 nm range. Over this depth,photons within the evanescent field and Fresnel zone areresponsible for molecular motion. The Fresnel zone is anintermediate zone corresponding to observation planes whereevanescent and confined propagating waves are present thatwill be detected in the far field after propagation (Fraunhofferdiffraction). In this region, it is possible to find a λ/2n-confinedoptical pattern. In order to detect a pure near field whereevanescent waves dominate, the polymer thickness must bereduced. This was successfully performed and is shown inFigure 5c; a sub-10 nm thick film of azobenzene-containingpolymer was used for ultrahigh-resolution near-field imaging ofcolloidal gold nanocubes.14 We analyzed the confinement ofboth electromagnetic hot and “cold” spots with a resolution ofλ/35 and emphasized the particularly high spatial confinementof cold spots. As an illustration, the top inset of Figure 5c showsan SEM image of the studied objects consisting of gold cubesmeasuring 60 nm on a side. The bottom inset image is adifferential AFM image obtained by subtracting the image takenbefore exposure (top AFM image) from the image taken afterexposure (bottom AFM image). It highlights the opticallyinduced molecular displacement regardless of the initialtopography. In this case, the in-plane incident polarizationwas parallel to a cube ridge, and the contrast revealed thelongitudinally polarized near-field dipolar emission orientedalong the incident polarization direction. The bottom inset ofFigure 5c also shows confined dark zones as a signature ofelectromagnetic cold spots. Both hot spots and cold spots wereprecisely studied as a function of cube orientation relative tothe incident field direction. The concept of a cold spot14

complements the well-known electromagnetic hot spot butproduces even more confined regions without charge densityvariations. Thus, ultraconfined cold spots offer a new route tohigh-resolution imaging and spectroscopy. The approach ofphotoactivated azobenzene nanomotors enabled the first clearobservation and investigation of electromagnetic near-field coldspots. In the future, the use of thinner and thinner films willrequire studies of the influence of the film thickness on theefficiency of the transport process.40 This study will define thelimit of our approach in terms of ultimate resolution.Continuing Our “Zoom-In” by Entering, f rom the Above

Nanoworld, into the Molecular World. To date, although

conformational change from the trans to the cis form of azodyes was already experimentally observed,41 it has not beenpossible to experimentally follow and address single moleculesundergoing displacement during laser irradiation. Simulations,however, have been performed that yield insight into this world.An example is shown in Figure 6; three different molecules(M1, M2, M3) move within the intensity distribution whoseprofile is represented in Figure 6a.20,34 The light polarization islinear and parallel to the intensity gradient. Each moleculediffers from the others by its initial location and orientationrepresented by a white arrow. Red dots represent the calculatedpaths followed by the molecules under illumination. Gray dotsrepresent the local intensity perceived by the molecules overtheir displacement. Figure 6b−d shows the total displacementand the EM intensity that each molecule encounters asnumerical iterations are performed. In general, molecules inintense regions easily move until they reach the low-intensityregion. This situation takes place after a subtle transient regime.For example, due to its initial orientation, molecule M3 firsttravels to a brighter region before escaping from this region to adarker zone. On the other hand, M2 undergoes an active andstraight displacement because it was initially in the highest-intensity region and was oriented almost parallel to the incidentfield. Finally, displacement of M1 is weak because it was initiallyin a dark fringe, such that its initial orientation had little impact.Relying on this multiscale aspect, it appears that individualmolecules do not simply migrate from high- to low-intensityregions. The microscopic and macroscopic matter accumulationis related to a statistical effect where molecules have variousmobilities as a function of their position and orientation,leading to a surface topography that involves a very largenumber of molecules. At the molecular scale, the displacementcan be more complex and is led by the local probability ofevents.Critical Analysis and Comparison with Other Methods.

Numerous methods have been developed to image EM fieldsat the nanoscale. Excellent reviews have been recentlypublished by A. Bouhelier et al.42 and K. A. Willets43 describingthe commonly used methods for plasmonic fields. Thesemethod include SNOM, PEEM, EELS (electron energy lossspectroscopy), and super-resolution optical far-field imagingbased on the use of point spread function fitting of a diffraction-limited spot in single-molecule fluorescence. From theseexhaustive reviews, it appears that our method has somedrawbacks; the first one is the spectral limitation induced by thedye absorption, the second one is that this is an indirectmethod of near-field measurement (an AFM step is needed

Figure 6. Case of three molecules (M1, M2, M3) of different initial position and orientation moving within the intensity distribution whose profile isrepresented in (a). The starting orientation of the molecules is given by the white arrows. (b−d) Field intensity and displacement path as a functionof time iteration for M1, M2, and M3, respectively. From ref 20.

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after the irradiation step), and finally, the interpretation of theresults requires numerical modeling.In this Perspective, we have discussed a photochemical

imaging approach based on azobenzene nanomotors. We cancontrast this approach with two other photochemicalapproaches. The first approach relies on nanoscale photo-polymerization triggered by the locally enhanced optical nearfield of a metal nanoparticle.15,16,44 A metal nanoparticle iscovered by a free radical photopolymerizable formulation(PPF) characterized by a threshold dose that must be overcometo induce the polymerization process. The polymerization isactivated by laser irradiation with wavelengths overlapping boththe PPF absorption spectrum (one-photon or multiphotonabsorption) and the plasmon resonance. The exposure dose ischosen to be smaller than the threshold dose, below which nopolymerization can occur. The field enhancement locallyinduced by the plasmon resonance will be the only regionthat surpasses the photopolymerization threshold to initiate thechain reaction leading to polymerization.15,16,44 This techniqueallows one to map and quantify (by comparing with thethreshold) the local field intensity around a nanometric metallicparticle. The second photochemical technique, recentlydeveloped by Quidant’s group,45 is based on the photolysisof PMMA by four-photon absorption. It allows one to map theelectromagnetic field around complex metallic nanostructureswith an excellent agreement with numerical calculations. Thefour-photon absorption acts as a threshold because theprobability of absorption is directly linked with the localintensity of the field. Unfortunately, no quantification of thelocal field has been demonstrated by this method. The majordifference between these two techniques and the PMMA-DR1based approach is that our technique allows one to discriminatebetween the different field components (in-plane and out-of-plane). The spectral range is basically equivalent and alwayslimited by the absorption of the photoactive molecule. Finally,the PMMA-DR1 based approach relies on a reversible effect;post exposure or heating of the sample allows one to retrieve itsinitial topography, enabling extensive studies of the samesample based on many successive exposures.Future Scope and Challenges. We have shown that light-

induced molecular movement of PMMA-DR1 is a very valuableapproach for a wide range of opportunities in subwavelengthimaging and nano- to microscale control of mass transport.

This approach can be distinguished from other types of opticalexposure processes for imaging by its vectorial transport nature,thus opening up new avenues for truly understanding thegoverning processes of field formation and dynamic evolution

in three dimensions in the condensed phase. Examples of whathas thus far been accomplished are the ability:• Map the optical near fields of plasmonic nanoparticles with

better than 20 nm resolution.3 A wide range of plasmonicnanostructures have been studied, and highly complex fieldshave been mapped, including nanoscale disks, rods, bowtieantenna, and cubes.• Discern the polarization of optical near fields, for example,

longitudinal versus transverse near fields, of plasmonicnanoparticles.4 This helps to reveal the source of the plasmonenhancement, such as whether the near field is due to apropagating surface plasmon polariton, as well as importantmorphology-dependent depolarization processes near the metalnanoparticle surface.• Monitor the temporal dynamics of formation and decay of

plasmonic “hot spots” in PMMA-DR1/hybrid metal nanostruc-tures.13

• Identify the unusual morphological impact on near fields ofplasmonic nanoparticles, such as the ultraconfined “cold spots”of gold nanocubes under select orientation conditions of thenanocubes relative to the incident light polarization.14

• Image the optical near field of an illuminated metallicscanning probe tip near a PMMA-DR1 film.30,46

• Use optical fields to manipulate mass transport andmolecular motion of PMMA-DR1.3,4,13,14,27,46 This approachhas been used to image molecular motion in complex fieldsincluding radial, azimuthal, and Hermite−Gaussian beams.27

In addition to these experimental advances, we have beenable to successfully model and predict the PMMA-DR1response to optical fields, laying the groundwork for futurestudies in this area.20−22 More specifically, the movement ofindividual DR1 molecules along their dipole direction,combined with a probability for excitation based on thealignment of the polarization direction with the transitiondipole, enables unusual control of molecular motion in threedimensions with complex fields. Of particular value is the factthat this movement is useful both on the macroscale usingdiffraction-limited light and at the nanoscale using, for example,the optical near fields of plasmonic nanoparticles. Thus, theability to assemble morphological features or patterns throughmolecular movement and “trapping” of molecules withincomplex fields at multiple length scales is realizable with thisapproach. These possibilities should be able to be straightfor-wardly accessed using programmable spatial light modulators toproduce a wide range of patterns, beginning with 1D and 2Dgratings and evolving to more complex morphological featuresat the macroscale. Conversely, using nanoscale evanescentfields, tiny numbers of DR1 molecules can be manipulated andplaced with a degree of precision not possible with conven-tional optical control methods.For the future, harnessing these optically driven molecular

motors to move larger species is an interesting prospect. Inparticular, one goal is to covalently attach nanoparticles to thepolymer backbone, in order to “piggyback” on the molecularDR1 motors. By attaching silica, metal, or high-dielectric-constant nanoparticles (TiO2, ZnO, Si, etc.), combined withillumination in 1D, 2D, or even 3D grating patterns, it may bepossible to assemble nanoparticles into the grating patternsduring the illumination process. This will have the effect ofcreating higher-index differentials for diffraction in the brightand dark regions of the illumination pattern. It also has thepotential to provide a new means to assemble more complexstructures of photonic interest, such as noncentrosymmetric

This approach can be distin-guished from other types ofoptical exposure processes for

imaging by its vectorial transportnature, thus opening up new

avenues for truly understandingthe governing processes of fieldformation and dynamic evolutionin three dimensions in the con-

densed phase.

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photonic band gap structures too complex to be createdthrough self-assembly alone,47 or to produce new metamaterialsfor “perfect” imaging or cloaking.48

Further work is needed to optimize and broaden theapplications of our approach. For example, the ultimate spatialresolution will have to be defined and quantified. van der Waalsinteraction at the nanoscale, molecular quenching, and thermaleffects could constitute some fundamental limits. As anotherexample, new azo molecules that possess the inchwormtranslational mechanism and have resonances at differentparts of the visible spectrum would enable more tunability inthe types of nanostructures that can be used to produce themolecular translational process. At present, only resonances inthe green spectral region are utilized. Similarly, changes in thepolymer backbone to optimize the viscosity or the glasstransition temperature could have significant impact on theultimate spatial resolution of this imaging and molecularmanipulation approach. New chemical functionalities could beadded in order to bind different materials for transport.Additional time-resolved studies could be performed onmultiple length scales, beginning with femtosecond studies ofthe initial excitation process followed by longer time studies ofthe dynamics of mass transport using different nanostructuresand/or DR1 materials. This would produce additionalinformation for optimizing the efficiency of molecularexcitation and transport and also enable the linearity of themorphology response for imaging purposes to be clarified. Theaddition of nonlinear chromophores could provide a means tofurther improve the spatial response of these materials astransport (trapping) may only occur in the very highest(lowest) regions of the optical interference pattern. Thus,although the PMMA-DR1 approach has produced novel opticalimaging and molecular transport advances, its potentialexpansion into new areas such as nanomotors, nonlinearresponses, and complex self-assembly, among others, has onlybegun to be explored.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: renaud.bachelot@utt.fr.

NotesThe authors declare no competing financial interest.Biographies

Jerome Plain graduated in Physics (1998) and completed his Ph.D.(europeus doctor) in Physics (2001) at the University of Poitiers andthe Autonomous University of Barcelona. He is now Full Professor ofPhysics at the University of Technology of Troyes where he managesthe nano’mat platform for nanofabrication and nanocharacterization ofmaterials (www.nanomat.eu) and leads the Molecular plasmonic andnanophotochemistry group of the LNIO (http://lnio.utt.fr). In recentyears, his interests were mainly focused on metal nanostructures andtheir applications.

Gary P. Wiederrecht is the Group Leader of the NanophotonicsGroup at the Center for Nanoscale Materials at Argonne NationalLaboratory. His research interests include the photochemistry andphotophysics of nanoparticles, hybrid nanostructures, optical energyconversion, and ultrafast nonlinear optical responses. http://nano.anl.gov/research/nanophotonics.html

Stephen K. Gray obtained a B.Sc. in Chemistry from CarletonUniversity, Ottawa, in 1977 and a Ph.D. in Chemistry from theUniversity of California, Berkeley in 1982, under the supervision of

William H. Miller. He is currently a Senior Scientist and Group Leaderof the Theory and Modeling Group at Argonne’s Center for NanoscaleMaterials, and his research focuses on understanding light interactionswith nanostructures. http://nano.anl.gov/research/theory.html

Pascal Royer is a Full Professor in Physics and is Director of Researchat the University of Technology of Troyes (UTT). He received aPh.D. in Physics from the University of Burgundy in 1987 and formedthe Laboratory of Nanotechnology, Instrumentation and Optics atUTT in 1994. His scientific interests are in the fields of nano-optics,plasmonics, and near-field optics. http://lnio.utt.fr

Renaud Bachelot is a Full Professor of Physics at the University ofTechnology of Troyes (UTT, France) where he leads the Laboratoryof Nanotechnology, Instrumentation and Optics involving more than65 people. He originally joined the laboratory in 1996 following hisPh.D. studies at the University of Paris 7. His research areas includenano-optics, near-field optics, nanophotonics, local light/polymerinteractions, scanning probe microscopy, nano-optoelectronics, andplasmonics. http://lnio.utt.fr

■ ACKNOWLEDGMENTS

This work was supported by the Partner University Fundsprogram (PUF2010). Use of the Center for NanoscaleMaterials was supported by the U.S. Department of Energy,Office of Science, Office of Basic Energy Sciences, underContract No. DE-AC02-06CH11357. This work was alsosupported by the European community FEDER fund and theRegion Champagne-Ardenne, Grants HYNNOV and NANO′-MAT (www.nanomat.eu). This work has been performed incooperation with the Labex ACTION program (ContractANR-11-LABX-01-01).

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