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Journal ID: 340, Article ID: 4730, Date: 2011-09-08, Proof No: 1

« APHB 340 layout: Large v.1.3.2 file: aphb4730.tex (ELE) class: spr-twocol-v1.3 v.2011/08/17 Prn:2011/09/07; 17:42 p. 1/14»« doctopic: OriginalPaper numbering style: ContentOnly reference style: aps»

Appl Phys BDOI 10.1007/s00340-011-4730-x

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Optical nanofibers and spectroscopy

R. Garcia-Fernandez · W. Alt · F. Bruse · C. Dan ·K. Karapetyan · O. Rehband · A. Stiebeiner ·U. Wiedemann · D. Meschede · A. Rauschenbeutel

Received: 4 May 2011 / Revised version: 6 July 2011© Springer-Verlag 2011

Abstract We review our recent progress in the productionand characterization of tapered optical fibers with a sub-wavelength diameter waist. Such fibers exhibit a pronouncedevanescent field and are therefore a useful tool for highlysensitive evanescent wave spectroscopy of adsorbates on thefiber waist or of the medium surrounding. We use a carefullydesigned flame pulling process that allows us to realize pre-set fiber diameter profiles. In order to determine the waistdiameter and to verify the fiber profile, we employ scanningelectron microscope measurements and a novel accurate insitu optical method based on harmonic generation. We useour fibers for linear and nonlinear absorption and fluores-cence spectroscopy of surface-adsorbed organic moleculesand investigate their agglomeration dynamics. Furthermore,we apply our spectroscopic method to quantum dots on thesurface of the fiber waist and to cesium vapor surround-ing the fiber. Finally, toward dispersive measurements, wepresent our first results on building and testing a single-fiberbimodal interferometer.

R. Garcia-Fernandez · O. Rehband · A. Stiebeiner ·A. RauschenbeutelInstitut für Physik, Johannes Gutenberg-Universität Mainz,55099 Mainz, Germany

A. Rauschenbeutele-mail: [email protected]

R. Garcia-Fernandez · A. Stiebeiner · A. RauschenbeutelVienna Center for Quantum Science and Technology,Atominstitut, TU Wien, Stadionallee 2, 1020 Wien, Austria

W. Alt · F. Bruse · C. Dan · K. Karapetyan · U. Wiedemann ·D. Meschede (�)Institut für Angewandte Physik, Universität Bonn, Wegelerstr 8,53115 Bonn, Germanye-mail: [email protected]

1 Introduction

Glass fibers are nowadays well established as the backboneof the modern communication society. In a standard glassfiber used for telecommunications, the light is guided in-side the core and is isolated from the environment by thecladding, allowing low loss and low disturbance transmis-sion over long distances [1]. In contrast, optical nanofibers[2] with a diameter smaller than the wavelength of theguided light exhibit a strong lateral confinement of theguided mode in conjunction with a pronounced evanescentfield surrounding the fiber [3]. These properties make opticalnanofibers a unique tool for efficient and controlled couplingof light with matter on or near their surface. A wide range ofnanofiber applications are emerging in fields like, e.g., op-tical sensing [4–6], nanofiber-based evanescent wave spec-troscopy [7, 8], nonlinear optics [9–12], cold atom physics[13–17], probing of nanocavities and cavity quantum elec-trodynamics (cQED) [18–21], coupling to nanocavity lasers[22], or single quantum dot spectroscopy [23]. For these ap-plications, high-quality nanofibers with a high diameter uni-formity and low surface roughness are required, renderingthe technical realization a demanding process.

A standard fabrication method for optical nanofibers isflame pulling of commercial single mode optical fibers re-sulting in tapered optical fibers (TOFs) with a nanofiberwaist [24–29]. Efficient coupling of the light into and outof the nanofiber waist requires a careful design of the ta-per profile [30, 31]. Therefore, we first describe the pro-duction of nanofibers with our computer controlled fiberpulling rig as well as their main properties in Sect. 2. In or-der to verify the accuracy and precision of our productionprocess, we present scanning electron microscope measure-ments of the TOF profile and compare the experimental re-sults with the theoretically predicted target profile. Further,

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Journal ID: 340, Article ID: 4730, Date: 2011-09-08, Proof No: 1, UNCORRECTED PROOF


R. Garcia-Fernandez et al.

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Fig. 1 Schematic of the fiberpulling rig. A standard glassfiber is clamped on two stackedtranslation stages, which moveand stretch the glass fiber whichis heated with the flame of astationary gas burner. Thepulling procedure is performedunder clean room conditions ina laminar flow box

in Sect. 3, we present a method for measuring the diame-ter of the nanofiber waist, which is based on second andthird harmonic generation and has an accuracy of better than2% [12].

In Sect. 4 of this paper, we focus on applications ofnanofibers for surface spectroscopy. Fiber-based evanescentwave spectroscopy (EWS) offers stability, fast response, andapplications with in situ measurements. Motivated by theseadvantages, fiber-based EWS sensors have been demon-strated [32]. It is remarkable that the sensitivity of nanofiberspectroscopy depends strongly on the fiber diameter d andthe fiber length L: At resonance, the absorption probabilityper surface adsorbed particle with absorption cross sectionσ can be roughly estimated to be σ/λ2 because, in the fiberwaist of a silica nanofiber, the light field can be confinedto an effective cross section on the order of λ2, similar to astrongly focused light beam. In contrast to the situation infree space, this confinement is, however, maintained alongthe entire fiber length L, and hence the typical depth of fo-cus of a free space beam is enhanced by 3 to 5 orders ofmagnitude. As we show in detail in Sect. 4.2, the spectro-scopic sensitivity η of the nanofiber scales as

η(λ) ≈ ηfree(λ)πdL

λ2= ηfree(λ)

πL

λ, (1)

where we assumed d = λ and ηfree is the value for a freespace beam. Therefore, the strongly confined light in thenanofiber can be considered as an extremely elongated “in-finite” focus. We demonstrate the versatility of fiber-basedEWS by a series of absorption and fluorescence measure-ments performed on systems like surface-adsorbed organicmolecules or quantum dots. Further, fiber-based EWS is alsosuited for spectroscopic measurements of atoms surround-ing the nanofiber, as shown in Sect. 5 on the example ofcesium vapor.

Finally, we use nanofibers for the realization of an in-terferometric sensor based on the interference of two co-propagating transversal modes along the TOF [33, 34]. Thefirst experimental results are presented in Sect. 6. This tech-nique is highly sensitive to changes in the refractive index inthe vicinity of the nanofiber surface and therefore opens theroute toward dispersive measurements on surface adsorbates[35].

2 Fabrication and design of nanofibers

In the following, we summarize the fabrication procedure ofour nanofibers.

2.1 Description of the fiber pulling rig

All our experiments are performed using the nanofiber waistof tapered optical fibers (TOFs). We produce the TOFs bystretching a standard optical fiber while heating it with atraveling hydrogen/oxygen flame [36]. A schematic of thefiber pulling rig is shown in Fig. 1. It consists of two lineartranslation stages, the translator and the stretcher, which al-low sub-micrometer positioning, and is equipped with pre-cisely parallel aligned V-groove holders to which the fiberis clamped and tightened. A computer-controlled hydro-gen/oxygen burner, located between these two holders, pro-vides a clean flame. The flame heats a section of the fiberwith a length of approximately 1 mm to a temperature ofabout 1500°C, making it soft and malleable. The transla-tor moves the fiber back and forth above the flame whilethe stretcher simultaneously elongates it. Due to volumeconservation, the fiber diameter is thus gradually reduced.We use an analytic algorithm in order to calculate the tra-jectories of the translator and of the stretcher to obtain apredetermined fiber shape. This process allows us to fab-ricate TOFs with a homogeneous waist of diameters downto 100 nm and a typical length of 1–10 mm [28]. The trans-mission of light through the fiber is continuously monitoredduring the pulling process, yielding valuable information forquality control of the taper and its overall transmission. Formonochromatic light of 850 nm wavelength and a nanofiberwith a 500-nm diameter waist, our fabrication procedureyields up to 98.7% of the initial fiber transmission.

2.2 Properties of the guided light

Figure 2 shows a schematic of a TOF with three sections ex-hibiting different propagation properties: the nanofiber waist(a) surrounded by the two taper transitions (b) and the orig-inal unstretched fiber (c). The intensity distribution of thecorresponding guided mode is schematically represented byfilled curves.

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Fig. 2 Schematic of a taperedoptical fiber (TOF) showing thelight propagation andconversion of the fundamentalmode (c) in the tapered region(b) to the nanofiber waist (a)and back again to the initialmode (c). The intensity profileof the guided mode isschematically represented by thefilled curves

In the unmodified part of the single mode TOF, thelight is guided inside the core via total internal reflectionat the core-cladding boundary. Due to the low refractive in-dex difference between core and cladding (ncore/ncladding ≈1.0035), this section is often designated as weakly guidingand Maxwell’s equations are approximately solved by lin-early polarized (LP) modes [37]. In the taper transition, bothcladding and core shrink. The decrease of the core diam-eter leads to a slight compression of the guided mode un-til the mode diameter reaches a minimum. Further reduc-tion of the core diameter leads to an expansion of the modeinto the cladding. The cladding then acts as the new guid-ing medium. In order to minimize transmission losses, thismode conversion should be adiabatic, i.e., there should beno coupling of the fundamental mode to higher transverseor radiative modes. This is achieved by maintaining shallowtaper angles of a few milliradians [30, 38].

Further decrease of the fiber diameter to the waist valuecauses the mode to be more and more laterally confined.The high refractive index difference at the cladding-air in-terface (ncladding/nair ≈ 1.5) allows a much tighter radialconfinement. Throughout the nanofiber waist, the diameterof the fiber remains constant in the sub-wavelength regime,and the guided mode exhibits tight radial confinement andstrong evanescent field. At the second taper transition, themode transformation is reversed. Note that through this pro-cess the weakly guided LP01 mode of the unstretched fiberis adiabatically transformed into the strongly guided HE11

mode of the nanofiber and back.The intensity distribution of a circularly or unpolarized

HE11 mode is plotted in Fig. 3 as a function of the distancefrom the fiber center. The intensity is maximal at the fibercenter and decreases toward the surface. Due to the high re-fractive index jump at the surface, the radial component ofthe electric field exhibits a correspondingly strong disconti-nuity, while the tangential components of the electric fieldremain continuous [3]. In the nanofiber, for visible wave-lengths the decay length of the evanescent field is typicallyon the order of few hundreds of nanometers. The strong in-tensity achieved at the fiber surface provides excellent con-ditions for high-sensitivity spectroscopic measurements andnonlinear optical experiments.

Fig. 3 Radial intensity distribution in units of 1/λ2 of the fundamentalmode HE11 guided through a nanofiber as a function of the distancefrom the fiber center in units of the wavelength. Calculated accordingto [3] for circularly or unpolarized light, a guided power of 10 nW,d/λ = 0.6 and a refractive index n = 1.46

Light propagation in a nanofiber is usually describedusing the concept of an effective refractive index neff =c/vphase = β/k0, where c is the vacuum speed of light, vphase

is the phase velocity of the mode, β is the mode propa-gation constant, and k0 = 2π/λ is the vacuum wavenum-ber [1, 37, 39, 40]. The propagation constant β is ob-tained by solving the transcendental equation arising fromthe Maxwell’s equations in conjunction with the bound-ary conditions applied to the nanofiber geometry [37]. Forthis purpose, we have created a Matlab toolbox that allowsus to automatically calculate all relevant parameters of thenanofiber guided modes [41]. As an example, Fig. 4 showsthe effective refractive indices of the lowest nanofiber modesas a function of the waist diameter. The single-mode andmultimode regime can be clearly distinguished in the fig-ure. The cutoff value at which only the fundamental HE11

mode propagates is wavelength-dependent and correspondsto d/λ ≈ 0.73 for a refractive index of nsilica = 1.45. Asthe fiber diameter decreases, a larger and larger fraction ofthe mode propagates outside of the glass, and the effec-tive refractive index of the mode decreases to unity. On theother hand, a fiber diameter increase leads to a mode guidedmostly entirely in the bulk and the effective refractive index

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Fig. 4 Effective refractiveindex for the fundamental mode(leftmost) and a few higherorder modes as a function of thefiber diameter in units ofwavelength. The calculation wasperformed assuming a refractiveindex of nsilica = 1.45;reproduced from [12]

approaches nsilica. Throughout this paper, with the exceptionof Sect. 6, adiabatic tapers are used, i.e., to a good approx-imation the light is guided exclusively in the fundamentalHE11 mode.

3 Characterization of the diameter profile

The knowledge of the fiber diameter profile is essential fora quantitative understanding of the light propagation and ofnonlinear effects in nanofibers. In order to characterize thepredetermined diameter profile used by the fiber pulling rig,we performed scanning electron microscope (SEM) mea-surements. Although this method can offer an accuracy of3%, it is time-consuming and does not allow one to reuse theTOF because it has to be put onto a substrate and needs tobe gold-coated [12, 42]. Another method for measuring thenanofiber uniformity with a precision of 2–3% has been pro-posed [43, 44] that, however, gives no information about theabsolute fiber diameter. We have developed a nondestructiveoptical method for diameter measurements with an accuracybetter than 2%.

3.1 Scanning electron microscope measurement

Figure 5 shows the diameter profile of a TOF with a 320-nm diameter nanofiber waist of 1-mm length measured witha SEM. It exhibits a three-fold linear taper transition withlocal taper angles of Ω1 = 3 mrad, Ω2 = 1 mrad, andΩ3 = 5 mrad with the slope changing at the fiber diame-ters d1 ≈ 84 μm and d2 ≈ 40 μm. The solid line indicatesthe fiber diameter profile as predicted by our algorithm andthe inset shows a magnification of the submicron-diameterregion of the profile containing the waist with a length of1 mm. The nanofiber was mounted on a gold coated sub-strate for the SEM measurement (see [31] for technical de-tails). In order to determine the local fiber diameter, we suc-cessively took electron micrographs along the TOF with a

Fig. 5 Comparison between a SEM measurement (points) and the pre-dicted diameter profile (solid line) of a TOF with a nanofiber waist;reproduced from [31]

step size varying between 0.25 and 1 mm. The measureddiameter profile closely follows the predicted profile withdeviations smaller than 10%. For the two linear sectionswith local taper angles Ω1 and Ω2, they even fall below5%. From this excellent agreement, we conclude that ourproduction procedure for nanofibers indeed realizes the pre-dicted profiles to a very good approximation.

Figure 6 shows the detailed measurement of the waist re-gion of a nanofiber using a Zeiss SUPRA 55 field-emissionSEM. Since the manufacturer guaranties an accuracy of only±5%, we have recalibrated the SEM for each measurementseries and thereby achieved an improvement of the absoluteaccuracy to below ±2% (see [12] for further technical de-tails). Our fiber pulling rig produces a waist with less than±1.5% diameter variation, sufficiently uniform for the ex-periments reported here. At the same time, the pulling al-gorithm predicted a waist diameter of 393 nm while theSEM measurement yield a value of 415 nm. This accuracy ofabout 5% agrees well with the global nanofiber shape mea-surement shown in Fig. 5.

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3.2 Optical measurement

The optical method for measuring the waist diameter isbased on second and third harmonic generation (SHG andTHG) in a bare silica nanofiber in air [45, 46]. Efficient har-monic generation requires that the phase-matching condi-tion is fulfilled, i.e., the effective refractive indices of thefundamental and harmonic waves have to be equal. Phase-matching for the two fundamental modes of both waves isdifficult to achieve due to material dispersion and the wave-length dependence of the effective indices of the modes in ananofiber (compare Fig. 4). However, the same effective re-fractive index dependence on the fiber diameter allows oneto fulfill the phase-matching condition if the harmonic lightpropagates in a higher transverse mode. If the wavelengthchanges, the effective refractive index curves shift and thephase-matching condition occurs at a different fiber diame-ter. There exists a one to one relation between the fiber di-ameter and the phase-matched wavelength for each mode.

Figure 7 shows a schematic of the experimental setup.Light from a tunable pulsed Ti:Sa laser (1 ps pulse duration,80 MHz repetition rate, 100 mW average power) is coupledinto one end of the TOF and its wavelength is scanned inorder to detect the SHG and THG peaks. A dichroic mirror atthe output of the TOF reflects the infrared fundamental light,which is measured with a power meter. The transmitted SHG

Fig. 6 SEM measurement of the nanofiber-waist region of a TOF. Theinset shows a vertically enlarged picture of the waist. Each point hasbeen measured with an accuracy of better than 7 nm; reproduced from[12]

and THG light is analyzed by a spectrometer (see [12] forfurther details).

Figure 8(a) shows the measured full response of the SHGand THG of a nanofiber. In Fig. 8(b), the phase-matchingcondition for harmonic generation is plotted. The nanofiberdiameter is determined from the position of the SHG andTHG peaks.

For the sample measured in Fig. 8, the fiber waist diam-eter is obtained as 425.7 nm with a resolution of 0.5 × 10−3

and an accuracy of <2%. Taking into account the trans-parency window of silica, our method allows to measurefiber waists with diameters down to 190 nm and up to 1 μm.

4 Applications in surface spectroscopy

This section demonstrates surface spectroscopy of differenttypes of nanofiber-adsorbed emitters. Further, we quantita-tively analyze the experimental results.

4.1 Experimental setup

Figure 9 shows the schematic experimental setup for fiber-based absorption and fluorescence spectroscopy. Depend-ing on the species under study, the particles are depositedonto the nanofiber waist using different approaches, e.g.,vapor deposition or immersion in a solution. The absorp-tion of the deposited particles is measured using a conven-tional absorption spectroscopy configuration with a tung-sten white light source and a spectrograph. Fluorescencespectra are obtained upon excitation by laser light cou-pled into the TOF after reflection from a glass plate. Thecounter-propagating fluorescence is collected after transmis-sion through the glass plate by the same spectrograph asfor the transmitted white light. This configuration avoids di-rect incidence of the excitation laser light into the spectro-graph. The residual backscattered and reflected excitationlaser light is suppressed by means of a razor edge filter,placed directly in front of the spectrograph’s entrance slit.

4.2 Theoretical considerations

In the following, we derive expressions for the measured ab-sorption and fluorescence signals to be expected from theset-up described above.

Fig. 7 Scheme of theexperimental setup for SHG andTHG measurement

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Fig. 8 (a) The spectral response of a nanofiber for SHG (dashedline) and THG (solid line) plotted vs. the fundamental wavelength.The spectrometer response of the THG measurement is scaled up forvisibility. The four peaks correspond to phase matching of HE11(ω)

to the following modes (from left to right): HE21(2ω), HE12(3ω),

HE31(3ω), TM01(2ω). (b) Wavelength-dependent phase-matching di-ameter. Dashed lines: SHG, solid lines: THG, dotted lines: modes notobserved due to lack of effective mode overlap with the fundamentalmode. The horizontal line indicates the diameter of the investigatedsample determined by this method. Reproduced from [12]

Fig. 9 Scheme of theexperimental setup forabsorption and fluorescencespectroscopy

We consider a nanofiber with diameter d and length L

covered with n emitters, yielding a surface coverage, i.e.,number of emitters per surface area, of θ = n/πdL. Theabsorbance of light at a wavelength λ propagating in thefundamental mode of this nanofiber is given by η(λ) =−lg(Psig(λ)/Pref(λ)), where Psig(λ) and Pref(λ) are thetransmitted powers at the excitation wavelength λ in thepresence and absence of emitters, respectively. Accordingto [7], the absorbance η(λ) can be approximated as

η(λ) ≈ nσ(λ)

ln(10)Aeff(λ)= θσ (λ)

ln 10

πdL

Aeff(λ), (2)

if the absorption cross section of the emitter σ(λ) is muchsmaller than the effective area of the guided fiber modeAeff(λ) which is defined by

Aeff(λ) = Pref(λ)

Isurf(λ). (3)

Isurf(λ) denotes the intensity of the evanescent field compo-nents at the fiber surface that actually couple to the emit-ters. Isurf(λ) does account for a possible preferential orien-tation of the transition dipole of the emitter which might oc-cur, e.g., when planar molecules lie flat at the fiber surface

[47]. In this case, their transition dipole moment is orientedperpendicular to the radial electric field component of theguided fiber mode. Therefore, only the axial and azimuthalcomponents of the electric field, calculated according to [3],contribute to Isurf(λ).

Equation (2) can be rewritten as a function of the ab-sorbance ηfree(λ) = θσ (λ)/ ln 10 of a freely propagatingbeam [7]

η(λ) = ηfree(λ)ξ(λ), (4)

where the factor ξ(λ) = πdL/Aeff gives the enhancement ofthe sensitivity of the fiber-based absorption measurement.With a length of the nanofiber L on the order of millime-ters and a diameter d in the submicron range, we thus ex-pect an increase by four orders of magnitude in the sensi-tivity in comparison with methods using freely propagatingbeams [7].

The absorbance is directly proportional to the number ofemitters n and inversely proportional to the effective areaAeff. As can be seen from the second term in (2), the quan-tity 1/Aeff determines the sensitivity for a given numberof emitters. It is plotted in Fig. 10 in units of 1/λ2 as afunction of the fiber diameter and reaches a maximum for

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Fig. 10 Plot of 1/Aeff andd/Aeff in units of 1/λ2 and 1/λ,respectively, as a function of thefiber diameter d in units of λ.Aeff is calculated taking intoaccount all three components ofthe electric field. Assuming arefractive index of 1.46 andignoring material dispersion,both plots hold universally forany λ

dmax = 0.508 · λ. It rapidly goes to zero for small diametersbecause then the mode is only weakly bound and Aeff di-verges. The third term in (2) shows that for spectroscopicmeasurements of thin layers, the quantity d/Aeff shouldbe maximized. This quantity is also plotted in Fig. 10 inunits of 1/λ as a function of d/λ, reaching a maximum fordmax = 0.584 · λ.

In order to derive an expression for the fluorescence sig-nal, we consider an emitter at position z along the fiberwaist that absorbs the fraction σ(λexc)/Aeff(λexc) of thepower Psig(λexc, z) of the excitation wavelength λexc. Forthe fiber diameter and the wavelengths considered in our ex-periments, about 20% of the fluorescence of a single dipoleemitter on the fiber surface is expected to be coupled backinto the fundamental guided mode of the fiber, 10% in eachdirection of the fiber [48]. As a result, the fluorescence emit-ted into the fiber mode by an emitter at a position z along thefiber waist is proportional to the absorbed power:

Pfl(λ, z) = C(λ)Pabs(λexc, z), (5)

where the proportionality factor C(λ) ∝ q(λ)/Aeff(λ) in-cludes the wavelength dependent fluorescence quantumyield of the emitter q(λ) and the average fractional emissionof the emitter into the guided fiber mode. The latter is an av-erage over all possible orientations of the emitter on the fibersurface. Furthermore, it is determined by the intensity of theevanescent field at the fiber surface which is proportionalto 1/Aeff; see (3). Note that the spectral overlap betweenthe absorption and emission spectra in our system results ina partial self-absorption of the emitted fluorescence by cir-cumjacent emitters before it reaches the output of the fiber.While the high sensitivity of our method allows us to per-form measurements in a regime of low surface coverages,self-absorption is nonnegligible at higher surface coverages.Detailed theoretical considerations in order to quantify thiseffect are presented in [8].

The expressions derived for the absorbance and fluores-cence in (2) and (5), respectively, are proportional to the

wavelength dependent coupling to the fiber mode charac-terized by 1/Aeff. The absorption and fluorescence spectrapresented throughout this paper have been corrected accord-ingly.

4.3 Organic molecules: linear excitation

As a model system, we use 3,4,9,10-perylene-tetracarbox-ylic dianhydride molecules (PTCDA) because of their sta-bility under evaporation at ambient conditions, their highquantum yield and the experimental and theoretical knowl-edge concerning their spectral characteristics [49]. More-over, PTCDA molecules significantly change their spectralproperties depending on their arrangement on the surface[50]. Therefore, these organic molecules are well suited forsensitivity studies.

The measurements were performed with a 320-nm di-ameter nanofiber with a length of 1 mm fabricated from aNufern 460-HP fiber. This diameter ensures that only thefundamental mode is guided in the fiber waist for wave-lengths longer than 450 nm, thereby matching the singlemode cutoff wavelength of the unprocessed fiber. Moreover,this diameter yields the maximum sensitivity for absorptionspectroscopy in the visible domain, as discussed in Sect. 4.2.The TOF used in this experiment has a transmission of up to70% in the wavelength range between 450 and 650 nm. Themolecules are deposited on the nanofiber waist by placing acrucible with PTCDA crystals below the fiber and by heat-ing it to 325°C. By convection, sublimated molecules arecarried to the nanofiber waist where they are adsorbed. Inter-laced measurements of absorption and fluorescence spectraare performed in order to determine the respective surfacecoverage (see (2)). The fluorescence spectra are obtainedupon excitation with a freely running diode laser at a wave-length of 406 nm. The background signal induced by the ex-citation light in the fiber has been subtracted in the spectra.For further technical details, see [8].

Figure 11 displays a series of in situ fluorescence and ab-sorption spectra recorded with about 8 μW laser power and

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Fig. 11 Fluorescence (left) andcorresponding absorption (right)spectra of surface-adsorbedPTCDA molecules duringdeposition. Five representativespectra within a range of surfacecoverages between 0.10 and1.28% of a ML are shown.Reproduced from [8]

about 3 nW white light power respectively. These powersare low enough not to saturate the molecules. For clarity,we chose five representative spectra taken during depositionof the molecules for different surface coverages. The shownabsorption spectra are the mean of two consecutive spec-tra, one recorded 420 ms before and the other 420 ms afterthe corresponding fluorescence spectrum. Within this timeinterval, the deposition rate remains roughly constant, thusenabling us to determine the surface coverages underlyingthe measured fluorescence spectra by this averaging process.Using σ = 4.1 × 10−16 cm2 [8] in (2), we infer from the ab-sorbance values ranging from 0.04 to 0.51 at the absorptionmaximum at 2.47 eV that 0.8 ×106 to 10.6×106 moleculescover the fiber waist. This corresponds to a surface cover-age of 0.8 × 1011 cm−2 to 10.6 × 1011 cm−2 or 0.10% to1.28% of a compact monolayer (ML) of flat lying PTCDAmolecules, arranged in the herringbone structure of the (102)plane of the PTCDA bulk crystal [51]. The absorption spec-tra show a clear vibronic progression and agree well withdifferential reflectance spectroscopy (DRS) spectra of sub-monolayers of PTCDA on mica [50]. The fluorescence spec-tra also show a vibronic progression as known for PTCDA insolution [52]. Note, that the absolute peak positions in both,absorption and fluorescence spectra, are slightly shifted incomparison with the literature due to the interaction of theadsorbed molecules with the fiber surface.

The mirror symmetry between absorption and fluores-cence spectra that is characteristic for numerous molecularsystems is retrieved after correction for the self-absorptioneffect. Figure 12 displays two of the spectra from Fig. 11for about 0.75% and 0.20% of a ML and the ones correctedfor self-absorption and further reemission of the reabsorbedfluorescence (see [8] for further details). Note that, althoughfor higher surface coverages like 0.75% of a ML the self-absorption changes the fluorescence spectra significantly,this effect is negligible for lower surfaces coverages like0.20% of a ML. Summarizing, our sensitivity is high enough

to perform studies for low surface coverages. At the sametime, the fact that we are able to correct for self-absorptionmakes our method applicable for a large range of surfacecoverages.

Our method yields absorption and fluorescence spectrawith a good signal to noise ratio for down to ∼106 PTCDAmolecules at room temperature. Under these conditions,thermal effects lead to strongly broadened molecular transi-tions with a FWHM of ∼850 cm−1 or ∼25 THz [53]. Whencooling the fiber to cryogenic temperatures below 4 K, thisthermal broadening could in principle be eliminated and 106

times narrower lines of a few 10 MHz should be obtainablewith organic dye molecules embedded in organic crystals[54]. Under these circumstances, our method would thusalso allow us to carry out spectroscopic studies on singlemolecules.

The high sensitivity of our method also allowed us tostudy the agglomeration dynamics of the adsorbed mole-cules on a second to minutes time scale at ambient con-ditions. Figure 13(a) shows absorption spectra measuredduring the ripening of the film after the molecular depo-sition has been stopped. We observe a continuous changeof the shape of the spectra, evolving from monomer-liketo oligomer-like. At the beginning of this process (up to23 min), the absorbance remains constant at the energies ofabout 2.34 eV and 2.67 eV. These so-called isosbestic pointsresult from the fact that, at these energies, the monomer andthe dimer phase of PTCDA have the same molar absorp-tivity. This observation thus indicates that the total num-ber of molecules remains nearly constant during the ripen-ing. Additional kinetic details of the ripening process canbe found in [7]. Note that reordering of PTCDA moleculeson a mica surface at ambient condition has already beenobserved. However, the measurement was not resolved intime [55]. The fast agglomeration process is attributed tothe presence of water at the surface that increases the mo-bility of the surface-adsorbed molecules and thus acceler-

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Fig. 12 Influence ofself-absorption and reemissionon the measured fluorescencesignal for 0.20% (a) and 0.75%(b) of a ML. For 0.75% of aML, the signal corrected forself-absorption differssignificantly from the measuredspectrum. For 0.20% of a ML,the difference is only marginal.The correction for reemissiondoes not have a significantinfluence on the spectral shapefor neither of the two surfacecoverages. The absorbance forboth surface coverages (dashedline) is shown for comparison ofthe spectral shape

ates their reordering. In order to desorb pollutants from thefiber surface, we set up an ultrahigh vacuum (UHV) cham-ber, allowing us to work with a more well-defined surface.Figure 13(b) displays the absorption spectra measured dur-ing the agglomeration process in the UHV environment.A comparison with the analogue process at ambient con-ditions (Fig. 13(a)) shows qualitatively the same agglomer-ation dynamics. However, at UHV conditions, the processis slowed down by about two orders of magnitude, therebyshowing that the mobility of the molecules in a vacuum en-vironment is significantly reduced in comparison with am-bient conditions. Both measurements have been performedwith the same nanofiber diameter and the molecular deposi-tion has been stopped at about the same surface coverage.

4.4 Organic molecules: two-photon excitation

The high light intensity and long interaction length (“the in-finite focus”) makes nanofibers a suitable platform for non-linear optical experiments. Here, we present two-photon ex-cited fluorescence (TPEF) measurements of organic mole-cules. The theory presented in Sect. 4.2 holds for linear ex-citation and it has to be slightly modified for two-photonexcitation. In particular, an emitter at position z along thefiber waist absorbs now a fraction σ2Ph(λexc)/Aeff(λexc) of

the two-photon power P 2sig(λexc, z) of the excitation wave-

length λexc, where σ2Ph(λexc) denotes the two-photon ab-sorption cross section of the emitter.

We use the fluorescent dye Rhodamine 6G (Rh6G),which exhibits a high fluorescence quantum yield [56]. Un-like PTCDA, most organic dye molecules undergo ther-mal decomposition by evaporation at atmospheric pres-sure. In order to extend the fiber surface spectroscopy to alarger variety of organic molecules, we developed a simpleapproach—the “drip method”. The molecules are dissolvedin a spectroscopic-grade solvent and a drop of this solutionis dripped onto the nanofiber waist using a pipette. A thinfilm of the solution covers the fiber surface. Subsequently,the solvent evaporates and the molecules remain adsorbed atthe fiber surface.

With slight modifications, the experimental setup corre-sponds to the one shown in Fig. 9. For excitation, we use apulsed Ti:Sa laser at 870 nm wavelength with a pulse du-ration of 1 ps, a repetition rate of 80 MHz and a 104-foldhigher peak intensity compared to the corresponding contin-uous wave intensity. Further, in order to prevent laser powerlosses, the glass plate has been exchanged by a dichroic mir-ror that separates the infrared light of the Ti:Sa laser from theemitted visible light from the TPEF measurement. We use a

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450-nm diameter nanofiber with a length of 5 mm fabricatedfrom a Fibercore SM800 fiber.

The molecules have been deposited with a 0.016% massconcentration solution of Rh6G in ethanol. The correspond-ing absorption spectrum of Rh6G adsorbed on the nanofiber

Fig. 13 Time evolution of the spectral absorption of a constant num-ber of molecules at ambient (a) and UHV (b) conditions, showing theagglomeration dynamics of the molecules on the fiber surface

is shown in Fig. 14. Using the peak absorbance value of1.47 and the absorption cross section of 6.7 × 10−16 cm2

at 2.37 eV (calculated according to [8, 23] and [57]), weestimate the number of Rh6G molecules adsorbed on thenanofiber waist to be 4 × 107. With the surface density of aML of Rh6G molecules adsorbed on silica of 5×1013 cm−2

[58], this corresponds to a surface coverage of 1.1% of aML. By exciting the molecules with an average power of20 mW of pulsed light, we detect the TPEF spectrum alsoplotted in Fig. 14. It shows the typical mirror-image relationbetween the absorbance and fluorescence spectra [59]. Theplotted fluorescence spectrum has been already corrected forthe self-absorption effect. Both absorption and fluorescencespectra, agree qualitatively well with the measurements per-formed in solution [57]. However, there is a shift in the abso-lute peak positions which can be attributed to the interactionof the molecules with the fiber surface.

The TPEF spectrum is obtained directly after startingthe excitation process since the TPEF intensity decreasesrapidly due to photobleaching of the molecules. In fact, ina separate measurement, the temporal behavior of the TPEFstrength has been recorded by replacing the spectrometerwith a photomultiplier. The result is shown in the inset ofFig. 14. Starting with an initial value of 2 × 107 adsorbedRh6G molecules, the TPEF vanishes within seconds. At thispoint, the absorbance is also zero and, therefore, no undam-aged molecules remain adsorbed at the fiber surface.

4.5 Quantum dot spectroscopy

We have further investigated whether the spectroscopicmethod used for organic molecules can be also applied toquantum dots. Quantum dots are nanometer-scale semicon-ductor particles with a high fluorescence quantum yield, amuch better resistance to photobleaching in comparison to

Fig. 14 Two-photon excitedfluorescence (solid line) andabsorption (dashed line)spectrum of 4 × 107 Rhodamine6G molecules (1.1% of a ML)adsorbed on a nanofiber. Inset:An independent measurement ofthe time evolution of the TPEFresponse, showingphotobleaching of 2 × 107

adsorbed Rhodamine 6Gmolecules (0.6% of a ML)

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Fig. 15 Fluorescence (solidline) and absorption (dashedline) spectrum of 3 × 105 CdSequantum dots (0.3% of a ML)adsorbed on an nanofiber

organic molecules and a fluorescence emission wavelengththat is size-dependent.

The experimental setup shown in Fig. 9 has been used.For excitation, we use a continuous-wave diode laser at awavelength of 405 nm. The measurement was performedusing a 450-nm diameter nanofiber with a waist length of5 mm fabricated from a Fibercore SM800 fiber.

A solution of CdSe quantum dots (PL-QD-O-570 fromPlasmachem GmbH, core diameter 3.3 nm, total diameter8.9 nm) dissolved in heptane with a mass concentration of0.036% was deposited by the drip method on the nanofiberwaist. The absorption spectrum is shown in Fig. 15. Fromthe absorbance value of 0.043 and the absorption cross sec-tion of 8 × 10−16 cm2 at 2.22 eV (calculated from the molarextinction coefficient of 2.1 × 105 M−1 cm−1 provided bythe manufacturer), we estimate that about 3 × 105 quantumdots were adsorbed on the nanofiber waist. The correspond-ing surface coverage of 0.3% of a ML is calculated by as-suming that the spherical quantum dots arrange in one fullmonolayer in the geometrically most compact form. The flu-orescence spectrum, obtained by exciting the quantum dotswith 36 nW of laser light at 405 nm, is also shown in Fig. 15.The fluorescence maximum at 2.15 eV differs slightly fromthe manufacturer specifications of (2.175 ± 0.020) eV forquantum dots dissolved in chloroform. This discrepancycould be attributed to the interaction of the quantum dotswith the nanofiber surface. Note that, for an absorbanceas low as 0.043, the effect of self-absorption is negligi-ble.

5 Cesium vapor spectroscopy

The strong evanescent field surrounding the nanofiber waistof a TOF allows light-matter interaction not only withsurface-adsorbed species but also with media surrounding

the fiber. This feature allowed us to perform hot cesium va-por spectroscopy with a nanofiber.

The experimental setup consists of a tunable single-modediode laser, a nanofiber in a vacuum chamber, and a siliconphotodiode as a detector. The chamber contains a reservoirof liquid cesium, which is heated to 70°C, thereby providinga cesium vapor pressure of approximately 10−4 mbar duringthe experiment. The rest of the chamber is heated to 80°C inorder to prevent cesium condensation anywhere outside thereservoir. The measurement was performed with a 450-nmdiameter nanofiber with a length of 5 mm fabricated from aFibercore SM800 fiber.

Figure 16 shows the measured transmission through thenanofiber for an input laser power of 20 nW while scan-ning around the cesium D2 resonance line at a wavelengthof 852.3 nm. The data has been fitted using a function whichis the sum of three Voigt profiles corresponding to the threehyperfine transitions (F = 3 to F ′ = 2, 3, 4) of the D2 line:

A(ν) =4∑

F ′=2

aF ′∫ ∞

−∞exp(−x2/(2σ 2))

σ√

2π

× 2γ

π((ν − νF ′ − x)2 + γ 2/4)dx, (6)

where ν is the frequency, ν3 = 351.7 THz the frequency ofthe 3 → 3 transition, ν4 − ν3 = 201.3 MHz and ν3 − ν2 =151.2 MHz the separations of the hyperfine cesium lev-els [60], γ and σ the widths of the Lorentzian and Gaus-sian components of the Voigt curve. The hyperfine transi-tion strength factors for the 3 → 2, 3 → 3, 3 → 4 transi-tions, have the values a2 = 5/14, a3 = 3/8, and a4 = 15/56,respectively [60].

The fit yields the values σ = 178 MHz and γ = 130 MHz.The Gaussian width agrees well with the theoretically ex-

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Fig. 16 Measured (points) and fitted (solid line) cesium absorptionspectra

pected value for Doppler broadening at a temperature of80°C:

σtheoretical = ν0

√kBT

mc2= 174 MHz, (7)

where ν0 = 351.73 THz is the frequency of the D2 line, kB

the Boltzmann constant, T = 353 K is the temperature, andm = 2.21 × 10−25 kg the mass of a cesium atom.

The inferred Lorentzian width is significantly larger thanthe natural line-width γlifetime = 1/(2πτ) = 5.2 MHz, whereτ is the D2 lifetime of 30.4 ns. This may be explained by thefact that the thermal atoms pass the evanescent field of thefiber within a typical transit time of about 1 ns.

These measurements show that nanofibers can also beused for high-resolution spectroscopic studies of gases andliquids surrounding the fiber waist. A similar experiment ofnanofiber-based vapor spectroscopy has been done with ru-bidium vapor [61].

6 Toward dispersive spectroscopy

Dispersive spectroscopy is interesting because it avoids de-structive effects like, e.g., molecule photobleaching.A Mach–Zehnder interferometer (MZI) is the natural can-didate for this purpose. With the nanofibers, we realize theMZI by modal interference. The interference is achieved us-ing the single-mode fiber pigtails of the TOF as the interfer-ometer input and output, the down-taper as a beam splitterand the up-taper as a recombiner. In comparison with thenanofiber described above, the taper slope here is steeper,i.e. nonadiabatic, in order to induce coupling between thefundamental and higher order modes. If the entire nanofiberis cylindrically symmetric around its axis, light fields can becoupled only between the modes with the same azimuthal

Fig. 17 Contour plot of the transversal intensity distribution of theHE11 and HE12 modes. The dashed line indicates the nanofiber sur-face. The calculation was performed for a 1.3-μm diameter nanofiberand linearly polarized light in the x-direction at a wavelength of850 nm

symmetry, e.g., from the HE11 mode to HE12, HE13, etc. Atthe same time, a waist diameter can be chosen such that theHE13 and the higher HE1n modes are cut off (see Fig. 4),while the two lowest modes HE11 and HE12 are guidedalong the waist. Since the output pigtail is a single-modefiber, the HE12 mode is not guided there. The output powerthus depends on the interference in the up-taper, determinedby the relative phase accumulated by the HE11 and HE12

modes as they propagate along the waist: light can be eithercoupled back into the HE11 mode and guided out, or to theHE12 mode and be lost by coupling to radiative modes ofthe up-taper. Therefore, the output power is given by:

Pout ∼ sin(ϕ11 − ϕ12) with ϕ1n = β1nL, (8)

where β = 2πneff/λ is the propagation constant of a mode,L is the propagation distance, ϕ1n is the phase accumulatedby the HE1n mode, and n = 1 or 2.

The amount of light outside the fiber body is differentfor the HE11 and HE12 modes. For example, for a waist di-ameter of 1.3 μm and a wavelength of 850 nm, 2% of theoptical power of the HE11 mode and 31% of the opticalpower of the HE12 mode is propagating in the evanescentfield. Figure 17 shows the calculated intensity distributionfor both modes. As a result, a dispersive medium aroundthe fiber will influence the phase accumulated by the HE12

mode more strongly than that for the HE11 mode, and thuscreate a measurable phase difference.

We have implemented and tested such an interferometer.For this purpose, we use a Fibercore SM800 fiber with a purefused silica cladding, which is single-mode for wavelengthsabove 800 nm. The pulling rig was programmed to producea taper slope of 16 mrad and a waist of 1.3 μm diameter and3 mm length. After pulling, the interferometer was tested inthe pulling rig by coupling a single-frequency diode laser at852 nm into the input pigtail of the fiber. The output wasdetected with a photodiode, and the stretcher stage of thepulling rig was used to elastically stretch the fiber by a fewten micrometers.

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Fig. 18 Interference fringes due to stretching of the fiber. The elonga-tion of the waist leads to an additional phase shift between the HE11and the HE12 modes. The slight modulation of the sinusoidal signalmay indicate a weak presence of further modes possibly excited due tonanofiber asymmetries

Figure 18 shows the resulting interference signal betweenthe HE11 and the HE12 modes [34]. The changes in the phasedifference are expected to be given by:

�(ϕ11 − ϕ12) = (β11 − β12)�L + �(β11 − β12)L, (9)

where the term �(β11 − β12) takes into account the influ-ence of both the Poisson and photoelastic effect on the prop-agation constants. The Poisson effect leads to the stretch-induced change of the fiber diameter. This, according toFig. 4, results in the change of the propagation constantsof both of the propagating modes. The photoelastic effectexplains the change of the material refractive index understrain. This change, in turn, is also influencing the propaga-tion constants [41].

The measurement outcome demonstrates the realizationof a single-nanofiber-based interferometer with good inter-ference contrast. A possible application of this device is tomeasure variations of the refractive index of the surroundingmedium. The latter will differently change the propagationconstants β, and thus lead to interference fringes which canthen be traced.

7 Outlook

In this review, we have shown applications of fiber-based ab-sorption and fluorescence spectroscopy. Our spectroscopictechnique could be useful for biosensing, extending the pos-sible applications from label-free detection by absorptionmeasurements [62] to label-based detection using fluores-cence measurements [63]. The fact that we are able to si-multaneously record absorption and fluorescence spectra fora given surface coverage, in conjunction with the stable and

reproducible fluorescence collection efficiency, makes ourmethod ideal for analyzing and quantifying dynamic pro-cesses at or near the nanofiber surface. Excitation and fluo-rescence collection via a single fiber mode yields an entirelyfiber-based method that can be used for spectroscopic stud-ies at remote locations.

We have also demonstrated our first results on the imple-mentation of a nanofiber-based interferometer. The next goalis to perform dispersive detection on emitters adsorbed at thefiber waist and to compare the results with the absorptionmethod. Further, the dispersive method should allow us todetect species that do not absorb in the available wavelengthrange or to carry out measurements on sensitive moleculeswhile avoiding photobleaching.

Acknowledgements This work was supported by the DeutscheForschungsgemeinschaft DFG (Research Unit 557), the Volkswa-gen Foundation (Lichtenberg Professorship), the European ScienceFoundation (European Young Investigator Award), and the EuropeanComission (STREP CHIMONO).

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