REPRINT

LASER & PHOTONICSREVIEWS

www.lpr-journal.org

LASER & PHOTONICSREVIEWS Laser Photonics Rev. 6, No. 5, 678–683 (2012) / DOI 10.1002/lpor.201200010

OR

IGIN

AL

PAPE

R

Abstract An innovative, simple and reliable method to fabri-cate micro-lasers by self-assembly of rod-shaped nanocrys-tals is demonstrated. Dot/rod core/shell CdSe/CdS nanorodsare used to form optical micro-resonators by exploiting theirself-organization into well-defined coffee stain rings. The fabri-cation process merely consists of capillary jet deposition of ananorod solution onto a glass substrate, and is scalable, eco-nomic, and highly reproducible. Upon optical pumping of themicro-resonators, laser emission in the red or in the blue-greenspectral region is obtained, demonstrating lasing both from coreand shell transitions, with low pumping thresholds. Modeling byfull-wave numerical simulations according to generalized (i. e.scattering) formulation of laser theory demonstrates lasing fromcomplex modes of the self-assembled cavity.

Self-assembled CdSe/CdS nanorod micro-lasers fabricatedfrom solution by capillary jet depositionMargherita Zavelani-Rossi1,*, Roman Krahne2,*, Giuseppe Della Valle1, Stefano Longhi1,Isabella R. Franchini3, Salvatore Girardo3, Francesco Scotognella1, Dario Pisignano4,Liberato Manna2, Guglielmo Lanzani1,5, and Francesco Tassone5

1. Introduction

Colloidal semiconductors nanocrystals (NCs) are attractingincreasing interest as functional materials in many photonicapplications due to their optical properties and to the possi-bility to control their electronic level structure by changingtheir composition, size and shape. Furthermore, they canbe fabricated at low cost and organized in ordered struc-tures over large scales by simple self-assembly. Prototypesof light-emitting diodes [1], solar cells [2], colour convert-ers [3] and lasers [4, 5] based on colloidal NCs have beenproposed, but so far not much developed. In particular forlasing applications zero and one-dimensional particles con-sisting of homo- or hetero-structures have been tested [4–8],and heterostructure systems have proven to be more advanta-geous due to additional control over their electronic proper-ties. One-dimensional particles [4, 6–8] offer the advantageof strong quantum confinement along two spatial directions,while retaining a large absorption cross section [9]. In thiscontext an interesting combination is represented by asym-metric CdSe/CdS core/shell dot/rod nanorods (NRs) whichhave unique optical properties [10, 11] like optical gain [12]that is particularly long living [13] due to substantial reduc-

tion of surface/interface defects [14] and because of suppres-sion of Auger non-radiative recombination [15]. Moreoversuch heterostructures show optical gain both from core andshell transitions in the red or in the green respectively [16].The challenge is to develop efficient laser devices by simpleand reliable fabrication techniques.

Colloidal NRs can be organized into ordered structuresover large areas on the micron and even millimeter scaleby self-assembly [10, 17–19], for example via depositionfrom solution onto planar surfaces and controlled solventevaporation or mediated by external electric fields [10]. Thecoffee stain effect, i. e. the fluid dynamics in an evaporatingdroplet [20, 21], can be exploited to obtain superstructuresthat are reminiscent of nematic/smectic liquid crystal phases.In this case a dense and highly ordered region is formedwithin the outer ring at the edge of the evaporated droplet,with the NRs aligned parallel to the ring edge [21]. Fromsuch regions polarized emission [10, 22], directionally de-pendent photoconductivity [17, 23], and lasing [13] havebeen observed.

Here we used CdSe/CdS NRs to fabricate micro-lasersby a solution-based technique, in a controllable and repro-ducible method that can be fully integrated in wet lab pro-

1 Dipartimento di Fisica, IFN-CNR, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano, Italy 2 Istituto Italiano di Tecnologia, via Morego30, 16163 Genova, Italy 3 NNL, CNR-Istituto Nanoscienze, Universita del Salento, via Arnesano 16, 73100 Lecce, Italy 4 NNL, CNR-IstitutoNanoscienze, Dipartimento di Matematica e Fisica ”Ennio De Giorgi”, Universita del Salento, Via Arnesano 16, Lecce 73100, Italy and Center forBiomolecular Nanotechnologies, IIT @UniLe, via Barsanti, 73010 Arnesano, Lecce, Italy 5 Center for Nano-Science and Technology, IIT@POLIMI,via G. Pascoli 70/3, 20133 Milano, Italy* Corresponding authors: e-mail: margherita.zavelani@fisi.polimi.it, roman.krahne@iit.it

© 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Laser Photonics Rev. 6, No. 5 (2012)

ORIGINALPAPER

679

cesses. We obtained laser emission in the red and blue-greenspectral region from NRs with different length, due to tran-sitions respectively from core states or shell states. Thespectral region of emission was determined essentially bythe NR length. We emphasize that in our innovative de-vices the NRs both constitute the resonator and the gainmedium, which enables a simple one-step fabrication pro-cess to obtain micron-sized lasers with low pump thresholdsand wide range of emission. Lasing in the coffee stain ringsis sustained by complex modes inside the gain mediumwhich can be modelled on the basis of generalized (i. e. scat-tering) formulation of laser theory in open and complexcavities [24, 25]

2. Fabrication of self-assembled CdSe/CdSNR microlasers

We used two representative NR samples synthesized by awet-chemical route [26] that had similar core size of 3 nmdiameter and different lengths of 20 nm (short) and 50 nm(long). The NRs were dissolved in toluene solution (concen-tration of 1–3 µM) and picoliter quantities were depositedonto glass substrates using an Eppendorf femtojet systemcoupled to an inverted optical microscope. The femtojetapparatus consisted of a capillary tube (apex diameter of20 µm) coupled to a compressor and enabled the flow controlof NR solution via tuning the pressure and time of the injec-tion pulse (typical values were 1000 hPa for 0.1–1 s). Theapex of the capillary tube was adjusted to a distance of lessthan 300 µm above the glass substrate that was previouslycleaned in acetone and isopropanol in ultrasonic baths andblow dried by nitrogen. Deposition of drops with picolitervolumes resulted in a single, well defined coffee stain ringthat had diameters in the hundred-µm range and widths of5–15 µm, as displayed in Figs. 1a,b by scanning electron mi-croscopy images. The AFM image in Fig. 2a demonstratesthat the ring shape was uniform over the length-scales ofseveral tens of micrometers with an almost constant heightof ∼1 µm. The cross section in Fig. 2c (solid line) revealswell-defined borders with a slope of approximately 45◦at the

Figure 1 (online color at: www.lpr-journal.org) Scanning electronmicroscopy (SEM) images of a self-assembled coffee stain con-sisting of densely packed NRs: (a) entire area of a typical coffeestain; (b,c) higher magnification images of the outer coffee stainring showing its well defined borders (b) and the alignment of thenanorods (c). Some NRs are highlighted by the yellow lines as aguide to the eye; (d) the central region of the coffee stain wherethe NRs are deposited with lower density and do not show largescale alignment. We note that in (d) the individual NRs can be bet-ter resolved by SEM with respect to (c) due to a reduced amountof residual organics in this region and the lower NR layer thick-ness. The inset in (d) illustrates the core/shell architecture. Redrectangles in (a) indicate the regions corresponding to zoomedimages (b–d).

inside and 10◦at the outside. The slope of the outer bordercorresponds to the contact angle of the evaporating dropletwith the glass substrate before the detachment of the contactline, and is similar for both profiles shown in Fig. 2c. Theheight of the ring was determined by the nanorod concen-tration in the solution and by the time that the contact lineremained pinned during the solvent evaporation process. Inthis respect the cleanliness of the substrate is very impor-tant, as dust particles favor the rupture of the contact line,and lead to less homogeneous rings as the one displayed inFig. 2b. We clearly note the irregularities in the ring surfacedue to the dust particles, and we also observe a genericallydifferent cross section profile in this case, evidenced by thedotted line in Fig. 2c, that is more symmetric and has a re-duced height with respect to the profile of the “clean” ring.To understand this, we briefly recall how the coffee stain

(a) (b) (c)

Figure 2 (online color at: www.lpr-journal.org) Typical atomic force microscopy images recorded in tapping mode of coffee stain ringsdeposited onto a clean surface (a), and onto a surface contaminated by dust particles (b). The coffee stain ring in (a) showed lasing,that in (b) did not show lasing. The cross section profile of the ring in (a) (solid line), and of the ring in (b) (dotted line) are displayedin (c).

www.lpr-journal.org © 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

680

LASER & PHOTONICSREVIEWS

M. Zavelani-Rossi et al.: Self-assembled CdSe/CdS nanorod lasers fabricated by capillary jet deposition

ring is formed: after drop deposition the pinning of the exter-nal contact line of the drop with the substrate surface duringsolvent evaporation leads to a microfluidic flow towards theexternal border that transports the NRs to this edge and re-sults in the ring formation. Therefore, the longer the contactline stays pinned the more material is accumulated in thecoffee stain ring. Eventually, the height of the accumulatedmaterial exceeds the height of the liquid surface, which thenforms a cone at the inside of the ring and shortly afterwardsdetaches [27]. This process determines the inner slope ofthe coffee stain ring, which in our case is around 45◦. Ir-regularities like dust particles destabilize the pinning of thecontact line and lead to an earlier detachment, which is nottriggered by the liquid cone and thus the shape of the profileis different. We emphasize that the achievement of a single,well defined, controllable coffee stain ring is unique to thecapillary jet deposition and represents a significant innova-tion with respect to previous works [13] where microliterquantities were used.

3. Lasing operation and characteristic:experimental results and discussion

Upon optical excitation of the coffee-stain assembled NRswe could observe laser emission. We used, as the pump,100-fs laser pulses at 400 nm, produced by second harmonicgeneration of an amplified Ti:sapphire laser system oper-ating at 1 KHz repetition rate, focused on a circular spotwith radius of ∼ 40 µm. The pump beam direction variedfrom 0◦to 45◦to the normal of the sample substrate and theemission was collected at angles varying from 90◦to 0◦, by

a bundle of optical fibers connected to a cooled spectrome-ter. All experiments were performed at ambient conditions.Typically at low excitation fluences we detected solely thephotoluminescence (PL) of the NRs, whereas for excitationfluences larger than a certain threshold value we observed anarrow peak, with a FWHM of <1 nm, which corresponds tolaser emission. We obtained laser emission solely from ringsthat were deposited onto clean surfaces (like the one shownin Fig. 2a) and which had a ring height larger than 600 nm.The surface irregularities of the ring displayed in Fig. 2b(that can be related to residual dust particles on the sub-strate) have diameters that are comparable to the emissionwavelength of the material, and consequently are possiblesources for large scattering losses in the micro-cavity thatprevent lasing. Figure 3 displays the emission properties ofa cavity made of short NRs. Here laser emission peaks at610 nm, which is related to recombination from states local-ized in the CdSe region, with a FWHM of 0.6 nm, close tothe resolution limit of 0.5 nm of our spectrometer. In Fig. 3bwe show the input-output characteristic, where we observea clear pump threshold followed by a linear dependence,which is characteristic of laser behavior.

In films consisting of long NRs the PL emission spec-trum (inset of Fig 4a) is characterized by two peaks, themain one at around 610 nm due to CdSe-core transitionsand the second at around 500 nm due to CdS-shell transi-tions, which shows amplified spontaneous emission [16].The emission properties of a resonator made of long NRsare displayed in Fig. 4: the laser emission spectrum has asingle narrow peak (FWHM of 0.7 nm) at 495.5 nm and theinput-output characteristic shows the typical laser behavior.To our knowledge this is the first time that laser action orig-

(a) (b)

Figure 3 (online color at: www.lpr-journal.org)Emission characteristics of an aged micro-resonator consisting of short NRs: (a) normalizedemission spectra at low (black dashed line) and athigh (red continuous line) pump fluences and (b)emission intensity as a function of pump fluences(red circles) with linear fit (black dashed line). Theinset in (a) shows the normalized emission spec-trum of a freshly prepared device for an incidentpump fluence of ∼0.22 mJ/cm2 on a spot with∼ 120 µm radius.

(a) (b)

Figure 4 (online color at: www.lpr-journal.org)Emission characteristics of a micro-resonator con-sisting of long NRs: (a) normalized emission spec-tra at low pump fluence (black dashed line) andat high pump fluence (green continuous line); (b)emission intensity as a function of pump fluences(green circles) and linear fit (black dashed line).The inset in (a) shows a normalized emission spec-trum recorded from a dense film of disordered longNRs that manifests amplified spontaneous emis-sion at ∼520 nm.

© 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.lpr-journal.org

Laser Photonics Rev. 6, No. 5 (2012)

ORIGINALPAPER

681

inating from shell states in core/shell systems is reported.Lasing from the shell material is particularly interesting forpractical device applications because it provides a novelroute for achieving green-blue spectral emission from a het-erostructured NCs that can also be used for emission in thered spectral range.

The optical quality of the self-assembled micro-cavityis apparent from the low laser thresholds observed in our ex-periments. In freshly prepared samples a threshold pump flu-ence around 0.2 mJ/cm2 was observed both for short (insetof Fig. 3a and Fig. 6) and long NRs (Fig. 4b). After ageingover long time scales of several months the threshold pumpfluence increased to ∼ 10 mJ/cm2 for short NR (Fig. 3b),an issue that we will address later. The threshold pump flu-ence for fresh samples (0.2 mJ/cm2) is lower than the best(0.5 mJ/cm2) [28] and the most recent (4 mJ/cm2) [29] val-ues reported for nanocrystal lasers. It is also lower than thevalue obtained from coffee-stain devices fabricated by dropcast deposition (3 mJ/cm2) [13]. The very low values of thepump threshold reported above became possible due to theuse of the femto-jet deposition technique for the fabricationof coffee-stain based lasers, which allows to fully exploitthe advantage of the smart device design that directly com-bines gain and feedback via a self-assembled cavity. In allother cases lasers based on ensembles of nanocrystals useexternal cavities to provide the optical feedback [4,5,28,29],thus introducing extra losses. In this respect our system isin some way similar to individual liquid microdroplets con-taining organic dyes which show lasing [30], but with someadvantages. First of all the coffee stain laser is a permanentsolid-state structure which can be integrated with other de-vices, moreover it provides single line emission instead ofthe general multimode emission that results from whisperinggallery modes, finally, the use of colloidal nanocrystals pro-vides robustness and a broad range of emission wavelengthachievable with simple fabrication control.

The peculiar shape of the structure obtained by our capil-lary jet deposition procedure does not give rise to a standardFabry-Perot resonator, rather to a highly multi-modal opencavity supporting complex modes. To ascertain the possi-bility of lasing in such a complex cavity, we adopted thegeneralized formulation of laser theory commonly used tomodel two-dimensional micro-cavity lasers [24, 25] andrandom lasers [24, 25, 31], according to which the lasermodes correspond to the scattering resonances (complexpoles) of the self-assembled medium, and thus to a diver-gence of the scattered power when the gain medium reachesthe laser threshold. Therefore, we performed finite elementmethod numerical simulations (employing a commercialsoftware, Comsol Multiphysics 3.5) of the optical scatter-ing exhibited by the self-assembled structure with the mea-sured cross-section profile reported in Fig. 2c by the solidline. For the sake of simplicity we assumed that the cross-section is invariant along the third spatial direction, wheremode confinement is simply achieved by gain-guiding ofthe finite pump spot size. This allowed us to reduce thecomplexity of the study to a two-dimensional problem withperfectly-matched layer boundary conditions at the edges ofthe (20x20 µm2) computation domain. As the incident wave,

we considered a circularly polarized plane wave imping-ing at an angle of 45◦with respect to the plane of the ringto guarantee excitation of all the resonances of the struc-ture. In our scattering simulations we assumed a complexrefractive index of the NR structure with variable imaginarypart nI, corresponding to different pumping conditions inthe laser experiment, whereas the real part of the refrac-tive index was estimated [32] to be nR = 2.9. Frequencypulling effects, which would just lead to a slight change inthe estimated laser threshold and emission frequency, wereneglected for the sake of simplicity. We computed the totalpower scattered by the structure as the flux of the Poynt-ing vector of the scattered fields across the boundaries ofthe computation domain. By inspecting the total scatteredpower as a function of the wavelength of the incident fieldand for different values of nI, a set of scattering resonances,corresponding to the laser modes, is revealed in the wave-length range 600–620 nm as displayed in Fig. 5a. Here thecavity mode at 614.2 nm achieves laser threshold for a gaincoefficient g = 2nIk0 = 1.75× 103 cm−1 (where k0 is thewavenumber).

The experimental gain coefficient g of the self-assembledNRs within the coffee stain rings can be derived from ab-sorption and pump-probe measurements. We used data ob-

(a)

(b)

Figure 5 (online color at: www.lpr-journal.org) (a) FEM numericalsimulations of the power spectrum scattered by the 2D micro-cavity as a function of the gain coefficient g and the wavelength λ .(b) Normalized intensity pattern of the optical field at resonance(λ = 614.2 nm, g = 1.75 × 103 cm−1). The cross section profile ofFig. 2c is shown by the white solid line.

www.lpr-journal.org © 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

682

LASER & PHOTONICSREVIEWS

M. Zavelani-Rossi et al.: Self-assembled CdSe/CdS nanorod lasers fabricated by capillary jet deposition

tained from the short NRs in solution [13,15] and we consid-ered the concentration of the solution and that of a packedfilm, as well as the anisotropy in absorption which is alsorelevant for core transitions [33]. With the condition of−∆α/α = (α0−α)/α0 = 2, which can be achieved in prop-erly designed semiconductor nanocystals [13, 15, 16, 34, 35],we estimated g to be around 2.0×103 cm−1. Here α0 and α

correspond to the absorption for linear and excited system,respectively. The above value for g is consistent with theexpectation reported by Kazes et al. [36] and it is in agree-ment with the value required for lasing reported above thatwas obtained by the numerical simulations of the complexlaser cavity.

Our simulations show also that the cavity consideredin Fig. 5b can support modes with spectral separation of4.0 THz (5 nm in Fig. 5a). This separation can be comparedto the free spectral range (FSR) of a hypothetical Fabry-Perot cavity with a length L: ∆νfsr = c0/2nRL (where c0 isthe speed of light in vacuum, and nR is the index of refrac-tion of the material filling the resonator). Considering L asthe maximum extension in x direction in the cross section ofFig. 5b we obtain ∆νfsr = 6.5 THz which is larger than thespectral separation observed in Fig. 5a. This corroboratesthe idea of a complex zig-zag path of the laser modes insidethe resonator that are longer than L and indicates that thelaser feedback is provided in the transverse plane of the ring.The multimode behaviour can also be observed in the ex-perimental emission spectra shown in Fig. 6. The calculatedintensity pattern in Fig. 5b indicates also that the decreasein height towards the outer edge of the ring results in asignificant enhancement of the light intensity and, conse-quently, the highest laser emission intensity originates fromthe sharp outer edge of the cavity, from which a large frac-tion is coupled into the underlying glass substrate. This lightis trapped and guided by the substrate and can be simplycollected from its side, or can be exploited for the integra-tion of these laser emitters into photonic integrated circuits

Figure 6 (online color at: www.lpr-journal.org) (a) Normalizedlaser emission spectra from micro-cavities made of short NRswith different deposition parameters. (b-e) Normalized laser emis-sion spectra of a series of micro-cavities fabricated with identicaldeposition parameters and (f) corresponding coffee stain ringSEM images (the scale bar is 200 µm). The pump conditions were0.6–2 mJ/cm2 in (a), and 0.3−0.6 mJ/cm2 in (b-e) on a circularspot with ∼ 120 µm radius.

or similar devices, paving the way for new architectures ofmicro-optical-systems.

Microcavities obtained with different deposition param-eters have different geometrical characteristics and showdifferent emission spectra, where the emission peak spannedthe whole gain spectrum of the NR material (Fig. 6a). Inparticular, we achieved emission over a spectral range of11 nm, pumping at excitation fluences near threshold. Onthe other hand, in Fig. 6f we show a series of rings depositedin sequence with identical deposition parameters, togetherwith their corresponding laser emission spectra (Fig. 6b–e)and observe the laser emission peak at precisely the samewavelength. This proves the potential of the capillary jettechnique as a solution-based, reproducible and efficientdeposition process for lasing applications at a specific wave-length. Furthermore, it can be easily integrated in mechani-cal systems, for example, in inkjet printers with micro nozzlepositioning controlled via micro-mechanical stages.

By pumping larger regions of different coffee stains,with a spot size of the pump beam of ∼ 120-µm radius, weobtained emission peaks with FWHM ranging from 0.8 nm(inset in Fig. 3a) to 3 nm (Fig. 6). The large peak width canbe ascribed to slight variations in the ring width, within thesame stain, giving rise to a sort of group of lasers in par-allel configuration. Indeed, dust particles on the substrateor agglomerates in the NR solution can introduce inhomo-geneities. On the other hand narrow emission peaks areobserved from very homogeneous rings, like for examplethe one in Fig. 2a that has a FWHM of 0.8 nm. Note thatin such cases the uniformity is of the order of few tens ofnanometers over areas of several tens of microns and that itdepends on the cleanliness of the substrate and the absenceof disruption during the solvent evaporation process. It isworth noting that these conditions can be further improvedin a dedicated fabrication procedure.

Another remarkable aspect of our NR micro-lasers isthat they preserve their properties over long storing times un-der ambient conditions. We found nearly unaltered emissioncharacteristics over time periods of several months, as dis-played in Fig. 3 where an aged sample maintained a narrowemission peak even though with a higher pump threshold.We conclude that aging only adds losses, most probably dueto degradation of the material on the surfaces of the cavitybut it does not alter the structure of the resonator. Anothercharacteristic point is the stability and resistance to degra-dation under laser action. At pump fluences slightly abovethreshold no appreciable degradation was detected over tensof minutes of operation.

4. Conclusions

We presented a new controllable and reproducible tech-nique to produce micro-lasers based on CdSe/CdS dot/rodNRs, with emission from core or shell transitions in thered or blue-green spectral region. The formation of suchmicro-cavities is driven by self-assembly and is obtainedby a one-step, simple and inexpensive capillary jet deposi-tion process. The deposition technique we demonstrated is

© 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.lpr-journal.org

Laser Photonics Rev. 6, No. 5 (2012)

ORIGINALPAPER

683

scalable and compatible with many substrates used in nano-photonics. This approach provides thus a new possibilityfor the integration of narrow band emitters, in the visiblespectral range, into device architectures such as lab-on-chipfor point of care diagnostics, or for optical components inlocal area network datacom structures.

Acknowledgements. L. Manna acknowledges partial financialsupport from European Union through the FP7 starting ERC grantNANO-ARCH (contract number 240111) and G. Lanzani fromproject ICARUS (grant agreement number: 237900).

Received: 13 March 2012, Revised: 29 May 2012,Accepted: 4 June 2012

Published online: 29 June 2012

Key words: Colloidal nanocystals, nanocrystal lasing, self as-sembly, nanorods.

References

[1] S. Coe, W. K. Woo, M. Bawendi, and V. Bulovic, Nature 420,800–803 (2002).

[2] D. J. Milliron, I. Gur, and A. P. Alivisatos, Mrs Bulletin 30,41–44 (2005).

[3] H. Puhringer, J. Roither, M. V. Kovalenko, M. Eibelhuber,T. Schwarzl, D. V. Talapin, and W. Heiss, Appl. Phys. Lett.97, 111115 (2010).

[4] M. Kazes, D. Y. Lewis, Y. Ebenstein, T. Mokari, and U. Banin,Adv. Mat. 14, 317–321 (2002).

[5] H. J. Eisler, V. C. Sundar, M. G. Bawendi, M. Walsh,H. I. Smith, and V. Klimov, Appl. Phys. Lett. 80, 4614–4616(2002).

[6] H. C. Hsu, C. Y. Wu, and W. F. Hsieh, J. Appl. Phys. 97,064315 (2005).

[7] M. A. Zimmler, J. Bao, F. Capasso, S. Muller, and C. Ron-ning, Appl. Phys. Lett. 93, 051101 (2008).

[8] J. P. Mondia, R. Sharma, J. Schafer, W. Smith, Y. P. Zhao,Z. H. Lu, and L. J. Wang, Appl. Phys. Lett. 93, 121102(2008).

[9] H. Htoon, J. A. Hollingworth, A. V. Malko, R. Dickerson,and V. I. Klimov, Appl. Phys. Lett. 82, 4776–4778 (2003).

[10] L. Carbone, C. Nobile, M. De Giorgi, F. D. Sala, G. Morello,P. Pompa, M. Hytch, E. Snoeck, A. Fiore, I. R. Franchini,M. Nadasan, A. F. Silvestre, L. Chiodo, S. Kudera, R. Cin-golani, R. Krahne, and L. Manna, Nano Lett. 7, 2942–2950(2007).

[11] F. Pisanello, L. Martiradonna, G. Lemenager, P. Spinicelli,A. Fiore, L. Manna, J. P. Hermier, R. Cingolani, E. Giacobino,M. De Vittorio, and A. Bramati, Appl. Phys. Lett. 96, 033101(2010).

[12] M. Saba, S. Minniberger, F. Quochi, J. Roither, M. Marceddu,A. Gocalinska, M. V. Kovalenko, D. V. Talapin, W. Heiss,A. Mura, and G. Bongiovanni, Adv. Mat. 21, 4942–4946(2009).

[13] M. Zavelani-Rossi, M. G. Lupo, R. Krahne, L. Manna, andG. Lanzani, Nanoscale 2, 931–935 (2010).

[14] A. Creti, M. Zavelani-Rossi, G. Lanzani, M. Anni, L. Manna,and M. Lomascolo, Phys. Rev. B 73, 165410 (2006).

[15] M. Zavelani-Rossi, M. G. Lupo, F. Tassone, L. Manna, andG. Lanzani, Nano Lett. 10, 3142–3150 (2010).

[16] R. Krahne, M. Zavelani-Rossi, M. G. Lupo, L. Manna, andG. Lanzani, Appl. Phys. Lett. 98, 063105 (2011).

[17] D. Steiner, D. Azulay, A. Aharoni, A. Salant, U. Banin, andO. Millo, Phys. Rev. B 80, 195308 (2009).

[18] M. Zanella, R. Gomes, M. Povia, C. Giannini, Y. Zhang,A. Riskin, M. Van Bael, Z. Hens, and L. Manna, Adv. Mat.23, 2205–2209 (2011).

[19] A. M. Hung, T. Oh, and J. N. Cha, Nanoscale 4, 1016–1020(2012).

[20] R. D. Deegan, O. Bakajin, T. F. Dupont, G. Huber,S. R. Nagel, and T. A. Witten, Nature 389, 827–829 (1997).

[21] C. Nobile, L. Carbone, A. Fiore, R. Cingolani, L. Manna,and R. Krahne, J. Phys. Cond. Matter 21, 264013 (2009).

[22] A. Rizzo, C. Nobile, M. Mazzeo, M. De Giorgi, A. Fiore,L. Carbone, R. Cingolani, L. Manna, and G. Gigli, Acs Nano3, 1506–1512 (2009).

[23] A. Persano, M. De Giorgi, A. Fiore, R. Cingolani, L. Manna,A. Cola, and R. Krahne, Acs Nano 4, 1646–1652 (2010).

[24] H. E. Tureci, A. D. Stone, and B. Collier, Phys. Rev. A 74,043822 (2006).

[25] T. Harayama, S. Sunada, and K. S. Ikeda, Phys. Rev. A 72,013803 (2005).

[26] D. V. Talapin, R. Koeppe, S. Gotzinger, A. Kornowski,J. M. Lupton, A. L. Rogach, O. Benson, J. Feldmann, andH. Weller, Nano Lett. 3, 1677–1681 (2003).

[27] S. Maheshwari, L. Zhang, Y. Zhu, and H.-C. Chang, Phys.Rev. Lett. 100, 044503 (2008).

[28] C. F. Zhang, F. Zhang, A. Cheng, B. Kimball, A. Y. Wang,and J. Xu, Appl. Phys. Lett. 95, 183109 (2009).

[29] Y. J. Chen, B. Guilhabert, J. Herrnsdorf, Y. F. Zhang,A. R. Mackintosh, R. A. Pethrick, E. Gu, N. Laurand, andM. D. Dawson, Appl. Phys. Lett. 99, 241103 (2011).

[30] K. F. W. H.-M Tzeng, M. B. Long, and R. K. Chang, Opt. Lett.9, 499 (1984).

[31] M. A. Noginov, G. Zhu, I. Fowlkes, and M. Bahoura, LaserPhys. Lett. 1, 291–293 (2004).

[32] T. M. Bieniewski and S. J. Czyak, J. Opt. Soc. Am. 53, 496(1963).

[33] A. Lutich, L. Carbone, S. Volchek, V. Yakovtseva, V. Sokol,L. Manna, and S. Gaponenko, Phys. Status Solidi-RapidResearch Letters 3, 151–153 (2009).

[34] F. Garcia-Santamaria, Y. F. Chen, J. Vela, R. D. Schaller,J. A. Hollingsworth, and V. I. Klimov, Nano Lett. 9, 3482–3488 (2009).

[35] A. V. Malko, A. A. Mikhailovsky, M. A. Petruska,J. A. Hollingsworth, and V. I. Klimov, J. Phys. Chem.B 108, 5250–5255 (2004).

[36] M. Kazes, D. Oron, I. Shweky, and U. Banin, J. Phys. Chem.C 111, 7898–7905 (2007).

www.lpr-journal.org © 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim