Scanning near-field optical microscopy images of microradiographs stored in lithiumfluoride films with an optical resolution of λ ∕ 12Alessandro Ustione, Antonio Cricenti, Francesca Bonfigli, Francesco Flora, Antonella Lai, Tiziana Marolo, RosaMaria Montereali, Giuseppe Baldacchini, Anatoly Faenov, Tatiana Pikuz, and Lucia Reale Citation: Applied Physics Letters 88, 141107 (2006); doi: 10.1063/1.2193654 View online: http://dx.doi.org/10.1063/1.2193654 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/88/14?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optical spectroscopy and microscopy of radiation-induced light-emitting point defects in lithium fluoride crystalsand films Low Temp. Phys. 38, 779 (2012); 10.1063/1.4740241 Point Light Sources Based on Color Centers in LiF Films for Scanning NearField Optical Microscopy AIP Conf. Proc. 709, 444 (2004); 10.1063/1.1764050 Multicolor images acquisition by scanning near-field optical microscopy J. Appl. Phys. 90, 4820 (2001); 10.1063/1.1408265 Nanospectrofluorometry inside single living cell by scanning near-field optical microscopy Appl. Phys. Lett. 79, 2489 (2001); 10.1063/1.1409586 Imaging organic device function with near-field scanning optical microscopy J. Chem. Phys. 112, 7811 (2000); 10.1063/1.481386

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Scanning near-field optical microscopy images of microradiographsstored in lithium fluoride films with an optical resolution of � /12

Alessandro Ustionea�

Istituto di Struttura della Materia—C.N.R., via fosso del cavaliere 100, 00133 Roma, Italy

Antonio CricentiIstituto di Struttura della Materia—C.N.R., via fosso del cavaliere 100, 00133 Roma, Italyand Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37212

Francesca Bonfigli, Francesco Flora, Antonella Lai, Tiziana Marolo,Rosa Maria Montereali, and Giuseppe BaldacchiniENEA, Advanced Physical Technologies, C. R. Frascati, via E. Fermi 45, 00044 Frascati, Italy

Anatoly Faenov and Tatiana PikuzMISDC of VNIIFTRI Mendeleevo, Moscow Region, 141570, Russia

Lucia RealeUniversità de L’Aquila e INFN, Physics Department, Coppito, 67100 L’Aquila, Italy

�Received 22 April 2005; accepted 15 March 2006; published online 6 April 2006�

Here we show a new, simple method to observe soft x-ray microradiographs of biological material.A thin film of lithium fluoride �LiF� works as image detector, storing the microradiograph obtainedexposing biological samples to extreme ultraviolet and soft x-ray radiations. To read the storedimage, collecting the optically stimulated visible luminescence emitted by the LiF active colorcenters locally produced by the x rays, a scanning near-field optical microscope is used with anoptical aperture of 50 nm, i.e., � /12, where � is the wavelength of the collectedphotoluminescence. © 2006 American Institute of Physics. �DOI: 10.1063/1.2193654�

High resolution soft x-ray microradiography of biologi-cal material has been hindered so far by the poor spatialresolution or dynamic range of the available detectors. Over-coming this obstacle requires monochromatic radiation fromx-ray synchrotron sources and diffractive lenses. Here weshow a method to observe microradiographs of biologicalsamples using a scanning near-field optical microscope�SNOM� with an optical resolution of 50 nm.

LiF is a well-known radiation-sensitive material, that canbe grown as a thin film on different substrates.1,2 Recently,LiF was used as image detector for x-ray microradiographyand x-ray microscopy3 due to the fact that extreme ultraviolet�EUV� radiation and soft x rays locally produce, in LiF,stable point defects,4 known as color centers �CCs�. Amongthe CCs family, the F3

+ and F2 CCs are of particular interestfor photonic applications and they emit visible light underselective optical excitation. In this experiment the EUV andsoft x rays were generated by a laser plasma source5–7 ob-tained by focusing an excimer laser emitting 1 J 10 ns pulseswith a repetition rate up to 10 Hz on a zinc target. The x-rayemission covers the spectral interval of 1500–20 eV�0.8–60 nm�. The exposure of a mosquito wing has beenobtained with 500 shots placing the sample in contact withthe radiation-sensitive salt layer and fixing the edge of thewing with glue on LiF-based detector. The plasma sourceemits soft x rays and EUV photons in flash bursts having aduration equal or less than the laser pulse width, within a 2�solid angle following a cos0,8��� angular intensity distribu-tion, and therefore irradiates the whole surface of thesamples placed in its proximity. Close to the normal of the

target ���8�� the x-ray flux is uniform within 1%. Thus, asignificant advantage of this technique is that a large area �upto 4�4 cm2 with a dose uniformity within 1% 15 cm awayfrom the target� can be irradiated in a short exposure time.The x-ray dose delivered to the sample is about 75 mJ/cm2.The biological sample is dried in the vacuum and after theexposure the biological sample has been easily removedfrom the LiF film where the wing image is stored.

The exposed LiF film stores a microradiograph of thebiological sample in a thin surface layer �estimated depth�50 nm�8 containing CCs. It can be read in a simple way byilluminating the film itself with a blue light, at around450 nm, in the middle of the almost overlapping absorptionbands of F3

+ and F2 CCs, and collecting their yellowish pho-toluminescence, which consists of two broad emission bandspeaked around 535 and 680 nm, respectively.2 LiF-filmbased radiation sensors overcome the limitations of x-raystandard detectors used in contact mode microscopy likephotographic emulsions �low spatial resolution� and poly-meric photoresists �low dynamic range�.3,9 Moreover, the LiFdetector does not need any development process and the op-tically stimulated luminescence �OSL� reading technique issimple and versatile. The spatial distribution of the so-created CCs will depend on the local structure of the biologi-cal material, and its detection will rely on the used readoutoptical technique.

Among the various kind of optical techniques, scanningnear-field optical microscopy �SNOM� offers the possibilityof investigating the optical properties of a sample with aresolution in the nanometric range,10–14 not limited to � /2.Its peculiarity to overpass the Rayleigh diffraction limit re-lays on the properties of the nonpropagating component ofthe electromagnetic field �its intensity decays exponentially

a�Author to whom correspondence should be addressed; electronic mail:alessandro.ustione@ism.cnr.it

APPLIED PHYSICS LETTERS 88, 141107 �2006�

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with the distance�. SNOM requires that a subwavelength ap-erture is kept at a distance of few nanometers from the in-vestigated surface and the smaller the aperture, the higher theresolution obtainable.

In this letter we have shown images of a microradio-graph of a mosquito �Diptera� wing, stored in a 120-nm-thick LiF film thermally evaporated on a silicon substrate.There are many different geometries that can be used in aSNOM15 setup. The one used in the present experiments isthe collection mode:16 the light coming from an Ar+ laser, atthe wavelength �=458 nm, is illuminating the whole area ofthe sample and the local fluorescence is revealed through asmall aperture, consisting of the sharp tip at the end of anuncoated, tapered optical fiber, placed at few nanometersfrom the illuminated surface. The near-field components arethus converted into far-field ones and propagated into theoptical fiber. A long-pass filter, with a cutoff wavelength of510 nm, is used to collect, through a photomultiplier, onlythe integrated fluorescence coming from the active CCs inLiF.

The electronic to control the scanning unit, the data ac-quisition system and the image elaboration are fully de-scribed elsewhere.17–19 In order to keep the aperture of thefiber at a small constant distance from the surface, a lateralpiezoelectric is used to dither the fiber and a bimorph piezo-electric detects the shear force between the vibrating tip andthe surface. We simultaneously acquire topographical �con-stant shear force� and optical �fluorescence� images to iden-tify possible artifacts. However, it is worth noting the factthat such samples are ideal candidates for SNOM studies,since the optical contribution arises from electronic differ-ences between the locally created CCs and the not-irradiatedblank LiF. No topographic contribution is expected so thatartifacts induced by the local topography can be completelyruled out.

It is also worth noting that the investigated sample is anoptically transparent film of LiF thermally evaporated onsilicon. Because the silicon substrate is not transparent in theenergy range of interest, this sample can only be studiedusing a SNOM working in the collection mode.

SNOM images of the microradiograph of a mosquitowing are shown in Figs. 1�a� �topography of the LiF film�and 1�b� �fluorescence�. The smooth surface of the LiF film,that has a typical roughness of �20 nm,20 is covered byparticles of various nature �debris, dust, etc.�, as can be ob-served in Fig. 1�a�. On the contrary, the corresponding fluo-rescence image, shown in Fig. 1�b�, clearly reveals the de-

tails of a small portion of the wing: little hairs, having widthbetween 0.5 and 1 �m and length between 6 and 10 �m, arespread all over the wing. Due to the hair higher density, theyabsorbed more soft x rays than the rest of the wing and sothey appear darker in the microradiograph �in fact, a layer of�10 �m of biological material is sufficient to completelystop soft x rays�. The hairs are present in pairs on the wingand they appear like overturned dark V shapes in the fluo-rescence image.

During the analysis of this sample, we encountered re-gions where metallic debris coming from the laser-plasmasource used in the exposing process is present on the surfaceof the LiF film �Fig. 2�a��. It can be distinguished from otherimpurities looking at the corresponding fluorescence image�Fig. 2�b��: it block the illuminating light and its shape ap-pears as a nonfluorescent object. Although not intentional,the debris presence has been crucial to determine the opticalresolution of our SNOM. In fact, this zinc debris blocked thecreation of the CCs and because it arrived on the film surfaceas melted material, it adhered very well to the film surface,so we can estimate its distance �D� from the LiF detectorcomparable with the LiF film roughness that is �20 nm. Inthis way the resulting diffraction blurring can be evaluated as�r���D�1/2 yielding a value between 4 and 34 nm, consid-ering the extreme x-ray wavelengths used in the exposureprocess �0.8–60 nm�. As they show very sharp edges, werepeated a scan across one of them: the area of this measureis enclosed in the white square shown in Fig. 2�b�. The re-sulting SNOM image �Fig. 2�c� is the fluorescence image�shows the sharp transition from a fluorescent to a nonfluo-rescent area. Along the AA� segment in Fig. 2�c�, we tracedthe intensity profile that is shown in Fig. 2�d�. From thisprofile it results that this edge has a width of �50 nm, andconsidering that the wavelength of the collected light rangesbetween 550 and 650 nm, we evaluate an edge resolution of

FIG. 1. SNOM images �40 �m�40 �m� of a microradiograph of a mos-quito wing stored in a LiF film. �a� Topographical image; the height �Z� ismeasured in nanometers. �b� Fluorescence image; the total integrated fluo-rescence intensity �I� is measured in arbitrary units. The dark overturned Vshapes that are pointed by the arrows are the radiographic images of smallhairs on the wing surface.

FIG. 2. Edge resolution: �a� 40 �m�40 �m topographical image, showingthe presence of zinc debris from the laser-plasma source, over the LiF filmsurface, �b� 40 �m�40 �m corresponding fluorescence image, showing thesharp contrast produced by the metallic debris. The white square enclose thearea over which we took the next scan. �c� A 4�4 �m fluorescence image;the AA� segment laying across the sharp transition indicates where the in-tensity profile was traced. �d� Intensity profile showing an edge resolution of�50 nm, corresponding to � /12.

141107-2 Ustione et al. Appl. Phys. Lett. 88, 141107 �2006�

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� /12 for our SNOM setup. This means that all the opticalimages shown before are collected with this resolution.

In conclusion, we have demonstrated that a microradio-graph of biological material, stored in LiF thin films, can beread by SNOM working in the collection mode with a reso-lution of 50 nm that corresponds to � /12. For its high opticalresolution, SNOM can fully exploit the advantages of animage detector based on active CCs in LiF and moreover theachieved results demonstrate that the SNOM technique is asuitable one to directly read microradiography based on OSLat nanometric scale, although the biological sample featuresand, probably, the mounting on the LiF detector, could limitthe final resolution of the obtained images.

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