Analysis of tissue-substrate adhesion by hyperspectral surface plasmon resonance microscopy.

The preparation of histology slides is a critical step in histopathology, and poor-quality histology slides with weak adhesion of tissue sections to the substrate often affect diagnostic accuracy and sometimes lead to diagnostic failure due to tissue section detachment. This issue has been of concern and some methods have been proposed to enhance tissue-substrate adhesion. Unfortunately, quantitative analysis of the adhesion between tissue sections and glass slides is still challenging. In this work, the adhesion of mouse brain tissue sections on gold-coated glass slides was analyzed using a laboratory-fabricated hyperspectral surface plasmon resonance microscopy (HSPRM) system that enabled single-pixel spectral SPR sensing and provided two-dimensional (2D) distribution of resonance wavelengths (RWs). The existence of the nanoscale water gap between the tissue section and the substrate was verified by fitting the RW measured in each pixel using the five-layer Fresnel reflection model. In addition, a 2D image of the tissue-substrate adhesion distance (AD) was obtained from the measured 2D distribution of RWs. The results showed that tissue-substrate AD was 20-35 nm in deionized water and 4-24 nm in saline solution. The HSPRM system used in this work has a wide wavelength range of 400-1000 nm and can perform highly sensitive and label-free detection over a large dynamic detection range with high spectral and spatial resolutions, showing significant potential applications in stain-free tissue imaging, quantitative analysis of tissue-substrate adhesion, accurate identification of tumor cells, and rapid histopathological diagnosis.

Fiber-optic bionic microphone based compact sound source localization system with extended directional range.

Traditional sound source localization (SSL) systems based on electret condenser microphone arrays are bulky because their localization accuracy depends on the size of the array. Inspired by the hearing mechanism of the parasitic fly Ormia ochracea, the localization accuracy of miniature bionic SSL devices breaks through the limitations of device size, but their ability to localize low-frequency sound sources over a wide angular range remains a challenge. In this work, a compact low-frequency SSL system with an extended directional range was prepared using two bionic micro-electro-mechanical system diaphragm based fiber-optic microphones, which form a non-coplanar array with a size of Φ44 mm × 13 mm. An algorithm for quantifying the azimuthal angle of a sound source is established for the prepared SSL system. Simulation and experimental results show that the prepared SSL system is capable of determining the propagation direction of acoustic signals with a frequency of less than 1 kHz in the azimuthal range from -90° to 90°, with a linear response in the range from -70° to 70°, and an angular measurement accuracy of the system within the range of ±7°.

Temperature-robust optical microphone with a compact grating interferometric module.

The high demand for advanced acoustic sensors has prompted optical microphones to become a current research hotspot; this is especially the case in light of the performance of existing electroacoustic microphones having reached the ceiling. In this work, a thermally stable optical microphone has been developed for sensitive detection of low-frequency acoustic signals. The microphone was prepared using a prestressed nickel diaphragm and a compact grating interferometric module. The adjacent surfaces of the diaphragm and grating form a short Fabry-Perot cavity, which makes the microphone robust to ambient temperature fluctuation due to the reduced thermal drift of its operating point relative to the quadrature point of the interferometer. The cavity length-operating wavelength relationship of the microphone operating at the quadrature point was obtained. The performance of the prepared microphone was tested using various methods. Experimental results show that the microphone enables stable operation at the quadrature point over a wide range of temperatures from 0°C to 60°C with low signal distortion and high sensitivity. The response of the prepared optical microphone to low-frequency drone noise was measured and compared with that obtained with a commercial electret condenser microphone.

A Grating Interferometric Acoustic Sensor Based on a Flexible Polymer Diaphragm.

This study presents a grating interferometric acoustic sensor based on a flexible polymer diaphragm. A flexible-diaphragm acoustic sensor based on grating interferometry (GI) is proposed through design, fabrication and experimental demonstration. A gold-coated polyethylene terephthalate diaphragm was used for the sensor prototype. The vibration of the diaphragm induces a change in GI cavity length, which is converted into an electrical signal by the photodetector. The experimental results show that the sensor prototype has a flat frequency response in the voice frequency band and the minimum detectable sound pressure can reach 164.8 µPa/√Hz. The sensor prototype has potential applications in speech acquisition and the measurement of water content in oil. This study provides a reference for the design of optical interferometric acoustic sensor with high performance.

Prototype Optical Bionic Microphone with a Dual-Channel Mach-Zehnder Interferometric Transducer.

A prototype optical bionic microphone with a dual-channel Mach-Zehnder interferometric (MZI) transducer was designed and prepared for the first time using a silicon diaphragm made by microelectromechanical system (MEMS) technology. The MEMS diaphragm mimicked the structure of the fly Ormia Ochracea's coupling eardrum, consisting of two square wings connected through a neck that is anchored via the two torsional beams to the silicon pedestal. The vibrational displacement of each wing at its distal edge relative to the silicon pedestal is detected with one channel of the dual-channel MZI transducer. The diaphragm at rest is coplanar with the silicon pedestal, resulting in an initial phase difference of zero for each channel of the dual-channel MZI transducer and consequently offering the microphone strong temperature robustness. The two channels of the prototype microphone show good consistency in their responses to incident sound signals; they have the rocking and bending resonance frequencies of 482 Hz and 1911 Hz, and their pressure sensitivities at a lower frequency exhibit an "8"-shaped directional dependence. The comparison indicates that the dual-channel MZI transducer-based bionic microphone proposed in this work is advantageous over the Fabry-Perot interferometric transducer-based counterparts extensively reported.

Fabrication of Glass Diaphragm Based Fiber-Optic Microphone for Sensitive Detection of Airborne and Waterborne Sounds.

A glass-diaphragm microphone was developed based on fiber-optic Fabry-Perot (FP) interferometry. The glass diaphragm was shaped into a wheel-like structure on a 150-µm-thick glass sheet by laser cutting, which consists of a glass disc connected to an outer glass ring by four identical glass beams. Such a structural diaphragm offers the microphone an open air chamber that reduces air damping and increases sensitivity and results in a cardioid direction pattern for the microphone response. The prepared microphone operates at 1550 nm wavelength, showing high stability in a range of temperature from 10 to 40 °C. The microphone has a resonance peak at 1152 Hz with a quality factor of 21, and its 3-dB cut-off frequency is 32 Hz. At normal incidence of 500 Hz sound, the pressure sensitivity of the microphone is 755 mV/Pa and the corresponding minimum detectable pressure is 251 µPa/Hz1/2. In addition to the above characteristics of the microphone in air, a preliminary investigation reveals that the microphone can also work stably under water for a long time due to the combination of the open-chamber and fiber-optic structures, and it has a large signal-to-noise ratio in response to waterborne sounds. The microphone prepared in this work is simple, inexpensive, and electromagnetically robust, showing great potential for low-frequency acoustic detection in air and under water.

AuAg alloy film-based colorful SPR imaging sensor for highly sensitive immunodetection of benzo[a]pyrene in water.

A colorful surface plasmon resonance imaging (SPRi) sensor with the hue-based enhanced sensitivity has been developed by using sputtered AuAg alloy thin film as the sensing layer. The condition for optimizing the hue-based sensitivity of the SPRi sensor was achieved, that is, the initial resonance wavelength is in the range from 595 to 610 nm. Under this condition, the hue-based refractive index sensitivity of the SPRi sensor was measured as high as Δhue/Δnc=29879/RIU. This sensitivity is 8 times higher than that obtained with a gold-layer SPRi sensor (Δhue/Δnc=3658/RIU) and 7.7 times as high in magnitude as the spectral sensitivity of the same sensor (ΔλR/Δnc=3897.6 nm/RIU). After functionalization of the AuAg alloy film with the monoclonal antibody, the sensor was used for quantitative immunodetection of highly carcinogenic benzo[a]pyrene (BaP) in water. According to the experimental results, the average hue of the SPR color image (300 pixels×300 pixels) experiences an initial rapid increase and then stabilizes 15 min after exposure of the functionalized AuAg alloy film to an aqueous BaP solution sample. The variation of average hue obtained at the equilibrium of surface immunoreaction is a linear function of BaP concentration with the slope being Δhue/Δc=132.2/(µg·L-1). A cooled CCD camera is able to distinguish a change of Δhue=1, offering the colorful SPRi sensor the BaP detection limit of 0.01 µg·L-1. The comparative measurements of the sensor's responses indicate that the hue variation obtained with 0.1 µg·L-1 of BaP is equal to that obtained either with 5 µg·L-1 of benzanthracene or with 5 µg·L-1 of benzofluoranthene, revealing the sensor's excellent specificity to BaP. The work demonstrated that the AuAg alloy film-based colorful SPRi sensor can be used not only for visualized analysis of molecular interaction at the surface but also for quantitative trace detection of small-molecule analytes.

An Optical MEMS Acoustic Sensor Based on Grating Interferometer.

Acoustic detection is of great significance because of its wide applications. This paper reports a Micro-Electro-Mechanical System (MEMS) acoustic sensor based on grating interferometer. In the MEMS structure, a diaphragm and a micro-grating made up the interference cavity. A short-cavity structure was designed and fabricated to reduce the impact of temperature on the cavity length in order to improve its stability against environment temperature variations. Besides this, through holes were designed in the substrate of the grating to reduce the air damping of the short-cavity structure. A silicon diaphragm with a 16.919 µm deep cavity and 2.4 µm period grating were fabricated by an improved MEMS process. The fabricated sensor chip was packaged on a conditioning circuit with a laser diode and a photodetector for acoustic detection. The output voltage signal in response to an acoustic wave is of high quality. The sensitivity of the acoustic sensor is up to -15.14 dB re 1 V/Pa @ 1 kHz. The output signal of the high-stability acoustic sensor almost unchanged as the environment temperature ranged from 5 °C to 55 °C.

A Potassium Ion-Exchanged Glass Optical Waveguide Sensor Locally Coated with a Crystal Violet-SiO2 Gel Film for Real-Time Detection of Organophosphorus Pesticides Simulant.

An optical waveguide (OWG) sensor was developed for real-time detection of diethyl chlorophosphate (DCP) vapor, which is a typical simulant for organophosphorus pesticides and chemical weapon agents. Silica gel, crystal violet (CV), and potassium ion-exchange (PIE) OWG were used to fabricate the sensor's device. In the real-time detection of the DCP vapor, the volume fraction of DCP vapor was recorded to be as low as 1.68 × 10-9. Moreover, the detection mechanism of CV-SiO2 gel film coated the PIE OWG sensor for DCP, which was evaluated by absorption spectra. These results demonstrated that the change of output light intensity of the OWG sensor significantly increased with the augment of the DCP concentration. Repeatability as well as selectivity of the sensors were tested using 0.042 × 10-6 and 26.32 × 10-6 volume fraction of the DCP vapor. No clear interference with the DCP detection was observed in the presence of other common solvents (e.g., acetone, methanol, dichloromethane, dimethylsulfoxide, and tetrahydrofuran), benzene series (e.g., benzene, toluene, chlorobenzene, and aniline), phosphorus-containing reagents (e.g., dimethyl methylphosphonate and trimethyl phosphate), acid, and basic gas (e.g., acetic acid and 25% ammonium hydroxide), which demonstrates that the OWG sensor could provide real-time, fast, and accurate measurement results for the detection of DCP.

Self-referenced antiresonant reflecting guidance mechanism for directional bending sensing with low temperature and strain crosstalk.

A sensitive one-dimensional vector bending fiber-optic sensor based on self-referenced antiresonant reflecting guidance mechanism has been proposed and experimentally demonstrated. Two symmetric air holes in the hollow-core photonic crystal fiber (HCPCF) were infiltrated with refractive index matching liquids with different refractive indices, which formed a self-referenced anti-resonant reflecting optical waveguide. The bending of the HCPCF induces a wavelength shift of lossy dip in the transmission spectrum. Specially, the one-dimensional bending orientation can be detected through the wavelength interval between two lossy dips due to the asymmetric refractive index change of the silica cladding for two resonators. The bending sensitivities are 4.86 and -4.84 nm/m-1 for the curvatures of the 0° and 180° bending orientations in a bending range from 0 to 0.88 m-1, respectively. Moreover, the temperature and strain crosstalk of the proposed sensor can be eliminated through the compensated self-referenced anti-resonant reflecting optical waveguide. The proposed fiber sensor can be used for the monitoring of the structural health of infrastructures.

Real-time fiber-optic anemometer based on a laser-heated few-layer graphene in an aligned graded-index fiber.

A real-time all-fiber anemometer based on laser-heated few-layer graphene in aligned graded-index fibers has been proposed and experimentally demonstrated. The proposed fiber-optic anemometer was composed of a pair of all-fiber collimators by using aligned graded-index fibers that was coated with the few-layer graphene. The few-layer graphene was heated through a heating light from a 532-nm laser, which changed the optical transmittance of signal light with the wavelength of 1550 nm. The wind speed can be measured through the transmission power of the signal light based on the wind cooling effects on the heated few-layer graphene, acting as a "hot-wire" anemometer. The experimental results show that the maximum sensitivity of the anemometer reaches -22.03 µW/(m/s), and a fast response time of as 0.064 s can be achieved. The proposed fiber sensor can be used for the real-time measurement of wind speed in the fields of environmental monitoring, oil exploration, oceanography research, etc.

Nanoporous Gold Films Prepared by a Combination of Sputtering and Dealloying for Trace Detection of Benzo[a]pyrene Based on Surface Plasmon Resonance Spectroscopy.

A wavelength-interrogated surface plasmon resonance (SPR) sensor based on a nanoporous gold (NPG) film has been fabricated for the sensitive detection of trace quantities of benzo[a]pyrene (BaP) in water. The large-area uniform NPG film was prepared by a two-step process that includes sputtering deposition of a 60-nm-thick AuAg alloy film on a glass substrate and chemical dealloying of the alloy film in nitric acid. For SPR sensor applications, the NPG film plays the dual roles of analyte enrichment and supporting surface plasmon waves, which leads to sensitivity enhancement. In this work, the as-prepared NPG film was first modified with 1-dodecanethiol molecules to make the film hydrophobic so as to improve BaP enrichment from water via hydrophobic interactions. The SPR sensor with the hydrophobic NPG film enables one to detect BaP at concentrations as low as 1 nmol·L-1. In response to this concentration of BaP the sensor produced a resonance-wavelength shift of ΔλR = 2.22 nm. After the NPG film was functionalized with mouse monoclonal IgG1 that is the antibody against BaP, the sensor's sensitivity was further improved and the BaP detection limit decreased further down to 5 pmol·L-1 (the corresponding ΔλR = 1.77 nm). In contrast, the conventional SPR sensor with an antibody-functionalized dense gold film can give a response of merely ΔλR = 0.9 nm for 100 pmol·L-1 BaP.

Waveguide-coupled directional Raman radiation for surface analysis.

Kretschmann-type waveguide structures, including Plasmon Waveguide (PW) and Resonant Mirror (RM), have been applied in interfacial Raman spectroscopy due to the following unique features: (1) unlike the classic surface enhanced Raman scattering (SERS) substrates made of either gold or silver, both PW and RM can be prepared using a large variety of inexpensive materials; (2) the field enhancement factors using these structures can be theoretically predicted and experimentally controlled, which enables us to manipulate the surface Raman sensitivity with high repeatability; (3) the use of transverse electric (TE) and transverse magnetic (TM) modes for Raman excitation allows us to evaluate the orientation of target molecules immobilized on the waveguide surface; (4) the unwanted impact of noble metals on the Raman fingerprints of target molecules, which is often observed for conventional SERS substrates, can be avoided upon the use of dielectric waveguides. In this paper, guided-mode-coupled directional Raman emission, which is an additional important feature of the waveguide Raman technique, was theoretically investigated based on the optical reciprocity theorem combined with the Fresnel equations. The simulation results indicate that the directional Raman emission from a dipole located within the field confinement and penetration depth of a guided mode depends on both the orientation of the dipole and its distance from the waveguide surface. Raman light from the TE-oriented dipoles is launched into the prism coupler at the TE-mode resonance angle and that from the non-TE-oriented dipoles propagates at the TM-mode resonance angle. The intensity of the guided-mode-excited Raman signal propagating at the mode resonance angle is proportional to the fourth power of the mode field (E(4)) at the depth of the dipole from the waveguide surface. This means that the guided-mode-excited and guided-mode-coupled directional Raman spectroscopy has a detection depth that is as small as a quarter of the evanescent-field penetration depth, indicating the excellent surface selectivity of this technique. The directional Raman emission also facilitates high-efficiency signal collection compared with conventional SERS. It is worth noting that Raman light from the dipoles confined in the core layer of a single-mode waveguide can be simultaneously coupled into both the guided mode and the substrate mode, especially the surface plasmon resonance (SPR) mode for PW.

Miniature Fourier transform spectrometer based on wavelength dependence of half-wave voltage of a LiNbO3 waveguide interferometer.

A simple and reliable spectrum-retrieval method was proposed for the development of miniature stationary Fourier transform (FT) spectrometers based on a LiNbO3 (LN) waveguide Mach-Zehnder interferometer (MZI) modulator. The method takes into account the wavelength dependence of the optical pathlength difference (OPD) and allows us to use a nonlinear voltage ramp to modulate the OPD. The method is based on the dispersion of the half-wave voltage, which was measured to be a monotonous polynomial function of the wavelength for the LN waveguide MZI used. With the measured dispersion of the half-wave voltage, the OPD, as a linear function of the modulating voltage, can be accurately determined at each wavelength in the near-infrared region in which the MZI used is a single-mode device. A prototype FT spectrometer was prepared using a LN waveguide MZI modulator based on the above method. The experimental results demonstrated that the spectrometer can be used for accurate determination of the laser wavelength and for liquid absorptiometry.

Nanoporous TiO2/polyion thin-film-coated long-period grating sensors for the direct measurement of low-molecular-weight analytes.

We present novel nanoporous TiO(2)/polyion thin-film-coated long-period fiber grating (LPFG) sensors for the direct measurement of low-molecular-weight chemicals by monitoring the resonance wavelength shift. The hybrid overlay films are prepared by a simple layer-by-layer deposition approach, which is mainly based on the electrostatic interaction of TiO(2) nanoparticles and polyions. By the alternate immersion of LPFG into dispersions of TiO(2) nanoparticles and polyions, respectively, the so-formed TiO(2)/polyion thin film exhibits a unique nanoporous internal structure and has a relative higher refractive index than LPFG cladding. In particular, the porosity of the thin film reduces the diffusion coefficient and enhances the permeability retention of low-molecular-weight analytes within the porous film. The increases in the refractive index of the LPFG overlay results in a distinguished modulation of the resonance wavelength. Therefore, the detection sensitivity of LPFG sensors has been greatly improved, according to theoretical simulation. After the structure of the TiO(2)/polyion thin film was optimized, glucose solutions as an example with a low concentration of 10(-7) M was easily detected and monitored at room temperature.

Wavelength-interrogated surface plasmon resonance sensor with mesoporous-silica-film-enhanced sensitivity to small molecules.

Sol-gel copolymer-templated mesoporous silica films with a thickness of 70 nm and interpore spacing of 4.34 nm were fabricated on gold layer covered glass substrates for application as a wavelength-interrogated surface plasmon resonance (SPR) sensor. The resonance wavelength (λ(R)) of the sensor with a solution sample was determined by absorptiometry at a given incident angle. A comparison between the experimental data obtained with the coated and uncoated SPR chips demonstrated that the mesoporous silica film effectively enhanced sensor response to individual adsorption of cysteamine molecules and lead(II) ions. An approximate proportional relationship between the resonance-wavelength shift of the sensor and the volume fraction of analyte molecules adsorbed in the mesoporous silica film was obtained by numerical simulation. Porosities of 0.865 and 0.785 for the two silica films used as well as the volume fractions of 0.048 and 0.116 for adsorbed lysozyme and cysteamine molecules were determined by fitting the simulation results to the experimental data. The adsorbed amount of cysteamine (∼0.5 nm) is equivalent to more than 16 full monolayers on the geometric surface of the mesoporous silica film used. In contrast, an equivalence of less than 2 full monolayers for adsorbed lysozyme molecules (3 nm × 3 nm × 4.5 nm) suggests that the mesoporous silica film has good size-selective adsorption capability due to its uniform pore size distribution. Cysteamine modification of the mesoporous silica film renders the SPR sensor able to detect lead(II) ions at concentrations as low as 1 nM.

Kinetics of competitive adsorption of ß-casein and methylene blue on hydrophilic glass.

The competitive adsorption of methylene blue (MB) and ß-casein on hydrophilic glass from an aqueous mixed solution was directly detected at the solution pH smaller than the protein isoelectric point (pI) by means of the waveguide-based broadband time-resolved evanescent wave absorption spectroscopy. The competitive adsorption causes the MB coverage to exponentially decrease with time from its peak value and prevents MB aggregation at the interface. The kinetic equation for the competitive adsorption of binary adsorbates was theoretically deduced based on the Langmuir model, and was used for creating the best fit to the experimental data. In the case of a fixed concentration of MB in the mixed solution, the best-fit parameter τ(-1) increases with the protein concentration at a specific pH and decreases with the solution pH at a given concentration of protein. The findings suggest that the ß-casein concentration in sub-µM level can be rapidly determined by the time-resolved waveguide absorptiometry based on the competitive adsorption of MB and protein.

Flexible hyperspectral surface plasmon resonance microscopy.

Optical techniques for visualization and quantification of chemical and biological analytes are always highly desirable. Here we show a hyperspectral surface plasmon resonance microscopy (HSPRM) system that uses a hyperspectral microscope to analyze the selected area of SPR image produced by a prism-based spectral SPR sensor. The HSPRM system enables monochromatic and polychromatic SPR imaging and single-pixel spectral SPR sensing, as well as two-dimensional quantification of thin films with the measured resonance-wavelength images. We performed pixel-by-pixel calibration of the incident angle to remove pixel-to-pixel differences in SPR sensitivity, and demonstrated the HSPRM's capabilities by using it to quantify monolayer graphene thickness distribution, inhomogeneous protein adsorption and single-cell adhesion. The HSPRM system has a wide spectral range from 400 nm to 1000 nm, an optional field of view from 0.884 mm2 to 0.003 mm2 and a high lateral resolution of 1.2 µm, demonstrating an innovative breakthrough in SPR sensor technology.

Determination of surface protein coverage by composite waveguide based polarimetric interferometry.

Deposition of a tapered thin film of Ta(2)O(5) onto a single-mode, polarization-insensitive slab glass waveguide enables the resultant structure to serve as a simple, inexpensive yet highly sensitive polarimetric interferometer for trace, even ultra-trace, detection of chemical and biochemical analytes. By comparing the measured refractive-index sensitivity with that simulated based on a four-layer homogeneous waveguide, the equivalent thickness for the tapered layer of Ta(2)O(5) and the sensitivity of the sensor to adlayer thickness were determined. Responses of the sensor to unspecific adsorption of bovine serum albumin (BSA) and to surface antibody-antigen interaction were investigated in situ and the corresponding surface coverages were obtained with the adlayer-thickness sensitivity. The interferometer sensor shows good long-term stability and its phase drift is lower than π over 10 h.

Spatially Resolved Spectroscopic Characterization of Nanostructured Films by Hyperspectral Dark-Field Microscopy.

Nanostructured films have been widely used for preparing various advanced thin-film devices because of their unique electrical, optical, and plasmonic characteristics associated with the nano-size effect. In situ, nondestructive and high-resolution characterization of nanostructured films is essential for optimizing thin-film device performance. In this work, such thin-film characterization was achieved using a hyperspectral dark-field microscope (HSDFM) that was constructed in our laboratory by integrating a hyperspectral imager with a commercial microscope. The HSDFM allows for high-resolution (Δλ = 0.4 nm) spectral analysis of nanostructured samples in the visible-near-infrared region with a spatial resolution as high as 45 nm × 45 nm (corresponding to a single pixel). Four typical samples were investigated with the HSDFM, including the gold nanoplate array, the self-assembled gold nanoparticle (GNP) sub-monolayer, the sol-gel nanoporous titanium dioxide (TiO2) film, and the layer-stacked molybdenum disulfide (MoS2) sheet. According to the experimental results, the plasmon resonance scattering bands for nanoplate clusters are identical with those for individual gold nanoplates, indicating that the gap between adjacent nanoplates is too large to allow plasmonic coupling between them. A different case was observed with the self-assembled GNP sub-monolayer in which the aggregated clusters with the internal plasmonic interaction show a considerable red-shift of the plasmon resonance band relative to the isolated single GNP. In addition, the protein adsorption on the nanoporous TiO2 film was observed to be inhomogeneous on the microscale, and the stepped boundaries of the MoS2 sheet were clearly observed. A quasi-linear dependence of the single-pixel light intensity on the step height was obtained by combining the HSDFM with atomic force microscopy. The minimum thickness detectable by the present HSDFM is 6.5 nm, corresponding to the 10-layer MoS2 film. The work demonstrated the outstanding applicability of the HSDFM for nanostructured film characterization.