Super-Resolution in Label-Free Photomodulated Reflectivity - Nano

Subscriber access provided by TULANE UNIVERSITY

Communication

Super-resolution in label-free photo-modulated reflectivity Omer Tzang, Alexander Pevzner, Robert Edward Marvel, Richard F. Haglund, and Ori Cheshnovsky Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl504640e • Publication Date (Web): 20 Jan 2015 Downloaded from http://pubs.acs.org on January 20, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Super-resolution in label-free photo-modulated reflectivity Omer. Tzang1,2, Alexander. Pevzner2, Robert. E. Marvel3, Richard.F. Haglund3, 4 ,Ori Cheshnovsky1,2 1

School of Chemistry, The Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978 ,Israel. 2

3

The Center for Nanoscience and Nanotechnology at Tel Aviv University.

Interdisciplinary Materials Science Program, Vanderbilt University, Nashville, Tennessee 37235-1807 USA

4

Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 372351406 USA

KEYWORDS: Super resolution, microscopy, label-free, semiconductors, ultrafast optics.

ABSTRACT:

We demonstrate a new, label-free, far-field super-resolution method based on an ultrafast pumpprobe scheme oriented towards nano-material imaging. A focused pump laser excites a diffraction-limited spatial temperature profile, and the nonlinear changes in reflectance are probed. Enhanced spatial resolution is demonstrated with nanofabricated silicon and vanadium oxide nanostructures. Using an air objective, resolution of 105nm was achieved, well beyond the 1 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

diffraction limit for the pump and probe beams and offering a novel kind of dedicated nanoscopy for materials. TEXT: The quest to understand structure, dynamics and function at the nanoscale has inspired new ultrahigh-resolution imaging techniques. In particular, optical microscopy has succeeded in surpassing the Abbé resolution limit (~λ/2) either by near field1 techniques, or by far-field superresolution (SR) techniques such as stimulated-emission depletion (STED)2,3, photo-activated localization microscopy (PALM)4, stochastic optical reconstruction microscopy (STORM)5,6 saturable absorption (SAX) 7 , structured illumination8, SR optical fluctuation imaging (SOFI)9, and quantum-emitter microscopy10. General algorithmic methods, which rely on sparse object distributions, have also been introduced11–13. These fluorescence-based techniques are very useful when functional groups can be reliably and selectively labeled, and have been established as major tools in biological and medical research. However, in many instances far-field, label-free microscopy is desirable. Label-free modalities can serve in vivo studies of tissue organs or animals, and circumvents the bleaching and optotoxicity of dyes. Label-free SR microscopy is of utmost necessity in monitoring nanomaterials, nano-electronic and electro-optical systems. Nevertheless, methods that do not rely on fluorescent labelling were introduced only recently. Wang and coworkers used ground-state depletion of the charge carriers in graphene-like structures in transmission mode14. Nedosekin and coworkers used nonlinear photo thermal microscopy in a fluid medium15.

2 ACS Paragon Plus Environment

Page 2 of 20

Page 3 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

In this work, we introduce a new far-field, label-free SR approach, which relies on inducing spatial distribution of physical properties (such as temperature or carrier concentration) by an ultrashort laser pulse within the photo-excited diffraction limited spot. By monitoring any property of the examined sample, which depends on this spatial distribution (e.g. temperature), spatial information beyond the diffraction limit may be extracted. This may include the temperature dependence of luminescence or Raman spectral shifts. Here, the reflectance of the photo-excited spot is probed instantaneously after excitation, using a second laser pulse. The key to realization of SR is detection of high-order nonlinearities in reflectance, induced by the photoexcitation. Our method does not depend on fluorescent or any other form of labeling. It presents the first general approach to SR that is suitable to the characterization of semiconductor and optoelectronic nanometric systems in any environment (vacuum, ambient, liquid) for semi-transparent and opaque, thin and thick systems alike. Photo-modulated reflectance originates from numerous physical effects16. We will focus our discussion on thermal excitation and the probing of thermoreflectance (TR). Thermoreflectance, which records changes of reflectance upon heating, is typically used to measure and then extract the thermal properties of materials using linear models17,18. Here, the nonlinear components of TR in respect to photo-excitation allow the dramatic narrowing of the effective point spread function (PSF). We demonstrate SR on fabricated silicon nanostructures, relying on the fact that Si, as any other reflective or scattering surface, contains nonlinear components in it reflectivity19–21.A special case, in which photo-excitation induces a phase transition, is also discussed: the highly abrupt


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

change of reflectance of vanadium oxide (VO2) upon its characteristic insulator-to-metal transition. Upon ultrafast photoexcitation, materials undergo several stages of relaxation before achieving a thermodynamic steady-state. Immediately after photoexcitation, reflectance changes originate mainly from changes in carrier concentrations.; carrier excitation (10-100fs) is followed by carrier–carrier and carrier–phonon scattering processes (10fs-10ps)22,23. Eventually, on a time scale of a few ps, the thermal transport can be treated classically. The instantaneous photoexcited spatial profile diffuses quickly and blurs in time. However, we demonstrate that in a ps timescale, it is possible to monitor the non-equilibrium state with high spatial resolution. The pump-probe time-delay window is therefore a key element in this SR method. Figure 1depicts a simulation of the photo-excitation temperature profile in silicon (1a) and its time evolution (1b). Note that the initial spatial-temperature distribution blurs within few ps (1c), thus reducing spatial resolution.


Page 4 of 20

Page 5 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 1: Principle of the SR method (a): Simulation of optical absorption and temperature distribution in a silicon wafer at pump-probe delay of 1ps. pump fluence is ~70mJ/cm2. (Simulation details in supporting information 5). (b) Experimental dependence of thermoreflectance in silicon on the pump-probe delay. (c) Simulation of time dependent temperature profile in silicon following photo-excitation. (d). Point spread function simulation. Red – probe pulse at 785nm. Blue- pump at 392nm. Green –Thermo reflectance at delay t=1ps. In this case the PSF conforms to the product of the pump and probe beam PSFs. Black – PSF resulting from 4th order nonlinearities in thermo reflectance.

The TR experimental setup is illustrated in Figure 12 (details in Methods). Briefly, a 785nm beam (probe) with 1ps pulse duration is frequency doubled to 392nm (pump). The beams are separated by a dichroic mirror. A variable delay line tunes the timing of the probe and a dichroic filter combines the beam into a reflection microscope. The pump beam at 392nm is modulated at 2.5 kHz using a chopper wheel or a tuning fork, both optimized to produce a pure sinusoidal excitation. The probe reflectance is monitored using a photodiode and a lock-in amplifier.

Figure 1: Diagram of the optical setup.

The fundamental assumption in our method is that the instantaneous temperature distribution mimics the 3D absorption profile of the pump. Consequently, the PSF of the linear TR is the product of the PSFs of the pump and probe beams. When utilizing a Gaussian pump beam with half the wavelength of the probe, the resolution enhancement over electrically heated samples


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with the same diffraction-limited probe beam is √5. (See Supporting Information 1 for discussion of this point.) However, our major interest lies in the contributions of high-order nonlinearities of TR to SR. In silicon nanostructures, the essential material in semiconductor industry, linear photo-modulated reflection has been studied extensivly22. In order to extract the nonlinear components of reflection due to photo-modulation at , we demodulate the reflection intensity at the corresponding harmonic frequencies , 2 , 3 … in a lock-in amplifier. We note that for low repetition rate excitation, another scheme for extracting high order non-linear components has been demonstrated27. The nth harmonic components of the reflectivity scale with the nth power of the excitation. Accordingly the related effective PSF scales as the nth power of the excitation PSF and effectively reduce spatial resolution like the Ѵn, below diffraction limit. (See Supporting Information 1, supporting Figure 5). SAX is a SR technique closely related in concept7. There, high-order harmonics of the ﬂuorescence, induced by saturating the modulated excitation, are detected. Here, the origins of nonlinear effects in the TR of silicon are to be determined, and are beyond the scope of this letter. We presume that multi-photon excitations or cooling by Auger recombination 24,25contribute to the nonlinearity. Note also that in laser-induced ablation the nonlinear response of matter has been used to achieve SR ablation with a diffraction limited focused beam26. Here we present a nondestructive SR method for imaging. Our experiments were performed on 100nm thick silicon layers patterned on sapphire (Figure 3b). At a pump fluence of ~100mJ/cm2, harmonic frequencies are discernible in TR. The estimated peak temperature of silicon at these conditions is ~700K (Supporting information-4). 6 ACS Paragon Plus Environment

Page 6 of 20

Page 7 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 3 3 depicts line scans orthogonal to 125nm silicon stripes with variable spacing. The resolution enhancement in the harmonics is clearly discernible.

Figure 3: SR line imaging of silicon on sapphire nanostructures. The sample consists of silicon double lines, 125nm wide, with gaps of 370nm and 270nm respectively. (a) – Scan with a 0.7 NA Objective using the probe reflection only (red), the 1st (black), 2nd (blue), and 5th (Green) harmonics of the modulation. (b) –HR-SEM image of the scanned sample in figure 3a. The distances between lines are marked on the image (supporting information Figure 8). (c) – Imaging with a 0.95NA objective using 1st, 2nd, and 3rd harmonics. The improvement in resolution is consistent with the increase of the NA and the harmonic order.

Based on the SEM images of the patterned strips, the deconvoluted PSF of the 5th harmonic performed with 0.7NA corresponds to 140±10nm FWHM. Using a 0.95NA objective, the resolution with the 3rd harmonic signal corresponds to 105±10nm. The improvement in resolution is consistent with the increase of the NA and the harmonic order. We have simulated the expected resolution of our experiment (see Supplement 1 and Supplement Figure 5). The obtained values of 134nm (0.7NA 5th harmonic) and 114 nm (0.95NA 3th harmonic) respectively, 7 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

correlate well with our experiment. Our best resolution provides 2x (4x) improvement over the diffraction limit of the pump (probe). As in SAX, the resolution can be further improved by recording higher-order nonlinearities. However, the practical limit of resolution is governed by SNR and the purity of pumping. Our approach is highly sensitive to the purity of the excitation wave and works best with a pure sinusoidal excitation. Direct excitation with higher harmonics introduces distortions in the image. The dips observable in Figure 3c originate from the interference of linear TR component at high harmonics, due to harmonic distortion in the pump, and the weak high-harmonic signals of the nonlinear TR at the margins of the silicon stripes. These components will be eliminated in a more sophisticated modulation scheme that we are developing. Photo exited damage is avoided by restricting the laser fluence to a level well below the damage threshold. Post measurement damage inspections were performed by optical microscopy and SEM; no discernible damage was identified. Total cooling of the sample between pulses, 12.5ns apart, is simulated and verified experimentally. The transient temperature increase of silicon during the excitation pulse in our TR experiments is (∆T≈485K) well below the melting point of silicon. Heating by the probe laser was estimated to be less than 5K. (Supporting Information 4). To demonstrate a direct and abrupt photo-induced change in TR, we choose VO2. At Tc=340K, VO2 undergoes a first-order structural phase transition (monoclinic to rutile) roughly coinciding with an insulator-to-metal transition28. Extensive experimental and theoretical studies have addressed the physical mechanisms responsible for the thermally induced transition29. Recently, the mechanism of the phase transition induced by ultra-fast photexcitation has also been studied in detail30–32. Above a critical pump fluence, VO2 undergoes a photo-induced transition into a 8 ACS Paragon Plus Environment

Page 8 of 20

Page 9 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

metallic state accompanied by a dramatic change in reflectivity within ~100fs after excitation. Subsequently, electronic energy is transfered into lattice heating via electron-phonon coupling on ps time scales, and the volume of the thermally induced (non-coherent) phase transition expands. Later, heat diffusion cools the excited volume, and the reverse transition into the insulator state occurs in hundreds to thousands of ps, at rates that depend inversely on the initial absorbed fluence and the thermal conductivity of the substrate. These photothermal properties of VO2 can be utilized to realize SR by fine-tuning the pump-laser fluence to be slightly above the phase-transition treshhold. Only the intense center of the pump beam will induce the insulator-to-metal phase-transition, invoking a sharp, non-linear response in the thermoreflectance and narrowing the PSF below the diffraction limit. Our experiments were performed on a patterned granular film of VO2 (~100nm thick), on a silicon substrate (Methods). The samples, containing ~150nm wide lines of nanoparticles, were scanned at varying pump fluences to characterize the onset of the photo-induced phase-transition using a 0.7NA objective. The VO2 sample comprises polycrystalline nanoparticles, and the phase transition may occur at somewhat different temperatures or laser fluences depending on nanoparticle size33. Consequently the contrast visibility of the particles varies, as verified in our experiments. In order to achieve SR, we have tuned the pump energy slightly above the onset of the phase transition on individual VO2 nanoparticles (Figure 4). Note the strong nonlinear response of TR to pump pulse energy (Figure 4). The best resolution (PSF of 165nm) is achieved only at pump energies slightly above the onset of the monoclinic-to-rutile phase transition (peak power 6.6mJ/cm2). At lower (3.2mJ/cm2) and higher (20mJ/cm2) energies, the particles appear larger (PSF of~280nm FWHM).


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4 : TR characterization of a single VO2 particle. A 270x200nm VO2 particle (size verified by SEM) was scanned over a series of pump fluences. (a): TR of the VO2 particle as a function of the pump energy. (b): Normalized cross sections along x axis of the TR scans (c, d, e), depicted in the bottom images. (c)– Scan above phase transition (20mJ/cm2). (d)- Scan at phase transition (6.6mJ/cm2). (e) –Scan below phase transition (3.2mJ/cm 2). The pump-probe delay was 1ps. The pump was modulated in a rectangular temporal waveform, with amplitude matching the optimal photo-excitation fluence. A 0.7NA objective was used.

Figure 5 depicts a TR scan of several VO2 nanoparticles (pump=8mJ/cm2) along with the corresponding SEM Images. Note that two particles separated by 70nm could be resolved. We have fitted a cross section in the TR scan to the SEM image, convoluted with a Gaussian PSF. The PSF was found to be between 160-190±10nm (FWHM), about twice the diffraction limit of the pump. Note that the FWHM of the focused pump beam at 392nm through a 0.7NA objective should amount to 290nm, while our measurement shows 360nm. The range of PSFs values reflects the variability of the nanoparticles in respect to phase transition as a function of size. This moderate variability of the PSF in granular VO2 nanoparticles introduces inhomogeneous resolution in the image. However for the newly developed single-crystal films and nanostructures of VO2, and devices based on single crystals, better homogeneity in resolution is expected.


Page 10 of 20

Page 11 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 5: VO2 TR area scan. (a)- TR scan. (b)- SEM images of the same area (See detailed in supporting information Figure 7) .(c) Experimental results compared to simulation: The sample was modeled (blue rectangles) as step function with longitudinal dimensions taken from the SEM image and heights proportional to the width in the SEM image to account for the higher reflection signal in wider particles . Black – cross section of the TR scan along the line in (a). The best fit to the experimental data is the red curve – convolution of the best PSF (red dotted Gaussian) and the sample model. A FWHM of 190±5nm can be evaluated from this figure. On the left side of the figure, the 70nm gap is discernible, and corresponds to a PSF of 160nm FWHM (blue dotted Gaussian). 0.7NA objective was used.

These experimental results conform to our simulations (supporting information 1). In summary, we have demonstrated a novel concept for SR imaging by utilizing the nonlinear response of TR. We have shown the applicability of the method to silicon nanostructures and the unique case of VO2 as it undergoes its structural phase transition. Further improvement in the absolute resolution can be expected by using a 1.4NA oil-immersion objective and shorter wavelengths for probe and pump. A high data-acquisition rate is inherent in the method due to the fast response of TR and its large cross section. Material differentiation can be implemented by using specific wavelength dependence of the absorption cross-sections, thermoreflectance coefficients and information about the phase response (see Supporting Information 2). 11 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

In comparison to other label-free methods presented recently, our method shows higher resolution (100nm, FWHM with a 0.95 NA objective), and is more general. Unlike the groundstate depletion method presented by Wang and coworkers14 (225nm FWHM with a 1.2 NA objective) our method can be applied to low absorbance and opaque materials, thin samples and thick samples alike. Unlike the nonlinear photo-thermal microscopy presented by Nedosekin and coworkers 15 (90±10nm FWHM with a 1.2 NA objective) it is not tied to fluidic media and can be applied in ambient, vacuum and fluid alike. Bubble formation, which is a key element in the nonlinear photothermal method, is erratic, and its unstable onset introduces noise. Moreover, once a cleaner excitation modulation is developed for our method, the resolution can be further improved by detecting higher harmonics. Our method can be applied to many materials, exploiting minute nonlinearities in photo-modulated reflection. Such nonlinearities have been reported in crystalline semiconductors19,20,34, on metal surfaces35 and on glass and liquid surfaces36. While the mechanism for nonlinearity varies, we postulate that many additional material systems possess the required nonlinearity in their reflection. Finally we wish to emphasize that our approach can be extended by probing any physical properties that depend on temperature, such as luminescence, Raman shift and absorption.

Methods Experimental setup The description refers to Fig. 2. A Ti-sapphire oscillator (Tsunami, Spectra physics) pumped by a 5W 532mn CW laser (Verdi, Coherent) was set to 1.5ps pulse length at 785nm central wavelength, with 10 nJ pulses at 80MHz. The laser beam is passed through an optical isolator (Thorlabs,IO-5-780-HP) and focused by a 100mm achromatic lens into a BBO crystal (L=5mm) to produce second harmonic 392nm pulses with 20% conversion efficiency. The fundamental 12 ACS Paragon Plus Environment

Page 12 of 20

Page 13 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

and second-harmonic beams were expanded and collimated by a 400mm lens and split by a dichroic filter (Semrock BLP01- 473R-25); each beam was then spatially filtered and further expanded. The time delay between pulses was adjusted by a variable delay line, using a retroreflector riding on a stepper motor translation stage (Standa 8MT175-100) with 1µm resolution. The pump beam at 392nm is modulated at 2.5 kHz using a chopper wheel or a tuning fork .The position of the chopper wheel is adjusted so that the width of slots at the point where it intersects the laser beam and the diameter of the beam are similar. Fine tuning of this geometry is important in order to optimize the excitation waveform to approach pure sinusoidal excitation. We also examined beam modulation using a tuning fork and received similar excitation waveform compared to the optimized chopper wheel. The direct harmonic distortions of our excitation was typically, 3%, 2%, 0.2%, 0.1% for the second, third, fourth and fifth harmonics. Typical TR signals were 10%, 4%,1%,0.5%, respectively.The method to generate precise sine wave excitation by interfering two AOMs, as used in SAX7, was considered and rejected. This approach is not suitable for pump probe experiments, since precise time tuning of the pump and probe pulses is needed. The pump and probe beams are combined by a dichroic mirror (Semrock SP01-785RU-25). The two spatially overlapped beams are focused by a 0.7NA air objective (Olympus LCPLFLN50xLCD) or a 0.95NA objective (Nikon CF Plan APO EPI 150X) on the sample surface. The epi-reflected probe beam is transmitted through a dichroic filter (OD8 for the pump beam) and detected by the trans-impedance silicon photodiode preamplifier (Thorlabs PDA100A) and a lock in amplifier (SRS 830). Timing was calibrated by the TR signal (supporting information Figure 9). Data are collected by an analog-to-digital converter (NI usb6000). The sample is scanned by an x-y stage, controlled with 10nm resolution (Thorlabs DRV517 and BPC303). 13 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Si on Sapphire Sample Fabrication: Stripes were prepared from 200nm Si on sapphire substrate. The pattern of 125nm lines with variable distances (800-200nm) was written by an e-beam system (Raith 150 EBL) on photoresist. After development, 40nm film of Ni was deposited, followed by a lift off process. Si on Sapphire lines was prepared by dry etching in RIE (Nextral 860) using CHF3 and O2. Finally, the Ni capping was chemically etched in solution. The quality of the resulting samples was verified using a SEM (FEI Quanta 200 FEG) as depicted in Fig. 5b and supporting information Figure 1.

VO2 Sample Fabrication Vanadium dioxide (VO2) structures (supporting information Figure 2) were fabricated on silicon using electron beam lithography and pulsed laser deposition (PLD). Substrates were coated with ~150nm of PMMA 495 A4 (Microchem) followed by a five minute bake at 180C. A JEOL 9300FS electron beam lithography system operating at 100kV was used to pattern the structures. MIBK/IPA 1:3 (Microchem) was used for development. Prior to deposition, the patterned substrates were cleaned with O2 plasma for 2 seconds. Amorphous VO1.7 was deposited by PLD using a Epion PLD-3000 system with a Lambda Physik (Coherent COMPex) excimer laser operating at 248 nm (KrF), 4 J/cm2 per pulse, 25 Hz repetition rate and 25 ns pulse duration. Prior to deposition, the chamber was pumped down to 9x10-6 Torr. A pure vanadium metal target was ablated in an ultra-high purity oxygen environment at 1.1x10-2 Torr with a 2 sccm flow rate. The average deposition rate was 0.3 angstroms/second. Following deposition, liftoff was performed in acetone. The patterned structures were annealed in a tube furnace in 250mTorr of O2 at 723K for 10 minutes. Annealing under these conditions crystallizes the asdeposited VO1.7 into stoichiometric, switching VO2. 14 ACS Paragon Plus Environment

Page 14 of 20

Page 15 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

ASSOCIATED CONTENT Supporting Information Available: The supporting information contains: SEM images of the

scanned samples; Definition of the resolution as presented in the paper; Phase image discussion; Input for resolution evaluation; Laser heating estimation; Heat conduction simulation; Temporal and spatial overlap of the beams. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *(O.C). E-mail: [email protected]. Tel: +972-3-640-8325 Author Contributions O.C. and O.T. conceived the methodology and designed the experiment. O.T. performed the experiment, analyzed the data and performed the simulations. R.M and R.H. prepared the VO2 sample. A.P. prepared the silicon sample. O.C. and O.T wrote the manuscript. All coauthors have read the manuscript and contributed to the final version. Funding Sources Filled in the forms of electronic submission as well as in the acknowledgement.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was support by the Israel Science foundation (grant 1716/13). Research at Vanderbilt University was supported by the National Science Foundation (DMR-1207507). We thank Yossi 15 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Rosenwaks for contributing his laser system. We thank Alfred Leitenstorfer, Alexej Pashkin and Uzi Even for useful discussions. The silicon fabrication was performed in the Tel Aviv University center for nanoscience and nanotechnology. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. ABBREVIATIONS SR, super-resolution. TR, thermo-reflectance. PSF, point spread function. FWHM, full width half maximum. NA, numerical aperture.VO2, vanadium dioxide. DM, dichroic mirror. BS, beam splitter. SEM, scanning electron microscope.


Page 16 of 20

Page 17 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

REFERENCES

(1)

Lewis, A.; Taha, H.; Strinkovski, A.; Manevitch, A.; Khatchatouriants, A.; Dekhter, R.; Ammann, E. Nat. Biotechnol. 2003, 21, 1378–1386.

(2)

Klar, T. a; Jakobs, S.; Dyba, M.; Egner, a; Hell, S. W. Proc. Natl. Acad. Sci. U. S. A. 2000, 97, 8206–8210.

(3)

Klar, , T a, Engel E , Hell S, W. Phys. Rev. E 2001, 64, 0666131.

(4)

Betzig, E.; Patterson, G. H.; Sougrat, R.; Lindwasser, O. W.; Olenych, S.; Bonifacino, J. S.; Davidson, M. W.; Lippincott-Schwartz, J.; Hess, H. F. Science (80-. ). 2006, 313, 1642–1645.

(5)

Henriques, R.; Griffiths, C.; Hesper Rego, E.; Mhlanga, M. M. Biopolymers 2011, 95, 322–331.

(6)

Rust, M. J.; Bates, M.; Zhuang, X. Nat. Methods 2006, 3, 793–795.

(7)

Fujita, K.; Kobayashi, M.; Kawano, S.; Yamanaka, M.; Kawata, S. Phys. Rev. Lett. 2007, 99, 228105.

(8)

Gustafsson, M. G. J. Microsc. 2000, 198, 82–87.

(9)

Dertinger, T.; Colyer, R.; Iyer, G.; Weiss, S.; Enderlein, J. Proc. Natl. Acad. Sci. 2009, 106, 22287–22292.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(10)

Schwartz, O.; Levitt, J. M.; Tenne, R.; Itzhakov, S.; Deutsch, Z.; Oron, D. Nano Lett. 2013, 13, 5832–5836.

(11)

Szameit, A.; Shechtman, Y.; Osherovich, E.; Bullkich, E.; Sidorenko, P.; Dana, H.; Steiner, S.; Kley, E. B.; Gazit, S.; Cohen-Hyams, T.; Shoham, S.; Zibulevsky, M.; Yavneh, I.; Eldar, Y. C.; Cohen, O.; Segev, M. Nat. Mater. 2012, 11, 455–459.

(12)

Shechtman, Y.; Eldar, Y. C.; Szameit, A.; Segev, M. Opt. Express 2011, 19, 14807– 14822.

(13)

Barsic, A.; Grover, G.; Piestun, R. Sci. Rep. 2014, 4, 5388.

(14)

Wang, P.; Slipchenko, M. N.; Mitchell, J.; Yang, C.; Potma, E. O.; Xu, X. Nat. Photonics 2013, 7, 449–453.

(15)

Nedosekin, D. a; Galanzha, E. I.; Dervishi, E.; Biris, A. S.; Zharov, V. P. Small 2014, 10, 135–142.

(16)

Bogdanowicz, J. Fundamental Study of Photomodulated Optical Reflectance towards Non-Destructive Carrier Profiling in Silicon, Leuven, 2011.

(17)

Cahill, D. G. Rev. Sci. Instrum. 2004, 75, 5119–5122.

(18)

Cahill, D. G.; Ford, W. K.; Goodson, K. E.; Mahan, G. D.; Majumdar, A.; Maris, H. J.; Merlin, R.; Phillpot, S. R. J. Appl. Phys. 2003, 93, 793.

(19)

Wagner, R. E.; Mandelis, A. Semicond. Sci. Technol. 1996, 11, 289–299.


Page 18 of 20

Page 19 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(20)

Wagner, R. E.; Mandelis, A. Semicond. Sci. Technol. 1996, 11, 300–307.

(21)

Lin, Q.; Painter, O. J.; Agrawal, G. P. Opt. Express 2007, 15, 16604.

(22)

Mazur, E. Interaction of Ultrashort Laser Pulses with Solids; 1996; pp. 1–54.

(23)

Sundaram, S. K.; Mazur, E. Nat. Mater. 2002, 1, 217–224.

(24)

Kar, A.; Upadhya, P. C.; Dayeh, S. A.; Picraux, S. T.; Taylor, A. J.; Prasankumar, R. P. IEEE J. Sel. Top. Quantum Electron. 2011, 17, 889–895.

(25)

Hopkins, P. E.; Barnat, E. V.; Cruz-Campa, J. L.; Grubbs, R. K.; Okandan, M.; Nielson, G. N. J. Appl. Phys. 2010, 107, 053713.

(26)

Phillips, H. M.; Callahan, D. L.; Sauerbrey, R.; Szaho, G.; Bor, Z. Appl. Phys. A 1992, 54, 158–165.

(27)

Danielli, A.; Maslov, K.; Garcia-Uribe, A.; Winkler, A. M.; Li, C.; Wang, L.; Chen, Y.; Dorn, G. W.; Wang, L. V. J. Biomed. Opt. 2014, 19, 086006.

(28)

Morin, F. J. Phys. Rev. Lett. 1959, 3, 34–36.

(29)

Basov, D. N.; Averitt, R. D.; van der Marel, D.; Dressel, M.; Haule, K. Rev. Mod. Phys. 2011, 83, 471–541.

(30)

Pashkin, A.; Kübler, C.; Ehrke, H.; Lopez, R.; Halabica, A.; Haglund, R. F.; Huber, R.; Leitenstorfer, A. Phys. Rev. B 2011, 83, 195120.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(31)

Kübler, C.; Ehrke, H.; Huber, R.; Lopez, R.; Halabica, a.; Haglund, R.; Leitenstorfer, a. Phys. Rev. Lett. 2007, 99, 116401.

(32)

Wall, S.; Foglia, L.; Wegkamp, D.; Appavoo, K.; Nag, J.; Haglund, R. .; Stahler, J.; Wolf, M. Phys. Rev. B 2013, 87.

(33)

Lopez, R.; Haynes, T.; Boatner, L.; Feldman, L.; Haglund, R. Phys. Rev. B 2002, 65, 224113.

(34)

Fournier, D.; Boccara, C.; Skumanich, A.; Amer, N. M. J. Appl. Phys. 1986, 59, 787.

(35)

Jha, S. S. Phys. Rev. Lett. 1965, 15, 412–414.

(36)

Altshuller, G. b.; Ermolaev, V. s.; Krylov, K. I.; Makarov, M. . Opt. Commun. 1985, 56, 131–135.

Insert Table of Contents Graphic and Synopsis Here


Page 20 of 20

Super-Resolution in Label-Free Photomodulated Reflectivity - Nano

Recommend Documents