Laboratory lecture in Introduction to Nanooptics (2012 SS)

Transcription

1 INO12_lab_lecture_ docx 1 Laboratory lecture in Introduction to Nanooptics (2012 SS) by Prof. Thomas PERTSCH at Friedrich-Schiller-Universität Jena in summer term July, 2012, 12:45 15:15 Institute of Applied Physics, Friedrich-Schiller-University Jena, Albert-Einstein-Strasse 15, Campus Beutenberg, Jena Meeting point is the seminar room of the institute. Please try to arrive there some minutes before the starting time. Learning objective: Getting an overview and some initial experience on the primary methods applied in the research field nano optics. After this laboratory lecture you should be able to answer the questions, given for each of the topics. All students will be arranged in 5 groups and each group will go through 5 teaching sessions in different laboratories, 30 minutes each. Laboratory lecture subjects # Lab Topic Instructor L1 156 seminar L2 103 chemistry L3 055 sem L4 153 spectroscopy L5 107 snom Design of plasmonic nanostructures by numerical modeling Synthesis of metallic nanoparticles for engineering of plasmonic resonances Characterization of photonic nanostructures by Scanning Electron Microscopy (SEM) Far field spectroscopy for characterization of the optical properties of plasmonic nanostructures Observation of surface plasmon polaritons by Scanning Nearfield Optical Microscopy (SNOM) Benny WALTHER Jessica RICHTER Holger HARTUNG Matthias FALKNER Angela KLEIN

2 INO12_lab_lecture_ docx 2 Directions to the institute of Applied Physics on Campus Beutenberg, Albert-Einstein-Strasse 15 Bus stop for lines 10, 11, 13 Institute of Applied Physics

3 INO12_lab_lecture_ docx 3 L1 - Design of plasmonic nanostructures by numerical modeling There is a variety of numerical tools for the simulation of the interaction of light with optical nanostructures. Such methods are e.g. Finite-Difference-Time-Domain (FDTD) simulation, Rigorous Coupled Wave Analysis (RCWA), Beam Propagation Method (BPM), mode solvers, band structure solvers, Finite Element techniques. The method of choice depends on the nanostructure and on the physical quantities which need to be computed. RSoft is a commercial tool which allows the use of a broad spectrum of methods to model a variety of physical parameters. The three-dimensional nanostructures are defined by the user in a graphical CAD environment. Upper left: Metamaterial in Fishnet geometry designed in the CAD environment of RSoft. Upper right: RSoft display of electric field during an FDTD simulation. Lower left: Retrieval of effective properties of the nanostructure. L1 - Questionnaire (1) Which physical quantities provided by simulations may help in finding out on the physical processes inside a nanostructure? (2) How is the permittivity ε(ω) of a dispersive material modeled in FDTD simulations? (3) By which factor does the computation time of a 3D-FDTD simulation rise when doubling the spatial resolution? (4) What are Perfectly Matched Layers (PMLs) used for? (5) Why can periodic structures be simulated more effectively than finite structures? (6) What kind of structure is most suitable for RCWA simulations? (7) Which quantities are necessary for the retrieval of the effective properties ε, µ?

4 INO12_lab_lecture_ docx 4 L2 Synthesis of metallic nanoparticles for engineering of plasmonic resonances UV-vis spectra of gold nanoparticles with different sizes reference:l.m. Liz-Marzán 2006 Langmuir 22, UV-vis spectra of silver nanoparticles with different shapes reference: J.J. Mock et al J Phys Chem 116, 15,

5 INO12_lab_lecture_ docx 5 Size distribution of gold nanoparticles (synthesized with protocol demonstrated in laboratory) Simplified chemical reactions metal ions (metal salt solution) are reduced by reducing agent (e.g. borohydride) to metal; the electrons for the reaction come from the reducing agent Au e - Au Ag + + e - Ag Student experiment Cleaning of glassware All glassware needs to be washed carefully. A) aqua regia: 3:1 mixture of conc. nitric acid (HNO 3 ) and conc. hydrochloric acid (HCl)

6 INO12_lab_lecture_ docx 6 CAUTION: strong acids (corrosive!); decomposition into toxic chlorine gas and nitric oxide gas; were gloves, goggles, and lab coat; work in chemical fume hood with sash down as far as possible; do NOT store in tightly closed containers From Wikipedia: decomposition of aqua regia Upon mixing of concentrated hydrochloric acid and concentrated nitric acid, chemical reactions occur. These reactions result in the volatile products nitrosyl chloride and chlorine as evidenced by the fuming nature and characteristic yellow color of aqua regia. As the volatile products escape from solution, the aqua regia loses its potency. HNO 3 (aq) + 3 HCl (aq) NOCl (g) + Cl 2 (g) + 2 H 2 O (l) 2 NOCl (g) 2 NO (g) + Cl 2 (g) B) rinse extensively with dh 2 O Nanoparticle synthesis 1) Gold nanoparticles synthesis reference: Duff 1993 Langmuir use: nucleation sites for nanoshell growth prepare a 28 mm HAuCl 4 solution (dissolve 140 mg HAuCl 4 in 14.8 ml dh 2 0) and age solution in dark for >30 min (this is enough for all experiments) prepare NaOH solution: 110 mg in 46 ml dh 2 O (enough for all experiments) reaction: (white-capped scintillation vials) ml dh 2 O add 1.93 ml NaOH solution add 23.3 µl THCP, wait 5 min rapidly add 0.7 ml HAuCl 4 solution color change to brown within 2 min 2) Silver nanoparticle synthesis reference: Lee/Meisel 1982 J Phys Chem (modified) use: signal enhancement in SERS freshly prepare a 5 mm AgNO 3 solution (dissolve mg AgNO 3 in 15 ml dh 2 O); weigh in AgNO 3 for each experiment and dissolve right before use

7 INO12_lab_lecture_ docx 7 freshly dissolve 3 mg NaBH 4 in 40 ml dh 2 O while stirring (NaBH 4 decomposes with time; you could weigh the solid into containers before and then just dissolve right before the experiment, substance dissolves very well) reaction: (50 ml beaker) 30 ml NaBH 4 solution add rapidly 10 ml AgNO 3 solution immediately color change to dark brown/green L2 - Questionaire 1. Which color do you expect for bigger and bigger particles? Why? 2. Does the shape matter: conical, spherical,? 3. Why is the resonance so broad and not sharp? 4. Why do you see a color? 5. What is a plasmon resonance? 6. expected from students: Why do people use gold or silver? Are there other materials that work?

8 INO12_lab_lecture_ docx 8 L3 - Characterization of photonic nanostructures by Scanning Electron Microscopy (SEM) Objectives - understand the working principle of an SEM - get a quick overview of the SEM operation parameters for imaging purposes - obtain a clear and focused SEM picture of o a silicon photonic crystal o a double-fishnet metamaterial - achieve a maximum material contrast highlighting the gold in the double-fishnet metamaterial during SEM operation - achieve information of the three-dimensionality of the photonic crystal by tilting and rotating the sample in the SEM specimen chamber (You will get technical support from experienced staff during SEM operation!) Sketch of electron extraction from a thermal emitter Remark: Alternative electron emission sources are Schottky emitters or cold field emitters.

9 INO12_lab_lecture_ docx 9 Sketch of operation principle of a scanning electron microscope (SEM) The operation of the SEM consists of applying a voltage between a conductive sample and filament, resulting in electron emission from the filament to the sample. This occurs in a vacuum environment ranging from 10-4 to Torr. The electrons are guided to the sample by a series of electromagnetic lenses in the electron column. The resolution and depth of field of the image are determined by the beam current and the final spot size, which are adjusted with one or more condenser lenses and the final, probe-forming objective lenses. The lenses are also used to shape the beam to minimize the effects of spherical aberration, chromatic aberration, diffraction, and astigmatism, see next figure.

10 INO12_lab_lecture_ docx 10 Typical imaging errors in an SEM

11 INO12_lab_lecture_ docx 11 Electron-matter interactions and energy spectrum of emitted electrons The electrons interact with the sample within a few nanometers to several microns of the surface, depending on beam parameters and sample type. Electrons are emitted from the sample primarily as either backscattered electrons (BSE) or secondary electrons (SE). Secondary electrons are the most common signal used for investigations of surface morphology. They are produced as a result of interactions between the beam electrons and weakly bound electrons in the conduction band of the sample. Some energy from the beam electrons is transferred to the conduction band electrons in the sample, providing enough energy for their escape from the sample surface as secondary electrons. Secondary electrons are low energy electrons (<50eV), so only those formed within the first few nanometers of the sample surface have enough energy to escape and be detected. High energy beam electrons which are scattered back out of the sample (backscattered electrons) can also form secondary electrons when they leave the surface. Since these electrons travel farther into the sample than the secondary electrons, they can emerge from the sample at a much larger distance away from the impact of the incident beam which makes their spatial distribution larger. Once these electrons escape from the sample surface, they are typically detected by an Everhart-Thornley scintillatorphotomultiplier detector. The SEM image formed is the result of the intensity of the secondary electron emission from the sample at each x,y data point during the rastering of the electron beam across the surface. Prototypical SEM images of biological species SEM image of an ant (left) and drosophila fly (right).

12 INO12_lab_lecture_ docx 12 L3 - Questionnaire 1. Why do you have to use an electron microscope to image nanoscale features? 2. Describe the operation principle of a scanning electron microscope (SEM). 3. Why does SEM operation require high vacuum standards? 4. What s the principle limit of the spatial resolution of an SEM? 5. Which properties should SEM samples necessarily possess? 6. Which other electron microscopes do exist and how do they differ from the SEM? 7. Which emission products can be expected from the interaction of high-energetic electrons with a solid state sample? 8. Name and explain at least two imaging errors during SEM operation and explain how they can be corrected.

13 INO12_lab_lecture_ docx 13 L4 - Far field spectroscopy for characterization of the optical properties of plasmonic nanostructures One of the fundamental experimental techniques for characterization of photonic nanostructures is the far field spectroscopy. In our labs we have three spectrometers which reveal the information about the reflectance, transmittance and with that about the absorption of the fabricated samples. These are the commercial grating spectrometer PerkinElmer Lambda950 (fig. 1), the commercial Fourier-transform infrared spectrometer (FTIR) Bruker Vertex 80v (fig. 2) with the integrated microscope Hyperion 2000 and the home build complex ellipsometric spectrometer CES (fig. 3). The last one provides a very special possibility: with this we are able to measure the transmission and reflection complex and also the complete complex transmission Jones matrix, which are worldwide unique opportunities. Fig. 1 Grating spectrometer PerkinElmer 950 Fig.2 FTIR Bruker Vertex 80v Fig. 3 complex ellipsometric spectrometer CES L4 - Questionnaire 1. What is the basic principle behind a FTIR spectrometer? 2. How does a grating spectrometer work? 3. What are the advantages of a FTIR spectrometer? 4. What defines the spectral resolution of a grating and a FTIR spectrometer? 5. What defines and limits the bandwidth of a FTIR spectrometer? 6. Why is it important to know the phase of a metamaterial? 7. Why do the spectra of the three spectrometers differ for the same test sample? 8. What is the problem of measuring a metamaterial with a microscope?

14 INO12_lab_lecture_ docx 14 L5 - Observation of surface plasmon polaritons by Scanning Nearfield Optical Microscopy (SNOM) In this experiment, we want to observe the propagation of SPPs on a metallic surface. The dispersion relation of SPPs lies below the light line (Fig. 1). This means that SPPs can not be directly excited by free-space light. We use a grating to obtain phase matching (i.e. provide the missing k). The grating period equals the SPP wavelength, so that SPPs are excited by illuminating the grating at normal incidence. We use collection-mode SNOM to observe the generated SPPs (Fig. 2). k Fig. 1: Dispersion relations of SPPs and free-space light. Fig. 2: Scheme of the experimental setup

15 INO12_lab_lecture_ docx 15 L5- Questionnaire 1. Why is there a diffraction limit in far-field microscopy? 2. Why can Scanning Near-Field Optical Microscopy (SNOM) overcome this diffraction limit? 3. How is the tip-sample distance kept constant during scanning? 4. Why does a SNOM measurement take relatively long? 5. Which different SNOM measurement modes do you know? 6. Why is SNOM a good method for imaging surface plasmon polaritons (SPPs)? 7. Name optical phenomena that are not well-suited for SNOM imaging. 8. Are the topographic and optical resolutions of the SNOM the same or different?