In apertureless scanning optical nearfield microscopy signals are usually very weak.
In our proof of principle measurement we try to measure a perturbation of the nearfield signal caused by the mechanical oscillation of the particle. Because the nearfield signal itself is rather weak, we need to optimize every aspect of our ex-periment in order to be able to detect a pump induced perturbation of the nearfield signal. The spectral properties of metal nano particles are strongly dependent on the particle size (see figure 3.3). The requirements for a sample that facilitates the demonstration a time resolved nearfield microscope are contradictory. On the one hand the nearfield should be maximised to have a high quality carrier signal and on the other hand we have seen that the transient cross sections vanish at the spec-tral position of the plasmon resonance (compare section 4.3). In order to obtain a large transient signal we therefore use T-Matrix calculations (compare chapter 2.6) to predict the best possible sample geometry. The calculation in figure 4.8 is ob-tained by calculating the scattering cross section in dependence of the radius for a
λ (nm)
500 550 600 650
ΔT/T (10 )
700 750
Simulation Measurement
0 1
extinction (arb. u) Darkfield spectrum
0 2.5
-2.5
-5
1.5*10-5
0 180
90
270
a) b)
Figure 4.7 a) Spectrally resolved pump probe measurement of a single gold nano disc with a radius of 50nm and 50nm height. The amplitude spectrum shows a dispersive line shape with a zero crossing at about the plasmon resonance (green). The differential transmission spectrum as calculated by T-Matrix simulation is shown in blue. b) The polar plot of the oscillation amplitude shows a constant phase over all measurements which is to be expected as the mechanical oscillation phase is determined by the geometry of the particle [51].
particle of 50nm height at a wavelength of 800nm for a ground state (csca,gs) and an excited state (csca,es). ∆csca is then defined as
∆csca =csca,es−csca,gs (4.10)
4.6. CONCLUSION 55
r (nm) 60 70
Δc /c ( 10 )
80 90 0
1.0
-1.0
-4
100 110
scasca
Figure 4.8 T-Matrix simulation of the differential scattering cross section of a gold disc of 50nm height and radius r calculated for a wavelength of 800nm. The curve shows a dispersive line shape that has a zero crossing at about 80nm disc radius. The extrema of the curve are located at 60-70nm and 90-100nm disc radius. The simulation predicts a maximal relative change of10−4 assuming a temperature change in the gold disc of∆T = 10K
4.6 Conclusion
Building a time resolved nearfield microscope combines two experimentally chal-lenging techniques. The goal is to demonstrate the combination of picosecond tem-poral resolution as well is 20nm spatial resolution. The model system used for the proof of principle measurement are the mechanical oscillation of metal nano par-ticles that are excited with a short pump pulse. We model the elastomechanical properties of our gold discs in Comsol and get a good agreement between mea-surement and simulation. In particular we find that the oscillation period of the first order breathing mode of a metal disc is proportional to the disc radius. We continue to investigate the spectral dependence of the differential scattering signal and find a dispersive line shape that has a zero crossing at the plasmon resonance.
The differential scattering measurements are backed up by T-Matrix calculations.
In the end we use T-Matrix calculation to optimize the gold disc geometry with respect to the differential scattering cross section. For a disc height of 50nm and a wavelength of 800nm we determine the optimal disc radius to be 90nm to 100nm.
CHAPTER 5
Apertureless Scanning Nearfield Optical Microscopy
In the last chapter pump probe measurements served as a characterization tool for the mechanical properties of gold nano discs, as well as a means to familiarize one self with the pump probe approach. Before the pump probe technique is combined with the nearfield microscope, nearfield measurements are carried out on gold nano discs in order to characterize the nearfield response of the gold nano discs as well as to get to know the behaviour of the nearfield microscope. Scanning nearfield optical microscopy (SNOM) is a technique that allows to optically investigate nano structures beyond the farfield resolution limit which states that the smallest dis-tance between two objects to be separable is given by [42]
∆d= λ
2·N A (5.1)
where λ is the wavelength of light and NA is the numerical aperture. This limit is circumvented in SNOM by placing a scatterer at a distance much smaller than the wavelength of light from the sample surface. At these distances the detector interacts with exponentially decaying nearfields. In this chapter we demonstrate the nearfield capabilities of the setup by measuring the nearfield distribution of a plasmonic particle. These measurements also serve as a characterization step on the way towards the timeresolved nearfield microscope. On the long run, the goal is to create a tool that has a temporal resolution on the order of 10fs as well as a spa-tial resolution of 10nm. There are two approaches to nearfield microscopy which utilize either aperture probes or apertureless probes. Aperture probes are typically realized by tapering a glass fiber which shrinks the fiber core down to a size of a few hundred nanometers and less, thereby creating a nanoscopic aperture. The aperture fiber can either be used in collection mode or excitation mode in which it serves as
57
a nanoscopic lightsource that can be used to efficiently funnel light into highly lo-calized modes [62]. The spatial resolution achievable with such an aperture SNOM is given by the aperture size which is typically limited to roughly 100nm. This limit is a result of the transmitted powerPtaperthrough the tapered fiber which is [35]
Ptaper ∝e−λd (5.2)
where d is the aperture diameter and λis the wavelength. In the excitation mode, where the probe light is delivered through the tapered fiber, the temporal resolution of a time resolved near field microscope is limited due to the pulse broadening occurring in the fiber. The temporal broadening is characterized by the GVD value (group velocity dispersion) and can be expressed as [63]
GV D= λ3 2πc2
d2n
d2λ (5.3)
where n is the refractive index.
200
50 100 150
output (fs)
200 input (fs)
50 150 100
Figure 5.1 Input pulse width compared to the output pulse width for a travel distance of 20mm through silica glass withnsilica= 1.4. The center wavelength of the pulse is 800nm.
For input pulse durations above 100fs the pulse broadening is negligible whereas a 25fs input pulse has already a width of roughly 100fs after exiting the glass.
Figure 5.1 shows the effect on the pulse duration of a 800nm center wavelength input pulse when traversing through 20mm of silica glass. The strong dispersion of a broad band pulse severly reduces temporal resolution.
When using the aperture SNOM in collection mode the temporal resolution of a broad band source can be conserved, as has already been demonstrated by Kuypers et al. [64]. Overall the aperture SNOM is capable of delivering very good temporal resolution in combination with a slightly enhanced spatial resolution that is given by the aperture diameter.
5.1. FEM FIELD SIMULATIONS 59 In apertureless SNOM (aSNOM) a sharp tip made of metal or dielectric is used as a local probe. Because the illumination and detection is realized in a free space con-figuration, the temporal resolution of the aSNOM is only determined by the light source used. Furthermore the spatial resolution of the aSNOM is independent of the wavelength of the light and only dependent on the radius of the apex of the tip.
A spatial resolution ofd= 100λ and better has already been demonstrated [65].
In summary the apertureless approach to SNOM is limited by the temporal reso-lution of the laser source and the spatial resoreso-lution is limited by the size of the tip apex which can be 10nm or even smaller. We favor the aSNOM approach as it is the experimental approach with the least theoretical limitations regarding achievable spatial and temporal resolution.
The apertureless SNOM used in this thesis is based on a design developed by R. Vogelgesang and coworkers [20]. The technical details of the apparatus have already been shown in section 3.5.
To characterize the nearfield response of the experiment we use a sample con-sisting of gold discs on a glass substrate. The disc dimensions are chosen such that the structure delivers a high differential nearfield signal (see 4.5) at a wavelength of 800nm. Detailed information on the sample design is given in section 4.5. The samples were created using a combination of electron beam lithography and metal evaporation process. All samples where manufactured by K. Lindfors.
5.1 FEM field simulations
Prior to the measurements the nearfield signals where simulated using the FEM based software Comsol Multiphysics [55, 66, 67]. The wave front in the focus of the objective is approximated as a plane wave. The total fieldEtot is the sum of the excitation fieldEbackand the scattered fieldEscat.
Etot =Eback+Escat (5.4)
In order to mimic the excitation field in the experiment the components of
Eback =
where Ey is the field amplitude, nef f is the refractive index of the surrounding medium,λ0is the wavelength andφis the angle between sample surface and prop-agation direction. Figure 5.2 shows a top view of the model structure which has been adapted from other works [68] and modified to suit the experiment. A gold disc sits in the center of a medium with refractive index nef f (blue). The outer shell of the model is given by a perfectly matched layer (PML) (outside of dashed white line) with a spherical scattering condition at the outer surface of the PML of kdir(r) = −n(r). To evaluate the fields, an integration plane parallel to the x-y direc-tion is implemented 10nm above the disc surface (not shown in the figure). For the model parameters we use a disc radius of 100nm and a disc height of 50nm. Fur-thermore we chooseφ = 18◦, nef f = 1.4, Ex = 0, Ey = 1V /mandλ0 = 800nm. The dielectric properties of gold are taken from the data by Johnson and Christy [26].
The PML mimics an infinitly extended simulation volume by dampening all im-pinging field components and thereby suppressing any reflections occurring from the boundaries of the shell. We use an adaptive mesh in order to get a good compro-mise between the number of elements, the computation speed and the simulation accuracy. From the simulation data we then extract the z-component of the
scat-perfectly matched layer (PML)
neff gold disc
effective medium
k Eb
y x
Figure 5.2 Top view of the Comsol model used for the calculation of the nearfields around a gold disc of 100nm radius and 50nm radius. The gold disc is embedded in an effective medium sphere. Backreflections from the model borders are suppressed by introducing a PML shell around the effective medium. The excitation wavelength of is 800nm.
tered field Esca in the evaluation plane. Figure 5.3 shows the field distribution of