In order to handle situations where more than a single dielectric interface is present, a generalization to (2.4) is available for general stratified media [74]. Although this framework can treat any number of layers with different refractive indices, only one more interface has to be introduced here, which is enough to describe a glass coverslip with adjacent heterogeneous media.
Fig. 2.6:An additional interface is introduced to account for a glass coverslip with a thickness of∆dand a refractive indexn2, wheren1,n3<n2.
The diffraction integral for three media can be written in the same form as (2.4):
E(3)(P) =i f n1
Whileκ is effectively identical to the previous case, the term Ψneeds to be further discussed, as the complex fieldA(3):
A(3)=√
now contributes an additional phase term introduced by the transmission coefficients
Ts,p= ts,p(1)ts,p(2) exp(ik2∆dcosΘ2) 1+rs,p(1)rs,p(2) exp(2ik2∆dcosΘ2)
(2.13)
based on the amplitude coefficients at the two interfaces
ts(j)= 2njcosΘj Note that for the cases which are to be considered here, it is possible to simplify (2.13).
In a typical setup,n1andn3will be situated between the refractive index of water (1.33) and that of the glass coverslip (about 1.51), which always forms the middle layer. In this case, the productr(1)s,prs,p(2)is much smaller than unity, and therefore the denominator of (2.13) can be omitted. The total aberration can then be written as
Ψtot =Ψ(3)+d1n2cosΘ2−d2n2cosΘ2 (2.16)
=d2(n3cosΘ3−n1cosΘ1) +∆d(n2cosΘ2−n1cosΘ1)
This in turn means that there are effectively two decoupled spherical aberration terms introduced by the coverslip and the sample layer, respectively. Both of these will have to be corrected for independently in the microscope objective. Due to technical con-siderations, it is likely that these corrections are implemented in the simplified form (2.16). Regarding the total aberration, it is interesting to note that the penetration depth d2and the refractive indexn3of the sample in principle cannot be perfectly corrected for with the single degree of freedom provided by the correction collar. This holds especially true for the marginal rays at high numerical apertures, which means that the manufacturer has to make some assumption about the sample and the depth range when designing the optics which are actuated by the correction collar. Regarding the coverslip, the microscope objective is usually corrected for a defined (#1.5, 170µm) thickness. As the dependence of the coverslip aberration onΘ1in (2.16) differs from that of the sample-induced term, the objective should be strictly used at that design thickness instead of correcting for a deferring coverslip thickness with the correction collar. This is especially true in cases wheren1an3differ largely.
The following calculations of the focal light intensity distributions were performed with the exact form of Ts,p (2.13) in order to assess the influence of its small non-rotational-symmetric variations on the zero-intensity spot of the STED PSFs. Objec-tive corrections where simulated by applying the inverted aberration terms according to (2.16). The apodization introduced by the angle-dependent transmission properties of the interfaces are thereby not affected. One has to keep in mind though, that the subtle details of the correction mechanisms are known only to the manufacturers. The presented calculations assume perfect correction mechanisms and therefore present a best-case study with respect to real-world experimental conditions.
2.3.1 Corrected objectives with glycerol and water immersion
Being equipped with a framework to handle more complex cases, two high-NA objec-tives with built-in correction mechanisms are evaluated in this section. One of them is a NA1.3 glycerol-immersion (n=1.46) objective and the second is a NA1.2 water-immersion objective. Both of them are corrected for a glass cover slip and feature a correction collar to compensate for sample-induced spherical aberrations. For the sim-ulations, a #1.5 (170µm) coverslip was added over the settings of the previous chapter.
As no total internal reflection will occur for these systems and the correction for spher-ical aberrations is assumed to be perfect, the performance of the objectives is expected to be uniform throughout the addressable depth range, if the collar is adjusted properly
1. However, besides absolute performance parameters as resolution and signal levels, it is important for the experimentalist to know the sensitivity of these parameters to non-optimum phase correction, which is likely to be the case in real-world setups. This was assessed by evaluating the effect of mismatches between the actual imaging depth and the depth that the correction collar is adjusted to.
B
correction depth mismatch / µm correction depth mismatch / µm
A
Fig. 2.7:If the correction collar setting and the actual NFP are not perfectly matched, loss of signal and resolution will occur. The sensitivity of the two objectives to non-optimal alignment in STED microscopy is evaluated here for the vortex phase-plate (A) and that used for 3D-STED (B).
The objectives were compared with the same peak intensity of the depletion PSF lead-ing to the same resolution enhancement factor. This is based on the reasonlead-ing that the STED resolution in living samples is often not limited by the total available STED laser power alone. As one usually tries to reduce the peak STED intensity in the sam-ple as far as possible to minimize the risk of disturbing the observed organism, the resolution obtainable for a fixed peak intensity is an important key figure for the exper-imentalist. The initial FWHM offset therefore corresponds to the ratio of the sizes of the diffraction-limited PSFs. In Fig. 2.7, the result is shown for the case of (A) lateral and (B) 3D resolution enhancement. Note that the presented curves are symmetric for positive and negative alignment errors, so only the positive branch is presented here.
1Fresnel losses are compensated by increasing the power at the back aperture
In the first case, if the maximum tolerable loss in terms of either resolution or signal is (arbitrarily) defined to be 10% of the initial value, the glycerol objective has to be aligned within±4µm of the optimal setting, whereas the water immersion objective tolerates±8µm . The same ratio between the two approximately applies for the 3D case, with tolerances of±2µm and±4µm . Note that these ranges also limit the extent of z-stacks that can be recorded without re-adjusting the correction collar.
Generally, the water objective performs significantly better in terms of alignment sensi-tivity, which is not surprising considering the lower refractive index difference between the immersion medium and the sample. However, this comes at the cost of a lower NA, which reduces the achievable resolution at a given peak intensity, especially along the optical axis. Additionally, one has to take into account how the absolute alignment re-quirements above translate to the experiment. Usually the correction collar is adjusted by optimizing either the fluorescence signal or the PSF shape while scanning a small structure in the sample in axial direction. It is clear that if changes are harder to sense, the alignment cannot necessarily be performed at a higher absolute precision. Consid-ering mechanical sensitivity, the full scale of the correction collar of the glycerol objec-tive was experimentally determined to correspond to a correction range of 100µm in the present setting. With ten scale parts on the collar, the highest alignment require-ment is therefore a little less than half a scale part, which is not difficult to achieve.
For the water immersion objective, on the other hand, the full correction range corre-sponds to a significantly larger range (>150µm), meaning that the required mechanical precision is about of the same order. For all practical matters, the two objectives should therefore be similarly well suited to image through layers of brain tissue.