Stimulated emission depletion (STED) resolves fluorescent features that are closer than the far-field optical diffraction limit by applying a spatially modulated light field keeping all but one of these features dark consecutively. For estimating the efficiency of transient fluorophore darkening, we developed analytical equations considering the spatio-temporal intensity profile of the STED beam. These equations provide a quick analysis and optimization of the resolution and contrast to be gained under various conditions, such as continuous wave or pulsed STED beams having different pulse durations. Particular emphasis is placed on fluorescence fluctuation methods such as correlation spectroscopy (FCS) using STED.
©2010 Optical Society of America
Stimulated emission depletion (STED) overcomes the diffraction limit in fluorescence microscopy by ensuring that a (large) part of fluorophores in the diffraction-limited focal region is non-fluorescent (dark) for a brief period of time in which the remaining fluorophores are detected. As a result, a subset of fluorescent (bright) fluorophores is separated from a subset of dark neighbors within the diffraction region. Transient fluorescence deactivation is attained by a beam of light, called the STED beam, inducing stimulated emission from the fluorescent state of the dye [1–3]. Fluorophores that are exposed to an intensity I STED of the STED beam that is well beyond a threshold I s (at which the rate of stimulated emission equals the spontaneous decay) spend virtually all time in the ground state even when illuminated by excitation light; their capability to emit fluorescence is ‘switched off’.
In the focal plane of a typical STED microscope an excitation light spot is overlaid with a STED light spot featuring a central intensity minimum (“zero”). In regions with high STED intensity I STED the spontaneous decay of fluorophores is largely suppressed, but this is clearly not the case for those fluorophores residing at or close to the central minimum. The more intense the STED beam, the narrower the region becomes in which the fluorophores are allowed to emit. For most fluorophores the lifetime of the fluorescent state is in the order of nanoseconds and the cross section for stimulated emission σ STED is about 10−17–10−16cm2. Hence a STED beam providing a time-averaged focal intensity turns the fluorophores off even when they are flooded by excitation photons.
To keep the fluorophores dark using a moderate average laser power, most STED microscopes (‘nanoscopes’) employ a train of light pulses for both excitation and STED [1–3]. Numerical calculations of the STED efficiency have been performed elsewhere [1,2,4,5]; in the following, we will show how to optimize the performance of STED microscopy by tuning the pulse width of the STED beam. For this purpose, we calculate the fluorescence deactivation (‘off-switching’) efficiency by using approximations suitable for obtaining an analytical expression for the STED nanoscopy performance.
2. STED nanoscopy
Figure 1 shows the Jablonski diagram of a typical organic fluorophore with the electronic ground singlet state S0, the electronic excited singlet state S1, the lowest triplet or other dark state T1, and vibrational sub-levels as well as the most relevant transitions between these states. After excitation from S0 and vibrational relaxation, the fluorophore is found in the base level of the excited state S1, from where it spontaneously decays to S0 or crosses to T1. If spontaneous decay takes place, a photon is emitted with a fluorescence quantum yield . On the other hand, the excited fluorophore can be sent down to a higher vibrational sub-level with a red-shifted light field inducing stimulated emission. If the stimulated emission rate is larger than the spontaneous decay, the fluorophore remains dark even under excitation irradiation; the residence in the excited state S1 is essentially disallowed by the presence of the STED beam. Interruption of the STED beam turns the dye on instantly, because S1 is allowed again. Keeping alternating features dark for a brief period of time enables the separation of features that are mutually closer than the diffraction limit. It is essential to realize that in the concept of STED microscopy, the sole purpose of the STED beam is to ensure that not all but just one of the features located within the diffraction region emits signal.
For estimating the fluorescence ability of a fluorophore under STED, we assume that the fluorophore is initially in S1 due to a short excitation pulse. Furthermore, we assume a rectangular STED pulse lasting for τ STED, and a stimulated emission rate , with k S1 denoting the spontaneous decay rate and being the saturation factor. ζ scales with the ratio of the pulse intensity I STED of the STED light and the saturation (threshold) intensity I s . The saturation intensity here defines the STED intensity at which the rate of stimulated emission equals the spontaneous decay rate k S1 (i.e., ζ = 1). We define as the photon flux, λ STED is the STED wavelength in free space and . Consequently, is a molecule-specific constant given by the cross section σ STED of stimulated emission and k S1. During the STED pulse, i.e. for 0 < t < τ STED, the relevant rate equations for the populations P S1 of S1 and of , respectively, are
The probability of spontaneous decay η ps is now obtained as F(γ)/F(0), which equals the fraction to which the fluorescence is suppressed by the STED pulse of intensity I STED (which is proportional to ζ and γ), and where is the fluorescence probability of the undisturbed molecule.
Here, k S1 τ STED and characterize the average number of spontaneous and the average number of possible stimulated de-excitation events during the STED pulse τ STED, respectively . k STED τ STED is a measure of the strength of the applied STED irradiation. For a constant pulse repetition rate 1/T, it scales linearly with the average STED power .
If the period T is long compared to the excited state lifetime, k S1 T is large and Eq. (3) simplifies to the first term, i.e.
For large , Eq. (4) becomes then
This is the case for long STED pulses, i.e. for the case of applying pulsed excitation in conjunction with continuous wave (CW) STED (τ STED = T). The equation also holds for large k S1, i.e. for fluorophores with short fluorescence lifetimes, in which case I s is large and the I STED necessary to achieve a certain fluorescence inhibition becomes very large as well. Likewise Eq. (5) also applies if large I STED, i.e. large ζ and γ are used. This condition also defines the maximum achievable fluorescence inhibition , which is determined by the finite vibrational relaxation rate k vib. For typical fluorophores and measurement conditions, the ratio k vib/k S1 ~103–104 (k S1 ~1/ns compared to k vib > 1/ps), hence η ps < 1‰ approaches almost complete fluorescence suppression. An infinite relaxation rate results in for . In most experiments however, because of a small though non-zero probability of S0 → S1 excitation by the STED light.
Equation (3) is applicable for pulsed excitation; STED can be either pulsed or CW. In case of CW excitation and CW STED, the ongoing excitation continuously works against the inhibition of the fluorescent state occupation, i.e. against the off-switching of the fluorophore. Neglecting the triplet state, the probability of spontaneous decay η CW writes as
This is the main equation for CW STED, which derives from the ratio of the average time periods a fluorophore requires for a spontaneous decay from S1 with and without STED. Without STED, this is the time for a full excitation–de-excitation cycle . With STED, more excitations by a factor of γ are required, that is a time of per spontaneous decay. For low excitation avoiding saturation of the S0 → S1 transition (k ex << k S1), Eq. (6) simplifies to the expression of Eq. (5). In this case of CW STED the same fluorescence inhibition is achieved regardless of applying pulsed or CW excitation. More detailed calculations taking into account the pulse delay as well as the excitation power and triplet state dependent response are summarized in the annexes.
Previous work has given different descriptions of the fluorescence inhibition efficiency η ps by STED . Assuming and regarding only the residual fluorescence emitted after the STED pulse, η ps was calculated from the single rate equation as the remaining population of S1 after the pulse duration τ STED.Equation (3) differs mainly in the aspects that it accounts for k vib and the pulse period T and that it includes the spontaneous fluorescence emitted during the STED pulse.
Pulse length dependence
Figure 2 outlines the estimated STED performance, i.e. the probability of spontaneous decay η ps versus ζ (i.e. pulse STED intensity) calculated for k S1 = 1/3.9ns, k vib = 5/ps and 80MHz pulse rate. These values are taken from the organic dye Atto647N in a supported lipid bilayer  and a mode-locked Ti:Sapphire laser. In Fig. 2a, the thin line shows the lower limit , which is achieved for pulsed excitation but continuous wave (CW) STED and infinite k vib. The bold lines indicate the performance for pulsed STED (Eq. (3)) showing an ever more pronounced knee, the shorter the STED pulses become. This is because the STED intensity has to counterbalance the short excited state depletion time before the suppression becomes efficient. Regardless of the pulse width, the suppression efficiency saturates at a lower bound (here η ps ≈5·10−5) due to the vibrational relaxation rate k vib, which sets the upper limit for the effective stimulated depopulation rate of S1 (Eq. (5)). In addition, the thin dotted lines of Fig. 2a and the lines of Fig. 2b trace η ps versus ζ and τ STED, respectively, for constant average STED powers that potentially stimulate the S1 de-excitation equals 1–16 times per pulse.
Increasing ζ or τ STED along the k STED τ STED isolines entails an according decrease of τ STED and ζ, respectively. For very long pulses, i.e. small ζ, the suppression of the fluorescent state is quite inefficient because spontaneous emission is as fast or faster. For very short pulses on the other hand, i.e. large ζ, the suppression becomes inefficient again because of the limited S1 de-excitation rate stemming from the finite vibrational relaxation rate k vib. Figure 2 reveals that a pulse width of τ STED ≈20–30ps would exploit the available STED power maximally and would yield the steepest decrease of η ps with increasing ζ. Both characteristics are preferable in STED imaging as outlined further on.
In many practical cases however, it turned out that a pulse width of τ STED ≈100–150 ps leads to better results. The following reasons can be cited. Firstly, longer pulses leave more time for the excited molecules to rotate, which helps minimizing polarization effects . Secondly, if pulsed diode lasers are used for excitation , the excitation pulse width of about 50–70ps and the synchronization jitter can be better countered with longer STED pulses. Thirdly, it has been shown that longer pulses are more advantageous with respect to photobleaching of the fluorophores , since these phenomena usually scale with higher orders of the intensity . Strongly related with the previous argument is the fact that longer pulses disfavor multiphoton excitation processes, including plain multiphoton excitation of relaxed molecules from the ground state .
Spatial resolution and STED imaging
In the focal plane of a typical STED microscope a diffraction-limited excitation light spot is overlaid with a STED light spot featuring a local intensity minimum at the center of the excitation spot. An example is the doughnut-shaped intensity pattern recalled in Fig. 3 . Consequently, fluorescence is inhibited everywhere but at the focal center and the more intense the STED beam, the more fluorophores and fluorescent features are switched off by the STED light and the narrower the region becomes in which the fluorophores are allowed to fluoresce. Figure 4 exemplifies calculated cross-sections through effective STED point-spread-functions (PSF) of such a confocal epi-fluorescence microscope with an oil immersion objective of numerical aperture (NA) of 1.40 and a fluorescent dye with a lifetime of 3.9ns [15,16]. The effective STED PSF describes the region in which the dyes are allowed to emit and subsequently be registered by the detector. More precisely, it gives the normalized probability with which the fluorophore is able to contribute to the measured signal at a given location in the sample. The excitation wavelength λ ex is 635nm and the STED wavelength λ STED is 770nm. Both beams overfill the objective aperture and are circularly polarized. The STED beam passes through a 2π phase ramp (vortex plate) for creating a doughnut with a central “zero”, i.e. a minimum with a relative intensity of ε times the intensity at the doughnut crest . The detection pinhole has a projected diameter of 500nm in the sample space, i.e. 0.9 × the Airy disc diameter. At low excitation intensity, the detected fluorescence brightness profile is the product of the excitation probability times the emission probability η times the detection efficiency.
Without STED, the detected fluorescence (λ fl = 670nm) has a brightness profile with a full width at half-maximum (FWHM) diameter of 232nm. Applying STED pulses or CW STED irradiation, the FWHM diameter is narrowed with increasing STED power in all cases and follows a similar dependence regardless of the STED modality as summarized in Fig. 5 .
For obtaining the same FWHM diameter, the CW STED power has to be increased about five fold (~1.5k S1 T) as compared to the average STED power in the pulsed case . As predicted  and experimentally verified , the inverse of the FWHM diameters of the STED foci follow a square root law. In the pulsed mode, the FWHM is about with the diffraction limited confocal FWHM0 ≈0.52λ ex/NA and where k STED τ STED is the number of potential stimulated emission events per molecule at the doughnut crest (if there were as many excitations). With 50ps long STED pulses and k STED τ STED = 50 at the crest, a FWHM of about 33nm is reached in the outlined example. In the vicinity of the “zero”, changing the pulse width τ STED does not significantly alter the profile as shown by the CW PSF, where the STED “pulse width” equals the excitation pulse period T = 12.5ns.
In Fig. 4, the dash-dotted lines show the effect of an imperfect “zero” with a residual intensity amounting to ε = 1% of the crest intensity. In the outlined examples, a vortex plate is used for creating the doughnut, which is rather tolerant to many experimental imperfections such as astigmatic or spherical wave front aberrations or lateral misalignments. Using a vortex plate, an imperfect zero typically stems from wrongly polarized light (c.f . for examples) and/or from asymmetric wave front aberrations. For instance, elliptically polarized light with an electric field varying by ~20% peak-to-peak results in ε = 1%. In this case, the peak brightness drops significantly, in particular at high STED power, but the diameter is hardly affected for short STED pulses. However, with CW STED the resolution deteriorates (with respect to the case of ε = 0) to a value of FWHM = 43nm as shown by the normalized profiles. Note, that along with the reduction of the fluorescence at the central peak, suppression of fluorescence is not complete in the focal periphery. While the contribution of this peripheral fluorescence is in principle low, it is less for short STED pulses and for ε = 0, i.e. the effective STED PSF is sharper and offers a better contrast. The improved performance in the case of short STED pulses results from the steep dependence of η ps with increasing ζ (Fig. 2).
3. STED nanoscopy for fluorescence fluctuation spectroscopy
So far, we measured the resolution by the FWHM of the central peak. Unfortunately, this measure may become inappropriate for very high resolution, i.e. small FWHM values. Because the emission of peripheral fluorophores cannot be suppressed completely, their contribution to the total signal becomes increasingly important in this case (Fig. 4). As long as STED nanoscopy is applied for imaging comparatively sparse features, the effective PSF in real space, respectively its optical transfer function (OTF) in Fourier space, can be usually exploited for reducing undesired image artifacts like blur or ghost images. Within the limits of the signal-to-noise ratio (SNR) of the image(s), image deconvolution allows improving the image quality by re-attributing faint peripheral signals to their original features. However, such image restoration methods are hardly (if ever) applicable for spectroscopic measurements, such as fluorescence correlation spectroscopy (FCS) [20,21] for instance, essentially because the fluorescence signal from randomly distributed and/or mobile fluorophores is collected and the spatial information (neighborhood) has to be ignored in favor of sufficient time resolution, i.e. the assumption of sparse and spatio-temporally resolved features fails. For example, previous STED-FCS measurements of fluorophores diffusing in open measurement volumes of nanoscale dimensions showed a decrease in signal-to-background ratio (SBR) with increasing resolution due to a growing contribution of peripheral fluorescence [8,22]. Besides increasing the noise in the FCS data, a poor SBR damps the cor-relation amplitude, impeding the determination of concentration and brightness values .
In order to quantify the performance of STED in FCS, we introduce a general measure based on the SBR within a homogeneous sample. Note that a spatially homogenous sample represents a well-defined worst case, i.e. non-structured and hence non-resolvable case in imaging, whereas it is virtually the default case in FCS (when the random molecular trajectories are averaged over time). The SBR shall denote the contrast between the signals from the brightest focal region(s) compared to the background from the dimmer region(s), where we apply an arbitrary threshold to separate the bright from the dim region(s). For 50ps pulsed and CW STED irradiation, Fig. 6 exemplifies the SBR versus the radii of the bright signal area for a threshold set at 10% of the maximum fluorescence intensity in the focal center. Without STED, the signal area has a radius of about 203nm and nearly 90% of the detected fluorescence is emitted within this area (SBR ≈8.6). With increasing STED power, the SBR decreases along with the radius of the signal area.
The decrease of the SBR is much more pronounced for the CW than for the pulsed STED mode. With a perfect “zero” (ε = 0), the SBR remains >3 for signal area radii >5nm in pulsed mode and 118nm with CW STED, respectively. This significant advantage of pulsed STED was experimentally observed when comparing STED-FCS measurements in pulsed and CW STED mode . With ε = 1% residual intensity in the “zero”, this radius increases sharply from 5nm to 30nm in the pulsed mode, because the bright central region is no longer able to surpass three-fold the background from the entire periphery. In this case, increasing the STED power further can even lead to a complete loss of signal, which is indicated by the return point on the curve in Fig. 6a, where the diameter increases because the first order side lobe (Airy ring) becomes part of the signal area. For the same imperfect “zero” in CW STED, the diameter hardly increases to about 119nm because the peripheral regions always contribute notable background (Fig. 4b).
With the SBR set up here, an effective size of the STED focal region for FCS may be obtained as the area or volume of the brightest focal region(s). Analogous to the FWHM, requiring a SBR = 1 yields a region of half-signal (RHS) from where 50% of the detected signal originates. Figure 6b compares the FWHM diameter with the RHS diameter , if the outlined system is used to investigate a two-dimensional sample, such as fluorophores diffusing in a membrane sheet in the focal plane. For low to medium STED power, the RHS diameter follows the FWHM with < 20% difference. If k STED τ STED increases beyond the maximum contrast between the “zero” and the crest region, the RHS diameter deviates and eventually grows again, which accounts for a too dim signal from the tiny effective STED PSF. For the outlined example, the minimal RHS diameters are about 4nm (50ps) and 29nm (50ps with ε = 1%) at k STED τ STED ≈104 and 140, respectively.
As pointed out and demonstrated, STED does not require pulsed operation . In the outlined case of a fluorophore with 3.9ns lifetime and a STED laser with 80MHz pulse rate, pulsed excitation but CW STED would just require about five times the average pulsed STED power in order to obtain equivalent FWHM diameters [24,25]. For FCS however, it became clear that the CW STED performs significantly worse because the fluorescence in the peripheral region is less efficiently suppressed. This is indicated by minimal CW RHS diameters of 40nm with perfect and 93nm with ε = 1% “zero”, respectively. With strong CW excitation, the situation may even become worse because fluorophores in the periphery cycle more often than those in the center (c.f. annexes).
In summary, we outlined a straightforward analytical model that allows a rather accurate estimation of the inhibition of spontaneous fluorescence by STED and the resulting brightness profiles in the effective STED PSF, i.e. in the region in which the fluorophore remains bright. This region scales down in diameter with a factor proportional to the square root of the number of stimulated emission events per molecule that are in principle possible (maximum statistical average) at the focal intensity maximum of the STED beam. We introduced a combined resolution and contrast measure, the region of half-signal (RHS), stating that the collected signal from within the tiny region in which the dye is allowed to fluoresce should preferably be at least as large as all background contributions from elsewhere, in particular from the focal periphery. For nearly perfect “zeros”, the FWHM diameter continuously shrinks with increasing STED power, whereas the RHS diameter decreases only as long as the fluorophores in the active region generate enough signal to overcome the background from their neighbors. The achievable RHS diameters are smaller in the pulsed than in the CW STED mode, because the pulsed mode better suppresses the excited state in the focal periphery for a given number of stimulating photons.
However, it is worth noting that remedies can be found. If pulsed excitation is used, time-gated detection improves the performance if the early spontaneous emission of the fluorophore is discarded . Likewise, it is important to understand that the outlined RHS ruler fully applies only to the most unfavorable case, i.e. a dense sample with many fluorophores in the focal periphery, each contributing. Whereas this assumption holds for most spectroscopic measurements such as FCS, which typically disregard spatial information in favor of time resolution, it represents a rare worst case in imaging, where the effective PSF (or the corresponding optical transfer function) can be fully exploited for improving the image based on the spatial information discriminating the neighboring features. In both cases, our simple model allows optimizing the experimental conditions for STED nanoscopy as well as taking into account the effective STED PSF for an improved analysis of the measurement data.
The probability of spontaneous decay η is given by the ratio of the fluorescence F(γ) under STED versus the fluorescence F(0) without STED. The emission rate per fluorophore k fl is the product of the fluorescence quantum yield qfl and the rate of spontaneous decays k from the excited state S1: F ∝ kfl = qflk.
The derivation of Eq. (3) and its simplifications are outlined in section 2 and summarized as case 2–4 in Table 1 (cumulative conditions). If the STED pulse follows the excitation pulse with a delay τ, spontaneous decays take place before the start of the STED pulse and only the remaining fraction of molecules are affected (case 1).
As long as the fluorescence response follows the excitation intensity linearly, the fluorescence emission is proportional to the product of the excitation intensity and the spontaneous emission probability. However, for larger excitation intensities, the ground state S0 of the fluorophore becomes depopulated during the excitation pulse (saturated excitation) and/or because of triplet state accumulation. For the sake of simplicity, we keep assuming (instantaneous excitation) and assume in addition (slow triplet decay).
Let be the population of the molecular state X just before the excitation pulse and just thereafter. Let n S0 be the number of excitations from the ground state S0 during the excitation pulse. The singlet state populations are then described by the following relations:Eq. (9) yields the triplet population and insertion into Eq. (8) yields then a relation for the population of the excited singlet state S1.Eq. (10) for and noting that the rate of spontaneous decays k ps is given by , we finally obtain
Equation (11) includes the effect of the triplet state accumulation, which is most pronounced at the local “zero” of the STED beam, where the spontaneous decays are most frequent. Therefore, if a fluorophore shows a significant triplet state population, the accumulation in the dark triplet state will lead to a significant reduction of the fluorophores brightness in the vicinity of the “zero”, much as if there would be some residual STED light.
In addition to the triplet state accumulation, saturated excitation occurs at higher excitation rates k ex because of the depopulation of the ground state S0 during the excitation pulse. The number of excitations n S0 is approximately given by , where we neglect spontaneous decays during the pulse according to our assumptions. However, for high excitation rates, the effective excitation rate should be used. Furthermore, if the STED pulse overlaps the excitation pulse, the (net) number of excitations is reduced to the dynamic equilibrium of the fluorescence excitation and suppression rates and reaches approximately
Validity of the effective STED rate
For the sake of simplicity and physical insight, we solved the rate Eqs. (1) at the dynamic equilibrium and found the effective STED rate . On the other hand, the exact solution of the time-varying population reads asEquation (13) can be simplified by noting that . However, it is not obvious to get Eq. (3) as the STED rate k STED in the doughnut crest may approach or even surpass k vib, which prohibits any further simplification. Therefore, Fig. 7 compares the exact results with those obtained by Eq. (3) (Fig. 2) showing the excellent agreement at all relevant conditions in the outlined examples. Notable differences occur only at very short STED pulses, at which the condition is violated and our assumption of a dynamic (pseudo-)equilibrium during the STED pulse fails.
Continuous wave excitation
Continuous wave (CW) excitation requires CW STED, which simplifies the calculations because pulse synchronization and timing is not required. The calculation of the spontaneous decay rate k CW and the probability of spontaneous decay η CW are straightforward. Let be the effective STED rate as defined before, and let be the corresponding intersystem crossing yield. At the dynamic equilibrium, the average cycle time τ CW is then given by the sum of the occupation times of the singlet and triplet states.Table 1. This convergence at low CW excitation rate can be expected because individual excitations become well separated in time and, therefore, the fluorophores undergo STED as if a pulsed excitation scheme were used.
Marcel Leutenegger is a fellow of the Alexander von Humboldt foundation.
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