1836 J. Phys. Chem. B, Vol. 106, No. 8, 2002
Letters
close to the fundamental and second harmonic frequencies. The
expression for resonant â has been described by Corn26 and
Simpson.16 Using the formalism by Simpson and two excited
states, we simplified resonant â as follows:
δin0mjn0mkn0
âijk
+
∑
ωn02 - (ω + iΓn0)2
{
n)1,2
min0(δnj 0mkn0 + mjn0δnk0)(ωn02 + 2ω2)
+
[ωn02 - (2ω + iΓn0)2][ωn0 - (ω + iΓn0) ]
2
2
}
m1i 2(mj20m1k0 + m1j 0m2k0)[ω10ω20 - ω2 + Γ10Γ20 - iω(Γ10 + Γ20)]
Figure 4. Second-harmonic generation (SHG) (a) and two-photon
fluorescence (2PF) images (b) collected at 800 nm excitation. SHG
images are scaled as follows: (i) p-in-p-out (×1.8), (ii) s-in-p-out
(×1.0), (iii) s-in-s-out (×1.7), and (iv) p-in-s-out (×2.5). 2PF images
are taken at (i) p-in-u(unpolarization)-out, (ii) s-in-u-out, (iii) s-in-
u-out, and (iv) p-in-u-out.
+
+
[ω102 - (ω + iΓ10)2][ω202 - (2ω + iΓ20)2]
{
m2i 0(mj10m1k2 + mj12m1k0)[ω10ω20 + 2ω2 - Γ10Γ20 + iω(2Γ10 + Γ20)]
[ω102 - (ω + iΓ10)2][ω202 - (2ω + iΓ20)2]
m1i 0(mj20m1k2 + mj12m2k0)[ω10ω20 + 2ω2 - Γ10Γ20 + iω(Γ10 + 2Γ20)]
quantitative physical picture of this magnetic effect has not been
established. However, to our best knowledge, this observation
of a magnetic influence on a molecular monolayer is without
precedent, although eq 1 does provide a physical basis for this
observation.
[ω202 - (ω + iΓ20)2][ω102 - (2ω + iΓ10)2]
}
(2)
In eq 2, subscripts 0, 1, and 2 correspond to the ground and the
first and second excited states. ωn0 and Γn0 are the energy and
the lifetime broadening term for the electronic transition between
excited (n) and ground (0) states. δinm and mni n are the
difference in the permanent dipole moment and the transition
dipole moment between the electronic state n and n′ along the
molecular axis i, respectively.
Nonlinear Optical Imaging. The absorption spectrum of the
TAZ in ethanol solution exhibits features around 290, 420, and
750 nm.8 The 750-nm feature has a negative solvatochroism,
shifting to 790 nm in toluene solution. Absorption spectroscopy
on spin-cast thin films of TAZ indicated that there was
appreciable absorption strength in the solid material at wave-
lengths longer than 800 nm. Therefore, we chose 800 and 860
nm as illumination wavelengths. Both wavelengths are on-
resonance at 2ω, but 860 nm is, at most, only weakly resonant
at ω, while 800 nm is more strongly resonant. Dispersed
fluorescence measurements from TAZ in CH2Cl2, excited at 446
nm, provided a guide for choosing an appropriate filter set for
2PF imaging. The protonated form of TAZ yields a strong
fluorescence feature near 750 nm, while the dominant fluores-
cence feature from the zwitterionic form is centered near 550
nm. Thus, we collected the 2PF image using filters to select
wavelengths between 450 and 650 nm.
The 2PF and SHG images, collected at various polarization
combinations and at ω ) 800 nm, are presented in Figure 4. In
our experiment, p-polarization is in the X-Z plane, while
s-polarization is parallel to the Y-axis. For the SHG images,
the signal level varied dramatically for different polarization
conditions, so some of the SHG data are scaled for clarity of
presentation.
The 2PF image reveals a stripe feature that is approximately
five times more intense than the flat background. The relative
intensity of the stripe can be attributed either to the direction
of the two-photon transition dipole moment or to a concentration
gradient of TAZ molecules. Topographic SPM analysis of these
domains revealed a height variation across a dark/bright stripe
boundary of only about 8 Å, which can explain a change in
molecular orientation but not a concentration gradient. This
orientation change was confirmed by SPM operating in surface
potential (SP) mode, which revealed an electric field (presum-
ably arising from a change in dipole orientations) at the bright/
dark stripe boundaries. The same stripe appears in the SHG
image, although other stripes appear in what are the background
regions of the 2PF image. All SHG features exhibited a strong
polarization-dependent intensity.
At 800 nm, both the fundamental and SHG frequencies are
resonant with absorption bands of the TAZ. The 2PF process
involves coherent two-photon absorption and incoherent one-
photon emission, which can be explained by the transition dipole
moment between electronic states. However, the SHG signal is
a completely coherent process and retains information origi-
nating from not only the transition dipole moments but also
the permanent dipole moment change as shown in eq 2.
Therefore, it is not surprising to observe additional features in
the SHG images.
We collected a full polarization data set of SHG signals at
ω ) 860 nm with the newly prepared sample. In this sample,
two long and wide adjacent stripe structures, identified with
BAM after the transfer to glass substrate and oriented nearly
parallel to the X-scan axis of the nonlinear optical microscope,
were imaged so that data on dark and bright stripes could be
collected in one experiment cycle. We arbitrarily labeled the
stripes with high- and low-SHG signal (as measured at s-input/
s-output polarizations), respectively, as bright and dark. Each
data point was separated from adjacent points by a distance of
5 µm to minimize photobleaching.
At seven output polarizations (Φout), SHG intensity was
measured as a function of input polarization (Φin) at every 10°
from 0° to 180°. A subset of that data is presented in Figure 5.
In these plots, p- and s-polarization correspond, respectively,
to 0° and 90°. Each plot shows two intensity lobes of varying
intensity depending on the input polarization, separated by 90°.
The polarization of an SHG signal, and its correlation to
molecular parameters, has been explicitly derived by Feller et
al.,29 and we used that formalism to fit the data.
The SHG polarization plots can be fitted with various
combinations of 18 independent molecular â tensor components
and Euler angles (θ,φ,ψ), which are shown in the Appendix. In
our particular experiment, the fundamental light (860 nm) and
the doubled light (430 nm) is 70 nm (0.13 eV) or 10 nm (0.07
eV) away from the first and second electronic excitation of TAZ.
The SHG signal depends on molecular hyperpolarizability
â. In our experimental condition, there are two excited states,
which locate 750 and 420 nm above the ground state and are