Fluorescent Holographic Fringes with a Surface Relief Structure Based on Merocyanine Aggregation Driven by Blue-violet Laser

Stability and integration are the goals for developing photonic devices. Spirooxazines have the property of photoinduced merocyanine-aggregation in polymer matrix, which can be applied to fluorescence emission and stable information storage. Although visible light coherent radiation with UV-assist has been used to achieve polarization-modulated holographic memory in spirooxazine doped PMMA films, the complexity of optical systems is increased and the aggregation ability of merocyanine is decreased. Here, we report that fluorescent holographic gratings with a surface relief structure can be inscribed in the film via sole irradiation of 403.4 nm. Time-dependent photo-anisotropy and holographic dynamics were both investigated with different power densities of the near-UV laser. The non-exponential photokinetics was explained by the sequential formation of mono- and aggregate-merocyanine molecules. The appearance of merocyanine aggregates is found to be beneficial to the long-term holographic memory with fluorescent emission. This work provides a research strategy for the integrity of storage, display and micro-fabrication of organic functional-devices.

The holographic kinetics presents an interesting oscillation behavior, which can be explained by the sequential formation of mono-merocyanine (MC mono ) and aggregate-merocyanine (MC agg ). Benefiting from the stability of MC agg , long-term hologram memory and fluorescence emission were both realized in the photochromic film.

Materials and Methods
Materials and film preparation. 1,3-Dihydro-1,3,3-trimethylspiro [2H-indole-2,3′-[3 H] phenanthr [9,10-b](1,4)oxazine (SO) with the purity of 98% and poly(methy methacrylate) (PMMA) were purchased from Aldrich. Molecular weight of PMMA is 400000~550000. SO and PMMA (weight ratio of 1:5) were full of dissolved in chloroform to obtain a mixed solution with the solution concentration of 10 mg/ml. After stirring for 30 min by a mixing machine, the solution of 200 μL was drop-coated on a clean slide glass to produce enough thick samples for effective holographic recording. A watch glass was buckled to the sample so that the evaporation rate can be controlled well. The photochromic film was obtained after the solvent evaporated at room temperature (300 K) for 12 h in the dark. The sample surface morphology (surface height change of less than 150 nm) and the resultant film thickness (~2.38 μm), were measured by a step profiler (KLA-Tencor) and Atomic Force Microscope (AFM, Bruker, Inc.), as shown in Figures S1(a) and (b) in the Supporting Information, respectively.
Optical measurement. The in situ UV-Vis absorption and the fluorescence emission spectra of the SO-doped PMMA film were obtained with a UV-Vis spectrophotometer and a fiber spectrophotometer (Ocean Optics), respectively.
Optical setup for photoinduced birefringence is shown in Fig. 1(a). Two laser beams were nearly parallel incident to the same position on the sample surface. The major elements of the experimental apparatus were a blue-violet laser (TOPTICA Photonics, 403.4 nm) as pumping source and an S-polarized red laser providing probe beam of 671 nm. A half-wave plate was rotated to a proper direction to adjust the polarization state of the pumping beam. The sample was fixed between two crossed polarizers in the path of the probe beam. It was designed to measure the effective phase retardation. The power density of the 671 nm beam was set as 1.42 mW/ cm 2 to reduce the destructive readout. The intersection angle between the polarization states of the pumping and probing beams was 45°. The transmittance of probe beams was monitored by a silicon photodiode interfaced with a computer.
Mono-color holographic grating recording with different writing power densities were carried out by the irradiation of two coherent S-polarized lights from the blue-violet laser on the center of the fixed photochromic film,  Fig. 1(b). The intersection angle between the two blue-violet beams was fixed at θ = 6.5°. The grating period can be calculated to be 3.56 μm by the Bragg's law of Λ = λ/2sin (θ/2), where λ represents the wavelength of the writing beams. In addition, dynamics of holographic grating was monitored using a red laser (671 nm, 2.85 mW/cm 2 , S-polarized). The probe beam was normal incident to the same spot with the writing-beams. The first-order diffraction signal was measured as a function of time by a silicon photodiode interfaced with a computer.
An inverted Confocal Laser Scanning Microscope (CLSM) was used (FluoView FV1000, Olympus, Tokyo, Japan) to observe holographic gratings, in bright and dark field channels, respectively. A 20-mW 488-nm laser (Cyan OEM, Spectra Physics) was chosen as the exciting beam. Surface relief structures for the fluorescent gratings were measured by AFM.

Results and Discussion
Formation of Merocyanine Aggregates. The absorption spectra of SO/chloroform solution and SO/ PMMA solid film were measured, as shown Figure S2 in the Supporting Information. However, little shift for the absorption peak position was found, indicating that there is no dye aggregation during sample preparation. Figure 2(a) shows that the differential absorption spectra in the UV-Vis region from 350 nm to 900 nm for the SO-doped polymer, excited with the blue-violet laser (57 mW/cm 2 ) for different times. Obvious absorbance accumulation appears around 589 nm and another absorption band emerges at ~400 nm under the long-time excitation. Figure 2(b) presents the excitation time dependence of visible absorbance at 400 nm and 589 nm. It is found that the absorption band in the yellow-green region can be enhanced by increasing the irradiation time until 90 min, and then presents gradual decrease. The near-UV absorbance has a similar temporal behavior to the former in the long-time excitation.
The decrease of absorption coefficients at 400 nm and 589 nm in the later excitation stage may be related to photo/thermal bleach and the secondary photo-chemical reaction. The transparent SO molecules are transformed to open ion-form by the C-O bond cleavage under the blue-violet light excitation, resulting in the formation of mono-MC molecules (MC mono ). The continuous increase of MC mono molecule population results in the enhancement of the absorbance at 589 nm at the initial stage. Meanwhile, the excited molecules can either be reverted to the colorless state thermally ( Figure S3a in the Supporting Information) or by visible excitation ( Figure S3b in the Supporting Information), or tend to be gathered as MC agg , resulting in the later decline of absorbance at 589 nm. Here, the blue-shift of the absorption peak from 589 nm to 400 nm indicates the formation of H-aggregation 17,35,36 . That means MC mono consumption, partly, contributes to the formation of MC agg . Chemical structures for SO, mono-and aggregate-forms of MC, and their inter-conversion are all shown in Fig. 2(c). In situ fluorescent spectra under the blue-violet excitation of 71.4 mW/cm 2 for different times were carried out, as show in Fig. 2(d). The fluorescence intensity centered at 559 nm is enhanced by almost 30 fold at the irradiation time of 180 min and then decreases gradually. Under the near-UV excitation, the MC agg with high energy level can release the luminous energy in visible region 37 , which, instead, induces the transformation from MC to SO and weakens the stability of MC agg , i.e., fluorescence quenching. Thus the declines both for the absorbance at ~400 nm and the fluorescence intensity at ~559 nm can be explained by the negative role of fluorescence after the long-time excitation.
The generation of carbon dioxide in the formation of merocyanine aggregates was in situ monitored using a gas chromatograph (SP-2100A, BFRL Co.). After the near-UV excitation (403.4 nm, 45 mW/cm 2 ) on the sample for 420 min, very small amount of carbon dioxide gas, only 13 ppm, was detected, indicating photodegradation process in our case can be ignored. Figure 3(a) presents the probe-transmittance versus time for the red light passing through two orthogonal polarizers (0°and 90°), under +45°polarized blue-violet irradiation with different pumping power densities. The transmitted signal, which is related to photo-anisotropy of the film, increases to the maximum value rapidly and decreases gradually versus pumping time. Evidently, the maximum values and the response rates of the transmittance can both be enhanced when increasing the power density of the 403.4 nm light. The transmittance peaks for different pumping power densities (14 mW/cm 2 , 43 mW/cm 2 and 100 mW/cm 2 ) are indicated as P14, P43 and P100 in the inset of Fig. 3(a). After these peaks, the transmitted intensities for different pumping power densities begin to drop gradually and almost reach the same value at ~22000s. The photo-anisotropy of the SO/PMMA film at the early stage of the pumping process can be explained as the principle of Angular Hole Burning (AHB) 38 , as shown in Fig. 3(b). Because of the insensitivity of SO molecule to the laser polarization state, the near-UV excitation may provoke the ring-opened reaction to produce a large population of the colored molecules in various orientations. The MC molecules with ion form are highly sensitive to linearly-polarized visible irradiation. Along the direction of electronic vector of optical field, positive and negative charge centers of MC are easier to be recovered to the original state, which can be proved by near-UV assisted dichroism experiments (see Figure S4 in the Supporting Information). In fact, due to the location of 403.4 nm at the edge of the visible region, the inverse excitation by blue-violet light may also act on the MC molecules of which the electric dipole distance lies in the direction of the laser polarization state [see Figure S5 Thus the photo-induced anisotropy from the formation of MC agg is not strong but stable which contributes to the final transmitted signal.

Photo-anisotropy by blue-violet excitation.
As the temporal evolution of 671 nm transmittance is similar to the curves of exponential growth with decay term, a dynamics model to explain the photoinduced anisotropy observed in Fig. 3(a) can be expressed as, where Δn i (i = momo and agg) is the maximum value of refractive index change resulted from the AHB effect of MC mono and the regular arrangement of MC agg , respectively. τ i (i = momo and agg) is the anisotropy-formation time constant. τ D,i (i = momo and agg) is anisotropy decay time constant. Using this model, theoretical fitting to the experimental curves in Fig. 3(a) for different pumping power densities of 14, 43 and 100 mW/cm 2 was carried out [see Figure S6 Table S1 in the Supporting Information. The theoretical fitting results indicate that the AHB effect of MC mono plays a major role in the whole process and contributes to the rapid process of photoinduced anisotropy, while the contribution of MC agg is much weaker and takes effect at the later stage.

Mono-color Holographic Dynamics.
Due to the stable green emission of MC agg , fluorescent micro-patterns were formed via holographic recording. Figure 4(a) shows the temporal evolution of the first-order diffractive intensity of the holographic grating inscribed by the mono-color laser with different power densities of 14, 43 and 100 mW/cm 2 . Clear oscillations of diffractive signal for all the laser power densities appear. Three maxima were observed in the whole holographic kinetics. The diffraction efficiency increases sharply to the first maximum in the initial stage (0~500s) followed by a slight decrease, and then climbs to the secondary high value before 3000s. The third one appears for a rather long recording time with the highest diffraction efficiency. After this point, the intensity of the diffractive signal begins to decrease and tends to be stable. Based on analysis of the temporal evolution of the near-UV light induced anisotropy, the photo-transformation process of spirooxazine molecules in the coherent linearly-polarized blue-violet excitation can also be divided into two stages: (i) the increase of MC mono molecules and (ii) their subsequent angle-selective inverse conversion in the bright region of the interference fringes, resulting in the quick formation of absorption gratings (AGs) and the gradually accumulated phase gratings (PGs), respectively, as shown in Fig. 4(b). With the population decline of the excited MC molecules, the AG is weakened, causing the slight decrease of diffraction efficiency in the first 500 s. However, the periodic distribution of anisotropy by the AHB effect of MC mono can be built up gradually which also contributes to the increase of diffraction efficiency. Comparison of the response time at peak position (P14, P43 and P100) between the photoinduced anisotropy and holographic dynamics for different near-UV laser power densities indicates that the second maximum of the diffraction efficiency comes from the formation of PGs, as shown in the inset graph of Fig. 4(a). After the long-time excitation, the alternate distributions of stable MC agg at bright regions and SO at dark regions in the interference fringes are produced to cause the appearance of the last diffractive maxima. As the anisotropy of MC agg is rather low, the last peak is ascribed to the other periodic change of absorption coefficient. In-situ transmittance measurement at 671 nm also proves the two increases of absorption coefficient and indicates the sequential formation of MC mono and MC agg [see Figure S5(c) in the Supporting Information]. Finally, the fringe contrast for both AG and PG are weakened by the negative role of the green fluorescence.
Fluorescent Holographic Dynamics Model. Based on the above analysis of the grating growth, the corresponding grating dynamic model is constructed as follows: η(t) is defined as diffraction efficiency. It has two components resulting from the diffraction on the PG (η p ) and AG (η α ) p Taking the thin sinusoidal gratings into account, the temporal evolution of diffraction efficiency can be expressed as the sum of the square of the first-order Bessel function of accumulated phase (ΔΓ) and the square of sinusoidal function of absorption coefficient (Δα), as followed: where d equ is the equivalent thickness of the thin film (2.38 μm in the case), λ is the wavelength of the probe beam (671 nm in the case), Δn the refractive index and Δd the thickness modulation. The PG kinetics is the result of the co-action of the angle-selective conversion of MC mono , the regular arrangement of MC agg and the possible surface relief modulation. The accumulated phase can be expressed as 2πd equ Δn equ (t)/λ, where Δn equ (t) is equivalent refractive index change taking the three factors mentioned above into account. The expression is reasonable for there is only one maximum in the process of photoinduced anisotropy. The simplified single-exponential growth process with decay term can be expressed as, For the sequential formation of MC mono and MC agg , the dynamic process of the AG can express by bi-exponential response:  where Δα i,max (i = momo, agg) is absorption coefficient with response time variation, τ i,α is recording time constant of absorption grating, τ Dα,i is erasure time constant of absorption grating. Consequently, the total diffraction efficiency dynamics can be expressed as follow: The solid lines in Fig. 4(a) show the kinetics descriptions according to Eq. (7), which agree well with the experimental results. The approximation parameters for all studied cases are gathered in Table 1. It was found that the amplitudes of the formed AG and PG, i.e., Δn equ,max , Δα mono,max and Δα agg,max are all enhanced with increasing the writing power densities, while all the time constants for either AG or PG exhibit the opposite behavior depending on the excitation intensity.
The theoretical fitting indicates the photochromism and anisotropy from SO to MC can both be accelerated by introducing high-power excitation light. The formation of MC agg contributes mainly to the AG growth although consuming rather long time. The excitation light with higher power induces more rapid decline of population of MC mono , however, provides better condition for the formation of MC agg . Due to the light-scattering of the fluorescence from MC agg , destructive interference between different bright regions in the holographic fringes is inevitable. The higher the writing power is, the faster the grating erase is.
It was also noticed that the maximum value of diffractive efficiency was achieved to be ~0.15%, which is still lower than that of other photochromic materials, such as azo 39 . The main reason may come from the limited conversion efficiency of SO to MC, the absorbance (exposure sensitivity) of the film at 403.4 nm, and the non-efficient construction of PGs which was proved by holographic recording with different polarization configurations (see Figure S7 in the Supporting Information).

Fluorescent Holographic Interference Fringes with a Surface Relief Structure.
Under the blue-violet excitation, the stable MC agg provides a possibility for high-efficiency patterned fluorescent emission. CLSM was used to observe, in bright and dark field channels, respectively, the holographic interference fringes in the SO-doped polymer film under different writing power densities. After holographic recording for ~8000s, a 20-mW 488-nm laser (Cyan OEM, Spectra Physics) was chosen as the exciting beam. As shown in Fig. 5(a), almost no periodic structures were observed in the dark field channel after the holographic excitation with the writing power density of 14 mW/cm 2 . However, in the bright field channel (Fig. 5d), holographic gratings were presented clearly. With the increase of the writing power density, the holographic fringes become more clearly in both bright and dark field channels. Surface relief structures for the fluorescent fringes were also measured by AFM, as shown in Fig. 5(g-i). The surface periodic modulation was found to be enhanced versus near-UV excitation intensity accordingly. The fluorescent intensities in the bright and dark regions of the holographic fringes (I max and I min , respectively) were determined by the software attached to the CLSM. The contrast of the holographic fringes (C) was calculated accordingly by the in following formula: The values of C were inserted in Fig. 5. Commonly, the variation of Δα is associated with C in the bright field channel, while the green fluorescent emission in the dark field channel is related to the formation of MC agg . That means the low-power near-UV excitation preferred to construct the alternate arrangement between MC mono and SO. Surface relief modulation (Δd) was also indicated in Fig. 5. The population of MC agg was increased by increasing the irradiation intensity which is beneficial to the formation of surface relief gratings (SRGs). It was also found that the diffraction efficiency of the fluorescent holographic grating can be maintained for a long time after the formation of MC agg (see Figure S8 in the Supporting Information). The results are consistent with the previous analysis of holographic kinetics. The formation of MC agg plays a role in the non-volatile holographic memory with high-efficiency fluorescent emission and surface relief modulation. The controllable photochemical reaction in SO-MC system also put a bright way to polarization-dependent biomedical detection, and can be applied to the integrated optoelectronic devices with abilities of storage, display and micro-fabrication.

Conclusions
The photoinduced anisotropy in SO/PMMA films excited by linearly-polarized blue-violet laser beams mainly results from the AHB effect of MC mono , while MC agg with antiparallel dipole molecule structure only generates a rather low anisotropic growth but plays an important role in the formation of absorption gratings with the help of long time excitation of coherent lights at 403.4 nm. Taking the measurements of in situ absorbance and fluorescence in the photochromic film under the 403.4 nm excitation into account, theoretical description for one phase grating and two sequentially-formed absorption gratings was determined. With the increase of writing light intensity, the amplitudes of absorption and phase gratings were both enhanced; meanwhile the erasure process of the holographic grating was also accelerated. It was found that the writing beams with higher power are helpful to produce MC agg so as to form more stable fluorescence holographic fringe with a surface relief structure, which agrees well with the results of CLSM in bright and dark field channels and AFM observation. The fluorescent periodic structures in blue-violet excitation provide a fabrication strategy for the photon-device with multi-capabilities of information processing.

Figure 5.
Holographic fringes in the SO-doped polymers observed by CLSM in dark (a-c) and bright (d-f) field channels, and by AFM (g-i) for different writing power densities (14,43 and 100 mW/cm 2 ). The florescent contrast and surface relief modulation of each holographic grating was calculated and labeled.