## 4 Conclusions  
In this paper, we studied the laser-induced absorbance change spectra for R6G and PIC instead of TPPS, which we used in the preceding reports [38; 39]. For both molecules, we compared the unsaturated aqueous solution samples and the PVA film samples about the effects of non-resonant laser-light irradiation and found the absorbance decrease and increase in the monomers and in the aggregates, respectively, for the aqueous samples and no absorbance change for the PVA samples. For R6G, which is known to form H-aggregates [41], the observed absorbance increase was expectedly located on the shorter-wavelength side of the H dimer's absorption, close to the H tetramer's absorption wavelength [47]. For PIC, which is known to form J-aggregates on the longer-wavelength side of the  
Figure 2: The absorbance change (solid line) and absorbance (dashed line) spectra of (**a**) pseudoisocyanine (PIC) aqueous solution in 0.3 mm-thick cell, where the irradiation intensity is 1.1 kW/cm\({}^{2}\), and of (**b**) PIC in PVA, where the irradiation intensity is 1.1 kW/cm\({}^{2}\). Both the absorbance and its change were measured with the multi lock-in amp. system.  
monomer in the equilibrium, on the other hand, we unexpectedly observed an absorbance increase on the shorter-wavelength side of the monomer, suggesting the formation of H-aggregates. The result for the PIC sample demonstrates that aggregation states that do not exist in the equilibrium are generated by light-induced force, as we discussed in Ref. [38].  
The observed absorbance increases for R6G and PIC are approximately 100 and 500 times larger, respectively, than the ratio of the optical gradient force potential to the thermal motion energy at room temperature. This magnitude (defined here as the aggregation efficiency) is much smaller than that of TPPS in our preceding report [38]. Therefore, the molecular aggregation by light-induced force occurs far more efficiently for TPPS molecules than R6G and PIC. Although the mechanism is still unknown and the efficiency is varied depending on the kinds of molecules, it is notable that the high-efficiency molecular aggregation by light-induced force is most likely a universal phenomenon.  
Multichannel lock-in nonlinear spectroscopy, which can detect changes in absorption spectra on the order of \(10^{-5}\), was the most suitable for the discovery of this phenomenon, where an extremely small amount of specific aggregation states is formed only under light irradiation. In the future, in order to elucidate the mechanism, it is desirable to observe not only the spectroscopic signature but also the spatial image of the aggregation products. Direct imaging of molecular aggregates is realized by AFM [50], X-ray/electron diffraction [51], and cryo-TEM [52], and nanoscale structures have been observed. In particular, cryo-TEM is a powerful method that allows observation of structures as they are in aqueous solution at room temperature, which are preserved by rapid freezing, and it is promising if the samples can be rapidly frozen under irradiation light. Even for this approach, there remain many challenges to overcome, one of which is to increase the aggregation fraction \(\Delta A/A\), which is \(10^{-3}\) at its highest (TPPS).  
Conceptualization, E.T.; methodology, E.T., M.S. and T.K.; validation, M.S.; formal analysis, M.S.; investigation, M.S.; writing--original draft preparation, M.S.; writing--review and editing, E.T. and T.K.; supervision, E.T. All authors have read and agreed to the published version of the manuscript. This research received no external funding. The authors acknowledge technical support and advice by [PERSON]. The authors declare no conflict of interest.