## 1 Introduction  
Since optical trapping and manipulation of a micro-meter size particle were achieved by [PERSON] and co-workers in 1970-80s [1; 2], laser trapping has been applied not only to biological cells [3; 4], but also to smaller particles such as biomolecules [5] and nanoparticles [6; 7; 8; 9]. Although there have also been reports on the possibility of single molecular trapping at room temperature [10; 11] and a manipulation method of single molecules with resonant laser irradiation has been theoretically proposed [12; 13; 14], stable trapping and manipulation of 1-nm-sized molecules at room temperature via light-gradient force is challenging. This is because the optical trapping potential (\((1/2)\alpha F^{2}\)) of molecules due to the light-induced force (gradient force) is much smaller than the kinetic energy of the thermal motion of the molecules (\(K_{B}T\)) at room temperature due to the tiny polarizability volume for \(\alpha\) of the molecules. In general, for stable trapping of particles or other moieties at room temperature, the trapping potential has to be deeper than the thermal energy at the temperature. Although the use of a sufficiently high-power laser is a possible choice to approach this condition, it can cause thermal effects and disturbs optical trapping. Therefore, recent research has focused on enhancing the laser-induced force by exploiting the surface plasmonic effects to overcome the thermal disturbance and limited focus size due to diffraction [15; 16]. In many cases, plasmon optical tweezers employ metal nano structures to enhance the optical force between metal nano-gaps [17; 18]. Another experimental method exploits the opto-thermoelectric field generated from opto-thermal fluidics [19]. Photophysical crystallization or aggregation with non-resonant laser irradiation has also been studied for decades [20; 21] and is applied for preparing protein single crystals for X-ray diffraction analysis of the atomic structure of proteins [22; 23; 24; 25]. The optical trapping force is also applied for laser crystallization and aggregation [26; 27]. In most of these studies, supersaturated solutions of solute molecules that form crystals or aggregates in equilibrium states were irradiated with laser light. In these experiments, the laser works as either a trigger for nucleation or an accelerator of crystal growth [28; 29]. Recently, there have been interesting reports on the crystallization or aggregation of organic molecules \"on air-liquid surfaces\" utilizing light-induced force as a direct trapping force instead of a mere trigger [30; 31; 32]. For molecular J-aggregates [33; 34], some reports have demonstrated the formation of J-aggregates in a saturated solution via laser irradiation [26; 27]. The \"bottom-up\" approach exploits the self-assembly nature of J-aggregates to form nanostructures, e.g., layer structures [35; 36; 37]. It is well known that the concentration of solutes and electrolytes, pH [38; 39; 40], temperature [41], and solution viscosity [42] contribute to formation of J-aggregates. Moreover, there is a collaborative effect on aggregation formation among the molecular hydrophobic effect and molecular-solvent and intermolecular hydrogen bonding [43; 44; 45], making the solvent effect on the aggregation phenomenon diverse. Additionally, it is difficult to control the size of micro-aggregates that configure macro-aggregates [46] because the generation of micro-aggregates occurs during the rapid change of the first-order phase transition and is highly dependent on the surrounding environment [41; 47]. Intermediate states in the formation process of micro-aggregates are considered to have a very short transient lifetime and are difficult to clearly identify. In our preceding paper [48], we proposed to employ an absorption spectrum to observe the changes in molecular aggregation states introduced by light-induced force.