Fig. 1. Schematic diagrams of experimental system and sample. (a) Schematic diagram of experimental system, L1, L2: beam expanding lens, F1: excitation filter, F2: emission filter, DBS: dichroic mirror, BS: beam splitter, L3, L4: focusing lens; (b) structure of sample chamber; (c) SEM image of gold nanocubes; (d) SEM image of gold nanospheres

Fig. 2. Dark field imaging of gold nanoparticles trapped by laser (scale bar is 5 μm). Arrow represents the position of the focal spot, circles represent gold nanoparticles. (a1)--(a5) Dark field imaging of gold nanocubes trapped by laser; (b1)--(b5) dark field imaging of gold nanospheres trapped by laser

Fig. 3. Raman signal intensities in different kinds of aqueous solution at different times. Raman spectra of 4-MBA in (a1) aqueous solution of gold nanocubes and (a2) aqueous solution of gold nanospheres; Raman spectra of 4-MBA in (b1) aqueous solution of gold nanocubes and (b2) aqueous solution of gold nanospheres at initial time, where vertical dashed lines make positions of two Raman characteristic peaks and their corrsponding Raman shifts are 1078 cm^-1 and 1594 cm^-1, respectively

Fig. 4. Raman signal (Raman shift is 1078 cm^-1) intensitis of 4-MBA in solutions of gold nanocubes and gold nanospheres vary with time after laser is turned on

Fig. 5. Optical forces on particles in two typical states. When electric field E (along the y-direction) is parallel to (a1) side or (b1) diagonal of gold nanocube,for y=0, (a2)(b2) relationship between F_x(x) and x (dot line), (a3)、(b3) relationship between F_z(x) and x (dot line); for x=0, (a2)(b2) relationship between F_y(y) and y (diamond line), (a3)(b3) relationship between F_z(y) and y (diamond line); (a4) (b4) relationship between F_x and D (dot line) when the center of gold nanocube is located at x=200 nm and y=0, relationship between F_y and D (diamond line) when the center of gold nanocube is located at x=0 and y=200 nm, and relationship between F_z and D (triangle line) when the center of gold nanocube is located at x=0 and y=0

Fig. 6. Velocity distributions of photothermal convection in cavity caused by aggregated gold nanocubes, arrows indicate directions of velocity. (a) Side view; (b) top view

Fig. 7. Distributions of f for different numbers of nanoparticles on x-y plane. (a1)--(a3) Distributions of f for single particle; (b1)--(b3) distributions of f for two particles; (c1)--(c3) distributions of f for 9 gold nanospheres; (d1)--(d3) distributions of f for 9 gold nanocubes

Fig. 8. f_SERS for different metal structures. f_SERS for different numbers of gold nanocubes and nanospheres. Data in dotted box are the results for the case that number of particles increases gradually while arrangement of particles with the previous number remains unchanged; (b) f_SERS corresponding to two gold nanocubes with typical structure of "edge close to surface" as shown in Fig. 7(b2) under different g. Inset shows schematic of simulated structure, where the electric field E of incident light beam is along the y-direction. Horizontal dashed line represents the sum of f_SERS of two single gold nanocubes (one of them is diagonal parallel to E and the other is edge parallel to E)

Fig. 9. Raman enhancement factor f_SERS for different M when g=20 nm. Circular curve and rectangular curve correspond to structure with two gold nanospheres and structure with two gold nanocubes, respectively (electric field E of incident laser along the y-direction). Ordinate value of circular curve is magnified 5 times to show the details