Dependence of Raman peak intensities with periodicities

From the Raman data collected one can deduce the relation between the periodicity (Px and Py)of a sample with its Raman peak intensities. The results are presented in the plots below with two polarization states. The Raman peaks were observed at 1310 cm⁻¹, 1365 cm⁻¹, 1510 cm⁻¹ when measured with 785 nm laser. The cor-respondence of the peaks in wavelength scales are 875 nm, 879 nm and 890 nm, respectively. These are the well-known Raman peaks of R6G [142]. The dependen-cies of Raman peak intensities with Px and Py periodicities are illustrated in Figure 5.9.

(a) (b)

Figure 5.9: Variation of Raman peak intensities with different periodicities when the radiation effect along y periodicity. The intensities are defined by the colorbar scale.

While there exist significant variations between the samples, some general trends can nevertheless be observed. Raman peak intensities decrease gradually with in-crease of Py periodicities, for instance; An array 500x580 shows peak 1, 2 and 3 with intensities of around 1600, 3600 and 3400 counts, respectively. For an array 500x584, peak 1, 2 and 3 has intensities of around 2100, 2300 and 2000 counts, respectively. Along Py direction from 580 to 600 nm in 4 nm steps, By taking the ratio of one peak intensity to another in the order y periodicity for all peaks, there is an average ratio of peak intensities of 0.97, 0.95 and 0.93 for peak 1, peak 2 and peak 3, respectively. The peak intensities have very small deviation in the Y period direction. Moreover, there is an anomaly increase of intensity from 580 nm to 584 nm period, mostly caused by the setup but the overall trend is consistent. In Px

periodicity for instance; the first row of 580 nm period from 510 nm to 540 nm on interval of 10 nm, the average ratio of Raman peak intensities are 0.92, 0.98 and 1 for peak 1, peak 2, and peak 3, respectively. As we can see peak 3 has relatively the same intensity across the 580 nm Py periodicity.

When the Raman laser is in y polarization, i.e., the sample is simply rotated by 90^◦, the dependence of Raman peaks intensity can be elucidated in Figure 5.10.

(a) (b)

Figure 5.10: Variation of Raman peak intensities with different periodicities when the radiation effect in x periodicity. The increase of intensities is defined by the colorbar scale.

Raman peak intensities in Py direction follows the same trend, however, in this case the average ratio of peak intensities for peak 1, peak 2 and peak 3 are 0.9, 0.76 and 0.67, respectively. There is a decrease of Raman peak intensities of about 7%, 19% and 29% for peak 1, peak 2 and peak 3, respectively. This is due to the change of polarization which is associated with the decrease of laser power intensity that

couples into the structure. In Px periodicity, for instance; the first row of 580 nm period from 510 nm to 540 nm on interval of 10 nm has relatively equal value of Raman peak intensities.

The variation of Raman peak intensities for array 500x580 and 520x600 from periodic, Random 1 and Random 2 array is illustrated using Figure5.11. We shall start the analysis from the random arrays.

(a) (b)

1300 1400 1500 1600 1700 1800 Raman shift (1/cm)

Periodic and random array 500x580 500x580 500x580Random1 500x580Random2

1200 1300 1400 1500 1600 1700 1800 Raman shift (1/cm)

Periodic and random array 520x600 520x600 520x600Random1 520x600Random2

Figure 5.11: Variation of Raman peak intensities from random and periodic arrays

The random particle arrays were designed in such a way that they have the same particle density as the ordered arrays, but in random locations. “Random 1” refers to the samples, where the particle positions were entirely random, for in-stance arbitrarily small gaps between particles. Small gaps are known to induce plasmonic hotspots, which in turn are capable of producing enormous Raman en-hancements. We wanted to rule out this effect by designing another set of random samples (“Random 2”), in which the inter-particle distance was kept above 50 nm to exclude plasmonic hotspots. Indeed, ”Random 1” samples give higher signals than

”Random 2” samples, implying plasmonic hotspots are likely contributing.

Looking at the signal for periodic array 500x580 nm in Figure 5.11(a), we can clearly see that the ordered arrays give a higher Raman signal. This highlights the radiation induced enhancement of both the Raman laser and the transitions.

This sample is showcased here, as its x-periodicity is best matched with the Raman excitation laser. X-direction related SLR resonance of the sample is around 830 nm

but there is still significant coupling at 785 nm, with approximately 30 percent in coupling efficiency. The samples y-periodicity matches well with the Raman peaks (y-direction related SLR resonance is at 890 nm, Raman peaks at 875 nm, 879 nm and 890 nm). Next, we turn our attention to 520x600 nm sample in (b). The overall enhancement is lower than in Figure 5.11 (a) and closer to that of the ”Random 1” sample. We rationalize this by noting that x periodicity 520 nm has x-direction related SLR resonance at 820 nm, which is not anymore resonant with the Raman excitation laser. Interestingly, there is a gradually increasing Raman enhancement as we move toward higher wavelengths. This effect is due to y direction related SLR being resonant with the third peak at 1510cm⁻¹ (y direction related SLR resonance is at 880 nm, and the third peak is at 890 nm). This SLR resonance dependency with periodicity is also elucidated in the transmission plots in Figure 5.12. This implies that due to narrow SLR resonance linewidth, we can selectively enhance particular Raman transitions by tuning the lattice periodicity.

(a) (b)

500x580

600 700 800 900

[nm]

600 700 800 900

[nm]

Figure 5.12: Transmission plots for 500x580 and 520x600 showing the SLR resonance effect in y periodicity