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Register a new userInvestigation of the SERS spectra of copper phthalocianine adsorbed on gallium phosphide substrates
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The SERS spectra of the phthalocianine molecule, adsorbed on the gallium phosphide substrate are investigated. It is demonstrated that there appear strong lines, which are forbidden in usual Raman scattering. Analysis of the spectra indicates that these lines are associated with a strong quadrupole light-molecule interaction and also by a strong enhancement of the tangential components of the electric field on the surface. As it was demonstrated earlier, the last effect is characteristic for SERS on semiconductor and dielectric substrates, where there is the enhancement not only of the normal, but of the tangential components of the electric field on the surface.
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The SERS spectrum of hydroquinone, adsorbed on nanoparticles of titanium dioxide is analyzed. It is pointed out that the enhancement is stronger for larger mean size of nanoparticles that is in an agreement with the electrostatic approximation. In addition it is found that there are the lines, which are forbidden in usual Raman spectra. Along with this there is the enhancement, caused both by the normal and tangential components of the electric field. This result is in agreement with the theory of SERS on semiconductor and dielectric substrates. Discovery of the forbidden lines indicates sufficiently large role of the strong quadrupole light-molecule interaction in such a system.
The SEHRS and SERS spactra of 4,4 - Bipyridine are analyzed on the base of the Dipole-Quadrupole theory for two possible geometries of the molecule. It is demonstrated that there appear strong lines caused by vibrations transforming after a unit irreducible representation both for the geometry with D2 and D2h symmetry groups, which may probably describe the symmetry properties of the molecule. Appearance of these lines is associated with a strong quadrupole light-molecule interaction, which arises in nano size rregions of sharp roughness of the metal. In addition, there are the lines caused by contributions from both the vibrations transforming after the unit irredicible representations A or Ag and the representations B1 or B1u, respectively, which describe transformational properties of the Ez component of the dipole moment, which is perpendicular to the surface for both geometries. This result is associated with a specific geometry of the molecule, when the indicated vibrations can be nearly degenerated and can not be resolved by the SEHRS and SERS spectra analysis. This issue is in a full compliance with the results of the SEHRS and SERS Dipole-Quadrupole theory.
Gallium phosphide (GaP) is an indirect bandgap semiconductor used widely in solid-state lighting. Despite numerous intriguing optical properties---including large $chi^{(2)}$ and $chi^{(3)}$ coefficients, a high refractive index ($>3$), and transparency from visible to long-infrared wavelengths ($0.55-11,mu$m)---its application as an integrated photonics material has been little studied. Here we introduce GaP-on-insulator as a platform for nonlinear photonics, exploiting a direct wafer bonding approach to realize integrated waveguides with 1.2 dB/cm loss in the telecommunications C-band (on par with Si-on-insulator). High quality $(Q> 10^5)$, grating-coupled ring resonators are fabricated and studied. Employing a modulation transfer approach, we obtain a direct experimental estimate of the nonlinear index of GaP at telecommunication wavelengths: $n_2=1.2(5)times 10^{-17},text{m}^2/text{W}$. We also observe Kerr frequency comb generation in resonators with engineered dispersion. Parametric threshold powers as low as 3 mW are realized, followed by broadband ($>100$ nm) frequency combs with sub-THz spacing, frequency-doubled combs and, in a separate device, efficient Raman lasing. These results signal the emergence of GaP-on-insulator as a novel platform for integrated nonlinear photonics.
It is demonstrated that the reason of SERS on dielectric and semiconductor substrates is the enhancement of the electric field in the regions of the tops of the surface roughness with very small radius, or a very large curvature. The enhancement depends on the dielectric constant of the substrate and is stronger for a larger dielectric constant. It is indicated that the enhancement on dielectrics and semiconductors is weaker than on metals with the same modulus of the dielectric constant. The result obtained is confirmed by experimental data on the enhancement coefficients obtained for various semiconductor and dielectric substrates.
We present an in-depth analysis of the experimental estimation of cross sections in Surface Enhanced Raman Scattering (SERS) by vibrational pumping. The paper highlights the advantages and disadvantages of the technique, pinpoints the main aspects and limitations, and provides the underlying physical concepts to interpret the experimental results. Examples for several commonly used SERS probes are given, and a discussion on future possible developments is also presented.
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