1 INTRODUCTION

The spectroscopy of the inelastic (Raman) scattering of light has become one of the most attractive methods of studying the dynamics of elementary excitations and structure in various condensed media starting from its independent discovery by Raman and Landsberg along with Mandel’shtam [1, 2]. From the quantum-chemical viewpoint, Raman scattering is due to the electronic transition of photons incident on a sample from the ground state into the excited state and passes through intermediate electronic states of electron-hole pairs. The latter form elementary excitations with the transition to the final state and emit corresponding energy-shifted scattered photons. Correspondingly, intermediate electronic states play a key role forming quantum states which determine the nature of the important mechanisms of the nonresonant and resonant behavior of scattering as well as interference in the excited and scattering channels.

In contrast with conventionally studied and interpreted two-band intensity amplification of the Raman scattering of light (see, e.g., [3, 4]), the phenomenon of the multiband resonant intensity amplification of the scattering of light was for the first time predicted and revealed experimentally initially for Mandel’shtam–Brillouin scattering of light by acoustic phonons using the example of ZnSe crystals [5]. Then a similar approach was found and studied for the case of the Raman scattering of light by optical phonons by the example of AgJ crystals [6]. Herewith, it was established on the basis of the appeared experimental possibility of performing rigorous quantitative intensity measurements for well spectrally resolved narrow lines of optical and acoustic phonons that the experimentally found dependences of the scattering intensity of light at lattice vibrations on the quantum energy of the excitation radiation are described based on the widely used two-band Loudon theory insufficiently well [3]. This model of resonant scattering [3] does not take into account the Coulomb interaction between intermediate electronic states considering them as the states of an unbound electron–hole pair. Atomic-like exciton states belonging to both discrete exciton bands and a continuous spectrum as well as the upper valence bands were rigorously and in detail described in the introduced theory of the multiband resonant amplification of the scattering intensity of light at lattice vibrations as intermediate electronic states [5, 6]. In the following years, the theory of the multiband resonant scattering of light and approaches developed in [5–7] were applied and confirmed in [8, 9]. Taking them into account, the results of previously performed investigations for a series of semiconductors of the group III–V [8, 9] were recalculated as well as confirmed for many semiconductor materials—from bulk (for example, [7–12]) to low-dimensional nanostructures (for example, [13–17]). The formation of numerous electronic levels in nitrogen-doped diamond crystals can condition the clearest manifestation of the mechanism of the multiband resonant inelastic scattering of light. In this work, we present the results of investigations into the Raman scattering of light by optical phonons for the case of an ensemble of optically active NV centers in diamond crystals with a nitrogen-substituted vacancy. This point defect in diamond in the presence of nitrogen comprises the vacancy of a carbon atom coupled with a nitrogen atom. Diamond crystals with such centers attract considerable interest in the context of their use in quantum technologies. Our data point to the revelation of the strong resonant intensity amplification of the scattering of light by optical phonons. It is established that resonance processes with electron transitions for nitrogen impurities with both zero-phonon-lines play the determining role in this case: for the neutral NV0 center and for the negatively charged NV– center.

2 EXPERIMENTAL

Excitation of the Raman scattering spectra of light was performed by the emission of the second harmonic of a continuous wave laser based on aluminum–yttrium garnet with the wavelength λi = 532.070 nm and a continuous wave helium–neon laser with λi = 632.817 nm according to the procedure presented in [18–20]. We used the sample of a single-crystal diamond plate 3.0 × 2.5 × 0.5 mm in size. The sample oriented along axis (100) was prepared by chemical vapor deposition (CVD). The nitrogen-impurity content was [Ns]0 < 1 ppm (part per million). The surface roughness of the polished surfaces was <30 nm. Measurements of the scattering spectra of light optical phonons were performed at room temperature in the scattering configuration to the right angle Z(XZ)Y with axes X, Y, and Z, respectively, along directions [110], [1\(\bar {1}\)0], and [001] under identical experimental conditions. The measured values of the frequencies were calibrated with the help of the recording of lines of the neon lamp at 572.923 and 692.947 nm.

3 RESULTS OF INVESTIGATIONS AND THEIR DISCUSSION

The typical Raman scattering spectra of light by transverse optical phonons of E(TO) symmetry for local \({{C}_{{3{v}}}}\) symmetry in the diamond crystal under study with a nitrogen-substituted vacancy are presented in Figs. 1 and 2 and are denoted as R. The spectrum in Fig. 1 is recorded under excitation by laser emission with the wavelength λi = 632.817 nm for the optical phonon at 1332.4 cm–1. Figure 2 shows the spectrum recorded under excitation by laser emission with the wavelength λi = 532.070 nm for the optical phonon at 1332.9 cm–1. Such spectra of optical phonons in Figs. 1 and 2 are recorded under identical experimental conditions (scattering configuration, temperature, excitation emission power, etc.). It is clearly seen from the presented spectra that new features associated with a substantial variation in the intensity of the scattering lines of light at optical phonons are observed in diamond with the nitrogen-substituted vacancy in the process of the Raman scattering of light under excitation by the corresponding emission wavelength of the used lasers. It is established that the resonant processes with two electronic transitions for optically active impurity centers of nitrogen play a determining role in this case: (i) with the zero-phonon-line for the neutral NV0 center at 575.468 nm in the input scattering channel under excitation by laser emission with the wavelength λi = 532.070 nm and (ii) for the negatively charged NV– center at 637.874 nm in the output scattering channel with the wavelength λi = 632.817 nm. Judging by the presented intensity scales in Figs. 1 and 2 as well as taking into account instrumental corrections for the energy dependence of the spectrometer sensitivity, the spectral width of the slits, optical absorption, and (1/λi)4 dependence of the scattering intensity, an unusual increase in the intensity of the scattering spectra of light by optical phonons by a factor of more than 2.5 is observed under spectrum excitation by laser emission with a wavelength of λi = 532.070 nm. Herewith, the neutral NV0 center gives a large contribution to the mechanism of resonant-scattering-intensity amplification in the input scattering channel.

Fig. 1.
figure 1

Scattering spectrum of light by optical phonons in a diamond crystal with a nitrogen-substituted vacancy recorded under excitation by laser emission with a wavelength of λi = 632.817 nm. The Gaussian contour on the spectral line of a neon lamp at a wavelength of 692.947 nm used for calibration also points to the magnitude of the spectral resolution ΓG = 0.9 cm–1.

Full size image
Fig. 2.
figure 2

Scattering spectrum of light by optical phonons in a diamond crystal with a nitrogen-substituted vacancy recorded under excitation the laser emission with a wavelength of λi =532.070 nm. The Gaussian contour on the spectral line of a neon lamp at a wavelength of 571.923 nm used for calibration also points to the magnitude of the spectral resolution ΓG = 1.6 cm–1.

Full size image

4 CONCLUSIONS

Thus, the phenomenon of the resonant amplification of the Raman scattering intensity of light at optical phonons is for the first time discovered by the example of a diamond crystal with a nitrogen-substituted vacancy under the resonant excitation of intracenter transitions of impurity centers. It is established that both the neutral NV0 center at the input scattering channel and the negatively charged NV– center in the output scattering channel play a key role in the formation of quantum states determining the nature of the important mechanism of the resonant-scattering behavior. However, it is revealed at the same time that a large contribution to the mechanism of the resonant amplification of the scattering intensity of light by optical phonons more than by a factor of 2.5 is due to the neutral NV0 center in the input scattering channel.