Second-order optical effects in organometallic nanocomposites induced by an acoustic field

Acoustically stimulated second-order optical effects in Ru-derivative nanocomposites were discovered. The alkynyl ruthenium derivatives were embedded in a polymethyl methacrylate (PMMA) polymer matrix. As second-order optical effects we studied second-harmonic generation (SHG) and linear electro-optics (LEO) phenomena. The physical insight of the effect observed consists in a coexistence of nanocofined chromophore levels and localized $d$ states of ruthenium. A transverse acoustic field favors the occurrence of charge density noncentrosymmetry required for observation of the second-order optical effects, particularly SHG. We have found that acoustically induced SHG and LEO for fundamental YAB-${\mathrm{Gd}}^{3+}$ laser light $(\lambda =1.76\mu \mathrm{m})$ increases and achieves a maximum value at acoustic power density of about $1.45\mathrm{W}∕{\mathrm{cm}}^2$. The values of the SHG for several Ru chromophores were higher than those for well-known inorganic crystals. With decreasing temperature, the SHG signal strongly increases below 55 K and correlates well with occurrence of “softlike” low-frequency anharmonic quasiphonon modes responsible for the phase transitions. The SHG maxima were observed at acoustic frequencies of about 13 kHz. Increasing of acoustical frequencies up to the megahertz range suppresses the observed phenomena. Comparing the obtained results with the acoustically induced Raman spectra at different temperatures one can conclude that the observed effects are due to acoustically induced electron-vibration anharmonicity, and are observed at temperatures below 55 K. Varying the chromophore content within the embedded matrices we were able to use effective nanoparticle sizes within the range 5–60 nm. It is clearly shown that the enhancement of the effective nanosize effectively suppresses the observed second-order optical effects.

Figure 1

Synthetic procedures to obtain the alkynyl derivatives (v) $\left[\mathrm{Ru}\mathrm{Cl}{\left(\mathrm{dppe}\right)}_2\right]\left[\mathrm{Tf}\mathrm{O}\right]$ 0.25 mmol, A1 0.30 mmol, $\mathrm{C}{\mathrm{H}}_2{\mathrm{Cl}}_2$ 40 mL, room temperature (RT) 20 h; (vi) 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) 0.50 mmol, $\mathrm{C}{\mathrm{H}}_2{\mathrm{Cl}}_2$ 40 mL, RT, 1 h; (vii) $\left[\mathrm{Ru}\mathrm{Cl}{\left(\mathrm{dppe}\right)}_2\right]\left[\mathrm{Tf}\mathrm{O}\right]$ 0.50 mmol, A2–4 0.60 mmol, $\mathrm{C}{\mathrm{H}}_2{\mathrm{Cl}}_2$ 50 mL, RT, 20 h; (vii) ${\mathrm{Et}}_3\mathrm{N}$ 1 mmol, $\mathrm{C}{\mathrm{H}}_2{\mathrm{Cl}}_2$ 20 mL, RT, 1 h.

Figure 2

General access to the terminal alkynes A1–4 (i) $\mathrm{Pd}{\mathrm{Cl}}_2{\left(\mathrm{P}{\mathrm{Ph}}_3\right)}_2$, CuI, trimethylsilylacetylene, THF, ${\mathrm{Et}}_3\mathrm{N}$, 50 °C, 24 h; (ii) ${\mathrm{Bu}}_4\mathrm{N}\mathrm{F}$ (solution 1M in THF), 1 eq. THF, RT, 1 h; (iii) $\left[\mathrm{Cr}{\left(\mathrm{C}\mathrm{O}\right)}_3\left({\eta }^6\text{−}{\mathrm{C}}_6{\mathrm{H}}_5\right)\mathrm{C}{\mathrm{H}}_2\mathrm{P}\left(\mathrm{O}\right){\left(\mathrm{O}\mathrm{C}{\mathrm{H}}_3\right)}_2\right]$, 1.1 eq. NaH, THF, 12 h, 65 °C, (iv) ${\mathrm{Bu}}_4\mathrm{N}\mathrm{F}$ (solution 1M in THF), 2 eq. THF, RT, 1 h.

Figure 3

Principal schema of the acoustically stimulated SHG experiment, BS, beam splitter; M1, M2, mirrors; P1, P2, polarizers; A, attenuator; S, specimens kept in liquid helium cryostat; SP, spectrograph; PM1, PM2, photomultipliers; PC, personnal computer; DL, delaying line; $\mathrm{\Phi }$, complement of the angle of incidence onto the sample; $\mathit{E}$, applied electric field.

Figure 4

Typical dependences of the acoustically induced optical SHG versus the effective specimen thickness at acoustical frequency 13 kHz and different acoustical powers: curve $B$, $0.79\mathrm{W}∕{\mathrm{cm}}^2$; curve $C$, $1.75\mathrm{W}∕{\mathrm{cm}}^2$ for the C4 sample for averaged nanoparticle size about 5 nm. The curve $D$ shows the same at acoustical power about $1.75\mathrm{W}∕{\mathrm{cm}}^2$ and averaged nanoparticle size about 50 nm. All the measurements are done at $T=4.2\mathrm{K}$.

Figure 5

(a) Dependence of the SHG and LEO on the acoustical power for different samples: SHG, C1 (▵), C2 (∎), C3 (◇), C4 (×); LEO, (▴) for C4. (b) Dependences of the effective second-order optical susceptibility defined from the SHG (×) and LEO (▴) versus averaged nanoparticle sizes for the C4 samples at 13 kHz and temperature about 5 K.

Figure 6

Typical dependences of the SHG (×) and LEO (▴) versus the temperature at optimal conditions.

Figure 7

Temperature-dependent DSC signal $\times {10}^3$ for the sample C4 achieved at the optimal conditions.

Figure 8

Behavior of the acoustically induced Raman modes at $T=5\mathrm{K}$ at different acoustical powers for C4: thin black line, $0.65\mathrm{W}∕{\mathrm{cm}}^2$; thick black line, $1.75\mathrm{W}∕{\mathrm{cm}}^2$ at 13 kHz acoustical frequencies. The gray line corresponds to the frequencies of the acoustical field about 1 MHz. All the measurements are done at the optimal conditions for the second-order optical susceptibilities.

Figure 9

Dependence of the acoustically induced second-order nonlinear optical susceptibility tensor component $d_{\mathit{yyy}}$ SHG versus the acoustical frequency at acoustical power $1.75\mathrm{W}∕{\mathrm{cm}}^2$ for the compound C1 at $T=5\mathrm{K}$.