# Nonlinear optical characterization of tetraphenylporphyrin in the picosecond regime

C. Cassagne, M. Chniti, Cid de Araújo, H. Belmabrouk, Georges Boudebs

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C. Cassagne, M. Chniti, Cid de Araújo, H. Belmabrouk, Georges Boudebs. Nonlinear optical characterization of tetraphenylporphyrin in the picosecond regime. 17th International Conference on Transparent Optical Networks (ICTON2015), Jul 2015, Budapest, Hungary. 10.1109/ICTON.2015.7193435. hal-02442883

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# Nonlinear Optical Characterization of Tetraphenylporphyrin in the Picosecond Regime

C. Cassagne<sup>(1)</sup>, M. Chniti<sup>(1,2)</sup>, Cid B. de Araújo<sup>(3)</sup>, H. Belmabrouk<sup>(2)</sup>, and G. Boudebs<sup>(1,*)</sup>

## ABSTRACT

Applying the D4σ-Z-scan technique with a laser delivering single pulses in the picosecond regime at 1064 nm, 532 nm, and 355 nm we report on the nonlinear (NL) optical properties of 5,10,15,20-Tetraphenyl-21H,23H-porphine (TPP) and 5,10,15,20-Tetraphenyl-21H,23H-porphine zinc (ZnTPP) solutions prepared in chlorobenzene. Accurate measurements of the NL coefficients for 4 concentrations in both compounds are provided. The results show no response in the IR for both materials, negative NL refraction for TPP in the green changing sign to positive for ZnTPP and positive NL refraction in the UV for both compositions. The NL absorption of these materials is quite huge in the visible range where applications for optical limiting are needed.

**Keywords:** porphyrin, nonlinear absorption, nonlinear refractive index, D4σ-Z-scan technique.

## 1. INTRODUCTION

Large Porphyrins (Pphs) are biological macromolecules that play a crucial role in various processes in nature such photosynthesis and oxygen transport in cells, as well as applications such as in solar photovoltaic devices [1] and photodynamic cancer therapy [2]. Synthesis and experimental investigation of nonlinear (NL) optical properties of Pphs have been reported by several groups [3, 4]. The large interest in the NL properties of Pphs is due to their unique electronic, optical and biochemical properties [5, 6] as well as the high stability and architectural flexibility that provide a very good basis to finely tune their optical response.

In the present article, we report on experiments with TPP and ZnTPP using an Nd: YAG laser operating in the picosecond regime at 10 Hz repetition rate to measure the nonlinear absorption (NLA) coefficients and NL refractive indices of the Pphs based solutions at three different wavelengths (1064 nm, 532 nm and 355 nm) and four different concentrations. The experiments were performed calculating the beam waist relative variation (BWRV) using D4σ intensity profile measurements combined to Z-scan method [7]. It is referred here as D4σ-Zscan technique [8]. It was possible to determine the sign and the magnitude of the NL refractive indices even in the presence of high NLA at relatively high concentration which is the case when dealing with Pphs. Simple linear relations were empirically obtained allowing a correlation between the NL parameters and the linear absorption coefficient.

## 2. EXPERIMENTAL

### 2.1 UV-VIS Absorption

<figure><img src="image_2.png" /><figcaption>Figure 1: Molar extinction coefficients ε versus wavelengths. Solid (red) line for TPP; dashed (blue) line for ZnTPP. Porphyrin concentrations for both compounds: (a) 10⁻⁴ mol/L; (b) 10⁻³ mol/L. The vertical straight lines indicate the laser wavelengths at 1064 nm, 532 nm and 355 nm. I, II, III, IV are the TPP Q-bands.</figcaption></figure>

The electronic absorption spectra were recorded in chlorobenzene solution. Figure 1(a) shows the optical absorption spectrum of TPP in solid (red) line and ZnTPP dashed (blue) line measured from 300 nm to 1100 nm, at room temperature. The strong absorption band centered at ≈ 420 nm (≈ 2.96 eV) is called Soret band (also

<footnote>(1) LUNAM Université, Université d'Angers, LPhiA, EA 4464, 2 Boulevard Lavoisier, 49045 Angers France</footnote>

<footnote>(2) Laboratory Electronic and Microelectronic, University of Monastir, Tunisia</footnote>

<footnote>(3) Departamento de Física, Universidade Federal de Pernambuco, 50670-901 Recife, PE, Brazil</footnote>

<footnote>(*) Tel: (33) 2.41.73.54.26, Fax: (33) 2.41.73.52.16, email: georges.boudebs@univ-angers.fr</footnote>
called B band). Figure 1(b) shows the spectra of the samples measured at concentration of 10⁻³M. The bands in the region from ≈ 500 nm to ≈ 700 nm are the Q bands that are attributed to transitions associated to the π-π* conjugate system. Also drawn in Fig. 1(a) are vertical lines that indicate the laser wavelengths used in the experiments. In Table 1 we indicated the wavelength corresponding to the maximum of absorption, λ<sub>max</sub>, and the corresponding molar extinction coefficient for each band.

## 2.2 Nonlinear Measurements

The experimental setup used to implement the D4σ-Z-scan technique is described in details in [8]. For the characterization of the nonlinear refraction and absorption, the solution were inside a 1 mm thick quartz cell. The NL measurements were made at 355 nm, 532 nm and 1064 nm. The results are shown in Tables 1 and 2. At 1064 nm the TPP and ZnTPP solutions did not show any NL response even for high concentrations. Experiments were made at 15 GW/cm² in solutions with concentration of 10⁻³ M and for pure chlorobenzene. The measured NL refraction was approximately equal for the Pph solutions and for pure chlorobenzene. It was found n₂=(0.16±0.05)×10⁻¹⁸m²/W and β < 0.02 cm/GW which is smaller than the sensitivity of the apparatus. At 532 nm and I₀ = 8.5 GW/cm², chlorobenzene shows a NL refractive index equal to n₂=(0.35±0.05)×10⁻¹⁸m²/W and β < 0.02 cm/GW. The NL refraction behavior of TPP and ZnTPP is illustrated in Figs. 2(a) and 2(b), respectively; the data presented were obtained at 2.1 GW/cm². The BWRV profile with the peak after the valley indicates that the TPP presents a negative NL refractive index exhibiting self-defocusing at this wavelength. The open-aperture Z-scan data, presented as insets of both figures, illustrate the NLA behavior. These results illustrate clearly the influence of Zn on the NL response at 532 nm: (i) the sign of n₂ is positive for ZnTPP while it is negative for TPP; (ii) the 2PA coefficient of ZnTPP is larger than in TPP. Note that in Table 1 the positive value of n₂ for TPP is due to the low concentration (0.12×10⁻³ M) showing the result of the competition between the positive NL refraction due to chlorobenzene alone and the small amount of TPP molecules dissolved inside.

<figure><img src="image_3.png" /><figcaption>Figure 2: Beam waist relative variation versus z, the sample position at 532 nm. Sample thickness: 1 mm. Pph solution concentration: 10⁻³ M. Laser intensity: I₀=2.1 GW/cm². (a) Sample: TPP (α=1126 m⁻¹); (b) sample: ZnTPP (α=1048 m⁻¹). The insets represent the open aperture Z-scan traces and the blue lines represent the best fit of the data.</figcaption></figure>

At 355 nm the experiments with pure chlorobenzene gives n₂=(0.55±0.1)×10⁻¹⁸m²/W and β=(0.35±0.06)cm/GW. Due to the high linear absorption coefficient (α = 4221 m⁻¹) of Pph solution at concentration 10⁻³ M, the NL measurement at low intensity was not possible because even in the linear regime, in order to detect a signal at the CCD, the needed laser intensity was high enough to induce unwanted NLA leading to non-reliable coefficient values. The measurement was possible starting from concentration of 0.5×10⁻³ with I₀ = 4 GW/cm². TPP and ZnTPP solutions presented positive n₂ (self-focusing) and strong NLA. The measured NL parameters for solutions with different concentrations of TPP and ZnTPP are shown in Table1 for λ = 532 nm and in Table 2 for λ = 355 nm. The 2PA cross section, σ₂, was calculated from β, the NL absorption coefficient, using the relation σ₂(cm⁴·s·photon⁻¹)=10³hνβ/N C, where N_A is the Avogadro number, C is the Pph concentration, h the Planck's constant and ν is the frequency of the incident light. The σ₂ values are indicated in Tables 1-2 in GM units (1 GM = 10⁻⁵⁰ cm⁴·s·photon⁻¹). Note that the values of σ₂ are not dependent on the Pph concentration, as expected.
<table><caption>Table 1. Linear and nonlinear parameters of TPP and ZnTPP (with different concentrations), at 532 nm. $\alpha_{532}$ is the linear absorption coefficient, $n_2$ is the effective NL refractive index, $\beta$ is the NLA coefficient, $I_0$ is the laser peak intensity and $\sigma_2$ is the 2PA cross-section in GM units.</caption><thead><tr><th></th><th>Concentration<br>(×10<sup>-3</sup>M)</th><th>α<sub>532</sub><br>(cm<sup>-1</sup>)</th><th>I<sub>0</sub><br>(GW/cm<sup>2</sup>)</th><th>n<sub>2</sub>×10<sup>-18</sup><br>(m<sup>2</sup>/W)</th><th>β<br>(cm/GW)</th><th>σ<sub>2</sub><br>(GM)</th></tr></thead><tbody><tr><td>Chlorobenzene</td><td></td><td></td><td>2.0 - 8.0</td><td>0.35 ± 0.05</td><td>&lt; 0.06</td><td></td></tr><tr><td rowspan="4">TPP</td><td>1</td><td>11.2</td><td rowspan="4">2.1</td><td>-2.7 ± 0.5</td><td>5.5 ± 0.8</td><td>3.4 × 10<sup>5</sup></td></tr><tr><td>0.48</td><td>5.45</td><td>-0.9 ± 0.3</td><td>2.8 ± 0.4</td><td>3.6 × 10<sup>5</sup></td></tr><tr><td>0.24</td><td>2.7</td><td>-0.26 ± 0.4</td><td>1.3 ± 0.3</td><td>3.4 × 10<sup>5</sup></td></tr><tr><td>0.12</td><td>1.4</td><td>0.19 ± 0.1</td><td>0.7 ± 0.2</td><td>3.6 × 10<sup>5</sup></td></tr><tr><td rowspan="4">ZnTPP</td><td>0.94</td><td>10.4</td><td rowspan="4">2.1</td><td>1.7 ± 0.3</td><td>7.8 ± 0.1</td><td>5.1 × 10<sup>5</sup></td></tr><tr><td>0.47</td><td>5.3</td><td>0.9 ± 0.2</td><td>4.0 ± 0.5</td><td>5.3 × 10<sup>5</sup></td></tr><tr><td>0.23</td><td>2.7</td><td>0.6 ± 0.1</td><td>2.0 ± 0.3</td><td>5.4 × 10<sup>5</sup></td></tr><tr><td>0.11</td><td>1.1</td><td>0.5 ± 0.1</td><td>1.0 ± 0.1</td><td>5.6 × 10<sup>5</sup></td></tr></tbody></table>

<table><caption>Table 2. Linear and nonlinear parameters of TPP and ZnTPP (with different concentrations), at 355 nm. $\alpha_{355}$ is the linear absorption coefficient, $n_2$ is the effective NL refractive index, $\beta$ is the NLA coefficient, $I_0$ is the laser peak intensity and $\sigma_2$ is the 2PA cross-section in GM units.</caption><thead><tr><th></th><th>Concentration<br>(×10<sup>-3</sup>M)</th><th>α<sub>355</sub><br>(cm<sup>-1</sup>)</th><th>I<sub>0</sub><br>(GW/cm<sup>2</sup>)</th><th>n<sub>2</sub>×10<sup>-18</sup><br>(m<sup>2</sup>/W)</th><th>β<br>(cm/GW)</th><th>σ<sub>2</sub><br>(GM)</th></tr></thead><tbody><tr><td>Chlorobenzene</td><td></td><td></td><td>7.6 - 25</td><td>0.5 ± 0.1</td><td>0.35 ± 0.06</td><td></td></tr><tr><td rowspan="4">TPP</td><td>1</td><td>42.2</td><td rowspan="4">4.0</td><td>-</td><td>-</td><td>-</td></tr><tr><td>0.48</td><td>20.4</td><td>2.8 ± 0.6</td><td>6.4 ± 0.8</td><td>1.2 × 10<sup>6</sup></td></tr><tr><td>0.24</td><td>10.2</td><td>1.4 ± 0.2</td><td>3.5 ± 0.3</td><td>1.3 × 10<sup>6</sup></td></tr><tr><td>0.12</td><td>5.2</td><td>0.8 ± 0.1</td><td>1.7 ± 0.2</td><td>1.3 × 10<sup>6</sup></td></tr><tr><td rowspan="4">ZnTPP</td><td>0.94</td><td>18.9</td><td rowspan="4">4.0</td><td>1.0 ± 0.2</td><td>7.3 ± 1</td><td>0.7 × 10<sup>6</sup></td></tr><tr><td>0.47</td><td>9.4</td><td>0.6 ± 0.1</td><td>3.4 ± 0.5</td><td>0.7 × 10<sup>6</sup></td></tr><tr><td>0.23</td><td>4.6</td><td>0.5 ± 0.1</td><td>1.8 ± 0.3</td><td>0.7 × 10<sup>6</sup></td></tr><tr><td>0.11</td><td>2.2</td><td>0.4 ± 0.1</td><td>1.2 ± 0.2</td><td>1.0 × 10<sup>6</sup></td></tr></tbody></table>

In Fig. 3, we summarized the values of the NL coefficients of TPP and ZnTPP at 532 nm and 355 nm as a function of the linear absorption coefficient, $\alpha$ (which is proportional to the Pph concentration). Figure 3(a) shows the variation of $n_2$ versus $\alpha$ for ZnTPP (data in blue color): empty squares at 355 nm, filled squares at 532 nm and for TPP (red data) empty circles at 355 nm, filled circles at 532 nm. At very low concentration (very low $\alpha$), the intersection of the straight lines with the vertical axis occurs for a positive value of $n_2$ corresponding to pure chlorobenzene (within the experimental error). Figure 3(b) shows the variation of $\beta$ versus $\alpha$ for ZnTPP (blue data): empty squares at 355 nm, filled squares at 532 nm and for TPP (red data) empty circles at 355 nm, filled circles at 532 nm. At very low concentration, the intersection of the straight lines with the vertical axis occurs at the value of $\beta$ corresponding to pure chlorobenzene. Notice that the maximum efficiency for optical limiting is obtained at 532 nm. Indeed, for a given $\alpha$ the NLA is higher at 532 nm for both Pphs compositions when compared to the contribution obtained at 355 nm as noticed by the effective slope of the linear fits.

<figure><img src="image_4.png" /><figcaption>Figure 3: (a) Values of $n_2$ versus the linear absorption coefficient for ZnTPP: (blue data) empty squares in the UV (355 nm), filled squares in the green (532 nm) and for TPP (red data) empty circles in the UV (355 nm), filled circles in the green (532 nm). (b) Values of $\beta$ versus the linear absorption coefficient for ZnTPP: (blue data) empty squares in the UV, filled squares in the green and for TPP (red data) empty circles in the UV, filled circles in the green. The dashed and solid lines are the linear fits.</figcaption></figure>

## 2.3 Discussion

To investigate the contribution of the solvent, D4σ-Z-scan experiments were performed with pure chlorobenzene. As expected, no significant NL absorption was detected at 1064 nm and 532 nm; at 355 nm. The measured $n_2$ values show a normal dispersion behavior with respect to wavelengths. This decreasing behavior
with $\lambda$ suggests the high reliability of the $n_2$ absolute values determined via the BWRV measurements [8] for transparent liquids far away from any absorption band. At 1064 nm both Pphs solutions did not show NLA because they are transparent at this wavelength. Although the energy of two laser photons falls into the Q bands region (see Fig. 1b) these bands are relatively weak. For excitation at 532 nm the energy of two photons corresponds to the energy of one photon at 266 nm that implies a large NLA.

Based on the results of [9] studying the same molecules and as in [10] we concluded that the effective third-order susceptibility governing the NL response of TPP and ZnTPP is due to a cascade process $\chi^{(1)}: \chi^{(1)}$ (acting as an effective third-order susceptibility). In this process the first $\chi^{(1)}$ is mainly related to the ground state absorption and the second $\chi^{(1)}$ is mainly influenced by the excited state refraction. The first excitation step is due to the bound electrons in the Pph molecules in the singlet $S_0$. The second step is dependent on the number of molecules in the excited singlet $S_1$ and is related to the change in the refractive index per unit photoexcited molecule. The population of the $S_1$ states will always be small due to their fast relaxation time [9].

Finally, the herein reported NLA coefficients when compared to other values found for organic compounds [11] are much larger. Furthermore the effective 2PA cross section values measured in the commercial Pphs are of the same order of magnitude than those reported for porphyrin supramolecules [12].

### 3. CONCLUSIONS

In summary, the third-order nonlinearity of TPP and ZnTPP was investigated applying the D4σ-Z-scan technique in the ps regime at 355 nm, 532 nm and 1064 nm. Beside a change of the NL refractive index from negative to positive due to the presence of Zn it was measured large NLA coefficients for both compounds. The NL behaviour observed indicates the large potential of TPP and ZnTPP for optical limiting applications because of their large 2PA cross-section of several thousand GM units. The NL response is attributed to an effective $\chi_{eff}^{(3)}$ dominated by a cascade process. The reported results indicate that ground state absorption and excited state refraction dominate in these experiments and therefore linear and effective 2PA strengths are linked.

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