Deposition of nano thick gold film - Laser pulse induced gold nanoparticles gratings

Chapter 5 Laser pulse induced gold nanoparticles gratings

5.1 Deposition of nano thick gold film

In this paper, we report the results of our experimental investigation on the optical properties of gold nano-particle gratings produced by a single shot of a pair Nd-YAG laser pulse beams. In our experimental investigations, the gratings were obtained via a spatially periodic laser heating of a thin gold film. The heating by the laser pulse leads to the formation of a periodic distribution of gold nano-particles. The surface morphology of gold nano-particle gratings was analyzed by using scanning electron microscope (SEM) and dark-filed optical microscope. It was noted that the grating configuration and the geometry of nano-particles were strongly dependent on the fluence of the laser pulse. We also investigated the diffraction property of the nano-particle gratings in the visible spectral regime.

In our experimental investigations, we first deposited a polymethy1 methacrylate (PMMA) layer with a thickness of 2.5 μm on a glass substrate via spin coating. We then thermally deposited a 6 nm thick gold film on the PMMA layer.

Figure 5.1 shows the experimental setup for the creation of a gold nano-particle grating using a Q-switch Nd-YAG pulse laser at 532 nm with a pulse width of 6 ns.

The fluences of the laser pulse were in 70 and 110 mJ/cm².

Figure 5.1: Fabrication process of nano thick gold film on PMMA substrate

Figure 5.2 shows the experimental and theoretical absorbance spectrum of gold film with nano-thickness of deposited. The experimental spectrum is consistent with the theoretical prediction. On the base of this consistent, the deposited thickness is confirmed.

Figure 5.2: (a) Experimental and (b) theoretical absorbance spectrum of gold film.

Experimental setup was shown in Figure 5.3. The incident beam was polarized perpendicular to the plane of incidence (s-polarization) by using a polarizer (P) and a half-wave plate (λ/2). The laser beam was split into two beams by using a beam splitter (BS). These two beams were directed toward the thin film sample by using a mirror (M), creating an interference pattern on the film surface. The angle (θ) between two beams was about 3 degrees. The gold film/PMMA sample was oriented so that the beams enter the sample from the glass substrate. After the exposure with the laser pulse, we examined the nano-particle grating by using a linearly polarized beam of He-Ne laser.

Figure 5.3: Experimental setup for the formation of nano-particle grating via a spatially periodic heating process on a thin gold film. The periodic intensity pattern is obtained via the interference of two Nd-YAG pulse beams.

5.2 Transformation between optical and thermal energy

Upon exposure of the laser pulse, the optical energy deposited in the gold film was converted to thermal energy. This led to an increase of the temperature in the gold film. The temperature gradient in the film plane along the direction of the fringe pattern can be estimated by assuming local heating only. In other words, we can ignore the heat conduction in a non-equilibrium state. This assumption is legitimate as the nano-particles are formed during the extreme short duration of pulse. The spatial intensity (W/m²) distribution of the optical beam at the golf film can be written [11]

⎟⎠ where the I0 is the intensity of each of the incident beams, Λ is the period of the fringe pattern and the x is the coordinate along the direction of the interference fringe pattern.

For a symmetric incidence, the period is given by Λ =λ 2sin(θ/2). For λ=532 nm, a period of Λ ~ 10 μm requires an angle of θ=3 degrees.

Assuming local heating only, the temperature (T) on the surface of gold film during the exposure can be written [12]

2 specific heat (J/kg·℃), respectively. These physical parameters of gold are listed in Table 1. It is important to note that the time t in Eq. (2) is limited to the 6ns-duration

of the laser pulse.

Figure 5.3 shows the spatial distribution of the beam intensity and the temperature on the target surface. We note that an estimated maximum surface temperature of 1450 and 2278 ℃ at the target surface were obtained from the laser pulse of 70 and 110 mJ/cm², respectively. These temperatures are in the range between the melting point of 1064 and boiling point of 2660 of gold. This ℃ ℃ temperature range is ideal for the formation of nano-particles. Due to the extremely short duration of the laser pulse, the energy deposited in the film leads to a non-equilibrium distribution of the thermal energy, resulting a rapid increase of temperature at the high optical intensity locations (bright fringes).

Figure 5.4: Spatial distributed optical density and temperature along interference pattern.

As a result, the gold film in the bright regions is converted into nano-particles, while the gold film in the dark regions remains intact. Thus, the exposure of the gold film with a periodic intensity pattern leads to the formation of a nano-particle grating.

This is illustrated in Figure 5.5

Table 5.1: Physical thermal properties of gold material at 25 ℃

Thermal conductivity(K) Density (ρ) Specific heat (C) Thermal diffusivity( k) 3.17 W/(cm· )℃ 19.32 (g/cm³) 0.128 J/(g· )℃ k=K/C (cm²/s)

Figure 5.5: Spatial distribution of the surface temperature due to the periodic variation of the incident beam intensity on the gold film.

In our experimental investigations, the PMMA layer played an important role in the formation of gold nano-particles. The absorption of optical energy at 532 nm in the PMMA layer is virtually zero. Furthermore, the PMMA layer also plays the role of an insulating layer preventing the flow of heat into the glass substrate. This ensures that the optical energy is deposited at the interface between the gold film and the PMMA layer.

5.3 Gold nanoparticles gratings

In our experimental investigation, we employed laser pulses at two different fluences (70 and 110 mJ/cm²). Figure 5.6 shows the scanning electron micrograph and dark-field optical micrograph of gold nano-particle grating induced by a laser pulse with a fluence of 70 mJ/cm². As shown in Figure 5.6 (a), the grating consists of a periodic bands of gold nano-parrticles, separated by a periodic array of stripes of gold film. In the region of stripes of gold films, the lower thermal energy due to the destructive interference of optical wave was insufficient to raise the temperature of the gold film to form gold nano-particles. The size of gold nanoparticles is in the range of ~ 100 nm in diameter, with a variation of about 20%. Several gold nano-particles were found on the stripes of gold film. They may be a result of sputtering from the nano-particles formed in the adjacent high-temperature regions.

The few gold nanoparticles appeared on this non-heated zone and the presence of gold films are consistent with the assumption of local heating with a rapid raise of the local temperature.

Figure 5.6: (a) Scanning electron and (b) dark-field optical micrograph of gold nano-particle gratings induced with a Nd-Yag laser pulse with a fluence of 70 mJ/cm².

Figure 5.6(b) shows the dark-field optical micrograph of the grating. The bright spots represent strong scattering of light from the gold nano-particles. The enhanced scattering of light at selected wavelength can be due to the excitation of localized surface plasmons in the gold nano-particles. The figure also shows that the nano-particles are indeed formed in the spatial regions of bright fringes. The few nano-particles found in the regions of dark fringes are possibly due to sputtering from neighboring bands of nano-particles.

Figure 5.7: (a) Scanning electron and (b) dark-field optical micrograph of gold nano-particle gratings induced with a Nd-Yag laser pulse with a fluence of 110 mJ/cm².

5.3.1 Diffraction Property of gold nanoparticles gratings

Figure 5.7 illustrates a similar set of micrographs showing the grating configuration using a laser pulse with a fluence of 110 mJ/cm². At such a higher fluence level, the residual energy in the regions of dark fringes can be sufficient to raise the local temperature so that nano-particles are formed. As a result, nano-particles are formed in both the bright regions and dark regions. This leads to a

nano-particle grating with alternating high and low nano-particle populations. In this case, the excitation of surface plasmons occurs in both regions of the grating.

Figure 5.8: First order diffraction efficiency of gold nano-particle gratings as a function of probe wavelength.

We also investigated the diffraction property of the nano-particle grating formed with two different fluence levels. A linearly polarized beam of monochromatic light was directed toward the gold nanoparticles grating. The polarization state of the incident beam was perpendicular to the x-direction. The first-order diffraction spot was measured by using an optical detector. The monochromatic beam was obtained by using a monochromator in conjunction with a tungsten lamp. Figure 5.8 shows the first order diffraction efficiencies of gold nano-particle gratings formed with laser pulses of fluence levels 70 and 110 mJ/cm². The diffraction efficiencies were measured in the spectral range from 450 nm to 750 nm. The two diffraction spectra are significantly different. Maximum diffraction efficiencies occur at around 680 and 620 nm, respectively. The significant difference can be due to the difference of the population distribution of the nano-particles in the two gratings. Since the presence of nano-particles of gold can lead to the excitation of surface plasmons and the resulting absorption and scattering of light, the nano-particle gratings can be modeled as an amplitude gratings.

Figure 5.9: Absorbance of a thin layer of gold nanoparticles with a size (diameter) of ~ 100 nm. The insert shows the SEM of an array of gold nano-particles distributed on the PMMA layer.

Figure 5.9 shows the measured absorbance (defined as A = - log (T/T0), where T and T0 are, respectively, the transmittances of the sample with and without the presence of a thin layer of 2D array of gold nano-particles on the PMMA layer) as a function of wavelength. The insert in Fig. 5.9 is the SEM of an array of the gold nano-particles. The density and size of the nano-particles are typical of that presented in the bright zone of Figure 5.6 (b). We note that the absorbance spectrum consists of a broad absorbance band with a peak at around 630 nm. The broad absorption band at 630 nm is possibly due to the excitation of surface plasmons on the gold nano-particles. The nano-particle gratings are due to the periodic variation of the population of nano-particles. In addition to the variation of population of nano-particles, the averaged size of the nano-particles can be spatially dependent. A periodic modulation of the scattering and/or absorption leads to an optical grating with a first order diffraction efficiency which peaks at around 630 nm. Further analysis is needed to include the size distribution, the presence of a phase grating due to the difference in the phase shift in the two regions of nano-particles.

In summary, we have carried out experimental investigation on the formation of gold nano-particles via the laser ablation of a thin film. Using a periodic variation of the laser intensity, we obtained nano-particle gratings via a single shot of Nd-YAG

laser pulse. The spatial variation of the local heating leads to a periodic variation of the population of nano-particles. We also investigated the diffraction property of the nano-particle gratings. The diffraction spectra show a strong wavelength dependence, possibly due to the presence of the gold nano-particles and the excitation of surface plasmons. The method of the formation of the nano-particles via the illumination of laser pulses may provide a fast and simple fabrication processes of metal nano-particle devices.

References

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“Photoinduced Conversion of Silver Nanospheres to Nanoprisms,” Science 294, 1901 (2001) 2. J. J. Mock, M. Barbic, D. R. Smith, D. A. Schultz, and S. Schultz, “Shape effects in plasmon

resonance of individual colloidal silver nanoparticles,” J. Chem. Phys. 116, 6755 (2002)

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Chapter 6

Surface plamson induced extra diffraction band of cholesteric liquid crystal

Liquid crystal (LC) material has been applied as environment of the metallic material to modulate the surface plasmon effect [1-3]. It is well-known that the LC materials characterized with the outstanding electrical-switching and optical birefringence. When applying a voltage on the LC cell, the reorientation of LC molecule affects the surface plasmon effect of metallic nanoparticle in the cell. One indium-tin-oxide (ITO) glass plate of the cholesteric liquid crystal (CLC) cell was covered with silver nanoparticles. With the application of a proper voltage, a well formed phase grating was constructed in the CLC cell. The CLC grating was probed by the polarized-monochromatic beam. In this study, we found the enhanced optical diffraction in the CLC grating.

6.1 Fundamental optical property of cholesteric liquid crystal

In this section, I will introduce the fundamental optical property of cholesteric liquid crystal and his preparation. When applying a proper voltage on the CLC cell, a well grating structure can be obtained in the cell. The diffraction pattern and theoretical analysis of diffraction efficiency are also discussed in this section.

6.1.1 Optical birefringence

Figure 6.1 and 6.2 show the correlation between light polarization and effective refraction index. If the polarization is parallel to the long axis of LC molecule, the extraordinary refraction index is available. If the polarization is perpendicular to the long axis of LC molecule, it would be ordinary refraction index. The polarization dependent effective refraction index leads a potential modulation on the optical constant of material.

Figure 6.1: Optical birefringence property of liquid crystal is dependent on the light polarization

Figure 6.2: Different angle between polarization and LC molecule lead to a variation of effective refraction index.

6.1.2 Diffraction property of cholesteric liquid crystal grating

Figure 6.3: With an application of a proper voltage, an ideal cholesteric liquid crystal grating is constructed in the CLC cell.

As a result of the helical twisting of the liquid crystal director, the dielectric property of the intrinsic CLC phase grating can be written [14]

⎟⎟ note the dielectric tensor is a periodic function of y. The helical twisting leads to a periodic variation of the dielectric tensor. The dielectric tensor in Eq. (1) can be conveniently written term representing an index grating which is responsible for the diffraction. Using Eqs.

(1, 2) we obtain

We now consider the incidence of a beam of polarized light along the z-axis.

The electric field of the incoming beam at the surface of the LC cell (z=0) can be written

where Eo is the amplitude, θ is the angle of polarization measured from the x-axis.

Inside the CLC medium, the incoming beam generates two modes of propagation at different speed. The electric field in the CLC medium can thus be written

where the first term represents an extraordinary wave Ee and the second term

represents an ordinary wave Eo.

To understand the diffraction property of the CLC grating, we consider the polarization (ΔP) produced by the electric field in the LC cell due to the presence of the CLC grating

Since ΔεEo=0, the ordinary wave is not interacting with the CLC index grating. The ordinary wave is polarized along the y-axis. The electric field Eo is perpendicular to the director of the liquid crystal, regardless of the twisting of the director. As a result, there's no index grating for the ordinary wave. The ordinary wave will propagate through the CLC medium unaffected by the CLC grating.

For the extraordinary wave, the diffracted wave (1st order) can thus be written (in Raman-Nath regime)

modulation of the index grating.

The diffraction efficiency (1st order) can thus be written

It is noted that the diffraction efficiency proportional to the square of Bessel function of the depth of phase modulation results in that proportions to the square of the cosine of the orientation angle of the polarization state. The diffraction efficiency is predicted to decrease as θ increases. In the Raman-Nath regime, the diffraction efficiency of high orders can be written

6.1.3 Diffraction pattern and diffraction efficiency of CLC grating

The CLC grating was probed by a Nd-YAG laser of 532 nm. The diffraction pattern in Figure 6.4 shows that the intensity of even diffraction orders is generally stronger than that of odd diffraction orders (up to m=4). The measured diffraction efficiencies are in good agreement with the results of our vector analysis. The value of δ was estimated as 9.91 with Δn = 0.1087, λ=532 nm, and d=7.7 μm. The Raman-Nath diffraction behavior was also verified due to the Q factor (^Q=²πλd nΛ²) of ~ 0.196, where the grating spacing Λ= 9 μm and the mean refractive index n = 1.627 [17, 18].

Figure 6.4: CLC grating was probed by a Nd-YAG laser of 532 nm, the calculated efficiency of ±4 diffraction order was shown in below.

In addition, we also investigate the diffraction efficiency of the CLC grating by using a polarized monochromatic beam in spectral range from 450 to 750 nm. In our experimental measurements, we employed a tungsten lamp and a monochromator.

The bandwidth Δλ of probe monochromatic beam was about 2.5 nm and the scanning rate was set 5 nm per step in the measurement. The diffraction signal collected in the optical detector was enhanced by using a locking amplifier.

Figure 6.5 shows the experimentally measured first-order (m=1) diffraction efficiency as a function of wavelength at the polarization angles of 0, 30, 60, and 90 degree (measured from the x axis). At the polarization angle of 0⁰, there are two peak diffractions at around 675 and 530 nm. The wavelength dependence and the oscillating behavior is a result of the Bessel function dependence on the modulation index

δ = 2 π Δ n

₁

d λ

Figure 6.5: Diffraction efficiency of the CLC grating probed by a polarized beam in spectral range from 450 to 750 nm.

The diffraction efficiency drops as θ increases. At the polarization angle of 90⁰, the diffraction efficiency should drop down to zero due to the vanishing of the

interaction with the birefringent index grating (according to Eq. (10)). However, in our measurement, a weak diffraction efficiency was still detected. This measured weak diffraction efficiency may be caused by a distortion of the CLC index grating at the boundaries where the liquid crystal molecules are usually strongly anchored by the alignment layer (SiO film).

Figure 6.6 shows the first order (m=1) diffraction efficiency as a function of wavelength at various polarization angles according to Eq. (10). The diffraction efficiency is a Bessel function of the modulation index δ which has a (1/λ) wavelength dependence. Peak diffraction occurs at 672 nm and 538 nm. The simple result of Eq. (9) assumes a uniform lossless CLC grating without considering the boundary anchoring and birefringence dispersion. The dispersion of ne and no as well as the distortion of CLC index grating at the boundaries are possible causes of the discrepancy. Further analysis is needed to include the effect of the molecular anchoring at the boundaries as well as absorption.

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