**2. Materials properties**

### **2.1 Norland Optical Adhesive 65 (NOA 65**®**)**

This polymer is typically used for putting lenses in metal mounts, bounding plastic to glass and cold blocking by cured process. The polymer cure process depends on intensity and wavelength of the UV radiation. Before being exposed to UV radiation, the polymer's adhesive is in liquid state because the monomers and photo initiators will not react with each other. When exposed to UV, the photo initiators undergo a change creating free radicals that react with monomers, producing monomer chains. In the cured state, the monomer chains convert to cross-linked polymer chains.

The absorbance spectra of the NOA 65® obtained with an UV-Vis spectrophotometer is show in Fig. 1, where we can observe that its absorbance displays a plateau in the visible region, showing a maximum absorption in 300 nm. Complementary to this plot, Fig. 2 show the spectral transmission for the UV-Vis-IR regions. The plot was obtained from (Norland Products Incorporate, 1999).

Fig. 1. Absorption spectra of NOA 65 in region UV-Vis.

Fig. 2. Transmission spectra for NOA 65.

concentrations between NOA 65 and CV, sample thickness, beams intensity ratio and spatial frequency. The material shows refraction index modulation, which is calculated using the Kogelnik`s theory. The results obtained are show by the behavior of diffraction efficiency

This polymer is typically used for putting lenses in metal mounts, bounding plastic to glass and cold blocking by cured process. The polymer cure process depends on intensity and wavelength of the UV radiation. Before being exposed to UV radiation, the polymer's adhesive is in liquid state because the monomers and photo initiators will not react with each other. When exposed to UV, the photo initiators undergo a change creating free radicals that react with monomers, producing monomer chains. In the cured state, the

The absorbance spectra of the NOA 65® obtained with an UV-Vis spectrophotometer is show in Fig. 1, where we can observe that its absorbance displays a plateau in the visible region, showing a maximum absorption in 300 nm. Complementary to this plot, Fig. 2 show the spectral transmission for the UV-Vis-IR regions. The plot was obtained from (Norland

versus energy.

**2. Materials properties** 

Products Incorporate, 1999).

**2.1 Norland Optical Adhesive 65 (NOA 65**®**)** 

monomer chains convert to cross-linked polymer chains.

Fig. 1. Absorption spectra of NOA 65 in region UV-Vis.

Fig. 2. Transmission spectra for NOA 65.

Table 1 shows some properties of NOA 65 whereas Table 2 shows the typical cure times according to Norland Products instructions.


Table 1. Typical properties of NOA 65.


Table 2. Typical cure times of NOA 65.

Fig. 3 shows the absorbance spectra obtained with a FTIR spectrophotometer showing absorption peaks, indicating the presence of some compounds Table 3 displays brief analysis of the NOA 65 IR spectrum. briefly analysis of the NOA 65® IR spectrum.

Fig. 3. Absorbance spectra in IR region

Norland Optical Adhesive 65® as Holographic Material 27

The mix of NOA 65® and CV dye composed the photosensitive material. We prepare three different concentrations varying the quantities of the NOA 65® and CV as mentioned in Table 4 when the compounds are mixed, we deposit them into different glass cells fabricated with two glass substrates separated by mylar (17 µm thickness), cellophane (27 µm thickness) or mica (110µ thickness), All the thicknesses used in this work were measured with an electronic micrometer. The mixture was introduced into the cell by the gravity

Concentration NOA 65® CV

Fig. 5. Photography of the emulsion between two glasses deposited by gravity technique.

C1 99.95 % 0.05 % C2 99.9 % 0.1 % C3 99.85 % 0.15 %

Fig. 4. Absorbance spectra of CV dye showing a peak at 580 nm.

Table 4. Relation of concentration between NOA 65 and CV.

**2.3 Photosensitive material** 

technique as is shown in Fig. 5

Table 3. IR analysis for NOA 65.

#### **2.2 Crystal violet dye**

The crystal violet dye (**CV**) is a dark green powder soluble in water, chloroform, isopropyl alcohol, but not in in ether and ethylic alcohol. The crystal violet dye can be used as antiseptic and a pH indicator for some substances. Its chemical composition is *C25 H30 ClN3* and molecular weigh 407.98. In Fig 4 we show its absorption spectra showing a peak in the spectral line at 591 nm, making a displacement of the absorption curve towards the yellow and orange color

vibration

Asymmetric vibration

Asymmetric

Asymmetric vibration 2900±45 For Secondary amide (NH-CO) not associated, they have a sharp and big band to 3460-3300

Amide in liquid phase, They exhibit a big band to 3270 and a weak band

2-Propenamide, n,n' methylenebis

metilenebisacrylamide.

interval appear aliphatic groups CH3 and CH2.

characteristic band of the

A double SH due to the intensity of this band has

been considered.

The vibration that appears to 255 corresponds to the vibration of the SH, reported as more

cm-1

Or

thiols.

vibration 2980 In 2982-2877 cm-1

to 3100-3070.

N-H Asymmetric

Wave number Group Strain Note

N-H 3346

CH3- 2982

CH2- 2877

Possible structure

The crystal violet dye (**CV**) is a dark green powder soluble in water, chloroform, isopropyl alcohol, but not in in ether and ethylic alcohol. The crystal violet dye can be used as antiseptic and a pH indicator for some substances. Its chemical composition is *C25 H30 ClN3* and molecular weigh 407.98. In Fig 4 we show its absorption spectra showing a peak in the spectral line at 591

nm, making a displacement of the absorption curve towards the yellow and orange color

4000 3500 3000

wave number cm -1

Possible structure

2982 2877 -CH2 - -CH3

Absorbance (u. arb)

3000 2800 2600 2400

Table 3. IR analysis for NOA 65.

Wave number cm-1

**2.2 Crystal violet dye** 


Absorbance (u. arb)

N-H

3346

3076

Fig. 4. Absorbance spectra of CV dye showing a peak at 580 nm.

### **2.3 Photosensitive material**

The mix of NOA 65® and CV dye composed the photosensitive material. We prepare three different concentrations varying the quantities of the NOA 65® and CV as mentioned in Table 4 when the compounds are mixed, we deposit them into different glass cells fabricated with two glass substrates separated by mylar (17 µm thickness), cellophane (27 µm thickness) or mica (110µ thickness), All the thicknesses used in this work were measured with an electronic micrometer. The mixture was introduced into the cell by the gravity technique as is shown in Fig. 5


Table 4. Relation of concentration between NOA 65 and CV.

Fig. 5. Photography of the emulsion between two glasses deposited by gravity technique.

Norland Optical Adhesive 65® as Holographic Material 29

To record the phase holographic gratings in the photosensitive material we use the experimental setup shown in Fig. 7. We use a beam from a He-Ne laser with emission line at λ=598 nm corresponding to yellow color which is separated into two beams by a beam splitter (BS). The beams are reflected by two mirrors (M1 and M2) toward the photosensitive material (PM) where an interference pattern is formed and recorded in real time by the

In order to realize the measurements of the diffraction efficiency of the holographic grating we use a He-Ne laser as reading beam with emission line at λ=545 nm, which corresponds to green color, it is because this line does not affect the grating´s recording process. The gratings we record in this photosensitive material correspond to phase holographic gratings

The diffraction efficiency is defined as +1 diffracted order intensity and incident beam

Where I1 is the +1 diffracted order intensity and Ii the incident beam intensity. This equation is

The Fig. 8 is a photography of the diffracted pattern produced by the grating recorded on the material with concentration C3, 110 μm thickness sample, an interference angle of 5 degrees

x100 (2)

η( ) % <sup>=</sup> I1 Ii

not considering the Fresnel losses because of the reflection in the photosensitive cell.

**3. Experimental process** 

period of 3 hours continuous.

by refraction index modulation.

Fig. 7. Setup to make diffraction grating in real time.

intensity ratio expressed as percentage as is represented in eq. 2.

Fig. 6 show the absorption spectra in the UV-Vis region of the photosensitive film. The curves correspond to the three concentrations that we prepare and all the concentrations have absorption located in 500 – 630 nm range with a peak in the spectral line at 591 nm.

Fig. 6. Absorption spectra of mix NOA 65 and CV for three concentrations of 110 μm thickness cell.

In Table 5 we show the absorption coefficients for the 110µm thickness sample obtained with the Beer´s law considering the spectrum of Fig. 5. In eq. 1 we write the absorption *A*, as function of the molar concentration *c*, thickness *l*, and the absorption coefficient α.

$$\mathbf{A} = \mathbf{c} \mathbf{l} \alpha \tag{1}$$


Table 5. Absorption coefficient for the mix NOA 65 and CV.

#### **3. Experimental process**

28 Holograms – Recording Materials and Applications

Fig. 6 show the absorption spectra in the UV-Vis region of the photosensitive film. The curves correspond to the three concentrations that we prepare and all the concentrations have absorption located in 500 – 630 nm range with a peak in the spectral line at 591 nm.

Fig. 6. Absorption spectra of mix NOA 65 and CV for three concentrations of 110 μm

function of the molar concentration *c*, thickness *l*, and the absorption coefficient α.

μ

Table 5. Absorption coefficient for the mix NOA 65 and CV.

In Table 5 we show the absorption coefficients for the 110µm thickness sample obtained with the Beer´s law considering the spectrum of Fig. 5. In eq. 1 we write the absorption *A*, as

> C1 110 0.004201745 0.001964058 C2 110 0.005589304 0.003213745 C3 110 0.00773117 0.006168739

A = clα (1)

*m)* α (λ=598 nm) α (λ=543 nm)

thickness cell.

Concentration *l (*

To record the phase holographic gratings in the photosensitive material we use the experimental setup shown in Fig. 7. We use a beam from a He-Ne laser with emission line at λ=598 nm corresponding to yellow color which is separated into two beams by a beam splitter (BS). The beams are reflected by two mirrors (M1 and M2) toward the photosensitive material (PM) where an interference pattern is formed and recorded in real time by the period of 3 hours continuous.

In order to realize the measurements of the diffraction efficiency of the holographic grating we use a He-Ne laser as reading beam with emission line at λ=545 nm, which corresponds to green color, it is because this line does not affect the grating´s recording process. The gratings we record in this photosensitive material correspond to phase holographic gratings by refraction index modulation.

Fig. 7. Setup to make diffraction grating in real time.

The diffraction efficiency is defined as +1 diffracted order intensity and incident beam intensity ratio expressed as percentage as is represented in eq. 2.

$$\text{m(\%)}=\frac{\text{I}\_1}{\text{I}\_i} \times 100\tag{2}$$

Where I1 is the +1 diffracted order intensity and Ii the incident beam intensity. This equation is not considering the Fresnel losses because of the reflection in the photosensitive cell.

The Fig. 8 is a photography of the diffracted pattern produced by the grating recorded on the material with concentration C3, 110 μm thickness sample, an interference angle of 5 degrees

Norland Optical Adhesive 65® as Holographic Material 31

In Fig. 10 we show the diffraction efficiency obtained for all the concentrations again and 27 μm of sample thickness. The maximum diffraction efficiency is for the concentration C3 and

Fig. 10. Diffraction efficiency for the three concentrations and 27 μm sample thickness

Fig. 11. Diffraction efficiency for the three concentrations, C3 show a major efficiency.

its value is 1.1%.

between the beams which according to Bragg Law produces a grating with spatial frequency of 146 *lines/mm* (the spatial frequency of the gratings is defined as the inverse of the period and is measured as lines per millimeter *lines/mm*). The central spot is called zero diffracted order, inside spots are called -1 and +1 diffracted orders (left and right respectively) and outside spots are -2 and +2 diffracted orders (again left and right spots respectively).

Fig. 8. Real time diffraction pattern showing first and second orders.

#### **3.1 Diffraction gratings in real time**

We recorded phase holographic gratings in all material concentrations and thickness and measured the diffraction efficiency. The Fig. 9 shows the diffraction efficiency of the holographic gratings recorded in the 17 μm thickness sample with all concentrations. The diffraction efficiency measurements were taken each 10 minutes after the exposition began. The curves show an increase of the diffraction efficiency concerning exposure energy obtaining η=0.53% as maximum for the concentration C3 (see table 4). These measurements were taken in real time for the +1 diffraction order only.

Fig. 9. Diffraction efficiency vs. energy of the three concentrations.

between the beams which according to Bragg Law produces a grating with spatial frequency of 146 *lines/mm* (the spatial frequency of the gratings is defined as the inverse of the period and is measured as lines per millimeter *lines/mm*). The central spot is called zero diffracted order, inside spots are called -1 and +1 diffracted orders (left and right respectively) and outside

We recorded phase holographic gratings in all material concentrations and thickness and measured the diffraction efficiency. The Fig. 9 shows the diffraction efficiency of the holographic gratings recorded in the 17 μm thickness sample with all concentrations. The diffraction efficiency measurements were taken each 10 minutes after the exposition began. The curves show an increase of the diffraction efficiency concerning exposure energy obtaining η=0.53% as maximum for the concentration C3 (see table 4). These measurements

spots are -2 and +2 diffracted orders (again left and right spots respectively).

Fig. 8. Real time diffraction pattern showing first and second orders.

were taken in real time for the +1 diffraction order only.

Fig. 9. Diffraction efficiency vs. energy of the three concentrations.

**3.1 Diffraction gratings in real time** 

In Fig. 10 we show the diffraction efficiency obtained for all the concentrations again and 27 μm of sample thickness. The maximum diffraction efficiency is for the concentration C3 and its value is 1.1%.

Fig. 10. Diffraction efficiency for the three concentrations and 27 μm sample thickness

Fig. 11. Diffraction efficiency for the three concentrations, C3 show a major efficiency.

Norland Optical Adhesive 65® as Holographic Material 33

Fig. 13. Diffraction efficiency in function beams intensities ratio. The best diffraction

Based on the measured values of the diffraction efficiency η (%) the modulation amplitude of the refraction index Δ*n* can be calculated using the Kogelnik´s theory according to the next equation:

<sup>Δ</sup><sup>n</sup> <sup>=</sup> <sup>λ</sup>cosθarcsin <sup>η</sup>

Where *d is the* grating´s thickness, λ is the reading beam wavelength (in this case λ = 545 nm) and θ is the incident angle of the reading beam (θ=0) in this case. The Fig. 14 shows the modulation amplitude of the refraction index for three thicknesses of the photosensitive material composed

by Norland Optical Adhesive 65® (NOA 65®) mixed with crystal violet dye (CV).

Fig. 14. Modulation amplitude for the refraction index for all thicknesses.

<sup>π</sup>d (3)

efficiency is obtained at 1:1 relation.

The Fig. 11 shows the results obtained for the 110 μm thickness sample and all the concentrations. Again we record the different gratings in the same conditions. The maximum diffraction efficiency is η=1.85% obtained at 0.225 J/cm2 using the concentration C3.

It can be observed in the Figs. 9-11, the thickness of the emulsion plays an important role for the grating modulation. We find that the sample of the thickness of 110 μm is adapted to do diffraction gratings because it presents the highest values of diffraction efficiencies.

Another important parameter for the diffraction gratins recorder is the spatial frequency, which can be modified if we change the interference angle between the beams in the setup shown in Fig. 7 this change allows us to obtain gratings with different period according to the Bragg´s law.

In this sense, we change the interference angle between the two beams and record holographic gratings in the 110 microns thickness sample using the concentration C3. The angles were fixed at 5, 10 and 15 degrees and the results are shown in Fig. 12. The higher diffraction efficiencies are obtained when the interference angle between the beams is 5° producing a grating with frequency of 146 *lines/mm* and has a value η=1.85%. For the 10 and 15 degrees interference angle the diffraction efficiency is very low and produce gratings with 292 *lines/mm* and 436 *lines/mm* respectively. These results suggest us that the material is of low resolution.

Fig. 12. Diffraction efficiencies for different frequencies.

Finally, we record gratings in the 110 microns thickness sample using the concentration C3 and the interference angle fixed at 5 degrees but we change the beams intensity ratio using the 1:1, 2:1 and 3:1 relations. In Fig. 13 we plot the diffraction efficiency obtained and, as we can see, the diffraction efficiency for the relation 3:1 and 2:1 are very low. The best choices to record holographic gratings in the proposed material are the 110 microns thickness sample prepared with the concentration C3, 5 degrees between the beams and the intensity ratio 1:1.

The Fig. 11 shows the results obtained for the 110 μm thickness sample and all the concentrations. Again we record the different gratings in the same conditions. The maximum diffraction efficiency is η=1.85% obtained at 0.225 J/cm2 using the concentration

It can be observed in the Figs. 9-11, the thickness of the emulsion plays an important role for the grating modulation. We find that the sample of the thickness of 110 μm is adapted to do

Another important parameter for the diffraction gratins recorder is the spatial frequency, which can be modified if we change the interference angle between the beams in the setup shown in Fig. 7 this change allows us to obtain gratings with different period according to

In this sense, we change the interference angle between the two beams and record holographic gratings in the 110 microns thickness sample using the concentration C3. The angles were fixed at 5, 10 and 15 degrees and the results are shown in Fig. 12. The higher diffraction efficiencies are obtained when the interference angle between the beams is 5° producing a grating with frequency of 146 *lines/mm* and has a value η=1.85%. For the 10 and 15 degrees interference angle the diffraction efficiency is very low and produce gratings with 292 *lines/mm* and 436 *lines/mm* respectively. These results suggest us that the material is

Finally, we record gratings in the 110 microns thickness sample using the concentration C3 and the interference angle fixed at 5 degrees but we change the beams intensity ratio using the 1:1, 2:1 and 3:1 relations. In Fig. 13 we plot the diffraction efficiency obtained and, as we can see, the diffraction efficiency for the relation 3:1 and 2:1 are very low. The best choices to record holographic gratings in the proposed material are the 110 microns thickness sample prepared with the concentration C3, 5 degrees between the beams and the intensity ratio 1:1.

diffraction gratings because it presents the highest values of diffraction efficiencies.

C3.

the Bragg´s law.

of low resolution.

Fig. 12. Diffraction efficiencies for different frequencies.

Fig. 13. Diffraction efficiency in function beams intensities ratio. The best diffraction efficiency is obtained at 1:1 relation.

Based on the measured values of the diffraction efficiency η (%) the modulation amplitude of the refraction index Δ*n* can be calculated using the Kogelnik´s theory according to the next equation:

$$
\Delta \mathbf{n} = \frac{\lambda \cos \theta \arcsin \sqrt{\eta}}{\pi \mathbf{d}} \tag{3}
$$

Where *d is the* grating´s thickness, λ is the reading beam wavelength (in this case λ = 545 nm) and θ is the incident angle of the reading beam (θ=0) in this case. The Fig. 14 shows the modulation amplitude of the refraction index for three thicknesses of the photosensitive material composed by Norland Optical Adhesive 65® (NOA 65®) mixed with crystal violet dye (CV).

Fig. 14. Modulation amplitude for the refraction index for all thicknesses.

Norland Optical Adhesive 65® as Holographic Material 35

Fig. 15 shows the diffraction efficiency and the room temperature during 3 hours. As can be seen, the temperatures have the same behavior and are 25 ºC approximately, the DE has a similar values but temperature fluctuations modifies it behavior. When the temperature is almost constant the DE has a softly grow, but when the temperature has some change, the DE show an anomalous behavior. The same behavior for 27 μm and 17 μm was observed as

Fig. 16. (a). Plot of the Diffraction efficiency to certain temperature during the recording process.

Fig. 16. (b). Temperature behavior during the recording process in the 27 μm thickness cell of two holographic gratings. The red and black line in both plots corresponds to same

is shown in Fig. 16 and 17.

experiment.

#### **4. The temperature as recording parameter**

The diffraction efficiency behavior of the holographic gratings recorded in photopolymers is due to several conditions such as monomer concentration, humidity, temperature, recorder beam intensity, polymerization velocity, mechanical vibrations, etc.

The temperature has an important role during the recording process in our photosensitive material as is shown below. When the material is exposed to interference pattern along 3 hours in small room temperature changes. We measured the temperature each 10 minutes in the photosensitive cell neighborhood and the diffraction efficiency at same time and the results are shown in Fig. 15 we prepare two cells to achieve this proof, so, the red line of the DE in Fig 15(a) corresponds a grating recorded while the room temperature has the behavior showed by red line in Fig. 15(b), using different cell, we record a new grating showing a DE behavior as is indicated by black line in Fig 15(a) while the room temperature has the behavior showed by black line in Fig. 15(b), we repeat this proof for 110 μm, 27 μm and 17 μm thickness cells using the C3 concentration.

Fig. 15. (a). Diffraction efficiency to certain temperature during the recording process.

Fig. 15. (b). State of the temperature during the recording process.

The diffraction efficiency behavior of the holographic gratings recorded in photopolymers is due to several conditions such as monomer concentration, humidity, temperature, recorder

The temperature has an important role during the recording process in our photosensitive material as is shown below. When the material is exposed to interference pattern along 3 hours in small room temperature changes. We measured the temperature each 10 minutes in the photosensitive cell neighborhood and the diffraction efficiency at same time and the results are shown in Fig. 15 we prepare two cells to achieve this proof, so, the red line of the DE in Fig 15(a) corresponds a grating recorded while the room temperature has the behavior showed by red line in Fig. 15(b), using different cell, we record a new grating showing a DE behavior as is indicated by black line in Fig 15(a) while the room temperature has the behavior showed by black line in Fig. 15(b), we repeat this proof for 110 μm, 27 μm and 17

Fig. 15. (a). Diffraction efficiency to certain temperature during the recording process.

Fig. 15. (b). State of the temperature during the recording process.

**4. The temperature as recording parameter** 

μm thickness cells using the C3 concentration.

beam intensity, polymerization velocity, mechanical vibrations, etc.

Fig. 15 shows the diffraction efficiency and the room temperature during 3 hours. As can be seen, the temperatures have the same behavior and are 25 ºC approximately, the DE has a similar values but temperature fluctuations modifies it behavior. When the temperature is almost constant the DE has a softly grow, but when the temperature has some change, the DE show an anomalous behavior. The same behavior for 27 μm and 17 μm was observed as is shown in Fig. 16 and 17.

Fig. 16. (a). Plot of the Diffraction efficiency to certain temperature during the recording process.

Fig. 16. (b). Temperature behavior during the recording process in the 27 μm thickness cell of two holographic gratings. The red and black line in both plots corresponds to same experiment.

Norland Optical Adhesive 65® as Holographic Material 37

Several types of holograms exist; these are of transmission, amplitude, phase, reflection,

A hologram can be done registering the interference pattern intensity between two beams, called a reference and an object beam, in a photosensitive material or holographic film as is

Fig. 18. Basic setup to hologram recording process, the beam reflected by the object is called

In particular, recording the interference pattern between the reference beam and the Fourier transform of an object produces the Fourier holograms. One of the main characteristics of this type of holograms is that the necessary area to record is small compared with other

Fig. 19. Schematically representation for Fourier holograms recording. BS: beam splitter, BE beam expander, CL: Collimating lens, TL: transforming lens, PM: photosensitive material,

types of holograms. The Fig. 19 we depicted the scheme to record Fourier holograms.

object beam and the reflected beam by the mirror is the reference beam.

computer generated holograms, Fourier holograms, etc., (Smith, 1975).

**5. Fourier holograms** 

shown in Fig. 18.

M: mirror.

Fig. 17. (a). Plot of the Diffraction efficiency to certain temperature during the recording process.

Fig. 17. (b). Temperature behavior during the recording process in the 17 μm thickness cell using the C3 concentration of two holographic gratings.

The temperature changes in the cases showed in Figs 15, 16, and 17 are due that we do not have temperature control in our laboratory. It is important to say; the gratings were recorded along different days. We can conclude that the temperature most be constant during the recording process about 25ºC to obtain a softly behavior and the highest DE values.
