Holographic recording

Prepared As-S thin film samples were used as a medium for classical holographic recording and for laser lithography (laser writing). Some of the exposures were done at IPHT Jena during visit(s) there, due to unique optical equipment available, the rest was done in-house. The holographic recording setup in IPHT Jena was situated on vibrations isolating Melles Griot optical table with air suspension mechanisms. The setup itself is depicted on fig. 17. The continuous wave Ar$^+$ laser COHERENT INNOVA model 304 served as the main recording beam source. The used wavelength was 514.5 nm. The laser was equipped with intra cavity etalon and the gaussian beam diameter was about 1.2 mm. The coherence length of this laser is on the order of tens meters so this factor can be neglected. The beam was controlled by 845 HP-02 remote-controlled electronic shutter system by Newport to prevent touching the optical table. Than the beam height adjusting system comprising of set of two mirrors was used. The Newport beamsplitter was used to split the beam into two. Two halfwave plates were used to control and adjust the outgoing beams ratio. Two Newport spatial filters were used to remove random fluctuations from the intensity profile of the laser beams. The schematic drawing of a spatial filter is on the figure 19. Two lenses were used to make the expanded beams parallel.

Figure 17: Holographic recording setup used at IPHT Jena
Description: 1 Ar$^{+}$laser COHERENT INNOVA 304; 2 He-Ne laser HN-40P-1; 3 Electronic Shutter; 4 Beamsplitter; 5 Spatial Filters; 6 Collimating Lens; 7 Dielectric Mirrors; 8 Iris Diaphragms; 9 Sample stage; 10 Mechanical Shutter; 11 Photodetector Newport 818-SL

All of mirrors (Newport) used in the recording setup were models with extremely high surface flatness ($\lambda/20$) and reflectivity, which results into a low wavefront distortion. They were placed on massive steel mounts to prevent vibrations and any motion. To control the beams diameters last components before the sample stage were two iris diaphragms. A beamblocker was located just behind a sample to protect operators. Beams ratio has been checked regularly and adjusted to be as close as possible to 1:1. Beam intensity was about 100  mW/cm$^2$ in each of two beams. The Newport hand-held Optical Power meter 840-C, with Newport 818-SL visible range silicon detector was used for these measurements. This detector is equipped with EEPROM calibration module compatible with power meter.

Figure 18: Holographic recording setup built at Pardubice
Description: 1 Ar$^{+}$laser ILA-120-1; 2 Electronic Shutter; 3 Beamsplitter; 4 Spatial Filters; 5 Collimating Lens; 6 Dielectric Mirrors; 7 Iris Diaphragms; 8 Sample stage

The setup used in our laboratories is similar to that one used in Jena. Our setup is shown on fig. 18 The whole setup is also placed on an optical table, as vibrations isolating/reducing component is used thick (15 cm) layer of soapy polystyrene. The recording beam source is Ar$^{+}$ laser system ILA-120-1 with water-cooling system by Carl Zeiss corp. The Newport dielectric mirror is then used to change beam direction. The beam is interrupted by 845 HP-02 remote controlled electronic shutter system by Newport to minimize operator-table contact time. On the stable fixed height platform are a Newport beamspliter cube and again two spatial filters to improve beams profiles and qualities.

Figure 19: Schematic drawing of a Spatial Filter [53]

Two achromatic lenses are used to make the beams parallel again. Three additional mirrors are used to redirect beams. All used mirrors are Newport VALUMAX models with dielectric coating and very high ($\lambda/10$) surface flatness to prevent wavefront distortion. Two iris diaphragms are used to control the final diameters of the beams. Various exposure times were tested on both setups as well as various recording angles. Applying equation 17 on page , it is possible to calculate different recording periods obtained.