1. Introduction

Since Lohmann et al. [1,2] firstly demonstrated the computer-generated holograms (CGHs), they have yielded many applications, such as optical interconnection [3], spatial filtering (optical data processing and optical computing), optical shop testing (generation of reference wave fronts for optical testing) and three-dimensional display [4]. Usually, CGHs are fabricated with standard large-scale integrated circuit technology. The process of fabrication is as follows. First, the designed holographic pattern is transferred onto a mask by using standard photolithography, e-beam lithography, or laser-beam lithography. The next step may involve the transfer of the mask pattern onto a substrate such as glass or a semiconductor material by using ultraviolet light exposure and subsequent reactive-ion etching, etc. Most of the processes must be carried out in a carefully controlled environment and are time consuming.

Recently, material processing using a femtosecond laser has attract great interests as it can achieve high-quality, damage-free processing [5]. Up to now, a lot of high-quality material processing has been achieved by using femtosecond laser ablation. Promising applications using this technique have also been demonstrated for photolithographic mask repair [6], metallic meshes machining [7], etc. Zergioti et al. have reported microetching and microdeposition of CGHs by using UV excimer femtosecond laser [8, 9]. In their researches, the mask was needed to project UV laser onto the substrate.

In this letter, we report the fabrication of CGHs on metal film deposited on glass substrate by a near infrared 800 nm femtosecond laser. After laser selectively etching by material ablation, the desired CGH was written on the substrate. The process required no mask, no pre-or post-treatment of the substrate.

2. Experimental

A commercial regeneratively amplified 800 nm Ti: Sapphire laser (Spitfire, Spectra-Physics) delivering pulses of 120 fs duration with a maximum power output of 700 mW at a repetition rate of 1 kHz was used in our experiments. A metallic aluminum film with a thickness of 200 nm evaporated on a silica glass substrate was used as a sample. The sample was mounted on a three dimensional translation stage, which was controlled by a computer. The laser beam was focused onto the sample by a microscope objective. The average laser power was adjusted by an ND filter. The sample was translated perpendicular to the laser beam.

With the use of the simulated algorithm, the Fourier-type binary holograms were designed. In this experiment, the alphabetic characters “SIOM” were designed with a square pattern of up to 128×128 pixel size and were encoded onto the film by serial writing (pixel-by-pixel) with a 10× microscope objective (NA 0.30).

Fabricated structures were observed by a charge coupled device (CCD) camera attached to an optical microscope and a scanning electron microscope (SEM). The diffraction features of the fabricated structures were evaluated by a He-Ne laser with a wavelength of 633 nm.

 

Fig. 1. The hole structures on metal film deposited on glass substrate fabricated by femtosecond laser ablation with different power (a) 50 mW, (b) 30 mW and (c) 1 mW. The inset is the plot of diameter of ablative spots as a function of the laser power.

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3. Results

Before writing the CGH, we first decided the diameters and quality of the ablative spots on the metal film in order to achieve well-defined patterns. We focused a 10× microscope objective (NA 0.30) onto the surface of metal film with varied laser power ranging from 1 to 50 mW. The ablative hole structures were shown in Fig.1. As can be seen, when laser powers were 50 and 30 mW, the crater structures with ill-defined margin appeared and the glass substrate was damaged (Fig. 1(a) and 1(b)). While when the laser power was down to 1 mW, the well-defined hole was achieved, leaving clear and damage-free glass surface. The inset in Fig.1 shows the diameter of ablated hole as a function of laser power.

 

Fig. 2. Optical and SEM images of a CGH fabricated by the femtosecond laser selective ablation of metal film deposited on glass substrate. (a) Optical image of the CGH; (b) SEM image of the CGH; (c) SEM image of magnified holographic dots in (b); (d) Back-scatter SEM image of magnified holographic dots in (b).

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By optimizing laser parameters, a CGH was written at 1 mW laser power. The optical and SEM images of the fabricated CGH are shown in Fig.2. The structure with 128×128 pixels was serially written within an area of 3×3 mm². The CGH consists of two-type of phase dots, bright represents π and black 0. Dots with a diameter about 10 µm and an interval about 20 µm were encoded onto the film and real hole structure can be confirmed by the back-scatter image (Fig. 2(d)).

For the fabricated structures, both transmission and reflection holograms can be realized. We coupled a He-Ne laser beam into the sample, which was inclined at 45° against the laser beam, the transmission and reflection diffraction patterns with the reconstruction of designed alphabetic characters “SIOM”, as shown in Fig. 3. First order diffracted beams on both vertical and horizontal direction can be observed (Fig. 3(a) and 3(b)). Here, we define the diffraction efficiency as the ratio of the intensity of first-order diffraction to that of incident beam. The efficiency of transmission beams was 6.67%, and that of reflection beam was 2.65%. So, the total efficiency was about 9.32%. It is important that transmission and reflection holograms can be fabricated within a single process. The ratio of transmitted and reflected beams can be controlled by adjusting the laser energy and focusing optics, etc.

In order to improve diffraction efficiency, increasing the thickness of metal film deposited on glass substrate and decreasing the interval of holographic dots should be a feasible method because diffraction efficiency is decided by grating parameter Q, defined by Kogelnik [10].

The grating parameter can be expressed as Q=2πλd/(nΛ ²), where Λ is the period of the gratings, d is the thickness of the diffraction grating, n is the refractive index, λ is the wavelength of the incident beam.

 

Fig. 3. Diffraction patterns with reconstruction of the CGH shown in Fig. 2. (a) Transmitted pattern; (b) Reflected pattern; (c) The experimental scheme for the reconstruction of the CGH. The inset in (c) gives the diffraction efficiency of the CGH.

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4. Conclusions

In summary, CGH have been fabricated on metal film by a 800 nm femtosecond laser point-by-point writing. The CGH can work as both transmission and reflection modes. The total diffraction efficiency of the fabricated CGH was 9.32%. The fabrication process required no mask, no pre- or post-treatment of the substrate. Using this technique, we expect that the novel optical functional components can be developed.