1. Introduction

Alkali halide crystals containing color centers (CCs) are well known active media in optically pumped tunable solid state lasers. Among such crystals, lithium fluoride (LiF) is particularly interesting as it can host laser-active defects that are both stable at room temperature and also provide emission in the visible and near-infrared [1–3]. As active stripe waveguides based on this technology have the potential to provide laser emission, tunable across the visible spectrum, considerable efforts have been made at developing efficient fabrication methods. Both color centers and waveguides have been fabricated by keV electron beam lithography [4], high intensity ion beams [5], as well as, gamma and X-ray radiation [6]. Although these traditional approaches are well established in industry, they limit material processing to the region near the surface of the sample.

Recently, an alternative approach was introduced that allows one not only to write waveguides parallel but also perpendicular to the surface of the host medium. In particular, femtosecond (fs) pulse lasers allow high band gap materials to absorb multiple photons at one time, thus generating color centers and refractive index changes with sub-micrometer spatial resolution. This ability to create three-dimensional objects allows various types of periodic structures such as diffractive optical elements, photonic bandgap material, pattern gratings in fibers and three-dimensional data storage to be fabricated [7, 8]. Femtosecond pulse lasers have been demonstrated to provide a direct, rapid and cost-effective means for inducing permanent index changes in glass and crystal [9, 10].

Most recently, fabrication of color center waveguide and grating in lithium fluoride was reported by the production of F₃ ⁺ center with focusing high-intensity near-infrared femtosecond laser pulses inside the lithium fluoride [11, 12]. Nonlinear ionization is the starting mechanism for color center formation, In order to match the LiF 11.8 eV band gap energy, an eight 800 nm photon process is necessary. It mean that high laser power is necessary(color center formation threshold in LiF around 2 TW/cm² [13]).

In this paper, we present the results of an experimental investigation in which active stripe waveguides and am active beam splitter have been fabricated by a fs Ti:sapphire laser in LiF crystal. Physical mechanism is not based on the production of color centers F₃ ⁺ and F₂ but on the conversion F₂ into F₃ ⁺ color center. A two-photon process is enough to induce this conversion. This single-step process should allow the development of three-dimensional integrated optics in bulk LiF crystal. Our results show it is possible to fabricate tunable integrated optical lasers and amplifiers by a single femtosecond laser beam lithography process in LiF crystals.

2. Experiment

A single LiF crystal (6x10x10mm³) was first colored by irradiation with a MeV electron beam. Waveguides were then fabricated by focusing the output of a Ti:Sapphire oscillator and regenerative amplifier (λ=800nm, pulse duration = 120fs, repetition rate = 1kHz, Spitfire, Spectra Physics,USA) into the crystal. A computer controlled three-axis translation stage (100nm resolution, Pi Inc, Germany) was used to move the LiF crystal between pulses.

During the writing of a waveguide, the crystal was translated parallel to the excitation beam at a rate of 1 mm/s (see Fig. 1). Waveguide formation was monitored by focusing a 405nm laser diode (Intensity<20mW, 1.6nm spectral width) co-linearly into the crystal. The resulting fluorescence was imaged onto a CCD camera. A commercial fluorescence spectrophotometer (Edinburgh instruments, Oxford, UK) was used obtain spectral information about before and after device formation. A CCD camera (CCD) monitors fluorescence generated by the laser diode. Waveguides were fabricated by moving the crystal in the positive z-direction (away from the laser beam). Two different lenses were used to focus light onto the crystal. Narrow waveguides were fabricated using a long working distance (WD=13mm) objective lens (NA=0.55). Broad (∼30μm) waveguides were fabricated using a standard 100mm focal length lens.

 

Fig. 1. Schematic diagram of the experimental configuration. A dichroic mirror (DM) is used to combine the output from the high peak power Ti:Sapphire laser (used for writing) and a low power short wavelength laser diode (used to monitor waveguide formation). A lens (F) co-focuses the beams into the LiF crystal which is mounted on a three dimensional translation stage.

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3. Results

Before laser processing, the crystal has an absorption and emission peak of 450nm and 680nm respectively, due to F₂ center generation by the MeV electron beam. After irradiation by the fs laser, the irradiated region becomes yellow and green. Coupling of a λ=405nm laser diode into this area results in strong yellow and green light emission (Very little emission is seen in the red). This is seen in Fig. 2 where microscope photographs of the fluorescence generated by two active structures –waveguide, beams splitter of waveguide – are presented. In each case the sections of the waveguide farthest from the beam are created first so as to ensure that the waveguide is of constant diameter along its width. (If sections of the waveguide closest to the laser source are created first, then the channel width decreases as one moves further from the source [14]). In addition, the characteristics of the waveguide itself were seen to depend strongly on the pulse energy of the irradiated pulse and the scan speed. Overexposure in the focal volume led to micro-explosions, void-like structures and cracks. Not unexpectedly, such structural damage was seen, as has been previously observed for passive waveguides [8], to deteriorate the waveguide’s characteristics. For the experiments performed here, pulse energies of 0.01mJ and scanning speeds of 1mm/s were found to be optimum. It should be noted that care was also taken to ensure that the focal spot of the fs laser beam remains within the crystal during irradiation, since, once the focus is on the surface of the crystal, surface damage occurred immediately.

 

Fig. 2. Images of the fluorescence generated by a 405nm laser coupled into structures written in LiF: F₂ crystal by a femtosecond laser. Each 100fs, 0.1mJ laser pulse is focused into the crystal using a NA=0.55 objective lens (see Fig. 1). (left) waveguide, (right) splitter. The intensity of the 405nm laser coupled into the waveguides was <1mW, while that into the splitter was <20mW. During fabrication, the crystal was moved at a rate of 1mm/s away from the laser beam.

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Color centers are generated in the complete crystal by writing a two dimensional matrix of waveguides separated by 30μm laterally and extending through the crystal. Spectra are obtained with 0.5nm resolution at room temperature. The appearance of the peak at 540nm after irradiation demonstrates that a F₃ ⁺ center has been formed by the laser pulse. Figure 3 illustrates the changes in fluorescent spectra as a result of processing by the fs laser beam. Before processing, the spectrum is single peaked at 680nm indicating that the presence of F₂ center. After processing, the contribution to the photo-luminescent (PL) spectra of the F₂ center is greatly reduced being replaced by a new peak centered at 538nm and 630nm. The good fit obtained by summing a Gaussian centered at 630nm and one centered at 538nm is evidence that the main affect of laser radiation is the conversion of F₂ → F₃ ⁺ centers. The peak at 538nm is that expected for an F₃ ⁺ center [5, 11–13]. The peek at 630nm was found in heavily colored sample in 1988 [15], its optical characteristics were similar with F₃ ⁺ center. It was contributed probably to the F₃ ²⁺ center [16]. It should be noted that the F₃ ⁺ center is stable in room temperature – a repeat measurement of the spectrum one month later indicated little change.

Photo-luminescent spectra of the waveguide were found to be strongly polarized. Shining the crystal with a 405nm LD, and then inserting a polarizer between the crystal and eyes, one can observe clearly intense green light at one polarization direction, and intense yellow light at another polarization direction. It could be contributed to the polarization of the excitation light [17], and regular distributions of color centers in the crystal plane [18]. According to our experiment, the intensity irradiated inside the crystal is about 3x10¹³W/cm². The electric field intensity is beyond 10⁷V/m. On the other hand, F₃ ⁺ centers are with charge. So F₃ ⁺ center is oriented regularly in the crystal under the electric field of laser, and its absorption and emission is anisotropic. In addition, the F₃ ⁺ center is in the <111> crystal plane of the LiF lattice, and the F₂ centers in <110> plane. Based on dipole irradiation theory, their fluorescence is dependent on azimuth.

 

Fig. 3. Fluorescence spectra of the crystal before (thick gray curve) and after (thick black curve) irradiation by fs laser pulses. The dotted lines represent the results of fitting the after radiation spectrum to two peaks (one centered at 538nm and the other at 630nm).

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4. Discussion

We would like to discuss the physical mechanism of the color center conversion. F₂ center has been investigated extensively. F₂ center can be ionized by the two-step process based on the fact excitation of F₂ center requires considerably lower energy than the corresponding transition of the F center. The dynamics of the ionization process can be expressed as $F_2+e\to F_2^{(*)}$ and $F_2^{*}+e\to F_2^{+}$ [19]. For F₂ center, which absorbs at 448nm, two photon absorption should generate this process. It is more effective than four photon absorption for F center ionization(5eV) and eight photon absorption for LiF crystal(11.8eV). The increase of the F₃ ⁺ center was been suggested from the F₂ ⁺ center by vacancy capture (F center), expressed as $F_2^{+}+F\to F_3^{+}$ [20]. In our experiment, pulse energies of 0.01mJ and focusing objective of NA=0.25 and pulse width of 120fs were used to create the waveguide. The radius of the waveguide is 5μm, resulting in a color center conversion threshold intensity, It=106.1 GW/cm², a 20 X decrease in threshold intensity compared to the performance of color center generation previously reported [13]

This technology can be used to produce photo-luminescent patterns based on active color centers in LiF crystal. Compared with the way of soft x-ray with copper meshes to produce luminescent patterns [13], femtosecond laser inducing laser active color centers can achieve more fine patterns with sub-micrometer spatial resolution and realize three-dimension images.

5. Conclusion

We have demonstrated that green, yellow, and red emitting active channel waveguide, waveguide arrays and splitter, realized by irradiated of a LiF: F₂ crystal with a near-infrared femtosecond laser, exhibit intense emitting properties in yellow and green spectral range using low pumping intensities. A major advantage of this technique for fabrication photonic elements is its flexibility, simplicity, and high spatial resolution. The results reported are highly promising for the fabrication of miniaturized active devices based on LiF crystal by near-infrared femtosecond laser.