Focused femtosecond laser pulses have been used to modify the optical properties of glass doped with CdSxSe1-x nanocrystals. Large positive refractive index changes have been observed and exploited for the fabrication of photonic devices. In particular, we report on highly confining optical waveguides and single and multi-layer volume diffraction gratings.
©2007 Optical Society of America
In recent years femtosecond lasers have been widely used for internal modification of transparent materials. The high peak power of ultrashort pulses induces a nonlinear absorption only at the focus of the laser beam, while leaving the rest of the material unchanged . By keeping the intensity just below the optical damage threshold, it is possible to change, locally and permanently, the refractive index of the material maintaining its optical quality . Therefore, a suitable translation of the sample with computer controlled motion stages allows one to fabricate customized refractive index patterns without photolithographic processes. The three-dimensional capabilities of femtosecond laser writing and its inherent simplicity have been proven in the straightforward fabrication of several photonic devices such as optical waveguides [2–5], waveguide splitters, couplers and interferometers [6–9], waveguide lasers and amplifiers [10, 11], Bragg gratings [12, 13] and volume gratings [14–17].
Despite the large number of devices demonstrated, the physical process underlying the refractive index increase is not yet completely understood, partially due to the fact that it is strongly material dependent. In fact, quite different refractive index profiles can be obtained with the same femtosecond laser on different materials [18, 19]. The main limitation of this fabrication technology is the refractive index change that, while suitable for index matching with standard optical fibers , is not comparable with that achievable in high-index-contrast waveguides . Waveguides have been fabricated in a wide range of glasses [3, 5–7, 18, 19], but the maximum achievable index change is however limited to about 1 × 10-2. Up to three times higher refractive index changes have been reported on chalcogenide  and PbS or Ge doped glasses [15,22], but nevertheless the corresponding waveguides were weakly confining and the transparency in the visible limited.
Composite materials consisting of semiconductor quantum dots embedded in a glass matrix show interesting optical properties that are quite different from those of bulk semiconductors . In particular, they show sharp-edge absorption spectra, red-shifted with respect to the undoped glass matrix. The optical properties of such glasses are significantly affected by the nanocrystal parameters, i.e. composition and size, as well as by the conditions of the surrounding glass. These properties can be modified by both preparation parameters and subsequent external factors like irradiation by intense light [24,25].
In this paper we use a femtosecond laser to modify a borosilicate glass with embedded CdSxSe1-x nanocrystals (OG530 glass from Schott Glass Inc., ∼ 1% volume fraction of nanocrystals in the matrix) to study the possibility of achieving higher refractive index changes on such nanostructured glass. The idea behind these experiments is that the amount of structural changes, that a light pulse can generate in a uniform glass without damaging it, is rather limited; as a consequence the refractive index change cannot be too high. The role of the nanoparticles in the glass should be that of enhancing the refractive index change. In fact, the optical properties of a quantum dot strongly vary for even small structural variation thus helping to increase the average refractive index of the nanostructured glass. The choice of the OG530 glass is due to a cut-off wavelength (530 nm) which is sufficiently low to allow some transparency in the visible, but sufficiently high to have a larger nonlinear absorption in the nanocrystals than in the glass matrix in order to favour the contribution of the formers to the refractive index increase. A positive refractive index change as high as 1.8 × 10-2 has been measured in the irradiated regions. Optical waveguides and volume diffraction gratings have been fabricated and optically characterized. A diffraction efficiency of 37% in the first order at 633 nm has been achieved, which is one of the highest values for diffraction gratings fabricated with femtosecond laser pulses [14–17].
2. Femtosecond laser irradiation
The schematic of the set-up used for femtosecond irradiation of the sample is reported in Fig. 1. The femtosecond laser used for the material modification is a diode-pumped cavity-dumped Yb:glass laser, that generates 350-fs pulses at 1040-nm wavelength with energies up to 0.5 μJ and a repetition rate of 600 kHz . The laser beam is focused by a microscope objective (50× long working distance, 0.6 numerical aperture) inside the sample; the latter is translated by a computer-controlled motion stage (Physik Instrumente, M-511.DD).
The OG530 glass is transparent for wavelengths longer than 530 nm. Thus, the linear absorption of the near infrared femtosecond laser light by the substrate is negligible. The high peak intensity achieved by focusing the femtosecond laser pulses, however, induces a nonlinear absorption mechanism consisting of a combination of multiphoton absorption and avalanche ionization . The occurrence of this phenomenon is experimentally indicated by the emission of white light from the electron plasma generated at the laser focus.
A first consequence of the irradiation of the nanostructured glass is a slight darkening of the glass color in the modified region. This is consistent with a red shift of the absorption spectra of the glass, measured after irradiation of the whole sample thickness and reported in Fig. 2 for a 1-mm-thick sample. The variation of the absorption spectrum corresponds to a refractive index variation through a Kramers-Kronig mechanism.
A thorough investigation about the physics of the interaction between femtosecond laser pulses and semiconductor nanocrystals has been reported in Ref. . This study demonstrates that upon femtosecond laser irradiation the nanoparticles do not change significantly their size or shape. The absorption and fluorescence changes are explained in terms of a nonlinear ionization of the nanocrystals with subsequent charge transfer to the glass matrix; this creates a static electric field across the nanocrystal, modifying its optical properties.
3. Waveguide fabrication
Waveguides were fabricated by focusing the femtosecond laser beam into the glass substrates at a 170-μm depth below the surface. The samples were translated with speeds ranging from 20 μm/s to 1.5 mm/s transversally with respect to the laser beam direction in order to obtain straight waveguides at a constant depth below the surface of the sample. The laser pulse energy was kept at the maximum value of 0.5 μJ. The refractive index profiles at 670 nm were studied using a refracted near-field profilometer (Rinck Elektronik, Germany).
The highest refractive index change is observed at a scanning speed of 200 μm/s. Figure 3(a) reports the refractive index profile of a waveguide cross section, written at such speed, showing a maximum index change of 1.8×10-2. The increased refractive index region surrounding the peak has a size of about 3 × 5 μm2 thus resulting in a rather small waveguide region. Guided modes both at 633 nm and at 1550 nm have been observed. Figure 3(b) shows the near field image of the guided mode at 1550 nm acquired with a Vidicon camera (Hamamatsu C-2400). The high index change allows a rather tight confinement of the guided mode (mode field diameter of 4 μm) with respect to waveguides fabricated with the same technique on plain glasses. The coupling losses to standard telecom fibers, estimated from the overlap integral between the fiber and the waveguide modes, are of 0.5 dB/facet. On the other hand, the measured insertion losses are of about 3.8 dB on a 1 cm waveguide, which means that the propagation losses approach 2.8 dB/cm. This figure is rather high with respect to typical propagation losses achieved on standard glasses with the same writing set-up . The reason can be related to the presence of nanocrystals in the glass; these, in spite of their small size (∼ 4 nm), cause light diffusion according to Rayleigh scattering, which is proportional to the size of the scattering element to the power of six. For such reason these waveguides can be employed only for short distance propagation but their high confinement and 3D deployment make them potentially interesting for high density optical circuits.
4. Volume gratings
The high refractive index change obtained in this nanostructured glass is also very interesting for the fabrication of diffractive structures. In fact, the high refractive index change yields a high diffraction efficiency, while the 3D capability of the fabrication technique allows to place the diffraction gratings directly in the volume and to easily stack more than one of them. On the other hand, Rayleigh scattering is negligible due to the very short propagation path in the material.
Much work has been done in the past to model the diffraction properties of refractive-index modulated gratings, and different theories apply according to the Q parameter of the grating, defined by Kogelnik  as:
where λ is the wavelength of the incident laser beam, d is the grating thickness, n 0 is the average refractive index and Λ is the grating period. In our gratings Q is always less than unit, thus they are defined as “thin”. In the case of a sinusoidal pure phase grating the diffraction efficiency of the m-order is:
where Jm denotes an integer-order Bessel function of the first kind. Two kinds of diffractive optical gratings were produced. The first one was a 2 ×2 mm2 volume grating at a depth of 170 μm below the surface with a line pitch of 10 μm. A microscope picture of this grating is shown in Fig. 4(a) where the homogeneity and uniform periodicity of the grating are appreciable. The transmission diffraction pattern generated by 633 nm laser light and the intensity profile of this pattern are shown in Fig. 4(b) and 4(c) respectively. The diffraction angles are in good agreement with the predictions based on diffraction theory (mλ = Λ sin θ, where θ is the diffraction angle).
The experimental diffraction efficiencies at λ = 633 nm for the zero, first and second orders are reported in Table 1. These values are fitted with theoretical calculations based on Eq.(2), where the refractive index change is taken equal to that presented above for the single waveguide, Δn = 1.8×10-2, and the grating thickness is the free parameter. An excellent agreement is achieved for a grating thickness d = 5.5 μm, giving the theoretical diffraction efficiencies reported in Table 1.
Equation (2) indicates that keeping the same Δn, but increasing the grating thickness to d = 10 μm, would indeed improve the first order diffraction efficiency. The 3D capability of this fabrication technique enables the production of multiple-layers of volume gratings with different distances between layers. In this way, it is possible to obtain a single diffractive layer with a higher depth or a stratified volume diffractive grating (alternating layers of gratings with layers of non-modified material).
To increase the diffraction efficiency, the single grating presented above was overwritten by another identical grating at a different depth z, but at the same x and y positions, in order to generate a double layer 1D grating as schematized in Fig. 5(a). The z-displacement between the two single layer gratings is 4.5 μm, which gives a “single” grating with a thickness in depth of 10 μm. The second layer was written closer to the surface than the first one, in order to avoid the effects of the previously modified regions on the subsequent laser writing.
The calculated diffraction efficiencies for this double-layer grating, for an incident wavelength of 633 nm, are reported in Table 1 for the zero, first and second orders. Figures 5(b) and (c) show the experimental transmission diffraction pattern, generated by 633 nm laser light, and the intensity profile of this pattern, respectively. With the double-layer grating it has been possible to concentrate most of the light in the first order with a diffraction efficiency of 37%, which is in good agreement with the calculated one. The prediction of the diffraction efficiency of zero and second orders is less accurate, this discrepancy can be attributed to the fact that the grating is not uniform in depth due to the small overlap between the two layers. In fact, the closest prediction is obtained with an effective depth of 9 μm (see Table 1).
We have reported on the use of femtosecond laser pulses to induce permanent modifications in glasses doped with semiconductor nanocrystals. Such modifications remained stable over a two years time. The presence of nanocrystals provides an enhanced refractive index variation with respect to homogeneous glasses, resulting in an increase as large as Δn = 1.8 × 10-2. Even larger index changes can be foreseen for a higher nanocrystal concentration in the glass.
Exploiting the large refractive index change, highly confining optical waveguides were fabricated, and their optical properties were studied. With the same technique, exploiting its 3D capabilities, single- and double-layer 1D volume gratings were fabricated and characterized. Diffraction efficiencies up to 74% in ± 1 orders were obtained. This work demonstrates that a suitable choice of the substrate material could overcome the main limitation of the femtosecond writing technique, i.e. a limited refractive index change, thus paving the way to engineering of smart substrates specific for this application. The possibility to produce high-index-contrast structures widens the already broad range of devices that can take advantage of the flexibility and 3D capability of the femtosecond writing technique.
We wish to acknowledge partial financial support by Fondazione Politecnico di Milano (Research Program “Advanced materials and devices for Photonics”).
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