We report on the design, fabrication and application of dispersive mirrors (DMs) with a SiO2 sculptured layer. The group delay dispersion (GDD) of the DMs are -200 fs2 in the spectral range from 680 to 920 nm. DMs with a multilayer structure consisting of a SiO2 sculptured layer with a low refractive index of ~1.09 fabricated by glancing-angle deposition (GLAD) on top of a conventional chirped film stack fabricated by dual-ion beam sputtering are designed and fabricated. The GDD oscillations of the DMs could be significantly reduced compared to the conventional chirped film stack without the SiO2 sculptured layer while maintaining a large GDD of several hundred femtoseconds squared and a wide dispersion bandwidth. For the first time, a single DM with a SiO2 sculptured layer has been successfully applied in a fiber chirped system without being used in DM-pair, which can realize the compression of laser pulses to 16fs.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Ultrafast lasers have seen their potential applications in attosecond pulse generation  and material processing . Dispersive mirrors (DMs) which play a pivotal role in generation and manipulation of few-femtosecond pulses are indispensable elements for advancing progress in ultrafast laser science. By simultaneously providing octave-spanning bandwidths and superior dispersion control [3–5], DMs have many applications, such as intra-cavity dispersion compensation in few-cycle lasers [6, 7] and facilitate external, nonlinear pulse compression [8–10].
Although the group delay dispersion (GDD) over unprecedented bandwidths can be controlled by DMs, DM technology suffers from unavoidable GDD oscillations , which may distort or broaden the temporal profiles of femtosecond (fs) laser pulses . Compression and generation of a short pulse requires that destruction of the pulse shape should be prevented and that the spectral phase should be flattened .
The origin of GDD ripples in conventional chirped mirrors can be traced back to impedance mismatches . As pointed out by N. Matuschek, double-chirped mirrors topped with an antireflection coating can provide a smooth GDD because of the interface match between the top layer structure and the external medium . The reflectance of the antireflection coating has to be less than 0.1% over the entire bandwidth in order to minimize the dispersion oscillations [16–18]. A nanostructure with low-refractive-index reduces the jump of refractive index from air to the DM structure, which has better impedance match with the air. For example, a low-index layer of 1.10 is used in DM design, which can obtain smooth GDD of −35fs2 with the residual GDD ripples in the order of ± 5 fs2 . Only theoretical optimization of the low-ripple-DM design with nanostructured layer has been made , a clear experimental demonstration of the fabrication process and test procedure about the low ripple DM with a low-index-layer has never been performed. An essential prerequisite for advancing this area is the ability to design and fabricate an oscillation-free GDD DM when the GDD value grows. To obtain the low-refractive-index layer, a great variety of methods, including sol-gel approach , lithographic process  and GLAD  have been reported. Compared with other fabrication techniques, GLAD offers the unique features of morphology sculpture and composition tenability, which can accurately control the morphologies and nanostructures [23-24].
In our work, we carry out an experimental study to design and fabricate the DMs consisting of a low-refractive-index SiO2 sculptured layer and a chirped film stack that provides a −200 fs2 GDD from 680 to 920 nm. Due to the low-refractive-index sculptured layer, better impedance match with the incident medium can be achieved and the GDD oscillations of the optimum DM can be obviously reduced compared with the chirped film stack without the SiO2 sculptured layer. The GDD value is much larger than the other DMs shared the same bandwidth and the low-refractive-index SiO2 sculptured layer has high homogeneity and accuracy. For the first time, a single DM with a SiO2 sculptured layer has been successfully applied in a fiber chirped system without being used in DM-pair, which can realize the compression of laser pulses to 16fs.
2. DM design approaches
The effect of GDD oscillations on the performance of DMs is fatal. Reducing the spectral ripples of the GDD in a large region of the GDD and a wide dispersion bandwidth is the design problem of the DMs. An improved structure that can dramatically reduce the dispersion oscillations of DMs is designed as shown in Fig. 1(a). The DM structure consists of two parts, the chirped film stack, which is close to the substrate, and the low-index layer, which is adjacent to the air interface.
Nb2O5 and SiO2 are chosen as the high- and low-index materials in our design, respectively. The substrate material is fused silica. The refractive index wavelength dependence of thin-film materials is described by the well-known Cauchy formula:Table 1. And the schematic refractive index profile of DM is shown in Fig. 1(b).
The OptiLayer software , which has powerful optimization algorithms, is used to design and optimize the DM. In the design, the reference wavelength is 800 nm. The target p-polarized reflectance R(p) for the design is 100%, and the target GDD value is −200 fs2 in the spectral range of interest at an angle of incidence of 5°. The gradual evolution algorithm  is utilized to optimize the layer thickness, and the needle optimization algorithm  is used to further optimize the structure to achieve our targets. Firstly, a chirped film stack is designed to meet the targets with multi-cavities structure as shown in Fig. 2(a). The initial DM design with a low-refractive-index SiO2 layer of ~1.09 added on the chirped film stack is shown in Fig. 2(b). And the structure (shown in Fig. 2(b)) is further optimized to achieve the targets.
As a result, a 96-layer DM is synthesized. The layer thicknesses and theoretical GDD of the final design are shown in Fig. 3(a) and 3(b), respectively. The total physical thickness of the coating is 13.3 µm and the physical thickness of the low-index layer is 191 nm. The reflectance exceeds 99.9% in the wavelength region of 680-920 nm. Figure 3(b) compares the GDD oscillations without and with the top low-index SiO2 sculptured layer. The specific optimum GDD value (red curve in Fig. 3(b)), which reaches −200 fs2 with shallow GDD ripples, is shown in the inset of Fig. 3(b). Further, the residual GDD ripples of the optimum DM is in the order of less than ± 0.5 fs2 deviations of GDD values from their nominal values, which is almost no GDD oscillations in the interest wavelength region. The GDD and reflectance curve of the optimum DM are presented in Fig. 3(b) (red curve). The optimum GDD exhibits sharply reduced oscillations compared with those of the mirror without the top low-index layer (black curve in Fig. 3(b)). At the same time, the overall dispersion value meets the design target. The top layer of SiO2 material has a much lower refractive index than typical SiO2 materials, ~1.09 within the design wavelength region. Because of the top low-refractive-index SiO2 layer, a better impedance match with the incident medium can be realized, and the GDD oscillations are dramatically dampened.
3. Fabrications of the DM
The key to reducing the GDD oscillations of a DM is to use a low-refractive-index layer whose impedance matched with the incident medium (air). GLAD has been shown to be capable of producing films with sculpture realizing a low refractive index. A SiO2 sculptured layer with low-index is prepared by electron-beam evaporation using GLAD at a glancing-angle of 88°. The O2 pressure is 2.3 × 10−2 Pa, and the deposition rate is 0.25 nm/s. The electron gun voltage is 6.0 kV, and the current is 150 mA. The deposition rate and thickness of the films are measured by a quartz-crystal sensor placed near the substrate.
The surface morphology and cross-sectional profile of the fabricated low-refractive-index SiO2 layer are shown in Fig. 4. A SiO2 sculptured layer whose deposition thickness is 1000.3 nm as measured by cross-sectional scanning electron microscopy (SEM) is grown on a fused silica substrate. The refractive index of the SiO2 sculptured layer is low, ~1.09 within the visible spectrum, which is fitted by the OptiChar software . The glancing-angle-deposited sculptured layer provides a solid foundation for subsequent fabrication of the DM.
The underlying chirped film stack is fabricated by dual-ion beam sputtering with time control, which provides high precision and excellent repeatability. Then a SiO2 sculptured layer with the same deposition parameters as the SiO2 layer shown in Fig. 4 is deposited on the chirped film stack in another deposition run, by means of the GLAD technique. After two deposition runs of dual-ion beam sputtering and GLAD, the integrated DM is fabricated, whose SEM profile is shown in Fig. 5. The cross-sectional view clearly shows that the glancing-angle-deposited SiO2 layer is located on the chirped film stack, which consists of interleaved high- and low-refractive-index materials, forming a buffered interface between the chirped film stack and the incident medium. The thickness of the top SiO2 layer is 192.3 nm, according to the SEM measurement, which is consistent with the design value.
4. Experimental results
The optical properties of the mirrors are fully characterized by measuring the reflectance and GDD. The reflectance spectra are measured by a PerkinElmer spectrophotometer (Lambda-1050) between 600 and 1600 nm and the GDD values of the DM are extracted from measurements made by a white-light interferometer .
As shown in Fig. 6(a), excellent correspondence between the theoretical and experimental reflectance data is achieved. The GDD values of the fabricated DMs before and after GLAD of the low-index layer are shown in Fig. 6(b). The black and red curves represent the practical effective GDD before and after deposition of the top low-index layer, respectively. Further, the theoretical GDDs of the design are shown by the dashed orange and green lines. It is obvious that the GDD oscillations decreased significantly, providing reliable evidence that the DM design approach minimizes the GDD oscillations of DMs well. In addition, we compare the measured GDD with the designed GDD of our prototypical DM. The residual ripples of the measured GDD are with ± 56.37fs2 root mean square relative deviations of GDD values from their nominal values, which is within allowable error range.
5. Compression application of the DM
To test the utility of the DM, we apply it into a fiber system for compressing pulses. The schematic layout of the experimental setup is shown in Fig. 7. The seed pulses are delivered by a Ti: sapphire laser of 1kHz repetition-rate pumped by pulsed light. They are coupled into 1m long hollow-core fiber filled with 250mbar argon for spectral broadening. After that the pulses are compressed with the DM. The pulse duration is measured by a homemade frequency-resolved optical gating (FROG).
In the compressor, instead of conventional gratings or fibers, the DM with a GDD of about −200 fs2 is employed to compress the pulses. A total of the DM for an angle of incidence of ~5° is utilized to compensate the GDD of −400 fs2 with 2 reflections. The function of Ag mirror is only to act as a reflection lens, it will not introduce extra dispersion. The compressed pulses are characterized by the homemade FROG. Assuming a Gaussian pulse shape, the pulse duration is at a level of 16 fs FWHM (full width at half maximum), as shown in Fig. 8.
The DM compressor has successfully compressed the broadened pulses to 16fs. It’s easier to adjust the angle of a single DM to meet the angle of incidence than the DM-pair. Fewer mirrors need to be adjusted, which reduces the errors. The advantages of the DM compressor in the fiber system is the alignment insensitivity and the flexibility that the DM-pair lack of, which reduces the difficulty of system adjusting efficiently.
The suppression of dispersion ripples in DMs is a key issue in the fabrication and application of mirror coatings in ultrafast lasers. In this paper, we report the entire design–fabrication–application chain of DMs with a SiO2 sculptured layer and a large GDD of −200 fs2 in the spectral range of 680 to 920 nm. A detailed experimental study about optimizing, fabricating and applying DMs consisting of a low-refractive-index sculptured layer and a chirped film stack is demonstrated. The low-refractive-index sculptured layer plays an important role in matching the impedance with the incident medium and GDD oscillations can be minimized evidently. The main advantage of the DMs is that, while a large GDD of several hundred femtoseconds squared and a wide dispersion bandwidth are maintained, smaller GDD oscillations can be obtained as compared to that obtained by the conventional complementary-pair approach and other single-DM approaches. DMs prepared by this method can be used under a wider range of conditions. For the first time, the single DM with a SiO2 sculptured layer has been successfully applied in a fiber chirped system without being used in DM-pair for the compression of laser pulses to 16fs. It provides tremendous space for improving the performance of DMs in ultrafast laser systems.
National Natural Science Foundation of China (NSAF) (U1630140); Youth Innovation Promotion Association, Chinese Academy of Sciences (CAS) (2017289).
We would like to thank Professor Ping Ma for providing the measurement apparatus.
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