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We report on the fabrication of Bragg gratings within rib-type waveguides of previously UV-cured inorganic-organic Ormocer hybrid polymers by applying the interferometric phase mask technique in conjunction with deep-UV laser radiation. The fabrication process as well as the influence of the applied laser fluence and the length of the Bragg grating on the characteristics of the Bragg grating’s transmission and reflection spectra are discussed and compared to numerical simulations and calculations. Depending on the applied laser fluence and the chosen grating length, waveguide Bragg gratings with strong reflectivities of up to 98 % and narrow bandwidths of down to 120 pm have been achieved.
© 2016 Optical Society of America
1. Introduction
The specific molecular composition of Ormocer hybrid polymers provides multiple advantageous characteristics in favor for the fabrication of optical elements [1–4]. The pristine oligomer allows for an UV induced radical polymerization and therefore a simple and cost-efficient processing, whereas the cured material features a duroplastic characteristic with high thermal and chemical stability as well as a very good optical transparency [3, 5]. Accordingly, the fabrication of various wafer scale optical elements such as waveguides, couplers and junctions using Ormocers has successfully been demonstrated [6, 7]. Besides these optical elements, wave-guide Bragg gratings represent another important structure of wafer scale optics, since they are versatilely applicable in various devices such as distributed feedback lasers, distributed Bragg reflectors, wavelength-division multiplexing systems [8–10] or as highly sensitive sensor elements as demonstrated by the authors and others elsewhere [11–13]. While there are some reports discussing the fabrication of geometrically structured surface-relief Bragg gratings in Ormocers [14, 15], to our best knowledge, the fabrication of Bragg gratings directly written into waveguides of Ormocers has yet only been demonstrated by the authors in [16]. Herein, the authors reported on the fabrication of waveguide Bragg gratings within embedded channel waveguides of OrmoCore, OrmoClad and OrmoComp, which were applied as highly sensitive temperature sensors. Opposite to surface-relief Bragg gratings, this specific type of Bragg grating is characterized by a periodic and volumetric refractive index modulation directly within the waveguide. This refractive index perturbation can be achieved by highly reproducible fabrication methods, such as interferometric techniques [17], point-by-point inscription [18] or phase mask technology [19], which furthermore allow a variable adaption of the grating period during the fabrication process and thus provide the inscription of even more complex grating structures such as blazed, super-structured or superimposed Bragg gratings [20–22].
In this report, we demonstrate the static phase mask technology in conjunction with deep-UV laser radiation for the fabrication of planar Bragg gratings within rib-type waveguides of already UV-cured OrmoComp hybrid polymers. The rib-type waveguides were achieved by UV-enhanced substrate conformal imprint lithography, a more recent advancement in the field of nanoimprint lithography, which consolidates a high resolution with large-area patterning and thus represents an attractive technology considering an economical production of wafer scale optics [23, 24]. The work emphasizes the waveguide Bragg grating’s fabrication process and the influence of fabrication parameters, such as the applied laser fluence and the length of the grating on the spectral characteristics of the Bragg grating. The results are discussed and compared to corresponding numerical simulations and calculations.
2. Experimental
2.1. Applied materials
The Ormocer hybrid polymer used in this study is OrmoComp (micro resist technology), which was applied as purchased. As substrate material phosphorus doped <100> silicon wafers were applied containing a 2 μm thermally grown SiO2 layer, which was treated by oxygen-plasma in order to firstly clean the substrates and secondly to guarantee a better adhesion of the hybrid polymer on the wafer surface.
2.2. Waveguide fabrication
Rib-type waveguide structures were fabricated using UV-enhanced substrate conformal imprint lithography (UV-SCIL). For the UV-SCIL process, a mask aligner (SÜSS MicroTec MA/BA8) was used, which has been extended by a SCIL imprinting toolset.
The rib-type waveguide structures were fabricated according to the process-chain depicted in Fig. 1.
After the OrmoComp was spin-coated onto the wafer, followed by a 2 minute softbake at 80 °C, the samples were structured and cured by UV-SCIL technology. During the imprint process [Fig. 2(a)], the UV-SCIL working stamp is unidirectionally and stepwise released from the stamp holder by applying a slight pressure of 20 mbar and thus inserted into the Ormocer by means of capillary action. Once the stamp is fully released, the hybrid polymer is cured by UV-light of a mercury vapor lamp at an exposure dose set to 2.4 J cm−2 (measured at i-line). For a better handling, subsequent to a 3 hour hardbake at 150 °C, the samples are cleaved into single chips.
Fig. 2 Schematic illustration of the UV-SCIL process (a) and SEM image of the ridge of a fabricated rib-type waveguide (b).
By this method, various rib-type waveguide structures with 6.5 μm ridge height and a varying slab height and ridge width are generated [Fig. 2(b)].
The UV-SCIL working stamp was fabricated by casting a silicon master structure (manufactured by a combination of photo lithography and reactive ion etching) into a poly(dimethylsiloxane) (PDMS) layer linked to a 300 μm thin glass backplane. A more detailed description of the fabrication of OrmoComp rib-type waveguide structures using UV-SCIL technology can be found elsewhere [25].
2.3. Refractive index modifiability of UV-cured OrmoComp
Due to the UV-SCIL process, the achieved OrmoComp hybrid polymer rib-type waveguides can be considered as fully UV-cured. However, in order to fabricate Bragg gratings using phase mask technology, a refractive index modification of the already UV-cured OrmoComp by further deep-UV irradiation is indispensable.
The photosensitive behavior of the precured Ormocer was therefore investigated by measuring the refractive index of the hybrid polymer irradiated by an increasing 248 nm laser fluence. The samples for this analysis were prepared according to the process-chain depicted in Fig. 3. After the OrmoComp was spin-coated onto the wafer, followed by a 2 minute softbake at 80 °C, the samples were cured by flood exposure using a mercury vapor lamp with an exposure dose set to 2 J cm−2 (measured at i-line). Subsequent to a 3 hour hardbake at 150 °C, the samples were cleaved into single chips of 12 mm × 12 mm dimension.
In order to investigate the refractive index modifiability, the precured Ormocer-On-Silicon samples were irradiated using a KrF excimer laser (Coherent BraggStar S-Industrial), with the laser beam being homogeneously expanded using a micro lens array. By this measure a fully covered illumination of the samples at a single-pulse fluency of FP = 1 mJ cm−2 was provided. For the measurement of the samples refractive indices, a multi-wavelength m-line spectrometer (Metricon 2010/M Prism Coupler) was applied.
2.4. Bragg grating fabrication
For the fabrication of Bragg gratings within the OrmoComp rib-type waveguides, a KrF excimer laser was applied. In the setup, the laser beam (λ = 248 nm, f = 50 Hz) is shaped into a rectangular geometry using a cylindrical lens arrangement and focused through a phase mask onto the precured waveguides at a single pulse fluence of FP = 8 mJ cm−2 [Fig. 4]. Due to the interfering +1 and −1 diffraction orders below the phase mask, a sinusoidal intensity pattern is formed within the waveguide, which leads to a local refractive index increase and hence the Bragg grating [19]. The applied phase masks (Ibsen Photonics) feature a grating area of 10 mm × 10 mm and corrugation periods Λd of 1020 nm and 1036.79 nm. The 0th order diffraction efficiencies of both phase masks are less than 3 %. According to the Bragg grating phase matching condition
where λB represents the reflected Bragg wavelength, neff the modal effective refractive index, m the Bragg grating’s diffraction order and Λ = Λd/2 the Bragg grating period [19, 26], Bragg reflections within the telecom wavelength range are ensured.Fig. 4 Schematic illustration of the Bragg grating inscription process (a) and a differential interference contrast microscopy image of a fabricated Bragg grating (b).
The inscription of the Bragg gratings within the UV-SCIL fabricated rib-type waveguides was carried out with the respective phase mask being in contact with the waveguide structure. The length of the Bragg gratings was defined by applying an amplitude mask of desired aperture on top of the phase mask.
2.5. Bragg grating characterization
Within the scope of characterization, the influence of the applied KrF excimer laser fluence during the fabrication process and the influence of the length of the Bragg grating on the spectral characteristics of the Bragg grating was analyzed.
Therefore, single mode optical fibers equipped with a standard FC/APC connector are butt-coupled and bonded to each facet of the waveguide using a V-groove fiber assembly. The spectral characteristics of the fabricated waveguide Bragg gratings were investigated by applying a combined source and detector interrogation system (Micron Optics sm125-500) which operates at telecommunication wavelengths in the range between 1510 nm and 1590 nm and features a sampling rate of 2 Hz with a resolution of 1 pm.
2.5.1. Influence of the KrF excimer laser fluence
In order to investigate the influence of the KrF excimer laser fluence on the spectral characteristic of the Bragg grating, an UV-SCIL fabricated rib-type OrmoComp waveguide, previously connected to two single mode optical fibers, was placed below the phase mask (Λd = 1036.79 nm) and irradiated, while monitoring the transmitted and reflected spectrum with the interrogation system. The applied laser parameters were chosen to λ = 248 nm, f = 2 Hz, FP = 8 mJ cm−2. The spectral response of the Bragg grating was later correlated to the respective applied laser fluence and the results were compared to corresponding calculation models.
2.5.2. Influence of the Bragg grating length
The influence of the Bragg grating length on the grating’s spectral characteristics was investigated by writing multiple Bragg gratings into a set of parallel oriented rib-type waveguides, while the applied phase mask (Λd = 1020 nm) was diagonally covered by an amplitude mask. By this measure, Bragg gratings of varying length were achieved. The applied laser parameters were chosen to λ = 248 nm, f = 50 Hz, FP = 8 mJ cm−2. By applying 500 pulses, the exposure dose was set to approx. 4 J cm−2. The spectral response was subsequently investigated with the interrogation system and correlated with the respective grating length. The influence of the Bragg grating length on the grating’s spectral characteristics was furthermore simulated by applying the simulation tool RSoft (synopsys) comprising the packages BeamPROP, which utilizes finite difference methods to solve approximations of the Helmholtz equation, and GratingMOD, which is based on Coupled-Mode Theory and the transfer matrix method.
3. Results and discussion
3.1. Refractive index modifiability of OrmoComp
The m-line spectroscopic investigations of the refractive index modifiability of previously UV-cured OrmoComp hybrid polymer by 248 nm KrF excimer laser radiation reveals a saturation-type exponential dependency of the refractive index on an increasing excimer laser fluence [Fig. 5].
Fig. 5 Relative refractive index increase of precured OrmoComp due to 248 nm KrF excimer laser radiation determined by multi-wavelength m-line spectroscopy. The error bars take into account the standard deviation of the single pulse fluence and the refractive indices measured for three independently irradiated samples.
The refractive index was found to rise sharply with increasing excimer laser fluence until, at a laser fluence of approx. 7 J cm−2, the refractive index merges into a saturation. The level of saturation and hence the highest index modification (valid for all wavelengths under investigation) was found to approx. 1.7×10−3. An additionally performed alteration of the light sources’ polarization state during the measurement revealed no detectable index anisotropy (birefringence) of the UV-cured and irradiated hybrid polymer.
Since OrmoComp contains poly(methyl acrylat) units [27] we assume a 248 nm UV-induced split-off of the ester side chain from the main chain to be the driving force behind the refractive index increase as it is analogously described in [28] for poly(methyl methacrylat). However, due to the higher complexity of OrmoComp further molecular mechanisms may contribute to the refractive index modification induced by 248 nm excimer laser radiation. Supplementary investigations are required to deepen the understanding of these mechanisms.
3.2. Influence of the excimer laser fluence on the Bragg grating’s spectrum
Based on the refractive index modifiability of precured OrmoComp, Bragg gratings were achieved within UV-SCIL fabricated rib-type waveguides by applying the static phase mask technology and 248 nm KrF excimer laser radiation.
In order to investigate the influence of the applied KrF excimer laser fluence on the spectral characteristics of the fabricated Bragg grating, the reflection as well as the transmission spectrum of the emerging Bragg grating were monitored during the inscription and correlated to the respective applied laser fluence.
Figure 6 shows the evolution of the Bragg peaks reflectivity η and its −3 dB bandwidth (full width at half maximum, FWHM) Δλ on the applied excimer laser fluence. The reflectivity was determined by means of the transmission dip Δϕt according to (2) as described in [29].
As depicted in Fig. 6, the reflectivity was found to increase quasi instantaneously with increasing excimer laser fluence and saturates at a reflectivity of about 55 % for fluences above 3 J cm−2. Please note, the relatively low reflectivity is caused by an insufficient coupling of the single mode fibers to the rib-type waveguide. The FWHM of the reflected Bragg peak was found to temporarily remain constant at a level of approx. 120 pm. At a laser fluence of 1.2 J cm−2, the FWHM increases instantaneously towards a saturation level of around 240 pm, which is reached at a laser fluence of approx. 6 J cm−2. Since the length of the Bragg grating remains constant, both, the reflectivity as well as the FWHM can decisively be attributed to an increasing refractive index contrast Δn between the illuminated region and the non-illuminated region of the waveguide below the phase mask [26, 30], which is driven by the increasing KrF excimer laser fluence during the inscription process (cf. chapter 3.1).Fig. 6 Reflectivity η and −3 dB bandwidth (FWHM) Δλ of the Bragg grating’s reflection due to an increasing KrF excimer laser fluence.
This assumption is supported by investigations on the Bragg wavelength shift [Fig. 7]. Under consideration of condition (1), the Bragg wavelength represents the effective refractive index neff of the grating structure, which should be proportional to the refractive index contrast Δn. The shift of λB and hence the effective refractive index neff increases almost linearly starting at a laser fluence of 1 J cm−2. At a laser fluence of 8 J cm−2 the shifts of λB and neff were found to be 0.53 nm and 5.1×10−4, respectively. Considering a refractive index increase of mainly the illuminated region of the Bragg grating and the propagating mode’s evanescent field to interact with the waveguide’s surrounding (which is air), the increase of neff lies in good agreement with the results depicted in Fig. 5.
Fig. 7 Reflected Bragg wavelength λB and the power transmitted through the Bragg grating containing rib-type waveguide ϕt due to an increasing KrF excimer laser fluence.
Furthermore, the power transmitted through the rib-type waveguide with included arising Bragg grating was monitored within the wavelength range between 1510 nm to 1590 nm and found to decrease with increasing KrF excimer laser fluence. However, for a laser fluence below 1.5 J cm−2, the transmitted power remains relatively constant, thus at this state, the Bragg grating causes no optical loss. Considering the definition of Bragg gratings as weak (Δn → 0) and strong gratings [31], where weak Bragg gratings only have a small influence on the guided mode [32], the arising Bragg grating can be declared as weak for an excimer laser fluence below 1.5 J cm−2 and with respect to the determined reflectivity [Fig. 6] as strong for an applied KrF excimer laser fluence above 3 J cm−2. The dependency of the transmitted power of the 10 mm long Bragg grating for a KrF excimer laser fluence above 1.5 J cm−2 was found to −0.2 dB/(J cm−2). Consequently, for a strong Bragg grating, fabricated with a KrF excimer laser fluence of 3 J cm−2 the Bragg grating caused optical loss will be 0.3 dB cm−1.
However, above a KrF excimer laser fluence of 8 J cm−2, the development of the reflectivity, the FWHM, the Bragg wavelength and the transmitted power reveal an omnipresent knee in the progressions of all observed parameters [Figs. 8 and 9]. For KrF excimer laser fluences above 8 J cm−2, we found the width of the Bragg reflection, the reflectivity and the Bragg grating caused optical loss to regress, while the Bragg wavelength shift continues to increase but merges into an inferior slope. With respect to the m-line spectroscopic investigations on the refractive index modifiability of precured OrmoComp hybrid polymer [Fig. 5], the Bragg wavelength shift during the Bragg grating inscription process should result in a saturation. This divergent behavior can be explained by the sinusoidal intensity profile of the phase mask caused interference along the waveguide [26]. We assume that both, the supposed illuminated as well as the supposed non-illuminated region of the waveguide below the phase mask experience a refractive index modification, where the refractive index of the illuminated region increases faster than the one of the non-illuminated region, and thus, the Bragg grating’s refractive index contrast Δn initially increases. At an applied KrF excimer laser fluence of approx. 6.5 J cm−2 however, we assume that the refractive index modification of the illuminated region is beginning to saturate (in analogy to the findings depicted in Fig. 5), whereas the non-illuminated region is still being modified. By this means, the Bragg grating’s refractive index contrast decreases again, resulting in the regress of η, Δλ and ϕt
Fig. 8 Reflectivity η and −3 dB bandwidth (FWHM) Δλ of the Bragg grating’s reflection due to an increasing KrF excimer laser fluence (extension of Fig. 6).
Fig. 9 Reflected Bragg wavelength λB and the power transmitted through the Bragg grating containing rib-type waveguide ϕt due to an increasing KrF excimer laser fluence (extension of Fig. 7).
A further indicator suggesting that these relations are driven by a reversing refractive index contrast Δn is the analogy of the development of η and Δλ on excimer laser fluences below 6.5 J cm−2 to the development of η and Δλ on fluences above 6.5 J cm−2. For instance, a laser fluence of 2.5 J cm−2 causes a reflectivity of 0.5 and a FWHM of 180 pm. The same reflectivity and FWHM, both are simultaneously reached at a laser fluence of 16 J cm−2, i.e. the increase and decay of η and Δλ are comparable to each other. However, this relationship does not apply for the regress of the optical loss, which accordingly should be more pronounced if caused only by a reversion of the refractive index contrast. A performed comparison of the rib-type waveguide’s surface subsequent to a Bragg grating inscription with a low and a high excimer laser fluence revealed a periodic modification of the waveguide’s surface, that was exposed to the higher laser fluence. Therefore, we assume that the thus developed surface profile causes a further optical loss which counteracts the regress of the optical loss caused by the descend refractive index contrast of the Bragg grating.
According to [30, 33], both the reflectivity η as well as the −3 dB bandwidth Δλ can be described as a function of the Bragg grating length and the refractive index contrast. Thus, the reflectivity η is expressed as
with the refractive index contrast Δn, the length of the Bragg grating lBG (which remains constant in the present case), the Bragg wavelength λB and the fraction of the guided mode that actually interacts with the Bragg grating M (which we assume to be around 0.35 due to the geometry of the waveguide). The −3 dB bandwidth is expressed as with the refractive index of the waveguide n0, the number of grating planes N (which remains constant in the present case) and a factor indicating the strength of a Bragg grating S (ranging between 0.5 for weak and 1 for strong gratings). Even though the conditions (3) and (4) are actually related to fiber based Bragg gratings, under consideration of the refractive index contrast to be a function of the KrF excimer laser fluence Δn(F) and taking into account the already determined constants, such as the Bragg grating’s length, its period or the expected maximum refractive index modification by KrF excimer laser irradiation, the conditions are utterly applicable to describe the here found relations. The appropriately performed calculations of the Bragg wavelength shift, the reflectivity and the FWHM based on the conditions (3) and (4) are depicted as dashed lines in Figs. 8 and 9.Based on these findings, the assumption of a decreasing refractive index contrast Δn due to an increasing refractive index of the supposedly non-illuminated region of the Bragg grating can be confirmed as the mechanism that leads to the regress of the −3 dB bandwidth and the reflectivity of the Bragg grating.
The investigations on the influence of the applied KrF excimer laser fluence on the Bragg grating parameters during the inscription process, clearly show the possibility of choosing a desired Bragg peak configuration simply by adjusting the applied KrF excimer laser fluence. However, for sensing applications a compromise has to be found between a high reflectivity and a small peak width. Based on our investigations, for a sufficient consensus a KrF excimer laser dosage of 3 J cm−2 is recommended.
3.3. Influence of the grating length on the Bragg grating’s spectrum
In further investigations, the influence of the Bragg grating length on the grating’s spectral characteristics was analyzed. Therefore, Bragg gratings of individual length were written into the UV-SCIL fabricated rib-type waveguides of OrmoComp, with the Bragg grating’s reflection and transmission spectra being subsequently monitored and correlated to the grating length.
As depicted in Fig. 10 we found the reflectivity of the fabricated Bragg gratings to increase with increasing length of the grating. Under consideration of the applied inscription parameters, a reflectivity of up to 98 % can be reached for Bragg gratings with a length of at least 6 mm.
Fig. 10 Reflectivity η and −3 dB bandwidth (FWHM) Δλ of the Bragg grating’s reflection due to an increasing length of the Bragg grating.
Furthermore, a good consistency between the experimentally obtained data and the numerical simulations achieved with the simulation tool RSoft can be found. Furthermore, Fig. 10 depicts the dependency of the FWHM of the Bragg grating’s reflection peak on the length of the Bragg grating, where a decrease of the FWHM with increasing length of the Bragg grating was found. Taking into account the applied inscription parameters, a saturation of the FWHM at approx. 280 pm for a grating length above 7 mm is in good agreement with the bandwidth determined in Fig. 6. As for the reflectivity, the experimentally determined −3 dB bandwidths show a good consistency to the simulated results. The small discrepancy between the simulated and experimental data can be attributed to an imperfect alignment of the single mode optical fiber to the Bragg grating containing rib-type waveguide.
The investigations clearly show the influence of the grating length on the spectral characteristics of the Bragg grating written into the UV-SCIL fabricated rib-type waveguides of Ormo-Comp. Considering the applied inscription parameters, for a Bragg grating with a preferably high reflectivity and a small bandwidth, a grating length above 6 mm is recommended.
3.4. Bragg gratings in OrmoComp rib-type waveguides
Taking into account the previously made investigations, highly reflective Bragg gratings with a narrow Bragg reflection have been achieved within precured UV-SCIL fabricated rib-type waveguides of OrmoComp hybrid polymers. Figure 11 shows the reflection and transmission spectra of an 8 mm long Bragg grating fabricated with a KrF excimer laser fluence of 4 J cm−2. The reflection peak features a small −3 dB bandwidth of Δλ = 238 nm and the Bragg grating’s transmission dip was found to Δϕt = 13.7 dB. Thus, taking into account condition (2), the fabricated Bragg grating features a reflectivity of η = 0.957, i.e. 95.7 %. Hence, by this method, Bragg gratings with a well-defined Bragg reflection have been fabricated, which are utterly sufficient for sensing applications [34].
Fig. 11 Reflection and transmission of a fabricated Bragg grating within a precured UVSCIL fabricated rib-type OrmoComp waveguide.
4. Conclusion
In this report, we demonstrated the static phase mask technology in conjunction with deep-UV laser radiation for the fabrication of Bragg gratings within rib-type waveguides of already UV-cured OrmoComp hybrid polymers. By this approach, Bragg gratings with strong reflectivities of up to 98 % and narrow bandwidths of down to 120 pm have been achieved within the waveguides fabricated by UV-SCIL technology.
The refractive index modifiability of the UV-cured Ormocer by 248 nm laser radiation was confirmed by multi-wavelength m-line spectroscopy, where the maximum refractive index increase was found to 1.7×10−3.
Furthermore, the influence of fabrication parameters, such as the applied laser fluence during the inscription process and the length of the fabricated Bragg grating on the grating’s spectral characteristics was demonstrated. Here, we found the reflected wavelength of the arising Bragg grating, the reflectivity and the −3 dB bandwidth to increase with increasing laser fluence, where the maximum reflectivity was reached at a laser fluence of approx. 3 J cm−2 and the maximum FWHM was reached at a laser fluence of approx. 6 J cm−2. The thereby investigated relationship of the wavelength shift, the reflectivity and the bandwidth on the applied laser fluence could be attributed to an increase of the refractive index contrast of the arising Bragg grating during the inscription process. Calculations on the influence of the refractive index contrast on the wavelength shift, the reflectivity and the FWHM confirmed the experimental results. The additionally monitored transmitted power through the waveguide revealed, that an appropriately fabricated strong Bragg grating is causing an optical loss of approx. 0.3 dB cm−1. The investigated influence of the Bragg grating’s length on the spectral characteristic of the Bragg grating was found to be expressed in an increasing reflectivity and a decreasing −3 dB bandwidth with increasing length of the Bragg grating. A reflectivity of up to 98 % was found for Bragg gratings with a length above 6 mm and the −3 dB bandwidth of the Bragg reflection was found to saturate at approx. 280 pm for a grating length above 7 mm. Numerical simulations on the influence of the grating length on the reflectivity and the FWHM confirmed the experimental results.
In conclusion, we could confirm the feasibility of fabricating Bragg gratings in waveguides of previously UV-cured OrmoComp by applying the static phase mask technology in conjunction with deep-UV laser radiation. Hence, these findings promote the utilization of inorganic-organic Ormocer hybrid polymers as material for the fabrication of advanced wafer scale optical elements such as distributed feedback lasers, distributed Bragg reflectors, wavelength-division multiplexing systems and for the fabrication of sensor elements.
Acknowledgments
This research has received funding from the Deutsche Forschungsgemeinschaft (DFG) under the grant numbers HE 5150/1-1 and FR 713/10-1. We thank our colleagues of the Applied Laser and Photonics Group Manuel Rosenberger and Steffen Hessler for fruitful discussions and Benedikt Adelmann for preparing the cleaving edges of the wafers by laser ablation.
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