On-chip Bragg grating waveguides and Fabry-Perot resonators for long-wave infrared operation up to 8.4 µm

: Taking advantage of unique molecular absorption lines in the mid-infrared fingerprint region and of the atmosphere transparency window (3-5 µm and 8-14 µm), mid-infrared silicon photonics has attracted more research activities with a great potential for applications in different areas, including spectroscopy, remote sensing, free-space communication and many others. However, the demonstration of resonant structures operating at long-wave infrared wavelengths still remains challenging. Here, we demonstrate Bragg grating-based Fabry-Perot resonators based on Ge-rich SiGe waveguides with broadband operation in the mid-infrared. Bragg grating waveguides are investigated first at different wavelengths from 5.4 µm up to 8.4 µm, showing a rejection band up to 21 dB. Integrated Fabry-Perot resonators are then demonstrated for the first time in the 8 µm-wavelength range, showing Q-factors as high as 2200. This first demonstration of integrated mid-infrared Fabry-Perot resonators paves the way towards resonance-enhanced sensing circuits and non-linear based devices at these wavelengths.


Introduction
In the recent years, mid-infrared (MIR) silicon photonics is attracting a lot of attention [1][2][3]. Taking advantage of the unique molecular absorption lines in the MIR range [4,5], silicon photonics has been proposed as a convincing solution for the development of highperformance and cost-effective MIR integrated sensors. A large number of applications are foreseen, for instance, real-time environmental monitoring [6,7], bio-sensing and medical diagnosis [8,9]. Interestingly Germanium (Ge) and Silicon Germanium (SiGe) alloys are strong candidates for extending the operation wavelength of silicon photonics in the mid-IR [10][11][12][13][14][15][16]. In this context Ge-rich SiGe graded index waveguides have been recently demonstrated as a promising platform benefiting from the wide transparency window of Ge to achieve deep-MIR operation, beyond 8 µm [17][18][19][20]. Furthermore, the strong 3rd order nonlinearity of SiGe shows a huge potential in the field of non-linear active devices for efficient optical frequency generation and conversion [11,21,22].
The integrated resonator is a key building block for the enhancement of on-chip sensing, spectroscopy and nonlinear optics. Si-based on-chip MIR resonators have been reported previously [23][24][25][26][27], however on-chip integrated resonant structures are still missing for wavelengths beyond 5.6 µm. We have thus chosen to use broadband Ge-rich SiGe graded index wavegu comparison w flexibility in t Because of th design of the keeping low l In this pa wavelength. demonstrated enhanced sens

Bragg gra
The Bragg gr Ge concentra thick wavegu µm, which pr wide range of waveguide is profile at a w platform is sh als inside region, λ B s the duty ency was orrugated numerical calculations, an optimized ratio of 70% was obtained to maximize the coupling efficiency, corresponding to W Etch = 3.5 µm. Such optimized performance of the device coupling efficiency is attributed to an improved overlap factor between the optical mode and the grating. Finally, the wideband characteristics of the graded SiGe guiding platform allow for efficient tailoring of the central operating wavelength of Bragg grating waveguides by simply modifying the grating period according to the Bragg condition. The simulated transmission spectral response of different Bragg grating waveguides are shown in Fig. 2(b) (blue curves). A 2D method and an eigenmode expansion (EME) solver was used for the calculation [29].
The SiGe graded-index waveguides were firstly grown using a low energy plasma enhanced chemical vapor deposition (LEPECVD) technique, which allows a tight control of the alloy composition in the growth direction [30]. The Bragg grating waveguides were fabricated using an electron beam lithography, followed by an inductive coupled plasma (ICP) etching. Firstly, a partial shallow-etch level of 0.4 µm was performed to define the Bragg grating, followed by a second etching step of 4 µm to define the waveguides. A scanning electronic microscope (SEM) image of the fabricated Bragg grating waveguide is shown in Fig. 2(a).
Fabricated devices were characterized using a free-space configuration with a tunable quantum cascade laser (QCL). Additional details about the testing set-up can be found in Ref [18]. Figure 2 Figure  2(c) shows the measured transmission of Bragg grating waveguide, comprising different lengths, i,e. different number of Bragg grating periods, while maintaining a constant grating period of 0.88 µm, providing rejection centered at λ = 6.4 µm. It can be seen that the minimum transmittance (i.e. rejection of the Bragg grating) decreases as the number of periods increases, which corresponds to an increase of the Bragg grating reflection. A maximum rejection of 21.6 dB was measured. The minimum transmittance has also been calculated as a function of the number of periods and compared to experiments in Fig. 2(d), obtaining comparable trends.

Fabry-Perot resonators
From the demonstration of MIR Bragg grating waveguides, it has been possible to design and fabricate Fabry-Perot resonators. This was implemented by integrating two identical Bragg gratings as reflecting mirrors, with a straight waveguide in between, thereby acting as a cavity. It is worth mentioning that despite being able to sweep the central wavelength of the Bragg grating at will over the studied wavelength range in the MIR (as seen in Fig. 2(b)), we decided to focus the demonstration of FP resonators especially around of λ ≈8 µm, where onchip resonant structures are crucially missing.
The schematic view of the fabricated FP resonator are shown in Fig. 3(a). Figure 3(b) shows a reference simulated transmittance of a FP resonator with following parameters: Λ = 1.1 µm, N = 280 and a cavity length L cav of 70 µm. The resonant peak is situated at a wavelength of 7.906 µm. Figure 3(c) shows the experimental results achieved for two FP resonators using Bragg mirrors with N = 280, L cav = 70 µm and different periods of 1.1 and 1.16 µm, operating at wavelengths of 7.95µm and 8.35 µm, respectively. The −3 dB resonance bandwidths are measured to be 5.3 and 6.5 nm, respectively. This corresponds to loaded Q-factors of 1514 and 1272, respectively. Bragg grating reflectivity values of around 88% and 85% are thus estimated. Both FP resonators exhibit comparable performance in terms of maxi and taking int mirror losses The cavity a smaller FSR using longer B Fig. 3(d), wh bandwidth at loaded Q-fact laser in pulse nm at 8 µm w nm, attaining measured Q-f grating reflec evaluate the i given by the f where n g is th Q int is about 4 Fig. 3

Conclusion
In conclusion, we demonstrated Bragg gratings waveguides and Bragg-grating-based Fabry-Perot resonators operating in the long-wave MIR region. Benefiting from the wideband waveguide design, and following the Bragg condition, the Bragg gratings and Fabry-Perot resonators were investigated over a wavelength range from 5.4 µm up to 8.4 µm. The Bragg grating structure is based on a top-surface waveguide corrugation that provides a rejection higher than 20 dB. We also implemented Fabry-Perot resonators by facing two Bragg grating mirrors one in front of each other within a certain distance to control the cavity length. The resonators are demonstrated up to 8.4 µm wavelength with a Q-factor higher than 1000 in all cases. A maximum Q-factor reaching 2200 is demonstrated at 7.95 µm wavelength by increasing the grating length. This first demonstration of resonators in such deep-MIR region paves the way to further investigation of new MIR resonance-enhanced sensing circuits in the molecular fingerprint region. Moreover, benefiting from the strong nonlinearity, the enhancement of non-linear effects in SiGe alloys is also anticipated using these new MIR cavities.