Letter
Vol. 4, No. 8 / August 2017 / Optica
893
Integrated continuous-wave aluminum nitride Raman laser XIANWEN LIU,1 CHANGZHENG SUN,1,* BING XIONG,1 LAI WANG,1 JIAN WANG,1 YANJUN HAN,1 ZHIBIAO HAO,1 HONGTAO LI,1 YI LUO,1 JIANCHANG YAN,2 TONGBO WEI,2 YUN ZHANG,2 AND JUNXI WANG2 1
Tsinghua National Laboratory for Information Science and Technology/State Key Lab on Integrated Optoelectronics, Department of Electronic Engineering, Tsinghua University, Beijing 100084, China 2 R&D Center for Semiconductor Lighting, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China *Corresponding author:
[email protected] Received 27 January 2017; revised 25 June 2017; accepted 1 July 2017 (Doc. ID 285799); published 27 July 2017
Wurtzite aluminum nitride (AlN) is known to exhibit six Raman-active optic phonons, making it appealing for Raman lasing. Here, we demonstrate continuous-wave Raman lasers with a low threshold and a high slope efficiency in high quality factor AlN-on-sapphire microrings. Stokes radiations around 1.7–1.8 μm and cascaded operation into 1.9–2.0 μm are identified with a telecom pump. Two types of Stokes lights with distinct Raman shifts and polarizations are recorded via selective excitation of corresponding optic phonons, in accordance with the Raman selection rules in AlN. The observed lasing behavior is satisfactorily accounted for by a theoretical analysis. Our results indicate that AlN-on-sapphire should be promising for integrated nonlinear optics. © 2017 Optical Society of America OCIS codes: (190.4390) Nonlinear optics, integrated optics; (290.5910) Scattering, stimulated Raman; (140.3550) Lasers, Raman; (140.3560) Lasers, ring; (140.3945) Microcavities. https://doi.org/10.1364/OPTICA.4.000893
Stimulated Raman scattering (SRS) is a nonlinear optical process that has been recognized as an attractive approach to extend the available spectral coverage of conventional laser sources [1]. It potentially offers coherent lights at any desired wavelength within the transparency window of Raman-active media. To relax the required pump levels for Raman lasing in a continuous-wave (CW) regime, microresonator-based geometries are preferred, because they not only significantly reduce the lasing threshold due to the intracavity power enhancement, but also enable device miniaturization [2–8]. Among them, planar-integrated CW Raman lasers with robust waveguides to feed the pump and extract Stokes radiation are of particular interest, and have been demonstrated only in cubic-structure silicon [6,7] and diamond [8]. However, the narrow bandgap (∼1.1 eV) of silicon makes it susceptible to significant two-photon absorption (TPA) at the telecom band [9], while the preparation of high-quality diamonds for integrated waveguide fabrication still remains a challenge. 2334-2536/17/080893-04 Journal © 2017 Optical Society of America
In contrast, wurtzite aluminum nitride (AlN) has an excellent crystalline quality and is readily accessible via the mature epitaxial growth technique. In addition, its enormous bandgap (∼6.2 eV) makes it immune to TPA. Moreover, AlN features six Raman-active high LO TO LO low [10,11], bephonons, i.e., ATO 1 , A1 , E 1 , E 1 , E 2 , and E 2 cause it belongs to space group C 46v . AlN epitaxially grown on sapphire substrate (nsapphire ∼1.75 at 1.55 μm) not only naturally forms a waveguide structure for guiding light, but also exhibits narrow-linewidth phonon spectra [11], thereby facilitating on-chip Raman laser generation. Since AlN harbors both intrinsic quadratic and cubic susceptibilities (χ 2 and χ 3 ) [12], AlN Raman lasers can potentially realize continuous electro-optic tuning [13] compared to the slow thermo-optic tuning counterpart in diamonds [8], which is expected to alleviate the thermal lensing effect [1] and phonon linewidth broadening at elevated temperature [14]. Currently, the phonon behavior in AlN is primarily employed to monitor film quality, stress, and free-carrier concentration [15]. To the best of our knowledge, their Raman lasing properties have not been explored either in bulk AlN or integrated waveguides. In this Letter, we demonstrate AlN-on-sapphire as a novel platform for on-chip CW Raman laser generation. The lasing behavior of optic phonons is investigated based on high quality factor (Q) AlN microrings and is found in good agreement with our Raman spectroscopy measurement. Additionally, Raman-induced hyperparametric oscillation and four-wave mixing (FWM) are observed at a certain pump regime due to the interplay between SRS and Kerr nonlinearity. In our experiment, single-crystalline AlN is prepared on c-plane (0001) sapphire by metal organic chemical vapor deposition (MOCVD) [16], with a measured refractive index of 2.1 at 1.55 μm. To enhance intensity-dependent Raman gain, we have designed and fabricated AlN microrings [Fig. 1(a)] with a cross section of 3.5 μm × 1.2 μm to minimize the clad-core interface scattering loss and thereby access high Q-factors [17]. For microresonator-based Raman lasers, it is essential to align the Stokes mode to its cavity resonances, i.e., the free spectrum range (FSR) of the cavity should be related to the Raman shift ΩR of the corresponding optic phonons as ΩR ≈ m · FSR, with m being an integer [18]. According to the Raman shifts of AlN, we have fabricated AlN microrings with three distinct FSRs by
Letter
Vol. 4, No. 8 / August 2017 / Optica
(a) ~1.73 m
s
p
~1.93 m SiO2
~1.56 m m-plane
AlN
R 1 m
Pump 1.0
0.8 0.6 TM00 0.4 Q = 1.2 × 106 L 6 0.2 Q0 = 2.3 × 10
QC = 2.6 × 106
0.0 1560.296
1560.300
156 Hz
Measured Fitted 1560.304
Wavelength (nm)
Transmission
Transmission
Sapphire
Stokes
1.0
(b)
a-plane
0.8 0.6
TE00 0.4 QL = 0.9 × 106
211 MHz
6 0.2 Q0 = 1.5 × 10 Measured QC = 2.2 × 106 Fitted 0.0 1559.095 1559.100 1559.105
(c)
Wavelength (nm)
Fig. 1. (a) Left: Energy-level diagram for the SRS process. Middle: Schematics of the structure and principle for microring-based AlN Raman lasers. Right: Scanning electron microscopy (SEM) image of the cleaved waveguide facet (embedded in 3 μm silica), exposing the a-plane (1120) of sapphire and m-plane (1010) of AlN. (b) and (c) Measured resonance linewidth and extracted Q-factors for TM00 and TE00 modes, respectively.
tuning their respective outer radii to be 60, 80, and 100 μm. We observe high-efficiency Raman lasers in devices with an 80 μm radius, while pure Kerr combs for others. By adopting these parameters, excellent spatial overlaps between the pump (∼1.56 μm), first-order 1st (∼1.73 μm) and second-order 2nd (∼1.93 μm) Stokes modes in Fig. 1(a) can also be ensured [19]. For our integrated AlN chips, the coupling bus waveguide width is designed to be 1.35 μm for single-mode operation, and is then tapered to 4 μm at the chip end facets to enhance the fiber-to-chip coupling efficiency [17]. The feeding waveguide is chosen to run perpendicular to the natural cleavage facet (i.e., m-plane) of AlN [right of Fig. 1(a)] to facilitate the cleavage. An unetched AlN layer of 418 nm is left at the bottom to increase the waveguide-to-microring coupling, thereby expanding nearly critical coupling gap to 700 nm. The transmission spectrum of the AlN chip reveals a low-insertion loss of ∼3 dB∕facet, a FSR of ∼279 and 286 GHz for fundamental transverse-magnetic (TM00 ) and transverse-electric (TE00 ) modes, respectively. Figures 1(b) and 1(c) illustrate the high-resolution linewidth measurements. A high loaded Q-factor (Q L ) of 1.2 × 106 and 0.9 × 106 is identified for TM00 and TE00 modes, respectively. The intrinsic and coupling Q-factors (Q 0 and Q C ) are then extracted as the microring is slightly under-coupled according to its phase response [20]. The device characterization is detailed in Supplement 1. The Raman scattering process is schematically depicted in the left section of Fig. 1(a) where a pump photon with frequency ωp is scattered into a lower frequency Stokes photon at ωs by releasing an optic phonon with corresponding ΩR ωp − ωs [1]. Phase matching is inherently fulfilled as the involved optic phonon is excited at the Brillouin zone center with a wavevector q ≈ 0 [10]. In our experiment, CW light from a tunable laser (Santec TSL-510, 1500–1630 nm, linewidth
