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We developed a technique that enables to replace a metallic waveguide cladding with a low-index (n $sim$ 1.4) material - CaF2 or BaF2 - that in addition is transparent from the mid-IR up to the visible range: elevated confinement is preserved while introducing an optical entryway through the substrate. Replacing the metallic backplane also allows double-side patterning of the active region. Using this approach, we demonstrate strong light-matter coupling between an intersubband transition (lambda $sim$ 10 microns) and a dispersive resonator, at 300 K and at 78 K. Finally, we evaluate the potential of this approach as a platform for waveguiding in the mid-IR spectral range, with numerical simulations that reveal losses in the 1-10 cm$^{-1}$ range.
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The main interest of group-III-nitride nanophotonic circuits is the integration of active structures and laser sources. A photonic platform of group-III-nitride microdisk lasers integrated on silicon and emitting in the blue spectral range is demonstrated. The active microdisks are side-coupled to suspended bus waveguides, and the coupled emission is guided and outcoupled to free space using grating couplers. A small gap size of less than 100 nm between the disk and the waveguide is required in the blue spectral range for optimal evanescent coupling. To avoid reabsorption of the microdisk emission in the waveguide, the quantum wells are etched away from the waveguide. Under continuous-wave excitation, loaded quality factors greater than 2000 are observed for the whispering gallery modes for devices with small gaps and large waveguide bending angles. Under pulsed excitation conditions, lasing is evidenced for 3 $mu$m diameter microdisks integrated in a full photonic circuit. We thus present a first demonstration of a III-nitride microlaser coupled to a nanophotonic circuit.
Ultraviolet microdisk lasers are integrated monolithically into photonic circuits using a III-nitride on silicon platform with gallium nitride (GaN) as the main waveguiding layer. The photonic circuits consist of a microdisk and a pulley waveguide terminated by out-coupling gratings. We measure quality factors up to 3500 under continuous-wave excitation. Lasing is observed from 374 nm to 399 nm under pulsed excitation, achieving low threshold energies of $0.14 ~text{mJ/cm}^2$ per pulse (threshold peak powers of $35 ~text{kW/cm}^2$). A large peak to background dynamic of around 200 is observed at the out-coupling grating for small gaps of 50 nm between the disk and waveguide. These devices operate at the limit of what can be achieved with GaN in terms of operation wavelength.
Integrating atomic quantum memories based on color centers in diamond with on-chip photonic devices would enable entanglement distribution over long distances. However, efforts towards integration have been challenging because color centers can be highly sensitive to their environment, and their properties degrade in nanofabricated structures. Here, we describe a heterogeneously integrated, on-chip, III-V diamond platform designed for neutral silicon vacancy (SiV0) centers in diamond that circumvents the need for etching the diamond substrate. Through evanescent coupling to SiV0 centers near the surface of diamond, the platform will enable Purcell enhancement of SiV0 emission and efficient frequency conversion to the telecommunication C-band. The proposed structures can be realized with readily available fabrication techniques.
Silicon nitride (Si3N4), as a complementary metal-oxide-semiconductor (CMOS) material, finds wide use in modern integrated circuit (IC) technology. The past decade has witnessed tremendous development of Si3N4 in photonic areas, with innovations in nonlinear photonics, optical sensing, etc. However, the lack of an integrated laser with high performance prohibits the large-scale integration of Si3N4 waveguides into complex photonic integrated circuits (PICs). Here, we demonstrate a novel III-V/Si/Si3N4 structure to enable efficient electrically pumped lasing in a Si3N4 based laser external cavity. The laser shows superior temperature stability and low phase noise compared with lasers purely dependent on semiconductors. Beyond this, the demonstrated multilayer heterogeneous integration provides a practical path to incorporate efficient optical gain with various low-refractive-index materials. Multilayer heterogeneous integration could extend the capabilities of semiconductor lasers to improve performance and enable a new class of devices such as integrated optical clocks and optical gyroscopes.
On-chip microlaser sources in the blue constitute an important building block for complex integrated photonic circuits on silicon. We have developed photonic circuits operating in the blue spectral range based on microdisks and bus waveguides in III-nitride on silicon. We report on the interplay between microdisk-waveguide coupling and its optical properties. We observe critical coupling and phase matching, i.e. the most efficient energy transfer scheme, for very short gap sizes and thin waveguides (g = 45 nm and w = 170 nm) in the spontaneous emission regime. Whispering gallery mode lasing is demonstrated for a wide range of parameters with a strong dependence of the threshold on the loaded quality factor. We show the dependence and high sensitivity of the output signal on the coupling. Lastly, we observe the impact of processing on the tuning of mode resonances due to the very short coupling distances. Such small footprint on-chip integrated microlasers providing maximum energy transfer into a photonic circuit have important potential applications for visible-light communication and lab-on-chip bio-sensors.
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