Nonlinearities in GaAs cavities with high CW input powers enabled by photo-oxidation quenching through ALD encapsulation

We demonstrate that encapsulation using Atomic Layer Deposition of GaAs nano-cavity resonator made of photonic crystal cavity enables to prevent photo-induced oxidation. This improvement allows injecting a large quantity of energy in the resonator without any degradation of the material, thus enabling spectral stability of the resonance. Using this, we prove eﬃcient second harmonic and third harmonic generation with a good plug eﬃciency ( η SHG = 8 . 3 × 10 − 5 W -1 and η THG = 1 . 2 × 10 − 3 W -2 ) and a large net output energy for both operation ( P outSHG = 0 . 2 nW and P outTHG = 8 pW).

Nonlinearities in GaAs cavities with high CW input powers enabled by photo-oxidation quenching through ALD encapsulation Gregory Moille, 1, 2, a) Sylvain Combrié, 2 Laurence Morgenroth, 3  We demonstrate that encapsulation using Atomic Layer Deposition of GaAs nanocavity resonator made of photonic crystal cavity enables to prevent photo-induced oxidation. This improvement allows injecting a large quantity of energy in the resonator without any degradation of the material, thus enabling spectral stability of the resonance. Using this, we prove efficient second harmonic and third harmonic generation with a good plug efficiency (η SHG = 8.3 × 10 −5 W -1 and η T HG = 1.2 × 10 −3 W -2 ) and a large net output energy for both operation (P out SHG = 0.2 nW and P out T HG = 8 pW). However, for large input power, thus large optical power density, resonators made of GaAs undergo photo-induced oxidation, blue-drifting the resonance wavelength irreversibly. This oxidation process is thermally activated and follow an exponential dependence with the device temperature 10 .
In this letter, we demonstrate non-linear effects in GaAs PhC cavity, namely Second Har-  (Fig. 2a). We find experimentally a resonance wavelength for the second order mode at 1508.2 nm with a Q factor of Q = 8100. The SH (Fig. 2b) and TH signals (Fig. 2c) are spectrally characterized using Optical Spectral Analyzers (OSA, Thorlabs, and Ocean Optics respectively), and the signal peaks lie at 754.1 nm and 502.75 nm respectively. Interestingly, the photo-luminescence (PL) can be easily detected as the excitation at 1508 nm leads to two-photon absorption, and the PL signal is spectrally separated enough from the SH one. Therefore both can be independently characterized.
Mode profiles of the SH and TH signals are observed using the setup shown in Fig. 1. The light is collected using a microscope objective (Olympus-M PLAN) with a large Numerical Aperture N A = 0.9 and a magnification of 100. The image is observed thanks to a high sensitivity cooled-down EMCCD camera (Andor) with a 512 × 512 pixels matrix. Each pixel has a size of 16 µm. The effective resolution obtained is 72 nm by pixel (i.e. magnification of 220). Appropriate filters are inserted in the setup to observe only the SHG (short pass filter at 1 µm) or the THG (band pass filter). photon-absorption at 1.5 µm. It is highly interesting to note that resolving the TH spatial mode allows resolving the resonant mode at Telecom wavelength. Indeed, as previous work pointed out 8 , the diffraction limit is greatly reduced using the TH signal compare to the Telecom one. The conversion efficiency of both SH and TH were characterized. Using the same setup previously shown in Fig. 1, it allows us to measure the output signal power. Indeed, the number of photons can be retrieved using the Andor camera, thus the detected power.
Here, only the area corresponding to the cavity is accounted. A normalization is performed taking into account all the different losses at the SH and TH wavelength (Table I) leading to −4.87 dB for the SH and −5.41 dB for TH signal. The insertion losses to the waveguide are estimated to be -7 dB. Due to thermal effect, mostly induced by carrier recombination, the input CW signal is tuned for each input power to match the shifted resonance. The plug efficiency conversion for the SHG operation found is P in /P 2 SHG = 8.3 × 10 −5 W -1 , while for the THG conversion is P in /P 3 T HG = 1.2 × 10 −3 W -2 (Fig. 4). Saturation effects appear at high input power, which is predicted by the Coupled Mode Theory (CMT) 19 .
These efficiency values are still below the state of the art of SHG and THG for nano- In regards with the Telecom wavelength here, which generates carriers through Two-Photon Absorption (TPA) just above the band-gap, the main oxidation effect is the thermally induced one through carrier recombination 28 , and enhanced by the high carrier surface recombination velocity 11,29 . Therefore, other processes can be considered as negligible.
To investigate further the oxidation process, we measure the temperature of the resonator, using two main different techniques. First by observing the modification of the photoluminescence spectra (Fig. 5d) using an OSA (Thorlabs), the band gap energy can be probed (which correspond to the energy of the peak of photo-luminescence minus k B T /2) and is directly linked to the temperature of the material as described in ref. 30. Secondly the spectral shift of the resonance measured by probing the THG (Fig. 5c), SHG (Fig. 5b) and the direct transmission at 1.5µm (Fig. 5a). Indeed, the GaAs the temperature rise induces a refractive index shift, following 31: Using FDTD computation, one can retrieve the dependence of the resonance wavelength with the refractive index of this specific device, such that: In conclusion, we have shown here high output power for SHG and THG in PhC resonator made of GaAs. Indeed, conform encapsulation of the material with Al 2 O 3 made by ALD enables to prevent photo-induced oxidation leading to an irreversible blue shift of the resonance. Therefore, high input power can be injected into the system, leading to high output power. Measurement of the system temperature was achieved by measuring the shift of the resonance wavelength and by measuring the photo-luminescence spectra. The system reaches a maximum temperature of 435 K, without any degradation of the resonator. By improving the conversion efficiency of SH and TH operations, such system can be of great promise for high output power SHG, for instance for f − 2f optical clock systems, and high output THG for efficient sub-wavelength imaging. This work was funded by Agence Nationale de la Recherche/DoD through contract ETHAN (ANR-15-ASTR-0014) and AUCTOPUSS ( ANR-12-ASTR-0014).