Second Harmonic Generation in AlGaAs/AlOx Random Structures

Our aim is to study the nonlinear properties of micro-structured devices, and focus on second harmonic generation in random dielectric structures. We recently proposed a new method to achieve highly efficient second harmonic generation in random structures. This method does not rely on phase matching, is insensitive to fabrication tolerances, and it provides excellent tunability in both incident angle and wavelength. While overall performance maybe not as high as in high-Q, doubly-resonant devices, yet we find an enhancement factor that is four orders of magnitude larger than what can be obtained from an unprocessed, nonlinear material, and in excess of one order of magnitude with respect to an ideally phase-matched, nonlinear material. We applied our theoretical results to design, realize and experimentally test a sample made of AlGaAs/AlOx layers grown in a GaAs substrate. Since the GaAs substrate exhibits strong absorption coefficient for the second harmonic field we will develop an experimental setup to detect the generated signal in reflection with respect to the direction of incidence of the pump. We focused our attention to second harmonic generation at 770 nm pumped at 1540 nm wavelength. Material refractive indices at given wavelengths are n(AlOx)=1.5, n(Al_(0.31)Ga_(0.69)As (1540 nm)) = 3.23 and n(A_l(0.31)Ga_(0.69)As (770 nm))=3.46. Following our design criteria we proposed a structure consisting of 38 layers. The total thickness is approximately 15 micron and the structure is designed to properly work with a pump at oblique incidence around 40 degrees with respect to the normal direction. Both fundamental and second harmonic fields are ppolarized. Our numerical calculation, in non depleted pump approximation, predicts an enhancement factor of 10 of the backward generated second harmonic signal with respect to what is achievable in transmission with a coherence length of a AlGaAs bulk with a line width of approximat- ely 0.6 nm, suitable for ps-pulse pumping. In order to reduce the possibility of cracks occurring during the oxidization process we also designed a structure of 7.5 micron length. Our calculations show that the new structure is about two order of magnitude less efficient than the previous one. Nevertheless a 3-4 order of magnitude enhancement is still predicted. Experimental tests are in progress.