Ming-Liang Ren

Exact iterative solution of second harmonic generation in quasi-phase-matched structures

A versatile and accurate approach that combines a numerical iteration technique and a transfer-matrix method (TMM) is developed to solve the general problem of second harmonic generation (SHG) with pump depletion in quasi-phase-matched (QPM) nonlinear optical structures. We derive the iterative formulae from the nonlinear coupled wave equations and obtain the intensity distribution of fundamental wave and second harmonic wave by TMM. The approach shows quick numerical convergence of iteration and maintains perfect conservation of total energy. The simulation results show that the model coincides with the one under undepleted pump approximation very well when the SHG efficiency is small (well below 15%) and agrees very well with the effective nonlinear susceptibility model in handling general SHG problems even when the conversion efficiency is high up to 100%. Our method is applicable to general nonlinear optical structures, such as periodic, quasi-periodic, and aperiodic QPM structures, photonic crystals, and micro-cavities that might involve complicated modulation on the linear and nonlinear susceptibility.

Giant enhancement of second harmonic generation in nonlinear photonic crystals with distributed Bragg reflector mirrors

We theoretically investigate second harmonic generation (SHG) in one-dimensional multilayer nonlinear photonic crystal (NPC) structures with distributed Bragg reflector (DBR) as mirrors. The NPC structures have periodic modulation on both the linear and second-order susceptibility. Three major physical mechanisms, quasi-phase matching (QPM) effect, slow light effect at photonic band gap edges, and cavity effect induced by DBR mirrors can be harnessed to enhance SHG. Selection of appropriate structural parameters can facilitate coexistence of these mechanisms to act collectively and constructively to create very high SHG conversion efficiency with an enhancement by up to seven orders of magnitude compared with the ordinary NPC where only QPM works.

Broadband second harmonic generation in one-dimensional randomized nonlinear photonic crystal

We study experimentally second harmonic generation in a one-dimensional nonlinear photonic crystal with randomized inverted-domain structure. We show that the randomness enables one to realize an efficient broadband emission of high-quality second harmonic beam.

Direct observation of amplified spontaneous emission of surface plasmon polaritons at metal/dielectric interfaces

We report on direct observation of amplified spontaneous emission (ASE) of surface plasmon polaritons (SPPs) at the interface of a silver film and a gain medium. Based on a typical Kretschmann configuration incorporated with Rhodamine 6G molecules, the growing ASE spectra of SPPs have been clearly identified by carefully conducting a pump-dependent angle-resolved spectral measurement. Spectral narrowing effects induced by the SPP amplification are also demonstrated. The observed phenomena are helpful in understanding the fundamental interactions between SPPs and gain medium, which could enable wide applications on plasmonic sources and sensors.

Giant enhancement of second harmonic generation by engineering double plasmonic resonances at nanoscale

We have investigated second harmonic generation (SHG) from Ag-coated LiNbO₃(LN) core-shell nanocuboids and found that giant SHG can occur via deliberately designed double plasmonic resonances. By controlling the aspect ratio, we can tune fundamental wave (FW) and SHG signal to match the longitudinal and transverse plasmonic modes simultaneously, and achieve giant enhancement of SHG by 3 × 10(5) in comparison to a bare LN nanocuboid and by about one order of magnitude to the case adopting only single plasmonic resonance. The underlying key physics is that the double-resonance nanoparticle enables greatly enhanced trapping and harvesting of incident FW energy, efficient internal transfer of optical energy from FW to the SHG signal, and much improved power to transport the SHG energy from the nanoparticle to the far-field region. The proposed double-resonance nanostructure can serve as an efficient subwavelength coherent light source through SHG and enable flexible engineering of light-matter interaction at nanoscale.

Experimental demonstration of super quasi-phase matching in nonlinear photonic crystal

We have demonstrated super quasi-phase matching (QPM) in a super periodically poled lithium niobate (PPLN), which is composed of multiple ordinary PPLN cells. When super QPM is achieved, the slight phase mismatch in each PPLN cell can be further compensated for, and the relevant second harmonic generation is facilitated greatly. This mechanism provides an insightful means to relieve the limitation imposed by sample fabrication inaccuracy and will open up a promising avenue toward highly efficient nonlinear interactions.

Plasmonic coupling effect between two gold nanospheres for efficient second-harmonic generation

The coupling effect between two gold nanospheres (GNSs) is investigated for second-harmonic generation (SHG) in this paper. When the two GNSs approach each other, the coupling effect becomes stronger, and it consequently leads to stronger enhancement of SHG. A strong coupling area between two GNSs is also found. We propose a physical quantity, the coupling strength to describe the coupling effect on SHG enhancement and show that it can fully illustrate the relationship between the SHG enhancement and the gap of two GNSs. Moreover, further calculations reveal the existence of hot spots of both fundamental wave (FW) and excited second harmonic wave in the intermediate region of two GNSs. Our study can not only help to understand the coupling effect on SHG precisely but also offer some possibility to design a nonlinear plasmonic nano-ruler.

Amplified spontaneous emission of surface plasmon polaritons with unusual angle-dependent response.

Loss issues are fundamentally crucial for the application of surface plasmon polaritons (SPPs). In this study the amplified spontaneous emission (ASE) of SPPs in a typical Kretschmann configuration is observed and shows an unusually broadened angular response with increased pump intensity. Theoretical models are further developed to verify the results and understand the amplification of SPPs in Fourier space.

Multi-directional Čerenkov second harmonic generation in two-dimensional nonlinear photonic crystal.

We study Čerenkov-type second-harmonic generation in a two-dimensional quasi-periodically poled LiNbO3 crystal. We employ a new geometry of interaction to observe simultaneous emission of multi-directional nonlinear Čerenkov radiation with comparable intensities. This opens a way to control the angle of Čerenkov emission by tailoring the nonlinearity of the material, which is otherwise intrinsically defined by dielectric constants of the medium and their dispersion.

Broadband cascading of second-order nonlinearity in randomized nonlinear photonic crystal

We study both experimentally and theoretically optical parametric process in one-dimensional nonlinear photonic crystal with random variation to the sign of the second-order nonlinear coefficient. We demonstrate that the structure randomness enables broadband third-harmonic generation via cascading of two second-order nonlinear parametric wave interactions with the overall conversion efficiency higher than previously recorded in fully disordered media such as the as-grown strontium barium niobate crystal. Furthermore, thanks to the one-dimensional modulation of the second-order nonlinearity, the emitted broadband harmonics preserve well the spatial intensity profile of the incident fundamental light beam, a feature critical for applications of optical frequency conversion but unavailable in two-dimensional fully random media due to the beam divergence caused by the simultaneous emission of noncollinear harmonic.