Hongxing Dai

Quasi-phase-matched second-harmonic generation in reflection from AlxGa1-xAs heterostructures

Quasi-phase-matched second-harmonic (SH) generation in reflection geometry is described and demonstrated. The SH intensity can be strongly enhanced by spatially modulating the optical properties of the nonlinear medium. This type of quasi-phase-matching is demonstrated by using an Al0.8Ga0.2As/GaAs heterostructure designed for λ = 1.06 μm incident light. The SH light intensity generated in reflection from the heterostructure is enhanced 70 times relative to the SH response of a homogeneous GaAs wafer. A resonant cavity design that employs this structure to make thin films with extremely high SH generation efficiencies is proposed.

Multilayer, nonlinear antiresonant waveguides for surface emitted optical mixing

A multilayered core nonlinear antiresonant reflecting optical waveguide (NARROW) was used to generate surface emitted frequency doubled light. High efficiencies were obtained with the NARROW while simultaneously keeping optimum antiresonant conditions at the fundamental wavelength.

An InGaAs/GaAs SQW laser integrated with a surface-emitting harmonic generator for DWDM applications

An In/sub 0.22/Ga/sub 0.78/As/GaAs single quantum-well (SQW) laser integrated monolithically with a surface-emitting harmonic generator (SEHG) segment was fabricated and tested. By coupling a 1.06- mu m Nd:YAG laser output into the active device, the green surface-emitting light at 0.51 mu m was observed as the result of nonlinear interactions within the SEHG segment. The nonlinear interaction cross section A/sup nl/ of the device was found to be 2.3*10/sup -7/ W/sup -1/, which agrees well with theory. The application of the active devices as a dense wavelength-division-multiplexing (DWDM) demultiplexer was demonstrated with a tunable Ti:sapphire laser to simulate the multiwavelength signal source and a CCD detector array to record the spatially separated signals of different wavelength. System resolution better than 2 nm was obtained for the device, which was limited by the short length of the SEHG segment. Potential for a DWDM channel spacing of 10 GHz is discussed.<<ETX>>

High-resolution surface-emitting spectrometer and deformation sensors with nonlinear waveguides.

We investigate the limits of frequency resolution attainable in a nonlinear waveguide optical spectrometer, including the effects that are due to surface distortions and waveguide inhomogeneities, and demonstrate that the frequency-resolving capability is directly scalable with the radiating aperture length. The resolution of the waveguide is diffraction limited, and therefore the far-field radiation pattern can be used to characterize the phase variations along the waveguide that are due to surface distortions. The use of this device as a highly sensitive deformation sensor is demonstrated by application of a distortion to the waveguide and confirmation of the far-field diffraction pattern generated.

Design optimization of multilayer nonlinear antiresonant waveguides for blue - green surface normal second-harmonic generation.

We present a self-consistent design optimization for nonlinear antiresonant optical waveguides with quasi-phase-matched AlxGa1−xAs multilayer cores. Design parameters for efficient surface normal second-harmonic generation at green and blue wavelengths are obtained.

Longitudinal and vertical emission laser integration with quasi-phase-matched harmonic layers

We review the progress in our physical understanding and modeling of integrated surface emitting harmonic generators and also present experimental results for the monolithic integration of active laser sources embedded with a quasi phased matched nonlinear heterostructure cavity for surface emission in the blue-green wavelength region.

Monolithic integration of surface-emitting harmonic generator with InGaAs/GaAs single-quantum-well laser

We demonstrate active surface-emitting sum-frequency generators based on monolithic integration of InGaAs/GaAs single quantum well lasers and multilayer AlxGa1-xAs nonlinear waveguides. As broad area lasers with uncoated facets, threshold current densities as low as 87.5 A/cm2 have been obtained for a cavity length of 3.3 mm at 300 K. A differential internal quantum efficiency of 71% and a waveguide loss of 3.2 cm-1 have been measured from the room temperature light-current (L-I) characteristics. With an active/passive segmented cavity operating at 0.984 micrometers , coherent sum-frequency light at 0.51 micrometers was radiated from the etched-off passive segment surface when coupling 1.06 micrometers light into the cavity as the TM guided wave to interact with the TE SQW laser mode. The measured conversion efficiency in terms of the nonlinear cross section Anl is about 2 X 10-7 W-1 which agrees well with the calculated value. Potential applications of the integrated devices are briefly addressed.

Harmonic generation in multilayered waveguides: theory, devices, and application (Invited Paper)

Ten years ago Normandin and Stegeman demonstrated a novel surface harmonic generation geometry but the cross sections were too low for applications. Using periodic multilayers in the waveguide, cross sections were increased by over a million. Theory refinements and new results are presented for large angle light deflectors, integrated intracavity doubling visible lasers, and high resolution (< 10 GHz) optical spectrometers. Applications in display, telecommunication, interconnect, and coding are also discussed.

Cross-section optimisation in NARROW harmonic surface emission waveguides

We recently demonstrated large second harmonic surface emission cross-sections in a nonlinear anti-resonant reflecting optical waveguide (NARROW). High efficiencies were obtained in thick films by simultaneously maintaining the anti-resonance conditions at the 1.06 /spl mu/m fundamental in the multilayer film and the harmonic 0.53 /spl mu/m resonance. Here we report on a full optimisation process, taking all material constants, linear and nonlinear, into account and optimising the thickness ratio and composition of each layer in the system.<<ETX>>

Intracavity and resonant external cavity laser frequency doubling using semiconductor heterostructures

Optical second-harmonic (SH) generation in nonlinear semiconductors can be used to convert infrared radiation from semiconductor laser sources to visible light. The high incident light intensities and spatial modulation of the linear and nonlinear optical properties occurring in AlxGa1-xAs vertical cavity heterostructures can lead to SH generation efficiencies many orders of magnitude larger than possible homogeneous GaAs. SH generation in vertical cavity heterostructures is analyzed using a model incorporating multilayer reflections and spatially varying nonlinear sources.