High symmetry in the nonlinear susceptibility tensor combined with lack of birefringence in zincblende crystals allow for efficient mixing of a wide range of polarization states. Quasi-phasematching (QPM) allows phasematching to be achieved without relying on specific polarization states. Polarization insensitive difference-frequency generation  as well as optical parametric oscillation using circularly polarized and depolarized pump sources  have been demonstrated in QPM GaAs. Here, we present the six coupled-wave equations that describe chi-2 nonlinear mixing in these materials. We account for the two orthogonal polarization states at each frequency, which leads to six instead of three coupled-wave equations. We calculate the effective nonlinear coefficient by projecting the nonlinear susceptibility tensor onto the field polarization directions.
We applied the couple-wave equations to describe optical parametric amplification (OPA) and oscillation (OPO) in non-birefringent crystals and present simulations for QPM zincblende crystals. We show that for both OPA and OPO, there may be back-conversion to the orthogonally polarized pump wave. This back-conversion process is phasematched. Also, because the wave orthogonal to the pump is initially unseeded, this wave can take on the phase to favor back-conversion well before the original pump becomes depleted. This back-conversion process is associated with reduced OPA and OPO gain as well as apparent rotation of the pump polarization angle.
 S. J. B. Yoo, et al., Appl. Phys. Lett. 68, 2609 (1996).
 P. S. Kuo, et al., Opt. Lett. 32, 2735 (2007).
Hybrid quantum networks will be based on nodes that operate at different wavelengths, requiring quantum channel standardization via quantum frequency conversion (QFC). QFC is typically based on highly efficient sum- or difference-frequency generation in second-order nonlinear materials, such as periodically poled lithium niobate waveguides. The presence of the strong pump beam in such a nonlinear medium leads to unwanted nonlinear processes that produce noise. One of these noise processes is spontaneous Raman scattering (SRS). Typically, the pump is chosen to be the longest wavelength in the second-order nonlinear mixing process so that noise photons at the signal wavelength are produced by the less efficient anti-Stokes Raman scattering process rather than the Stokes scattering process. Since SRS is a temperature-dependent process, lowering the temperature reduces the Raman-scattered photons. We discuss the theory of temperature-dependent Raman scattering and present experimental results of the temperature dependence of dark count rates in a guided-wave QFC device.
We have developed an entangled photon pair source based on a domain-engineered, type-II periodically poled lithium niobate crystal that produces signal and idler photons at 1533 nm and 1567 nm. We characterized the spectral correlations of the generated entangled photons using fiber-assisted signal-photon spectroscopy. We observed interference between the two down-conversion paths after erasing polarization distinguishability of the down-converted photons. The observed interference signature is closely related to the spectral correlations between photons in a Hong- Ou-Mandel interferometer. These measurements suggest good indistinguishability between the two downconversion paths, which is required for high entanglement visibility.
We characterize spontaneous parametric downconversion in a domain-engineered, type-II periodically poled lithium niobate (PPLN) crystal using seeded emission and single-photon techniques. Using continuous-wave (CW) pumping at 775 nm wavelength, the signal and idler are at 1532.5 nm and 1567.5 nm, respectively. The domain-engineered crystal simultaneously phasematches signal and idler pairs: [H(1532.5 nm), V(1567.5 nm)] and [V(1532.5 nm), H(1567.5 nm)]. We observe the tuning curves of these processes through difference-frequency generation and through CW fiberassisted, single-photon spectroscopy. These measurements indicate good matching in amplitude and bandwidth of the two processes and that the crystal can in principle be used effectively to generate polarization-entangled photon pairs.
Spontaneous parametric down-conversion (SPDC) is a common method to generate entangled photon pairs for use in quantum communications. The generated single photon linewidth is a critical issue for photon-atom interactions in quantum memory applications. We compare the linewidths of greatly non-degenerate single photon pairs from SPDC generated in the single-pass case and the singly-resonant cavity case. For a 6 mm periodically poled lithium niobate (PPLN) crystal, the linewidth of the generated signal photons is reduced from 1 THz in the single pass case to tens of MHz in the singly-resonant cavity case, while the brightness within the modal lineiwdth is increased by a factor of the cavity finesse, though the overall SPDC generation rate remains unchanged.
Quantum memory is a key device in the implementation of quantum repeaters for quantum communications and quantum networks. We demonstrated a quantum memory based on electromagnetically-induced transparency (EIT) in a warm cesium atomic cell. The quantum memory system can avoid the need for helium temperature apparatus and it is low cost for bulk scalability.
We describe the design and application of domain-engineered, periodically poled lithium niobate (PPLN) for use to
produce entangled photons and for other tools in quantum information and communications. By specially designing and
controlling the PPLN poling pattern, multiple nonlinear optical processes can be simultaneously phasematched. This
capability can be used to generate polarization-entangled photon pairs through type-II spontaneous parametric
downconversion. The single PPLN crystal is designed to produce both the |HV〉 and |VH〉 states where the
downconverted photons are distinguishable by wavelengths, which enables generation of post-selection-free,
polarization-entangled twin photons. We describe the design and fabrication of the PPLN crystal, and initial
experimental results for downconversion of a 775 nm pump to 1532 nm and 1567 nm orthogonally polarized photons.
We also discuss other applications of engineered optical frequency conversion for quantum information including the
use of dual-wavelength upconversion as a beamsplitter to route or analyze photons.
We propose a scheme to generate polarization-entangled photon pairs by spontaneous parametric downconversion in a phase-modulated, type-II, quasi-phasematched (QPM) crystal. Instead of using two distinct crystals to generate |HV〉and |VH〉states, the phase-modulated QPM grating allows both states to be generated simultaneously in a distributed fashion throughout the nonlinear crystal. Temporal compensation is still needed to correct for effects of birefringence in the crystal. The distributed generation of the polarization-entangled photons is compared to generation using two sequential crystals.
A tunable waveguide-based frequency up-conversion detector is used for single photon level near infrared (IR) spectroscopic measurements. Applications include direct spectroscopic measurement of week near IR signals and remote bi-photon spectroscopy. We have demonstrated direct spectroscopy of single photon near IR signals from a greatly attenuated laser and a single photon source. We further applied the up-conversion spectrometer for frequency correlated bi-photon spectroscopy using a single photon source of non-degenerate photon pairs at 1310 nm (near IR) and 895 nm. In correlated bi-photon spectroscopy, the spectral function at one wavelength range of a remote object can be reproduced by locally measuring another (near IR) wavelength range using the up-conversion spectrometer and monitoring the coincidence counts. A near IR single photon detection efficiency of 32 % has been achieved with the up-conversion spectrometer. The spectral resolution of the system is approximately 0.2 nm at 1310 nm based on the acceptance width of the up-conversion chip used. In bi-photon spectroscopy, the spectral resolution for the correlated photons at 895 nm is approximately 0.1 nm. The sensitivity achieved using the up-conversion detector is -126 dBm at 1310 nm.
Upconversion of 1.3-micron photons and detection using silicon avalanche photodiodes (Si APDs) can produce high
photon detection efficiencies (PDEs) with low dark count rates. We demonstrate a novel two-channel device based on a
phase-modulated, periodically poled LiNbO3 waveguide that mixes 1302-nm signal photons with two pump beams at
1556 and 1571 nm. Both channels showed high PDEs with very low dark counts. Using wavelength- to time-division
multiplexing in this dual-channel device, we produced clock rates that exceed the timing-jitter-limited rates of a system
based on one Si APD. Higher clock rates are of interest for improved quantum communication systems.
A Volume Bragg Grating (VBG) can be used to efficiently extract a narrow bandwidth, highly collimated beam from an otherwise broad spectrum beam. We use a VBG to extract a narrow bandwidth of signal spectrum from a broadband Spontaneous Parametric Down-Conversion source to optimally match the narrow detection bandwidth of our idler upconversion detector. Improved coincidence count rates and visibility can be achieved when limiting signal-spectrum detection to the narrow signal bandwidth whose photons are correlated with a narrow idler-spectrum bandwidth that has been selected by the up-conversion detector. We compare coincidence count rate and visibility for when the entire signal spectrum is detected and when the spectrum has been filtered by the VBG. We further relax the collection techniques and show that following the VBG, the coincidence count rate improves with minimal loss in visibility compared to when the entire spectrum is detected. We introduce our initial efforts at using the VBG to further narrow the signal spectrum by placing it inside a multipass cavity. Additionally, we further adapt the single photon level up-conversion spectrometer, previously developed for idler spectrum measurement, to indirectly measure the single photon level signal spectrum. We verify its capability for several different wavelength and linewidth selections.
We have efficiently generated tunable terahertz (THz) radiation using intracavity parametric down-conversion in gallium
arsenide (GaAs). We used three types of micro-structured GaAs to quasi-phase-match the interaction: optically
contacted, orientation-patterned, and diffusion-bonded GaAs. The room-temperature GaAs was placed in an optical
parametric oscillator (OPO) cavity, and the THz wave was generated by difference-frequency mixing between the OPO
signal and idler waves. 250-GHz-bandwidth radiation was generated with frequencies spanning 0.4-3.5 THz. We
measured two orders of optical cascading generated by the mixing of optical and THz waves. In a doubly resonant
oscillator (DRO) configuration, the efficiency increased by 21 times over the singly resonant oscillator (SRO)
performance with an optical-to-THz efficiency of 10-4 and average THz power of 1 mW.
Zincblende semiconductors (GaAs, GaP) show great potential for quasi-phase-matched (QPM) THz generation because
of their small (20 times less than in lithium niobate) absorption coefficient at terahertz frequencies, small mismatch
between the optical group and THz phase velocities, high thermal conductivity, and decent electro-optical coefficient.
Terahertz-wave generation was demonstrated recently in QPM GaAs, using optical rectification of femtosecond pulses.
Here we report on a new efficient widely tunable (0.5-3.5 THz) source of THz radiation based on quasi-phase-matched
GaAs crystal. The source is based on difference frequency generation inside the cavity of a synchronously pumped near-degenerate
picosecond OPO and takes advantage of resonantly enhanced both the signal and the idler waves. THz average power as high as 1 mW was achieved in a compact setup.
In photoacoustic (optoacoustic) medical imaging, short laser pulses irradiate absorbing structures found in tissue, such as blood vessels, causing brief thermal expansions that in turn generate ultrasound waves. These ultrasound waves which correspond to the optical absorption distribution were imaged using a two dimensional array of capacitive micromachined ultrasonic transducers (CMUTs). Advantages of CMUT technology for photoacoustic imaging include the ease of integration with electronics, ability to fabricate large two dimensional arrays, arrays with arbitrary geometries, wide-bandwidth arrays and high-frequency arrays. In this study, a phantom consisting of three 0.86-mm inner diameter polyethylene tubes inside a tissue mimicking material was imaged using a 16 x 16 element CMUT array. The center tube was filled with India-ink to provide optical contrast. Traditional pulse-echo data as well as photoacoustic image data were taken. 2D cross-sectional slices and 3D volume rendered images are shown. Simple array tiling was attempted, whereby a 48 x 48 element array was simulated, to illustrate the advantages of larger arrays. Finally, the sensitivity of the photoacoustics setup to the concentration of ink in the tube was also explored. For the sensitivity experiment a different phantom consisting of only one 1.14-mm inner diameter polyethylene tube inside a tissue mimicking material was used. The concentration of the ink inside the tube was varied and images were taken.
We have demonstrated all-epitaxially fabricated orientation-patterned AlGaAs waveguides with reduced waveguide core corrugation for the quasi-phase-matched second harmonic generation (SHG) pumped at 1.55 μm. The attenuation coefficient is measured to be ~4.5 dB/m at 1.55 μm, and ~9.7 dB/cm at 780 nm. The conversion efficiency at continuous wave operation is 43%W-1 with an 8-mm long waveguide.
We demonstrate an optical parametric oscillator (OPO) based on GaAs. The OPO utilized an all-epitaxially-grown orientation-patterned GaAs (OP-GaAs) crystal, 0.5-mm-thick, 5-mm-wide, and 11-mm-long, with a domain reversal period of 61.2 microns. By tuning either the near-IR pump wavelength between 1.75 and 2 microns, or the temperature of the GaAs crystal, the mid-IR output tuned between 2 and 11 microns, limited only by the spectral range of the OPO mirrors. The pump threshold of the singly-resonant OPO was 16 micro-J for the 6-ns pump pulses, and the photon conversion slope efficiency reached 54%. Also, we show experimentally the possibility of pump-polarization-independent frequency conversion in GaAs.
We demonstrate an optical parametric oscillator (OPO) based on GaAs. The OPO utilized an all-epitaxially-grown orientation-patterned GaAs (OP-GaAs) crystal, 0.5-mm-thick, 5-mm-wide, and 11-mm-long, with a domain reversal period of 61.2 μm. By tuning either the near-IR pump wavelength between 1.75 and 2 μm, or the temperature of the GaAs crystal, the mid-IR output tuned between 2 and 11 μm, limited only by the spectral range of the OPO mirrors. The pump threshold of the singly-resonant OPO was 16 μJ for the 6-ns pump pulses, and the photon conversion slope efficiency reached 54%. Also, we show experimentally the possibility of pump-polarization-independent frequency conversion in GaAs.