Spatiotemporal optical vortex reconnections of multi-vortices.

Vortex reconnections are ubiquitous events found in diverse media. Here we show that vortex reconnections also occur between spatiotemporal vortices in optical waves. Since vortices exhibit orbital angular momentum (OAM), the reconnections of optical vortices create a variety of connected OAM states. Dispersion and diffraction can cause different reconnection pairs, depending on the orientation of the vortices. The transverse crossing of two vortices with a topological charge of one can produce unique vortex loop reconnection patterns. Higher topological charges result in arrays of vortex loops and connection points. Crossing of three vortices produces spherical structures made of three symmetrical vortex arms. A three vortices reconnection with higher topological charges develops complicated patterns similar to turbulence cascade phenomena in other media. Studying optical vortex interactions may bring insight into vortex reconnections in other fields. We also provide experimental results of two-vortex loop interaction.

Continuously variable Fourier filters fabricated using varying angle glancing angle deposition for chip-scale spectroscopy.

Low-cost spectroscopy has received a great deal of attention in recent years in applications such as food inspection, disease detection, and manufacturing. Current spectroscopic systems rely on multiple optical components, making them mechanically fragile systems. In our previous work, we demonstrated the use of Fourier filtering using thin dielectric films. The sampling effect from the cavity resonances can be used to decompose a signal into its Fourier components. Although the thin films were deposited directly on the face of the detectors, filters of varying thicknesses were needed, which required multiple lithographic processes. To overcome this challenge, in this work, we use a continuously variable filters deposited by a single-step electron-beam evaporation technique. We demonstrate a novel, to our knowledge, method that utilizes the glancing angle deposition technique with a continuously varying angle in order to produce tens of variable Fourier filters in a single deposition run. To prove this technique, we deposit this variable filter on a 38-channel linear detector and show the results from this device.

Development of spatially variant photonic crystals to control light in the near-infrared spectrum.

Spatially Variant Photonic Crystals (SVPCs) have shown the ability to control the propagation and direction of light in the near-infrared spectrum. Using a novel approach for simplified modeling and fabrication techniques, we designed unique, spatially-varied, unit-cell structures to develop photonic crystals that maintain self-collimation and direction of light for desired beam tuning applications. The finite-difference time-domain technique is used to predict the self-collimation and beam-bending capabilities of our SVPCs. These SVPC designs and the simulation results are verified in laboratory testing. The experimental evidence shows that two-dimensional SVPCs can achieve self-collimation and direct light through sharp bends. The simplicity and quality of these designs show their potential for widespread implementation in modern devices. These SVPCs will serve as a unique solution to optical systems for optical computing, multiplexing, data transfer, and more.

Power-Dependent Investigation of Photo-Response from GeSn-Based p-i-n Photodetector Operating at High Power Density.

We report an investigation on the photo-response from a GeSn-based photodetector using a tunable laser with a range of incident light power. An exponential increase in photocurrent and an exponential decay of responsivity with increase in incident optical power intensity were observed at higher optical power range. Time-resolved measurement provided evidence that indicated monomolecular and bimolecular recombination mechanisms for the photo-generated carriers for different incident optical power intensities. This investigation establishes the appropriate range of optical power intensity for GeSn-based photodetector operation.

Bio-inspired spatially variant photonic crystals for self-collimation and beam-steering applications in the near-infrared spectrum.

The self-collimation of light through Photonic Crystals (PCs) due to their optical properties and through a special geometric structure offers a new form of beam steering with highly optical control capabilities for a range of different applications. The objective of this work is to understand self-collimation and bending of light beams through bio-inspired Spatially Variant Photonic Crystals (SVPCs) made from dielectric materials such as silicon dioxide and common polymers used in three-dimensional printing like SU-8. Based upon natural PCs found in animals such as butterflies and fish, the PCs developed in this work can be used to manipulate different wavelengths of light for optical communications, multiplexing, and beam-tuning devices for light detection and ranging applications. In this paper, we show the optical properties and potential applications of two different SVPC designs that can control light through a 90-degree bend and optical logic gates. These two-dimensional SVPC designs were optimized for operation in the near-infrared range of approximately 800-1000 nm for the 90-degree bend and 700-1000 nm for the optical logic gate. These SVPCs were shown to provide high transmission through desired regions with low reflection and absorption of light to prove the potential benefits of these structures for future optical systems.

Low control-power wavelength conversion on a silicon chip.

We demonstrate controlled wavelength conversion on a silicon chip based on four-wave mixing Bragg scattering (FWM-BS). A total conversion efficiency of 5% is achieved with strongly unbalanced pumps and a controlling peak power of 55 mW, while the efficiency is over 15% when using less asymmetric pumps. The numerical simulation agrees with the experimental results. Both time domain and spectral domain noise measurements show as low as 2 dB signal-to-noise ratio (SNR) penalty because of the strong pump noise, two-photon absorption, and free-carrier absorption in silicon. We discuss how the scheme can be used to implement an all-optically controlled high-speed switch.

Theoertical investigation of quantum waveform shaping for single photon emitters.

We investigate a new technique for quantum-compatible waveform shaping that extends the time lens method, and relies only on phase operations. Under realistic experimental conditions, we show that it is possible to both temporally compress and shape optical waveforms in the nanosecond to tens of picoseconds range, which is generally difficult to achieve using standard dispersive pulse-shaping techniques.

SU-8 nanoimprint fabrication of wire-grid polarizers using deep-UV interference lithography.

We describe a new fabrication method for making wire-grid polarizers for the visible and near-IR based on deep-UV interference lithography, nanoimprint, and glancing angle deposition. We fabricated aluminum wire grids with periods ranging from 375 to 230 nm with heights from 145 to 110 nm, respectively. The measured extinction ratio was as high as 220:1 at 1064 nm. The performance of the polarizer is limited by the roughness and porosity of the Al film and the underlying SU-8 structure. This method allows patterning of wire grids on any substrate material, which makes this an attractive method for fabricating wire-grid micropolarizers in a timely and cost-effective manner.

Spectral broadening and shaping of nanosecond pulses: toward shaping of single photons from quantum emitters.

We experimentally demonstrate spectral broadening and shaping of exponentially-decaying nanosecond pulses via nonlinear mixing with a phase-modulated pump in a periodically poled lithium niobate (PPLN) waveguide. 1550 nm pump light is imprinted with a temporal phase and used to upconvert a weak 980 nm pulse to 600 nm while simultaneously broadening the spectrum to that of a Lorentzian pulse up to 10 times shorter. While the current experimental demonstration is for spectral shaping, we also provide a numerical study showing the feasibility of subsequent spectral phase correction to achieve temporal compression and reshaping of a 1 ns mono-exponentially decaying pulse to a 250 ps Lorentzian, which would constitute a complete spectrotemporal waveform shaping protocol. This method, which uses quantum frequency conversion in PPLN with >100:1 signal-to-noise ratio, is compatible with single photon states of light.

A chip-scale, telecommunications-band frequency conversion interface for quantum emitters.

We describe a chip-scale, telecommunications-band frequency conversion interface designed for low-noise operation at wavelengths desirable for common single photon emitters. Four-wave-mixing Bragg scattering in silicon nitride waveguides is used to demonstrate frequency upconversion and downconversion between the 980 nm and 1550 nm wavelength regions, with signal-to-background levels > 10 and conversion efficiency of ≈ -60 dB at low continuous wave input pump powers (< 50 mW). Finite element simulations and the split-step Fourier method indicate that increased input powers of ≈ 10 W (produced by amplified nanosecond pulses, for example) will result in a conversion efficiency > 25 % in existing geometries. Finally, we present waveguide designs that can be used to connect shorter wavelength (637 nm to 852 nm) quantum emitters with 1550 nm.

Improving the performance of bright quantum dot single photon sources using temporal filtering via amplitude modulation.

Single epitaxially-grown semiconductor quantum dots have great potential as single photon sources for photonic quantum technologies, though in practice devices often exhibit nonideal behavior. Here, we demonstrate that amplitude modulation can improve the performance of quantum-dot-based sources. Starting with a bright source consisting of a single quantum dot in a fiber-coupled microdisk cavity, we use synchronized amplitude modulation to temporally filter the emitted light. We observe that the single photon purity, temporal overlap between successive emission events, and indistinguishability can be greatly improved with this technique. As this method can be applied to any triggered single photon source, independent of geometry and after device fabrication, it is a flexible approach to improve the performance of systems based on single solid-state quantum emitters, which often suffer from excess dephasing and multi-photon background emission.

Experimental wavelength-division-multiplexed photon-pair distribution.

We have experimentally implemented the distribution of photon pairs produced by spontaneous parametric downconversion through telecom dense-wavelength-division-multiplexing filters. Using the measured counts and coincidences between symmetric channels, we evaluate the maximum fringe visibility that can be obtained with polarization-entangled photons and compare different filter technologies.

Two-photon interference using background-free quantum frequency conversion of single photons emitted by an InAs quantum dot.

We show that quantum frequency conversion (QFC) can overcome the spectral distinguishability common to inhomogeneously broadened solid-state quantum emitters. QFC is implemented by combining single photons from an InAs/GaAs quantum dot (QD) at 980 nm with a 1550 nm pump laser in a periodically poled lithium niobate (PPLN) waveguide to generate photons at 600 nm with a signal-to-background ratio exceeding 100:1. Photon correlation and two-photon interference measurements confirm that both the single photon character and wave packet interference of individual QD states are preserved during frequency conversion. Finally, we convert two spectrally separate QD transitions to the same wavelength in a single PPLN waveguide and show that the resulting field exhibits nonclassical two-photon interference.

Low-noise chip-based frequency conversion by four-wave-mixing Bragg scattering in SiN(x) waveguides.

Low-noise, tunable wavelength-conversion through nondegenerate four-wave mixing Bragg scattering in SiN(x) waveguides is experimentally demonstrated. Finite element method simulations of waveguide dispersion are used with the split-step Fourier method to predict device performance. Two 1550 nm wavelength band pulsed pumps are used to achieve tunable conversion of a 980 nm signal over a range of 5 nm with a peak conversion efficiency of ≈5%. The demonstrated Bragg scattering process is suitable for frequency conversion of quantum states of light.

Generation of pulsed and continuous-wave squeezed light with 87Rb vapor.

We present experimental studies on the generation of pulsed and continuous-wave squeezed vacuum via nonlinear rotation of the polarization ellipse in a (87)Rb vapor. Squeezing is observed for a wide range of input powers and pump detunings on the D1 line, while only excess noise is present on the D2 line. The maximum continuous-wave squeezing observed is -1.4 +/- 0.1 dB (-2.0 dB corrected for losses). We measure -1.1 dB squeezing at the resonance frequency of the (85)Rb F = 3 --> F' transition, which may allow the storage of squeezed light generated by (87)Rb in a (85)Rb quantum memory. Using a pulsed pump, pulsed squeezed light with -1 dB of squeezing for 200 ns pulse widths is observed at 1 MHz repetition rate.

Theoretical and experimental investigation of broadband cascaded four-wave mixing in high-Q microspheres.

We analyze the process of cascaded four-wave mixing in a high-Q microcavity and show that under conditions of suitable cavity-mode dispersion, broadband frequency combs can be generated. We experimentally demonstrate broadband, cascaded four-wave mixing parametric oscillation in the anomalous group-velocity dispersion regime of a high-Q silica microsphere with an overall bandwidth greater than 200 nm.

Silicon-waveguide-coupled high-Q chalcogenide microspheres.

We fabricate high-Q arsenic triselenide glass microspheres through a three-step resistive heating process. We demonstrate quality factors greater than 2 x 10(6) at 1550 nm and achieve efficient coupling via a novel scheme utilizing index-engineered unclad silicon nanowires. We find that at powers above 1 mW the microspheres exhibit high thermal instability, which limits their application for resonator-enhanced nonlinear optical processes.

Absorption of ultrashort optical pulses in water.

We investigate the linear propagation of 800 and 1530 nm ultrashort optical pulses in water. For all pulse repetition rates studied, we observe pure exponential decay down to a transmission of 2.5 x 10(-5). We further demonstrate that previous observations of nonmonoexponential decay and pulse splitup in broadband pulses are consistent with Beer's law in the purely linear regime.