On-chip phase-shift induced control of supercontinuum generation in a dual-core Si3N4 waveguide.

We investigate on-chip spectral control of supercontinuum generation, taking advantage of the additional spatial degree of freedom in strongly-coupled dual-core waveguides. Using numerical integration of the multi-mode generalized nonlinear Schrödinger equation, we show that, with proper waveguide cross-section design, selective excitation of supermodes can vary the dispersion to its extremes, i.e., all-normal or anomalous dispersion can be selected via phase shifting in a Mach-Zehnder input circuit. The resulting control allows to provide vastly different supercontinuum spectra with the same waveguide circuit.

Hybrid-integrated diode laser in the visible spectral range.

Generating visible light with wide tunability and high coherence based on photonic integrated circuits is of high interest for applications in biophotonics, precision metrology, and quantum technology. Here we present, to our knowledge, the first demonstration of a hybrid-integrated diode laser in the visible spectral range. Using an AlGaInP optical amplifier coupled to a low-loss Si3N4 feedback circuit based on microring resonators, we obtain a spectral coverage of 10.8 nm around 684.4 nm wavelength with up to 4.8 mW output power. The measured intrinsic linewidth is 2.3±0.2kHz.

Temporal model for quasi-phase matching in high-order harmonic generation.

We present a model for quasi-phase matching (QPM) in high-order harmonic generation (HHG). Using a one-dimensional description, we analyze the time-dependent, ultrafast wave-vector balance to calculate the on-axis harmonic output versus time, from which we obtain the output pulse energy. Considering, as an example, periodically patterned argon gas, as may be provided with a grid in a cluster jet, we calculate the harmonic output during different time intervals within the drive laser pulse duration. We find that identifying a suitable single spatial period is not straightforward due to the complex and ultrafast plasma dynamics that underlies HHG at increased intensities. The maximum on-axis harmonic pulse energy is obtained when choosing the QPM period to phase match HHG at the leading edge of the drive laser pulse.

Spectral control of high-harmonic generation via drive laser pulse shaping in a wide-diameter capillary.

We experimentally investigate spectral control of high-harmonic generation in a wide-diameter (508 µm) capillary that allows using significantly lower gas pressures coupled with elevated drive laser energies to achieve higher harmonic energies. Using phase shaping to change the linear chirp of the drive laser pulses, we observe wavelength tuning of the high-harmonic output to both larger and smaller values. Comparing tuning via the gas pressure with the amount of blue shift in the transmitted drive laser spectrum, we conclude that both adiabatic and non-adiabatic effects cause pulse-shaping induced tuning of high harmonics. We obtain a fractional wavelength tuning, Δλ/λ, in the range from -0.007 to + 0.01, which is comparable to what is achieved with standard capillaries of smaller diameter and higher pressures.

Single-shot fluctuations in waveguided high-harmonic generation.

For exploring the application potential of coherent soft x-ray (SXR) and extreme ultraviolet radiation (XUV) provided by high-harmonic generation, it is important to characterize the central output parameters. Of specific importance are pulse-to-pulse (shot-to-shot) fluctuations of the high-harmonic output energy, fluctuations of the direction of the emission (pointing instabilities), and fluctuations of the beam divergence and shape that reduce the spatial coherence. We present the first single-shot measurements of waveguided high-harmonic generation in a waveguided (capillary-based) geometry. Using a capillary waveguide filled with Argon gas as the nonlinear medium, we provide the first characterization of shot-to-shot fluctuations of the pulse energy, of the divergence and of the beam pointing. We record the strength of these fluctuations vs. two basic input parameters, which are the drive laser pulse energy and the gas pressure in the capillary waveguide. In correlation measurements between single-shot drive laser beam profiles and single-shot high-harmonic beam profiles we prove the absence of drive laser beam-pointing-induced fluctuations in the high-harmonic output. We attribute the main source of high-harmonic fluctuations to ionization-induced nonlinear mode mixing during propagation of the drive laser pulse inside the capillary waveguide.

Method to map individual electromagnetic field components inside a photonic crystal.

We present a method to map the absolute electromagnetic field strength inside photonic crystals. We apply the method to map the dominant electric field component Ez of a two-dimensional photonic crystal slab at microwave frequencies. The slab is placed between two mirrors to select Bloch standing waves and a subwavelength spherical scatterer is scanned inside the resulting resonator. The resonant Bloch frequencies shift depending on the electric field at the position of the scatterer. To map the electric field component Ez we measure the frequency shift in the reflection and transmission spectrum of the slab versus the scatterer position. Very good agreement is found between measurements and calculations without any adjustable parameters.

Time-dependent, three-dimensional simulation of free-electron-laser oscillators.

We describe a procedure for the simulation of free-electron-laser (FEL) oscillators. The simulation uses a combination of the MEDUSA simulation code for the FEL interaction and the OPC code to model the resonator. The simulations are compared with recent observations of the oscillator at the Thomas Jefferson National Accelerator Facility and are in substantial agreement with the experiment.

High-efficiency mid-infrared ZnGeP2 optical parametric oscillator directly pumped by a lamp-pumped, Q-switched CrTmHo:YAG laser.

We report a singly resonant optical parametric oscillator (SRO) based on a ZnGeP(2) crystal directly pumped by a lamp-pumped Q-switched CrTmHo:YAG laser. The IR was tunable from 4.7 to 7.8 microm via crystal angle tuning. A maximum optical to optical efficiency of 56% was obtained from the pump (2.09 microm) to total IR at a pump energy of 6.5 mJ. The corresponding idler energy was 1.45 mJ. The SRO was measured to have a slope efficiency of 64% and a threshold of 1 mJ. The spatial beam quality of the idler, characterized by the M(2) parameter, was 1.38 when the SRO was pumped at 2.5 times threshold. These results show that ZnGeP(2) optical parametric oscillators directly pumped by a CrTmHo:YAG laser can be operated efficiently, while maintaining good IR beam quality.