arXiv subjects

K. -J. Boller

Publications and source records attributed to K. -J. Boller.

14 recordsLinked to original sources

arXiv2025

Blue-shifted dispersive waves and broadband UV emission using dual-core SiN waveguides

We show that using strongly coupled dual-core waveguides for supercontinuum generation shifts the wavelength of the high-frequency dispersive waves towards shorter wavelengths, as compared to generation in a single-core waveguide having the same core dimensions. In a demonstration experiment, we launch ultrashort infrared pump pulses at 1-$μ$m wavelength (285-THz frequency) into silicon nitride waveguides, where soliton formation and fission leads to generation of dispersive waves in the visible range. Efficient input coupling and controlled excitation of the fundamental supermodes of the dual-core waveguide is provided with adiabatic tapers and a dual-prong input structure. For the dual-core waveguide, the short-wavelength dispersive wave is located at 540~nm (green, 555~THz), which is blue-shifted by 80~nm (70~THz) compared to that of the single-core waveguide. Simultaneously, the dual-core waveguide generates broadband radiation spanning from the blue into the UV range, reaching to below 350~nm (above 855~THz), with typically a spectral density 25~dB below that of the dispersive wave. The broadband component can be addressed to third harmonic generation and is not observed in single-core supercontinuum generation. Numerical modeling shows good agreement with experimental measurements. The demonstrated dual-core approach and dedicated input coupling appear to hold promise also for other waveguide structures, independent of specific materials or core dimensions, by providing shorter wavelengths than with the respective single-core waveguide.

physics.optics↗

arXiv2025

Milliwatt-level UV generation using sidewall poled lithium niobate

Integrated coherent sources of ultra-violet (UV) light are essential for a wide range of applications, from ion-based quantum computing and optical clocks to gas sensing and microscopy. Conventional approaches that rely on UV gain materials face limitations in terms of wavelength versatility; in response frequency upconversion approaches that leverage various optical nonlinearities have received considerable attention. Among these, the integrated thin-film lithium niobate (TFLN) photonic platform shows particular promise owing to lithium niobate's transparency into the UV range, its strong second order nonlinearity, and high optical confinement. However, to date, the high propagation losses and lack of reliable techniques for consistent poling of cm-long waveguides with small poling periods have severely limited the utility of this platform. Here we present a sidewall poled lithium niobate (SPLN) waveguide approach that overcomes these obstacles and results in a more than two orders of magnitude increase in generated UV power compared to the state-of-the-art. Our UV SPLN waveguides feature record-low propagation losses of 2.3 dB/cm, complete domain inversion of the waveguide cross-section, and an optimum 50% duty cycle, resulting in a record-high normalized conversion efficiency of 5050 %W$^{-1}$cm$^{-2}$, and 4.2 mW of generated on-chip power at 390 nm wavelength. This advancement makes the TFLN photonic platform a viable option for high-quality on-chip UV generation, benefiting emerging applications.

physics.optics↗

arXiv2023

Hybrid integrated near UV lasers using the deep-UV Al2O3 platform

Hybrid integrated diode lasers have so far been realized using silicon, polymer, and silicon nitride (Si3N4) waveguide platforms for extending on-chip tunable light engines from the infrared throughout the visible range. Here we demonstrate the first hybrid integrated laser using the aluminum oxide (Al2O3) deep-UV capable waveguide platform. By permanently coupling low-loss Al2O3 frequency-tunable Vernier feedback circuits with GaN double-pass amplifiers in a hermetically sealed housing, we demonstrate the first extended cavity diode laser (ECDL) in the near UV. The laser shows a maximum fiber-coupled output power of 0.74 mW, corresponding to about 3.5 mW on chip, and tunes more than 4.4 nm in wavelength from 408.1 nm to 403.7 nm. Integrating stable, single-mode and tunable lasers into a deep-UV platform opens a new path for chip-integrated photonic applications.

physics.optics↗

arXiv2017

Cluster size dependence of high-order harmonic generation

We investigate high-order harmonic generation (HHG) from noble gas clusters in a supersonic gas jet. To identify the contribution of harmonic generation from clusters versus that from gas monomers, we measure the high-order harmonic output over a broad range of the total atomic number density in the jet (from 3*10^16 cm^{-3} to 3x10^18 cm{-3}) at two different reservoir temperatures (303 K and 363 K). For the firrst time in the evaluation of the harmonic yield in such measurements, the variation of the liquid mass fraction, g, versus pressure and temperature is taken into consideration, which we determine, reliably and consistently, to be below 20% within our range of experimental parameters. By comparing the measured harmonic yield from a thin jet with the calculated corresponding yield from monomers alone, we find an increased emission of the harmonics when the average cluster size is less than 3000. Using g, under the assumption that the emission from monomers and clusters add up coherently, we calculate the ratio of the average single-atom response of an atom within a cluster to that of a monomer and find an enhancement of around 10 for very small average cluster size (~200). We do not find any dependence of the cut-off frequency on the composition of the cluster jet. This implies that HHG in clusters is based on electrons that return to their parent ions and not to neighbouring ions in the cluster. To fully employ the enhanced average single-atom response found for small average cluster sizes (~200), the nozzle producing the cluster jet must provide a large liquid mass fraction at these small cluster sizes for increasing the harmonic yield. Moreover, cluster jets may allow for quasi-phase matching, as the higher mass of clusters allows for a higher density contrast in spatially structuring the nonlinear medium.

physics.optics↗

arXiv2015

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.

physics.plasm-ph↗

arXiv2013

High precision wavelength estimation method for integrated optics

A novel and simple approach to optical wavelength measurement is presented in this paper. The working principle is demonstrated using a tunable waveguide micro ring resonator and single photodiode. The initial calibration is done with a set of known wavelengths and resonator tunings. The combined spectral sensitivity function of the resonator and photodiode at each tuning voltage was modeled by a neural network. For determining the unknown wavelengths, the resonator was tuned with a set of heating voltages and the corresponding photodiode signals are collected. The unknown wavelength was estimated, based on the collected photodiode signals, the calibrated neural networks, and an optimization algorithm. The wavelength estimate method provides a high spectral precision of about 8 pm (5*10^(-6) at 1550 nm) in the wavelength range between 1549 nm to 1553 nm. A higher precision of 5 pm (3*10^(-6)) is achieved in the range between 1550.3 nm to 1550.8 nm, which is a factor of five improved compared to a simple lookup of data. The importance of our approach is that it strongly simplifies the optical system and enables optical integration. The approach is also of general importance, because it may be applicable to all wavelength monitoring devices which show an adjustable wavelength response.

physics.optics↗

arXiv2012

Mapping 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 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.

physics.optics↗

arXiv2012

25 kHz narrow spectral bandwidth of a wavelength tunable diode laser with a short waveguide-based external cavity

We report on the spectral properties of a diode laser with a tunable external cavity in integrated optics. Even though the external cavity is short compared to other small-bandwidth external cavity lasers, the spectral bandwidth of this tunable laser is as small as 25 kHz (FWHM), at a side-mode suppression ratio (SMSR) of 50 dB. Our laser is also able to access preset wavelengths in as little as 200 us and able to tune over the full telecom C-band (1530 nm - 1565 nm).

physics.optics↗

arXiv2010

Modeling of mode-locking in a laser with spatially separate gain media

We present a novel laser mode-locking scheme and discuss its unusual properties and feasibility using a theoretical model. A large set of single-frequency continuous-wave lasers oscillate by amplification in spatially separated gain media. They are mutually phase-locked by nonlinear feedback from a common saturable absorber. As a result, ultra short pulses are generated. The new scheme offers three significant benefits: the light that is amplified in each medium is continuous wave, thereby avoiding issues related to group velocity dispersion and nonlinear effects that can perturb the pulse shape. The set of frequencies on which the laser oscillates, and therefore the pulse repetition rate, is controlled by the geometry of resonator-internal optical elements, not by the cavity length. Finally, the bandwidth of the laser can be controlled by switching gain modules on and off. This scheme offers a route to mode-locked lasers with high average output power, repetition rates that can be scaled into the THz range, and a bandwidth that can be dynamically controlled. The approach is particularly suited for implementation using semiconductor diode laser arrays.

physics.optics↗

arXiv2009

Generation of stable ultra-relativistic attosecond electron bunches via the laser wakefield acceleration mechanism

In recent experiments ultra-relativistic femtosecond electron bunches were generated by a Laser Wakefield Accelerator (LWFA) in different regimes. Here we predict that even attosecond bunches can be generated by an LWFA due to the fast betatron phase mixing within a femtosecond electron bunch. The attosecond bunches are stable outside the LWFA and can propagate in vacuum many tens of centimeter without significant change in their duration. Our calculations show that evidence for the formation of attosecond bunches can be found in the spectrum of coherent betatron radiation from LWFA's.

physics.acc-ph↗

arXiv2008

Electron bunch injection at an angle into a laser wakefield

External injection of electron bunches longer than the plasma wavelength in a laser wakefield accelerator can lead to the generation of femtosecond ultrarelativistic bunches with a couple of percent energy spread. Extensive study has been done on external electron bunch (e.g. one generated by a photo-cathode rf linac) injection in a laser wakefield for different configurations. In this paper we investigate a new way of external injection where the electron bunch is injected at a small angle into the wakefield. This way one can avoid the ponderomotive scattering as well as the vacuum-plasma transition region, which tend to destroy the injected bunch. In our simulations, the effect of the laser pulse dynamics is also taken into account. It is shown that injection at an angle can provide compressed and accelerated electron bunches with less than 2% energy spread. Another advantage of this scheme is that it has less stringent requirements in terms of the size of the injected bunch and there is the potential to trap more charge.

physics.plasm-ph↗

arXiv2008

Electromagnetic radiation from a laser wakefield accelerator

Coherent and incoherent electromagnetic radiation emitted from a laser wakefield accelerator is calculated based on Lienard-Wiechert potentials. It is found that at wavelengths longer than the bunch length, the radiation is coherent. The coherent radiation, which typically lies in the infrared range, shows features that reveal details of the acceleration process and properties of the electron bunch, such as its duration, charge, energy, and offset with respect to the wakefield axis. The incoherent range of the spectrum, which extends to the X-ray frequency range, consists of rather broad peaks, which are caused by the acceleration.

physics.acc-ph↗

arXiv2007

Dynamics of Femtosecond Electron Bunches

In the laser wakefield accelerator (LWFA) a short intense laser pulse, with a duration of the order of a plasma wave period, excites an unusually strong plasma wake wave (laser wakefield). Recent experiments on laser wakefield acceleration [Nature (London) 431, p.535, p.538, p.541 (2004)] demonstrated generation of ultra-short (with a duration of a few femtoseconds) relativistic electron bunches with relatively low energy spread of the order of a few percent. We have studied the dynamics of such bunches in vacuum and in laser wakefield. The results show strong bunch dynamics already on a few millimeters propagation distance in both cases. In vacuum, the bunch radius and emittance quickly grow. The latter worsens the focusability of the bunch. We found that when a femtosecond bunch is accelerated in a channel-guided laser wakefield, for realistic bunch lengths, the bunch length is approximately conserved. However, the spread in betatron frequencies leads to fast betatron phase mixing in the bunch envelope for on-axis injection. When bunch is injected in a laser wakefield off-axis, the bunch decoherence results in considerable increase in the normalized bunch emittance, and, in some cases, in increase in the energy spread, after acceleration. We also discuss a possible two-stage laser wakefield accelerator.

physics.plasm-ph↗

arXiv2006

Ponderomotive scattering of an electron-bunch before injection into a laser wakefield

For the purpose of laser wakefield acceleration, it turned out that also the injection of electron bunches longer than a plasma wavelength can generate accelerated femtosecond bunches with relatively low energy spread. This is of high interest because such injecting bunches can be provided, e.g., by state-of-the-art photo cathode RF guns. Here we point out that when an e-bunch is injected in the wakefield it is important to take into account the ponderomotive scattering of the injecting bunch by the laser pulse in the vacuum region located in front of the plasma. At low energies of the injected bunch this scattering results in a significant drop of the collection efficiency. Larger collection efficiency can by reached with lower intensity laser pulses and relatively high injection energies. We also estimate the minimum trapping energy for the injected electrons and the length of the trapped bunch.

physics.plasm-ph↗