Mid-infrared continuous-wave parametric amplification in chalcogenide microstructured fibers

The persistent growth of interest in the middle infrared (MIR) is stimulating the development of sources and components. Novel waveguides and fibers for the efficient use of nonlinear effects in the MIR are being intensively studied. Highly nonlinear silica fibers have enabled record performances of highly versatile parametric processes in the telecommunication band. However, no waveguiding platforms (to our knowledge) have yet solved the trade-off among high nonlinearity, low propagation losses and dispersion in the MIR. As single waveguide designs have not yet hit this particular optimal point, only pulsed–pumped demonstrations have been carried out, hindering any application requiring narrow linewidth, continuous-wave (c.w.) operation, or signal modulation. Here, we show MIR c.w. parametric amplification in a Ge10As22Se68 tapered fiber. Leveraging state-of-the-art fabrication techniques, we use a photonic crystal fiber (PCF) geometry combining high nonlinearity and low dispersion, while maintaining single mode and low losses in the short-wave IR and MIR. We experimentally demonstrate 5 dB signal amplification and 3 dB idler conversion efficiency using only 125 mW of pump in the 2 μm wavelength range. Our result is not only the first c.w. parametric amplification measured at 2 μm in any waveguide, but it also establishes GeAsSe PCF tapers as the most promising all-fibered, high-efficiency parametric converter for advanced applications in the MIR. © 2017 Optical Society of America

Development of coherent mid-infrared source using chalcogenide photonic crystal fibers

Sida Xing

Many molecule bonds have vibration frequencies in the mid-infrared band. Thus, this band is of great interest to molecular spectroscopy, material processing and medical applications. However, many optical materials typically used for laser sources experience high losses due to the presence of such vibration bonds making the design of mid-infrared coherent sources challenging. Such task is even more difficult when considering designing all-fiber sources which offer additional compactness and robustness. The goal of this project is to address such challenge by designing all-fiber mid-infrared sources based on chalcogenide fibers and nonlinear effects. Using degenerated parametric conversion of a 2 um pump and a telecom band signal laser, one can generate an idler in the mid-infrared band. As a first step, the linear and nonlinear performances of several chalcogenide fibers were compared. The results prove the chalcogenide photonic crystal fiber design to be the ideal platform for this project as it offers high nonlinearity and low loss. Numerical simulations provide further improvement of photonic crystal geometry. Our improved designs allow for an extreme shaping of the dispersion to optimize the four-wave-mixing process within the thulium /holmium fiber laser band. In the meantime, several cavity configurations were tested to boost the thulium/holmium laser performance, that are required for pumping of our chalcogenide fibers. Polarizer-free cavity designs result more compact laser footprint, which moreover enables a higher slope efficiency. Such lasers enable precise measurements of sample fiber optical parameters. The combined efforts on fiber and laser optimization leads to efficient continuous-wave parametric conversion. Thanks to the excellent mid-infrared transmittance of chalcogenide glass, this high efficiency can be extended to longer pumps in mid-infrared. However, fiber fuse, a result of heat dissipative solitons, is the main constrain for reaching better conversion efficiency. For ultrafast applications, a sub-cm length chalcogenide photonic crystal fiber generates flat-top, linearly chirped supercontinuum in the normal dispersion region. This pulse length could be compressed to two optical cycles by linear compression. Once again, the experimental results match perfectly with the simulations. Overall, we show in this thesis that chalcogenide PCFs are promising for highly efficient and low threshold nonlinear optics in the middle infrared, offering new options for light generation in this critical wavelength band.

EPFL2019

Experimental and numerical investigation of two-dimensional photonic crystals for application in integrated optics

David Leuenberger

Two-dimensional (2D) photonic crystals (PhCs) at near-infrared wavelengths are promising candidates for novel integrated optics applications. The main focus being on 'all PhC' monolithically integrated optical circuits. Because of well established fabrication technologies particular emphasis is on 'quasi' 2D PhCs where a 2D lattice of air holes is etched through a planar step-index waveguide providing the optical confinement in the third dimension. This approach has been successfully demonstrated both in GaAs and InP-based systems. We have studied vertical low-index-contrast structures both in GaAs and InP. These structures suffer inherently from radiation losses that are strongly affected by the air hole depth and shape. A semi-analytical 'ε"-model' enables out-of-plane scattering to be described by an effective imaginary dielectric constant in the air-holes which can be used in 2D finite difference time domain (FDTD) calculations. The model, once validated by 3D FDTD, allows us to deduce from optical simple slab transmission measurements the structural parameters (e.g. size, depth and form of the hole). The experimental transmission spectra are obtained with the so-called 'internal light source technique' (ILS), which allows for quantitative normalised transmission measurements, thus eliminating the uncertainties due to the in-out coupling arising in alternative characterisation techniques, i.e. end-fire. The ILS technique proved to be a versatile method without stringent limits for assessing the fabrication parameters (air-filling factor and loss) but also proved to be suitable for testing building blocks. In this thesis different building blocks were fabricated in the GaAs system and analysed using diverse modeling techniques (e.g. plane wave expansion, FDTD and coupled mode theory). The first building block was used to study contra-directional coupling of the fundamental mode with higher-order modes in channel waveguides. It is an extension of the work on waveguides with symmetric periodic corrugation to the case of multi periodicity. We showed that by breaking the translational symmetry in these structures referred to as hybrid waveguides, the number of mini-stopbands is increased and not, as one could have expected, decreased. A second building block analysed to guide light around the corner, are cavity resonant bends (CRBs). As our analysis showd that CRBs work best with non-degenerate cavity modes, different low-symmetry order cavities were studied with differ coupling coefficients between cavity and waveguides. Unfortunately, the resulting peak transmission and overall performance was below our expectations. An important building block in integrated optics are directional couplers. They have a potential for applications such as 3dB-intensity splitting or channel add-drop filtering. To reduce the coupling length, transmission measurements of a series of coupler structures of different lengths were taken. In the case of two W3 waveguides separated by a single row, the coupling length proved to be very dependent on the hole diameter in the single row. For this case the experimentally determined coupling length, which has been validated by both PWE and FDTD simulations, was of the order of 350 periods which corresponds to an absolute length of about 80 μm, a value that is small for conventional optics, but rather large for the domain of PhC optics where due to the unusually high propagation losses the integrated circuit has to be kept small. This value could, however, be reduced by a factor of at least three when reducing the ratio between the barrier hole and the nominal hole radius to 0.5. Finally we studied the waveguides needed to connect the different building blocks. The standard solution are channel waveguides, line-defects obtained by omitting a number of rows of holes. An alternative approach are coupled cavity waveguides (CCWs). They not only offer the potential for waveguiding but may also be used for non-linear optics by exploiting the special dispersion properties of CCWs (e.g. low group-velocity). A tight-binding model has been set-up that describes the interaction between the neighbouring cavities and that allows one to deduce the dispersion properties of the chain from the single cavity field distribution.

EPFL2004

Coherence properties of microcavity polaritons

Stefan Kundermann

This thesis presents the coherence properties of polaritons in semiconductor microcavities. Semiconductor microcavities are microstructures in which the exciton ground state of a semiconductor quantum well is coupled to a photonic mode of a microresonator. The strong coupling mixes the character of excitons and photons, giving rise to the lower and upper polariton branches, quasiparticles with an unusual energetic dispersion relation due to the extreme mass difference between exciton and photon. Particularly special is the dispersion of the lower polariton, which forms a dip in the 2-dimensional k-space around the lowest energy state with zero in-plane momentum. In this dip, which can be seen as a trap in momentum space, the polaritons are efficiently isolated from dephasing mechanisms involving phonons. Polaritons can be resonantly excited at desired points on the polariton dispersion by shining on the microcavity laser light at the appropriate angle and wavelength. Polaritons can interact and scatter pairwise with each other conserving energy and in plane momentum k, a process similar to parametric scattering of photons in a nonlinear crystal. One polariton from a pump reservoir scatters down to the signal state at k = 0 (corresponding to normal incidence) and a second takes away the excess energy and momentum of the first and scatters up to the idler position ({kP, kP} → {0, 2kP }). This process can be stimulated by a small amount of signal polaritons injected with a probe laser beam at normal incidence. Here the coherence properties of the polariton parametric scattering have been investigated using spectroscopy techniques sensitive to the optical phase, for example coherent control with phase-locked femtosecond probe pulses. Just above the threshold for the stimulated parametric scattering, the parametric amplification process is given by the linear superposition of the individual amplification processes of each probe pulse. The emission of signal, pump, and idler can be controlled by tuning the relative phase of the 150fs-long probe pulses, which are separated by a few picoseconds in time. Experiments are presented that deal with the real-time dynamics of the parametric scattering in the spontaneous and the stimulated regime. It is shown, that in the spontaneous regime the scattering is started by a small amount of polaritons which have relaxed to the band bottom by emitting phonons. In the regime where polariton scattering is stimulated by an external probe, the rise of the signal intensity is delayed with respect to the arrival time of both pump and probe, a feature that can be attributed to the complex phase-matching mechanism for the parametric scattering. In the second part of the thesis, the spontaneous build up of a macroscopic coherence in a CdTe microcavity under non-resonant laser excitation is analysed. The build up of a long-range spatial coherence easily exceeding the thermal wavelength of the polaritons is shown. This is the hallmark of Bose-Einstein condensation and the proof of a macroscopic wavefunction. Experimental data on the statistical distribution of the polaritons in time, the polarisation of the non-linear emission, and the quantum transition from a thermal to a coherent state1 confirm that Bose-Einstein condensation of microcavity polaritons has been observed. We regard these observations as the first bullet-proof evidence for spontaneous Bose-Einstein condensation in a solid state system, a phenomenon that has been the subject to many investigations and controversies during the past four decades. ------------------------------ 1 The data about the statistical distribution, the polarisation, and the transition from a thermal to a coherent state is by courtesy of Jacek Kasprzak of the University of Grenoble.

EPFL2006