Laser linewidth | EPFL Graph Search

Laser linewidth is the spectral linewidth of a laser beam. Two of the most distinctive characteristics of laser emission are spatial coherence and spectral coherence. While spatial coherence is related to the beam divergence of the laser, spectral coherence is evaluated by measuring the linewidth of laser radiation. Theory History: First derivation of the laser linewidth The first human-made coherent light source was a maser. The acronym MASER stands for "Microwave Amplification by Stimulated Emission of Radiation". More precisely, it was the ammonia maser operating at 12.5 mm wavelength that was demonstrated by Gordon, Zeiger, and Townes in 1954. One year later the same authors derived theoretically the linewidth of their device by making the reasonable approximations that their ammonia maser Notably, their derivation was entirely semi-classical, describing the ammonia molecules as quantum emitters and assuming classical electromagne

Interaction of Confined Polaritons in Microcavity Structures

Morteza Navadeh Toupchi

A polariton is a quasiparticle formed from the coupling of a confined photon in a cavity to electronic excitation, like exciton in a semiconductor. This dissertation reports on series of experiments in confined polariton interaction by design, fabrication, and characterization of semiconductor microcavity structures operating in strong or weak coupling regime. In the first part of the thesis, we mainly concentrate on the optical study of the 2D microcavity sample, including spin-dependent lower-upper polariton cross interactions by pump-probe spectroscopy technique, supported by theoretical analyses and numerical simulations based on Gross-Pitaevskii equations. In particular, we present a scattering resonance behavior via an exciton molecule (biexciton) when polaritons from both the upper and lower branches with anti-parallel spins are involved through a polaritonic cross Feshbach resonance. This demonstration will permit the control of the polariton interbranch scattering. The second part of the thesis is dedicated to the design and fabrication of the potentials where the photonic part of polaritons is confined laterally by adjusting the thickness of the cavity layer locally in so-called mesa structures. By engineering a periodic lattice of mesas on a two-dimensional microcavity, it is possible to couple confined polariton modes of nearby mesas to establish an optical lattice analogous to the crystalline semiconductorsâ electronic band structures. We especially demonstrate the localization of light with a lasing mode at the edge of the Brillouin zone in a two-dimensional triangular lattice. We produce a self-trapping of light by optically inducing a local breaking of the strong-coupling regime of excitons to photons. In the weak coupling regime, we control the confined modes by the shape of the generated defect. We also reveal a controllable localization degree and experimental signature of the Anderson localization in microcavity polaritons by inducing positional disorder in the triangular lattice. The last part is devoted to the fabrication of sub-micron size mesas to enhance polariton interaction by confining them tightly and discuss the quantum correlation of polaritons by a Hanbury Brown and Twiss (HBT) setting toward polariton blockade.

EPFL2021

Gigahertz free-space electro-optic modulators based on Mie resonances

Ileana-Cristina Benea-Chelmus

Electro-optic modulators are essential for sensing, metrology and telecommunications. Most target fiber applications. Instead, metasurface-based architectures that modulate free-space light at gigahertz (GHz) speeds can boost flat optics technology by microwave electronics for active optics, diffractive computing or optoelectronic control. Current realizations are bulky or have low modulation efficiencies. Here, we demonstrate a hybrid silicon-organic metasurface platform that leverages Mie resonances for efficient electro-optic modulation at GHz speeds. We exploit quasi bound states in the continuum (BIC) that provide narrow linewidth (Q = 550 at lambda(res) = 1594 nm), light confinement to the non-linear material, tunability by design and voltage and GHz-speed electrodes. Key to the achieved modulation of Delta T/T-max = 67% are molecules with r(33) = 100 pm/V and optical field optimization for low-loss. We demonstrate DC tuning of the resonant frequency of quasi-BIC by Delta lambda(res) = 11 nm, surpassing its linewidth, and modulation up to 5 GHz (f(EO,- 3dB) = 3 GHz). Guided mode resonances tune by Delta lambda(res) = 20 nm. Our hybrid platform may incorporate free-space nanostructures of any geometry or material, by application of the active layer post-fabrication.

NATURE PORTFOLIO2022

Self-Injection Locking of a Gain-Switched Laser Diode

Andrey Voloshin

We experimentally observe a self-injection locking regime of the gain-switched laser to a high-Q optical microresonator. We reveal that the comb generated by the gain-switched laser experiences a dramatic reduction of comb-teeth linewidths in this regime. We demonstrate the Lorentzian linewidth of the comb teeth of sub-kHz scale as narrow as for a nonswitched self-injection locked laser. Such a setup allows generation of high-contrast electrically tunable optical frequency combs with tunable comb-line spacing in a wide range from 10 kHz up to 10 GHz. The characteristics of the generated combs are studied for various modulation parameters & mdash;modulation frequency and amplitude, and for parameters, defining the efficiency of the self-injection locking & mdash;locking phase, coupling efficiency, and pump frequency detuning.

AMER PHYSICAL SOC2021

Interaction of Confined Polaritons in Microcavity Structures

Morteza Navadeh Toupchi

EPFL2021