Unidirectional Laser Emission Research Articles

Extremely high Q and unidirectional laser emission due to combination of the Kolmogorov-Arnold-Moser barrier and the chaotic sea in a dielectric microdisk.

Emission characteristics of an oval-shaped microcavity laser are studied. In experiments, modes localized on periodic orbits emit unidirectionally with a narrow in-plane divergence angle of about 12deg. The origin of high directionality is elucidated by means of classical ray dynamics. Wave calculations show that the Q-factors of the resonances are higher than 108. We explain this extraordinary high Q-factor in relation with a dynamical barrier region where Kolmogorov-Arnold-Moser curves significantly obstruct leakages of resonances.

Coupled microcavities with unidirectional single mode via femtosecond laser direct-writing

Optical microcavities play a key role in both fundamental research on light-matter interaction and also applications such as integrated optics and sensors. Among them, whisper gallery mode (WGM) microcavity outstands itself by low loss, high Q-factor and high sensitivity to their dielectric environment. It can be found to have a variety of applications, including nonlinear optics, quantum electrodynamics, bio-sensors, low-threshold lasers, etc. However, the multi-mode nature of WGM microcavity is inconsistent with the basic requirements for these applications, i.e., a single-mode output and tunable wavelength. Therefore, the modulation of whisper gallery mode towards a unidirectional single-mode output is meaningful for both studying cavity dynamics and developing the above-mentioned applications. Here in this paper a brief review is carried out on the study of coupled dye-doped polymer microcavity processed by femtosecond laser direct-writing (FSLDW). The content covers fabrication, microcavity structure design, lasing and coupling mechanism study. The powerful patterning ability of FSLDW can realize complex three-dimensional microcavity structure design, which follows two schemes. One is to integrate a filter port to a microcavity. The other is to bring two or more microcavities in close proximity to each other for coupling. Based on such schemes, three kinds of microcavity structures, which are stacked microdisks, a microdisk integrated with gratings and stacked spiral-ring and circular-ring microcavity, are developed for the mode modulation. It is shown that all the three kinds of structures support unidirectional single-mode emissions with low lasing threshold. For the case of the stacked microdisks, the coupling can have a vernier effect among their modes and hence the mode selection. For the case of the microdisk cavity integrated with gratings, the gratings work as a filter port to select a certain mode according to their own period. For the case of the stacked spiral-ring and circular-ring microcavities, it is the structure asymmetry of the former that leads to the single-mode output. The mode modulations based on the mentioned microcavity structures have successfully maintained the high Q-factor of WGMs, which makes these cavities promising unidirectional single-mode microlasers. Combining with theoretical simulations, it is confirmed that the mode coupling between the microcavities (or between gratings and a microcavity) is responsible for the mode selection. Moreover, the unique structure design can break the rotational symmetry of the microcavity and hence achieve unidirectional laser emission. By careful designing and processing, successful modulationscan be achieved on a series of polymer microcavities. With both high Q-factor and good lasing directionality, these microcavity lasers could be well explored in integrated optical systems and organic optoelectronic devices.

Unidirectional lasing in semiconductor microring lasers at an exceptional point [Invited

Recent experiments demonstrated that chiral symmetry breaking at an exceptional point (EP) is a viable route to achieve unidirectional laser emission in microring lasers. By a detailed semiconductor laser rate equation model, we show here that unidirectional laser emission at an EP is a robust regime. Slight deviations from the EP condition can break preferential unidirectional lasing near threshold via a Hopf instability. However, above a “second” laser threshold, unidirectional emission is restored.

Front Cover: Whispering‐gallery microcavities with unidirectional laser emission (Laser Photonics Rev. 10(1)/2016)

Laser & Photonics ReviewsVolume 10, Issue 1 p. 1-1 Front CoverFree Access Front Cover: Whispering-gallery microcavities with unidirectional laser emission (Laser Photonics Rev. 10(1)/2016) First published: 12 January 2016 https://doi.org/10.1002/lpor.201670015Citations: 4AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract Optical whispering gallery microcavities enhance significantly the light-matter interaction, and become an excellent platform for studying both fundamental and applied physics. This review summarizes the progress in such microcavities towards unidirectional laser emission and important applications of high-performance photonic devices. (Picture: Xue-Feng Jiang et al.,10.1002/lpor.201500163 pp. 40–61, in this issue) Citing Literature Volume10, Issue1January 2016Pages 1-1 RelatedInformation

Whispering‐gallery microcavities with unidirectional laser emission

AbstractOptical whispering‐gallery mode (WGM) microcavities featuring ultrahigh Q factors and small mode volumes enhance significantly the interaction between light and matter, becoming an excellent platform for achieving ultralow‐threshold microlasers. However, the emission of traditional WGMs is isotropic due to the rotational symmetry of cavity geometries, which hinders the potential photonics applications. In this review, the progress in WGM microcavities towards unidirectional laser emission is summarized. When a subwavelength scatterer is placed on the boundary of the microcavity, the unidirectional emission occurs due to the collimation effect of the microcavity‐enhanced scattering field. Furthermore, microcavities deformed from the circular shapes can not only produce the chaos‐assisted unidirectional emission, but also maintain high Q factors by special design and fabrication processes. Finally, gratings along the circumference of the WGM microdisk or microring can scatter the WGMs in the vertical direction. The review also lists several important applications of these types of microcavities, such as wide‐band laser illumination source, free‐space coupling, evanescent‐field enhancement, optical energy storage, and sensing. image

PT-symmetric microring laser-absorber.

The lasing and coherent perfect absorption (CPA) properties of PT-symmetric microrings with mixed index and gain gratings, externally coupled to a bus waveguide, are theoretically investigated. For a complex grating at the PT-symmetry breaking point, perfect unidirectional (either clockwise or counterclockwise) laser emission can be realized, but the grating does not discriminate longitudinal modes and CPA can not be simultaneously achieved. Above the grating the PT-symmetry breaking point, single-mode emission, and simultaneous CPA can be obtained, with unbalanced and controllable excitation of clockwise and counterclockwise modes in the ring.

Whispering-gallery mode resonators for highly unidirectional laser action

Optical microcavities can be designed to take advantage of total internal reflection, which results in resonators supporting whispering-gallery modes (WGMs) with a high-quality factor (Q factor). One of the crucial problems of these devices for practical applications such as designing microcavity lasers, however, is that their emission is nondirectional due to their radial symmetry, in addition to their inefficient power output coupling. Here we report the design of elliptical resonators with a wavelength-size notch at the boundary, which support in-plane highly unidirectional laser emission from WGMs. The notch acts as a small scatterer such that the Q factor of the WGMs is still very high. Using midinfrared (λ ∼ 10 μm) injection quantum cascade lasers as a model system, an in-plane beam divergence as small as 6 deg with a peak optical power of ∼5 mW at room temperature has been demonstrated. The beam divergence is insensitive to the pumping current and to the notch geometry, demonstrating the robustness of this resonator design. The latter is scalable to the visible and the near infrared, thus opening the door to very low-threshold, highly unidirectional microcavity diode lasers.

Angular emission characteristics of quantum cascade spiral microlasers

We perform ray and wave simulations of passive and active spiral-shaped optical microcavities, comparing our results to experimental data obtained with mid-infrared quantum cascade spiral microlasers. Focusing on the angular emission characteristics, we find that both ray and wave simulations are consistent with the experimental data, showing richly-featured, multidirectional far-field emission patterns in the case of uniform pumping and TM-polarized light. Active cavity simulations using the Schr odinger-Bloch model indicate that selective pumping of the quantum cascade spiral microlasers near the resonator boundary will yield unidirectional laser emission.

Unidirectional single-frequency lasing from a ring-spiral coupled microcavity laser

A ring-spiral coupled microcavity laser was fabricated. The ring resonator works as an oscillator and has a diameter that is slightly less than that of the spiral cavity. The spiral provides wavelength selective feedback and emits light out from the notch. As a result, the coupled cavity generates unidirectional single-frequency laser emission. Its lasing threshold is also significantly lower than a single spiral-shaped microcavity laser.

Unidirectional laser emission from π-conjugated polymer microcavities with broken symmetry

We report unidirectional laser emission from π-conjugated polymer microcavities with broken symmetry geometries such as spiral and microdisk containing a “line defect,” in comparison with plain microdisk cavity having isotropic emission. We found that the laser emission directionality contrast ratio is 8–10 and far field lateral divergence angle is 12°–15° for both broken symmetry geometries, with no significant increase in the laser threshold intensity. Fourier transform analysis of the laser emission spectra shows that unlike microdisks with line defect, the variation of light trajectories in the spiral microcavities leads to less defined laser modes.

Unidirectional laser emission from polymer-based spiral microdisks

Lasing emission has been observed in optically pumped DCM-poly(methyl methacrylate)-based spiral-shaped micropillar cavities. By transverse ring shaping of the pump beam, unidirectional laser emission is being clearly favored. The resulting improved far-field directionality evidenced by our imaging technique is shown to originate from the notch of the spiral microcavities.

Current-injection spiral-shaped microcavity disk laser diodes with unidirectional emission

A spiral-shaped microcavity heterojunction laser diode fabricated with InGaN multiple quantum wells is demonstrated to operate under current injection conditions and emit unidirectionally. Room-temperature laser operation was achieved for microcavity disk radii ranging from 50 to 350 μm and threshold current densities as low as 4.6 kA/cm2. Unidirectional laser emission is clearly revealed in the far-field pattern with the lateral divergence angle ranging from 60° to 75°. Output power of more than 25 mW was obtained for emission wavelengths near 400 nm.