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Register a new userReconsidering the design of planar plasmonic lasers: gain, gap layers, and mode competition
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Because surface plasmons can be confined below the diffraction limit, metallic lasers that support plasmonic modes can provide miniaturized sources of electromagnetic waves. Such devices often exploit a multilayer design, in which a semiconductor gain layer is placed near a metallic interface with a gap layer in between. However, despite many experimental demonstrations, key considerations for these planar metallic lasers remain understudied, leading to incorrect conclusions about the optimal design. Here, we pursue a detailed experimental and theoretical study of planar metallic lasers to explore the effect of design parameters on the lasing behavior. We print semiconductor nanoplatelets as a gain layer of controllable thickness onto alumina-coated silver films with integrated planar Fabry-Perot cavities. Lasing behavior is then monitored with spectrally and polarization-resolved far-field imaging. The results are compared with a theoretical waveguide model and a detailed rate-equation model, which consider both plasmonic and photonic modes. We show that the nature of the lasing mode is dictated by the gain-layer thickness. Moreover, by explicitly treating gain in our waveguide model, we find that, contrary to conventional wisdom, a gap layer with high refractive index is advantageous for plasmonic lasing. Additionally, our rate-equation model reveals a regime where plasmonic and photonic modes compete within the same device, raising the possibility of facile, active mode switching. These findings provide guidance for future designs of metallic lasers and could lead to on-chip lasers with controlled photonic and plasmonic output, switchable at high speeds.
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Although nanolasers typically have low Q-factors and high lasing thresholds, they have been successfully implemented with various gain media. Intuitively, it seems that an increase in the gain coefficient would improve the characteristics of nanolasers. For a plasmonic nanolaser, in particular, a distributed-feed-back (DFB) laser, we propose a self-consistent model that takes into account both spontaneous emission and the multimode character of laser generation to show that for a given pumping strength, the gain coefficient has an optimal value at which the radiation intensity is at a maximum and the radiation linewidth is at a minimum.
We consider design optimization of passively mode-locked two-section semiconductor lasers that incorporate intracavity grating spectral filters. Our goal is to develop a method for finding the optimal wavelength location for the filter in order to maximize the region of stable mode-locking as a function of drive current and reverse bias in the absorber section. In order to account for material dispersion in the two sections of the laser, we use analytic approximations for the gain and absorption as a function of carrier density and frequency. Fits to measured gain and absorption curves then provide inputs for numerical simulations based on a large signal accurate delay-differential model of the mode-locked laser. We show how a unique set of model parameters for each value of the drive current and reverse bias voltage can be selected based on the variation of the net gain along branches of steady-state solutions of the model. We demonstrate the validity of this approach by demonstrating qualitative agreement between numerical simulations and the measured current-voltage phase-space of a two-section Fabry-Perot laser. We then show how to adapt this method to determine an optimum location for the spectral filter in a notional device with the same material composition, based on the targeted locking range, and accounting for the modal selectivity of the filter.
We investigate correlations between orthogonally polarized cavity modes of a bimodal micropillar laser with a single layer of self-assembled quantum dots in the active region. While one emission mode of the microlaser demonstrates a characteristic s-shaped input-output curve, the output intensity of the second mode saturates and even decreases with increasing injection current above threshold. Measuring the photon auto-correlation function g^{(2)}(tau) of the light emission confirms the onset of lasing in the first mode with g^{(2)}(0) approaching unity above threshold. In contrast, strong photon bunching associated with super-thermal values of g^{(2)}(0) is detected for the other mode for currents above threshold. This behavior is attributed to gain competition of the two modes induced by the common gain material, which is confirmed by photon crosscorrelation measurements revealing a clear anti-correlation between emission events of the two modes. The experimental studies are in excellent qualitative agreement with theoretical studies based on a microscopic semiconductor theory, which we extend to the case of two modes interacting with the common gain medium. Moreover, we treat the problem by an extended birth-death model for two interacting modes, which reveals, that the photon probability distribution of each mode has a double peak structure, indicating switching behavior of the modes for the pump rates around threshold.
It is shown theoretically that a nonchiral, two-dimensional array of metallic spheres exhibits optical activity as manifested in calculations of circular dichroism. The metallic spheres occupy the sites of a rectangular lattice and for off-normal incidence they show a strong circular-dichroism effect around the surface plasmon frequencies. The optical activity is a result of the rectangular symmetry of the lattice which gives rise to different polarizations modes of the crystal along the two orthogonal primitive lattice vectors. These two polarization modes result in a net polar vector, which forms a chiral triad with the wavevector and the vector normal to the plane of spheres. The formation of this chiral triad is responsible for the observed circular dichroism, although the structure itself is intrinsically nonchiral.
A laser is based on the electromagnetic modes of its resonator, which provides the feedback required for oscillation. Enormous progress has been made in controlling the interactions of longitudinal modes in lasers with a single transverse mode. For example, the field of ultrafast science has been built on lasers that lock many longitudinal modes together to form ultrashort light pulses. However, coherent superposition of many longitudinal and transverse modes in a laser has received little attention. The multitude of disparate frequency spacings, strong dispersions, and complex nonlinear interactions among modes greatly favor decoherence over the emergence of order. Here we report the locking of multiple transverse and longitudinal modes in fiber lasers to generate ultrafast spatiotemporal pulses. We construct multimode fiber cavities using graded-index multimode fiber (GRIN MMF). This causes spatial and longitudinal mode dispersions to be comparable. These dispersions are counteracted by strong intracavity spatial and spectral filtering. Under these conditions, we achieve spatiotemporal, or multimode (MM), mode-locking. A variety of other multimode nonlinear dynamical processes can also be observed. Multimode fiber lasers thus open new directions in studies of three-dimensional nonlinear wave propagation. Lasers that generate controllable spatiotemporal fields, with orders-of-magnitude increases in peak power over existing designs, should be possible. These should increase laser utility in many established applications and facilitate new ones.
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