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Topological hybrid silicon microlasers

Zhao, Han, Miao, Pei, Teimourpour, Mohammad H., Malzard, Simon, El-Ganainy, Ramy, Schomerus, Henning, Feng, Liang·2018·Nature Communications
DOI 10.1038/s41467-018-03434-2PMC PMC5841408
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Abstract

Topological physics provides a robust framework for strategically controlling wave confinement and propagation dynamics. However, current implementations have been restricted to the limited design parameter space defined by passive topological structures. Active systems provide a more general framework where different fundamental symmetry paradigms, such as those arising from non-Hermiticity and nonlinear interaction, can generate a new landscape for topological physics and its applications. Here, we bridge this gap and present an experimental investigation of an active topological photonic system, demonstrating a topological hybrid silicon microlaser array respecting the charge-conjugation symmetry. The created new symmetry features favour the lasing of a protected zero mode, where robust single-mode laser action in the desired state prevails even with intentionally introduced perturbations. The demonstrated microlaser is hybrid implemented on a silicon-on-insulator substrate, and is thereby readily suitable for integrated silicon photonics with applications in optical communication and computing.

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Fig. 1 Topological hybrid silicon microlaser. a Schematic of a topological laser array made of nine microring resonators with alternating weak (t1) and strong (t2) couplings, emulating an SSH model. A layer of 10-nm Cr (shown in yellow) is deposited on top of every second element to introduce distributed gain and loss. The red halos represent the intensity profile of the oscillating zero mode that resides at the central site and decays exponentially away from the centre, with zero intensity in every second element. b Spectral features of the topological laser array, highlighting the lasing selectivity of the topological zero mode. Due to the charge-conjugation symmetry, the spectrum is symmetric around the imaginary axis. The blue circles represent the calculated eigenvalues of all the nine supermodes of the laser array. c SEM pictures of the fabricated structure consisting of nine rings on a hybrid III–V/silicon platform. Each ring has inner and outer radii of 3.5 and 4.5 µm, respectively, and is made of 500 nm-thick InGaAsP quantum well layers on top of a 220 nm silicon layer grown on a silicon-on-insulator (SOI) substrate. The alternating 200 and 300 nm edge-to-edge separations between adjacent rings realise the weak coupling t1 = 78 GHz and the strong coupling t2 = 134 GHz, compositing an estimated Hermitian topological band gap of 112 GHz in the absence of loss. In the top panel of c, we highlighted the Cr layer with artificial yellow rings for better visualisation of the arrayed structure, since the Cr layer is only ~10 nm thick and difficult to be clearly seen, especially in pictures of low magnification. Scale bars in c: low magnification: 10 µm; high magnification: 2 µm
Fig. 1 Topological hybrid silicon microlaser. a Schematic of a topological laser array made of nine microring resonators with alternating weak (t1) and strong (t2) couplings, emulating an SSH model. A layer of 10-nm Cr (shown in yellow) is deposited on top of every second element to introduce distributed gain and loss. The red halos represent the intensity profile of the oscillating zero mode that resides at the central site and decays exponentially away from the centre, with zero intensity in every second element. b Spectral features of the topological laser array, highlighting the lasing selectivity of the topological zero mode. Due to the charge-conjugation symmetry, the spectrum is symmetric around the imaginary axis. The blue circles represent the calculated eigenvalues of all the nine supermodes of the laser array. c SEM pictures of the fabricated structure consisting of nine rings on a hybrid III–V/silicon platform. Each ring has inner and outer radii of 3.5 and 4.5 µm, respectively, and is made of 500 nm-thick InGaAsP quantum well layers on top of a 220 nm silicon layer grown on a silicon-on-insulator (SOI) substrate. The alternating 200 and 300 nm edge-to-edge separations between adjacent rings realise the weak coupling t1 = 78 GHz and the strong coupling t2 = 134 GHz, compositing an estimated Hermitian topological band gap of 112 GHz in the absence of loss. In the top panel of c, we highlighted the Cr layer with artificial yellow rings for better visualisation of the arrayed structure, since the Cr layer is only ~10 nm thick and difficult to be clearly seen, especially in pictures of low magnification. Scale bars in c: low magnification: 10 µm; high magnification: 2 µmCC BY · PubMed Central
Fig. 2 Topological laser action. a Lasing spectrum of the structure of the topological hybrid silicon microlaser as a function of the pumping power much below the threshold (top panel), approaching the threshold (middle panel) and well above the threshold (bottom panel). As expected, as the pump power is increased, the output emission experiences a transition from broadband photoluminescence to a single narrow band lasing with an increased extinction ratio. b Pump dependence of the laser emission intensity, demonstrating the fingerprint of single-mode lasing: only one threshold with no other kinks in the light–light curve. Blue dots are experimental data and the red lines are linear fits with least squares of the data before and after the lasing threshold, respectively
Fig. 2 Topological laser action. a Lasing spectrum of the structure of the topological hybrid silicon microlaser as a function of the pumping power much below the threshold (top panel), approaching the threshold (middle panel) and well above the threshold (bottom panel). As expected, as the pump power is increased, the output emission experiences a transition from broadband photoluminescence to a single narrow band lasing with an increased extinction ratio. b Pump dependence of the laser emission intensity, demonstrating the fingerprint of single-mode lasing: only one threshold with no other kinks in the light–light curve. Blue dots are experimental data and the red lines are linear fits with least squares of the data before and after the lasing threshold, respectivelyCC BY · PubMed Central
Fig. 3 Intrinsic single-supermode topological laser action. a Multimode lasing from an identically sized microlaser array as the topological microlaser, but without on-top Cr deposition on every second ring to introduce the distributed gain/loss profile. b Measured lasing mode profile of the topological microlaser without on-top Cr deposition. c Single-supermode lasing from the topological microlaser under the same pumping condition. d Measured lasing mode profile of the topological microlaser with the distributed gain/loss profile. The broad spectrum in a is due to mode competition between the transverse and longitudinal modes of the single-ring resonators, each of which forms its own collective state. In c, the distributed gain and loss judiciously spoils the quality factors of the strongly hybridised modes, topologically favoring only the zero mode centred around λ = 1523 nm
Fig. 3 Intrinsic single-supermode topological laser action. a Multimode lasing from an identically sized microlaser array as the topological microlaser, but without on-top Cr deposition on every second ring to introduce the distributed gain/loss profile. b Measured lasing mode profile of the topological microlaser without on-top Cr deposition. c Single-supermode lasing from the topological microlaser under the same pumping condition. d Measured lasing mode profile of the topological microlaser with the distributed gain/loss profile. The broad spectrum in a is due to mode competition between the transverse and longitudinal modes of the single-ring resonators, each of which forms its own collective state. In c, the distributed gain and loss judiciously spoils the quality factors of the strongly hybridised modes, topologically favoring only the zero mode centred around λ = 1523 nmCC BY · PubMed Central
Fig. 4 Effect of disorder on the topological microlaser. a Numerical calculations for the spectrum of a disordered lattice when an onsite perturbation is introduced to the third site from the right (highlighted in red), as indicated schematically at the top of the same panel. b The mode profile of the topological eigenstate, normalised with identity norm calculated from the perturbed tight-binding Hamiltonian, where the general characteristics of the zero mode persist with addition of small light intensities on the previously dark sublattice (labelled in yellow marks). c SEM image of the perturbed topological laser array with an on-top thick layer of polymer covering the corresponding ring. Scale bar: 10 µm. d Measured emission spectrum of the perturbed laser array showing the maintained single-mode lasing feature against the introduced onsite perturbation. e Measured mode profile of the perturbed laser array. Despite the introduced perturbation, single-mode lasing and its spatial profile remain in the whole range of pump power (up to 2.4 GW m−2 that is ~3 times the lasing threshold) used in our experiment. This robustness arises from the topological robustness of the defect even with nonlinearity above the threshold (see Supplementary Note 2)
Fig. 4 Effect of disorder on the topological microlaser. a Numerical calculations for the spectrum of a disordered lattice when an onsite perturbation is introduced to the third site from the right (highlighted in red), as indicated schematically at the top of the same panel. b The mode profile of the topological eigenstate, normalised with identity norm calculated from the perturbed tight-binding Hamiltonian, where the general characteristics of the zero mode persist with addition of small light intensities on the previously dark sublattice (labelled in yellow marks). c SEM image of the perturbed topological laser array with an on-top thick layer of polymer covering the corresponding ring. Scale bar: 10 µm. d Measured emission spectrum of the perturbed laser array showing the maintained single-mode lasing feature against the introduced onsite perturbation. e Measured mode profile of the perturbed laser array. Despite the introduced perturbation, single-mode lasing and its spatial profile remain in the whole range of pump power (up to 2.4 GW m−2 that is ~3 times the lasing threshold) used in our experiment. This robustness arises from the topological robustness of the defect even with nonlinearity above the threshold (see Supplementary Note 2)CC BY · PubMed Central

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Metadata

Authors
Zhao, H., Miao, P., Teimourpour, M. H., Malzard, S., El-Ganainy, R., Schomerus, H., & Feng, L.
Venue
Nature Communications
Publisher
Springer Science and Business Media LLC
Published
2018-03-07
Volume
9
Issue
1
ISSN
2041-1723
DOI
10.1038/s41467-018-03434-2
PMC
PMC5841408
Cited by
429
References
3
Language
en
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