Light in a laser is confined in the form of standing waves. By engineering such waves, scientists have designed an optical system that enhances this confinement, producing a compact laser that emits a high-quality beam. See Letter p.196
Confining light without absorbing it is a key problem for lasers. Conventional lasers use optical cavities1 that trap light using mirrors, total internal reflection or scattering in periodic microstructures such as photonic crystals. For these systems, the quality factor — a measure of the efficiency of the reflecting boundaries — is limited, and depends only weakly on the geometry of the cavity. An alternative way to confine light is to use exotic optical 'modes' called bound states in the continuum (BICs), which can give rise to extremely large quality factors, depending on the systems' geometry. On page 196, Kodigala et al.2 report the efficient trapping of light in a patterned membrane that supports BICs. The authors demonstrate a compact laser that operates at telecommunication wavelengths, which could have many applications.
BICs were first theorized in the early days of quantum mechanics, when it was discovered3 that certain potential-energy profiles (potentials) could support spatially localized states of electrons that have energies larger than the maximum energy of the potential. For several decades, these states were considered to be only a mathematical curiosity. However, the advanced theory of resonances4 revealed that BICs can occur in many systems that support waves and wave propagation. Indeed, a 'phase-matching' condition was found5 under which the coupling of two waves interacting predominantly outside a potential leads to destructive interference, causing the waves to be trapped by the potential. Because of the generality of this effect, BICs are not restricted to quantum-mechanical systems — they are general wave phenomena6 found in electromagnetic waves, water and elastic waves, and in acoustic waves in air, and have been shown to exist as a special type of surface state7.
Mathematically speaking, BICs occur in systems in which at least one of the dimensions extends to infinity6. This is not the case for any realistic system. However, by using a cavity that supports several standing waves, the cavity's structure can be tuned until the system most closely satisfies the condition required for BICs in the corresponding infinite system. This 'supercavity' regime is entered when the quality factor no longer varies slowly, as in a conventional cavity, but instead grows rapidly, following the trend of a BIC before it reaches a maximum value, owing to the finite size of the cavity (Fig. 1a). Such an enhanced quality factor leads to a dramatic increase in the lifetime of the trapped waves that can be used for lasing.
To take advantage of this effect, Kodigala and colleagues fabricated a photonic membrane that consisted of dielectric (insulating) cylinders made of indium gallium arsenide phosphide (InxGa1−xAsyP1−y). The cylinders were connected to one other by bridges, to ensure mechanical stability of the membrane, and arranged in a square lattice (Fig. 1b). The authors selected three standing waves in this structure, and altered the parameters that describe the structure's geometry to realize a supercavity. They designed their laser to emit light at the wavelength most commonly used for telecommunications (about 1,550 nanometres).
Kodigala and colleagues 'pumped' their device with optical radiation of wavelength 1,064 nm, at room temperature. They then measured the spectrum of emitted light as a function of the pumping intensity and found that the device demonstrates robust lasing, which is characterized by a narrow spectral linewidth when the pumping power is above a threshold of 56 microwatts. The authors obtained such lasing for lattices of varying size (from 8 × 8 to 20 × 20 arrays) and cylinder radii ranging from 500 to 550 nm.
The authors' work is not the first demonstration of lasing in such systems. In 2014, lasers based on photonic-crystal membranes that produced high-quality beams were reported8. However, although the enhanced lasing was closely related to that of BICs, the researchers did not realize this fact and did not optimize their system's standing waves to achieve the conditions required for a supercavity. By contrast, Kodigala et al. took advantage of the BIC properties of their membrane and demonstrated a very compact laser (with a width of 9,600 nm, a thickness of 300 nm and a membrane consisting of as few as 8 × 8 dielectric cylinders).
Kodigala and collaborators' BIC lasing was expected, having been predicted theoretically. Their work will be useful for fabricating compact lasers that have applications in optical nanoantennae, optical computing and biosensing. For the next breakthrough, we should go to the nanoscale, where, as the physicist Richard Feynman said9, “There's plenty of room at the bottom”. We anticipate that BIC nanolasers could be realized in dielectric resonators that have a high refractive index, supporting electric and magnetic 'Mie' resonances10.Footnote 1
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High-Q Fano resonance based on degenerate modes in a single dielectric point-defect photonic crystal cavity with x–y asymmetry
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