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arXiv:2003.07352 · Physical Review Letters

Nanoscale Coherent Light Source

Raphael Holzinger, David Plankensteiner, Laurin Ostermann, Helmut Ritsch

Published Jun 26, 2020• 31 citations
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Abstract

A laser is composed of an optical resonator and a gain medium. When stimulated emission dominates mirror losses, the emitted light becomes coherent. We propose a new class of coherent light sources based on wavelength sized regular structures of quantum emitters whose eigenmodes form high-Q resonators. Incoherent pumping of few atoms induces light emission with spatial and temporal coherence. We show that an atomic nanoring with a single gain atom at the center behaves like a thresholdless laser, featuring a narrow linewidth. Symmetric subradiant excitations provide optimal operating conditions.

DOI
10.1103/physrevlett.124.253603

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A laser is composed of an optical resonator and a gain medium. When stimulated emission dominates mirror losses, the emitted light becomes coherent. We propose a new class of coherent light sources based on wavelength sized regular structures of quantum emitters whose eigenmodes form high-Q resonators. Incoherent pumping of few atoms induces light emission with spatial and temporal coherence. We show that an atomic nanoring with a single gain atom at the center behaves like a thresholdless laser, featuring a narrow linewidth. Symmetric subradiant excitations provide optimal operating conditions.
Subwavelength arrays of quantum emitters feature unique and largely design-able nonlinear optical properties. As a generic example we study a sub-wavelength sized ring of identical dipoles with an extra identical absorbing emitter at the center. For a 9-ring one finds the most efficient antenna configuration to direct single incoming photons to the center without re-emission. Interestingly, for very tiny structures sizes below a tenth of a wavelength, a full quantum description predicts an even larger absorption enhancement than a mean field model using a classical dipole approximation. We identify the origin of the enhancement in the appearance of a collective dark state with dominant center occupation. By special design of the center absorber one thus can harness the same efficiency enhancement also at different wavelengths and for other geometric structures. On the one hand this idea could be the basis of a new generation of highly efficient and selective nano antennas, while on the other hand, it could be an important piece towards understanding the surprising efficiency of natural light harvesting molecules. Adding gain via active dipoles in such nano ring systems allows to design minimalist laser like classical light sources. In the nonlinear operating regime at stronger pump fields these systems transform to non-classical light sources with tailor-able spatio-temporal emission upon coherent illumination.
Semiconductor quantum dots (QDs) are commonly considered as artificial atoms due to the 3D confinement of the electrons. These nanostructures are very promising for the realization of integrated devices such as single photon sources for quantum cryptography applications [1,2]. However, contrarily to genuine atoms, QDs are condensed matter systems and they consequently suffer from the coupling to their environment [3] which degrades the quality of the emitted photons in view of quantum cryptography applications. One way to circumvent this coupling is to excite strictly resonantly the QD at low temperature [4-10]. This difficult task has been achieved by designing an original excitation scheme to spatially decouple the excitation and detection paths. We first report that experimental studies of the resonant emission (RE) of single QDs can be strongly limited, even impossible, since most of the QDs show a very strong quenching of the RE of the fundamental transition. This issue is a broadly met experimental situation which is highly problematic for cavity quantum electrodynamics experiments in single QDs. We propose an efficient way to overcome this issue based on the use of an ultra-weak non-resonant laser that optically gates the QD RE [8] which controls the residual doping inherent to the QDs fabrication. It induces a complete recovery of the properties of an artificial atom, and the single QDs behave as quasi-ideal two-level systems. Up to now [7,9], the measurements on the QDs RE reported in the literature mainly focus on the incoherent fluorescence signal for achieving efficient single photon emission. In this regime, the spectrum linewidth is always limited to the radiative limit. We present experimental studies on the coherent laser light scattering by a single QD [7]. Besides the antibunching effect showing the non-classical nature of the emitted light, we observe that the QD emission spectrum is determined by the spectrum of the resonant excitation laser. By reducing the resonant excitation power, the so-called Mollow triplet shrinks to a single narrow Lorentzian line, while the relative intensity of this narrow peak increases and tends to overwhelm the RE spectrum. This implies that single QDs can produce single photons with a coherence time that is not limited anymore by the QD electronic properties, resulting in what we name an " ultra-coherent " character [7]. In conclusion, the resulting ultra-coherent single photon source promises high degrees of indistinguishability of the emitted photons which is a crucial requirement for quantum information applications. References [1] P. Michler et al., Science 290 (2000). [6] S. Ates et al., PRL 103 (2009). [2] C. Santori et al., Nature 419 (2002). [7] H. S. Nguyen et al., APL 99 (2011). [3] A. Berthelot et al., Nat. Phys. 2 (2006). [8] H. S. Nguyen et al., PRL 108 (2012). [4] A. Muller et al., PRL 99 (2007). [9] C. Matthiesen et al., PRL 1082 (2012). [5] R. Melet et al., PRB 78 (2008). [10] A. Reinhard et al., Nat. Phot. 6 (2012).
We discuss the prospects for a class of laser-like sources of atoms where the stimulated accumulation of atoms in the fundamental mode of a matter wave resonator is achieved by optical pumping. Precooled atoms in an internal state a which is not affected by the resonator trapping potential are excited into a state e by means of laser light. Once the atom is in state e it decays into the electronic ground state g by means of spontaneous emission, thereby occupying one of the resonator modes v equals 0, 1, ... Neglecting reabsorption of the spontaneously emitted photon, the probability for a transition into mode v is proportional to 1 + nv (`gain'), where nv is the number of atoms in mode v prior to the transition. Including linear losses this model shows threshold behavior in the fundamental mode v equals 0, mode competition and Poissonian atom statistics above threshold. We also analyze the `small-signal gain' including reabsorption of the spontaneously emitted photons. We demonstrate that the threshold for matter wave amplification can only be reached in small 3D resonators of linear size comparable to the wavelength of the spontaneous photon, or in resonators with effectively reduced dimensionality.
Regular arrays of two-level emitters at distances smaller than that of the transition wavelength collectively scatter, absorb, and emit photons. The strong inter-particle dipole coupling creates large energy shifts of the collective delocalized excitations, which generates a highly nonlinear response at the single and few photon level. This should allow us to implement nanoscale non-classical light sources via weak coherent illumination. At the generic tailored examples of regular chains or polygons, we show that the fields emitted perpendicular to the illumination direction exhibit a strong directional confinement with genuine quantum properties as antibunching. For short interparticle distances, superradiant directional emission can enhance the radiated intensity by an order of magnitude compared to a single atom focused to a strongly confined solid angle but still keeping the anti-bunching parameter at the level of g(2)(0)≈10−2.
We predict collective free-space lasing in a dense nanoscopic emitter arrangement where dipole-dipole coupled atomic emitters synchronize their emission and exhibit lasing behavior without the need for an optical resonator. At the example of a subwavelength-spaced linear emitter chain with varying fractions of pumped and unpumped emitters, we present a comprehensive study of this mirrorless lasing phenomenon. The total radiated power transitions from subradiant suppression under weak pumping to superradiant enhancement at stronger pumping, while exhibiting directional emission confined to a narrow spatial angle. At the same time multiple independent spectral emission lines below the lasing threshold merge toward a single narrow spectral line at high pump power. The most substantial enhancement and line narrowing occur when a fraction of unpumped atoms is present. We show that this leads to superradiant lasing near the bare atomic frequency, making the system a promising candidate for a minimalist active optical frequency reference.
A ring of dipole-coupled quantum emitters positioned at subwavelength distances possesses only a few radiant but many subradiant collective excitations with lifetimes growing exponentially with the atom number. These exhibit a three-dimensionally confined spatial radiation field pattern and form nanoscale high-QQQ optical resonators. Tailoring size, orientation, and distance between two rings allows for increasing the ratio of coherent ring-to-ring energy transfer versus free-space emission by orders of magnitude. In particular, the more dark and delocalized over several sites are excitons, the higher is their fidelity for transport to a second ring.