An almost lightless laser

Abstract

Lasers are often described in terms of a light field circulating in an optical resonator system. Now a laser has been demonstrated in which the field resides primarily in the atomic medium that is used to generate the light.Atomic clocks that operate on optical transitions in trapped ions or atoms are the most accurate instruments ever made by mankind [1][2][3][4]. In an atomic clock, the quantum mechanical phase between two atomic levels oscillates at a frequency given by the energy difference between those levels. The atom's oscillation is detected by a laser, and today's best clocks are limited by the frequency stability of that readout laser. Recently Murray Holland and colleagues have proposed an ultrastable laser based on an atomic gain medium with a very narrow frequency response [5]. The paper by Bohnet et al. on p. xxx of this issue [6] now reports the first prototype and characterizes key features of such a system. Atomic transitions used for optical clocks have linewidths at the Millihertz level and standard freerunning lasers are not sufficiently stable in frequency to directly interrogate the ultranarrow atomic transition. Rather, the world's best clock lasers are stabilized to a meticulously crafted and controlled reference optical resonator. Currently such reference resonators achieve relative frequency stability below 10 -15 , corresponding to a change in the resonator length by less than the radius of a proton. At this level, the stability of the reference resonator is limited by a fundamental process -thermal noise in the mirrors that leads to fluctuations in the length of the resonator, and thus in the frequency of the laser locked to it. Further progress, though difficult, may be possible by cryogenic operation and use of mirror materials with improved mechanical properties.As an alternative, theorists at Boulder have proposed a laser operating in an unusual parameter regime where the linewidth of the atomic gain medium is much smaller than the linewidth of the laser resonator. In such a system, that may be termed superradiant laser following Dicke's early proposal for a mirrorless laser [7], the laser oscillation is stored predominantly inside the atoms themselves, rather than in the light field circulating inside the resonator. This makes the laser frequency largely immune to cavity length changes (see Fig. 1), with an isolation that is given by the ratio of atomic to resonator linewidths. For their system Bohnet et al. measure an immunity factor exceeding 10 4 .To realize narrow-frequency gain in a medium consisting of rubidium atoms, a species that is easy to laser cool but where no suitable transition is readily available, Bohnet et al. resort to a trick: A narrow atomic line can be mimicked by applying an external laser to weakly drive a transition between two long-lived atomic ground states via a detuned excited state, see Fig. 1c. In this case, the gain profile, and thus the emitted laser light, is not absolutely narrow in frequency, but only when measured relative to the driving lase...

A superradiant clock laser on a magic wavelength optical lattice

“…Therefore, it has been suggested recently [2] and to some extent demonstrated experimentally [3,4] that an atomic clock transition could be used as a narrow band gain medium to run a laser. Due to the very feeble individual dipole moments of the atoms such a device can only be operated in the strong collective coupling regime, where superradiant emission into the field mode provides for the necessary gain [5]. In this domain of operation a huge collective dipole constituted by a large number of atoms, which are synchronized via their common coupling to the cavity field [6,7], will build up.…”

Superradiant Emission from Coherent Excitons in van Der Waals Heterostructures

“…[1,2,5,13] Furthermore, unlike the conventional lasers, all emitted photons take part in the resultant generation of the emission lines, enhancing the laser's efficiency. [1][2][3]9] Crucially, in the case of an SRE, in theory, only a few photons can stimulate coherent emission. This means SRE may not even exhibit a threshold power density value.…”

“…Superradiant emitters (SREs) are a new generation of light sources in which an ensemble of identical emitters produces a standing-wave of electric-dipoles that stimulate the emission from the system via collective light-matter interactions under a common pumping source. [1][2][3][4] The synchronized dipole oscillation leads to a strong amplification of the system's emission [5,6] that maintains a robust phase coherence and spectral 1L-TMDs possess a direct bandgap, it suffers from huge surface damping, which significantly influences the emission intensity making it challenging to obtain superradiant coupling of the excitons. In our design, employing the dielectric spacer in TMD/hBN/TMD heterostructure, the hBN layer preserves the direct bandgap nature of TMD layers and reduces the surface damping of the excitons while keeping them in close proximity.…”

“…Superradiant emitters (SREs) are a new generation of light sources in which an ensemble of identical emitters produces a standing‐wave of electric‐dipoles that stimulate the emission from the system via collective light‐matter interactions under a common pumping source. [ 1–4 ] The synchronized dipole oscillation leads to a strong amplification of the system's emission [ 5,6 ] that maintains a robust phase coherence and spectral purity, [ 7,8 ] which are required to circumvent the existing challenges of spectral broadening in conventional lasers [ 1,9 ] and appealing for countless applications, including fundamental sciences and cutting‐edge technologies. [ 10–12 ] SRE using an ensemble of suspended quantum dots, alkaline‐earth atoms etc., has been achieved recently.…”

Mesoscale cavities in hollow-core waveguides for quantum optics with atomic ensembles

“…Brought to you by | MIT Libraries Authenticated Download Date | 5/11/18 1:03 PM precision measurements of fundamental laws governing the universe [80]. Importantly though, the cavity required operation of this laser is of low finesse and typically operated with cooperativities C = g 2 /κγ < 1 .…”