We propose a new light source based on having alkaline-earth atoms in an optical lattice collectively emit photons on an ultra-narrow clock transition into the mode of a high Q-resonator. The resultant optical radiation has an extremely narrow linewidth in the mHz range, even smaller than that of the clock transition itself due to collective effects. A power level of order 10 −12 W is possible, sufficient for phase-locking a slave optical local oscillator. Realizing this light source has the potential to improve the stability of the best clocks by two orders of magnitude.PACS numbers: 42.50. Nn, 06.30.6v, 37.10.Jk, 37.30.+i, 46.62.Eh Time and frequencies are the quantities that we can measure with the highest accuracy by far. From this fact derives the importance of clocks and frequency standards for many applications in technology and fundamental science. Some applications directly relying on atomic clocks are GPS, synchronization of data and communication networks, precise measurements of the gravitational potential of the earth, radio astronomy, tests of theories of gravity, and tests of the fundamental laws of physics.With the advent of octave spanning optical frequency combs [1,2] it has become feasible to use atomic transitions in the optical domain to build atomic clocks. Optical clocks based on ions [3] and ultracold neutral atoms confined in optical lattices [4] have recently demonstrated a precision of about 1 part in 10 15 at one second and a total fractional uncertainty of 10 −16 [4] or below [3], surpassing the primary cesium microwave standards [5,6].The state-of-the-art optical atomic clocks do not achieve the full stability that is in principle afforded by the atomic transitions on which they are founded. These transitions could have natural line-Qs of order 10 18 , exceeding the fractional stability of the clocks by a factor of ∼ 100. The main obstacle that prevents us from reaping the full benefit of the ultra-narrow clock transitions is the linewidth of the lasers used to interrogate these transitions. These lasers are stabilized against carefully designed passive high-Q cavities and achieve linewidths < 1 Hz, making them the most stable coherent sources of radiation. It is mainly the thermal noise of the reference cavity mirrors that prevent a further linewidth reduction [7] and substantially reducing this noise is hard [8].An elegant solution to these problems would be to directly extract light emitted from the ultra-narrow clock transition [9]. That light could then be used as an optical phase reference, circumventing the need for an ultra stable reference cavity. Unfortunately, the fluorescence light emitted on a clock transition is too weak for practical applications. For instance, for 10 6 fully inverted 87 Sr atoms the power of the spontaneously emitted light is of the order of 10 −16 W.The key observation that motivates this work is that if we could coerce the ensemble of atoms to emit the energy stored in them collectively rather than individually, the resulting power of order 10 −...

The discovery and characterization of exoplanets around nearby stars is driven by profound scientific questions about the uniqueness of Earth and our Solar System, and the conditions under which life could exist elsewhere in our Galaxy. Doppler spectroscopy, or the radial velocity (RV) technique, has been used extensively to identify hundreds of exoplanets, but with notable challenges in detecting terrestrial mass planets orbiting within habitable zones. We describe infrared RV spectroscopy at the 10 m Hobby-Eberly telescope that leverages a 30 GHz electro-optic laser frequency comb with nanophotonic supercontinuum to calibrate the Habitable Zone Planet Finder spectrograph. Demonstrated instrument precision <10 cm/s and stellar RVs approaching 1 m/s open the path to discovery and confirmation of habitable zone planets around M-dwarfs, the most ubiquitous type of stars in our Galaxy. Fig.1. Instrumentation for precision infrared astronomical RV spectroscopy. (A) Starlight is collected by the Hobby-Eberly telescope and directed to an optical fiber. Lasers, electro-optics and nanophotonics are used to generate an optical frequency comb with teeth spaced by 30 GHz and stabilized to an atomic clock. Both the starlight and frequency comb light are coupled to the highly-stabilized Habitable Zone Planet Finder (HPF) spectrograph where minute wavelength changes in the stellar spectrum are tracked with the precise calibration grid provided by the laser frequency comb. (B) Components for frequency comb generation. (upper) A fiber-optic integrated electro-optic modulator and (lower) silicon nitride chip (5 mm × 3 mm) on which nanophotonic waveguides are patterned. Light is coupled into a waveguide from the left and supercontinuum is extracted from the right with a lensed fiber. (C) The HPF spectrograph, opened and showing the camera optics on the left, echelle grating on the right, and relay mirrors in front. The spectrograph footprint is approximately 1.5 m × 3 m. (D) The 10 m Hobby-Eberly telescope at the McDonald Observatory in southwest Texas.

Modern nonlinear optical materials allow light of one wavelength be efficiently converted into light at another wavelength. However, designing nonlinear optical materials to operate with ultrashort pulses is difficult, because it is necessary to match both the phase velocities and group velocities of the light. Here we show that self-organized nonlinear gratings can be formed with femtosecond pulses propagating through nanophotonic waveguides, providing simultaneous group-velocity matching and quasi-phase-matching for second harmonic generation. We record the first direct microscopy images of photo-induced nonlinear gratings, and demonstrate how these waveguides enable simultaneous χ (2) and χ (3) nonlinear processes, which we utilize to stabilize a laser frequency comb. Finally, we derive the equations that govern self-organized grating formation for femtosecond pulses and explain the crucial role of group-velocity matching. In the future, such nanophotonic waveguides could enable scalable, reconfigurable nonlinear optical systems. arXiv:1806.07547v1 [physics.optics]

We explore the dynamical response of dissipative Kerr solitons to changes in pump power and detuning and show how thermal and nonlinear processes couple these parameters to the frequency-comb degrees of freedom. Our experiments are enabled by a Pound-Drever-Hall (PDH) stabilization approach that provides on-demand, radio-frequency control of the frequency comb. PDH locking not only guides Kerr-soliton formation from a cold microresonator but opens a path to decouple the repetition and carrier-envelope-offset frequencies. In particular, we demonstrate phase stabilization of both Kerr-comb degrees of freedom to a fractional frequency precision below 10^{-16}, compatible with optical-time-keeping technology. Moreover, we investigate the fundamental role that residual laser-resonator detuning noise plays in the spectral purity of microwave generation with Kerr combs.

Controlling femtosecond optical pulses with temporal precision better than one cycle of the carrier field has a profound impact on measuring and manipulating interactions between light and matter. We explore pulses that are carved from a continuous-wave laser via electro-optic modulation and realize the regime of sub-cycle optical control without a mode-locked resonator. Our ultrafast source, with a repetition rate of 10 GHz, is derived from an optical-cavity-stabilized laser and a microwave-cavity-stabilized electronic oscillator. Sub-cycle timing jitter of the pulse train is achieved by coherently linking the laser and oscillator through carrier-envelope phase stabilization enabled by a photonic-chip supercontinuum that spans up to 1.9 octaves across the near infrared. Moreover, the techniques we report are relevant for other ultrafast lasers with repetition rates up to 30 GHz and may allow stable few-cycle pulses to be produced by a wider range of sources.Ultrafast lasers produce femtosecond-duration pulses of light and can operate as frequency combs to provide a time and frequency reference spanning the optical and microwave domains [1]. For applications across science and technology, ultrafast pulse-trains with repetition frequencies of 10 GHz and higher are needed for sampling or exciting high-speed or transient events and making precision measurements across octaves of bandwidth. Yet simultaneously creating broad spectral coverage, low-noise performance, and timing synchronization into the femtosecond domain and below is an unmet challenge. One approach to producing multigigahertz pulse trains is through electro-optic modulation (EOM) of a continuous-wave (CW) laser [2, 3]. These optical pulse generators, henceforth referred to as "EOM combs", first gained interest nearly fifty years ago due to their broad tunability, reliability, high power per mode, and spectral flatness [4,5,6,7]. However, they exhibit electronic-oscillator-limited noise that has prevented femtosecond-level timing stabilization, and they have been limited to relatively narrow spectral bandwidths due to the technical difficulties of broadening gigahertz-rate, low-energy pulses.Here we present broadly applicable techniques for resolving the challenges associated with multi-gigahertz pulse trains originating from EOM combs and other ultrafast sources, such as Kerr microresonators [8] and modelocked semiconductor lasers [9]. In particular, we generate electro-optic pulse trains at 10 and 30 GHz with approximately 1-ps pulse durations, and show how to spectrally broaden them to octave bandwidths and to temporally compress them to less than three optical cycles (15 fs) in nanophotonic silicon-nitride (Si 3 N 4 , henceforth SiN) waveguides. To deliver a stable ultrafast source timed with sub-cycle precision, * david.carlson@nist.gov † scott.papp@nist.gov our work introduces an EOM-comb configuration with optical-cavity stabilization of the CW laser and high-Q microwave-cavity stabilization of a 10-GHz electronic oscillator. The microwave oscil...