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Register a new userNoncollinear enhancement cavity for record-high out-coupling efficiency of an extreme-UV frequency comb
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We demonstrate a femtosecond enhancement cavity with a crossed-beam geometry for efficient generation and extraction of extreme-ultraviolet (XUV) frequency combs at a 154 MHz repetition rate. We achieve a record-high out-coupled power of 600 {mu}W, directly usable for spectroscopy, at a wavelength of 97 nm. This corresponds to a >60% out-coupling efficiency. The XUV power scaling and generation efficiency are similar to that achieved with a single Gaussian-mode fundamental beam inside a collinear enhancement cavity. The noncollinear geometry also opens the door for the generation of isolated attosecond pulses at >100 MHz repetition rate.
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Laser-driven high-order harmonic generation (HHG) provides tabletop sources of broadband extreme-ultraviolet (XUV) light with excellent spatial and temporal coherence. These sources are typically operated at low repetition rates, $f_{rep}lesssim$100 kHz, where phase-matched frequency conversion into the XUV is readily achieved. However, there are many applications that demand the improved counting statistics or frequency-comb precision afforded by operation at high repetition rates, $f_{rep}$ > 10 MHz. Unfortunately, at such high $f_{rep}$, phase matching is prevented by the accumulated steady-state plasma in the generation volume, setting stringent limitations on the XUV average power. Here, we use gas mixtures at high temperatures as the generation medium to increase the translational velocity of the gas, thereby reducing the steady-state plasma in the laser focus. This allows phase-matched XUV emission inside a femtosecond enhancement cavity at a repetition rate of 77 MHz, enabling a record generated power of $sim$2 mW in a single harmonic order. This power scaling opens up many demanding applications, including XUV frequency-comb spectroscopy of few-electron atoms and ions for precision tests of fundamental physical laws and constants.
Optomechanical devices operated at their quantum limit open novel perspectives for the ultrasensitive determination of mass and displacement, and also in the broader field of quantum technologies. The access to higher frequencies implies operation at higher temperatures and stronger immunity to environmental noise. We propose and demonstrate here a new concept of quantum well photoelastic comb for the efficient coupling of light to optomechanical resonances at hundreds of GHz in semiconductor hybrid resonators. A purposely designed ultra-high resolution Raman spectroscopy set-up is exploited to evidence the transfer of spectral weight from the mode at 60 GHz to modes at 190-230 GHz, corresponding to the $8^{th}$ and $10^{th}$ overtone of the fundamental breathing mode of the light-sound cavities. The coupling to mechanical frequencies two orders of magnitude larger than alternative approaches is attained without reduction of the optomechanical constant $g_0$. The wavelength dependence of the optomechanical coupling further proves the role of resonant photoelastic interaction, highlighting the potentiality to access strong-coupling regimes. The experimental results show that electrostrictive forces allow for the design of devices optimized to selectively couple to specific mechanical modes. Our proposal opens up exciting opportunities towards the implementation of novel approaches applicable in quantum and ultra-high frequency information technologies.
A Watt-level continuous and single frequency blue laser at 461 nm is obtained by frequency-doubling an amplified diode laser operating at 922 nm via a LBO crystal in a resonant Fabry-P{e}rot cavity. We achieved a best optical conversion efficiency equal to 87% with more than 1 W output power in the blue, and limited by the available input power. The frequency-converted beam is characterized in terms of long term power stability, residual intensity noise, and geometrical shape. The blue beam has a linewidth of the order of 1 MHz, and we used it to magneto-optically trap $^{88}$Sr atoms on the 5s$^{2},^{1}$S$_0$ -- 5s5p$,^{1}$P$_1$ transition. The low-finesse, linear-cavity doubling system is very robust, maintains the lock for several days, and is compatible with a tenfold increase of the power levels which could be obtained with fully-fibered amplifiers and large mode area fibers.
We have developed an extreme ultraviolet (XUV) frequency comb for performing ultra-high precision spectroscopy on the many XUV transitions found in highly charged ions (HCI). Femtosecond pulses from a 100 MHz phase-stabilized near-infrared frequency comb are amplified and then fed into a femtosecond enhancement cavity (fsEC) inside an ultra-high vacuum chamber. The low-dispersion fsEC coherently superposes several hundred incident pulses and, with a single cylindrical optical element, fully compensates astigmatism at the $w_0=15mu$m waist cavity focus. With a gas jet installed there, intensities reaching $sim10^{14}$ W/cm$^2$ generate coherent high harmonics with a comb spectrum at 100 MHz rate. We couple out of the fsEC harmonics from the 7th up to the 35th (42 eV; 30 nm) to be used in upcoming experiments on HCI frequency metrology.
Microresonator-based soliton frequency combs - microcombs - have recently emerged to offer low-noise, photonic-chip sources for optical measurements. Owing to nonlinear-optical physics, microcombs can be built with various materials and tuned or stabilized with a consistent framework. Some applications require phase stabilization, including optical-frequency synthesis and measurements, optical-frequency division, and optical clocks. Partially stabilized microcombs can also benefit applications, such as oscillators, ranging, dual-comb spectroscopy, wavelength calibration, and optical communications. Broad optical bandwidth, brightness, coherence, and frequency stability have made frequency-comb sources important for studying comb-matter interactions with atoms and molecules. Here, we explore direct microcomb atomic spectroscopy, utilizing a cascaded, two-photon 1529-nm atomic transition of rubidium. Both the microcomb and the atomic vapor are implemented with planar fabrication techniques to support integration. By fine and simultaneous control of the repetition rate and carrier-envelope-offset frequency of the soliton microcomb, we obtain direct sub-Doppler and hyperfine spectroscopy of the $4^2D_{5/2}$ manifold. Moreover, the entire set of microcomb modes are stabilized to this atomic transition, yielding absolute optical-frequency fluctuations of the microcomb at the kilohertz-level over a few seconds and < 1 MHz day-to-day accuracy. Our work demonstrates atomic spectroscopy with microcombs and provides a rubidium-stabilized microcomb laser source, operating across the 1550 nm band for sensing, dimensional metrology, and communication.
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