A deep-UV optical frequency comb at 205 nm

Interleaved electro-optic dual comb generation to expand bandwidth and scan rate for molecular spectroscopy and dynamics studies near 1.6 µm

A chirped-pulse interleaving method is reported for generation of dual optical frequency combs based on electro-optic phase modulators (EOM) in a free-running all-fiber based system. Methods are discussed to easily modify the linear scan rate and comb resolution by more than three orders of magnitude and to significantly increase the spectral bandwidth coverage. The agility of the technique is shown to both capture complex line shapes and to magnify rapid passage effects in spectroscopic and molecular dynamics studies of CO₂. These methods are well-suited for applications in the areas of remote sensing of greenhouse gas emissions, molecular reaction dynamics, and sub-Doppler studies across the wide spectral regions accessible to EOMs.

Deep-ultraviolet femtosecond laser source at 243 nm for hydrogen spectroscopy

This paper reports on the generation of a 100 MHz repetition rate, 1.7 mW average power and femtosecond deep-ultraviolet (DUV) 243 nm laser source. The infra-red output of a broadband Titanium-Sapphire (TiSa) laser containing 729 nm light is mixed with its second harmonic in a β-barium borate (BBO) crystal. By manipulating the group delay dispersion (GDD), we customize the spectral shape of TiSa resonator to improve conversion efficiency. This DUV laser is employed for direct frequency comb spectroscopy of hydrogen.

Development of a deep-ultraviolet pulse laser source operating at 234 nm for direct cooling of Al+ ion clocks

We report on the development of a 250-MHz 234 nm deep-ultraviolet pulse source based on a flexible wavelength-conversion scheme. The scheme is based on a frequency-doubled optical parametric oscillator (FD-OPO) together with a cascaded frequency conversion process. We use a χ⁽²⁾ nonlinear envelope equation to guide the design of an intra-cavity OPO crystal, demonstrating a flexible broadband tunable feature and providing as high as watt-level of a frequency-doubled signal output centered at 850 nm, which is served as an input wave for the cascaded frequency conversion process. As much as 3.0 mW of an average power at 234 nm is obtained, with an rms power stability of better than 1% over 20 minutes. This deep-ultraviolet pulse laser source can be used for many applications in quantum optics and for direct laser cooling of Al⁺ ion clocks.

Two-photon frequency comb spectroscopy of atomic hydrogen

We have performed two-photon ultraviolet direct frequency comb spectroscopy on the 1S-3S transition in atomic hydrogen to illuminate the so-called proton radius puzzle and to demonstrate the potential of this method. The proton radius puzzle is a significant discrepancy between data obtained with muonic hydrogen and regular atomic hydrogen that could not be explained within the framework of quantum electrodynamics. By combining our result [f _1S-3S = 2,922,743,278,665.79(72) kilohertz] with a previous measurement of the 1S-2S transition frequency, we obtained new values for the Rydberg constant [R _∞ = 10,973,731.568226(38) per meter] and the proton charge radius [r _p = 0.8482(38) femtometers]. This result favors the muonic value over the world-average data as presented by the most recent published CODATA 2014 adjustment.

Towards DCS in the UV Spectral Range for Remote Sensing of Atmospheric Trace Gases

The development of increasingly sensitive and robust instruments and new methodologies are essential to improve our understanding of the Earth’s climate and air pollution. In this context, Dual-Comb spectroscopy (DCS) has been successfully demonstrated as a remote laser-based instrument to probe infrared absorbing species such as greenhouse gases. We present here a study of the sensitivity of Dual-Comb spectroscopy to remotely monitor atmospheric gases focusing on molecules that absorb in the ultraviolet domain, where the most reactive molecules of the atmosphere (OH, HONO, BrO...) have their highest absorption cross-sections. We assess the achievable signal-to-noise ratio (SNR) and the corresponding minimum absorption sensitivity of DCS in the ultraviolet range. We propose a potential light source for remote sensing UV-DCS and discuss the degree of immunity of UV-DCS to atmospheric turbulences. We show that the characteristics of the currently available UV sources are compatible with the unambiguous identification of UV absorbing gases by UV-DCS.

A Deep Ultraviolet Mode-locked Laser Based on a Neural Network

Deep ultraviolet lasers based on the phenomenon of mode-locking have been used widely in many areas in recent years, for example, in semiconductors, the environment and biomedicine. In the development of a mode-locked deep ultraviolet laser, one of the most important aspects is to optimize the multiple parameters of the complex system. Traditional optimization methods require experimenters with more optimization experience, which limits the wide application of the lasers. In this study, we optimize the deep ultraviolet mode-locked laser system using an online neural network to solve this problem. The neural network helps us control the position of the crystal, the length of the cavity, the position of the focusing lens and the temperature of the frequency doubling crystal. We generate a deep ultraviolet mode-locked laser with a power of 18 mW and a spectral center at 205 nm. This result is greatly improved compared to previous results with the same pump power. This technology provides a universal solution to multiparameter problems in the optimization of lasers.

Generating Ultrabroadband Deep-UV Radiation and Sub-10 nm Gap by Hybrid-Morphology Gold Antennas

We experimentally investigate the interaction between hybrid-morphology gold optical antennas and a few-cycle Ti:sapphire laser up to ablative intensities, demonstrating rich nonlinear plasmonic effects and promising applications in coherent frequency upconversion and nanofabrication technology. The two-dimensional array of hybrid antennas consists of elliptical apertures combined with bowties in its minor axis. The plasmonic resonance frequency of the bowties is red-shifted with respect to the laser central frequency and thus mainly enhances the third harmonic spectrum at long wavelengths. The gold film between two neighboring elliptical apertures forms an hourglass-shaped structure, which acts as a "plasmonic lens" and thus strongly reinforces surface currents into a small area. This enhanced surface current produces a rotating magnetic field that deeply penetrates into the substrate. At resonant frequency, the magnetic field is further intensified by the bowties. The resonant frequency of the hourglass is blueshifted with respect to the laser central frequency. Consequently, it spectacularly extends the third harmonic spectrum toward short wavelengths. The resultant third harmonic signal ranges from 230 to 300 nm, much broader than the emission from a sapphire crystal. In addition, the concentration of surface current within the neck of the hourglass antenna results in a structural modification through laser ablation, producing sub-10 nm sharp metallic gaps. Moreover, after laser illumination the optical field hotspots are imprinted around the antennas, allowing us to confirm the subwavelength enhancement of the electric near-field intensity.

Beyond 100 THz-spanning ultraviolet frequency combs in a non-centrosymmetric crystalline waveguide

Ultraviolet frequency combs enable applications ranging from precision spectroscopy to atomic clocks by addressing electronic transitions of atoms and molecules. Access to ultraviolet light via integrated nonlinear optics is usually hampered by the strong material dispersion and large waveguide attention in ultraviolet regions. Here we demonstrate a simple route to chip-scale ultraviolet comb generators, simultaneously showing a gap-free frequency span of 128 terahertz and high conversion efficiency. This process relies on adiabatic quadratic frequency translation of a near-visible supercontinuum sourced by an ultrafast fiber laser. The simultaneous cubic and quadratic nonlinear processes are implemented in single-crystalline aluminum nitride thin films, where chirp-modulated taper waveguides are patterned to ensure a broad phase matching. The heterodyne characterization suggests that both the near-visible and ultraviolet supercontinuum combs maintain high coherence. Our approach is also adaptable to other non-centrosymmetric photonic platforms for ultrafast nonlinear optics with scalable bandwidth.

Cooling of atoms using an optical frequency comb

We report on laser cooling of neutral rubidium atoms by using a single mode of a frequency comb. Cooling is achieved on a dipole-allowed transition at 780 nm in a one-dimensional retro-reflected beam geometry. Temperatures are measured using standard time-of-flight imaging. We show the dependence of the temperature on the cooling time, intensity and detuning of the frequency comb. The lowest temperature achieved is approximately equal to the Doppler temperature and is limited by the intensity of the comb mode driving the cooling transition. Additionally, we verify the analogy between frequency comb and continuous-wave laser cooling. Our work is a step towards laser cooling of atoms with strong cycling transitions in the vacuum ultraviolet, such as hydrogen, deuterium and antihydrogen, where generation of continuous-wave laser light is limited by current laser technology. Achieving efficient cooling at these wavelengths would significantly improve the precision of optical frequency standards, enable measurements of fundamental constants with unprecedented accuracy, improve tests of charge, parity, and time reversal symmetry, and open the way to achieving quantum degeneracy width new atomic species.

Phase-dependent interference between frequency doubled comb lines in a χ(2) phase-matched aluminum nitride microring

Nonlinear optical conversion with frequency combs is important for self-referencing and for generating shorter wavelength combs. Here we demonstrate efficient frequency comb doubling through the combination of second-harmonic generation (SHG) and sum-frequency generation (SFG) of an input comb with a high Q, phase-matched χ⁽²⁾ microring resonator. Phase coherence of the SHG and SFG nonlinear conversion processes is confirmed by sinusoidal phase-dependent interference between frequency doubled comb lines.

Carrier-envelope phase drift measurement of picosecond pulses by an all-linear-optical means

Carrier-envelope phase (CEP) drift of a pulse train of 2 ps pulses has been measured by a multiple beam interferometer. The round trip time of the interferometer is slightly mistuned from the pulse sequence, leading to spectral interference fringes. We extract the pulse-to-pulse CEP drift from the position of the spectral interference pattern. The length of the interferometer has been actively stabilized to ±10  nm, which sets the ultimate limit on the accuracy of the measurement to 78 mrad, while the CEP-drift (rms) noise of the measurement was 127 mrad (at 800 nm).

Coherent phase lock of a 9 μm quantum cascade laser to a 2 μm thulium optical frequency comb

We demonstrate coherent phase locking of a room-temperature continuous-wave quantum cascade laser (QCL) at 9.1 μm to a Tm-fiber laser frequency comb centered at 2 μm, with an integrated residual phase error of 0.9 rad (30 mHz to 1.5 MHz). This resulted in a QCL linewidth reduction from 525 to 25 kHz at 1 ms observation time, limited by the linewidth of the free-running frequency comb.

Absolute frequency spectroscopy of CO2 lines at around 2.09 μm by combined use of an Er:fiber comb and a Ho:YLF amplifier

The low-frequency tail of an octave-spanning supercontinuum (SC) generated by an Er:fiber comb is enhanced by a multipass Ho:YLF amplifier and used in a sum-frequency-generation scheme to obtain absolute referencing of a single-mode Tm-Ho:YAG laser tunable around 2.09 μm. By tuning the comb repetition frequency, the probing laser is scanned across the absorption lines of a CO(2) gas sample and highly accurate absorption profiles are measured. This approach can be readily scaled to any wavelength above ~2 μm.

1 W average-power 100 MHz repetition-rate 259 nm femtosecond deep ultraviolet pulse generation from ytterbium fiber amplifier

We demonstrate 1W average-power ultraviolet (UV) femtosecond (fs) ultrashort pulse generation at a wavelength of 259 nm and a repetition rate as high as 100 MHz by quadrupling a fs ytterbium-fiber laser. A cavity-enhanced design is employed for efficient frequency doubling to the UV region. The optical-to-optical efficiency of UV output to the pump diode is 2.6%.