Dual-comb spectroscopy is an emerging new spectroscopic tool that exploits the frequency resolution, frequency accuracy, broad bandwidth, and brightness of frequency combs for ultrahigh-resolution, high-sensitivity broadband spectroscopy. By using two coherent frequency combs, dual-comb spectroscopy allows a sample’s spectral response to be measured on a comb tooth-by-tooth basis rapidly and without the size constraints or instrument response limitations of conventional spectrometers. This review describes dual-comb spectroscopy and summarizes the current state of the art. As frequency comb technology progresses, dual-comb spectroscopy will continue to mature and could surpass conventional broadband spectroscopy for a wide range of laboratory and field applications.

Dual-comb spectroscopy

Vertical-external-cavity surface-emitting lasers (VECSELs) are the most versatile laser sources, combining unique features such as wide spectral coverage, ultrashort pulse operation, low noise properties, high output power, high brightness and compact form-factor. This paper reviews the recent technological developments of VECSELs in connection with the new milestones that continue to pave the way towards their use in numerous applications. Significant attention is devoted to the fabrication of VECSEL gain mirrors in challenging wavelength regions, especially at the yellow and red wavelengths. The reviewed fabrication approaches address wafer-bonded VECSEL structures as well as the use of hybrid mirror structures. Moreover, a comprehensive summary of VECSEL characterization methods is presented; the discussion covers different stages of VECSEL development and different operation regimes, pointing out specific characterization techniques for each of them. Finally, several emerging applications are discussed, with emphasis on the unique application objectives that VECSELs render possible, for example in atom and molecular physics, dermatology and spectroscopy.

https://iopscience.iop.org/article/10.1088/1361-6463/aa7bfd/pdf

Optically pumped VECSELs: review of technology and progress

Previous work has shown that use of a passive enhancement cavity designed for ultrashort pulses can enable the up-conversion of the fs frequency comb into the extreme ultraviolet (XUV) spectral region utilizing the highly nonlinear process of high harmonic generation This promising approach for an efficient source of highly coherent light in this difficult to reach spectral region promises to be a unique tool for precision spectroscopy and temporally resolved measurements Yet to date, this approach has not been extensively utilized due in part to the low powers so far achieved and in part due to the challenges in directly probing electronic transitions with the frequency comb itself We report on a dramatically improved XUV frequency comb producing record power levels to date in the 50-150 nm spectral region based on intracavity high harmonic generation We measure up to 77 μW at the 11th harmonic of the fundamental (72 nm) with μW levels down to the 15th harmonic (53nm) Phase-matching and related design considerations unique to intracavity high harmonic generation are discussed, guided by numerical simulations which provide insight into the role played by intracavity ionization dynamics We further propose and analyze dual-comb spectroscopy in the XUV and show that the power levels reported here permit this approach for the first time Dual-comb spectroscopy in this physically rich spectral region promises to enable the study of a significantly broader range of atomic and molecular spectra with unprecedented precision and accuracy

Optimizing intracavity high harmonic generation for XUV fs frequency combs

We review the current state of tabletop extreme ultraviolet (XUV) sources based on high harmonic generation (HHG) in femtosecond enhancement cavities (fsEC). Recent developments have enabled generation of high photon flux (1014 photons s?1) in the XUV, at high repetition rates (>50?MHz) and spanning the spectral region from 40 to 120?nm. This level of performance has enabled precision spectroscopy with XUV frequency combs and promises further applications in XUV spectroscopic and photoemission studies. We discuss the theory of operation and experimental details of the fsEC and XUV generation based on HHG, including current technical challenges to increasing the photon flux and maximum photon energy produced by this type of system. Current and future applications for these sources are also discussed.

https://iopscience.iop.org/article/10.1088/0953-4075/45/14/142001/pdf

XUV frequency combs via femtosecond enhancement cavities

We experimentally and numerically investigate the intracavity ionization of a dilute gas target by an ultrashort pulse inside a femtosecond enhancement cavity. Numerical simulations detail how the dynamic ionization of the gas target limits the achievable peak intensity of the evolving intracavity pulse beyond that of linear cavity losses, setting a constraint on the strength of the nonlinear interaction that can be sustained in such optical cavities. Experimental measurements combined with numerical simulations predict ionization levels in a femtosecond enhancement cavity for the first time. We demonstrate how the resonant response of the femtosecond enhancement cavity can itself be used as a sensitive probe of optical nonlinearities at high intensities.

Intracavity ionization and pulse formation in femtosecond enhancement cavities.

We have developed a stable, high-power, single-frequency optically pumped external-cavity semiconductor laser system and generate up to 125 mW of power at 253.7 nm using successive frequency doubling stages. We demonstrate precision scanning and control of the laser frequency in the UV to be used for cooling and trapping of mercury atoms. With active frequency stabilization, a linewidth of 1 W) optically pumped semiconductor laser system. The results demonstrate the utility of these devices for precision spectroscopy at deep-UV wavelengths.

Doppler-free spectroscopy of mercury at 253.7 nm using a high-power, frequency-quadrupled, optically pumped external-cavity semiconductor laser

We demonstrate a continuous wave, single-frequency terahertz (THz) source emitting 1.9 THz. The linewidth is less than 100 kHz and the generated THz output power exceeds 100 μW. The THz source is based on parametric difference frequency generation within a nonlinear crystal located in an optical enhancement cavity. Two single-frequency vertical-external-cavity source-emitting lasers with emission wavelengths spaced by 6.8 nm are phase locked to the external cavity and provide pump photons for the nonlinear downconversion. It is demonstrated that the THz source can be used as a local oscillator to drive a receiver used in astronomy applications.

Narrow linewidth single-frequency terahertz source based on difference frequency generation of vertical-external-cavity source-emitting lasers in an external resonance cavity

We demonstrate a scalable approach for the generation of high average power femtosecond (fs) pulse trains from Ti:sapphire by optically injection locking a resonant amplification cavity. We generate up to 7 W average power with over 30% optical extraction efficiency in a 68 fs pulse train operating at 95 MHz. This master oscillator power amplifier approach allows independent optimization of the fs laser while enabling efficient amplification to high average powers. The technique also enables coherent synchronization among multiple fs laser sources.

Generation of high-power frequency combs from injection-locked femtosecond amplification cavities.

We demonstrate a continuous wave, single frequency terahertz (THz) source based on parametric difference frequency generation within a nonlinear crystal located in an optical enhancement cavity. Two single-frequency VECSELs with emission wavelengths spaced by 6.8 nm are phase locked to the external cavity and are used as pump sources for the nonlinear down conversion. The emitting THz radiation is centered at 1.9 THz and has a linewidth of less than 100 kHz. The output power of the source exceeds 100 μW. We show that the THz source can be used as local oscillator to drive a receiver used in astronomy applications.

Terahertz generation by difference frequency conversion of two single-frequency VECSELs in an external resonance cavity

We demonstrate the utility of optically pumped semiconductor lasers (OPSLs) in the eld of precision atomic spectroscopy. We have constructed an OPSL for the purpose of laser-cooling and trapping neutral Hg atoms. The OPSL lases at 1015 nm and is frequency quadrupled to provide the trapping light for the ground state cooling transition. We report up to 1.5 W of stable, single-frequency output power with a linewidth of < 70 kHz with active feedback. From the OPSL we generate deep-UV light at 253.7 nm used to form a neutral Hg magneto-optical trap (MOT). We present details of the MOT. We also report initial results for spectroscopy of the 61S0 - 63P0 clock transition in the Hg199 isotope.

Justin Paul

Papers

Doppler-free spectroscopy of mercury at 253.7 nm using a high-power, frequency-quadrupled, optically pumped external-cavity semiconductor laser

Narrow linewidth single-frequency terahertz source based on difference frequency generation of vertical-external-cavity source-emitting lasers in an external resonance cavity

Generation of high-power frequency combs from injection-locked femtosecond amplification cavities.

Terahertz generation by difference frequency conversion of two single-frequency VECSELs in an external resonance cavity

Optically pumped external-cavity semiconductor lasers for precision spectroscopy and laser cooling of atomic Hg