Jonas Morgenweg

High-energy, high-repetition-rate picosecond pulses from a quasi-CW diode-pumped Nd:YAG system.

Summary form only given. High-harmonic generation offers the possibility to generate table-top coherent radiation in the soft X-ray regime [1]. This enables high-resolution microscopy of biological and solid state samples. The high intensity required for HHG can be generated by amplifying pulses from a Ti:sapphire oscillator in an optical parametric chirped-pulse amplification (OPCPA) scheme, enabling the production of few-cycle pulses with terawatt peak powers [2]. To maximize the achievable flux of soft X-ray HHG, a high repetition rate pump system for OPCPA is required.Here we report on a new pump laser system based on quasi-CW diode pumped Nd:YAG modules. This system produces 60 ps pulses, with an energy of 80 mJ at 532 nm wavelength, at a repetition rate of 300 Hz. The transition from CW to quasi-CW diode pumping reduces the thermal load on the crystal rods and allows much more efficient pumping and high gain. This enables us to achieve an order of magnitude higher repetition rate compared to flashlamp pumped systems, while maintaining similar pulse energies [2]. One of the issues with side-pumped Nd:YAG rods is the occurrence of thermal birefringence, causing a radially dependent polarization [3]. After a single pass through a Nd:YAG rod, this can result in up to 30% depolarization, and therefore a similar intensity loss at polarizing optics. A double-pass scheme with a Faraday rotator almost completely compensates for this effect [4]. To limit the peak intensity while keeping the pulse energy as high as possible, a top-hat beam profile is used. Therefore relay-imaging is implemented between every critical surface.A schematic of the pump laser is shown in Fig. 1. A Nd:YV04 oscillator produces 10 ps pulses, which are amplified to 1 mJ in a regenerative amplifier. The pulses are stretched in this amplifier to 60 ps by applying an intracavity etalon. The output beam is enlarged by a telescope and then passed through an aperture to create a top-hat beam profile, reducing the pulse energy to 0.1 mJ. These pulses are then used as a seed for the postamplifiers. The aperture is relay-imaged in every pass through the Nd:YAG rods (taking into account thermal lensing), and then relay-imaged on a BBO crystal for efficient second-harmonic generation. The maximum pulse energy (80 mJ at 532 nm) is currently limited by the damage threshold of the second Nd:YAG rod. This pump laser system forms an ideal starting point for the development of an OPCPA system that produces terawatt pulses at high repetition rate. As such, it is an important step towards the production of high-flux soft-Xray radiation with a table-top source.

Ramsey-comb spectroscopy with intense ultrashort laser pulses

Frequency combs provide a broad series of well-calibrated spectral lines for highly precise metrology and spectroscopy, but this usually involves a trade-off between power and accuracy. A comb created by adjusting the time delay between two optical pulses now enables both. This so-called Ramsey comb could probe fundamental problems such as determining the size of the proton.

Multi-delay, phase coherent pulse pair generation for precision Ramsey-frequency comb spectroscopy

We demonstrate the generation of phase-stable mJ-pulse pairs at programmable inter-pulse delays up to hundreds of nanoseconds. A detailed investigation of potential sources for phase shifts during the parametric amplification of the selected pulses from a Ti:Sapphire frequency comb is presented, both numerically and experimentally. It is shown that within the statistical error of the phase measurement of 10 mrad, there is no dependence of the differential phase shift over the investigated inter-pulse delay range of more than 300 ns. In combination with nonlinear upconversion of the amplified pulses, the presented system will potentially enable short wavelength (<100 nm), multi-transition Ramsey-frequency comb spectroscopy at the kHz-level.

Direct frequency-comb spectroscopy of a dipole-forbidden clock transition in trapped 40Ca+ ions.

We demonstrate direct frequency-comb (FC) spectroscopy of the dipole-forbidden 4s(2)S(1/2)-3d(2)D(5/2) transition in trapped (40)Ca(+) ions using an unamplified FC laser. The excitation is detected with nearly 100% efficiency using a shelving scheme in combination with single-ion imaging. The method demonstrated here has the potential to reach hertz-level accuracy, if a hertz-level linewidth FC is used in combination with confinement in the Lamb-Dicke regime.

Widely tunable extreme UV frequency comb generation

Extreme UV (XUV) frequency comb generation in the wavelength range of 51 to 85 nm is reported based on high-order harmonic generation of two consecutive IR frequency comb pulses that were amplified in an optical parametric chirped pulse amplifier. The versatility of the system is demonstrated by recording direct XUV frequency comb excitation signals in He, Ne, and Ar with visibilities of up to 61%.

Multi-delay, phase-coherent pulse pair generation for precision Ramsey-comb spectroscopy

The realization of optical frequency combs has revolutionized the field of precision metrology [1]. However, these combs typically operate in the near-infrared, while many interesting atomic transitions require wavelengths shorter than 100 nm. Hence high peak power (frequency comb) pulses are required for efficient nonlinear up-conversion of comb lasers. Our approach to realize this goal is based on selective amplification of two frequency comb pulses in a parametric amplifier, and we demonstrated MHz-level extreme ultra-violet frequency comb spectroscopy [2]. The fixed delay of the amplified pulse pairs (maximum ~10 ns) used in [2] limited the achievable accuracy and restricted the method to single-transition recording, as in traditional Ramsey spectroscopy [3]. In addition, the phase-shift introduced by the parametric amplifier had to be monitored carefully to correct the Ramsey-like signals.

Ramsey-comb spectroscopy

Summary form only given. We demonstrate the new concept of a “Ramsey comb”, which is based on Ramsey spectroscopy [1] over multiple time zones using a frequency comb laser [2]. In contrast to Ramseys well-established technique of separated oscillating fields for ultra-high precision metrology [1], the Ramsey-comb technique can measure multiple transitions simultaneously with high accuracy. Moreover, the system is powerful enough to drive multiphoton transitions, and the wavelength range is easily extended by nonlinear optics.A prerequisite for measuring optical Ramsey-fringes are coherent pulse-pairs. We have developed a new system capable of producing phase-coherent, multi-delay pulse pairs at the mJ-level by selective amplification of frequency comb pulses [3]. The macro-delay of these pulse pairs can be changed in steps of ~8 ns, while the exact delay can be fine-adjusted at the attosecond-level and with less than a few mrad of phase error. As a proof of principle, we present kHz-level spectroscopy on four two-photon (5S-7S) transitions in Rb. Fig. 1, upper row, shows part of the fluorescence signal measured with a photo-multiplier after excitation with the amplified comb pulses. The signal is a beating of the four atomic transitions, and was recorded via scanning of the inter-pulse delays by a few femtoseconds, at different macro-delays. In the lower row of Fig. 1 the calculated (via a Fourier-transform) spectrum is shown, looking quite similar to full rep-rate comb excitation.We developed a robust fitting method, which, as opposed to for example traditional frequency comb spectroscopy, does not depend on the exact lineshapes of the transitions. Instead, the position and strengths of the atomic transitions are obtained by only fitting the individual phases of the Ramsey signals. In addition, Ramsey-comb spectroscopy is shown to be largely insensitive to the well known light-shift effect. We achieved an accuracy on the investigated Rb transition frequencies of ~10 kHz (relative accuracy of 1.3x10-11), challenging the most accurate spectroscopic measurements that have been performed on this system. Because of the high pulse energy of the amplified pulse-pairs (1 million times higher then what is typical available from an oscillator), the presented system enables ultra-high precision spectroscopy on transitions that are typically difficult for direct frequency comb excitation. As an example, we measured the weak 6S1/2 - 9S1/2 two-photon transition in Cs, improving the accuracy of the transition frequencies by an order of magnitude.

Ramsey-comb spectroscopy with amplified frequency comb pulse pairs

We demonstrate kHz-level “Ramsey-comb” spectroscopy using two amplified frequency comb pulses. This concept enables significantly improved accuracies on two-photon transitions as shown with rubidium and cesium, and is extendible to XUV wavelengths.

Direct frequency metrology Up To PHz frequencies

The capability of frequency-comb (FC) lasers to precisely measure optical frequencies has been extended to the to the extreme ultraviolet (XUV, wavelengths shorter than 100 nm), corresponding to frequencies of multiple PHz. We demonstrate “broad frequency comb generation” for a wavelength range of 51–85 nm. Our method is based on amplification and coherent up-conversion of a pair of pulses originating from a near-infrared femtosecond FC laser. Excitation of argon, neon, and helium with these upconverted laser pulses in the XUV lead to Ramsey-like signals with up to 61% contrast. From these signals an accuracy of 6 MHz has been achieved in the determination of the ground state ionization energy of helium at 51 nm. Further improvement to a kHz-level accuracy is expected based on a new pump laser for the employed parametric amplification system.

Frequency comb metrology at PHz frequencies: Precision in the extreme ultraviolet

The capability of frequency-comb (FC) lasers to precisely measure optical frequencies is extended to the multiple-PHz domain. This frequency region, which covers the extreme ultraviolet (XUV, wavelengths shorter than 100 nm), was previously not accessible to these devices. Frequency comb generation is shown for 51–85 nm by amplification and coherent up-conversion of a pair of pulses originating from a near-infrared femtosecond FC laser. Moreover, Ramsey-like signals with up to 61% contrast are observed when the XUV comb is scanned over transitions in argon, neon and helium, resulting in an 8-fold improved determination of the ground state ionization energy of helium.