Itan Barmes

High-precision spectroscopy with counterpropagating femtosecond pulses.

An experimental realization of high-precision direct frequency comb spectroscopy using counterpropagating femtosecond pulses on two-photon atomic transitions is presented. The Doppler broadened background signal, hampering precision spectroscopy with ultrashort pulses, is effectively eliminated with a simple pulse shaping method. As a result, all four 5S-7S two-photon transitions in a rubidium vapor are determined with both statistical and systematic uncertainties below 10(-11), which is an order of magnitude better than previous experiments on these transitions.

Spatial and spectral coherent control with frequency combs

Coherent control is a powerful tool for controlling light–matter interactions in time and frequency. Now, scientists show that counter-propagating broadband pulses can be used to generate fully controlled spatial excitation patterns. This spatial control approach also reduces decoherence, providing a high-frequency resolution similar to that of an optical frequency comb.

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.

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%.

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.

Full frequency comb model for the temporal talbot effect

We present a new approach to study repetition-rate multiplication of pulse trains via the Talbot-effect. Periodic functions are utilized to represent the spectral phase, enabling the calculation of temporal phase patterns for the first time.

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.

Doppler-free two-photon direct frequency comb spectroscopy with coherent control

We demonstrate a method to eliminate Doppler effects in femtosecond pulse excitation using coherent control. This enables high-precision direct frequency comb spectroscopy using significantly simplified setups and is compatible with XUV applications.

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.