United Time-Frequency Spectroscopy for Dynamics and Global Structure

Abstract

Ultrashort laser pulses have thus far been used in two distinct modes. In the time domain, the pulses have allowed probing and manipulation of dynamics on a subpicosecond time scale. More recently, phase stabilization has produced optical frequency combs with absolute frequency reference across a broad bandwidth. Here we combine these two applications in a spectroscopic study of rubidium atoms. A wide-bandwidth, phase-stabilized femtosecond laser is used to monitor the real-time dynamic evolution of population transfer. Coherent pulse accumulation and quantum interference effects are observed and well modeled by theory. At the same time, the narrow linewidth of individual comb lines permits a precise and efficient determination of the global energy-level structure, providing a direct connection among the optical, terahertz, and radio-frequency domains. The mechanical action of the optical frequency comb on the atomic sample is explored and controlled, leading to precision spectroscopy with an appreciable reduction in systematic errors.Ultrashort laser pulses have given a remarkably detailed picture of photophysical dynamics. In studies of alkali atoms (1) and diatomics (2) in particular, coherent wave packet motion has been observed and even actively controlled. However, the broad bandwidth of these pulses has prevented a simultaneous high-precision measurement of state energies. At the expense of losing any direct observation or control of coherent dynamics, precision spectroscopy enabled by continuous wave (cw) lasers has been one of the most important fields of modern scientific research, providing the experimental underpinning of quantum mechanics and quantum electrodynamics.This trade-off between the time and frequency domains might seem fundamental, but in fact it results from pulse-to-pulse phase fluctuations in laser operation. The recent introduction of phase-stabilized, widebandwidth frequency combs based on modelocked femtosecond lasers has provided a direct connection between these two domains (3, 4). Many laboratories have constructed frequency combs that establish optical frequency markers directly linked to a microwave or optical standard, covering a variety of spectral intervals. Atomic and molecular structural information can now be probed over a broad spectral range, with vastly improved measurement precision and accuracy enabled by this absolute frequency-based approach (5). One of the direct applications is the development of optical atomic clocks (6-8). To date, however, traditional cw laserbased spectroscopic approaches have been essential to all of these experiments, with frequency combs serving only as reference rulers (9).Here we take advantage of the phasestable femtosecond pulse train to bridge the fields of high-resolution spectroscopy and ultrafast dynamics. This approach of direct frequency comb spectroscopy (DFCS) uses light from a comb of appropriate structure to directly interrogate a multitude of atomic levels and to study time-dependent quantum coherence. DFCS allows time-resol...

Short-pulse photoassociation in rubidium below theD1line

“…In addition to controlling nuclear dynamics, a femtosecond laser can be employed for high-resolution spectroscopy in ultracold systems by utilizing it as a frequency comb [21]. These two limits have recently been combined in a spectroscopic study with coherent pulse accumulation [22]. The appeal of such an approach lies in the fact that with a femtosecond frequency comb it is possible to continuously switch between the two limits, selecting the desired time and frequency resolution.…”

Propagation of two short laser pulse trains in a Λ-type three-level medium under conditions of electromagnetically induced transparency

“…Obviously, larger differences between pulse propagations with symmetric and asymmetric detunings are obtained for energies that do not satisfy the one-photon resonance condition [9], since the twophoton Raman resonance is always fulfilled for symmetric detunings. There is one exception, when a similar behavior of the pulse propagation leading to EIT is obtained for asymmetric detunings that do not correspond to one-photon resonance, δ p = −δ c = (2 m p + 1) ω r /2, because the twophoton Raman detuning is again a multiple of the repetition angular frequency and therefore the twophoton resonance condition is satisfied, as shows, in Fig.…”

“…By using a comb laser, which means an ultrafast laser pulse train with a high repetition rate, one can carry out the ultrahigh resolution spectroscopy. Recently it has been shown that the accumulation ef-fects of coherence by a laser pulse train play an important role for coherent control of atomic or molecular systems [9,10]. The use of a chirped laser pulse train is another way for coherent control of population transfer [11].…”

A digitally generated ultrafine optical frequency comb for spectral measurements with 0.01-pm resolution and 0.7-µs response time

“…For example, individual comb components can be tuned to scan over the frequency gap. [16][17][18][19][20][21] Other methods include spatially separating the comb lines using gratings, 21 Fourier transform spectroscopy of a single OFC 22 and dual-mode spectroscopy using two OFCs. 23,24 Nevertheless, few efforts have been made to decrease the comb spacing to improve the spectral resolution.…”