A mode-locked femtosecond laser is used to physically map the laser frequency comb into the velocity comb of the excited Rb atoms at room temperature. Upon resonant excitation by discrete optical frequencies the velocity distribution of the excited Rb atoms shows comblike structure. Simultaneously, velocity-selective population transfer occurs between ground-state hyperfine levels. Both facts are observed by modified direct frequency comb spectroscopy and verified by a detailed density matrix treatment of the multilevel system in resonant field.

We correlate conformation and dynamics of the semirigid polyelectrolytes deoxyribonucleic acid (DNA) and hyaluronic acid (HA) in the semidilute regime, across a broad concentration range (10^–3 – 10² g/L). Salt-free polyelectrolytes are distinct from uncharged polymers as they presumably form a rather rigid, isotropic mesh. The polyelectrolyte characteristic mesh size is known as de Gennes correlation length ξ. We directly probed the mesh formed by DNA and HA, by employing fluorescence correlation spectroscopy (FCS) to measure the diffusion coefficient of fluorescently labeled DNA fragments added in trace amounts. For the salt-free solutions we found that the DNA or HA mesh size has to be 2–3 times larger than the fragments for them to start to diffuse freely (as if in the dilute solution). For a tighter mesh (concentrations 0.1–1 g/L), the fragment diffusion coefficient is only half the free diffusion value. Conversely, fragments show the free diffusion coefficient—as if there is no mesh—in DNA or HA in 10 mM buffer. This complies well with the fact that the ξ fades for polyelectrolytes with added salt. The diffusion coefficient falls off further when the mesh size gets smaller than the fragment size (above 1 g/L) and a similar value is reached in buffer as well as in pure water, respectively, at the highest measured concentrations (10 g/L). We also performed small-angle X-ray scattering (SAXS) on HA and DNA (range 3–130 g/L) to complement our previous dielectric spectroscopy (DS) studies (range 0.01–5 g/L). Combined, these methods provide reference values of the de Gennes length ξ across the range studied by FCS.

The bone healing process following osteotomy may vary according to the type of surgical instrumentation. The aim of the present in vivo study was to determine thermal changes of the bone tissue following osteotomies performed by Er:YAG laser ablation in contact and non-contact modes, piezoelectric surgery, and surgical drill using an infrared thermographic camera. For each measurement, the temperature before the osteotomy-baseline (Tbase) and the maximal temperature measured during osteotomy (Tmax) were determined. Mean temperature (ΔT) values were calculated for each osteotomy technique. The significance of the difference of the registered temperature between groups was assessed by the ANOVA test for repeated measures. Mean baseline temperature (Tbase) was 27.9 ± 0.3 °C for contact Er:YAG laser, 29.9 ± 0.3 °C for non-contact Er:YAG laser, 29.4 ± 0.3 °C for piezosurgery, and 28.3 ± 0.3 °C for surgical drill. Mean maximum temperature (Tmax) was 29.9 ± 0.5 °C (ΔT = 1.9 ± 0.3 °C) for contact Er:YAG laser, 79.1 ± 4.6 °C (ΔT = 49.1 ± 4.4 °C) for non-contact Er:YAG laser, 29.1 ± 0.2 °C (ΔT = −0.2 ± 0.3 °C) for piezosurgery, and 27.3 ± 0.4 °C (ΔT = −0.9 ± 0.4 °C) for surgical drill. Statistically significant temperature changes were observed for the non-contact laser. The results of the study showed beneficial effects of the osteotomy performed by the Er:YAG laser used in the contact mode of working as well as for piezosurgery, reducing the potential overheating of the bone tissue as determined by means of infrared thermography.

We study resonantly enhanced cascaded two-photon transitions in Doppler-broadened rubidium vapor, induced by an optical frequency comb. Cascaded two-step excitation of atoms from the ground state 5$S$ ${}_{1/2}$ to the final 5$D$ ${}_{5/2}$ state via an intermediate 5$P$ ${}_{3/2}$ state as well as the influence of the frequency comb repetition rate on the two-photon fluorescence signal have been investigated. It is shown that the careful choice of $f_R$ leads to enhancement of the two-photon signal with considerable increase of the final $5D_{5/2}$-state population due to velocity-selective double resonance. Since the double-resonance condition for two different rubidium isotopes is fulfilled for different $f_R$, the possibility of isotope-selective two-photon excitation is explored. The dependence of the two-photon signal on external magnetic field values and different repetition rates of the optical frequency comb is also investigated.

We study Doppler cooling of atoms using a frequency comb and develop a theory to quantitatively describe the process by investigating the interaction of two-level atoms with two counterpropagating pulse trains. We derive the radiative force on atoms and show that each comb mode effectively acts as a cooling laser, thereby indicating that simultaneous laser cooling in multiple cooling channels can be achieved using a single-frequency comb source. Possible experimental applications involving alkali-metal atoms are illustrated.

We investigated the coherence accumulation in Rb atoms excited by a train of femtosecond pulses. The coherence accumulation results in the velocity selective optical pumping of Rb hyperfine levels, observed by a cw probe laser. The effects of the pulse train excitation on the cw probe laser transmission were investigated, depending on the probe laser power. We observed the probe laser absorption increase in the strong probe case, followed by the reduction of the modulations in the probe laser absorption. Results show that accumulation of population and coherence can be effectively reduced and eventually destroyed by increasing the cw laser intensity. A strong cw laser can therefore serve as a switch from the pulse-train to pulse-by-pulse type of interaction of Rb atoms with the fs laser. We developed a density-matrix based theoretical model of the eight-level Rb atoms interacting with the fs and cw laser fields, and present agreement with the experimental results.