Czesław Radzewicz

DIRECT MEASUREMENT OF THE WIGNER FUNCTION BY PHOTON COUNTING

We report a direct measurement of the Wigner function characterizing the quantum state of a light mode. The experimental scheme is based on the representation of the Wigner function as an expectation value of a displaced photon number parity operator. This allowed us to scan the phase space point-by-point, and obtain the complete Wigner function without using any numerical reconstruction algorithms.

Pulsed squeezed light: Simultaneous squeezing of multiple modes

We analyze the spectral properties of squeezed light produced by means of pulsed, single-pass degenerate parametric down-conversion. The multimode output of this process can be decomposed into characteristic modes undergoing independent squeezing evolution akin to the Schmidt decomposition of the biphoton spectrum. The main features of this decomposition can be understood using a simple analytical model developed in the perturbative regime. In the strong pumping regime, for which the perturbative approach is not valid, we present a numerical analysis, specializing to the case of one-dimensional propagation in a beta-barium borate waveguide. Characterization of the squeezing modes provides us with an insight necessary for optimizing homodyne detection of squeezing. For a weak parametric process, efficient squeezing is found in a broad range of local oscillator modes, whereas the intense generation regime places much more stringent conditions on the local oscillator. We point out that without meeting these conditions, the detected squeezing can actually diminish with the increasing pumping strength, and we expose physical reasons behind this inefficiency.

Coherent supercontinuum generation up to 2.3 µm in all-solid soft-glass photonic crystal fibers with flat all-normal dispersion

Supercontinuum spanning over an octave from 900 - 2300 nm is reported in an all-normal dispersion, soft glass photonic crystal fiber. The all-solid microstructured fiber was engineered to achieve a normal dispersion profile flattened to within -50 to -30 ps/nm/km in the wavelength range of 1100 - 2700 nm. Under pumping with 75 fs pulses centered at 1550 nm, the recorded spectral flatness is 7 dB in the 930 - 2170 nm range, and significantly less if cladding modes present in the uncoated photonic crystal fiber are removed. To the best of our knowledge, this is the first report of an octave-spanning, all-normal dispersion supercontinuum generation in a non-silica microstructured fiber, where the spectrum long-wavelength edge is red-shifted to as far as 2300 nm. This is also an important step in moving the concept of ultrafast coherent supercontinuum generation in all-normal dispersion fibers further towards the mid-infrared spectral region.

Ground-and Excited-State Tautomerization Rates in Porphycenes

The rates of double hydrogen transfer in the ground and excited electronic states have been measured for porphycene and its derivatives by using a new method based on pump-probe polarization spectroscopy. Changing the strength of two intramolecular hydrogen bonds by altering the NHN distance leads to differences in the tautomerization rate exceeding three orders of magnitude. The reaction is considerably slower in the lowest electronically excited state. A correlation was found between the tautomerization rates and (1)H chemical shifts of the internal protons.

Unusual, Solvent Viscosity-Controlled Tautomerism and Photophysics: Meso-Alkylated Porphycenes

Stationary and time-resolved studies of 9,10,19,20-tetramethylporphycene and 9,10,19,20-tetra-n-propylporphycene in condensed phases reveal the coexistence of trans and cis tautomeric forms. Two cis configurations, cis-1 and cis-2, play a crucial role in understanding the excited-state deactivation and tautomer conversion dynamics. The trans-trans tautomerization, involving intramolecular transfer of two hydrogen atoms, is extremely rapid (k ≥ 10(13) s(-1)), both in the ground and lowest electronically excited states. The cis-1-trans conversion rate, even though the process is thermodynamically more favorable, is much slower and solvent-dependent. This is explained by the coupling of alkyl group rotation with the hydrogen motion. Excited-state deactivation is controlled by solvent viscosity: the S(1) depopulation rate decreases by more than 2 orders of magnitude when the chromophore is transferred from a low-viscosity solution to a polymer film. Such behavior confirms a model for excited state deactivation in porphycene, which postulates that a conical intersection exists along the single hydrogen transfer path leading from the trans to a high energy cis-2 tautomeric form. For this process, the tautomerization coordinate includes not only hydrogen translocation but also large-amplitude twisting of the two protonated pyrrole moieties attached to the opposite sides of the ethylene bridge.

Experimental Demonstration of Entanglement-Enhanced Classical Communication over a Quantum Channel with Correlated Noise

We present an experiment demonstrating the entanglement enhanced capacity of a quantum channel with correlated noise, modeled by a fiber optic link exhibiting fluctuating birefringence. In this setting, introducing entanglement between two photons is required to maximize the amount of information that can be encoded into their joint polarization degree of freedom. We demonstrated this effect using a fiber-coupled source of entangled photon pairs based on spontaneous parametric down-conversion, and a linear-optics Bell state measurement. The obtained experimental classical capacity with entangled states is equal to 0.82+/-0.04 per a photon pair, and it exceeds approximately 2.5 times the theoretical upper limit when no quantum correlations are allowed.

Photography of Shock Waves During Excimer Laser Ablation of the Cornea: Effect of Helium Gas on Propagation Velocity

Shadow photography of shock waves excited by means of a xenon chloride excimer laser was performed to determine the shock wave propagation velocity in air, nitrogen and helium. Energy densities between 500 and 2,000 mJ/ cm2 were used to ablate a rotating rubber cylindrical target and porcine corneas. In ablating the rubber cylinder, a shock wave velocity of 3.3 km/s was generated in air and nitrogen at 40 ns; this decreased to 1.4 km/s at 320 ns. When helium was blown on the target, the velocity increased by a factor of approximately two, to 5.9 km/s at 40 ns and 2.7 km/s at 320 ns. We suggest that blowing helium on the surface of the cornea during excimer laser ablation may speed the dissipation of high-energy acoustic waves and gaseous particles, and thus reduce the exposure and transfer of heat energy to the surrounding tissue.

Joint spectrum of photon pairs measured by coincidence Fourier spectroscopy

We propose and demonstrate a method for measuring the joint spectrum of photon pairs via Fourier spectroscopy. The biphoton spectral intensity is computed from a two-dimensional interferogram of coincidence counts. The method has been implemented for a type-I downconversion source using a pair of common-path Mach-Zehnder interferometers based on Soleil compensators. The experimental results agree well with calculated frequency correlations that take into account the effects of coupling into single-mode fibers. The Fourier method is advantageous over scanning spectrometry when detectors exhibit high dark count rates leading to dominant additive noise.

Simple all-PM-fiber laser mode-locked with a nonlinear loop mirror

In this Letter, we present a figure-eight all-PM-fiber laser oscillator design with a nonlinear optical loop mirror as an artificial saturable absorber. Unlike previous constructions using the same mode-locking technique, our cavity is constructed entirely of polarization-maintaining (PM) fibers, making the oscillator more resistant to thermal and mechanical perturbations. Two simple and robust laser configurations that differ by the output coupling ratio (70% or 30%) are presented. The first configuration delivers high energy pulses of 3.5 nJ, and the second configuration delivers pulses of 1.6 nJ at a common repetition rate of 15 MHz. In either configuration, the pulsed operation is stable, and the laser operates in a single pulse train regime, even for pump powers approaching twice the power required for mode-locking. We have also observed that, at higher intracavity powers, stimulated Raman scattering plays a significant role.

Passively modelocked, diode-pumped Yb:KYW femtosecond oscillator with 1 GHz repetition rate.

We demonstrate a high repetition rate, single mode fiber-coupled diode pumped, Yb:KYW laser in a four mirror ring cavity configuration and study its performance in soft aperture, Kerr lens mode-locked operation at around 1.04 microm.