G. P. Banfi

Traveling-wave parametric generation of widely tunable, highly coherent femtosecond light pulses

We report on the generation of ultrashort tunable pulses with a cavityless traveling-wave scheme consisting of a parametric superfluorescence seed source and a parametric amplifier. We show that the traveling-wave approach, with its advantages of simplicity and direct generation of tunable energetic single pulses, can be used in the femtosecond regime, and to this end we discuss the performances that were obtained with pump pulses of ≈1-ps and 200-fs duration at wavelengths of 0.53 and 0.6 μm, respectively. Of particular interest is the β-barium borate-based traveling-wave parametric generator (type-II phase matching), since it offers the possibility of generating nearly transform-limited pulses that are continuously tunable within a wide spectral range to as high as 3 μm in the IR. With a diffraction-limited pump at 0.53 μm. we obtained tunable pulses in a 1.2× diffraction-limited beam, which could be focused, with an f/20 optics lens, to an intensity of 1013 GW/cm2. A temperature-tuned lithium triborate-based femtosecond parametric generator, with its smaller group-velocity dispersion and absence of walk-off, can operate at a pump energy of as low as 30 μJ in a 200-fs pulse.

Nonlinear optical properties of semiconductor nanocrystals

This review is devoted to the description of recent experimental results concerning the nonlinear optical properties of semiconductor-doped glasses SDGs with particular emphasis on the regime in which the energy of the incident photon is smaller than the energy gap. A considerable theoretical and experimental effort has been devoted in the last 10years to the fundamental aspects of quantumconfined structures, which have properties somewhat intermediate between the bulk crystals and atoms or molecules. From this point of view, SDGs represent an easily available test system, and optical techniques have been a major diagnostic tool. Luminescence and absorption spectroscopy were extensively used to characterize the electronic states. The experiments aimed at the measurement of the real and imaginary parts of the third-order optical susceptibility of SDGs below the bandgap are described in some detail, and the results obtained with different techniques are compared. Besides the intrinsic fast nonlinearity due ...

WAVELENGTH SHIFTING AND AMPLIFICATION OF OPTICAL PULSES THROUGH CASCADED SECOND-ORDER PROCESSES IN PERIODICALLY POLED LITHIUM NIOBATE

We investigate the efficiency of wavelength shifting (generation of a pulse at the wavelength λp−Δλ from a signal at λp+Δλ under the action of a pump at λp) and parametric amplification through a cascaded second-order process in a periodically poled lithium niobate crystal. Our results show that, in a 19-mm-long sample, at λp=1.8u2009μm and Δλ as large as 60 nm, wavelength shifting with unit gain can be obtained with a pump intensity of 6u2009MW/cm2, while amplification by a factor 10 requires 18u2009MW/cm2.

Self-diffraction through cascaded second-order frequency-mixing effects in β-barium borate

The large effective third-order nonlinearity produced by cascaded second-order processes is evidenced by the spatial self-diffraction of two beams interacting in a β-barium borate crystal. In this configuration, both the real and imaginary parts of the induced third-order susceptibility play a role, and the effect reaches its maximum around the phase-matching condition.

Optical frequency mixing through cascaded second‐order processes in β‐barium borate

We show that the large effective optical nonlinearity, which is produced by cascaded second‐order processes, allows us to accomplish the frequency conversion of a signal pulse with a pump pulse of moderate intensity. In a 1 cm long β‐barium borate crystal, the cascaded effect enhances by 5 orders of magnitude the efficiency of the frequency mixing process. The enhancement is larger than 104 even in non‐phase‐matched condition for the intermediate second‐harmonic field, when the imaginary part of the induced nonlinear susceptibility is negligible.

Third‐order nonlinearity of semiconductor doped glasses at frequencies below band gap

We measured, through nearly degenerate three‐wave mixing, the third‐order nonlinear optical susceptibility of semiconductor doped glasses at frequencies below band gap. With typical 1.5%–5% volume fractions, the CdTe and CdS1−xSex nanocrystals give a contribution to the nonlinearity of the composite that is comparable to that of the glass matrix. The magnitude and the band‐gap dependence of the nonlinear susceptibility of the nanocrystals are similar to those of the bulk semiconductors.

OPTICAL NONLINEARITY OF SEMICONDUCTOR-DOPED GLASSES AT FREQUENCIES BELOW THE BAND-GAP - THE ROLE OF FREE-CARRIERS

We have studied, through time-resolved degenerate-four-wave mixing, the nonlinear response below the band gap of glasses doped with CdTe and CdS1−xSex nanocrystals by using 30-ps pulses from a Nd:YAG laser in the intensity range 0.4 – 1.6 GW/cm2. We found that the third-order nonlinearity is overshadowed by a fifth-order one when the band gap of the semiconductor crystallites became smaller than twice the photon energy. The fifth-order response, which is due to the refraction from the free carriers generated by two-photon absorption, does not saturate up to the highest intensity (corresponding to an electron–hole density in the crystallites N ≈ 1019 cm−3) and decays on a nanosecond time scale. We derive σ, the ratio between the refractive-index change and N (σ = 3 − 4 × 10−21 cm3 for CdTe nanocrystals) and compare it with the values for bulk semiconductors.

INDUCED 'THIRD-ORDER' NONLINEARITY VIA CASCADED SECOND-ORDER EFFECTS IN ORGANIC CRYSTALS OF MBA-NP

Abstract The enhancement of frequency mixing and phase-modulation due to cascaded second-order effects is studied in the organic MBA-NP crystal. In a 3 mm-thick sample, we observe a 10 4 increase of the frequency mixing yield, which implies an effective ‘third-order’ susceptibility of 10 -19 m 2 /V 2 .

Frequency shifting through cascaded second-order processes in a N-(4-nitrophenyl)-L-prolinol crystal

We exploit the high second-order susceptibility of the organic crystal N-(4-nitrophenyl)-L-prolinol to accomplish, through a cascaded second-order process, wavelength conversion of a signal pulse (from 1.16 to 1.14 microm) under the action of a pump pulse (at 1.15 microm). In a 2.8-mm-thick crystal, wavelength conversion with unit gain was obtained with a pump peak intensity as low as 9 MW/cm(2) . At low intensities, in the limit of negligible conversion where the cascading effect can be described through an effective third-order susceptibility, we derive |(x)((3))(eff) | approximately 2.4 x 10(-17) m(2)/V(2), which is ~10(2) larger than the nonresonant (x)((3)) of conjugated polymers or semiconductors.

Femtosecond traveling-wave parametric generation with lithium triborate

Ultrashort pulses, of the same time duration as the pump pulse, have been obtained with a traveling-wave parametric generator employing a temperature-tuned lithium triborate crystal in noncritical phase matching. Starting from a 190-fs pulse at 0.6 μm generated by a dye laser, we achieved an overall conversion efficiency of 10-15% in the tuning range 0.85–0.97 μm (and the corresponding range of 1.6–2.1 μm) with a pump energy of only 30 μJ. The easy operation in the femtosecond regime and the minimal energy requirements that we found are due to the small group-velocity mismatch of lithium triborate at this pump wavelength and to the absence of walk-off.