D. Fortusini

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=1.8 μm and Δλ as large as 60 nm, wavelength shifting with unit gain can be obtained with a pump intensity of 6 MW/cm2, while amplification by a factor 10 requires 18 MW/cm2.

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.

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.

TWO-PHOTON ABSORPTION SPECTRUM OF 3-BUTOXYCARBONYLMETHYLURETHANE POLYDIACETYLENE THIN FILMS

We report the two-photon absorption (TPA) spectrum of a film of poly[4,6-dacadiyne-1,10-diolbis(3-butoxycarbonyl-methyl-urethane)]—commonly known as poly(3-BCMU)—in the wavelength range λ=1.6–0.7 μm. We find that the TPA coefficient of the film, which was prepared by spin coating, increases from ≈2 cm/GW at 1.45 μm to 170 cm/GW at 0.8 μm. The main features of the spectrum can be accounted for by a three-level model with a two-photon state positioned at 2.8 eV.

Two-photon absorption coefficient measurements based on widely tunable femtosecond pulses from parametric generation

We discuss an experimental arrangement for fast and reliable measurements of the two-photon absorption coefficient and its wavelength dispersion. Widely tunable pulses, with sufficient power (and therefore intensity) to detect the nonlinear absorption in samples with a small (L is the sample thickness), are provided by travelling-wave parametric generation. The short time duration of the pulses minimizes the accumulation of carriers excited by two-photon absorption (TPA), or by an eventual linear mechanism, which could otherwise produce an appreciable additional absorption which would be difficult to take account of. To produce a relative calibration of the set-up, we suggest the use of second-order crystals which, when phase-matched for second harmonic generation, mimic a two-photon absorber. We give examples of measurements on semiconductors and on organic thin films, and we report the TPA spectra of CdTe and CdSe semiconductor nanocrystals in a glass matrix.

Measurement and prediction of phase-matching conditions in the nonlinear optical crystal N-(4-nitrophenyl)-L-prolinol

We report an accurate set of Sellmeier coefficients for the N-(4-nitrophenyl)-L-prolinol (NPP) crystal, obtained by improving a previously proposed set through the measurement of the phase-matching angle for second harmonic generation. Our Sellmeier set describes well the phase-matching conditions for various nonlinear optical interactions, covering almost completely the transmission range of the crystal.

Low-power frequency conversion through parametric interaction and cascaded processes in a N-(4-nitrophenyl)-l-prolinol crystal

Abstract Crystals of N -(4-nitrophenyl)- l -prolinol (NPP) of large size and of high optical quality are grown in methanol solution starting from toluene nucleated seeds. Measurements are conducted on a 2.8 mm thick specimen obtained by cleaving a thick slice parallel to the (101) plane. This type of cut allows access to the dominant phase-matching orientation, including the noncritical one. We exploit the high second-order susceptibility of NPP to produce various frequency conversion processes (second harmonic, parametric generation and nearly-degenerate wavelength conversion through cascading) at a very low input intensity. In particular, frequency conversion of a signal pulse (from 1.16 to 1.14 μm) under the action of a pump pulse (at 1.15 μm) with unit gain is obtained with a pump intensity as low as 9 MW/cm 2 , while parametric generation from quantum noise, with 10% conversion efficiency, is obtained in a single pass of the pump pulse focussed to an intensity around 10 8 W/cm 2 .

Measurement of the two-photon absorption coefficient of semiconductor nanocrystals by using tunable femtosecond pulses

By using femtosecond pulses from traveling-wave parametric generation we have accurately measured the absolute value and the frequency dispersion of the two-photon absorption coefficient of semiconductor nanocrystals embedded in a glass matrix. Comparison is made with bulk semiconductors.

Molecular weight induced order in poly-3BCMU spun films

Poly-3-butoxycarbonylmethylurethane (poly-3BCMU) spun films were formed from polymer batches with different molecular weight (from about 100,000 to 375,000 g/mol). The two-photon absorption (TPA) spectrum of poly-3BCMU films was measured in the near infrared from 800 to 1350 nm by using femtosecond pulses. Although the TPA spectrum features were the same in all investigated films, the TPA coefficient β was two times larger in the films obtained from the higher molecular weight batches. As the number of repeat units in all polymer batches was well above the saturation limit of the hyperpolarisability γ in conjugated chains, the observed behaviour can be explained with the assumption that a more ordered and planar arrangement of the polymer chains is promoted in the polymer films with higher molecular weight. This hypothesis is confirmed by the measurement of the film refractive index and birefringence.

Second order cascaded frequency shifting and signal amplification in organic and inorganic crystals

We report wavelength shifting (generation of a pulse at the wavelength (lambda) p - (Delta) (lambda) from a signal at (lambda) p + (Delta) (lambda) under the action of a pump at (lambda) p) and parametric amplification through a cascaded second order process in a periodically poled lithium niobate crystal and also in an organic crystal N-(4-nitrophenyl)-L- prolinol (NPP). NPP could convert signal pulse (from 1.16 to 1.14 micrometer) with unit gain under the action of a pump pulse (at 1.15 micrometer) of peak intensity as low as 9 MW/cm2. In the limit of negligible conversion, where the cascading effect can be described through an effective (chi) (3), we derive for NPP(chi) eff(3) approximately equals 2.4 X 10-17 m2/V2, which is approximately 102 larger than (chi) (3) of conjugated polymers or semiconductors. In a 19 mm long PPLN sample, at (lambda) p equals 1.8 micrometer and (Delta) (lambda) as large as 60 nm, we could obtain unit gain with a pump intensity of 6 MW/cm2, while amplification by a factor of 10 requires 18 MW/cm2. We also present a theoretical comparison between frequency mixing and cascading.