Rafael Quintero-Torres

PICOSECOND ALL-OPTICAL SWITCHING IN A FABRY-PEROT CAVITY CONTAINING POLYDIACETYLENE

Off‐resonant nonlinear optical measurements have been made in a Fabry–Perot cavity containing a polydiacetylene (PTS) single crystal. The cavity length was controlled to select specific bias points on the Fabry‐Perot fringe and the measurements were made by a pump‐probe technique using 90 ps pulses at 1.06 μm wavelength. The changes in transmission at selected bias points were measured as a function of time delay between the pump and the probe. The measured response time was found to be pulse‐width limited and the maximum changes in transmission were as large as 50% at low optical intensities (∼10 MW/cm2). The sign of n2 was determined from the direction of the fringe shift and was found to be negative. All‐optical switching (82 MHz) at a picosecond time scale is demonstrated for an organic material.

Measurement of the nonlinear refractive index of polydiacetylene using Michelson interferometry and z-scan

We have performed a detailed measurement of the sign of the nonlinear refractive index of PTS-polydiacetylene using Michelson interferometry and z-scan at 1.06 μm wavelength. The measurement was performed using 60 ps laser pulses at 10 Hz repetition rate that avoided any significant thermal contribution. The sign of n2 was found to be negative. The negative sign was explained considering saturation of the exciton absorption for off-resonant pumping. The saturation far outweighs two photon resonances in determining sign and magnitude of n2 in the off-resonant domain within the wavelength range 700–1064 nm.

Photoluminescence dynamics in solid formulations of colloidal PbSe quantum dots: Three-dimensional versus two-dimensional films

Time-resolved photoluminescence spectroscopy is used to compare decay dynamics of colloidal PbSe quantum dots as (i) dilute dispersions in hexanes, (ii) thick, dense, emulsive films, and (iii) sub-monolayer films on silicon surfaces. Accounting for the effect of each dielectric environment on exciton radiative decay rates, we deduce the non-radiative decay rate of excitons increases going from solution to thick films by ≈6 fold and to sub-monolayer films by ≈7–16 fold. This is attributed to surface passivation degradation due to a lack of mobile ligands when the particles are taken out of solution.

Ultralow-pump-threshold laser diode pumped Cr:LiSAF laser

Operation of Cr:LiSAF laser pumped by 670 nm high brightness single stripe (width of 50 μm) laser diode with pump-threshold as low as 650 μw is reported. The lasing threshold was observed simultaneously with threshold of operation of laser diode. For short pulse generation we designed and tested different multiple quantum well saturable absorbers. The continuous mode-locked operation was achieved with tunability from 846 to 875 nm.

NONLINEAR OPTICAL STUDIES OF SINGLE CRYSTAL FILMS OF N-(4-NITROPHENYL)-L-PROLINOL

Single crystal films of N–(4‐nitrophenyl)‐L–prolinol have been prepared by modified shear method and the second order optical susceptibility has been measured by second harmonic generation. The small thickness and high nonlinear coefficient of the films allowed second order optical measurements without considering phase matching. The magnitudes of the tensor elements of second order optical susceptibility were determined by detailed polarization selective measurements. The maximum effective value of the nonlinearity, dmaxeff was found to be approximately 18 times the deff of LiNbO3. The value of d21 is equal to 97 (±5) pm/V and d22 is equal to 30 (±5) pm/V at 1064 nm. Our analyses show that both type‐I and type‐II phase matching should be possible in waveguides of this material.

Correlation between photoinduced absorption and large off‐resonant nonlinear refractive index of polydiacetylene

We have calculated the off‐resonant nonlinear refractive indices of PTS and TCDU polydiacetylenes using photoinduced absorption data and applying the Kramers–Kronig transformation. The results are in good agreement with the recently published experimental measurements. This is the first time that a correlation between photoinduced absorption and the off‐resonant nonlinear refractive index has been established for polydiacetylenes. The calculated values are: n2∼1.3×10−5 cm2/MW for PTS, and ∼2.2×10−6 cm2/MW for TCDU at 1.06‐μm wavelength. The sign of n2 is positive at this wavelength.

High-sensitivity scatterometer with a bidirectional ring laser

We present a technique for the absolute measurement of very low-level scattering. The method is absolute in that it relies on fundamental physics and does not require calibration against standards maintained by the National Institute of Standards. In a bi-directional mode-locked ring laser, a difference in longitudinal mode frequency between the two senses of circulation of the intracavity pulses can be measured as a beat note between the corresponding outputs. A very small amount of light backscattered by a sample (located at the pulse crossing) from one sense of circulation into the other, causes the beat note to vanish. The threshold in mode frequency difference that causes locking is a measure of the scattering amplitude.

Sign and magnitudes of the off-resonant nonlinearities of polydiacetylene crystal measured by z-scan

We have used the z-scan technique to determine the sign and magnitude of the off-resonant n2 and two photon absorption coefficient of PTS-polydiacetylene. The sign of n2 is negative and the magnitudes of nonlinearities are consistent with our previous reports on waveguide measurements.

Photoluminescence dynamics at fast and slow time scale in PbSe nanocrystals

Photoluminescence as a function of time, wavelength and temperature is used to elucidate the relaxation of energy in PbSe as a function of the structural arrangement of the nanocrystals. The contribution of radiation emission, Förster exchange and nonradiative relaxation are accounted in processes from 77 to 300 K and in the wavelength from 1.65 to 1.20 um and in temporal scales from 1 ns to 10 us. Important contribution to the nonradiative relaxation with the mesoscopic arrangement of the nanocrystals and evidence of the Föster exchange independent of temperature and around 25 ns is measured. The general description of relaxation is used as well to correlate with the complex interaction between nanocrystals.

Ultralow-pump-threshold laser-diode-pumped continuous Cr:LiSAF laser

Operation of laser diode pumped Cr:LiSAF laser in 1 m long V-resonator with pump-threshold as low as 650 uW is reported. This threshold was observed simultaneously with threshold of single stripe (width of 50 um) 670 nm laser diode used as a pumping source.