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Self-phase modulation in polydiacetylene single crystal measured at 720-1064nm
Optics Letters
|November 28, 2007
Summary
This study measured spectral broadening in poly-[2, 4 hexadiyne-1, 6 diol-bis-(p -toluene sulfonate)] (PTS) crystals. The nonlinear refractive index (n(2)) and two-photon absorption coefficient (α(2)) were determined, showing n(2) decreases with increasing wavelength.
Area of Science:
- Nonlinear Optics
- Materials Science
- Condensed Matter Physics
Background:
- Poly-[2, 4 hexadiyne-1, 6 diol-bis-(p -toluene sulfonate)] (PTS) is a conjugated polymer with potential nonlinear optical applications.
- Understanding spectral broadening mechanisms is crucial for optimizing nonlinear optical devices.
Purpose of the Study:
- To quantitatively measure spectral broadening in PTS single crystals due to self-phase modulation.
- To determine the nonlinear refractive index (n(2)) and two-photon absorption coefficient (α(2)) of PTS across a range of wavelengths.
- To investigate the influence of group-velocity dispersion on spectral broadening.
Main Methods:
- Employed a Ti:sapphire laser (200-fs pulses, 720-920 nm) and a Nd:YAG laser (50-ps pulses, 1064 nm) for spectral broadening measurements.
- Calculated the nonlinear refractive index (n(2)) from measured phase shifts.
- Determined the two-photon absorption coefficient (α(2)) using nonlinear transmission measurements.
- Estimated and evaluated the effect of group-velocity dispersion.
Main Results:
- Observed significant spectral broadening in PTS single crystals attributed to self-phase modulation.
- Determined the nonlinear refractive index (n(2)) to be as large as 3.9x10^-5 cm²/MW at 720 nm.
- Found that the magnitude of n(2) monotonically decreases as wavelength increases away from resonance.
- Confirmed that two-photon absorption has a negligible contribution to n(2) at off-resonant wavelengths up to 1064 nm.
- Group-velocity dispersion was found to have a negligible effect on the observed spectral broadening.
Conclusions:
- Self-phase modulation is a dominant mechanism for spectral broadening in PTS crystals.
- PTS exhibits a strong, wavelength-dependent nonlinear refractive index.
- The material's nonlinear optical properties are suitable for applications in specific wavelength ranges, with minimal influence from two-photon absorption off-resonance.
