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Starch characterizes ultrashort pulses

08 Jun 2004

Starch could simplify the optical setup required to determine the exact temporal and spectral shape of ultrashort pulses.

Starch is an ideal non-linear medium for characterizing ultrashort pulses, according to researchers in Spain. By simply sandwiching a starch suspension between two microscope coverslips, the team says it can fully characterize ultrashort pulses in the sample plane of a non-linear microscope.

“We are able to determine the exact temporal and spectral shape of the pulse as well as the exact phase at the focal plane of a high numerical aperture (NA) lens,” Iain Cormack, a researcher based at the ICFO - Institut de Ciències Fotòniques in Barcelona told Optics.org. “Traditional techniques can only infer the pulse duration and provide little phase information.”

Armed with this extra information, the IFCO group hopes to optimize the pulses used in specific non-linear microscopy techniques. "We plan to tailor the exact pulse profile to investigate which phase profile produces a maximum fluorescence signal," explained Cormack."This is important in life science where imaging of living cells requires the minimum amount of light to avoid photodamage."

The team believes its approach solves the two main problems associated with pulse measurement through a high NA objective lens. The first problem is finding a suitable non-linear material.

The material must be able to cope with the large range of incident angles at the sample plane; be polarization insensitive and have a large spectral bandwidth. According to Cormack, starch meets all these criteria. There are also some practical advantages: starch is non-toxic and inexpensive as well as easy to store, obtain and handle.

“In the past, researchers have been forced to use extremely thin and expensive non-linear crystals and carefully control the polarisation on the light entering the microscope,” said Cormack. “Our technique does not require such complexity.”

The technique involves passing the pulses through a Michelson interferometer. A telescope expands the recombined output from the interferometer to fully fill the aperture of the high NA objective lens. The pulses generate a second harmonic signal from the starch, which is passed to a spectrometer for analysis.

This is where a second problem occurs in that a collinear geometry is imposed. As no suitable retrieval algorithm exists for this geometry, the team has developed a pre-processing technique that allows a conventional retrieval algorithm to fully characterize the pulse.

The researchers have successfully characterized ultrashort pulses from two sources: a mode-locked Ti:Sapphire emitting at 830 nm and an optical parametric oscillator operating at 1100 nm. The NA of the lenses used were 1.25NA and 0.85NA respectively.

Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.

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