Tissue produces harmonics with ultrashort pulses

Feb. 1, 1997

Using chicken skin, muscle, and fat tissue, researchers from the City University of New York and New York State Center for Advanced Technology in Photonics have generated second- and third-harmonic signals of femtosecond and picosecond laser pulse.

Laurie Ann Peach

Experimental setup for measuring second-harmonic signals from chicken samples includes a spectrometer coupled to a photomultiplier and a computer-controlled lock-in amplifier system. Long-pass filter in front of the focusing lens reduces possible harmonic contributions from on-line optics. Short-bandpass filter at the entrance of the spectrometer eliminates scattering from the fundamental beam.

Using chicken skin, muscle, and fat tissue, Robert R. Alfano, Yici Guo, and associates from the City College and Graduate School of the City University of New York and New York State Center for Advanced Technology in Photonics have generated second- and third-harmonic signals of femtosecond and picosecond laser pulses with conversion efficiencies in the range of 10^-7 to 10^-10. The researchers believe the results show the potential for using light as a noninvasive diagnostic and therapeutic tool for biomedical applications such as detecting cancer-infected tissue.

Alfano and Guo applied nonlinear, two-photon spectroscopy to animal tissue to develop a diagnostic method using near-infrared wavelengths. As the photons enter the tissue sample they scatter, producing new frequencies. Red photons penetrate further into the tissue than UV light.

Ultrashort pulses were used to analyze highly scattering tissue samples—in this case, a wet 3 3 2-cm sample (frozen, then thawed) from a chicken thigh (see figure). A Kerr-lens self-modelocked Ti:sapphire laser generated 120-fs pulses with a repetition rate of 76 MHz. The wavelength was adjusted to 810 nm, and the average output power set at 1.4 W. The beam was focused onto the chicken sample at an incident angle of about 45°. Tests were also performed with 30-ps pulses at 1064 nm.¹

Different biostructures such as chicken skin, chicken muscle, and chicken fat interfaces were studied. The chicken skin exhibited the largest second-harmonic-generation signal, followed by chicken muscle, then by chicken fat. For third-harmonic generation, chicken skin produced a slightly higher signal than chicken fat, and chicken fat produced a signal higher than muscle.

The next facet of the work will determine if this method can be applied to detection of highly scattering biological tissues, such as those infected with cancer. The work was supported by the US Air Force Office of Scientific Research and Mediscience Technology (Cherry Hill, NJ).

REFERENCE

1. Yici Guo et al., Appl. Opt. 35(34), 6810 (Dec. 1, 1996).