PROCEEDINGS OF SPIE
SPIEDigitalLibrary.org/conference-proceedings-of-spie
Laser Action in dye-infused biological
tissue
Lihong V. Wang, Da Liu, Nancy He, Steven L. Jacques,
Sharon L. Thomsen
Lihong V. Wang, Da Liu, Nancy He, Steven L. Jacques, Sharon L. Thomsen,
"Laser Action in dye-infused biological tissue," Proc. SPIE 2624, Laser-Tissue
Interaction and Tissue Optics, (10 January 1996); doi: 10.1117/12.229544
Event: BiOS Europe '95, 1995, Barcelona, Spain
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 12/11/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use
Laser Action in Dye-Infused Biological Tissue
Lihong Wang,* Da Liu, Nancy He, Steven L. Jacques, Sharon L. Thomsen
Laser Biology Research Laboratory, Box 17
University of Texas M. D. Anderson Cancer Center, 15 15 Holcombe Boulevard
Houston, TX 77030, USA
Abstract
The narrowing of the spectral linewidth and the increasing of the peak intensity
characteristic of laser action was observed in emission spectra of dye-infused biological tissues.
The fresh tissue was infused with a solution of Rhodamine 640 perchiorate in ethanol and then
excited with frequency-doubled Q-switched Nd:YAG laser pulses. The sharp spectral peaks of
laser action in tissues may find applications in detection of superficial disease.
Key Words: Tissue Optics, Laser Action, Fluorescence, Amplification of Spontaneous
Emission, Turbid Media.
*
To
whom correspondence should be addressed.
Assistant Professor.
Emails: lihong@laser.mda.uth.tmc.edu
Phone: (713) 745-1742
Fax:
(713) 792-3995.
2
ISPIE Vol. 2624
O-8194-1988-5/96/$6.OO
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 12/11/2018
Terms of Use: https://www.spiedigitallibrary.org/terms-of-use
Introduction
Laser action in turbid media made of Ti02 in methanol mixed with dye solution was
first observed by Lawandy et al. recently, where the dye solution was Rhodamine 640
perchiorate dissolved in methanol.' The turbid media were excited with a pulsed laser, and a
surprisingly low threshold excitation energy was required to generate the laser action. The
mechanism for the laser action was speculated to be scattering-enhanced amplification of
spontaneous emission.2 The scattering property of the turbid media increased the path length of
light in the gain medium, which consisted of the dye molecules excited by the pulsed laser.
When the path length exceeded a critical gain length, spectral narrowing of the spontaneous
emission linewidth occurred. It has been known that the spectral linewidth was inversely
proportional to the square root of the light propagation length for both homogeneously and
inhomogeneously broadened transitions under unsaturated-gain conditions.3
The above experiment was conducted in physical turbid media. Knowing that most
biological tissues are intrinsically scattering, we hypothesized that the laser action could be
generated in biological tissues and hoped that the sharp and strong spectral peak could result in
high-sensitivity diagnostic techniques in medicine. This article presents our experimental
findings on laser action in biological tissue. We have replaced the methanol with ethanol as the
solvent of the dye because ethanol is less biologically toxic than methanol. Other investigators
have also independently studied laser action in biological tissues.4
Methods and Materials
Dye solutions were prepared using Rhodamine 640 perchiorate (molecular weight, 591)
mixed in a 70% solution of ethanol. The concentration of dye will be expressed in the units of
M. Muscle tissue was freshly excised from killed Fisher 344 rats and then divided into
multiple pieces weighing 540 10 mg each. A total of 0.2 ml of dye solution was injected into
various sites of each piece of tissue and allowed to diffuse evenly for 10 mm. The actual
absorbed amount was 0.09
0.017 ml, which was measured by weighing the tissue before
and 10 mm. after the dye injection. The dye solution that leaked out of the tissue was
excluded.
The tissue sample was placed on a plastic dish and covered with a thin piece of glass to
prevent the tissue from drying (Fig. 1). Then the dish was placed on a height-adjustable
platform so the spot area of the laser beam on the sample surface could be varied. A linearly
polarized frequency-doubled (532 nm) Q-switched Nd:YAG laser pulse of lO-ns duration was
repeated at a rate of 10 Hz. Two prisms routed the laser light, which was focused by a lens of
SPIE
Vol. 2624 /3
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 12/11/2018
Terms of Use: https://www.spiedigitallibrary.org/terms-of-use
10-cm focal length and normally incident to the sample surface. The pulse energy was
monitored by splitting a portion of the laser beam into an energy detector connected to an
oscilloscope. An optical fiber (core diameter, 600 jim; N.A., 0.44) of 45° incidence collected
the emission spectra near the samples, where the distance between the tip of the optical fiber
and the incident point of the excitation laser beam on the tissue surface was 1.5 cm. The
polarization plane of the excitation laser light was perpendicular to the plane formed by the
incident laser light and the collection optical fiber. An optical multichannel analyzer (OMA)
system that was synchronized with the laser pulses analyzed the spectra that passed a 570-nm
long-pass filter, a narrow slit, and a spectrograph. A computer averaged the spectra over five
measurements and displayed the averaged spectra.
Results
Figure 2 compares the emission spectra of the dye-infused muscle tissues with
respectively low and high excitation energy. The dye concentration in the ethanol solution was
1.69 x i0 M. The spot area of the laser beam on the sample surface was 1.47 mm2. When
the excitation energy of the laser beam on the dye-infused muscle tissue was increased from
4
/ SPIE Vol. 2624
Beam Splitter Lens
Fig. 1. Schematic of the experimental set-up. The thick and thin arrows represent
optical and electronic signals, respectively. The thickness of the glass layer was 0.016
cm.
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 12/11/2018
Terms of Use: https://www.spiedigitallibrary.org/terms-of-use