Versatile multimodal optical modality based on Brillouin light scattering and photoacoustic effect
A multimodal optical technique that can measure the mechanical, optical, and acoustical properties of the sample at microscopic resolution, which is based on the integration of a Brillouin microscope and a photoacoustic (PA) microscopy is provided. The multimodal technique not only can acquire co-registered Brillouin and PA signals of the sample but also allows us to utilize the sound speed measurements by PA to quantify the sample's refractive index, which is an essential property of the material and cannot be measured by either technique individually. We demonstrated the colocalization of Brillouin and time-resolved PA signals by measuring the interface of kerosene and 1% CuSO4 aqueous solution. In addition, we measured the refractive index of saline solutions with a precision of 0.003 and validated the result against published data. This multimodal modality could open new way for charactering biological cell and tissue in physiological and pathological conditions.
1 . A multimodal optical apparatus for imaging a sample to provide comprehensive mechanical properties, the multimodal optical apparatus comprising:
a sample cell that holds the sample;
a first light source for providing a Brillouin light beam to the sample for Brillouin scattering;
a first optical system for directing and modifying the Brillouin light beam, the first optical system providing a p-polarized light beam to the sample;
a second light source for providing a photoacoustic light beam to the sample for photoacoustic excitation;
a second optical system for directing and modifying the photoacoustic light beam, the second optical system providing a s-polarized light beam to the sample;
a third optical system for coupling the Brillouin light beam to the photoacoustic light beam as a coupled light beam after the first optical system and the second optical system, the third optical system directing the coupled light beam to the sample;
a Brillouin spectrometer for collecting a Brillouin signal generated from the sample; and
an acoustic detector for detecting a photoacoustic signal generated from the sample.
2 . The multimodal optical apparatus of claim 1 wherein the Brillouin spectrometer collects the Brillouin signal at a 90° geometry.
3 . The multimodal optical apparatus of claim 1 wherein the acoustic detector is a hydrophone or a microphone.
4 . The multimodal optical apparatus of claim 1 wherein the first optical system comprises:
an isolator to reject back-reflection;
a variable neutral density filter adjusting output power of the first light source;
a halfwave plate (HWP 1 ) for adjusting polarization state of the Brillouin light beam to be p-polarized; and
a lens system for expanding the Brillouin light beam.
5 . The multimodal optical apparatus of claim 4 , wherein the lens system includes a pair of lenses.
6 . The multimodal optical apparatus of claim 1 , wherein the first light source is a continuous wave laser.
7 . The multimodal optical apparatus of claim 1 wherein the second light source is a tunable pulsed laser.
8 . The multimodal optical apparatus of claim 1 , wherein the second optical system includes a polarized beam splitter (PBS 1 ) to obtain a linearly polarized beam and a half-wave plate (HWP 2 ) for adjusting polarization orientation of the photoacoustic light beam to s-polarization.
9 . The multimodal optical apparatus of claim 1 , wherein the third optical system includes a polarized beam splitter (PBS 2 ) that couples the Brillouin light beam to the photoacoustic light beam.
10 . The multimodal optical apparatus of claim 9 , wherein the third optical system includes a half-wave plate (HWP 3 ) to further adjust the Brillouin light beam and PA beam to s-polarized and p-polarized, respectively.
11 . The multimodal optical apparatus of claim 1 , wherein the third optical system includes a mirror M 1 for directing the coupled light beam to the sample.
12 . The multimodal optical apparatus of claim 1 , wherein the Brillouin spectrometer includes virtually imaged phased arrays (VIPA) for measuring the Brillouin signal.
13 . The multimodal optical apparatus of claim 1 , wherein the sample is a biological sample.
14 . The multimodal optical apparatus of claim 1 , further comprising a computing device configured to receive Brillouin signals from the Brillouin spectrometer and photoacoustic measurements from the acoustic detector.
15 . The multimodal optical apparatus of claim 14 , wherein the computing device is configured to determine mechanical, optical, and acoustical properties of the sample at microscopic resolution.
16 . A multimodal optical apparatus for imaging a sample to provide comprehensive mechanical properties, the multimodal optical apparatus comprising:
a sample cell that holds the sample;
a first light source for providing a Brillouin light beam to the sample for Brillouin scattering;
a first optical system for directing and modifying the Brillouin light beam, the first optical system providing a p-polarized light beam to the sample;
a second light source comprising a pulsed laser for providing a photoacoustic light beam to the sample for photoacoustic excitation;
a second optical system for directing and modifying the photoacoustic light beam, the second optical system providing a s-polarized light beam to the sample;
a third optical system for coupling the Brillouin light beam to the photoacoustic light beam as a coupled light beam after the first optical system and the second optical system, the third optical system directing the coupled light beam to the sample;
a Brillouin spectrometer for collecting a Brillouin signal generated from the sample; and
an acoustic detector for detecting a photoacoustic signal generated from the sample, wherein the photoacoustic signal comprises acoustic waves induced by thermoelastic expansion in the sample.