TUNABLE LASER SOURCE  REVIEW

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PHOTON ETC'S TUNABLE LASER SOURCE

a versatile tool for state-of-the-art characterization

The variety of tunable light sources one can find seems at first limitless, but when trying to choose the right one for the right application, the limitations of each technology becomes apparent. The parameters to consider can also be confusing. With this in mind, we have gathered below a list of real applications to show how the versatile tunable light source developed by Photon etc’s can be of help. 

The basic idea behind the Tunable Laser Source (TLS) is to use a filtering device to select one wavelength out of a supercontinuum source, also known as a “white laser”. These pulsed light sources are now a common tool for various applications, from flow cytometry to lifetime experiments, but in many cases, only a small portion of the light emitted is needed, hence the use of a tunable filter.

The Laser Line Tunable Filter (LLTF) is a tunable bandpass filter based on volume holographic gratings. Placed in a certain configuration, these resonant glass gratings allow the selection of a portion of the supercontinuum output without affecting its polarization. Unlike other commercially available filters, it combines a very high isolation (up to 60dB or OD6) and a wide tunability: from 400 nm to 1000 nm or 1000 nm to 2300 nm within a single filter with bandwidths (FWHM) of 2.5 nm and 5 nm respectively. Also, the excellent pointing stability of the filtered beam allows an efficient recoupling to both mono- or multimode fibers.

Here are detailed more specific applications of Photon etc’s TLS:

Detecting breast cancer cells using gold nanoparticles1

Gold plasmonic nanoparticles (AuNPs) are used extensively as biomarkers, and are a viable candidate for a variety of other biological applications. However, it has been proven that their small size and the complex environment in which they navigate make their observation and characterization quite a challenge. In order to address this issue Patskovsky et al. [1] used a hyperspectral dark field microscope in backscattering configuration, replacing the usual white light illumination with by a tunable laser source. With this set-up, the group of researchers was able to sweep the illumination over the range 400 nm to 1000 nm. The wide range of wavelengths and high output power of the source were essential parameters for this study. They could indeed follow the spatial position and distribution along the z axis of the AuNPs targeting CD44, a cell surface receptor actively expressed in cancer stem cells. The hyperspectral imaging set-up described here can also be helpful in a wide range of biological applications requiring a combination of spatial and spectral information.

Testing the Gemini Planet Imager's coronagraph 2

The Gemini Planet Imager (GPI) is an astronomical instrument made to detect giant planets in nearby star system. The GPI uses a coronagraph in order to eliminate 99% of the coherent starlight. Before sending the GPI at Gemini South (located in Chilean Andes), it was crucial to test the coronagraph by reproducing the experimental conditions in which it would serve. The light source required to measure its performances had to be nearly achromatic, tunable across the GPI's wavelength domain (near-IR 0.95-2.4 µm), in addition to being powerful and collimated. Most of the light sources match one or two of these requirements but only Photon etc's unequalled and efficient tunable laser source combines all three above. The wide spectral range of the TLS and its high output power were exploited for sensitivity measurements of the imager. It helped determine that at its most efficient wavelength range the imager could detect a planet slightly more massive than Jupiter revolving around a 100 million year old Sun-like star.

Single-walled carbon nanotubes thermopile 3

Photon etc’s TLS is also a powerful tool for the characterization of carbon nanomaterials. St-Antoine et al. [3] used this technology to measure the spectral response of the photovoltage of a single-walled carbon nanotubes thermopile. It was also use for the acquisition of basic absorption spectra. The light source opened the possibility of studying the wavelength dependency of the researchers’ device, particularity in the visible and near infrared ranges, from 500nm to 1800nm. The combination of high power and small bandwidth (FWHM~2 nm) was indeed perfectly suited for the fine characterization of single-walled nanotubes. In addition to the above requirements, the source offers the possibility of high efficiency fiber coupling to allow the illumination of a sample within a confined vacuum chamber.

Solar cell external quantum efficiency mapping 4

In the race for higher solar cell efficiency, a better understanding of their fundamental electronic properties is paramount. With that in mind, Lombez et al. [4] investigated the spatial variations in the spectral response of CuInGa(S,Se)2 solar cells. In this study, Photon etc's TLS served as the illumination source for measurements of light beam induced current (LBIC) at different excitation wavelengths. The LBIC experiment allowed an estimation of the external quantum efficiency (EQE) at different positions of the sample. The LBIC measurements at enough positions on the sample allowed a map reconstruction of EQE. To carry out successfully this experiment, the illumination source needed both a wide spectral range and a high output power, delivered in a diffraction limited point source to achieve the best spatial resolution. Combining all of the above requirements, Photon etc’s TLS was chosen to excite the sample, mounted on a piezoelectric stage, to map the EQE for a large range of wavelengths.

Hyperspectral imaging for early detection of retinal diseases

In terms of impact on society, one of the most promising applications of such a technology is retinal imaging. To make sure it reaches its full potential, Optina Diagnostics' was founded and launched the Metabolic Hyperspectral Retinal Camera (MHRC), a high-contrast hyperspectral retinal imaging based on Photon etc’s TLS. The latter provides a fibered output illumination over a wide spectral range (400 - 1000 nm) with a small bandwidth (~2 nm) and an outstanding isolation (70dB or OD7). Those requirements are essential to achieve an efficient illumination of the entire retina. Hyperspectral imaging allows the identification of molecular content in each points of the retina. The objective of the instrument is to detect early signs of retina anomalies such as age-related macular degeneration (AMD) hence minimizing the possible loss of vision.

Mechanical properties of polypropylene blends 5

To improve the characterization of mechanical properties of solid polymers, Chaudemanche et al. [5] have developed a new experimental procedure coupling Raman spectroscopy and a system called VideoTraction. In a recent publication, they analyzed deformations’ micromechanisms in different polypropylene blends. To conduct this project, they performed tensile test, also known as tension test, during which they observed the evolution of the integrated Raman intensity. In order to extract information out of the Raman data, they implemented an incoherent light transport experiment, and determined the turbidity of the polymer as a function of stress. The light source was then focused on a turbid specimen and a CCD camera collected the halo of the backscattered light coming from the sample surface. A high pointing stability, a low bandwidth and wide spectral range were all necessary requirements for the success of this experiment. At last, the high wavelength isolation (70dB or OD7) was exploited since it allowed them to probe the scattering light at the exact same wavelength that was previously used for the Raman experiment.

APPLICATIONS OF THE TLS

  • Photoluminescence (PL)

  • Photoluminescence excitation (PLE)

  • Reflection/Transmission Spectroscopy

  • Hyperspectral imaging

  • Photovoltaic

  • Photochemistry

  • Steady State Pump-Probe experiment

  • Fluorescence spectroscopy

  • Detector calibration

  • Photobiology

  • Solar Simulators

  • Photo-Activation

REFERENCES

[1] Sergiy Patskovsky, Eric Bergeron, David Rioux, and Michel Meunier, Hyperspectral darkfield microscopy of PEGylated gold nanoparticles targeting CD44-expressing cancer cells, Biophotonics, 2013.

[2] Rémi Soummer, Anand Sivaramakrishnan, Ben R. Oppenheimer, Robin Roberts, Douglas Brenner, Alexis Carlotti, Laurent Pueyo, Bruce Macintosh, Brian Bauman, Les Saddlemyer, David Palmer, Darren Erickson, Christophe Dorrer, Kris Caputa, Christian Marois, Kent Wallace, Emily Griffiths, Jacob Mey, The Gemini Planet Imager coronagraph testbed, Proc. SPIE 7440, 2009.

[3] Benoit C. St-Antoine, David Ménard, and Richard Martel, Sigle-Walled. Carbon Nanotube Thermopile For Broadband Light Detection, Nano Letters, vol 11, 609-613, 2011.

[4] L. Lombez, D. Ory, M. Paire, A. Delamarre, G. El. Hajje, J. F. Guillemoles, Micrometric investigation of external quantum efficiency in microcrystalline CuInGa(S,Se)2 solar cells, Thin Solid Films, 2014.

[5] S. Chaudemanche, M. Ponçot, S. Adnré, A. Dahoun and P. Bourson, Evolution of the Raman backscattered intensity used to analyse the micromechanisms of deformation of various polypropylene blends in situ during a uniaxial tensile test, J. Raman Spectrosc. 2014.

Article written by Marc Verhaegen and Laura-Isabelle Dion-Bertrand

Marc Verhaegen Ph.D.
Chief Technical Officer (CTO)
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Laura-Isabelle Dion-Bertrand
Application scientist
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