In the last years, the Mid Infrared (MIR) spectral region has attracted the attention of many areas of science and technology, opening the way to important applications, such as molecular imaging, remote sensing, free- space communication and environmental monitoring. However, the development of new sources of light, such as quantum cascade laser, was not followed by an adequate improvement in the MIR detection system, able to exceed the current challenges. Here we demonstrate the single-photon counting capability of a new detection system, based on efficient up-converter modules, by proving the correlated nature of twin photons pairs at about 3.1μm, opening the way to the extension of quantum optics experiments in the MIR.
Upconversion of incoherent mid-infrared radiation to near visible wavelengths, offers very attractive sensitivity compared to conventional means of infrared detection. Incoherent light, focused into a nonlinear crystal, results in noncollinear phase matching of a narrow range of wavelengths for each angle of propagation. Non-collinear phase matching has been an area of limited attention for many years due to inherent incompatibility with tightly focused laser beams typically used for most second order processes in order to achieve acceptable conversion efficiency. The development of periodically poled crystals have allowed for non-critical collinear phase matching of most wavelengths, virtually eliminating the need for non-collinear phase matching. When considering upconversion of thermal light, spectral radiance is limited due to the finite temperature of the Planck radiation source. It is, however, straightforward to increase the incoherent power by increasing the receiving aperture of the upconversion unit i.e. the diameter of the upconversion laser beam. Hence, the optimal conversion efficiency for incoherent light is not achieved by tightly focused beams. In this paper we show that filling the nonlinear crystal with as large a pump beam as possible yields the best conversion as this allows for upconversion of large angles of incoming incoherent light. We present results of non-collinear mixing and how it affects spectral and spatial resolution in the image and compare against experiments. We finally discuss how it can be used to design and predict system performance and how incoherent upconversion can be used for mid-IR spectroscopy and imaging.
Mid-infrared microscopy and spectroscopy is interesting due to its medical, biological and chemical applications. Spectromicroscopy can be used for histopathology, sample analysis and diagnosis. The ability to do spectromicroscopy in the 2.5 to 4.5 μm wavelength range where many organic molecules have their fundamental vibrations, with the addition of sufficient spectroscopic resolution to resolve these bands, can e.g. potentially allow for diagnostics without the need for staining of the sample. On a longer timeframe, mid-IR spectromicroscopy has the potential for in-vivo diagnostics, combining morphological and spectral imaging. Recent developments in nonlinear frequency upconversion, have demonstrated the potential to perform both imaging and spectroscopy in the mid-IR range at unparalleled low levels of illumination, the low upconversion detector noise being orders of magnitude below competing technologies. With these applications in mind, we have incorporated microscopy optics into an image upconversion system, achieving near diffraction limited spatial resolution in the 3 μm range. Spectroscopic information is further acquired by appropriate control of the phase match condition of the upconversion process. Multispectral images for a region of interest can be obtained by XY-scanning this region of interest within the field of view of the mid-IR upconversion system. Thus, the whole region of interest can be imaged with all available converter wavelengths, and the spectral representation becomes equal for all points in the image. In addition, the range of converted/imaged wavelengths can be tuned continuously by changing the temperature of the crystal, or discretely by using a different poling channel in the PPLN crystal.
We present a novel approach for mid infrared (mid-IR) spectral analysis using upconversion technology applied in a diffuse reflectance setup. We demonstrate experimentally that mid-IR spectral features in the 2.6-4 μm range using different test samples (e.g. zeolites) can be obtained. The results are in good agreement with published data. We believe that the benefit of low noise upconversion methods combined with spectral analysis will provide an alternative approach to e.g. mid-IR Fourier Transform microscopy. We discuss in detail the experimental aspects of the proposed method. The upconversion unit consists of a PP:LN crystal situated as an intracavity component in a Nd:YVO4 laser. Mixing incoming spectrally and spatially incoherent light from the test sample with the high power intracavity beam of the Nd:YVO4 laser results in enhanced conversion efficiency. The upconverted light is spectrally located in the near infrared (NIR) wavelength region easily accessible for low noise Silicon CCD camera technology. Thus the room temperature upconversion unit and the Silicon CCD camera replaces noisy mid infrared detectors used in existing Fourier Transform Infrared Spectroscopy. We demonstrate specifically that upconversion methods can be deployed using a diffuse reflectance setup where the test sample is irradiated by a thermal light source, i.e. a globar. The diffuse reflectance geometry is particularly well suited when a transmission setup cannot be used. This situation may happen for highly scattering or absorbing samples.
We present a new background free method for in situ gas detection that combines degenerate four-wave mixing with an infra-red light detector based on parametric frequency upconversion of infra-red light. The system is demonstrated at mid infrared wavelengths for low concentration measurements of acetylene diluted in a N2 gas flow at ambient conditions. It is demonstrated that the system is able to cover more than 100 nm in scanning range and detect concentrations as low as 3 ppm based on the R9e line. A major issue in small signal measurements is scattered light and it is showed how a spatial analysis can be used to reduce this level.
Low noise upconversion of IR images by three-wave mixing, can be performed with high efficiency when mixing the object radiation with a powerful laser field inside a highly non-linear crystal such as periodically poled Lithium Niobate. Since IR cameras are expensive and have high levels of intrinsic noise, we suggest to convert the wavelength from the mid infrared to the visible/NIR wavelength for simple detection using CCD cameras. The intrinsic noise in cameras has two main contributions. First, read noise originating from the charge to signal read-out electronics. This noise source is usually measured in number of electrons. The second noise source is usually referred to as dark noise, which is the background signal generated over time. Dark noise is usually measured in electrons per pixel per second. For silicon cameras certain models like EM-CCD have close to zero read noise, whereas high-end IR cameras have read noise of hundreds of electrons. The dark noise for infrared cameras based on semiconductor materials is also substantially higher than for silicon cameras, typical values being millions of electrons per pixel per second for cryogenically cooled cameras whereas peltier cooled CCD cameras have dark noise measured in fractions of electrons per pixel per second. An ideal solution thus suggest the combination of an efficient low noise image wavelength conversion system combined with low noise silicon based cameras for low noise imaging in the IR region. We discuss image upconversion as a means to do low noise conversion of IR light to visible light. We demonstrate system noise performance orders of magnitude lower than existing cryogenic cooled IR cameras.
We present high resolution upconversion of incoherent infrared radiation by means of sum-frequency mixing with a laser followed by simple CCD Si-camera detection. Noise associated with upconversion is, in strong contrast to room temperature direct mid-IR detection, extremely small, thus very faint signals can be analyzed. The obtainable frequency resolution is usually in the nm range where sub nm resolution is preferred in many applications, like gas spectroscopy. In this work we demonstrate how to obtain sub nm resolution when using upconversion. In the presented realization one object point is imaged through the upconverter. Assuming homogeneous spherical emission from the object point, the upconverted radiation will carry the spectral information as con-centric rings. From the optical path length and dispersion properties of the nonlinear material, the acceptance bandwidth of the upconversion process is calculated. It is then straightforward to deduce the spectral information of the light emitted from the object point by a simple analysis of the upconverted radiation. In order to increase resolution, a scanning Fabry-Perot etalon is inserted in a collimated geometry of the upconverted light generated by the crystal. The etalon is designed with a free-spectral range larger than the bandwidth of the upconversion process. Hence, the spectral resolution is now set by the finesse of the etalon. Based on this approach a spectral resolution of 0.2 nm has been reached around 2.9 μm. We demonstrate high resolution spectral performance by observing emission from hot water vapor in a butane gas burner.
It has recently been shown that it is possible to upconvert infrared images to the near infrared region with high quantum efficiency and low noise by three-wave mixing with a laser field [1]. If the mixing laser is single-frequency, the upconverted image is simply a band-pass filtered version of the infrared object field, with a bandwidth corresponding given by the acceptance parameter of the conversion process, and a center frequency given by the phase-match condition. Tuning of the phase-matched wavelengths has previously been demonstrated by changing the temperature [2] or angle [3 Keywords: Infrared imaging, nonlinear frequency conversion, diode lasers, upconversion ] of the nonlinear material. Unfortunately, temperature tuning is slow, and angle tuning typically results in alignment issues. Here we present a novel approach where the wavelength of the mixing field is used as a tuning parameter, allowing for fast tuning and hence potentially fast image acquisition, paving the way for upconversion based real time multispectral imaging. In the present realization the upconversion module consists of an external cavity tapered diode laser in a Littrow configuration with a computer controlled feedback grating. The output from a tunable laser is used as seed for a fiber amplifier system, boosting the power to approx. 3 W over the tuning range from 1025 to 1085 nm. Using a periodically poled lithium niobate crystal, the infrared wavelength that can be phase-matched is tunable over more than 200 nm. Using a crystal with multiple poling periods allows for upconversion within the entire transparency range of the nonlinear material.
We present a technique increasing the space bandwidth product of a nonlinear image upconversion process used for
spectral imaging. The technique exploits the strong dependency of the phase-matching condition in sum frequency
generation (SFG) on the angle of propagation of the interacting fields with respect to the optical axis. Appropriate
scanning of the phase-match condition (Δk=0) while acquiring images, allow us to perform monochromatic image
reconstruction with a significantly increased space bandwidth product. We derive the theory for the image reconstruction
process and demonstrate acquisition of images with >10 fold increase in space bandwidth product, i.e. the number of
pixel elements, when compared to upconversion of images using fixed phase-match conditions.
We present a simple theoretical model for 2 dimensional (2-D) image up-conversion of incoherent light. While image
upconversion has been known for more than 40 years, the technology has been hindered by very low conversion
quantum efficiency (~10-7). We show that our implementation compared to previous work can result in a feasible
system: Using intracavity upconversion and Quasi Phase Matching (QPM) nonlinear materials provide increased
conversion efficiency. Using a QPM crystal and choosing the wavelengths so the first order term in the phasematch
wavelength acceptance vanishes, results in very large wavelength acceptance. This work describes how the bandwidth
acceptance can be predicted and designed. This gives promise of a new way to make infrared imaging devices with
tunable spectral sensitivity.
An optimized method for continuous wave 2-dimensional (2-D) upconversion of incoherent or thermal light is
demonstrated and quantified. Using standard resolution targets a resolution of 200x1000 pixels is obtained. The
suggested method is viewed in scope of modern CCD cameras operating in the near infrared (NIR) portion of the
electromagnetic spectrum. The key is optimization of the upconversion process. This include Quasi-Phase-Matching
leading to higher effective nonlinearities and elimination of walk-off, an intra-cavity design enhancing the upconversion
process, and finally the use of modern NIR CCD detectors. Furthermore, we discuss the exceptionally good depth of
field possible for imaging systems based on the proposed method.
The effect of a 1070-nm continuous and pulsed wave ytterbium fiber laser on the growth of Saccharomyces cerevisiae single cells is investigated over a time span of 4 to 5 h. The cells are subjected to optical traps consisting of two counterpropagating plane wave beams with a uniform flux along the x, y axis. Even at the lowest continuous power investigated-i.e., 0.7 mW-the growth of S. cerevisiae cell clusters is markedly inhibited. The minimum power required to successfully trap single S. cerevisiae cells in three dimensions is estimated to be 3.5 mW. No threshold power for the photodamage, but instead a continuous response to the increased accumulated dose is found in the regime investigated from 0.7 to 2.6 mW. Furthermore, by keeping the delivered dose constant and varying the exposure time and power-i.e. pulsing-we find that the growth of S. cerevisiae cells is increasingly inhibited with increasing power. These results indicate that growth of S. cerevisiae is dependent on both the power as well as the accumulated dose at 1070 nm.
We have studied the effect of a 1070 nm continuous wave Ytterbium fiber laser on exponentially growing
Saccharomyces cerevisiae yeast cells over a span of 4 hours. The cells were immobilized onto Concanavalin A covered
microscope slides and the growth was measured using the area increase of the cells in 2D. Using a continuous dual beam
plane wave with a uniform spatial intensity distribution, we found that a continuous radiant flux through a single cell as
low as 0.5 mW in 1.5 hours significantly changed the growth and division rate of S. cerevisiae. With the dual beam setup
used we were able to successfully manipulate single S. cerevisiae cells in 3 dimensions with a minimum flux thorough
the cell of 3.5 mW. In the regime investigated from 0.7 mW to 2.6 mW we found no threshold for the photo damage, but
rather a continuous response to the increased accumulated dose.
The generation of optical landscapes may be regarded as a communication system: A user sends information to a
designated region in space where the information is represented using optical parameters like intensity. Information
capacity has been used for understanding superresolution in optical systems and we adapt this concept to gain useful
insights for characterizing techniques for generating optical landscapes. In particular, we investigate the information
capacity of the generalized phase contrast method and computer-generated holography. We also consider the invariance
of information capacity and discuss its implications for the generation of dynamic optical landscapes.
Optical trapping by use of multiple counter-propagating beam traps has not been widely implemented outside optical engineering laboratories. One, if not the primary, reason for this is the relatively complex calibration procedures involved in connection with this optical geometry. In this talk, we present automated solutions to all the calibration issues, which in effect results in a turn-key counter-propagating multi-beam 3D trapping system. These results allow a wider audience to utilize counter-propagating beam trapping systems. The calibrated system can be used to independently manipulate a plurality of cells real-time in a large 3D working area. Optionally, the system can be extended to allow for use of various spectroscopic methods concurrently with optical manipulation/trapping.
The generalized phase contrast method (GPC) can produce a rich variety of optical landscapes from an incident flattop
beam. Here we show that the GPC can generate various intensity distributions directly from an incident Gaussian
illumination. This is illustrated by using GPC-based implementation of phase-only apertures that efficiently redirect the
available photons from an initial bell-shaped intensity distribution into desired configurations. GPC can reshape a
Gaussian beam into patterns having sharp intensity transitions and a flat phase profile in the bright regions with superior
energy efficiency over amplitude masks. Eliminating the initial Gaussian-to-flattop beamshaping requirement can be
beneficial for various applications employing GPC-based patterns.
We outline the specifications of a portable Bio-photonics Workstation we have developed that utilizes just a single spatial light modulator to generate an array of up to 100 reconfigurable laser-traps with adjustable power ratios making 3D real-time optical manipulation possible with the click of a laptop mouse. We employ a simple patented optical mapping approach from a fast spatial light modulator to obtain reconfigurable intensity patterns corresponding to two independently addressable regions relayed to the sample volume where the optical manipulation of a plurality of nano-featured micro-objects takes place. The stand-alone Biophotonics Workstation is currently being tested by external partners with micro-biologic and chemistry expertise.
We demonstrate how optical trapping and manipulation can be used to assemble microstructures. The microstructures we
show being automatically recognized and manipulated are produced using the two-photon polymerization (2PP)
technique with submicron resolution. In this work, we show identical shape-complementary puzzle pieces being
manipulated in a fluidic environment forming space-filling tessellations. By implementation of image analysis to detect
the puzzle pieces, we developed a system capable of assembling a puzzle with no user interaction required. This allows
for automatic gathering of sparsely scattered objects by optical trapping when combined with a computer controlled
motorized sample stage.
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