Ultrafast modulation, switching and optical wave mixing are essential functionalities for various CMOS compatible photonic integrated circuits with applications for optical communication, signal processing and computing. We explore second and third order nonlinear susceptibilities of various inhomogeneous thin film compositions and characterize their properties using a Maker fringes setup and ultrashort femtosecond scale laser pulses. Specifically, we demonstrate enhanced effective second order nonlinear response by engineering the compositions to create a strong internal DC electric fields (~20 pm/V) as well as synthesizing silicon rich silicon nitride films with high second order nonlinear polarizability (~8 pm/V) in as grown films.
Neurophotonics was launched in 2014 coinciding with the launch of the BRAIN Initiative focused on development of technologies for advancement of neuroscience. For the last seven years, Neurophotonics’ agenda has been well aligned with this focus on neurotechnologies featuring new optical methods and tools applicable to brain studies. While the BRAIN Initiative 2.0 is pivoting towards applications of these novel tools in the quest to understand the brain, in this article we review an extensive and diverse toolkit of novel methods to explore brain function that have emerged from the BRAIN Initiative and related large-scale efforts for measurement and manipulation of brain structure and function. Here, we focus on neurophotonic tools mostly applicable to animal studies. A companion article, scheduled to appear later this year, will cover diffuse optical imaging methods applicable to noninvasive human studies. For each domain, we outline the current state-of-the-art of the respective technologies, identify the areas where innovation is needed and provide an outlook for the future directions.
In classical Two photon microscopy (TPM), fluorescence excitation happens via absorption of two photons with the same energy. However, the energies of the two photons do not need to be the same: the sum of their energies must be equal to the total energy required for the ground state to excited state transition. This feature allows for non-degenerate two-photon excitation (ND-TPE), where excitation occurs via simultaneous absorption of two photons of different energies derived from two laser beams. ND-TPE has been exploited in fluorescence microscopy to extend the range of excitation wavelengths , increase resolution, increase penetration depth, and mitigate excitation outside of the focal volume.We use non-degenerate two-photon excitation where the two excitation beams are displaced in space outside the focal volume to increase the signal-to-background ratio (SBR), overcoming the fundamental penetration depth limit of conventional two-photon microscopy.
In non-degenerate two-photon excitation (ND-TPE), electronic transition of fluorophores happens via absorption of two photons with different energies. This contrasts with conventional - or degenerate - two-photon excitation (D-TPE), where two photons with identical energies are absorbed. ND-TPE can improve performance of two-photon microscopy by extending the excitation wavelength range, reducing out-of-focus excitation, and increasing resolution and penetration depth. However, a systematic study of fluorophore performance under ND-TPE is missing, which is critical for the selection of optimal excitation wavelength combinations. It is a well-known fact that degenerate two-photon absorption spectra often deviate from theoretical predictions based on one-photon absorption spectra. Therefore, it is not clear whether non-degenerate two-photon absorption spectra are predictable from the corresponding degenerate spectra. Using our sensitive fluorescence-based spectroscopy technique, we measured non-degenerate two-photon absorption cross-sections (ND-TPACS) of several commonly used fluorophores and generated 2-dimensional ND-TPACS maps. We observed that the shape of the measured ND-TPACS spectra follows the spectra of the degenerate two-photon absorption cross-sections (D-TPACS). However, ND-TPACS are higher in magnitude, which is predicted by the “resonant enhancement” phenomenon. Therefore, we show that ND-TPACS spectra are predictable from the corresponding degenerate D-TPACS spectra under consideration of resonant enhancement. Predictability of ND-TPACS spectra is an important finding that helps choosing the optimal combination of wavelengths for ND-TPE of a given fluorophore without prior experimental measurement of ND-TPACS.
In non-degenerate 2-photon excitation (ND-2PE) microscopy, a fluorophore simultaneously absorbs two photons of different energies. We performed a ND-2PE study of fluorescent proteins and synthetic dyes (eg.eGFP, FITC, and etc.) continuously varying energies and numbers of both photons to create two-dimensional map of fluorescence landscapes. By using the best photon energy combination from our two-dimensional map, we found an increase in detected fluorescent image brightness with ND-2PE as we imaged cortical neurons labeled with enhanced green fluorescent protein (eGFP). It should be noted that the photons corresponding to longer wavelength will penetrate deeper into the tissue at reduced scattering. Additionally, using non-overlapping spatial modes carrying the photons at different energies will significantly reduce out of focus fluorescence from the large number of low energy photons, and by a proper choice of the number of high energy photons the ND-2PE fluorescence can be obtained from deep tissue. Experimentally, we strategically displaced two laser beams until they reached the sample plane such that the unwanted background in the excitation beam path was suppressed. In contrast, these two pump beams were well overlapped at focus which still produced sufficient number fluorescence photons for detection. In our experiment, the temporal alignment was achieved with optical delay line in the optical path of IR beam. With this technique we demonstrated experimentally that ND-2PE with side-by-side beams provided a better signal to background ratio in the scattering phantom as compared with D-2PE. The excitation volume of ND-2PE with side-by-side beam was also investigated and determined to be comparable in size with that of the D-2PE.
We investigate the utility of non-degenerate 2-photon excitation (ND-2PE) as a strategy for extending the 2-photon imaging depth. For the ND-2PE scheme, two pulsed, synchronized laser sources of different wavelength each provide a photon for the 2-photon absorption process. By independently tuning their wavelengths, we are able to tune the excitation to tissue transparency windows while maintaining resonant fluorescence excitation. These transparency windows reduce excitation power loss resulting from scattering. In addition, by having two sources we are able to displace the beams in space except at their common focus; thus, reducing background fluorescence excitation. Finally, we show that ND-2PE inherently results in increased 2-photon absorption cross sections, resulting in increased fluorescence intensity. By combining beam displacement, tissue transparency and increased absorption cross sections, we achieve increased imaging depths as compared to degenerate 2-photon excitation with commonly used fluorophores.
Non-degenerate 2-photon excitation of a fluorophore with two laser beams of different photon energies may offer independent degree of freedom in tuning of the photon flux (i.e., the power) for each beam. Wereport a practical demonstration that the emission intensity of a fluorophore excited in the non-degenerate regime in scattering medium is more efficient than the commonly used degenerate 2-photon excitation. In our experiments we use spatially and temporally aligned Ti:Sapphiremode-locked laser and optical parametric oscillator beams operating at near infrared (NIR) and short-wavelength infrared (SWIR) optical frequencies, respectively. The non-degenerate 2-photon excitation mechanism takes advantage of the infrared wavelengths used in 3-photon microscopy to achieve increased penetration depth, while preserving relatively high 2-photon excitation cross section, exceeding that achievable with the 3-photon excitation. Importantly, independent control of power for each beam implies that the flux requirement for the higher photon energy NIR beam, which experiences higher scattering in biological tissue, can be relaxed at the expense of increasing the flux of the lower photon energy SWIR beam which experiences lower scattering, thus promising deeper penetration with higher efficiency of excitation.Applications for in vivo brain imaging will be also discussed.
Yttrium aluminium garnet (YAG) has been widely used as a solid-state laser host because of its superior optical, thermal,
mechanical properties, as well as its plurality in hosting active ions with a wide range of ionic radii. Drawing YAG into
single crystalline fiber has the potential to further scale up the attainable power level with high mode quality. The recent
advancement on the codrawing laser-heated pedestal growth (CDLHPG) technique can produce glass-clad YAG
crystalline fibers for laser applications. The drawing speed can reach 10 cm/min for mass production. The CDLHPG
technique has shown advantages on transition-metal ion doped YAG and short-fluorescent-lifetime ion doped YAG host.
Compared to silica fiber lasers, the crystalline core offers high emission cross section for transition metal ions because of
the unique local matrix. The challenges on the development of
glass-clad YAG fibers, including core crystallinity,
diameter uniformity, dopant segregation, residual strain, post-growth thermal treatment, and the thermal expansion
coefficient mismatch between the crystalline core and glass clad are discussed. Chromium, ytterbium, and neodymium
ions doped YAG fiber lasers have been successfully achieved with high efficiency and low threshold power. Power
scaling with a clad-pump/side-coupling scheme using single clad or double clad YAG fibers is also discussed.
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