I. V. Fedotov

Tailoring the soliton output of a photonic crystal fiber for enhanced two-photon excited luminescence response from fluorescent protein biomarkers and neuron activity reporters

Dual-cladding photonic crystal fibers (PCFs) with two zero-dispersion points are used to enhance the two-photon excited luminescence (TPL) response from fluorescent protein biomarkers and neuron activity reporters in dye-cell experiments and in in vivo work on transgenic mice and tadpoles. The soliton transmission of ultrashort pulses through a PCF suppresses dispersion-induced temporal pulse spreading, maintaining a high level of field intensity needed for efficient TPL excitation. The soliton self-frequency shift, stabilized against laser power fluctuations by a specific PCF dispersion design, is employed to accurately match the wavelength of the soliton PCF output with the two-photon absorption spectrum of dye or fluorescent protein biomarker molecules, enhancing their TPL response and allowing the laser damage of biotissues to be avoided.

Electron spin manipulation and readout through an optical fiber

The electron spin of nitrogen--vacancy (NV) centers in diamond offers a solid-state quantum bit and enables high-precision magnetic-field sensing on the nanoscale. Implementation of these approaches in a fiber format would offer unique opportunities for a broad range of technologies ranging from quantum information to neuroscience and bioimaging. Here, we demonstrate an ultracompact fiber-optic probe where a diamond microcrystal with a well-defined orientation of spin quantization NV axes is attached to the fiber tip, allowing the electron spins of NV centers to be manipulated, polarized, and read out through a fiber-optic waveguide integrated with a two-wire microwave transmission line. The microwave field transmitted through this line is used to manipulate the orientation of electron spins in NV centers through the electron-spin resonance tuned by an external magnetic field. The electron spin is then optically initialized and read out, with the initializing laser radiation and the photoluminescence spin-readout return from NV centers delivered by the same optical fiber.

Powerful wavelength-tunable ultrashort solitons in a solid-core photonic-crystal fiber

A solid-core photonic-crystal fiber (PCF) with an effective mode area of 20 microm2 is used to demonstrate the generation of sub-100-kW, 30-70 fs wavelength-tunable solitons within a wavelength range of 1300-1800 nm at a repetition rate of 18 MHz. An energy of 2.9 nJ per pulse is achieved for a 35 fs soliton PCF output centered at 1770 nm. Our numerical analysis supports experimental findings and suggests that frequency-shifted solitons in solid-core PCFs can be scaled up to a submegawatt level of peak powers.

Fiber-optic magnetic-field imaging

We demonstrate a scanning fiber-optic probe for magnetic-field imaging where nitrogen-vacancy (NV) centers are coupled to an optical fiber integrated with a two-wire microwave transmission line. The electron spin of NV centers in a diamond microcrystal attached to the tip of the fiber probe is manipulated by a frequency-modulated microwave field and is initialized by laser radiation transmitted through the optical tract of the fiber probe. The two-dimensional profile of the magnetic field is imaged with a high speed and high sensitivity using the photoluminescence spin-readout return from NV centers, captured and delivered by the same optical fiber.

Implantable fiber-optic interface for parallel multisite long-term optical dynamic brain interrogation in freely moving mice

Seeing the big picture of functional responses within large neural networks in a freely functioning brain is crucial for understanding the cellular mechanisms behind the higher nervous activity, including the most complex brain functions, such as cognition and memory. As a breakthrough toward meeting this challenge, implantable fiber-optic interfaces integrating advanced optogenetic technologies and cutting-edge fiber-optic solutions have been demonstrated, enabling a long-term optogenetic manipulation of neural circuits in freely moving mice. Here, we show that a specifically designed implantable fiber-optic interface provides a powerful tool for parallel long-term optical interrogation of distinctly separate, functionally different sites in the brain of freely moving mice. This interface allows the same groups of neurons lying deeply in the brain of a freely behaving mouse to be reproducibly accessed and optically interrogated over many weeks, providing a long-term dynamic detection of genome activity in response to a broad variety of pharmacological and physiological stimuli.

Spectral compression of frequency-shifting solitons in a photonic-crystal fiber.

A highly nonlinear photonic-crystal fiber is used to demonstrate spectral compression of femtosecond light pulses combined with a tunable soliton frequency shift. A spectral compression ratio of 6.5 is achieved for solitons produced by unamplified 50 fs 1270 nm Cr:forsterite laser pulses shifted by the Raman effect to a central wavelength of 1.58 microm.

Inhomogeneous broadening of optically detected magnetic resonance of the ensembles of nitrogen-vacancy centers in diamond by interstitial carbon atoms

We study the impact of the negatively charged nitrogen-vacancy (NV–) center density on the lattice strain resulting in the splitting of the optically detected magnetic resonance of HPHT diamond. A simple model, taking into account the presence of the interstitial carbon atoms, acting like a wedge force on the crystal lattice, explains the broadening and splitting of the optically detected magnetic resonance of the ensemble of NV– centers at densities within the range of 1013 ÷ 1014 cm−3. This model uses a complete generalized spin Hamiltonian, takes into account the strain-effect of each center in the ensemble and gives good agreement with experimental data.

Fiber-based thermometry using optically detected magnetic resonance

Fiber-optic probes coupled with nitrogen-vacancy (NV) centers in diamond and integrated with a microwave transmission line are shown to enable fiber-format optical thermometry. Temperature measurements with an accuracy of 0.02 K are performed by combining this NV-diamond fiber thermometer with a properly optimized differential lock-in detection technique.

Ionization penalty in nonlinear Raman neuroimaging.

Light-assisted ionization accompanying coherent anti-Stokes Raman scattering (CARS) of ultrashort laser pulses in brain tissue is shown to manifest itself in a detectable blueshift of the anti-Stokes signal. This blueshift can serve as an indicator of ionization processes in CARS-based neuroimaging.

Fiber-optic control and thermometry of single-cell thermosensation logic

Thermal activation of transient receptor potential (TRP) cation channels is one of the most striking examples of temperature-controlled processes in cell biology. As the evidence indicating the fundamental role of such processes in thermosensation builds at a fast pace, adequately accurate tools that would allow heat receptor logic behind thermosensation to be examined on a single-cell level are in great demand. Here, we demonstrate a specifically designed fiber-optic probe that enables thermal activation with simultaneous online thermometry of individual cells expressing genetically encoded TRP channels. This probe integrates a fiber-optic tract for the delivery of laser light with a two-wire microwave transmission line. A diamond microcrystal fixed on the fiber tip is heated by laser radiation transmitted through the fiber, providing a local heating of a cell culture, enabling a well-controlled TRP-assisted thermal activation of cells. Online local temperature measurements are performed by using the temperature-dependent frequency shift of optically detected magnetic resonance, induced by coupling the microwave field, delivered by the microwave transmission line, to nitrogen—vacancy centers in the diamond microcrystal. Activation of TRP channels is verified by using genetically encoded fluorescence indicators, visualizing an increase in the calcium flow through activated TRP channels.