Rachel Grange

Three-dimensional harmonic holographic microcopy using nanoparticles as probes for cell imaging

Luminescent markers play a key role in imaging techniques for life science since they provide a contrast mechanism between signal and background. We describe a new type of marker using second harmonic generation (SHG) from noncentrosymmetric BaTiO(3) nanocrystals. These nanoparticles are attractive due to their stable, non-saturating and coherent signal with a femtosecond-scale response time and broad flexibility in the choice of excitation wavelength. We obtained monodispersed BaTiO(3) nanoparticles in colloidal suspensions by coating the particle surface with amine groups. We characterized the SHG efficiency of 90-nm BaTiO(3) particles experimentally and theoretically. Moreover, we use the coherent SHG signal from BaTiO(3) nanoparticles for three-dimensional (3D) imaging without scanning. We built a harmonic holographic (H(2)) microscope which records digital holograms at the second harmonic frequency. For the first time, high-resolution 3D distributions of these SHG markers in mammalian cells are successfully captured and interpreted by the H(2) microscope.

Imaging through turbid layers by scanning the phase conjugated second harmonic radiation from a nanoparticle

We demonstrate imaging through a turbid layer by using digital phase conjugation of the second harmonic field radiated from a beacon nanoparticle. We show that the phase-conjugated focus can be displaced from its initial position by illuminating the same region of the turbid layer with an angular offset. An image is obtained by scanning the phase-conjugated focus through the turbid layer in a region around the nanoparticle. We obtain a clear image of the target by measuring the light transmitted through it when scanning the focused beam.

Digital phase conjugation of second harmonic radiation emitted by nanoparticles in turbid media

We demonstrate focusing coherent light on a nanoparticle through turbid media based on digital optical phase conjugation of second harmonic generation (SHG) field from the nanoparticle. A SHG active nanoparticle inside a turbid medium was excited at the fundamental frequency and emitted SHG field as a point source. The SHG emission was scattered by the turbid medium, and the scattered field was recorded by off-axis digital holography. A phase-conjugated beam was then generated by using a phase-only spatial light modulator and sent back through the turbid medium, which formed a nearly ideal focus on the nanoparticle.

Lithium niobate nanowires synthesis, optical properties, and manipulation

Free-standing lithium niobate nanowires (LiNbO_3) are synthesized by the hydrothermal route. The polarization response of the second harmonic generation (SHG) signal is measured in a single nanowire and used to identify the crystal orientation by matching with bulk LiNbO_3 nonlinear optical susceptibility. The electrical manipulation of a LiNbO_3 nanowire and its monitoring through the SHG signal in a fluidic setup are demonstrated.

Bioconjugation of barium titanate nanocrystals with immunoglobulin G antibody for second harmonic radiation imaging probes

The second harmonic generation (SHG) active nanocrystals have been demonstrated as attractive imaging probes in nonlinear microscopy due to their coherent, non-bleaching and non-blinking signals with a broad flexibility in the choice of excitation wavelength. For the use of these nanocrystals as biomarkers, it is essential to prepare a chemical interface for specific labeling. We developed a specific labeling scheme for barium titanate (BaTiO3) nanocrystals which we use as second harmonic radiation imaging probes. The specificity was achieved by covalently coupling antibodies onto the nanocrystals. We demonstrate highly specific labeling of the nanocrystal conjugates in an antibody microarray and also the membrane proteins of live biological cells in vitro. The development of surface functionalization and bioconjugation of SHG active nanocrystals provides the opportunities of applying them to biological studies.

Solid-state Er:Yb:glass laser mode-locked by using single-wall carbon nanotube thin film

We design single-wall carbon nanotube (SWNT) thin-film saturable absorbers (SAs) integrated onto semiconductor distributed Bragg reflectors for mode-locking solid-state Er:Yb:glass lasers. We characterize the low nonsaturable loss, high-damage-threshold SWNT SAs and verify their operation up to a pulse fluence of 2 mJ/cm(2). We demonstrate passive fundamental continuous-wave mode locking with and without group-delay dispersion compensation. Without compensation the laser produces chirped 1.8 ps pulses with a spectral width of 3.8 nm. With compensation, we obtain 261 fs Fourier-transform-limited pulses with a spectral width of 9.6 nm.

Low-loss GaInNAs saturable absorber mode locking a 1.3-μm solid-state laser

We have demonstrated stable self-starting passive cw mode locking of a solid-state laser at about 1.3 μm using a GaInNAs semiconductor saturable absorber mirror (SESAM). GaInNAs SESAMs show negligible nonsaturable losses, low saturation fluences (11 μJ/cm2) and picosecond decay times which make them well-suited for self-starting and stable cw mode locking. Sub-10-ps pulses were produced with a Nd:YLF laser at 1314 nm. The incorporation of about 2% nitrogen into InGaAs redshifts the absorption edge above 1330 nm and reduces the strain in the saturable absorber grown on a GaAs/AlAs Bragg mirror. Final absorption edge adjustments have been made with thermal annealing which blueshifts the absorption edge.

Second-harmonic generation of single BaTiO3 nanoparticles down to 22 nm diameter.

We investigate the second-harmonic generation (SHG) signal from single BaTiO3 nanoparticles of diameters varying from 70 nm down to 22 nm with a far-field optical microscope coupled to an infrared femtosecond laser. An atomic force microscope is first used to localize the individual particles and to accurately determine their sizes. Power and polarization-dependent measurements on the individual nanoparticles reveal a diameter range between 30 and 20 nm, where deviations from bulk nonlinear optical properties occur. For 22 nm diameter particles, the tetragonal crystal structure is not applicable anymore and competing effects due to the surface to volume ratio or crystallographic modifications are taking place. The demonstration of SHG from such small nanoparticles opens up the possibilities of using them as bright coherent biomarkers. Moreover, our work shows that measuring the SHG of individual nanoparticles reveals critical material properties, opening up new possibilities to investigate ferroelectricity at the nanoscale.

Pulse energy scaling to 5 μJ from a femtosecond thin disk laser

We report an increase in pulse energy to 5.1μJ obtained directly from a femtosecond diode-pumped Yb:YAG thin disk laser without external amplification. Stable passive mode locking was obtained with a semiconductor saturable absorber mirror (SESAM). The laser delivers 63W of average output power in a nearly diffraction-limited beam (M2=1.1) at a center wavelength of 1030nm. The pulse repetition rate is 12.3MHz, and the pulses have a duration of 800fs, which results in a peak power of 5.6MW. The laser was operated in a box flooded with helium because the nonlinearity of air was found to be a limiting factor for the stability of the pulse formation at increasing pulse energies.

Fabrication of nanoscale lithium niobate waveguides for second-harmonic generation

Nanoscale waveguides are basic building blocks of integrated optical devices. Especially, waveguides made from nonlinear optical materials, such as lithium niobate, allow access to a broad range of applications using second-order nonlinear frequency conversion processes. Based on a lithium niobate on insulator substrate, millimeter-long nanoscale waveguides were fabricated with widths as small as 200 nm. The fabrication was done by means of potassium hydroxide-assisted ion-beam-enhanced etching. The waveguides were optically characterized in the near infrared wavelength range showing phase-matched second-harmonic generation.