Wide-field multiphoton imaging through scattering media without correction.

Optical approaches to fluorescent, spectroscopic, and morphological imaging have made exceptional advances in the last decade. Super-resolution imaging and wide-field multiphoton imaging are now underpinning major advances across the biomedical sciences. While the advances have been startling, the key unmet challenge to date in all forms of optical imaging is to penetrate deeper. A number of schemes implement aberration correction or the use of complex photonics to address this need. In contrast, we approach this challenge by implementing a scheme that requires no a priori information about the medium nor its properties. Exploiting temporal focusing and single-pixel detection in our innovative scheme, we obtain wide-field two-photon images through various turbid media including a scattering phantom and tissue reaching a depth of up to seven scattering mean free path lengths. Our results show that it competes favorably with standard point-scanning two-photon imaging, with up to a fivefold improvement in signal-to-background ratio while showing significantly lower photobleaching.

Real-time detection of S(1D2) photofragments produced from the 1B2(1Σu+) state of CS2 by vacuum ultraviolet photoelectron imaging using 133 nm probe pulses.

Ultrafast photodissociation dynamics from the 1B2(1Σu+) state of CS2 are studied by time-resolved photoelectron imaging using the fourth (4ω, 198 nm) and sixth (6ω, 133 nm) harmonics of a femtosecond Ti:sapphire laser. The 1B2 state of CS2 was prepared with the 4ω pulses, and subsequent dynamics were probed using the 6ω vacuum ultraviolet (VUV) pulses. The VUV pulses enabled real-time detection of S(1D2) photofragments, produced via CS2*(1B2(1Σu+)) → CS(X 1Σ+) + S(1D2). The photoionization signal of dissociating CS2*(1B2(1Σu+)) molecules starts to decrease at about 100 fs, while the S(1D2) fragments appear with a finite (ca. 400 fs) delay time after the pump pulse. Also discussed is the configuration interaction of the 1B2(1Σu+) state based on relative photoionization cross-sections to different cationic states.

Harnessing speckle for a sub-femtometre resolved broadband wavemeter and laser stabilization.

The accurate determination and control of the wavelength of light is fundamental to many fields of science. Speckle patterns resulting from the interference of multiple reflections in disordered media are well-known to scramble the information content of light by complex but linear processes. However, these patterns are, in fact, exceptionally rich in information about the illuminating source. We use a fibre-coupled integrating sphere to generate wavelength-dependent speckle patterns, in combination with algorithms based on the transmission matrix method and principal component analysis, to realize a broadband and sensitive wavemeter. We demonstrate sub-femtometre wavelength resolution at a centre wavelength of 780 nm, and a broad calibrated measurement range from 488 to 1,064 nm. This compares favourably to the performance of conventional wavemeters. Using this speckle wavemeter as part of a feedback loop, we stabilize a 780 nm diode laser to achieve a linewidth better than 1 MHz.

Full observation of ultrafast cascaded radiationless transitions from S2(ππ(∗)) state of pyrazine using vacuum ultraviolet photoelectron imaging.

A photoexcited molecule undergoes multiple deactivation and reaction processes simultaneously or sequentially, which have been observed by combinations of various experimental methods. However, a single experimental method that enables complete observation of the photo-induced dynamics would be of great assistance for such studies. Here we report a full observation of cascaded electronic dephasing from S2(ππ(*)) in pyrazine (C4N2H4) by time-resolved photoelectron imaging (TRPEI) using 9.3-eV vacuum ultraviolet pulses with a sub-20 fs time duration. While we previously demonstrated a real-time observation of the ultrafast S2(ππ(*)) → S1(nπ(*)) internal conversion in pyrazine using TRPEI with UV pulses, this study presents a complete observation of the dynamics including radiationless transitions from S1 to S0 (internal conversion) and T1(nπ(*)) (intersystem crossing). Also discussed are the role of (1)Au(nπ(*)) in the internal conversion and the configuration interaction of the S2(ππ(*)) electronic wave function.

Wide-field three-dimensional optical imaging using temporal focusing for holographically trapped microparticles.

A contemporary challenge across the natural sciences is the simultaneous optical imaging or stimulation of small numbers of cells or colloidal particles organized into arbitrary geometries. We demonstrate the use of temporal focusing with holographic optical tweezers in order to achieve depth-resolved two-photon imaging of trapped objects arranged in arbitrary three-dimensional (3D) geometries using a single objective. Trapping allows for the independent position control of multiple objects by holographic beam shaping. Temporal focusing of ultrashort pulses provides the wide-field two-photon depth-selective activation of fluorescent samples. We demonstrate the wide-field depth-resolved illumination of both trapped fluorescent beads and trapped HL60 cells in suspension with full 3D positioning control. These approaches are compatible with implementation through scattering media and can be beneficial for emergent studies in colloidal science and particularly optogenetics, offering targeted photoactivation over a wide area with micrometer-precision depth control.

Generation of sub-17 fs vacuum ultraviolet pulses at 133 nm using cascaded four-wave mixing through filamentation in Ne.

The sixth harmonic (6ω, 133 nm) of a Ti:sapphire laser is generated using cascaded four-wave mixing in filamentation propagation of the fundamental (ω) and the second harmonic (2ω) pulses through Ne gas. The method provides the 6ω pulse energy higher than 5 nJ/pulse at 1 kHz and a pulse duration shorter than 17 fs without dispersion compensation.

Simultaneous generation of sub-20 fs deep and vacuum ultraviolet pulses in a single filamentation cell and application to time-resolved photoelectron imaging.

Sub-20 fs pulses of the third, fourth, and fifth harmonics of a Ti:sapphire laser are simultaneously generated using cascaded four-wave mixing in filamentation propagation of the fundamental frequency and the second harmonic pulses in Ne gas. Reflective optics under vacuum are employed after the four-wave mixing to minimize material dispersion of the optical pulses. The cross-correlation between 198 and 159 nm pulses of 18 fs is achieved without dispersion compensation. This new light source is applied to time-resolved photoelectron imaging of carbon disulfide (CS2).

Ultrafast dynamics of aniline following 269-238 nm excitation and the role of the S2(pi3s/pi sigma) state.

Femtosecond time-resolved photoelectron imaging is employed to investigate ultrafast electronic relaxation in aniline, a prototypical aromatic amine. The molecule is excited at wavelengths between 269 and 238 nm. We observe that the S2(pi3s/pi sigma*) state is populated directly during the excitation process at all wavelengths and that the population bifurcates to two decay pathways. One of these involves ultrafast relaxation from the Rydberg component of S2(pi3s/pi sigma*) to the S1(Pi Pi)* state, from which it relaxes back to the electronic ground state on a much longer timescale. The other appears to involve motion along the pi sigma* dissociative potential energy surface. At higher excitation energies, the dominant excitation is to the S3(pi pi*) state, which undergoes extremely efficient electronic relaxation back to the ground state. Our study supports some conclusions reached from H-atom photofragment translational spectroscopy measurements and pump-probe photoionization measurements and contradicts some others.

Shedding new light on the role of the Rydberg state in the photochemistry of aniline.

Efficient electronic relaxation following the absorption of ultraviolet light is crucial for the photostability of biological chromophores, so understanding the microscopic details of the decay pathways is of considerable interest. Here, we employ femtosecond time-resolved photoelectron imaging to investigate the ultrafast intramolecular dynamics of aniline, a prototypical aromatic amine, following excitation just below the second absorption maximum. We find that both the second ππ* state and the Rydberg state are populated during the excitation process. Surprisingly, the dominant non-radiative decay pathway is an ultrafast relaxation mechanism that transfers population straight back to the electronic ground-state. The vibrational energy resolution and photoelectron angular distributions obtained in our experiments reveal an interesting bifurcation of the Rydberg population to two non-radiative decay channels. The existence of these competing non-radiative relaxation channels in aniline illustrates how its photostability arises from a subtle balance between dynamics on different electronically excited states and importantly between Rydberg and valence states.