We present results from our Ho:YLF regenerative amplifier (RA) producing up to 6.9 mJ at a repetition rate of 1 kHz and up to 12.9 mJ at a repetition rate of 10 Hz. At 1 kHz, we observe strongly bifurcating pulses, starting with certain round trip number, but the measurements identify a highly stable operation point that lies “hidden” beyond the instability region. This operation point allows the extraction of highly stable and high energetic output pulses. Suppression of bifurcation in our system is presented for repetition rates below 750 Hz and Ho:YLF crystal holder temperatures of 2.5 °C. We furthermore present a stability optimization routine for our Ho:YLF RA that was operated close to gain depletion at a repetition rate of 100 Hz. By varying the Ho:YLF crystal holder temperature the gain depletion level could be fine adjusted, resulting in a highly stable RA system with measured pulse fluctuations of only 0.35 %.
The advances made in femtosecond electron sources over the last thirty years have been remarkable. In particular, the
development of ultrabright femtosecond electron sources has made possible the observation of molecular motion in
labile organic materials and it is paving the way towards the study of complex protein systems. The principle of radio
frequency (RF) rebunching cavities for the compression of ultrabright electron pulses is presented, alongside with a
recent demonstration of its capabilities in capturing the relevant photoinduced dynamics in weakly scattering organic
systems. Organic and biological systems can easily decompose or lose crystallinity as a consequence of cumulative
heating effects or the formation of side reaction photoproducts. Hence, source brightness plays a crucial role in achieving
sufficient signal-to-noise ratio before degradation effects become noticeable on the structural properties of the material.
The current brightness of electron sources in addition to the high scattering cross section of keV-MeV electrons have
made femtosecond electron diffraction a powerful tool for the study of materials composed by low-Z elements.
The Institute for Optical Sciences at the University of Toronto is an association of faculty members from various departments with research interests in optics. The institute has an extensive program of academic activities, for graduate and undergraduate students, as well as public outreach. For undergraduate students, we have a course on holography. We provide opportunities for students to gain optics experience through research by providing access to summer research positions and by enrolling them in the Research Skills Program, a summer course teaching the basic skills needed in research. For graduate students, we offer the Distinguished Visiting Scientists program, where world-renowned researchers come for a week, giving a series of 3 lectures and interacting closely with students and professors. The extended stay allows the program to run like a mini-course. We launched a Collaborative Master’s Program in Optics, where students earn a degree from their home department, along with a certification of participation in the collaborative program. Physics, Chemistry and Engineering students attending together are exposed to the various points of view on optics, ranging from the pure to the applied sciences. For the general public, we offer the Stoicheff Lecture, a yearly public lecture on optics, organized with the Royal Canadian Institute. Our institute also initiated Science Rendezvous, a yearly public celebration of science across the Greater Toronto Area, with lab tours, demonstrations, and other opportunities to learn about science and those who are actively advancing it. This year, this event attracted over 20,000 attendees.
Holography is one of the most intuitive methods to teach optics, covering many concepts of introductory optics courses, in a visual manner. At the same time it provides a bridge between sciences and art. For these reasons, the Institute for Optical Sciences at the University of Toronto in collaboration with the Ontario College of Art and Design (OCAD) has started an undergraduate course on holography. This course is unique from a number of perspectives. It is a collaboration between two Toronto post secondary education institutions. Also, it enrolls both science and art students, and teaches both the artistic and scientific aspects of holography. Besides the direct learning outcome of the course material, an equally important gain is for art and science students to work together on projects, learning from each others’ strengths. The course is completely hands-on, with students given individual access to the holography studio (under the supervision of a teaching assistant) to complete the required projects in the course. The projects are complemented with lectures that cover the necessary concepts in holography, such as wave propagation, interference and diffraction. The students also receive an introduction to other uses of interference and diffraction. Since the course is taken by art as well as science students, the lectures are delivered very conceptually. Students produced some stunning holograms as part of their projects and rated the course very positively with enthusiastic reviews.
We observe the electric fields caused by charge distributions during femtosecond laser ablation from a silicon
(100) surface. Femtosecond electron pulses passing near the ablation site serve as a probe of the electric field
generated by the emitted charges and countercharges on the sample surface. The density map of the electron pulse
downstream from the sample contains information about the charge distributions. We invert this information
by fitting the beam maps using a simple charge distribution model. Under the present excitation conditions
(390 nm, 150 fs, 5.6 J/cm<sup>2</sup>), we observe the emission of 5.3×10<sup>11</sup> electrons/cm<sup>2</sup> within 3 ps of the excitation
pulse, leading to self-acceleration of the emitted electrons to 2% of the speed of light. Preliminary experiments
on a metal sample display even faster dynamics.
We describe the activities of the Institute for Optical Sciences (IOS) at the University of Toronto towards the establishment of a Master’s Program in Optics. The IOS was formed as a collaboration between faculty members interested in optics from the four departments of Physics, Chemistry, Electrical and Computer Engineering and Materials Science and Engineering. One of its goals is to serve as unifying entity for graduate and undergraduate programs in optical sciences. The details of the proposed graduate program will be discussed. It will be set up in the form of a collaborative university program, where students must satisfy the requirements of one of the four home departments, as well as a set of IOS-specific requirements of the program. IOS-specific activities include attending the Distinguished Visiting Scientist Series, participation in a best-research-practice mini-course, where essential research skills are discussed, as well as participation in an annual internal conference. The benefits of this interdisciplinary program, for students, faculty and relevant industries are discussed. The students will benefit from a wider exposure and a more coherent curriculum. The IOS will also serve as local community within the campus to which students could belong and network. Faculty, on the other hand, will benefit from a reduced teaching load, as redundancies among the departments will be removed.
Femtosecond (fs) ablation is mediated via electron avalanche and multiphoton ionization and is characterized by very
precise cutting and undetectable thermal damage in biological tissues. We have used a 775nm, 150 fs, 1kHz laser system
compared to two conventional bone cutting techniques using carbide and diamond tip burs in a mice calvarial wound
healing model. Wound healing was evaluated using micro computerized tomographs and histological techniques. Good
healing outcomes were found for fs laser surgery in comparison to the conventional surgical methods. However, the
degree of healing was highly variable in all treatment groups. The realization of healing comparable to that observed for
conventional surgical tools demonstrates the possible use of fs lasers for clinical surgery involving small bones where a
much higher degree of precision is required than that possible with current methods.
The ability to watch atoms move in real time - to directly observe transition states - has been referred to as "making the molecular movie". Femtosecond electron diffraction is ideally suited for this purpose since it records the atomic structure of the sample with sub-Angstrom spatial resolution and femtosecond temporal resolution. Many-body simulations of ultrashort electron pulse propagation dynamics allowed the development of sources for femtosecond electron pulses with sufficient number density to perform near single shot structure determinations, a requirement for studies of irreversible processes. We have obtained atomic level views of melting of thin films of aluminum and gold under strongly driven conditions. The results are consistent with a thermally driven phase transition and the observed time scales reflect the different electron-phonon coupling constants for these metals.
Recent technical advances in electron gun design have further improved the temporal resolution of femtosecond electron diffraction. New electron pulse characterization techniques use direct laser-electron interaction and electron-electron interaction to determine the temporal overlap of the pump and probe pulses as well as the time resolution of the system. These advances have made femtosecond electron diffraction capable of observing transition states in molecular systems. The camera for "making the molecular movie" is now in hand.
The key point to scale the output power of diode-pumped solid-sate (DPSS) laser is to solve the thermally induced problems such as fracture of the material by thermal stress, degradation of the beam quality and efficiency by thermally induced birefringence and aberration of the thermal lens, etc. For end-pumped solid state lasers, the gain medium can be constructed in a format of a thin disk or a composite rod to scale the output power. This concept has been successfully used to scale DPSS laser output powers by 1~2 order, depending on the laser material and the beam quality of the output. In a thin disk laser, pump induced heat flows predominantly along the thickness of the laser disk and the thermal lens is eliminated to first order. However, a conventional thin disk laser requires a complex and expensive multipass pump setup to maximize pump absorption for the thinnest crystal possible to minimize the residual thermal lens. Alternatively, using a composite rod in a conventional end-pumped DPSS laser elevates the maximum allowable pump power by
~50%, since the interface between the doped and undoped region of the gain medium provides a heat buffering effect and the maximum thermal stress is reduced. Our anvil-cell disk laser, which clamps the gain medium between the heat sink and a sapphire window, combines the benefits of both the thin-disk laser and lasers using composite rods but with the ability to further optimize material properties. In addition, the portion of thermal lens due to bulge of the gain
medium can be compensated by pressure tuning. The complexity and cost on pump setup can be greatly reduced with this relatively simple design. In this work we demonstrated a reliable high power Nd:YVO<sub>4</sub> laser which delivered 26.2 W of laser output at M<sup>2</sup>=3. Results of intracavity frequency doubling with this laser are also reported.
The <i>all-trans</i> to <i>13-cis</i> isomerization of the retinal chromophore in bacteriorhodopsin (bR) plays an essential role in Nature (e.g., in photosynthesis of halobacteria). bR is a candidate for optical nanodevices driven by laser pulses, and a prospective material for optical memory storage devices and photoswitches. From the viewpoint of possible applications of bR in nanodevices we performed an experimental study of the isomerization yield by excitation with tailored laser pulses, using a coherent control approach. With specially shaped excitation pulses (found in optimization experiments) we are able to manipulate the <i>13-cis</i> yield in bR over an absolute range of 60% (30% enhancement as well as 30% suppression in comparison to excitation with a transform-limited pulse) while keeping the absorbed excitation energy at a constant level.
The picosecond barrier to high brightness electron pulses has been broken. Electron diffraction harbors great potential for providing atomic resolution to structural changes at critical points — a real-time view of atomic motions during structural transitions. Femtosecond electron pulses of sufficient number density to execute nearly single-shot structure determinations are needed. This requirement places severe constraints on the electron pulse propagation. A new photoactivated electron gun design has been developed based on an N-body numerical simulation and mean-field calculation of the electron wavepacket propagation that is capable of less than 600 femtosecond electron pulses with high enough brightness to provide structural details in the small shot number limit. Time-resolved diffraction studies with this new instrument have focused on strongly driven solid-liquid phase transitions of aluminum as a model problem of a structural transition. The signal to noise and available diffraction orders were sufficiently high to give direct access to fluctuations leading to the disordering or melting process and the associated radial distribution function.
This work gives atomic level details of a solid-liquid phase transition, i.e., we can literally watch the atoms move during melting. The promise of atomically resolving transition state processes is at hand and applications along this line will be discussed.
Photonics Research Ontario (PRO) is an Ontario Provincial Center of Excellence supporting a broad range of laser- processing activities within its photonics program. These activities are centered at the University of Toronto, and split between an industrial-user facility and the individual research programs of principal investors. The combined effort furnishes forefront laser system and advanced optical tools to explore novel processing applications in photonic, biomedical, and microelectronic areas. Facilities include laser micromachining stations, excimer-based mask-projection stations, extremely short wavelength lasers such as the molecular fluorine laser, and ultrafast laser systems. The latter two advanced laser offer interesting advantages and contrast in processing 'difficult' materials through linear and nonlinear absorption processes, respectively. These laser systems provide fine precision and strong interaction with a wide range of materials, including 'transparent' glasses, and also ceramics and metals. Applications fall broadly into several areas: wafer-level circuit trimming, high-resolution ultrasonic transducers, and the shaping of optical waveguides and Bragg-gratings for photonic components. This paper summarizes the laser-processing infrastructure and research activities at PRO.
Femtosecond time resolved two photon photoemission and above- threshold photoemission (ATP) have been used to characterize the dynamics of photoexcited electrons at single crystal Cu surfaces. The two photon photoemission studies measure nonradiative relaxation pathways of electrons near the surface, and the ATP studies demonstrate that photoemission occurs even when using light that is far below the work function. These studies provide important information regarding the extent and duration of the interaction of photoexcited electrons with surface adsorbates.
Optical Kerr effect (OKE) studies of myoglobin and water at wavelength of 627 nanometers with 45 femtosecond pulses have been performed. The nonresonant response of water consists of electronic, translational, librational and relaxational components. The low frequency components observed in the water OKE study are in qualitative agreement with previous depolarized light scattering (DLS) studies and molecular dynamics (MD) simulations. Operation on the edge of the Q-band of the myoglobin resulted in the pulse broadening to 100 fs. Two relaxational components, 195 fs and 2.4 +/- .2 ps were observed in both the carboxy and deoxy myoglobin samples studied. The 195 fs component is assigned to the OKE response of water while the 2.4 ps component is related to the transient birefringence of the myoglobin. A discussion of the origin of the transient signal as well as the calculation leading to the assignment to birefringence as opposed to dichroism is included. With this interpretation, the observed dynamics are related to the low frequency modes of the protein. Information on these modes is needed to understand the initial events that direct functionally important structural changes.
Solid-state diode pump lasers are placing new design criteria for intracavity electro-optics. Potassium titanyl phosphate (KTP) material properties and limitations for Q-switching and modelocking diode pumped systems are discussed. Specific applications include low jitter synchronization to an electronic reference (phase modulation) and high average power Q- switching and regenerative pulse amplification (amplitude modulation).
Surface grating and reflective electro-optic sampling are used to characterize interfacial hole transfer dynamics at n-GaAs(100)/[Se<SUP>-2</SUP>/Se<SUB>n</SUB><SUP>-1</SUP>] aqueous interface. Complementary femtosecond optical Kerr studies of the response of water to field changes are presented as a probe of the intermolecular solvent modes coupled to the reaction coordinate.
A recently developed class of digital filters known as morphological pseudoconvolutions are applied to scanning tunneling microscopy (STM) images. These filters use morphological filtering to improve the characteristics of both moving mean and moving median filters. They filter equally in both the x and y directions, so as not to introduce artifacts, and they have an adjustable parameter that allows the user to restore the observed image completely as the parameter tends to infinity. Very few assumptions are made concerning image and noise content, only the shape of typical data being taken into account. These filters are shown to outperform, both visually and in the mean square error (MSE) sense, previously introduced Wiener filtering techniques. The filters are compared on typical STM type images, using both modeled and actual data. The technique is general, and has been shown to perform very well on all types of STM and Atomic Force Microscopy (AFM) images.
The surface restricted transient grating technique has been found to be sensitive to the Fermi levelpinning surface
states at the atomic interface of the native oxide layer of (100) GaAs. The sensitivity to these states is better than
io of a monolayer. The high sensitivity arises from a surface enhancement effect that is attributedto the delocalized
two dimensional character of the electronic state at the surface. The surface enhancement is eliminatedby photoinduced
removal of the oxide layer and hole transfer to Se2 ions adsorbed to the surface. These resultssupport the
assignment of the signal to electronic factors associated with surface state species. The coulombic bindingenergy of
the minority hole carrier, into a 2-d hydrogenic state centered around these negatively charged surfacestates, is .16 eV.
This coulombic trapping must be the first step in the surface state trappingprocess and rationalizes the picosecond
surface state trapping dynamics observed at GaAs surfaces. In addition, the in-situ studies of hole transfer to Se2 at
liquid junctions found the hole transfer time to be less than 30 psec. Relative to the thermalization time scale of space
charge accelerated hole carriers, this result demonstrates that hot hole transfer contributes at least a fewpercent. to
this surface reaction mechanism.