Femtosecond Imaging

Near Infrared Tunable Femtosecond Cr:Forsterite (Cr:F) Laser system at 1250 nm with Ytterbium Fiber Laser Pump: For Research and Industrial Applications

The femtosecond Cr:Forsterite (Cr:F) laser system from Del Mar Ventures is a mode-locked ultrashort laser producing pulses in near infrared ~1250 nm range. A laser is mode-locked when many longitudinal modes inside the laser cavity are held in phase by constructive interference producing the femtosecond (10 ^-15s) pulse.¹  Forsterite based on Cr⁴⁺ are the first tunable lasers operating in 1150 to 1300 nm range.^2,3,4,5  The extremely short time duration of a femtosecond pulse gives enormous peak powers and power densities. Femtosecond lasers are being used in a rapidly growing number of applications, including ultrafast photochemistry, photophysics, photoablation, micromachining, imaging condensed matter, semiconductor device physics, and other areas.

The Cr:F gain medium is pumped by a 6 -10W Ytterbium Fiber Laser giving an all solid state laser system that is an affordable source of femtosecond pulses in 1230 - 1270nm region. The combination of Ytterbium Fiber Laser and Cr:F oscillator gives pulses in the sub-65 femtosecond range at a repetition rate of 120/76 MHz and delivers power between 180-250mW.

The femtosecond Cr:Forsterite laser is tunable over wavelengths from 1230 to 1270 nm, making it ideal for imaging condensed matter and biomedical applications.^6,7  Frequency doubling can produce wave lengths in the visible at ~630 nm and supercontinuum generation produce pulses in the infrared and visible range.

Multiphoton Confocal Microscopy Using a Femtosecond Cr:Forsterite Laser

Publications related to femtosecond imaging

Femtosecond imaging describes several techniques.

A) Imaging with femtosecond resolution using femtosecond pulses
This is an imaging technique that can be used to photograph ultrafast processes with time resolution determined by the duration of pump and probe laser pulses.

Femtosecond imaging of melting and evaporation at a photoexcited silicon surface
M. C. Downer, R. L. Fork, C. V. Shank
JOSA B, Volume 2, Issue 4, 595- April 1985

2) Two-photon and three-photon imaging using femtosecond pulses.

Advantages of Two-Photon Imaging
http://www.photonics.com/spectra/tech/XQ/ASP/techid.810/QX/read.htm

Advantages of Two-Photon Imaging BUFFALO, N.Y. -- During the past decade, photon scanning tunneling microscopy has achieved resolution better than 100 nm, overcoming the optical diffraction limits. Recently, scientists at the State University of New York at Buffalo developed a two-photon system that has better signal-to-noise ratio and optical contrast than the one-photon microscopes.     Photon scanning tunneling microscopy is an optical fluorescence method in which a laser excites one side of a thin sample while a fiber probe scans the opposite surface and collects the emitted photons. The aluminum-coated fiber probe has an apex diameter of about 200 nm. As with other scanning probe microscope systems, it is rastered using a piezo-tube scanner while its tip remains a few nanometers above the sample surface.     A Hamamatsu R943-02 photomultiplier detects the photons that are collected by the probe. The photons are processed to create an image or sent to a Kaiser Optical Systems spectrograph equipped with a Princeton Instruments CCD camera. The excitation source is an 800-nm Ti:sapphire laser with an average of 12 mW. The laser beam passes through a prism, reflecting from the surface below the sample. The beam focuses on the sample, which is mounted with an index-matching oil on the prism. Researchers at the State University of New York at Buffalo are using an experimental setup such as this for two-photon fluorescence imaging and spectroscopy. The system can be used to spatially and spectrally probe emitting regions on the nanometer scale.     According to team leader Paras N. Prasad, two-photon imaging has many advantages over one-photon imaging. One is that you can get better spatial resolution because of the quadratic density dependence of a two-photon process. A single-photon process can collect information only very close to the surface. "We can look through the layers of paint, for example, all the way to the substrate. We can see corrosion, delamination and other effects," he said.     Some materials can absorb two photons to create an excited state, and a photon emitted after the absorption of two near-infrared photons can be in the visible or ultraviolet range. Most substances do not fluoresce when exposed to near-infrared radiation, so they do not contribute to the background in a two-photon setup.     "One-photon excitation also results in significant spreading of excitation, and this limits resolution," Prasad said. Two-photon scanning tunneling microscopy also has lower sample damage from near-IR radiation, he added.     Prasad's group, whose results were published in the Jan. 3 issue of Applied Physics Letters, is concentrating on the development of other two-photon-absorbing materials, including multibranched large molecular compounds, dyes, erbium-doped nanoparticles and semiconductor quantum dots. Prasad also plans to develop a kit that can convert a conventional optical microscope into a two-photon laser scanning microscope at an affordable price. by Dr. James P. Smith

Two-photon imaging in living brain slices.

(http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10356355&dopt=Abstract)

Mainen ZF, Maletic-Savatic M, Shi SH, Hayashi Y, Malinow R, Svoboda K.

Cold Spring Harbor Laboratory, 1 Bungtown Road, Cold Spring Harbor, New York 11724, USA.

Two-photon excitation laser scanning microscopy (TPLSM) has become the tool of choice for high-resolution fluorescence imaging in intact neural tissues. Compared with other optical techniques, TPLSM allows high-resolution imaging and efficient detection of fluorescence signal with minimal photobleaching and phototoxicity. The advantages of TPLSM are especially pronounced in highly scattering environments such as the brain slice. Here we describe our approaches to imaging various aspects of synaptic function in living brain slices. To combine several imaging modes together with patch-clamp electrophysiological recordings we found it advantageous to custom-build an upright microscope. Our design goals were primarily experimental convenience and efficient collection of fluorescence. We describe our TPLSM imaging system and its performance in detail. We present dynamic measurements of neuronal morphology of neurons expressing green fluorescent protein (GFP) and GFP fusion proteins as well as functional imaging of calcium dynamics in individual dendritic spines. Although our microscope is a custom instrument, its key advantages can be easily implemented as a modification of commercial laser scanning microscopes.
 

Selective two-photon microscopy with shaped femtosecond pulses

http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-14-1695

Igor Pastirk, Johanna M. Dela Cruz, Katherine A. Walowicz, Vadim V. Lozovoy, and Marcos Dantus, Michigan State University

Abstract
Selective two-photon excitation of fluorescent probe molecules using phase-only modulated ultrashort 15-fs laser pulses is demonstrated. The spectral phase required to achieve the maximum contrast in the excitation of different probe molecules or identical probe molecules in different micro-chemical environments is designed according to the principles of multiphoton intrapulse interference (MII). The MII method modulates the probabilities with which specific spectral components in the excitation pulse contribute to the two-photon absorption process due to the dependence of the absorption on the power spectrum of E2(t) [1-3]. Images obtained from a number of samples using the multiphoton microscope are presented.