Photonics is emerging as a multidisciplinary new frontier of science and technology and is capturing the imagination of scientists and engineers worldwide because of its potential applications to many areas of present and future information and image processing technologies. Photonics is the analog of electronics in that it describes the technology in which photons instead electrons are used to acquire, store, transmit, and process information. Nonlinear optics provides key operational functions needed for the implementation of photonics technology.
Two-photon absorption (TPA or 2PA) or two-photon excitation or non-linear absorption is the simultaneous absorption of two photons of identical or different frequencies in order to excite a molecule from one state (usually the ground state) to higher energy, most commonly an excited electronic state. The absorption of two photons with different frequencies is called non-degenerate two-photon absorption.
Since TPA depend on the simultaneous absorption of two photons, the probability of TPA is proportional to the square of the light intensity, thus it is a nonlinear optical process. The energy difference between the involved lower and upper states of the molecule is equal or smaller than the sum of the photon energies of the two photons absorbed. Two-photon absorption is a third-order process, with absorption cross-section typically several orders of magnitude smaller than one-photon absorption cross-section.
The high-density disks encode, manipulate, and retrieve information at the molecular level, as opposed to current semiconductor techniques, which rely on the miniaturization of bulk devices such as integrated circuits. NJIT researchers have demonstrated a two-photon 3D optical data storage system consisting of a bichromophoric mixture of diarylethene and fluorene derivative as the storage medium.
The principle for this novel two-photon 3D optical storage device was based on a bichromophoric mixture consisting of diarylethene and fluorene derivative, suitable for recording data in thick storage media. The open and closed forms of diarylethene codify the binary information. The read-out method is based on the modulation of the two-photon fluorescence emission of fluorene by the closed-form of diarylethene.
Binary information bits were recorded throughout all three dimensions of the storage medium by two-photon localized excitation on the diarylethene molecules, transforming the closed-form of diarylethene into the open form. The readout method is based on the modulation of the two-photon fluorescence emission of fluorene by the closed-form of diarylethene.
Multiphoton lithography is a technique for creating small features in a photosensitive material, without the use of complex optical systems or photomasks. This method relies on a multi-photon absorption process in a material that is transparent at the wavelength of the laser used for creating the pattern. By scanning and properly modulating the laser, a chemical change (usually polymerization) occurs at the focal spot of the laser and can be controlled to create an arbitrary three-dimensional periodic or non-periodic pattern. This method has been used for rapid prototyping of structures with fine features.
Two-photon absorption is a third-order with respect to the third-order optical susceptibility and a second-order process with respect to light intensity. For this reason it is a non-linear process several orders of magnitude weaker than linear absorption, thus very high light intensities are required to increase the number of such rare events. For example, tightly focused laser beams provide the needed intensities.
Here, pulsed laser sources are preferred as they deliver high-intensity pulses while depositing a relatively low average energy. To enable 3D structuring, the light source must be adequately adapted to the photoresist in that single-photon absorption is highly suppressed while two-photon absorption is favoured. This condition is met if and only if the resist is highly transparent for the laser light’s output wavelength λ and, simultaneously, absorbing at λ/2. As a result, a given sample relative to the focused laser beam can be scanned while changing the resist’s solubility only in a confined volume.
The materials employed in multiphoton lithography are those normally used in conventional photolithography techniques. They can be found in liquid-viscous, gel or solid-state, in relation to the fabrication need. Liquid resists imply more complex sample fixing processes, during the fabrication step, while the preparation of the resins themselves may be easier and faster. In contrast, solid resists can be handled in an easier way, but they require complex and time-consuming processes. The photopolymers always include a prepolymer (the monomer) and, considering the final application, a photoinitiator, as a catalyzer for the polymerization reaction. In addition, we can find such polymerization inhibitors (useful to stabilize resins both reducing the obtained voxel), solvents (which may simplify casting procedures), thickens (so-called “fillers”) and other additives (as pigments and so on) which aim to functionalize the photopolymer.
Multiphoton microscopy (MPM) is a powerful fluorescent microscopy technique that can produce 3D images of biological structures and processes at unprecedented depth scales. By combining two photons with half the energy to excite fluorophores (both synthetic and natural) in a specimen, MPM can reveal dynamic biological processes occurring deep within living tissue. As well as providing 3D images at unprecedented depths, MPM can reduce photobleaching, the damage caused by extended illumination with the excitation wavelengths.
Multi-photon fluorescence microscopy has become a key technology in biological imaging enabling three-dimensional, noninvasive studies of biological tissue on the submicron scale.
The contrast mechanism in multi-photon microscopy is based on the excitation of fluorophores by 2 or more photons, typically in the infrared spectral range. Upon excitation, the fluorophores relax back by emitting a photon in the visible which is detected. In contrast to conventional linear fluorescence microscopy, where the molecules are excited with a single photon in the visible, the nonlinear character and the excitation wavelength in multi-photon microscopy offer several advantages: (i) the reduced absorption of infrared photons leads to a larger probing depth and enables in-vivo imaging due to lower absorption damage. (ii) Since the fluorescence excitation is limited to the focal plane of the microscope, no spatial filtering is required.
Important laser requirements in multi-photon microscopy
Multi-photon microscopy is a nonlinear microscopy technique and as such the image brightness and quality dramatically depends on the peak power of the excitation laser. The peak power is determined by the laser power, pulse duration, repetition rate, and most importantly also by the temporal pulse quality. Optimizing the pulse duration and the laser power inside the multi-photon microscope is also a critical aspect in multi-photon microscopy. Typically, this calls for elaborate and large optical setups that include optics for pulse compression (group-delay dispersion (GDD) compensation) to ensure a minimum pulse duration at the sample and fast optical modulators for power control and beam blanking.