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Spectroscopy and Applications of Orbital Angular Momentum

From quantum electrodynamics, we know that Electromagnetic (EM) waves carry spin angular momentum (SAM). Its analog in classical electrodynamics is polarization (linear or circular). However, a new property of photons was recently discovered that relate to electromagnetic (EM) vortices. Such a vortex beam has a specific helicity and an associated angular momentum which is “orbital” in nature. This orbital angular momentum (OAM) of the beam is called a “twisted” or “helical” property of the beam. General spectroscopy techniques conventionally involve circularly polarized light in which a plane polarized state is understood as a superposition of circular polarizations with opposite handedness. The right- and left- handedness of circularly polarized light indicates its spin angular momentum (SAM). However, in general an EM beam can be engineered to have both SAM and OAM and such beams are called vector beams. The vector beams can be used for new spectroscopy techniques with very specific interaction signatures with matter. The interaction of OAM beams with chiral molecules creates an azimuthal flow of momentum, with a specific signature.

Our recent experiments performed by NxGen at Ultrafast Spectroscopy labs at CUNY support the existence of measurable OAM light-matter interactions using such a beam for studies of various concentrations of glucose. These experiments suggest that not only does the interaction exist, but it appears to be stronger than with polarimetric techniques. The recently published articles in JAMA and Diabetes Care suggest that HbA1C is not useful as a marker to reliably predict future risk for developing Cardiovascular disease or other disease conditions in both Diabetics and Non-Diabetics. Therefore, there is an immediate need for a biomarker that can predict future complications like cardiovascular, neurological and kidney diseases due to sustained elevated levels of blood sugar (i.e. diabetes). We also know that the Glycation products are the potential molecular elements involved the long-term pathologies of Diabetes complications. Therefore, the glycation products of low molecular proteins and peptides in blood, body fluids and tissues need to be identified and investigated for their potential as predictive biomarkers of long-term diabetes complications and this can be done using OAM signatures.

We have studied the glycation induced changes in human hemoglobin (HbA1 -α2β2) using absorption, fluorescence, Raman and time resolved fluorescence spectroscopic methods. What we have seen is that the non-enzymatic glycosylation of hemoglobin changes the chemistry of its native chromophore and fluorophore moieties and as a result its tertiary structure gets altered significantly. These post translational molecular modifications would induce changes in optical properties like absorption, fluorescence emissions, fluorescence lifetimes and Raman fingerprints. We believe that specific OAM signatures can further improve clinical and research work in this area. There are half a dozen bioscience applications of OAM.

1 - Non-invasive real-time Glucose concentration measurements (OAM does not interfere with ambient radiation noise and therefore the signature is very distinct).

 2 - An OAM biomarker beyond HbA1C that can predict future complications like cardiovascular, neurological and kidney diseases due to sustained elevated levels of blood sugar (i.e. diabetes).

3 - OAM Optical Biopsy for real-time cancer cell detection.

4 - New multi-parameter spectroscopy that combine OAM with conventional absorption, emissions, fluorescence and Raman.

5 - A precision quantum wrench for new gene editing techniques as photons with OAM carry mechanical angular momentum and can exert mechanical torque to molecules to perform editing.

6 - New quantum mechanical selection rules for twisted light-matter interaction for new techniques in cellular biology and pharmacology.

7 - New interaction Hamiltonians have been constructed by NxGen.


Multi-Parameter Spectroscopy with Structured Photons

When light interacts with matter, changes in the dipole moment of its molecules create infrared absorption bands, while changes in their polarizability produce Raman bands. Such infrared and Raman spectroscopies can work in conjunction with incident photons that carry OAM. The sequence of observed energy bands arises from specific molecular vibrations which collectively produce a unique spectral signature indicative of each type of molecule. Such spectroscopy can work in conjunction with incident photons that carry OAM. Certain vibrational modes occurring in Raman spectroscopy are forbidden in infrared spectroscopy, while other vibrational modes may be observed using both techniques or a multi-parameter technique using OAM.

Real time spectroscopy may be accomplished using multiple parameters such as polarization, wavelength, and orbital angular momentum (OAM) of light. 3D spectroscopy gives consumers numerous applications including useful real time chemical and biological information. Raman and infrared spectroscopy can complement one another in conjunction with OAM and polarization. When an incident photon interacts with the electric dipole of a molecule, this form of vibrionic spectroscopy is often classically viewed as a perturbation of the molecule’s electric field. Quantum mechanically, however, the scattering event is described as an excitation to a virtual energy state lower in energy than a real electronic transition with nearly coincident decay and change in vibrational energy. Since the intensity of Raman scattering is low, heat produced by the dissipation of vibrational energy does not create a significant rise in material temperature. Such Raman spectroscopy can work in conjunction with incident photons that carry OAM. Stokes-shifted scattering events are typically observed in Raman spectroscopy since at room temperature the excited vibrational states are low and the electron originates in the ground state. Information from Raman beams that have optical vortices adds a new degree of spectroscopic capability when coupled with polarized and non-polarized Raman spectroscopy. Also, Terahertz spectroscopy is done in the far-infrared frequency and is therefore useful for identifying far-infrared vibrational modes in molecules.

THz spectroscopy provides a higher signal-to-noise ratio and wider dynamic range than far-infrared spectroscopy due the use of bright light sources and sensitive detectors. This provides for selective detection of weak inter- and intra-molecular vibrational modes commonly occurring in biological and chemical processes which are not active in IR-spectroscopy. Such THz spectroscopy can work in conjunction with incident photons that carry OAM. Two-dimensional THz absorption properties of samples are characterized by a THz imaging technique. This technique uses THz-TDS based on picosecond pulses, THz-wave parametric oscillator, quantum cascade laser, or optically pumped terahertz laser. Such 2D THz spectroscopy can work in conjunction with incident photons that carry OAM. A THz spectrometer can mechanically scan a sample in two dimensions, but the time of each scan scales with sample size. Real time THz imaging can be conducted with an array of THz wave detectors composed of electro-optic crystals or a pyroelectric camera. Such THz spectroscopy can work in conjunction with incident photons that carry OAM.

Fluorescence spectroscopy results in emission and excitation spectra. In emission fluoroscopy, the exciting radiation is held at a fixed wavelength and the emitted fluorescent intensity is measured as a function of emission wavelength. Such fluorescence spectroscopy can also work in conjunction with incident photons that carry OAM. Current chiral optics use circularly polarized light in which a plane polarized state is understood as a superposition of circular polarizations with opposite handedness. The right- and left- handedness of circularly polarized light indicates its spin angular momentum (SAM), in addition to the polarization, one can use helicity of the associated electromagnetic field vectors. The combined technique would have specific signatures for different materials. Delocalized OAM within solid materials associated with the envelope wavefunction in a Bloch framework, which may be spatially macroscopic in extent, may be distinguished from local OAM associated with atoms. A variety of light-matter interactions involving OAM optical beams indicate a broad range of possibilities including OAM transfer between acoustic and photonic modes in optical fibers. OAM-based Raman sideband generation, and the manipulation of colloidal particles manipulation with optical OAM “tweezers" offer new possibilities in spectroscopy and even gene editing. Applications of such multi-parameter spectroscopy can be used in different industries including food (identification of food spoilage), Nanoscale Material Development for Defense and National Security, chemical industry, pharmaceutical industry, medical industry for identification of infections, cancer cells, organic compounds and many more.


Fractional OAM and Applications in Spectroscopy

The orbital angular momentum of light beams is a consequence of their azimuthal phase structure. Light beams have a phase factor exp(imφ), where m is an integer and φ is the azimuthal angle and they carry orbital angular momentum (OAM) of mћ per photon along the beam axis. These light beams can be generated in the laboratory by optical devices, such as spiral phase plates or holograms, which manipulate the phase of the beam. In cases where such a device generates a light beam with an integer value of m, the resulting phase structure has the form of |m| intertwined helicities of equal phase. For integer values of m, the chosen height of the phase step generated by the optical device is equal to the mean value of the OAM in the resulting beam. Spiral phase steps with fractional step height as well as spatial holograms can be used to generate light beams with fractional OAM states. In these implementations, the generating optical device imposes a phase change of exp(iMφ) where M is not restricted to integer values. The phase structure of such beams shows a far more complex pattern. A series of optical vortices with alternating charge is created in a dark line across the direction of the phase discontinuity imprinted by the optical device. In order to obtain the mean value of the orbital angular momentum of these beams, one must average over the vortex pattern. This mean value coincides with the phase step only for the integer and half integer values.  

Molecular spectroscopy using OAM twisted beams can leverage Fractional OAM states as a molecular signature along with other intensity signatures (i.e. change of eccentricity, shift of center of mass, and rotation of the elliptical intensity) as well as phase signature (i.e. changes in the phase of the scattered beam) and specific formation of helicity distributed spectrum.

Other spectroscopy techniques include the pump-probe magneto-orbital approach. Here Laguerre-Gauss optical pump pulses impart OAM to the electronic states of a material and subsequent dynamics can be studied with femto second time resolution. The excitation uses vortex modes that distribute angular momentum over a macroscopic area determined by the spot size, and the optical probe studies the chiral imbalance of vortex modes reflected off the sample. There will be transients that evolve on time scales distinctly different from population and spin relaxation, but with large lifetimes. The method of optical orientation of electronic spin by circularly polarized photons has been used to study spin angular momentum in solid state materials. The process relies on spin-orbit coupling to transfer angular momentum from the spin of photons to the spin of electrons. This can impact the spintronics industry. What is proposed here is a spectroscopy technique that focuses instead on delocalized OAM in solids. Specifically, one can distinguish between delocalized OAM associated with the envelope wavefunction, which may be macroscopic in spatial extent, and local OAM associated with atomic sites, which is incorporated into the effective spin and electronic states. The first type of angular momentum is of fundamental interest to orbital coherent systems like quantum Hall layers, superconductors, and topological insulators.

Techniques to study non-equilibrium delocalized OAM in these and other systems would create opportunities to improve our understanding of scattering and quantum coherence of chiral electronic states, with potential implications for a material discovery. We have been studying the interaction of LG light with Glucose and Beta Amyloid and these experiments are the initial spectroscopy applications of OAM, but we have also studied the transfer of OAM between photonic modes in an optical fiber, the generation of Raman sidebands carrying OAM,  and can extend the study to OAM using a plasmonic lens, the study of optically coherent OAM in excitons using four-wave mixing, and application of linearly polarized light to create a 2D plasmonic analog to OAM light in a patterned thin metallic film. There is also the possibility of OAM light producing spin polarized photoelectrons for efficient semiconductors.

For integer OAM values, a theoretical description may exist which provides the way to treat the angle itself as quantum mechanical Hermitian operator. The description can provide the underlying theory for an uncertainty relation for angle and angular momentum.

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