In previous work, we have introduced an analytical approach that utilizes the dispersion relation for an infinite periodic multilayer structure to predict the performance of finite multilayer structures. We have validated the accuracy of our predictions by demonstrating numerical agreement with other established simulation methods, such as the transfer matrix method, and through experimental confirmation. In this work, we employ dispersion relations to first illustrate that metallo-dielectric structures, as opposed to multilayer dielectric-dielectric structures, can efficiently yield a sharp-edge transmittance spectrum profile, with control over both sides of the bandpass cutoff edges. Our approach also enables the calculation of effective permittivity without relying on traditional homogenization techniques. Next, utilizing the concept of effective permittivity, we illustrate that increasing the thickness of specific dielectric layers within MD structures leads to narrower passbands without significant loss in transmission, demonstrating the potential of this approach for engineering the transmittance spectrum of bandpass filters in the visible and near-IR regions. The capability to achieve a sharp-edge filter with a limited number of layers further underscores the cost-effectiveness of such bandpass filters.
Cadmium silicon phosphide, CdSiP2 (CSP), crystals have good nonlinear optical properties resulting in their use in optical parametric generation (OPO and OPA) of mid-infrared light. One common limitation on the performance of OPO materials is residual optical absorption which often results from point defects formed during crystal growth. Electron paramagnetic resonance (EPR) is a powerful technique for identifying and tracking point defects in materials. By correlating behaviors of native point defects exposed to 1064 nm light using EPR with changes in optical absorption bands, models are proposed for three of the observed broad optical absorption bands.
Unlike any other technology, planar optical components based on geometrical phase modulation can combine fundamentally 100% efficiency in broad spectral bandwidths and for wide range of angles. Their truly planar and smooth structure ensures no haze and provides an opportunity for anti-reflection coatings further enhancing transmission. As, essentially, diffractive waveplates, they allow integration of multiple layers for complex spectral and polarization engineering. Achieving perfection is not easy though since it requires meeting tight tolerances on a multitude of fabrication processes and materials. We will present the results of tolerance analysis for different architectures of geo-phase optics, and will discuss the performance of some of most challenging components we have been fabricating.
Liquid crystalline (LC) materials, particularly, polymers allow generating molecular orientation patterns with submicrometer resolution. By that, materials and fabrication technology have been developed to reach half-wave (HW) retardation for visible and even infrared wavelengths thus ensuring feasibility of geometrical phase optical components that are practically 100% efficient. Such LC polymer films allow rich architectures for designing spectrally selective optical components and polarization-independent systems. Thin LC films with customized optical functions make promising basis for high throughput and high definition display systems.
We show applications of our analytical approach to predict the performance of multilayer metallo-dielectric bandpass filters, which also enables estimation of their effective permittivity without relying on homogenization techniques. The approach is based on the one-dimensional dispersion relation for an infinite metallo-dielectric structure that accounts for the complex nature of the permittivities for the metal and dielectric constituents. The dispersion relation clearly reveals the band structure (often comprising multiple passbands), directly provides transmittance characteristics such as center wavelengths and bandwidths and enables the calculation of effective propagation constant and effective attenuation. In this work, we evaluate the dispersion relations for metallo-dielectric structures with complex refractive index data for the metal, viz., Ag, acquired from different sources to show the differences in the center wavelength and the cutoff wavelengths. We verify the accuracy of our method numerically by comparing the transmittance spectrum of finite metallo-dielectric structures using the transfer matrix method. We also plot the dispersion relation using Al as the metal and show the differences in the dispersion relations of the infinite structure and the transmittances of the finite structures relative to Ag. Extension to determination of dispersion relations for other polarizations, viz., transverse magnetic, is discussed, along with corresponding transmittance spectra for oblique incidence.
Cadmium silicon phosphide, CdSiP2 (CSP), exhibits the highest d-coefficient (d36 = 85 pm/V) among all practical nonlinear optical crystals. Its large band gap of 2.45 eV allows for 1-micron pumping with widely-available Nd- and Yb-based laser sources, and its dispersion properties are such that a 1-um pump yields non-critically phase-matched temperature-tunable output between 6.2-6.5 um (an attractive range for minimally-invasive laser surgery). However, residual 1-um absorption losses in CSP are not insignificant (0.16-0.2 cm-1). In this work we focused on identifying, and ultimately minimizing, the point defects responsible for these losses by correlating EPR spectra with polarized absorption near 1-um.
CdSiP2 (CSP) is a nonlinear optical material used for mid-infrared generation. For nonlinear optical materials, absorption bands associated with point defects often limit output power. We use electron paramagnetic resonance (EPR) to monitor paramagnetic charge states of defects. In CSP crystals, EPR shows singly ionized silicon vacancies (VSi-) initially present are eliminated by exposure to 1064 nm light. Our results suggest that 1064 nm light converts VSi- acceptors to nonparamagnetic doubly ionized (VSi2-) and neutral (VSi0) charge states. A thermal activation energy of 0.23 eV describes the recovery of the VSi- signal including at room temperature.
Optics manufacture is not easy – a piece of glass needs to be through water and fire, literally. And then more… It is even harsher for emerging metasurface technologies, involving solvents, developers, ion-etching, etc. The fourth generation optics (4G Optics), making feasible for thin planar films to be competitive with conventional optics in most applications and enabling performance features practically prohibited for other planar optics technologies that suffer from low efficiency, haze, and small area, it also is making a historical breakthrough in optics manufacturing by dramatically reducing its impact on environment along with time and cost. The 4G optics makes it possible a 1000x reduction of required material quantities compared to high quality optics of similar functionality, and at least a 1000x reduction of manufacture time, along with customization and reconfigurability of optical functions, and opportunities of recycling both the materials and substrates.
Chalcogenide phase change materials (PCMs) are uniquely suited for spectral tuning applications due to their contrasting dielectric material properties. Recent headway has been made towards realizing tunable photonic devices using twodimensional, sub-wavelength resonators by carefully designing geometries that optimize optical, electrical, and thermal performances using multi-physics analyses and machine learning. In this paper, we tackle two other essential aspects for creating application-specific, tunable PCM devices: (1) scalability of the device size and (2) high-throughput fabrication techniques. We employ a deep ultraviolet (DUV) stepper projection lithography to manufacture over 100 densely packed GST metasurfaces, each with a sample size of 5×7 mm2, all on a 4-inch Al2O3 wafer. These metasurface structures were discovered using artificial neural network (ANN) techniques and confirmed by finite-difference-time domain calculations. The primary structures under investigation were nanobar configurations enabling amplitude modulation at short-wave infrared wavelengths to realize efficient optical switches for free space optical multiplexing. The DUV fabrication technique can easily be extended to other metasurface geometries to demonstrate multi-functional, non-volatile photonic devices.
CdSiP2 (CSP) is a nonlinear optical crystal developed as a wider-band-gap analog of ZnGeP2 (ZGP) to enable mid-infrared generation. A direct comparison of the performance of ZGP and CSP crystals in mid-IR generating OPOs was performed with a 4 W Tm:YAP pump laser. CSP was shown to outperform ZGP in this configuration. A ring OPO using CSP with a 2.09 micron pump and 80W of power was used to generate 27 W of mid-IR light demonstrating CSP’s viability for high average power generation. An OPO seeded OPA was then used to directly compare CSP and ZGP with this same source as well as with an upgraded 140W source.
A method for writing programmable volume phase gratings into photorefractive materials using visible wavelengths in a transmission geometry, and then subsequently probing these gratings in a reflection geometry using infrared (IR) wavelengths to achieve specific angles of reflection of the probe beam is analyzed. The programmable features of these gratings include grating spacing and tilt, or K-vector magnitude and orientation. Relationships have been derived between the incidence angles of the writing beams and the corresponding reflection angles of the IR probe beam. More specifically, for a fixed angle of incidence of the probe beam, two unique writing beam angles can be found which generate a grating with the correct spacing and tilt to reflect and steer the probe beam through a wide field of desired angles.
It is well known in the literature that for a two photon nonlinear absorbing dye to be the most effective, high concentrations are needed. The problem is that most photophysical studies in solution are done at low concentration and in a solution. These low concentration studies are important for understanding inherent materials properties but it is also important to understand what happens in a material at high concentration. In addition to this, efforts have been made to study the effects of incorporating a dye into a solid matrix environment to better understand the constraints this environment has to a given material. Preliminary results for an epoxy system reveal the formation of excimers (excited state dimers) with an increase in concentration. Excimers are forming from the triplet excited state of the E1-BTF. A rate constant for this formation is 2.6 × 105 M-1 s-1. While rather slow, at greater than 50 mM concentration the excimer is readily formed with <90% efficiency. This must be considered when making nonlinear absorption measurements since the excimer will certainly contribute to the overall nonlinearity.
In order to understand electronic and conformational effects on structure-spectroscopic property relationships in platinum acetylides, we synthesized a model series of chromophores trans-Pt(PBu3)2(CCPhenyl-X)2, where X = NH2, OCH3, diphenylamino, t-Bu, methyl, H, F, benzothiazole, trifluoromethyl, CN and nitro. We collected linear spectra, including ground state absorption, phosphorescence and phosphorescence excitation spectra. We also performed DFT and TDDFT calculations on the ground and excited state properties of these compounds. The calculations and experimental data show the excited state properties are a function of the electronic properties of the substituents and the molecular conformation.
Previously, we presented the experimental evidence for a degenerate frequency two beam coupling (TBC) in two photon absorbing (2PA) organic solutions. It has been well established that the two critical requirements for TBC are a nonlinear refractive index with a finite lifetime and that the interacting fields must have non-degenerate frequencies. However, degenerate frequency coupling has been shown for fields containing a time-dependent phase, i.e. a frequency chirp. This chirp can either be intrinsic to the fields or induced by self- and cross- phase modulation (S/XPM). For nanosecond pulses, the relatively small intrinsic chirp of the fields is negligible compared to the strong cumulative effects of population redistribution which generates large S/XPM. A S/XPM-mediated theoretical treatment for degenerate frequency TBC is presented along with numerical simulations using known nonlinear optical parameters to model the experimental results.
The third-order nonlinear susceptibility of crystalline Cadmium Magnesium Telluride (CdMgTe) was studies using a spatially resolved Irradiance Scan method including picosecond and nanosecond laser pulse widths at 1064nm. The samples were placed in a loosely focused beam, and a series of individual laser pulses at different energies were collected. The transmitted beam was reimaged to a CCD with a microscope objective providing a detailed objective function for numerical simulations. The nonlinear transmission results were modeled by way of a split-step nonlinear beam propagation method including diffraction, nonlinear absorption, and refraction arising from bound electrons and light-generated free carriers. The angular dependence of the third order susceptibility with respect to the electric field is also represented along with laser-induced damage thresholds.
To develop a structure-spectroscopic property relationship in platinum acetylides having poly(aromatic hydrocarbon)
ligands, we synthesized a series of chromophores with systematic variation in the number of fused aromatic rings(nFAR)
and ligand topology(polyacene(L), polyphenanthrene(Z) or compact(C)). We measured ground state absorption,
fluorescence and phosphorescence spectra. We also performed nanosecond and picosecond flash photolysis
experiments. To extend the range of compounds in the structure-property relationship, we did DFT calculations on an
expanded series of chromophores to calculate the S1 and T1 state energies. In both the DFT results and experiment, the
ground state and phosphorescence spectra are a function of both nFAR and ligand topology. In the L chromophores, the
S1 and T1 state energies decrease linearly with nFAR. In contrast the S1 and T1 state energies of the Z chromophores
oscillate with increasing nFAR. The C chromophores have behavior intermediate between the L and Z chromophores.
The picosecond transient spectra show complex behavior, having spectra reflecting intersystem crossing, vibrational
cooling and solvent relaxation processes. The nanosecond transient spectra result from the T1 - Tn transition. The timeresolved
spectra show no systematic variation with structure, showing more complex behavior than previously studied
platinum acetylides having phenylene ethynylene ligands.
To learn about excited state geometry in biphenyl-containing platinum acetylides, we synthesized a series of
compounds that have biphenyl ligands. The ligands consisted of biphenyl(I), the hindered 2'-methyl biphenyl(III) and
planar fluorenyl(IV) groups. We also synthesized a "half" complex(II) consisting of one ligand attached to the platinum
atom. The optical properties of these compounds were measured by ground state absorption, phosphorescence, ultrafast
transient absorption and nanosecond transient absorption spectroscopy. DFT calculations were performed to determine
the ground state and triplet state geometries and the lowest triplet energy. TDDFT calculations were performed to
determine singlet excited state energies. Compared to the reference compound I, ground state spectra show a blue shift
in II and III and red shift in IV, showing the singlet energy is sensitive to conjugation and biphenyl twist angle.
Comparison of the phosphorescence spectra of I and II shows the triplet exciton is confined to one ligand. The time
behavior of the ultrafast excited state absorption spectrum of I shows a red shift within 1 ps from the initial spectrum.
This behavior is not seen in IV. The different behavior suggests formation of the triplet state of I is accompanied by
conversion from a non-planar to a planar conformation while IV retains a planar conformation.
There has been much interest in the development of two-photon absorbing materials and many efforts to understand the
nonlinear absorption properties of these dyes. We have recently explored a new type of two photon absorbing dye
containing a platinum center with ligands that vary in length that contain electron withdrawing benzothiazole. With
increased π-π* conjugation we expect to observe a red shift in the absorption properties of the material. We have
investigated the photophysical properties of the platinum chromophores using a variety of experimental techniques.
Previously we determined that the singlet and triplet excited states are responsible for nearly all of the nonlinearity in the
nanosecond regime accept the two photon mechanism that is primarily used for excitation. Therefore we would like to
tune the photophysical properties of both the singlet and triplet excited state in these materials. To our surprise we found
there is quite a bit of red shifting due to a metal-to-ligand charge transfer from the platinum to the ligand rather than the
expected shifting due to increased π-π* conjugation. However, with increased ligand length the chromophore does take
on more π-π* character.
There has been much interest in the development of two-photon absorbing materials and many efforts to understand the
nonlinear absorption properties of these dyes but this area is still not well understood. A computational model has been
developed in our lab to understand the nanosecond nonlinear absorption properties that incorporate all of the measured
one-photon photophysical parameters of a class of materials called AFX. We have investigated the nonlinear and
photophysical properties of the AFX chromophores including the two-photon absorption cross-section, the excited state
cross-section, the intersystem crossing quantum yield, and the singlet and triplet excited state lifetimes using a variety of
experimental techniques that include UV-visible, fluorescence and phosphorescence spectroscopy, time correlated single
photon counting, ultrafast transient absorption, and nanosecond laser flash photolysis. The model accurately predicts the
nanosecond nonlinear transmittance data using experimentally measured parameters. Much of the strong nonlinear
absorption has been shown to be due to excited state absorption from both the singlet and triplet excited states. Based on
this understanding of the nonlinear absorption and the importance of singlet and triplet excited states we have begun to
develop new two-photon absorbing molecules within the AFX class as well as linked to other classes of nonlinear
absorbing molecules. This opens up the possibilities of new materials with unique and interesting properties.
Specifically we have been working on a new class of two-photon absorbing molecules linked to platinum poly-ynes. In
the platinum poly-yne chromophores the photophysics are more complicated and we have just started to understand what
drives both the linear and non-linear photophysical properties.
Extensive measurements and modeling of several two photon absorbing materials are described. These are used to elucidate the relative significance of various relaxation and excitation processes that come into play in nonlinear transmission (NLT) and two photon absorption cross section measurements. A reliable measurement of the one photon absorption cross sections at energies 0.5 to ~1.7 eV below the fundamental transition are presented with Voigt function fits that enable the determination of the Gaussian and Lorentzian line widths. Both a numerical model and an analytical model are developed neither of which use any adjustable parameters in comparing calculated NLT results to data. Both models fit the data relatively well over the full range of the experiment. The analytical model captures the primary causes of the nonlinear absorption in the low intensity regime and demonstrates that the nonlinear transmittance can be estimated as a simple effective three-photon process. The numerical model calculates the spatial and time dependence of three state populations and all of the transitions between these states. This model improves the quality of the nonlinear transmission fit which is due to the inclusion of the ground state absorption. Additionally an observation of a strong, long lived transient which is quenched by oxygen suggests multiphoton ionization is happening at low intensities. Thus the full range of constraints applicable to all measurements of the two photon cross section are presented.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.