Molecular spectroscopy with optical frequency combs

Over recent years there has been a growing interest in molecular semiconductors of the oligoacene family. The interest has been spurred by the unique properties of these materials for applications for electronic devices, namely being °exible, cheap and exhibit- ing relatively high charge-carrier mobilities. Despite this interest, the nature of charge transport in the materials has remained the subject of intense debate. When these oligoacenes are used for device applications, they will often be evaporated as a ¯lm onto a substrate, since the fabrication process and the handling of these thin ¯lms is easier and cheaper than using single crystals. Besides the fundamental bulk properties, for the usage in devices it is also of high importance to know how the properties of these materials di®er at the interface with metals, since the substrates and contacts are often metallic. Therefore in this work both the vibrational and electronic properties of thin ¯lms of pentacene have been investigated as well as the intrinsic charge transport mechanism for the single-crystalline bulk material of pentacene and rubrene. The ¯rst main focus in this study was on the electronic and vibrational properties of thin layers of pentacene (ranging from 1 monolayer to 6 layers of pentacene molecules) evaporated on metal surfaces. These pentacene layers have been investigated using High Resolution Electron Energy Loss Spectroscopy (HREELS), which is a surface sensitive technique that probes both the vibrational and electronic states of the top layers of the sample by detecting the loss in energy of a beam of monochromatic electrons. The main advantages of this technique in comparison to techniques like IR and Raman spectroscopy are the surface sensitivity, the high spectral resolution (FWHM of 13 cm¡1), the broad energy range (from lattice phonons to surface plasmons and interband transitions) and the ability to measure as a function of wave vector transfer. Pentacene was grown upon Ag(111) and Au(100) surfaces, forming thin layers of var- ious thicknesses. The obtained vibrational spectra were compared to the vibrational modes for an isolated molecule calculated using DFT methods. The spectra of ¯lms of over 4 molecular layers showed contributions from all the optical active modes predicted by the calculation as is expected for a non-frustrated Van Der Waals bonded crystalline material. Furthermore the spectra showed low energetic features related to the lattice phonons. Both inter- and intramolecular modes showed little to no dispersion. The spec- tra for a monolayer deviated strongly from the vibrational structure of the multi-layers. Not only were the intermolecular modes absent, all intramolecular optical active modes with a dipole moment parallel to the interface were screened by the metal surface. In the case of a monolayer of pentacene on Ag(111) the interaction with the surface is so strong that the symmetric Ag modes became optically active as a result of charge transfer from the metallic substrate. For the intermediate regime (layers of 2 to 4 molecules) the spectra showed several contributions that cannot be explained using the calculated data on the symmetry and energy of the intramolecular modes. With HREELS not only the vibrational aspects of the thin ¯lms were studied but also the electronic transitions. The spectra for layers of more than four molecular layers could be modeled by combining calculations on the position of both excitonic and band levels and calculations of the vibronic ¯ne structure. For a monolayer of pentacene, states within the electronic gap were observed, the so-called Charge Neutrality Levels, which are substrate dependent. With increasing thickness the observed presence and position of the exciton states di®ered, thereby giving more insight in the structure of pentacene near the interface. Terahertz Time Domain Spectroscopy (THz TDS) is a technique that allows determin- ing the complex frequency dependent conductivity and is a powerful tool to investigate the charge transport mechanism in materials. THz TDS was used to study the charge trans- port as a function of temperature for single crystals of both pentacene and rubrene. For pentacene the conductivity spectra at various temperatures showed a strong absorption for the low energetic vibrational modes and thereby gave evidence for the model proposed by A. Troisi and G. Orlandi. This model describes the charge transport behaviour by one single mechanism: the large °uctuations in the intermolecular coupling of electronic states caused by thermal molecular motions. Furthermore it is shown that the charge density, and as a result the interaction between charges, plays an important role in the conductivity of these materials. In conclusion, this work shows that vibrational aspects are a key factor in under- standing the charge transport in oligoacene materials, and that good knowledge of the properties of these materials at the interface is essential, since di®erences in both struc- ture and electronic interaction with the substrate give rise to di®erent vibrational and electronic states in oligoacene materials.

We explore the advantages of a free-form dual-comb spectroscopy (DCS) platform based on time-programmable frequency combs for real-time, penalty-free apodized scanning. In traditional DCS, the fundamental spectral point spacing, which equals the comb repetition rate, can be excessively fine for many applications. While fine point spacing is not itself problematic, it comes with the penalty of excess acquisition time. Post-processing apodization (windowing) can be applied to tailor the resolution to the sample, but only with a deadtime penalty proportional to the degree of apodization. The excess acquisition time remains. With free-form DCS, this deadtime is avoided by programming a real-time apodization pattern that dynamically reverses the pulse periods between the dual frequency combs. In this way, one can tailor the spectrometer’s resolution and update rate to different applications without penalty. We show the operation of a free-form DCS system where the spectral resolution is varied from the intrinsic fine 160 MHz up to 822 GHz by applying tailored real-time apodization. Because there is no deadtime penalty, the spectral signal-to-noise ratio increases linearly with resolution by 5000× over this range, as opposed to the square root increase observed for post-processing apodization in traditional DCS. We explore the flexibility to change resolution and update rate to perform hyperspectral imaging at slow camera frame rates, where the penalty-free apodization allows for optimal use of each frame. We obtain dual-comb hyperspectral movies at a 20 Hz spectrum update rate with broad optical spectral coverage of over 10 THz.

The distributed Bragg reflectors (DBRs) are important optical components in a wide range of applications, such as Fabry-Perot optical resonators, hyperspectral sensors, micro-cavity structures, and waveguide lasers. In this thesis, we have developed, designed, and fabricated highly reflecting, and mechanically stable DBRs comprised of electron-beam deposited zinc selenide (ZnSe) and barium fluoride (BaF2) thin films and explored its use in optical filtering and hyperspectral sensing applications. Most published results are in the short-wavelength infrared (SWIR) and near-infrared (NIR) with limited results in the mid-wavelength infrared (MWIR). In this work, we have fabricated ZnSe/BaF2-based DBRs in the MWIR region. The fabricated DBRs showed a central wavelength of 4 µm and reflection exceeding 93%. A Fabry-Perot optical filter, that is comprised of two parallel DBR mirrors with half-wavelength optical cavity thickness was used to partially transmit and reflect the incident light at the target wavelength. The assembled fixed-cavity Fabry-Perot optical filters achieved a full-width half maximum (FWHM) of ~ 450 nm. We further showed that the FWHM of the optical filters can be reduced by ~ 50 % with the addition of nano-thick silicon monoxide (SiO) interface grading layer between every pair of the quarter-wave optical thick (QWOT) layers in the stack. The SiO interface grading layer acts as an anti-reflection coating layer to reduce reflection loss in the DBR multilayer structure. The SiO interface grading layer, ~ 80 nm thick, also has the potential to stop atomic inter-diffusion between the layers to minimize free carrier absorption loss. The principal thesis goal is to design and assemble fixed-cavity interface-graded Fabry-Perot optical filters (with SiO interfacial layers) that optimized optical transmission and spectral resolution. The thesis also proposes a low-cost micro-cavity structure that can be formed by embedding two-dimensional atomic crystals, such as black phosphorous into a DBR cavity structure. The proposed design can be used to increase absorption in 2D materials, which has the potential to increase the efficiency of next generation MWIR photodetectors and infrared sensors.

A multilayered dielectric structure, namely a one dimensional photonic crystal (1DPC), is proposed as a suitable platform for photon management, due to the low absorption of the dielectric materials. When properly designed, a 1DPC can sustain surface electromagnetic modes called Bloch Surface Waves (BSWs). In this PhD Thesis it is shown how light coupled to BSW can be focused or guided by means of ultrathin polymeric refractive structures directly patterned on the surface. Moreover, by patterning the surface with surface relief gratings, far-field radiation can be efficiently coupled to the surface modes, thus providing an enhanced electromagnetic field at the truncation interface of the 1DPC. By shaping the grating in a circular symmetry, light can be in-plane focused into a sub-wavelength spot. The same structure can be used to re-shape the radiation pattern of dipolar emitters. It is shown that an emitter lying on the surface of the 1DPC couples to the photonic structure and the fluorescence radiated couple with the surface modes. The so called BSW-coupled fluorescence propagates along the surface with low losses and a well-defined wavevector. By means of surface diffraction gratings properly designed, fluorescence can be extracted along any direction, thus improving the fluorescence collection with no need of high numerical aperture optics or critical alignements. A novel method for evaluating the enhancement gained with such photonic structures on the extraction efficiency is proposed. Such method is capable of providing at the same time spatial resolution, angular resolution and spectral resolution. A biosensing experiment to detect small amounts of labeled proteins is provided, in order to show the sensing capabilities of the photonic structure

In this thesis, geometry is used as a basis for conducting experiments aimed at growing and arranging inorganic minerals on curved interfaces. Mineralisation is directed using crystalline and liquid-crystalline metallic soaps and surfactant/water systems as templates.¶ A review of the history, syntheses, structure and liquid crystallinity of metallic soaps and other amphiphiles is presented as a foundation to understanding the interfacial architectures in mesostructured template systems in general. ...

Spectral imaging has been extensively applied in many fields, including agriculture, environmental monitoring, biomedical diagnostics, etc. Thanks to the advances in sensor technology, spectral imaging systems nowadays provide finer and finer spectral resolution needed to characterize the spectral properties of materials. The high spectral resolution, however, raises an issue as the difference in spectral information between two adjacent wavelength bands is typically very small. As a result, much of the data in a scene seems to be redundant. However, critical information is embedded that often can be used to identify materials. This thesis aims at facilitating the analysis in spectral imaging by making use of pattern recognition techniques, on the one hand, to improve visualization, and on the other hand, to directly solve classification problems.

For many remote sensing applications it is preferable to have images with both high spectral and spatial resolutions. On this regards, hyperspectral and multispectral images have complementary characteristics in terms of spectral and spatial resolutions. In this paper we propose an approach for the fusion of low spatial resolution hyperspectral images with high spatial resolution multispectral images in order to obtain superresolution (spatial and spectral) hyperspectral images. The proposed approach is based on the assumption that, since both hyperspectral and multispectral images acquired on the same scene, the corresponding endmembers should be the same. On a first step the hyperspectral image is spectrally down-sampled in order to match the multispectral one. Then an endmember extraction algorithm is performed on the down-sampled hyperspectral image and the successive abundance estimation is performed on the multispectral one. Finally, the extracted endmembers are up-sampled back to the original hyperspectral space and then used to reconstruct the superresolution hyperspectral image according to the abundances obtained from the multispectral image.

Hyperspectral vision beyond 3D: A review

Solar non-thermal high-energy particle flows are one of the main triggering sources of the disastrous space weather events. Their main observational manifestation is radio Type Ⅲ bursts with fast frequency drifting rate. This work introduces the main advances of observational instruments and scientific researches on the solar radio Type Ⅲ bursts at the space very low frequency (< 30 MHz SVLF) in the world, including the space-based or lunar-based solar SVLF spectral polarimeters with high temporal and spectral resolutions. We also discuss carefully the existing problems on the solar radio observations and related researches, also including the main scientific aims and prospective of the space-based or lunar-based solar SVLF observations.

This paper presents a simulation method of hyper-spectral dynamic scene and image sequence for hyper-spectral equipment evaluation and target detection algorithm. Because of high spectral resolution, strong band continuity, anti-interference and other advantages, in recent years, hyper-spectral imaging technology has been rapidly developed and is widely used in many areas such as optoelectronic target detection, military defense and remote sensing systems. Digital imaging simulation, as a crucial part of hardware in loop simulation, can be applied to testing and evaluation hyper-spectral imaging equipment with lower development cost and shorter development period. Meanwhile, visual simulation can produce a lot of original image data under various conditions for hyper-spectral image feature extraction and classification algorithm. Based on radiation physic model and material characteristic parameters this paper proposes a generation method of digital scene. By building multiple sensor models under different bands and different bandwidths, hyper-spectral scenes in visible, MWIR, LWIR band, with spectral resolution 0.01μm, 0.05μm and 0.1μm have been simulated in this paper. The final dynamic scenes have high real-time and realistic, with frequency up to 100 HZ. By means of saving all the scene gray data in the same viewpoint image sequence is obtained. The analysis results show whether in the infrared band or the visible band, the grayscale variations of simulated hyper-spectral images are consistent with the theoretical analysis results.

Abstract. Hyperspectral image enhancement has been a concern for the remote sensing society for detailed end member detection. Hyperspectral remote sensor collects images in hundreds of narrow, continuous spectral channels, whereas multispectral remote sensor collects images in relatively broader wavelength bands. However, the spatial resolution of the hyperspectral sensor image is comparatively lower than that of the multispectral. As a result, spectral signatures from different end members originate within a pixel, known as mixed pixels. This paper presents an approach for obtaining an image which has the spatial resolution of the multispectral image and spectral resolution of the hyperspectral image, by fusion of hyperspectral and multispectral image. The proposed methodology also addresses the band remapping problem, which arises due to different regions of spectral coverage by multispectral and hyperspectral images. Therefore we apply algorithms to restore the spatial information of the hyperspectral image by fusing hyperspectral bands with only those bands which come under each multispectral band range. The proposed methodology is applied over Henry Island, of the Sunderban eco-geographic province. The data is collected by the Hyperion hyperspectral sensor and LISS IV multispectral sensor.

Since Adam and Gibbs proposed their theory of cooperatively rearranging regions in 1965 [AG65] our understanding of the liquid-glass transition is connected to the concept of cooperativity. This theory implies a temperaturedependent radius of cooperativity, which diverges at the glass transition, the so-called divergent length scale. While the necessity of a divergent time scale for the glass transition is widely accepted nowadays (e.g. in advanced theories like mode-coupling theory), the existence of the divergent length scale is still subject to controversy. The principal question of its existence can be addressed by experiments concerning glass forming liquids in confinement, since the dynamics of a confined glass is expected to deviate strongly from the dynamics of a bulk glass as soon as the confining radius approximately matches the length scale of cooperativity. Nuclear resonant scattering is an experimental method suited to study the fast and slow dynamics of a glass with atomic resolution. First experiments on the model glass-former dibutylphthalate:ferrocene confined in controlled pore glasses yielded evidence for a shift of the glass transition to higher temperatures, but were unable to discern whether this was caused by the surface-glass interaction or induced by the geometrical confinement [ASF01]. In this work nuclear resonant scattering experiments were performed using the oriented channels of mesoporous silicon as a confining matrix; Mosbauer spectroscopy is applied as a complementary experimental method. The temperature dependencies of experimental parameters like the relaxation time and the Lamb-Mosbauer factor yielded new insights into the glass transition and the connected molecular dynamics. In all cases shifts of the modecoupling critical temperature TC to higher temperatures were observed. The

1. Introduction Frequency combs have been investigated in the laboratory over the course of the last 25 years in a wide range of implementations and applications including but not limited to precision metrology, optical atomic clocks, LIDAR, low-phase-noise RF generation, and precision spectroscopy [1]. While the Nobel prize winning technology of frequency combs have shown their usefulness in a variety of applications, there have been few demonstrations of this technology in real-world applications [2, 3]. Here we present a mode-locked oscillator that has been designed to be environmentally robust and low cost, while maintaining suitability for use in frequency comb applications. Vescent Photonics is designing and manufacturing fiber mode-locked oscillators and frequency combs for government customers whose application deployments include satellites and terrestrial moving platforms. These designs allow for repetition rate matching at the time of manufacture, which is an important consideration for integration of this technology into optical atomic clocks, dual comb spectroscopy, and low-phase-noise radio frequency generation. Vescent currently is manufacturing oscillators at 100 and 200 MHz repetition rates, and these designs could be modified to give repetition rates between 80 and 250 MHz. While performance and environmental robustness are necessary metrics for real-world applications of frequency combs, the cost of these devices can also be a barrier to wide-spread deployment. Vescent Photonics is diligently working on optimizing all three parameters (performance, environmental robustness, and cost) to bring a deployable mode-locked laser and frequency comb solution to market. 2. Mode-locked oscillator performance The compact mode-locked oscillators presented here have been designed to be low-cost, low-volume, highperformance, and environmentally robust for field-deployed applications. Figure 1a shows the mechanical enclosure for the fiber oscillator measuring 0.12 L (10.75 cm x 6.35 cm x 1.72 cm) in volume; it should be noted that pump diodes have not been included in this packaging due to thermal considerations. An enclosure for a frequency comb system (optic only) has also been designed with a volume of 0.62 L (19.05 cm x 11.30 cm x 2.86 cm). Two length actuators are included in this design; a fast PZT with &gt;200 kHz bandwidth that allows 150 Hz of tuning on a 100 MHz repetition rate, and slow temperature control of the fiber with a ~10 s thermal time constant and &gt;40 kHz of tuning on a 100 MHz repetition rate. The oscillator optical spectrum is included in Figure 1b. Average optical powers out of the oscillators are on the order of a few milliwatts, and pulses are close to transform limited (~250 fs). Erbium-doped fiber amplifiers and pulse compression stages are routinely added to the oscillator outputs to achieve average powers &gt;200 mW and pulse durations &lt;100 fs. While these parameters are useful for a wide range of mode-locked laser applications, it also allows the detection of the carrier envelope offset frequency ( , a fundamental comb parameter) in an f-2f interferometer with greater than 40 dB SNR in 300 kHz resolution bandwidth (Figure 1c). To show suitability of these fiber mode-locked oscillators in frequency comb applications, fractional Allan Deviations of the in-loop stabilized signals have been included in Figure 1d where the absolute frequency deviations have been divided by the distance between the frequency comb lock points. Typical optical clock performance has also been plotted, where the performance of the locks is more than suitable for this application. 3. Summary A compact, low-cost, environmentally robust mode-locked oscillator has been shown to be suitable for frequency comb experiments. Detection and stabilization of the fundamental comb parameters is suitable for optical atomic clocks, as well as a host of real-world frequency comb applications including dual comb spectroscopy, low phase noise RF generation, and optical metrology.

Intensity Clamping During Femtosecond Laser Filamentation

Hyperspectral images (HSI) have high spectral and low spatial resolutions. However, multispectral images (MSI) usually have low spectral and high spatial resolutions. In various applications HSI with high spectral and spatial resolutions are required. In this paper, a new method for spatial resolution enhancement of HSI using high resolution MSI based on sparse coding and linear spectral unmixing (SCLSU) is introduced. In the proposed method (SCLSU), high spectral resolution features of HSI and high spatial resolution features of MSI are fused. In this case, the sparse representation of some high resolution MSI and linear spectral unmixing (LSU) model of HSI and MSI is simultaneously used in order to construct high resolution HSI (HRHSI). The fusion process of HSI and MSI is formulated as an ill-posed inverse problem. It is solved by the Split Augmented Lagrangian Shrinkage Algorithm (SALSA) and an orthogonal matching pursuit (OMP) algorithm. Finally, the proposed algorithm is applied to the Hyperion and ALI datasets. Compared with the other state-of-the-art algorithms such as Coupled Nonnegative Matrix Factorization (CNMF) and local spectral unmixing, the SCLSU has significantly increased the spatial resolution and in addition the spectral content of HSI is well maintained.