Full-width At Half-maximum Bandwidth Research Articles

Optical design of reflective Bragg mirrors for EUV photolithography

In this article. we investigate the optical characteristics of reflective extreme-ultraviolet (EUV) mirrors for the application of lithography for 7 nm node and below. By using the conventional quarter-wavelength stacked Bragg reflective configuration of Si-based metal multilayers centered at soft x-ray 13.5 nm, the maximum reflection is numerically computed to be approximately 0.7 at nearly normal incidence, which is consistent to the typical values reported by Advanced Semiconductor Materials Lithography (ASML) for Mo/Si stacked multilayers. The full width half maximum (FWHM) bandwidth is 0.65∼0.78 nm. In our numerical simulation, moreover, we find Nb/Si reflective mirrors can be potentially utilized to replace Mo/Si stacked multilayers in order to harvest more EUV light inside the scanners.

Passively mode-locked fiber lasers with broadband FeOOH saturable absorber

In this contribution, we synthesized iron oxide hydroxide (FeOOH) nanomaterials and successfully demonstrated the application of FeOOH-based saturable absorbers in ultrafast photonics at 1 μm and 1.5 μm. Various characterizations were conducted to analyze the morphological and structural features of FeOOH nanomaterials. Subsequently, a tapered fiber coated with FeOOH was integrated into the resonator as a saturable absorber, exhibiting the significant saturable absorption effect with modulation depths of 1.26 % at 1 μm and 2.61 % at 1.5 μm, respectively. In an Yb-doped fiber laser, stable noise-like mode-locking pulses were achieved with a pulse duration of 519 fs at the operating wavelength of 1037.2 nm with a full-width at half-maximum (FWHM) spectral bandwidth of 4.7 nm, yielding a maximum single pulse energy of 1.23 nJ with a high signal-to-noise ratio (SNR) of 47.8 dB. Meanwhile, the conventional soliton pulses were produced in the passively mode-locked Er-doped fiber laser, featuring a pulse duration of 1.02 ps, and an SNR of 42.5 dB at a peak wavelength of 1559.25 nm with a FWHM bandwidth of 2.8 nm. This work demonstrates the considerable nonlinear optical properties of FeOOH nanomaterials in the near-infrared range, proposing a promising saturable absorber for ultrafast photonics applications.

Hybrid fiber–solid-state laser with 3D-printed intracavity lenses

Microscale 3D-printing has revolutionized micro-optical applications ranging from endoscopy, imaging, to quantum technologies. In all these applications, miniaturization is key, and in combination with the nearly unlimited design space, it is opening novel, to the best of our knowledge, avenues. Here, we push the limits of miniaturization and durability by realizing the first fiber laser system with intra-cavity on-fiber 3D-printed optics. We demonstrate stable laser operation at over 20 mW output power at 1063.4 nm with a full width half maximum (FWHM) bandwidth of 0.11 nm and a maximum output power of 37 mW. Furthermore, we investigate the power stability and degradation of 3D-printed optics at Watt power levels. The intriguing possibilities afforded by free-form microscale 3D-printed optics allow us to combine the gain in a solid-state crystal with fiber guidance in a hybrid laser concept. Therefore, our novel ansatz enables the compact integration of a bulk active media in fiber platforms at substantial power levels.

A new accurate low-cost instrument for fast synchronized spatial measurements of light spectra

Abstract. We developed a cost-effective Fast-Response Optical Spectroscopy Time-synchronized instrument (FROST). FROST can measure 18 light spectra in 18 wavebands ranging from 400 to 950 nm with a 20 nm full-width half-maximum bandwidth. The FROST 10 Hz measurement frequency is time-synchronized by a global navigation satellite system (GNSS) timing pulse, and therefore multiple instruments can be deployed to measure spatial variation in solar radiation in perfect synchronization. We show that FROST is capable of measuring global horizontal irradiance (GHI) despite its limited spectral range. It is very capable of measuring photosynthetic active radiation (PAR) because 11 of its 18 wavebands are situated within the 400-to-700 nm range. A digital filter can be applied to these 11 wavebands to derive the photosynthetic photon flux density (PPFD) and retain information on the spectral composition of PAR. The 940 nm waveband can be used to derive information about atmospheric moisture. We showed that the silicon sensor has undetectable zero offsets for solar irradiance settings and that the temperature dependency as tested in an oven between 15 and 46 ∘C appears very low (−250 ppm K−1). For solar irradiance applications, the main uncertainty is caused by our polytetrafluoroethylene (PTFE) diffuser (Teflon), a common type of diffuser material for cosine-corrected spectral measurements. The oven experiments showed a significant jump in PTFE transmission of 2 % when increasing its temperature beyond 21 ∘C. The FROST total cost (< EUR 200) is much lower than that of current field spectroradiometers, PAR sensors, or pyranometers, and includes a mounting tripod, solar power supply, data logger and GNSS, and waterproof housing. FROST is a fully standalone measurement solution. It can be deployed anywhere with its own power supply and can be installed in vertical in-canopy profiles as well. This low cost makes it feasible to study spatial variation in solar irradiance using large-grid high-density sensor setups or to use FROST to replace existing PAR sensors for detailed spectral information.

Non-collinear generation of ultra-broadband parametric fluorescence photon pairs using chirped quasi-phase matching slab waveguides.

Many optical quantum applications rely on broadband frequency correlated photon pair sources. We previously reported a scheme for collinear emission of high-efficiency and ultra-broadband photon pairs using chirped quasi-phase matching (QPM) periodically poled stoichiometric lithium tantalate (PPSLT) ridge waveguides. However, collinearly emitted photon pairs cannot be directly adopted for applications that are based on two-photon interference, such as quantum optical coherence tomography (QOCT). In this work, we developed a chirped QPM device with a slab waveguide structure. This device was designed to produce spatially separable (photon pair non-collinear emission) parametric fluorescence photon pairs with an ultra-broadband bandwidth in an extremely efficient manner. Using a non-chirped QPM slab waveguide, we observed a photon pair spectrum with a full-width-at-half-maximum (FWHM) bandwidth of 26 nm. When using a 3% chirped QPM slab waveguide, the FWHM bandwidth of the spectrum increased to 190 nm, and the base-to-base width is 308 nm. We also confirmed a generation efficiency of 2.4×106 pairs/(μW·s) using the non-chirped device, and a efficiency of 8×105 pairs/(μW·s) using the 3% chirped device under non-collinear emission conditions after single-mode fiber coupling. This is, to the best of our knowledge, the first report of frequency correlated photon pairs generation using slab waveguide device as a source. In addition, using slab waveguides as photon pair sources, we performed two-photon interference experiments with the non-chirped device and obtained a Hong-Ou-Mandel (HOM) dip with a FWHM of 7.7 μm and visibility of 98%. When using the 3% chirped device as photon pair source, the HOM measurement gave a 2 μm FWHM dip and 74% visibility.

Compact and Efficient Ring Resonator–Based Plasmonic Lens with Multiple Functionalities

A novel plasmonic lens based on a metal–insulator–metal bus waveguide coupled to a ring resonator is proposed and numerically investigated. The inner edge of the ring resonator was perforated with nanogrooves so that light emitted by these nanogrooves was sharply focused at the center of the ring. The structure relies on interference and resonance theory. The principle of the lens can be extended to a device 1 × N demultiplexer as well as a 1 × N power splitter. These functionalities were validated through finite element method simulations. Results show that the lens had a sharp focus, without any smearing, and a full width at half maximum (FWHM) intensity of around 240 nm. For the 1 × N demultiplexer, the selectivity was high with an FWHM bandwidth of less than 20 nm and crosstalk of less than − 10 dB, whereas the 1 × N power splitter had a narrow bandwidth and was able to split power into N equal parts with negligible imbalance. The proposed focusing structure is compact, and the simulation results show that the structure performs the various functionalities with high efficiency. Due to this, these structures will be of utmost utility in future all-optical signal processing systems.

The influence of Er3+ ions on the spectroscopic and lasing characteristics of alkaline earth titanium borate glasses for photonic applications

Alkaline earth titanium borate glasses doped with different ratios of Er3+ ions were fabricated using standard melt quenching technique. The density of the glasses increased from 3.652 to 3.762 g/cm3 as the ratio of Er3+ ions increased from 0 to 5.640 × 1020 cm−3. The gaussian deconvolution of the Raman and FTIR spectra was investigated, and the distinctive spectral vibrational bands were assigned. The UV–Vis–NIR absorption spectra of the erbium doped glasses exhibit nine bands located at 1531, 980, 808, 654, 520, 490, 446, 408, and 377 nm due to the electronic transitions of Er3+ ions from the ground level 4I15/2 to the higher excited levels 4I13/2, 4I11/2, 4I9/2, 4F9/2, 2H11/2, 4F7/2, 4F3/2, 2G9/2, and 4G11/2, respectively. The structural and luminescent properties of the erbium doped glasses were investigated by calculating Judd – Ofelt intensity parameters, radiative transition probabilities, branching ratio, lifetime, full width at half maximum (FWHM), and gain bandwidth (FWHM × σemis). The intensity parameters followed the trend as Ω2> Ω6> Ω4 for all glasses. The values of Ω2 decreased from 5.2079 × 10−20 to 3.8133 × 10−20 cm2 and that of Ω4 decreased from 0.7402 × 10−20 to 0.5939 × 10−20 cm2 as the ratio of Er3+ ions increased. The high laser efficiency of the studied glasses were verified by the high values of the radiative lifetime, branching ratio, FWHM, and gain bandwidth. The gain spectra for BaTiBEr3 and BaTiBEr5 glasses covered the conventional (1530–1565 nm) and long (1565–1625 nm) communication bands. Moreover, the high values of the FWHM and gain bandwidth for BaTiBEr3 and BaTiBEr5 glasses, suggested their suitability for photonic applications in the near infrared region and broadband optical fiber amplifiers.

Portable boom-type ultrahigh-resolution OCT with an integrated imaging probe for supine position retinal imaging.

To expand the clinical applications and improve the ease of use of ultrahigh-resolution optical coherence tomography (UHR-OCT), we developed a portable boom-type ophthalmic UHR-OCT operating in supine position that can be used for pediatric subjects, bedridden patients and perioperative conditions. By integrating the OCT sample arm probe with real-time iris display and automatic focusing electric lens for easy alignment, coupling the probe on a self-locking multi-directional manipulator to reduce motion artifacts and operator fatigue, and installing the OCT module on a moveable cart for system mobility, our customized portable boom-type UHR-OCT enables non-contact, high-resolution and high-stability retinal examinations to be performed on subjects in supine position. The spectral-domain UHR-OCT operates at a wavelength of 845 nm with 130 nm FWHM (full width at half maximum) bandwidth, achieving an axial resolution of ≈2.3µm in tissue with an A-line acquisition rate up to 128 kHz. A high-definition two-dimensional (2D) raster protocol was used for high-quality cross-sectional imaging while a cube volume three-dimensional (3D) scan was used for three-dimensional imaging and en-face reconstruction, resolving major layer structures of the retina. The feasibility of the system was demonstrated by performing supine position 2D/3D retinal imaging on healthy human subjects, sedated infants, and non-sedated awake neonates.

Highly efficient cryogenic Yb:YLF regenerative amplifier with 250 W average power.

We report an efficient diode-pumped high-power cryogenic regenerative amplifier operating at 1019 nm employing the c axis of Yb:YLF. Compared to the usually selected 1017 nm transition of the a axis, the c-axis 1019 nm line has a three-fold higher emission cross section and still possesses a full-width at half-maximum (FWHM) of 6.5 nm at 125 K. The chirped-pulse amplifier system is seeded by a fiber front-end with energy of 30 nJ and a stretched pulse width of 2 ns. In regenerative amplification studies, using the advantage of higher gain in the c axis, we have achieved record average powers up to 370 W with an extraction efficiency of 78% at a 50 kHz repetition rate. The output pulses were centered on 1019.15 nm with a FWHM bandwidth of 1.25 nm, which supports sub-1.5 ps pulse durations. The output beam maintained a TEM00 beam profile at output power levels below 250 W with an M2 below 1.2. Above this power level, the thermally induced lensing in Yb:YLF created a multimode output beam. The thermal lens was rather dynamic and deteriorated the system stability above a 250 W average power level.

Oblique and Polarization Independent Metasurface-Based Absorber for Bio-Sensing Applications

In this paper, a submillimetric/THz metasurface polarization/oblique independent absorber is designed and characterized. The absorber is designed using a periodic arrangement of a modified split ring resonator unit cell. The symmetric absorber is insensitive to the variation of incident angles and polarization angles. The results confirm that perfect absorption is achieved (close to 100%) for either TM or TE polarization at 300 GHz. Moreover, the absorber is efficient for normal and oblique incident angle (above 90% for 45° angle of incidence). In all the presented cases, the full width half maximum bandwidth is 20 GHz. The absorber unit cell size is only 0.0052 λ 0 × 0.0052 λ 0 and it has a low-profile thickness = 0.0002 λ 0, at the operating frequency. Also, an absorber-based biosensor is presented. The sensing has proved the coating analyte thickness can be detected by shifting the absorption frequency. The maximum sensitivity is 24.9% RIU−1 with a low-profile maximum of 50 µm thickness. Analytical formulas for the frequency shift have been extracted. The presented results are discussed based on 3D full-wave EM simulations.

Engineering tunability through electro-optic effects to manifest a multifunctional metadevice.

Ultrafast modulation of the refractive index exhibits either linear or nonlinear electro-optic (EO) effect, which is extensively utilized in tunable photonic circuits. Silicon, a mature material for on-chip devices, lacks a strong electro-optic (EO) Pockels effect. Utilization of the Pockels effect alters the intrinsic property (refractive index) of the material that manifests tunability and offers expanded functionalities. Driven by the limited space constraints in data storage, sensing, and imaging applications, we propose an electrically tunable meta-device whose resonance wavelength and focal length can be tuned by varying applied electric fields in the visible range. The fundamental unit of a metalens is the barium titanate (BTO) diffractive optical element placed on indium titanium oxide (which serves as an electrode) with SiO2 as the substrate. The metalens' tunability is characterized by point spread function (PSF), full-width half-maximum (FWHM), and imaging bandwidth that demonstrates the tuning of resonance wavelength and focal length. Moreover, polarization-insensitive meta-holograms are realized at a wavelength of 633 nm without utilizing propagation and Pancharatnam–Berry (PB) phase. The proposed study can find exciting applications in machine vision, broadband microscopy, and spectroscopy.

Design of grating based narrow band reflector on SOI waveguide

A grating-based narrowband wavelength selective reflector on silicon on insulator waveguide is proposed here. The 500 nm thick silicon waveguide is used in the reflector design. The design consists of dual-etch layers for different grating sets separated by the center cavity. The duty-cycle, grating-period, and side etching depth are optimized for achieving high reflection and narrow bandwidth. The proposed reflector shows a reflection of 96 % with a full-width half-maximum bandwidth of 1.5 nm at 1550 nm wavelengths.

An ultra-thin quad-band metamaterial inspired absorber using symmetric bent-arrow shaped resonator for sensing and imaging in defense applications

In this paper, a novel compact quad-band polarization insensitive metamaterial absorber has been proposed to be employable in the microwave frequency regime. The unit-cell geometry comprises of four symmetric bent-arrow shaped resonators, where each arrow has been bounded by an open ring. The resultant structure is further surrounded by a closed ring to obtain an extra resonance band. Full-wave simulation with normal incidence depicts quad-band operation with absorption peaks at 3.24 GHz (S-band), 6.55 GHz (C-band), 15.22 GHz (Ku-Band), 15.94 GHz (Ku-band) and absorptivity levels of 99.57%, 99.94%, 96.10%, 98.65% correspondingly. It also shows full width half maximum (FWHM) bandwidth of 100 MHz, 200 MHz and 1210 MHz in the first, second and third bands respectively. Furthermore, the proposed structure is based on four-fold symmetry therefore exhibits polarization-insensitive behaviour unlike conventional absorbers. The structure is fabricated on 1 mm FR4 Glass Epoxy substrate equivalent to 0.0108 λ 0 hence, can be used as absorber coating for planar surfaces. The designed absorber has been fabricated and experimental results were in good agreement with the simulated responses enabling its wide application in various technologies like stealth technology, radar cross section reduction, anechoic chambers, electromagnetic interference/electromagnetic compatibility, and radio frequency identification.

Two-color emission of BaWO4 crystals pumped by 472 nm 0.11 ps laser pulses for frequency conversion into the longwave IR domain

The frequency conversion of Ti:Sapphire laser second harmonic pulses into two-color emission using a tandem of BaWO4 Raman crystals was experimentally demonstrated with an internal efficiency of ∼17%. The output emission contained symmetrical Stokes and anti-Stokes spectral components separated by 902 cm−1 with a full-width half-maximum bandwidth of ≥110 cm−1 each. The applicability of this two-color emission was numerically verified for the production of longwave infrared ultrashort pulses suitable for seeding a high-pressure CO2 laser amplifier via difference frequency generation in LiGaS2 crystal.

Optical Filter Switchable Between Bandstop and Bandpass Responses in SOI Wafer

An optical filter switchable between bandpass and bandstop responses is proposed by analyzing frequency response in the complex plane. The device is fabricated based on silicon-on-insulator wafer. Experimental results show that the bandstop response has an extinction ration of exceeding 62.5 dB and a full width at half maximum (FWHM) bandwidth of 971 MHz, and the bandpass response has a rejection ration of 19.5 dB and a FWHM bandwidth of 840 MHz, respectively. The device also exhibits good tunability for both bandstop and bandpass responses.

Wavelength-encoded multi-channel spectral domain optical coherence tomography (SD-OCT) for spatiotemporally resolved imaging

In conventional spectral domain optical coherence tomography (SD-OCT), a beam is focused to one spot on the surface of the sample to acquire depth information beneath that spot. Adjacent spots are serially scanned to obtain a tomographic image of the sample; thus, available OCT techniques can take significant time to scan large objects. This paper illustrates an SD-OCT system involving spectral-slicing of a broadband spectrum to enable the simultaneous acquisition from multiple lateral positions of the sample. Using a wavelength encoding of spatial information, where each lateral location is associated with a specific portion of a source spectrum, multiple lateral spots are interrogated concurrently to improve the imaging speed of SD-OCT. A triple-channel SD-OCT system was devised using a broadband source of centre wavelength 840 nm and full width half maximum (FWHM) bandwidth of 50 nm. Both sample and reference arm beams were split into three channels using two dichroic splitters of 50% cut-on wavelength at 825 nm and 848 nm, resulting in a FWHM spectral distribution of 13.5 nm, 19 nm and 17.5 nm in the three channels, respectively. The imaging performance of the technique was demonstrated by concurrently acquiring tomographic images of a 3D-printed phantom. Despite the relative drop in the per-channel axial resolution and depth sensitivity, this new approach presents a straightforward way of interrogating a sample simultaneously in multiple locations, provides the additional advantage of allowing different delay lines for each spectral section and offers the potential of such a system to be used in a wide range of applications that demand a small product foot print.

A set square design metamaterial absorber for X-band applications

ABSTRACT This paper presents a set square design which serves as a wideband Metamaterial Absorber (MA) for X-band applications. The simulation results show that the absorber has dB absorption bandwidth of 2.51 GHz (8.36–10.87 GHz) and the Full Width Half Maximum (FWHM) bandwidth of 3.55 GHz (7.47–11.02 GHz). MA proposed in this paper uses as a substrate having thickness of 1.6 mm and overall dimensions of 10 mm × 10 mm with center frequency at 9.61 GHz. The absorption mechanism which depends on effective permittivity () and effective permeability () is explained and plotted.

Selection of the Oscillation Mode in Multi-Mode Oscillators by Initializing the Cavity Signal

We demonstrate a new method to select the oscillation mode in multi-mode oscillators with an instantaneous gain saturation. The method is based on initializing the cavity signal by feeding into the cavity an external signal for a duration that can be as short as the cavity delay. After the external signal is turned off, the steady state oscillation mode is exactly determined by the frequency of the external signal. By changing the external signal frequency, any specific mode can be selected over a broad continuous operating frequency region, which can be wider than the full width at half maximum (FWHM) bandwidth of the cavity filter. The method is not based on injection-locking effect and therefore the frequency of the external signal can be arbitrary chosen in a broad operating bandwidth, and it is not limited by the narrow locking range of the oscillator. When the frequency difference between the external signal and the central frequency of the filter is small, in comparison with the filter FWHM bandwidth, the selected mode is the mode with the nearest frequency to that of the external signal. However, as this frequency difference is increased, the selected mode is slightly shifted toward the filter central frequency. These results were experimentally demonstrated in an optoelectronic oscillator (OEO) and were compared to a comprehensive numerical simulation. A good quantitative agreement between theory and experiments were obtained. A fundamental limit on the minimum duration, which is required to initialize the cavity signal is given.

Narrow-Band 4.3<italic>μ</italic>m Plasmonic Schottky-Barrier Photodetector for CO2 Sensing

Using an embedded platinum silicide (PtSi) grating, a Schottky-barrier photodetector structure for narrow-band detection of mid-infrared (mid-IR) radiation is proposed and designed. Schottky-barrier photodetectors designed for the infrared wavelengths suffer from poor efficiency and photoresponse. We propose a photodetector based on PtSi-Si Schottky junction with increased interface area where hot carriers are more likely to cross the junction in favor of a higher photoresponse. Furthermore, a guided-mode resonance of the presented periodic structure is coupled to its Fabry–Perot resonance to achieve narrow-band absorption. The proposed photodetector absorbs 84% of incident power at a wavelength of 4.3 μm with a full-width half-maximum bandwidth of 94 nm. High photoresponsivity and compatibility with complementary metal oxide semiconductor process render the proposed structure a low-cost photodetector in the mid-IR range for CO2 sensing.

Silicon-on-insulator-based microwave photonic filter with widely adjustable bandwidth

We demonstrate a silicon-based microwave photonic filter (MPF) with flattop passband and adjustable bandwidth. The proposed MPF is realized by using a 10th-order microring resonator (MRR) and a photodetector, both of which are integrated on a photonic chip. The full width at half-maximum (FWHM) bandwidth of the optical filter achieved at the drop port of the 10th-order MRR is 21.6 GHz. The ripple of the passband is less than 0.3 dB, while the rejection ratio is 32 dB. By adjusting the deviation of the optical carrier wavelength from the center wavelength of the optical bandpass filter, the bandwidth of the MPF can be greatly changed. In the experiment, the FWHM bandwidth of the proposed MPF is tuned from 5.3 to 19.5 GHz, and the rejection ratio is higher than 30 dB.