From Nanometers to Light Years: Exploration of the Early Universe with Gamma-ray Bursts and Development of Photonic Spectrographs for Astronomy

Recent space- and ground-based studies of the circumgalactic medium (CGM) around nearby galaxies have revealed the dynamic interplay between the galaxy ecosystem and surrounding CGM using bright background quasars. In this thesis, we extend this investigation to higher redshifts by using the bright afterglows of gamma-ray bursts (GRBs) as background sources probing the CGM of their own host galaxies. We compiled a sample of 27 high-resolution (R $>$ 6000) rest-frame UV spectra of GRB afterglows in a redshift range (2 $\lesssim$ z $\lesssim$ 6); we call this the `CGM-GRB sample'. We find stronger blue wings in high-ionization species (Si IV, C IV) compared to the low-ionization species (Si II, Fe II), indicative of the presence of ubiquitous warm outflows in the GRB hosts at high redshifts. Using kinematic models, we estimated typical values of CGM properties (for the sample) such as CGM mass (10$^{9.8}$ $\mathrm{M_{\odot}}$) and outflow launch velocity (300 km s$^{-1}$). Further, by comparing our results with previous C IV absorption studies, we find a possible CGM-galaxy co-evolution. Over the course of evolution of present-day galaxies with $\mathrm{M_{*}}$ $>$ $10^{10}$ $\mathrm{M_{\odot}}$, the ratio of C IV mass in the CGM to the stellar mass remains fairly uniform, such that log$\mathrm{(M_{C IV}/M_{*})} \sim -4.5$ within $\pm$0.5 dex from $z\sim4$ to $z\sim0$, suggesting CGM-galaxy co-evolution. Next, we embarked on a search for possible relations between the outflow properties and those of the host galaxies such as $\mathrm{M_*}$, star formation rate (SFR), and specific SFR (= SFR/$\mathrm{M_*}$). To estimate the total SFR, we first investigated the degree of dust obscuration in the massive GRB hosts in our sample by comparing radio- and UV-based star-formation rates. We inferred that the GRB hosts in our sample are not heavily dust obscured, and hence, their SFRs can be estimated reliably using the established dust-correction methods. For the outflow-galaxy correlations, we focused on three outflow properties $-$ outflow column density ($\mathrm{N_{out}}$), maximum outflow velocity ($\mathrm{V_{max}}$), and normalized maximum velocity ($\mathrm{V_{norm}}$ = $\mathrm{V_{max}/V_{circ, halo}}$, where $\mathrm{V_{circ, halo}}$ is the halo circular velocity). We observe clear trends of $\mathrm{N_{out}}$ and $\mathrm{V_{max}}$ with increasing SFR in high-ion-traced outflows. These correlations indicate that these high-ion outflows are driven by star formation at these redshifts (in the mass range $\mathrm{log(M_*/M_{\odot})}$ $\sim$ $9-11$). We also, for the first time, observe a strong ($>$ 3$\sigma$) trend of normalized velocity decreasing with halo mass and increasing with sSFR at high redshifts, suggesting that outflows from low-mass halos and high sSFR galaxies are most likely to escape and enrich the outer CGM and IGM with metals. Thus, we demonstrate GRB afterglows as a method to uncover CGM-galaxy co-evolution and outflow-galaxy correlations at high redshifts, which constitute an important piece of the galaxy growth puzzle and cosmic metal enrichment. Next, we set out to develop a new tool $-$ an on-chip photonic spectrograph $-$ which will eventually expand our investigation to the first galaxies in the universe ($z > 6$). Astrophotonics is the application of versatile photonic technologies to channel, manipulate, and disperse guided light to efficiently achieve various scientific objectives in astronomy in a miniaturized form factor. We used the concept of arrayed waveguide gratings (AWG) to develop an on-chip photonic spectrograph in the H band ($1.45-1.65$ $\mu m$) with a moderate resolving power of $\sim$1500, a peak throughput of $\sim$23\%, and a size of only 1.5 cm $\times$ 1.5 cm. Various practical aspects of implementing AWGs as astronomical spectrographs are also discussed, including a) the coupling of the light between the fibers and AWGs, b) cleaving at the output focal plane of the AWG to provide continuous wavelength coverage, and c) a multi-input AWG design to receive light from multiple single-mode fibers at a time and produce a combined spectrum. Finally, we built a cross-dispersion setup which will orthogonally separate the overlapping spectral orders in the AWG and thus image the full spectrum on the detector. The AWG will be incorporated with this setup in the near future to get the spectrograph ready for our first on-sky test. The work conducted in this thesis is a crucial stepping stone towards building a high-throughput, miniaturized spectrograph for the next generation of ground-, balloon-, and space-based telescopes. With the nano-scale fabrication on a chip, we are poised to unravel the mysteries of galaxies billions of light years away, making this thesis a truly \textit{`Nanometers to Light Years'} journey

Development of high resolution arrayed waveguide grating spectrometers for astronomical applications: first results

Astrophotonics is the next-generation approach that provides the means to miniaturize near-infrared (NIR) spectrometers for upcoming large telescopes and make them more robust and inexpensive. The target requirements for our spectrograph are: a resolving power of about 3000, wide spectral range (J and H bands), free spectral range of about 30 nm, high on-chip throughput of about 80% (-1dB) and low crosstalk (high contrast ratio) between adjacent on-chip wavelength channels of less than 1% (-20dB). A promising photonic technology to achieve these requirements is Arrayed Waveguide Gratings (AWGs). We have developed our first generation of AWG devices using a silica-on-silicon substrate with a very thin layer of silicon-nitride in the core of our waveguides. The waveguide bending losses are minimized by optimizing the geometry of the waveguides. Our first generation of AWG devices are designed for H band and have a resolving power of around 1500 and free spectral range of about 10 nm around a central wavelength of 1600 nm. The devices have a footprint of only 12 mm x 6 mm. They are broadband (1450-1650 nm), have a peak on-chip throughput of about 80% (-1 dB) and contrast ratio of about 1.5% (-18 dB). These results confirm the robustness of our design, fabrication and simulation methods. Currently, the devices are designed for Transverse Electric (TE) polarization and all the results are for TE mode. We are developing separate J- and H-band AWGs with higher resolving power, higher throughput and lower crosstalk over a wider free spectral range to make them better suited for astronomical applications.Comment: 12 pages, 13 figures, 3 tables. SPIE Astronomical Telescopes and Instrumentation, Edinburgh (26 June - 1 July, 2016

Development of an integrated near-IR astrophotonic spectrograph

Here, we present an astrophotonic spectrograph in the near-IR H-band (1.45 -1.65 μm) and a spectral resolution (λ/δλ) of 1500. The main dispersing element of the spectrograph is a photonic chip based on Arrayed-Waveguide- Grating technology. The 1D spectrum produced on the focal plane of the AWG contains overlapping spectral orders, each spanning a 10 nm band in wavelength. These spectral orders are cross-dispersed in the perpendicular direction using a cross-dispersion setup which consists of collimating lenses and a prism and the 2D spectrum is thus imaged onto a near-IR detector. Here, as a proof of concept, we use a few-mode photonic lantern to capture the light and feed the emanating single-mode outputs to the AWG chip for dispersion. The total size of the setup is 50 cm x 30 cm x 20 cm³, nearly the size of a shoebox. This spectrograph will pave the way for future miniaturized integrated photonic spectrographs on large telescopes, particularly for building future photonic multi-object spectrographs

Development of an integrated near-IR astrophotonic spectrograph

Here, we present an astrophotonic spectrograph in the near-IR H-band (1.45 -1.65 μm) and a spectral resolution (λ/δλ) of 1500. The main dispersing element of the spectrograph is a photonic chip based on Arrayed-Waveguide- Grating technology. The 1D spectrum produced on the focal plane of the AWG contains overlapping spectral orders, each spanning a 10 nm band in wavelength. These spectral orders are cross-dispersed in the perpendicular direction using a cross-dispersion setup which consists of collimating lenses and a prism and the 2D spectrum is thus imaged onto a near-IR detector. Here, as a proof of concept, we use a few-mode photonic lantern to capture the light and feed the emanating single-mode outputs to the AWG chip for dispersion. The total size of the setup is 50 cm x 30 cm x 20 cm³, nearly the size of a shoebox. This spectrograph will pave the way for future miniaturized integrated photonic spectrographs on large telescopes, particularly for building future photonic multi-object spectrographs

Flattening laser frequency comb spectra with a high dynamic range, broadband spectral shaper on-a-chip

Spectral shaping is critical to many fields of science. In astronomy for example, the detection of exoplanets via the Doppler effect hinges on the ability to calibrate a high resolution spectrograph. Laser frequency combs can be used for this, but the wildly varying intensity across the spectrum can make it impossible to optimally utilize the entire comb, leading to a reduced overall precision of calibration. To circumvent this, astronomical applications of laser frequency combs rely on a bulk optic setup which can flatten the output spectrum before sending it to the spectrograph. Such flatteners require complex and expensive optical elements like spatial light modulators and have non-negligible bench top footprints. Here we present an alternative in the form of an all-photonic spectral shaper that can be used to flatten the spectrum of a laser frequency comb. The device consists of a circuit etched into a silicon nitride wafer that supports an arrayed-waveguide grating to disperse the light over hundreds of nanometers in wavelength, followed by Mach-Zehnder interferometers to control the amplitude of each channel, thermo-optic phase modulators to phase the channels and a second arrayed-waveguide grating to recombine the spectrum. The demonstrator device operates from 1400 to 1800 nm (covering the astronomical H band), with twenty 20 nm wide channels. The device allows for nearly 40 dBs of dynamic modulation of the spectrum via the Mach-Zehnders , which is greater than that offered by most spatial light modulators. With a superluminescent diode, we reduced the static spectral variation to ~3 dB, limited by the properties of the components used in the circuit and on a laser frequency comb we managed to reduce the modulation to 5 dBs, sufficient for astronomical applications.Comment: 15 pages, 10 figures. arXiv admin note: substantial text overlap with arXiv:2209.0945