Developing a New Generation of Integrated Micro-Spec Far Infrared Spectrometers for the EXperiment for Cryogenic Large-Aperture Intensity Mapping (EXCLAIM)

TL;DR

This paper presents the design, fabrication, and development of advanced integrated micro-spec far-infrared spectrometers for the EXCLAIM space mission, featuring improved spectral resolution and integration on a superconducting photonic chip.

Contribution

It introduces a new high-resolution $ m bc$-Spec spectrometer design with integrated detection, suitable for space-based far-infrared astronomy, advancing beyond previous prototypes.

Findings

Spectrometers cover 555-714 μm range with R=512 at 638 μm.

Fabrication involves flip-wafer-bonding on a 150 mm Si wafer.

Designed to operate at 100 mK with 355 Al KID detectors.

Abstract

The current state of far-infrared astronomy drives the need to develop compact, sensitive spectrometers for future space and ground-based instruments. Here we present details of the $\rm \mu$-Spec spectrometers currently in development for the far-infrared balloon mission EXCLAIM. The spectrometers are designed to cover the $\rm 555 - 714\ \mu$m range with a resolution of $\rm R\ =\ \lambda / \Delta\lambda\ =\ 512$ at the $\rm 638\ \mu$m band center. The spectrometer design incorporates a Rowland grating spectrometer implemented in a parallel plate waveguide on a low-loss single-crystal Si chip, employing Nb microstrip planar transmission lines and thin-film Al kinetic inductance detectors (KIDs). The EXCLAIM $\rm \mu$-Spec design is an advancement upon a successful $\rm R = 64\ \mu$-Spec prototype, and can be considered a sub-mm superconducting photonic integrated circuit (PIC) that combines spectral dispersion and detection. The design operates in a single $M{=}2$ grating order, allowing one spectrometer to cover the full EXCLAIM band without requiring a multi-order focal plane. The EXCLAIM instrument will fly six spectrometers, which are fabricated on a single 150 mm diameter Si wafer. Fabrication involves a flip-wafer-bonding process with patterning of the superconducting layers on both sides of the Si dielectric. The spectrometers are designed to operate at 100 mK, and will include 355 Al KID detectors targeting a goal of NEP ${\sim}8\times10^{-19}$ $\rm W/\sqrt{Hz}$. We summarize the design, fabrication, and ongoing development of these $\rm \mu$-Spec spectrometers for EXCLAIM.