An optical humidity sensor: A compact photonic chip integrated with artificial opal

Highlights

• The compact photonic chip has a small size and provides two essential functions of light emission and photodetection.

• The optical sensor has a highly linear and rapid response in a large humidity detective range of RH 3.8−90%.

• The optical humidity sensor is cost-effective and can be applied to in-situ monitoring of human respiration rate.

Abstract

A novel, miniaturized and easy-to-use optical sensor enabling accurate humidity measurement in real-time has been proposed in current study. This is achieved by the compact integration of a photonic chip with an artificial opal layer. The III-nitride chip with a size of 1 × 1 × 0.21 mm³ simultaneously provides two essential functions, i.e., photoemission and photodetection. The integrated opal film is comprised of silica powders with a hydrophilic nature, and its humidity-dependent light reflection can be precisely monitored by the on-chip detector without any aids of external optics. The optical sensing device shows a highly linear response in the relative humidity (RH) range of 3.8–90% and a sensitivity of 0.046–0.051 μA/%RH. The real-time monitoring function of the sensor has been demonstrated by mounting it on a facial mask which is used under various human breathing conditions. With the advantages of its compact size, ease of operation, high linearity and repeatability, large dynamic range, and fast response time, this cost-effective chip-scale optical sensor shows great potential in a wide range of practical humidity-sensing applications.

Introduction

Humidity monitoring is of crucial importance for a vast range of industrial and commercial applications, such as food processing, pharmaceutical preparation, air conditioning, agriculture, and electronic manufacturing. Recently, increasing attention has been paid to in-situ monitoring of human respiration rate by tracking the humidity in the inhaled and exhaled air [1], [2], [3], [4]. For instance, the frequency and depth of humidity-related breathing signals can be analyzed to provide potential assistance in health assessment, such as noninvasive diagnosis of sleep apnea, asthma, and cardiac arrest [5], and physiological monitoring [6]. To achieve this goal, a humidity sensor with rapid response and high sensitivity is desired. Optical sensing based on fiber optics is an effective means to detect the variation of refractive index induced by humidity changes [7], [8], [9], [10]. To enhance the sensitivity of the optical sensor to humidity, lots of efforts have been devoted to surface modification and a variety of humidity-sensitive coating layers has been reported, such as gelatin [11], [12], polyvinyl alcohol [13], [14], agarose gel [15], [16], metal oxides [17], [18], and polymers [19], [20].

Attributed to the ease of one-step integration and the large-area manufacturability, artificial opal is a promising photonic material for humidity sensing. Typically, artificial opals are solid colloidal crystals composed of self-assembled spheres of uniform size [21], [22], or their reversed replicas as inverse opals [23], [24]. They behave as photonic crystals, in which the stop-band prohibits the propagation of light in a specific frequency range and shifts according to changes in relative humidity (RH). Despite their recent advances in rapid response and high sensitivity, the quantitative analysis of RH readings still heavily relies on expensive and bulky spectrometers to determine the spectral shifts [25], [26]. In addition, effective light coupling involves the assembly of optical devices with external components and precise optical alignment is required. Therefore, the resulted bulky system may be difficult for large-scale manufacturing and high-density integration.

With the growing demand for portable devices in lab-on-a-chip and wearable applications, there is a great need to develop a miniaturized and low-cost humidity sensor by eliminating external components. In this work, we propose a fully integrated chip-scale humidity sensor based on the integration of an opal film onto a III-nitride chip. Noticeably, GaN and its alloys have been considered to be ideal materials for constructing light-emitting devices because of their high efficiency, high stability, and long lifespan [27], [28], [29]. In addition to the light emitter, another indispensable light detector is integrated on the same chip through a monolithic design. Covered on the chip surface, the opal film is composed of amorphous silica particles with the hydrophilic feature which is conducive to the rapid absorption of water vapor. The reflectance characteristic of opal film is investigated to verify the feasibility of the chip-scale optical sensor for rapid probing of surrounding RH.