**Photonic Crystal Laser Based Gas Sensor**

Marcus Wolff, Henry Bruhns, Johannes Koeth, Wolfgang Zeller and Lars Naehle

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/57147

#### **1. Introduction**

concentration and Determination of Zinc Traces with Flame Atomic Absorption

[11] Puszta Z.I. In situ NMR Spectroscopic Observation of a Catalytic Intermediate in Phosphine – Catalizer Cyclo-Oligomerization of Isocyanates. Angewandte Chemie

[12] Davletbaeva I.M., Shkodich V.F., Ekimova E.O., Gumerov A.M. The Study of Polyi‐ socyanate Groups Opening Initiated by Polyoxyethylene glycolate of Potassium.

[13] Davletbaeva I.M., Shkodich V.F., Gumerov A.M., et al. Intermolecular Interactions in Metal Containing Polymers Based on 2,4-toluene diisocyanate and Open-Chain Ana‐ logues of Crown Ethers. Journal of Polymer Science Part A: Polymer Chemistry 2010;

[14] Davletbaev R.S., Akhmetshina A.I., Avdeeva D.N., Gumerov A.M., Davletbaeva I.M. Investigation of Features of Interaction of Anionic Macroinitiators with 2,4-toluene diisocyanate. Vestnik of Kazan State Technological University Journal 2012; 20

[15] Davletbaev R.S., Akhmetshina A.I., Gumerov A.M., Sharifullin R.R., Davletbaeva I.M. The Influence of Solvent Nature on the Mechanism of the Reaction of Anionic Macroinitiators and Aromatic Isocyanates. Butlerov Communications 2013; 9(35)

[17] Savvin S.B., Dedkova V.P., Shvoeva O.P. Sorption-spectroscopic and Test Methods for the Determination of Metal Ions on the Solid-phase of Ion-exchange Materials.

[18] Costela A., Garcia- Moreno I., Sastre R. Materials for Solid-state Dye Lasers.. In: H. S. Nalwa (ed.) Handbook of Advanced Electronic and Photonic Materials and Devices.

[19] Hamdan A.S. Al-Shamiri, Maram T.H. Abou Kana, Azzouz I.M., Badr Y.A. Photophysical properties and quantum yield of some laser dyes in new polymer host. of

[20] Burger K. Organic Reagents in Metal Analysis. Elsevier Science & Technology; 1973.

[16] Tager A. Physical Chemistry of Polymers. Moscow: Mir Publishers; 1972.

Russian Chemical Reviews 2000; 3(69) 187-201.

San Diego: Academic Press; 2001. p161-208.

some laser. Optics & Laser Technology 2009; 4(41) 415-418.

Journal of Polymer Science Part A: Polymer Chemistry 2007; 8(45) 1494-1501.

[10] Szycher M., editor. Szycher's Handbook of Polyurethanes. CRC Press; 2012.

Spectrometry. Talanta 2002; 8(56) 491-498.

66 Optical Sensors - New Developments and Practical Applications

International Edition 2006; 1(45) 107-110.

4(48) 592-598.

131-133.

9-13.

The development of new radiation source technologies has a major impact on the progression of optical trace gas detection [1]. Especially semiconductor diode lasers have proven to be extraordinarily suitable devices for spectroscopic sensors. Their small size and their low acquisition cost are here valuable properties. However, it is particularly advantageous that their emission can spectrally be tuned simply via their operating temperature and operating current. Furthermore, diode lasers can be directly modulated via their injection current. Therefore, they represent particularly suitable radiation sources for photoacoustic spectrosco‐ py because this technique is based on the absorption of modulated radiation and its transfor‐ mation into a sound wave. As an offset-free technique it enables extremely high detection sensitivity [2].

Continuous-wave (cw) single-frequency diode lasers, like distributed feedback (DFB) lasers, are particularly suitable for spectroscopy because they avoid any cross-sensitivity and enable very selective gas detection [3,4]. DFB devices were originally developed for the telecommu‐ nication industry and can conveniently be operated at room temperature. Meanwhile, available emission wavelengths cover the entire near-infrared spectral range (800 nm – 3000 nm). Most recently, DFB lasers operating in the mid-infrared were introduced (> 3000 nm) [5]. The alternative concept of interband cascade lasers (ICL) covers almost the complete midinfrared from 3 µm to 6 µm [6]. This wavelength range is extraordinarily important for trace gas detection since many molecules have their strong fundamental vibrational absorption bands in this region, enabling extremely high detection limits. These devices close the gap to quantum cascade lasers (QCL), which are currently available with single-frequency emission wavelengths starting around 4 µm (multimode at 3 µm) [7].

© 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

However, semiconductor lasers suffer from the considerable weakness that their spectral tuning range is limited to a few nanometers only [4]. As a result of that, the analysis of a gas mixture typically requires the use of a separate laser for each gas component. This makes tunable diode laser spectroscopy expensive and too complex for many applications. A wider spectral tuning range would solve the problem and is, therefore, highly desirable for further practical applications.

Photons in a material with a periodic dielectric constant (index of refraction) experience similar effects. By varying the lattice parameters of these so called photonic crystals, photonic bandgap structures can be obtained that prohibit the propagation of light into the crystal structure. This effect can be used to define optical waveguide structures with very strong lateral light confinement compared to conventional ridge waveguides (of the order of the wavelength) as

Photonic Crystal Laser Based Gas Sensor http://dx.doi.org/10.5772/57147 69

In order for a photon to interact with its periodic environment, its wavelength must be comparable to the periodicity of the lattice. For visible to near-infrared radiation the lattice constant must be in the range of 100 nm to 1 µm. Figure 1 depicts a three-dimensional schematic diagram of photonic crystal structures (courtesy of nanoplus Nanosystems and Technologies

The photonic crystal laser we are employing in our optical gas sensor is based on an InAs/ InGaAs quantum dash-in-a-well structure grown on InP substrate by gas source molecu‐ lar beam epitaxy. A short period superlattice structure is used for photon confinement. Due to their wide spectral gain bandwidth dash-in-a-well structures are particularly well suited for wavelength tuning applications. The photonic crystal is realized by air-semiconductor structures defined by electron beam lithography and etched into the laser structure by an inductively coupled plasma etch step. The structure comprises a hexagonal lattice with a lattice period of 485 nm and an air fill factor of 30%. The air-filled holes in the semiconduc‐ tor material exhibit a diameter of 350 nm and a depth of 3 µm. These lattice parameters are chosen so that the middle of the photonic bandgap coincides with the device's emission wavelength. The photonic crystal structures are forming multiple lateral and longitudinal photon confinements as well as two independently contacted laser segments. A wave‐ guide is defined by seven rows of missing holes in a ГK oriented photonic crystal. The photonic crystal mirror at the rear end of the laser diode is oriented in ГM direction in order to present a smooth boundary to the traveling light mode, thus, reducing scattering losses. Coupling of the two laser cavities is provided by a photonic crystal coupling section consisting of two rows of etched holes. The output mirror is formed by a single row of holes. A scanning electron micrograph of the rear, middle and front part of the photonic crystal laser fabricated by nanoplus Nanosystems and Technologies GmbH in Gerbrunn,

The photonic crystal laser integrates two longitudinally coupled Fabry-Pérot (FP) resonators. The main advantage of this principle called coupled cavities is that sophisticated grating structures which are difficult and expensive to implement are avoided. Instead, due to the inverse length dependence of mode spacing in a FP resonator, different mode spacings can be realized by implementing different cavity lengths in each segment. Table 1 lists the lengths and according mode spacings of the two photonic crystal laser segments. The coupled cavity

well as optical filters and wavelength selective reflectors [9,10].

GmbH). The yellow blocks on top represent contacts.

Germany can be seen in Figure 2 [11,12].

laser diode is emitting radiation in the 1.9 µm wavelength range.

*2.1.2. Device structure*

This study presents a new photoacoustic gas detection scheme based on a widely tunable coupled-cavity photonic crystal laser operating around 1900 nm. Chapter 2 covers the structure and design of this custom-made room temperature continuous-wave laser followed by measurements of the unique performance characteristics. The new device was designed to enable sensitive detection of water vapor (H2O). Chapter 3 describes the photonic crystal laser based gas sensor and presents photoacoustic measurements of water spectra. The concluding Chapter 4 summarizes the results.
