**1. Introduction**

64 Photonic Crystals – Innovative Systems, Lasers and Waveguides

Zhang H. Y. ; Zhang Y. P. ; Wang P. et al. (2007). *Frequency response in photonic* 

101, No. 1, pp. 013111, 2007

*heterostructures consisting of single-negative materials*. Journal of Applied Physics, Vol.

The mid-infrared (mid-IR) region, covering the electromagnetic spectrum from about 2.5 to 25 μm wavelength, is of unique interest for many applications, especially for molecular spectroscopy, because the vibrational frequencies of almost all target molecules and hydrocarbons are in this spectral region, such as NO, NO2, CO, CO2, HF, and CH4. The specific application areas for molecular spectroscopy include scientific research, vehicle exhaust investigation, atmospheric pollution monitoring, medical diagnostics and biological & chemical weapon detection. Besides molecular spectroscopy applications, there are also some other potential applications, such as free space optical communications, etc. (Tittel, Richter, & Fried, 2003)

For all these applications, typical requirements for mid-IR lasers are manifold. First of all, to reach high spectral selectivity performance, a narrow spectral linewidth is needed. Secondly, optical output power should be larger than 100 μW to lift detector noise limits. (Tacke M. , 2001) Thirdly, high beam quality is favorable for optimum coupling with the gas sampling cell. Also, room temperature or thermoelectric-cooled operation is desirable, considering the additional weight, volumes and costs with the additional cryogenic cooling equipment. Furthermore, a compact set-up is preferable for hand-held *in situ* measurement in the field.

Two dimensional (2D) photonic crystal lasers, potentially have great practical applications in the mid-IR region due to their unique features in, for example, surface emission, circular beams, low threshold operation, miniaturization and simplicity of on-chip monolithic integration, etc. The studies of surface emitting 2D photonic crystal lasers started in and have expanded worldwide since the middle of the 1990s. The first 2D photonic crystal semiconductor laser of 1.55 μm emission peak was demonstrated in 1999. (Painter, et al., 1999) After that, this type of semiconductor lasers has been developed rapidly in near IR region. (Park, Hwang, Huh, Ryu, & Lee, 2001; Loncar, Yoshie, Scherer, Gogna, & Qiu, 2002; Altug, Englund, & Vuckovic, 2006) Unfortunately, mainly restricted by the etch-induced surface recombination, the research development of 2D photonic crystal semiconductor lasers in mid-IR range of electromagnetic spectrum was relatively slow. Even so, due to their intrinsic advantages addressed previously, this type of lasers operating in mid-IR range is

Mid-Infrared Surface-Emitting Two Dimensional Photonic Crystal Semiconductor Lasers 67

Fortunately, 2D photonic crystal technology allows quantum cascade lasers to achieve surface emission. The first mid-IR surface emitting 2D photonic crystal QCL was realized in 2003. (Colombelli F. , et al., 2003) In this demonstration, the 2D arrays were fabricated with the values of lattice spacing *a* from 2.69 to 3.00 μm, and the ratio of holes radius *r* to *a* from 0.28 to 0.32. As presented in figure 2B and figure 2C, the etched photonic crystal pattern penetrated through the active region down into the bottom substrate, which reduces the diffraction of radiation into the substrate and consequently provides strong in-plane optical feedback. Usually, in diode lasers, deep-etching induced surface recombination would substantially increase the leakage currents. (Colombelli F. , et al., 2003) But this serious

After processing, electroluminescence (EL) measurements were conducted at a temperature of 10 K and the light spectra were acquired using a Nicolet Fourier transform infrared (FTIR) spectrometer and a nitrogen-cooled HgCdTe detector. From the measurement, for the designed structure with a set of values of a and r having the A, B, and C flat-band regions overlapping the gain spectrum, three line-narrowing emission peaks emerge from the EL

It is worth to mention that, no defect mode emission has been demonstrated, because of the little difference observed between the emission spectra with and without a central defect design. (Colombelli F. , et al., 2003) Therefore, the device should be classified as the bandedge mode photonic crystal lasers. Simply explaining, the optical gain is intensified by reducing the group velocity of light, supported by a basic relation about light amplification

*<sup>g</sup> gNv* ,

where γ is the net stimulated-emission rate, g is the optical gain, N is the photon density, and vg is the group velocity of light. It has been proven that the gain enhancement is strongly correlated with the group velocity, which decreases rapidly in the vicinity of the

Fig. 2. (A) Theoretical simulation of 2D in-plane transverse magnetic (TM) photonic band structure. The flat-band region (A, B, and C) are indicated by dark grey horizontal bands. The red thick line indicates the highly localized defect modes. (B) Top-view scanning electron microscope (SEM) image of an etched device with a central defect. (C) Cross-section

SEM of a portion of a photonic crystal QC device. (Colombelli F. , et al., 2003)

obstacle does not affect in this case, because QCL are unipolar devices.

spectrum, as shown in Figure 3A.

in which (H.C. Casey & Panish, 1978)

photonic band edges. (Nojima, 1998)

still very desirable. Therefore, some major steps of the progress are very important and need to be paid close attention nowadays.

In this chapter, we will review the development of the mid-IR surface emitting 2D photonic crystal semiconductor lasers in recent decade. The performance of the reported mid-IR 2D photonic crystal lasers with different structure design and different material system is going to be introduced and compared. Their major advantages and disadvantages will also be discussed.
