Characterization procedures for VECSELs can be categorized into two areas: the first area is characterization of a VECSEL gain mirror, while the second phase is characterization of VECSEL laser properties.
2.3.1 VECSEL gain mirror characterization
Upon growth, a VECSEL wafer undergoes material characterization in order to obtain information, such as wavelength and the intensity distribution of photoluminescence (PL) across a wafer, wafer reflectivity, surface roughness, crystal quality, etc. PL measurement is one of the most basic and useful procedures of evaluation of optical quality. PL wavelength is also measured during intermediate calibrations of growth parameters in order to optimize the operation wavelength in terms of detuning from the cavity resonance. Intensity of PL signal can be used for a post-growth material quality assessment, since PL intensity is usually proportional to semiconductor crystal quality. Generally speaking, assessment of PL signals can be made by collection from the front surface (PL measured in a growth direction) or from the edge of the semiconductor wafer (measured from a cleaved facet). The front PL is easier to measure using routine equipment but it is filtered by the DBR
and cavity resonance [Tropper et. al., 2006]. Therefore, edge PL is typically employed for a precise PL peak wavelength analysis of a VECSEL. Fig. 7 demonstrates comparison between front and edge PL measured from the same VECSEL gain mirror structure.
Figure 7. Normalized front PL and edge PL signals measured from the same VECSEL structure, described in [P2]. Due to presence of a microcavity resonator (created between DBR and air/semiconductor surface) the spectrum of front PL is narrower and shifted in comparison to the edge PL.
Another method of assessing structure on a presence of defects is PL signal imaging via mapping [Hein et. al., 2012]. Fig 8 demonstrates such PL mapping taken through an intracavity heat spreader bonded to a gain mirror surface. The excitation was done with a 532 nm collimated laser beam, which was filtered out from the photograph by means of a long-pass filter. Such a method renders possible surface quality investigation and inspection of the structure for the presence of dark lines and dark spots, which may originate due to strain or misfit dislocations.
Figure 8. A stitched CCD photograph of large area PL of a VECSEL surface taken through an aperture of copper mount. The VECSEL was excited with 532 nm unfocused beam, which was filtered out by means of a long-pass filter (600 nm), transmitting only PL. Dark spots/non-radiative recombination centers (left) can be seen as well as a dark line (bottom). Diagonal black and white fringes originate from interference inside a diamond heat spreader bonded to VECSEL surface.
Time-resolved PL (TRPL) is instrumental in carrier lifetime measurement, indirectly evidencing crystal quality of a structure [Cooley et. al., 1998]. Moreover, temperature-dependent reflectivity (TDR) is employed for determining an exact value of the micro cavity resonance. TDR is measuring the reflectivity of a structure as a function of temperature. This information of the exact location and spectral shift rate of the cavity resonance is essential for laser operation optimization under specific pumping and, therefore, temperatures. Fig. 9 demonstrates the TDR measurement of a structure, where the microcavity resonance was determined to be at 747 nm, and the lowest dip in the reflectivity curve was measured at a temperature of 75°C. In this particular case (AlGaAs QW VECSEL described in chapter 3), the TDR indicated that the detuning of the structure is negative, meaning that the microcavity resonance is located at the shorter wavelength in regards to the gain peak at the operating temperature, which brings additional losses to the laser operation.
Figure 9. Temperature dependent reflectivity measurement of a VECSEL structure (described in chapter 3). In this particular case, micro cavity resonance was measured at ~747 nm (marked with a dot).
Additional techniques used routinely in semiconductor fabrication include: scanning electron microscopy (SEM) imaging of a cross-section of a VECSEL for assessing the layer sharpness and precise measurement of the layers thicknesses (see Fig. 23), atomic force microscopy (AFM) for imaging the surface morphology and for measuring wafer roughness (see Fig. 36), and X-ray diffraction (XRD) for examining the crystal quality and estimation of the accumulated strain [Ranta et. al., 2011].
2.3.2 Characterization of VECSEL laser operation
Lasing parameters of VECSELs can be categorized into two subclasses: i) tailorable parameters and ii) the intrinsic features of the gain mirror. Output power and tuning can be referred to as the tailorable features, since various design techniques can be applied in order to adjust these parameters. For example, output power can be increased via increasing the pumping spot area. In that case, thermal roll-over will be the limiting factor for further output power increase, which is dictated by the heat transfer dynamics inside the active region and the heat spreader. Thus, the thermal roll-over can be thought of as an intrinsic parameter of the gain mirror (although this is also affected by the heat spreader technology). Furthermore, an optimal
output coupling ratio plays an important role in achieving the highest output power values. Thus, several output coupling ratios are typically tested in order to find out an optimal value for a particular structure and parameters. Combined intrinsic optical losses of a VECSEL can be estimated by means of the Findlay-Clay analysis [Findlay et. al., 1966] or the Caird plot [Hartke 2008]. In addition, active loss mechanisms can be detected, for instance, parasitic or lateral lasing that can occur under high pumping powers in a lateral in-growth-plane resonator, which substantially decreases lasing efficiency due to carrier depletion [Chernikov et.
al., 2010]. Lateral parasitic lasing can be detected either by means of a CCD camera, or by optical spectrum measurements. Lateral lasing usually occurs at longer wavelengths, for which an un-pumped gain region is transparent. Lateral lasing is detectable by macrophotography during operation.
Laser spectra can be seen both a tailorable and intrinsic feature since free-running spectrum is defined by the intrinsic parameters (e.g. bandgap), but at the same time is susceptible to changes in a temperature and pump power or to a presence of intracavity elements, such as, etalons or birefringent filters.
Another example of tailorable VECSEL parameter with high relevance for application is the laser beam divergence or beam quality, described by the M2 factor.
Since the lowest order transverse laser mode possess the lowest divergence (TEM00), measured M2 factor can be used for the characterization of the transverse mode content of the laser output. As a rule of thumb, laser beams with M2 factor lower than 1.5 are composed from a single transverse mode, or fundamental mode. Due to an uncomplicated mode adjustment of a VECSEL, laser operation can be easily changed from fundamental mode to multimode operation, thus increasing the M2
factor.