Is the Halo laser a CO2 or an Erbium / Yag laser?

Halo is a hybrid ablative and non-ablative fractional laser. It is one part of a comprehesive quiver of lasers. Like any device, it has risk, and those will depend largely on who is operating it. To ensure you are receiving the highest level of care, seek out a modernly trained, new-school dermatologic surgeon, oculoplastic surgeon, facial plastic surgeon or plastic surgeon who is board certified and fellowship trained in one of these “core four” cosmetic specialties. Membership in organizations like the American Academy of Facial Plastic and Reconstructive Surgery help to identify a highly trained surgeon.Cameron Chesnut #realself hall of fame Physician 

The Halo Laser is a Hybrid Fractional laser which means that it it selectively treats a fraction of the skin in a way to limit the downtime and improve healing. This laser can target two different depths of the skin, both superficial and deep and improves a number of concerns including lines, wrinkles, pores, sun damage, pigmentation etc. This device works similarly to Fraxel and can be customized to each patients specific concern. 

Halo is a completely unique fractionated laser system that works to improve many, many skin conditions: brown spots, redness, fine lines, wrinkles, acne scarring and many many more. What makes this fractionated laser unique is that it combines a superficial ablative fractionated injury using erbium 2940nm wavelength with a deeper, non-ablative, 1470nm diode injury. Furthermore it stacks these 2 different injuries right on top of each other. No other laser available works or operates in this manner. Non-ablative lasers have been used for years with remarkable safety parameters. Halo, despite being more capable than older systems, is no different. It is remarkably safe for the great majority of patients so long as your provider understands how to use it and when its appropriate to modify the settings to increase safety. As with any laser, there are risks that should be discussed fully in consultation prior to treatment but "burns" in the sense that we discuss IPL or hair removal lasers are not really a part of that profile. The laser will not cause fat atrophy, it does not penetrate deep enough nor does it generate enough heat to harm fat. 

Halo is a tunable hybrid fractional laser that uses an ablative 2940 (erbium) and non ablative 1470 laser. The combination can help address skin texture, pigment, abnormal vessels, collagen and elastin atrophy and reverse UV damage.
It is safe to use on all skin types at the correct settings  and like any laser it is possible to burn someone at the wrong settings. Halo has a much wider window of safety then most lasers. It also has many of the same benefits as more intensive laser resurfacing without the same downtime or need for antibiotics. Halo cannot cause fat atrophy.  
The best way to determine if Halo is right for you is to schedule a consultation with a doctor specializing in aesthetic laser therapies and review your aesthetic concerns with them.