Holmium is a rare earth metal or lanthanide, twenty times more abundant than silver in nature. It has properties similar to erbium, another lanthanide. In its elemental form, the element is a silvery-white metal, soft and malleable. Pure holmium is slowly attacked by water and oxygen and dissolves in acids. It is stable at room temperature and in dry air.
But like other rare-earth metals, Holmium isn’t found in its pure form in the environment. In nature, Holmium largely exists in trivalently oxidized forms, with Ho3+ ions, in minerals such as gadolinite and monazite. These ions have fluorescent properties, just like erbium and other rare-earth ions. This optical fluorescence is very useful in some laser applications.
Holmium is also used in the strongest magnets and as control rods in nuclear reactors. Holmium reserves are estimated to be around 400,000 tonnes. Global annual production is around 10 tonnes. The main mining areas include India, China, Brazil, Australia, USA and Sri Lanka.
Element or Material | Holmium, Ho3+ |
Host crystal | Ya3Al5O12 |
Laser type | Solid |
Co-dopants | Cr3+, Tm3+ |
Lasing output | 2.1 micrometer |
Upper laser level | 5/7 |
Lower laser level | 5/8 |
PHYSICAL AND CHEMICAL PROPERTIES OF HOLMIUM | |
Atomic number | 67 |
Atomic mass | 164.93 g.mol-1 |
Molecular formula | Ho |
Oxidation state | 3+ |
Radiation wavelength of Ho3+ | 2100nm |
Appearance | Silvery |
Crystal structure | Hexagonal close-packed |
Melting point | 1474 degrees C |
Boiling point | 2695 degrees C |
Density (room temp.) | 8.8 g.cm-3 at 20 degrees C |
Electronegativity | 1.23 Paulings |
Electrical resistivity | 87 microhm-cm at 25 degrees C |
Thermal conductivity | 0.162 W.cm.K at 298.2 Kelvin |
Thermal expansion (at room temp.) | poly: 11.2 mu m/(m.K) |
Young’s modulus | 64.8 GPa |
Vickers hardness | 481 MPa |
Brinell hardness | 746 MPa |
Odor | Odorless |
What are Holmium rods?
Holmium rods are solid state laser gain media that may be co-doped with elements like Chromium and Thulium. Holmium-doped yttrium lithium fluoride (YLF), yttrium aluminum garnet (YAG) and yttrium iron garnet (YIG) are popularly used in laser systems. Both Ho:YAG single crystals and ceramics are widely used in applications.
Holmium lasers emit at 2.1 micrometer. This eye-safe wavelength and high transmittance in the atmosphere makes it attractive in industrial applications. The easy absorption of Ho3+ ions in water make holmium lasers suitable for medical applications.
Ho:YAG (doped from 0.25% up to 2%) is particularly promising in ultra-short-pulse and tunable lasers. It can be pumped with laser diodes or thulium-based fiber laser systems in a broad absorption band with a peak at 1907 nm.
Ho:YAG also has a low quantum defect, which relates to low energy loss. As a result, it’s desirable for high-power lasers. Low energy loss equates with small heat loads. Ho:YAG’s attractive thermochemical properties adds to the list and makes scaling high power outputs easier.
For efficiency in laser operation at 2.1 micrometers, Holmium can only be exploited directly at around 1.9 micrometer. Until recently, it was difficult to find laser diodes in the right wavelength ranges to directly pump Ho3+ ions. As a result, early holmium laser systems were co-doped with thulium. When excited, Thulium ions would help to further excite Holmium ions.
In recent years, thulium fibre lasers have become popular for pumping holmium ions in Ho:YAG and Ho:YLF.
Advanced manufacturing technique
The rapid growth and precise machining of large-sized crystals with high laser damage thresholds (LDT) are tough challenges in the laser manufacturing industry. LDT or LIDT (Laser induced damage threshold) as defined in the ISO 21254 is the highest fluence of laser radiation on the optical component for which the probability of damage is zero. Especially in higher power applications such as in surgery, it is important to use high quality, high LDT holmium laser rods so that the laser system is not compromised.
One of the problems of lasing in the 2 microns wavelength range is poor lasing efficiency. At room temperature, the thermal population of the upper laser level of the holmium laser is relatively low. In YAG at room temperature, only 10% of holmium ions excited to 5I7 populate the Stark level or the upper laser level. The number is 46% for thulium, which also has a lower temperature dependence. Co-doping with thulium offers the benefits of the cross-relaxation process of thulium ions. Slightly doping with Chromium leads to better luminescence properties of Cr3+ ions.
Manufacturers must develop advanced proprietary crystal growing technology to overcome these poor lasing efficiencies. Decades of fabrication and polishing experience leads to the development of low absorbing, high damage threshold optical coatings.
Advanced manufacturing techniques in crystal growth and anti-reflective coatings help us to design YAG laser crystals doped with holmium and other dopants like chromium and thulium. Low energy of the wavelength is usually compensated by proprietary processes that yield better efficiency. This technology allows the development of crystals with low scattering losses, high optical quality and high efficiency. Holmium co-doped YAG rods are available at lengths up to 150 mm long.
Application of Holmium rods
Solid state lasers doped with Ho3+ have several potential uses: medical applications, LIDAR ranging applications, light detection, eye-safe rangefinders, and pump sources for optical parametric oscillators (OPO). The 2 micron wavelength is eye-safe and has properties of transmission through the atmosphere that make it attractive for a number of uses.
The Ho:YAG laser is a solid state pulsed laser absorbed by water and any tissue that contains water. This makes it highly useful in medical and dental applications. The laser is minimally invasive. Like traditional CO2 lasers, Ho:YAG lasers offer precise cutting with minimal damage to surrounding tissues. These lasers can be used to remove wrinkles, scars, birthmarks, warts, sun damage and other skin conditions.
These solid state lasers can be used in liquid-filled environments like blood and saline. Holmium lasers are being increasingly used for incisions of the upper urinary tract, for prostate ablations, dissolving stones, etc.
Ho:YAG also has an edge over traditional lasers since it can be fashioned into optical fibers for endoscopic use. Holmium-doped YAG laser rods and optical fibers are used in laparoscopy, urology, general surgery, lithotripsy, orthopedic surgery, angioplasty, and dentistry. Double laser rod systems double output power for many applications, such as in dissolving urinary stones.
Because of the wide application, these lasers are called the ‘Swiss Army Knife’ of lasers.
