A Practical Guide Paper on Bulk and PLD Thin-Film Metals Commonly Used as Photocathodes in RF and SRF Guns

Abstract

This paper serves as a comprehensive and practical resource to guide researchers in selecting suitable metals for use as photocathodes in radio-frequency (RF) and superconducting radio-frequency (SRF) electron guns. It offers an in-depth review of bulk and thin-film metals commonly employed in many applications. The investigation includes the photoemission, optical, chemical, mechanical, and physical properties of metallic materials used in photocathodes, with a particular focus on key performance parameters such as quantum efficiency, operational lifetime, chemical inertness, thermal emittance, response time, dark current, and work function. In addition to these primary attributes, this study examines essential parameters such as surface roughness, morphology, injector compatibility, manufacturing techniques, and the impact of chemical environmental factors on overall performance. The aim is to provide researchers with detailed insights to make well-informed decisions on materials and device selection. The holistic approach of this work associates, in tabular format, all photo-emissive, optical, mechanical, physical, and chemical properties of bulk and thin-film metallic photocathodes with experimental data, aspiring to provide unique tools for maximizing the effectiveness of laser cleaning treatment.

1. Introduction

Historically, significant research has been devoted to improving the performance and longevity of metallic photocathodes as RF and SRF guns [1,2,3,4] through surface treatments [5,6,7,8,9], protective coatings [10,11], and operational optimizations by engineered configurations of photocathodes [12,13,14]. However, only a few reviews have comprehensively examined how environmental factors—such as exposure to reactive gases or contaminants—affect material durability and efficiency. Our study emphasizes such practical issues, allowing our findings to provide a valuable guide to real-world applications.

In this paper, we focus on six metals, copper (Cu), magnesium (Mg), yttrium (Y), niobium (Nb), lead (Pb), and barium (Ba), which are well suited for RF and SRF gun photocathode applications, owing to their quantum efficiency (QE), operational lifetime, thermal emittance, and compatibility with injector systems [15,16,17,18]. While previous reviews have examined the above materials in detail [19,20], our work provides a more comprehensive analysis by addressing often-overlooked factors, such as surface roughness, fabrication methods, environmental conditions, and laser cleaning effects, all presented in tabular format to guide the reader.

A key aspect of the present study is the comparative evaluation of bulk vs. thin-film photocathodes. Bulk metals offer superior oxidation resistance and longer operational lifetimes, whereas thin-film photocathodes, particularly in hybrid and non-conventional configurations [21,22,23], often exhibit a QE higher than their bulk counterparts [24,25]. By analyzing these trade-offs, we aim to provide valuable insights into materials selection for RF and SRF gun systems. Such optimization of photocathode performance will contribute significantly to relevant beneficiary technologies and applications, including particle accelerator physics [26,27,28], free-electron lasers (FELs) [29,30,31,32,33,34,35,36,37,38,39,40], inverse Compton scattering sources [41], energy recovery linacs (ERLs) [42,43,44,45,46,47,48], ultrafast electron microscopy and diffraction (UEM/D) systems [49,50], and other high-brightness electron source applications [51,52,53].

2. Highlights on Metallic Photocathodes

The main performance parameters of photocathodes based on the aforementioned copper (Cu), magnesium (Mg), yttrium (Y), niobium (Nb), lead (Pb), and barium (Ba) metals have been thoroughly and precisely detailed in numerous review articles [54,55,56]. However, this paper complements and expands upon those by presenting and analyzing a complete compilation of all experimental results obtained to date.

Table 1 summarizes the relevant photo-emissive properties of metallic photocathodes in both bulk and thin-film forms, Table 2 highlights the optical properties, and Table 3 details the most salient chemical, physical, and electrical properties of the metals under study. Table 1 presents the photo-emissive properties of the above metals, denoting those exhibiting the lowest work function (WF) values: Ba (2.5 eV) and Y (3.1 eV). WF values are primarily influenced by the electronic structure, surface morphology, and surface chemical contamination, such as hydrides and oxides formed on the solid metallic surface. Despite its high WF (4.6 eV), Cu remains the most used metal due to its extremely low electrical resistivity (16.8 (nΩ × m)) and high thermal conductivity (400 W/(m × K)). The thermal emittance of Cu is relatively low (0.6 mm × mrad) as compared to other metals [57,58,59,60]; however, the very high work function limits its QE value (2 × 10⁻⁵ at 266 nm) and requires excitation by deep UV laser pulses, such as the fourth harmonic of Nd:YAG at 266 nm (4.66 eV) or the third harmonic of Ti–sapphire at 267 nm (4.64 eV). In contrast, cathodes based on Y (WF = 3.1 eV) can utilize the third harmonic of Nd:YAG at 355 nm (3.5 eV) as the drive laser, offering more available energy and a higher operational stability. Additionally, the photoemission induced by visible radiation reduces the thermal emittance of the photoelectron beams. Regarding the QE of other metals, it is evident that Y has the highest value (3.0 × 10⁻³), followed by Mg (7.6 × 10⁻⁴) and Ba (7.6 × 10⁻⁴). Table 1 also reports the QE values of Cu, Mg, Y, and Pb in thin-film form. These values are either similar to or higher than those corresponding to the respective bulk metals, likely due to lower optical reflection [61,62].

The operational lifetime of photocathodes mainly depends on the vacuum quality in which they operate. To prevent degradation of photocathode lifetime caused by contamination processes, such as the growth of hydride and oxide layers on the cathode surface, even in ultra-high vacuum (UHV, <10⁻⁷ Pa) systems, RF and SRF guns should be nearly free of hydrogen- and oxygen-containing molecules, like H₂, H₂O, CO₂, and O₂ [63,64,65,66,67,68,69,70]. Table 1 specifies the recommended vacuum levels for each metallic photocathode in gun systems. Photocathodes based on bulk metals generally have a much higher operational lifetime than their thin-film counterparts. The high chemical reactivity of Mg, Y, and Ba is mainly due to their low electronegativity, as shown in Table 2. Finally, all metals under study exhibit fast response times, significantly less than 1 ps [71,72]. Therefore, the temporal length of the electron bunches is closely related to the driving laser pulse duration. Photocathodes based on samarium (Sm) are not included in this guide because of the limited literature available, despite the low WF (2.7 eV) and relatively high QE (7.0 × 10⁻⁴) [73] of the material. The lack of attention to this lanthanide element in accelerator science and technology is mainly due to its very low thermal conductivity (13.3 W/(m × K)) and extremely high electrical resistivity (940 nΩ × m). Such properties rule out Sm as an efficient photocathode material for RF cavities. For the same reasons, photocathodes based on these metals do not find immediate application in RF and SRF cavities.

Table 1. Photo-emissive properties of the selected metals (the list of all symbols is given in

Table A1. List of symbols associated with the photo-emissive, chemical, physical, and optical properties of the metallic photocathodes.

Definition	Symbol
Boiling point	Te
Dark current	Id
Dark lifetime	τd
Density	ρ
Electrical conductivity	σe
Electrical resistivity	ρe
Electronegativity	χ
Electron charge	e
Electron diffusivity	De
Electron mobility	μe
Extinction coefficient for bulk	kb
Extinction coefficient for thin films	ktf
First ionization energy	Ei
Latent heat of evaporation	ΔHe
Latent heat of fusion	ΔHf
Melting point	Tm
Number density of free electrons	Ne
Operational lifetime for bulk	τob
Operational lifetime for thin film	τotf
Optical penetration depth	δ
Optical reflection coefficient for bulk	Rb
Optical reflection coefficient for thin film	Rtf
Quantum efficiency	QE
Radiation wavelength	λ
Recommended vacuum level in the gun system	Pbg
Refractive index for bulk	nb
Refractive index for thin film	ntf
Response time	τr
Roughness	Ra
Thermal capacity	C
Thermal emittance for bulk	εb
Thermal emittance for thin film	εtf
Thermal conductivity	σT
Thermal diffusivity	DT
Work function	WF

).

	Cu	Mg	Y	Pb	Nb	Ba
WF (eV)	4.6 [74]	3.6 [9]	3.1 [75]	4.2 [76]	4.3 [76]	2.5 [77]
QE (at 266 nm) Bulk	2.0 × 10−5 * [78]	7.6 × 10−4 [79]	3.0 × 10−3 [79]	7 × 10−5 [80]	3.2 × 10−6 at 248 nm [81]	7.6 × 10−4 at 248 nm [82] 1 × 10−3 at 337 nm [83]
Thin film	6.0 × 10−5 [84]	1.8 × 10−3 [79]	3.3 × 10−4 [79]	8.0 × 10−5 [80]	NA	NA
τob (years) [19] τotf (months)	No limit <1 [84]	~1 2–3 [85]	<1 3–5 **	>1 4–6 **	No limit 3–5 **	<<1 <1 **
τd (years)	No limit	>2	>1	>1	No limit	~1
τr (ps)	<<1	<<1	<<1	<<1	<<1	<<1
Id (nA)	53 at 14.6 MV/m [86]	20 at 90 MV/m [84]	NA	240 at 14.5 MV/m [87]	NA	NA
εb (mm × mrad)	0.6 [88,89] 0.35 [90]	0.4 [91] 0.5 [92]	NA	0.8 ** [39]	0.6 [93]	NA
εtf (mm × mrad)	NA	0.85 [85]	0.76 [94]	NA	NA	NA
Pbg (Pa)	<10−7	<10−8	<10−8	<10−6	<10−6	<10−8

* Highest QE values up to 4.5 × 10⁻⁵ have been obtained using BPS-172 cleaning technique, as reported in Ref [95]. ** Expected value.

Table 1. Photo-emissive properties of the selected metals (the list of all symbols is given in Table A1).

	Cu	Mg	Y	Pb	Nb	Ba
WF (eV)	4.6 [74]	3.6 [9]	3.1 [75]	4.2 [76]	4.3 [76]	2.5 [77]
QE (at 266 nm) Bulk	2.0 × 10−5 * [78]	7.6 × 10−4 [79]	3.0 × 10−3 [79]	7 × 10−5 [80]	3.2 × 10−6 at 248 nm [81]	7.6 × 10−4 at 248 nm [82] 1 × 10−3 at 337 nm [83]
Thin film	6.0 × 10−5 [84]	1.8 × 10−3 [79]	3.3 × 10−4 [79]	8.0 × 10−5 [80]	NA	NA
τob (years) [19] τotf (months)	No limit <1 [84]	~1 2–3 [85]	<1 3–5 **	>1 4–6 **	No limit 3–5 **	<<1 <1 **
τd (years)	No limit	>2	>1	>1	No limit	~1
τr (ps)	<<1	<<1	<<1	<<1	<<1	<<1
Id (nA)	53 at 14.6 MV/m [86]	20 at 90 MV/m [84]	NA	240 at 14.5 MV/m [87]	NA	NA
εb (mm × mrad)	0.6 [88,89] 0.35 [90]	0.4 [91] 0.5 [92]	NA	0.8 ** [39]	0.6 [93]	NA
εtf (mm × mrad)	NA	0.85 [85]	0.76 [94]	NA	NA	NA
Pbg (Pa)	<10−7	<10−8	<10−8	<10−6	<10−6	<10−8

* Highest QE values up to 4.5 × 10⁻⁵ have been obtained using BPS-172 cleaning technique, as reported in Ref [95]. ** Expected value.

The scarce literature on the photo-emissive and physical properties of the above metals in thin-film form is primarily attributed to their high chemical reactivity that makes both their preparation and characterization extremely difficult. Moreover, the thermal emittance and WF values given in Table 1 are not fully comparable, as they were obtained under different conditions, even if the most commonly used technique to measure the WF is the photoelectric method [96]. It is well known that the photo-emissive properties of cathodes depend on various factors, including surface morphology (impurities and crystallinity), laser-driving wavelength, QE non-uniformity, applied electric field, space charge, laser spot size, polarization of the laser beam, and energy spread, as reported by Dowell and Schmerge [97].

Table 2. Relevant optical properties of the metals under study in both bulk and thin-film forms and roughness values of thin films.

	Cu	Mg	Y	Pb	Nb	Ba
δ (nm) λ (nm)	14 280 [98]	12 266 [99]	18 266 [99]	9 280 [100]	11 400 [101]	NA
nb (bulk)	1.16 at 450 nm 0.15 at 600 nm [102]	0.18 at 405 nm [96] 0.37 at 589 nm [102]	NA	1.78 700 nm [103]	2.52 at 405 nm 2.92 at 589 nm [104]	NA
kb (bulk)	2.4 at 450 nm 3.29 at 600 nm [102]	3.57 at 405 nm [96] 4.42 at 589 nm [102]	NA	3.57 at 700 nm [103]	2.63 at 405 nm 2.87 at 589 nm [104]	NA
* Rb (Φ = 0) %	55 at 450 nm 95 at 600 nm	97 at 405 nm 93 at 589 nm	45 at 405 nm 53 at 578 nm	65 at 700 nm	48 at 405 nm 50 at 589 nm	43 at 400 nm 53 578 nm [77]
ntf	0.87 at 450 nm 0.186 at 600 nm [102]	0.52 at 405 nm 0.48 at 589 nm [105]	0.8 at 405 nm 1.3 at 578 nm [106]	1.68 at 700 nm [102]	NA	0.82 at 405 nm 0.88 at 578 nm [102]
ktf	2.20 at 450 nm 2.98 at 600 nm [102]	2.05 at 405 nm 3.71 at 589 nm [105]	1.6 at 405 nm 2.4 at 578 nm [106]	3.67 at 700 nm [102]	NA	1.07 at 405 nm 1.52 at 578 nm [105]
* Rtf (Φ = 0) %	58 at 450 nm 93 at 600 nm	68 at 405 nm 88 at 589 nm	45 at 405 nm 53 at 578 nm	67 at 700 nm	NA	26 at 405 nm 40 at 578 nm
Ra (nm)	7.5 [107]	3 [108]	2 [109]	14 [110]	9 [111]	NA

* Calculated: R = [(n_o − n)² + k²]/[(n_o + n)² + k²], with n₀ index of refraction in air.

Table 2. Relevant optical properties of the metals under study in both bulk and thin-film forms and roughness values of thin films.

	Cu	Mg	Y	Pb	Nb	Ba
δ (nm) λ (nm)	14 280 [98]	12 266 [99]	18 266 [99]	9 280 [100]	11 400 [101]	NA
nb (bulk)	1.16 at 450 nm 0.15 at 600 nm [102]	0.18 at 405 nm [96] 0.37 at 589 nm [102]	NA	1.78 700 nm [103]	2.52 at 405 nm 2.92 at 589 nm [104]	NA
kb (bulk)	2.4 at 450 nm 3.29 at 600 nm [102]	3.57 at 405 nm [96] 4.42 at 589 nm [102]	NA	3.57 at 700 nm [103]	2.63 at 405 nm 2.87 at 589 nm [104]	NA
* Rb (Φ = 0) %	55 at 450 nm 95 at 600 nm	97 at 405 nm 93 at 589 nm	45 at 405 nm 53 at 578 nm	65 at 700 nm	48 at 405 nm 50 at 589 nm	43 at 400 nm 53 578 nm [77]
ntf	0.87 at 450 nm 0.186 at 600 nm [102]	0.52 at 405 nm 0.48 at 589 nm [105]	0.8 at 405 nm 1.3 at 578 nm [106]	1.68 at 700 nm [102]	NA	0.82 at 405 nm 0.88 at 578 nm [102]
ktf	2.20 at 450 nm 2.98 at 600 nm [102]	2.05 at 405 nm 3.71 at 589 nm [105]	1.6 at 405 nm 2.4 at 578 nm [106]	3.67 at 700 nm [102]	NA	1.07 at 405 nm 1.52 at 578 nm [105]
* Rtf (Φ = 0) %	58 at 450 nm 93 at 600 nm	68 at 405 nm 88 at 589 nm	45 at 405 nm 53 at 578 nm	67 at 700 nm	NA	26 at 405 nm 40 at 578 nm
Ra (nm)	7.5 [107]	3 [108]	2 [109]	14 [110]	9 [111]	NA

* Calculated: R = [(n_o − n)² + k²]/[(n_o + n)² + k²], with n₀ index of refraction in air.

Table 3. Chemical, physical, and electrical properties in bulk form.

	Cu	Mg	Y	Pb	Nb	Ba
Tm (K)	1358	923	1799	601	2750	1000
Te (K)	2835	1363	3203	2022	5017	2118
ρ (g/cm3)	8.92	1.74	4.47	11.34	8.57	3.51
Electronegativity (Pauling) [112]	1.90	1.31	1.22	1.80	1.60	0.89
Chemical reactivity	VERY LOW	MEDIUM	MEDIUM	VERY LOW	LOW	VERY HIGH
Ei (eV)	7.19	7.12	5.94	6.41	6.99	8.69
ΔHf (kJ/mol)	13.3	8.5	11.4	4.8	30.0	7.1
ΔHe (kJ/mol)	300	128	363	179	690	142
σT [W/(m × K)] [112]	400	156	17.2	35.3	53.7	18.0
C [J/(kg × K)]	385	1020	300	129	265	204
DT (mm2/s)	116.0 [106]	87.9 *	12.8 *	23.0 [113]	23.6 *	25.1 *
ρe (nΩ × m) [112]	16.8	43.9	596	208	152	332
Ke (MS/m)	59.5	22.7	1.7	4.8	6.6	3.0
Ne (×1022 cm−3)	8.5	8.6	6.0	13.2	5.6	3.15
μe (cm2/(V × s) **	43.8	1.6	1.8	2.3	7.5	5.9
De *** (cm2/s)	1.2	0.04	0.05	0.06	0.19	0.15
Electronic shell structure	[Ar] 3d104s1	[Ar] 3d104s1	[Kr] 5s24d1	[Kr] 5s24d1	[Kr] 5s14d4	[Xe] 6s2

* Calculated: D_T = σ_T/(ρ × C). ** Calculated: μ_e = σ_e/(N_e × e), with e electron charge. *** Calculated: D_e = μ_e × k_B × T/e, where k_B is the Boltzmann constant.

Table 3. Chemical, physical, and electrical properties in bulk form.

	Cu	Mg	Y	Pb	Nb	Ba
Tm (K)	1358	923	1799	601	2750	1000
Te (K)	2835	1363	3203	2022	5017	2118
ρ (g/cm3)	8.92	1.74	4.47	11.34	8.57	3.51
Electronegativity (Pauling) [112]	1.90	1.31	1.22	1.80	1.60	0.89
Chemical reactivity	VERY LOW	MEDIUM	MEDIUM	VERY LOW	LOW	VERY HIGH
Ei (eV)	7.19	7.12	5.94	6.41	6.99	8.69
ΔHf (kJ/mol)	13.3	8.5	11.4	4.8	30.0	7.1
ΔHe (kJ/mol)	300	128	363	179	690	142
σT [W/(m × K)] [112]	400	156	17.2	35.3	53.7	18.0
C [J/(kg × K)]	385	1020	300	129	265	204
DT (mm2/s)	116.0 [106]	87.9 *	12.8 *	23.0 [113]	23.6 *	25.1 *
ρe (nΩ × m) [112]	16.8	43.9	596	208	152	332
Ke (MS/m)	59.5	22.7	1.7	4.8	6.6	3.0
Ne (×1022 cm−3)	8.5	8.6	6.0	13.2	5.6	3.15
μe (cm2/(V × s) **	43.8	1.6	1.8	2.3	7.5	5.9
De *** (cm2/s)	1.2	0.04	0.05	0.06	0.19	0.15
Electronic shell structure	[Ar] 3d104s1	[Ar] 3d104s1	[Kr] 5s24d1	[Kr] 5s24d1	[Kr] 5s14d4	[Xe] 6s2

* Calculated: D_T = σ_T/(ρ × C). ** Calculated: μ_e = σ_e/(N_e × e), with e electron charge. *** Calculated: D_e = μ_e × k_B × T/e, where k_B is the Boltzmann constant.

Some of the values reported in the above table are not cited, as they can be found easily online. However, others were calculated using the formulas provided below the table.

3. Main Factors That Influence the Photo-Emissive Properties of Metallic Photocathodes

The key factors influencing the photo-emissive properties of metallic photocathodes include surface morphology, chemical environment, surface contaminants, the reflection coefficient, and fabrication methods.

Surface morphology has a significant impact on dark current, thermal emittance, and QE. The dark current or field emission of electrons is a crucial parameter that reduces photocathode performance. It is an unwanted current that flows in an RF gun in the absence of incident radiation [114]. A smoother surface obtained by dedicated cleaning treatment typically results in lower dark current and thermal emittance values. For instance, as reported in a study conducted at the DESY test facility, the dark current was reduced by an order of magnitude after a targeted cleaning treatment with dry ice [39]. Yet, Benjamin et al. showed that oxidized surfaces could reduce the dark current due to a slight increase in the work function and a reduced mobility of electric charges in the oxide layer [115]. Similarly, a study by Zhou et al. reported a significant improvement in thermal emittance following cleaning processes designed to remove contaminants, such as oxides and hydrides, from the metal surface [5]. However, the final surface morphology depends on the cleaning technique and the experimental conditions employed. For example, in the case of laser cleaning, both the type of photocathode and the laser parameters are fundamental in determining the resulting morphology, which can sometimes be considerably rough. Surface morphology also plays a key role in QE, as shown in Table 1, where photocathodes based on bulk metals consistently exhibit lower QE values than their thin-film counterparts [24,25]. In cases where the photocathodes are prepared with Mg and Ba, the discrepancy is likely attributed to the reflection coefficient, which tends to decrease as surface roughness increases, as observed in thin films [61,62]. For the rest of the metals under study, no significant variations have been observed (see Table 2). Additionally, the adsorption of atomic and molecular species can alter the surface band structure, causing variations in the performance of metallic photocathodes [116]. Changes in surface morphology are often a result of cleaning treatments. Among the various available methods, laser irradiation has proven to be the most effective and widely used technique for surface cleaning of photocathodes—a topic that will be discussed in detail later in this paper.

The chemical environment is another critical factor that influences not only the QE but also the operational lifetime of photocathodes. Electronegative gases, such as H₂ and O₂, and vapors, like H₂O and hydrocarbons, even in ultra-high vacuum cavities [117], can promote hydration and oxidation of the photocathode surface. These contaminant layers reduce QE values because of their relatively high work function, which inhibits or reduces the photoemission process. Eun Ha Choi et al. [118] investigated the WF of MgO for different crystal orientations, reporting values between 4.22 eV and 5.07 eV, all being significantly higher than the WF of a pure Mg surface. The WF of yttrium hydride is 4.76 eV higher than that of pure yttrium, as reported by T. Mongstad et al. [119]. However, various research groups have also demonstrated that ultra-thin oxide films (a few atomic layers) formed on metal substrates can enhance the photo-emissive properties of metallic photocathodes [120,121,122,123]. The explanation for this unexpected behavior, not included here for the sake of brevity, is thoroughly and comprehensively discussed in the cited references. Figure 1, Figure 2 and Figure 3 illustrate QE variations as a function of exposure time to the gas environment.

Thermal and laser cleaning treatments are the most used techniques to remove the surface contaminants from photocathodes. Other methods include mechanical, ultrasonic, and chemical cleaning [126,127], as well as hydrogen ion (H⁺) [128] and atomic hydrogen (H*) [129] or argon ion (Ar⁺) sputter-cleaning [130]. Thermal cleaning is highly effective in removing adsorbates, especially under ultra-high vacuum conditions; however, it can simultaneously accelerate the formation of oxides and hydrides due to the relatively high surface temperature. Therefore, finding a delicate balance between the desorption of atomic and molecular species and their chemical reactions with the metallic surface is crucial. The temperature of thermal cleaning technique required to remove surface contamination depends on the chemical reactivity of the material to be treated. For photocathodes based on alkaline earth metals, such as Mg and Ba, and to a lesser extent, Y, a temperature of about 200 °C is a good compromise between effective cleaning and reduced formation of unwanted oxides and hydrides on the surface [131]. On the contrary, for photocathodes based on metals with high chemical inertness, such as Cu bulk, the treatment temperature can reach 800 °C [132].

The cleaning process parameters by sputtering and magnetron sputtering vary depending on the component being cleaned and the type of material being sputtered. However, typical ion energy values used in sputtering and magnetron sputtering for metallic surface cleaning are about a few keV [130].

Laser cleaning treatment appears to be the most suitable technique; it is particularly useful when dealing with thin-film photocathodes. However, it is fundamental to determine the appropriate laser energy density (laser fluence) to prevent the ablation of the underlying metallic substrate. This cleaning technique must be performed at laser fluence just above the laser ablation threshold of contaminants with a physical process called “gentle” ablation. In this experimental condition, a small amount of material is ablated, minimizing damage beneath the contaminant layers. Typical laser parameters for laser cleaning treatment are a laser spot size of tenths of mm with energy of about tens of μJ (power density and laser energy density of some GW/cm² and tens of mJ/cm², respectively) and a laser repetition rate in the range of 5–10 Hz with hundreds or thousands of laser pulses per site [133]. Concerning other traditional cleaning treatments, they have been successfully applied to bulk metal surfaces and are not commonly used to clean photocathodes based on thin films.

Table 4. Effectiveness of the laser cleaning procedure on some metallic photocathodes under study (reproduced and modified from ref. [54]).

Mg	Before LCT	5.0 × 10−4	[134]
	After 200 lpps *	8.9 × 10−4	[124]
	After 300 lpps	1.1 × 10−3	[124]
	After 400 lpps	1.4 × 10−3	[124]
	After 500 1pps	1.8 × 10−3	[124]
Y	Before LCT	~10−5	[74]
	After 100 lpps	1.2 × 10−4	[74]
	After 400 lpps	2.8 × 10−4	[74]
	After 900 lpps	3.3 × 10−4	[74]
Pb	Before LCT	3.0 × 10−5	[80]
	After thousands lpps	8.0 × 10−5	[80]

* lpps: laser pulses per site.

highlights the effectiveness of laser cleaning treatment for various metallic photocathodes. The improvement in QE values after the cleaning procedure is clearly evident. The laser parameters corresponding to the results shown in Table 4 are detailed in the respective cited references.

The reflection coefficient plays a significant role in determining the QE of metallic photocathodes. The high reflectivity of metallic surfaces considerably limits the QE of photocathodes. Several research groups have also demonstrated that cleaning treatments typically increase the surface roughness, which, in turn, decreases the surface reflectance. As a result, while QE improves, the emittance unfortunately also increases [124,134,135,136,137,138]. However, it should be emphasized that the exact value of the reflection coefficient may vary depending on factors such as surface and cleaning condition, oxidation, and measurement technique.

A variety of technologies have been developed to fabricate metallic photocathodes with notable photo-emissive properties. The most used metallic photocathodes are either based on bulk disks or thin films deposited on bulk materials, such as Cu or Nb. Devices based on bulk disks are primarily prepared using techniques such as friction welding [139], hot isostatic pressing [140], and press fitting [141]. In these methods, a bulk disk with relatively high QE, typically made of Mg, Y, or Pb, is pressed into a Cu or Nb disk. This configuration provides a higher QE compared to a pure metallic disk and ensures good compatibility with RF and SRF cavities. However, it has been observed that such configurations can encounter issues with RF breakdown at the metallic disk interface, as shown in Figure 4 [140].

Other smart configurations have been engineered to create photocathodes based on thin films. The most frequently cited ones found in the literature are plasmonic photocathodes [142], hybrid configurations [21,23], and non-conventional configurations [22]. In these setups, metallic thin films with relatively high QE are deposited using various physical vapor techniques, including sputtering [143,144], pulsed laser deposition (PLD) [124,145], electroplating [146,147], magnetron sputtering [148], thermal evaporation [149], and ion implantation [150]. Among the numerous thin-film deposition techniques, the ones most used to prepare photocathodes based on metallic thin films to be used in RF and SRF guns are sputtering, magnetron sputtering, PLD, and ion implantation. Magnetron sputtering, using a magnetic field in a conventional sputtering system, results in a more advanced and effective deposition technique. The main advantages of the magnetron sputtering technique are the fast deposition speed, even on large surfaces. Moreover, the thin films deposited by this technique are highly adherent to the substrate [151]. The plasma instabilities and complexity of the deposition system are the core shortcomings. Films grown by PLD technique have a high adherence to the substrate due to the high kinetic energy of the ablated materials even at room temperature, but it shows a low deposition rate (tenths of Å per pulse) [152]. Characteristic laser parameters used in a conventional PLD system for the growth of a metallic film are laser fluence of about 10 J/cm² and laser spot size = 1 mm in ns regime, a few J/cm² and laser spot size = 300 μm in the ps regime, and repetition rate = 10 Hz and tens of thousands of laser pulses to deposit 500 nm thick film [133,134]. Most thin-film depositions occur at room temperature and at a background pressure of less than 10⁻⁵ Pa.

The thermal evaporation technique enables the deposition of highly uniform thin films across large surface areas. However, the adherence of the film to the surface is poor, compromising its use in RF and SRF guns as a photocathode [149]. On the contrary, the ion implantation method creates films that are very adherent, but it requires expensive equipment [153].

Figure 5a–c illustrate the commonly used engineered photocathode configurations. In detail, Figure 5a shows the press-fitting configuration, where a high-QE metal disk, usually Mg, Y, or Ba, is pressed into a bulk Cu disk, which is highly compatible with RF cavities. In a similar manner, a Pb disk is inserted into a Nb bulk for use in SRF cavities because of its good compatibility. Figure 5b displays the hybrid configuration, in which a high-QE thin film (Mg, Y, or Ba) is deposited on a bulk Cu substrate for RF cavities, and a Pb thin film is deposited on a Nb bulk disk to serve as a photocathode in SRF cavities. Finally, Figure 5c illustrates the non-conventional configuration, where a bulk of high-QE metal is completely covered by a thin film of Cu or Pb, except for the central part, which serves as the emitting area in RF and SRF cavities.

The high quality of the PLD thin-film deposition process is illustrated in Figure 6 and Figure 7. In both figures, the sharp boundary between the substrate and the thin film is analyzed using the SEM and EDX techniques [22,154].

Table 5. Compatibility of photocathodes based on thin films in the hybrid configuration.

	Mg Thin Film on Cu Back-Flange	Y Thin Film on Cu Back-Flange	Pb Thin Film on Nb Back-Flange
Compatibility with a Cu RF gun	High	Medium	-
Compatibility with a Nb SRF gun	-	-	High

and

Table 6. Compatibility of photocathodes based on thin films in the non-conventional configuration.

	Mg Bulk Cathode with Cu-Mask	Y Bulk Cathode with Cu-Mask	Pb Bulk Cathode with Nb-Mask
Compatibility with a Cu RF gun	Very high	High	-
Compatibility with a Nb SRF gun	-	-	Very high

highlight the good compatibility of metal thin films, with the RF and SRF cavities based on experimentally engineered photocathode configurations. The strong adhesion of the films to the substrate [78] adds further advantages, which promote their use in real-world photo-emitting devices.

4. Conclusions

In conclusion, considering the operational lifetime, the best photocathodes for RF and SRF cavities are based on Cu and Nb bulk materials. However, these devices exhibit a relatively low QE, and various engineered configurations have been proposed aiming to enhance QE. One approach involves deposition of a high-QE thin film on the aforementioned metals, resulting in a hybrid configuration, which has been explored by several research groups. Unfortunately, this configuration shortens the operational lifetime due to film thinning caused by continuous operation and the required cleaning treatments. Additionally, a further issue relates to their reduced compatibility with the cavities. Therefore, an optimal solution could be the non-conventional configuration, which could simultaneously ensure both a long operational lifetime and a high QE. Such a configuration seems to be particularly promising, offering two important advantages: the high QE and the low WF characteristic of metallic thin films (commonly Mg, Y, Ba, or Pb), while maintaining high electrical compatibility when installed into RF and SRF guns made of Cu bulk and Nb bulk. In this case, no modification of the cavity performance is observable. Nevertheless, a shortcoming of this configuration lies in the preparation of the photocathode. Depositing a uniform thin film over a large surface area is significantly more complex from a technological point of view. Besides the nature of the metal itself, the photo-emissive properties of the particular photocathode also depend on laser parameters, the fabrication techniques applied, and its surface morphology and reflectivity. A further critical factor is the chemical environment in which these devices operate. To minimize the impact of oxides and hydrides on the QE value, several cleaning procedures must be employed. Among them, thermal and laser cleaning treatments are the most significant for optimizing photocathode performance. All the above considerations apply to both types of photocathodes designed either for RF or SRF guns.